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Tectonic models for development of the Svalbard Caledonides depend on reliable assessment of the metamorphic evolution of the various basement provinces involved. The Mosselhalvoya Group (MG) and the Atomfjella Complex (AC) have previously been assigned to the Nordaustlandet and West Ny-Friesland terranes, respectively. New analytical data and petrographic observations indicate that both units experienced two-stage metamorphism under similar pressure-temperature (P-T) conditions. Two stages of amphibolite facies metamorphism (Ml and M2) are clearly recorded by garnet and staurolite porphyroblast textures. The results of thermodynamic phase equilibrium modeling indicate that peak M2 metamorphism occurred at ~ 7-7.5 kbar and 590-600 [degrees]C in both units. Zirconium-in-rutile trace element thermometry confirms the temperature estimates for Ml and M2 stages of metamorphism. Monazite chemical Th-U-Pb dates from the MG resolve a two-stage garnet growth at 444 [+ or -] 7 Ma (Ml) and 423 [+ or -] 6 Ma (M2). In contrast, monazite dated in the AC defines a single age of 420 [+ or -] 4 Ma interpreted as M2 growth. We suggest M2 was coeval with early strike-slip motion along the Billefjorden Fault Zone, whereas Ml reflects initial tectonic burial of the studied units. The similarity in metamorphic history between the both units suggests that the boundary between them is a subordinate thrust fault within the Atomfjella thrust stack rather than a major boundary separating the Nordaustlandet and West Ny-Friesland terranes. The MG should be included within the West Ny-Friesland terrane and the tectonic boundary with the Nordaustlandet terrane is likely the Eolussletta Shear Zone. Key words: Mosselhalvoya Group, Atomfjella Complex, West Ny-Friesland terrane, Nordaustlandet terrane, thermodynamic phase equilibrium modelling, monazite, 1. Introduction The Svalbard Caledonides are traditionally divided into three main basement blocks, terranes, or provinces with differing structural history, sedimentary record, and tectono-thermal evolution (Harland 1997). The Northeastern, Northwestern, [...]

The Nemiscau subprovince is a metasedimentary rocks-dominated sequence of the Archean eastern Superior Province. It is bounded by the gneissic and tonalite-trondhjemite-granodiorite (TTG) rocks-dominated La Grande and Opatica subprovinces. The Nemiscau consists of variably migmatized metasedimentary rocks and felsic to intermediate gneisses and plutonic suites. Maflc-to-ultramaflc metavolcanic rocks occur along its northern and southern boundaries. Previous structural and metamorphic studies suggested that it was the result of subduction-related, accretionary and collisional tectonics with adjacent plutonic terranes during the Kenorean orogeny. This study integrates various sets of structural, metamorphic, and U-Pb geochronological data suggesting a long-lasting tectonometamorphic evolution between ca. 2843 and 2598 Ma. Four tectonometamorphic events have been recognized. The first event ([D.sub.1]) is only locally preserved and occurred shortly after the deposition of the oldest volcanic sequences of the Nemiscau (ca. 2756-2736 Ma) under amphibolite facies metamorphic conditions ([M.sub.1]). The kinematic analysis of shear zones bounding the supracrustal sequences of both the Nemiscau and La Grande subprovinces suggests their relative crustal sinking as compared to TTG of the La Grande and Opatica subprovinces, shortly after deposition (at [less than or equal to] 2724 and [less than or equal to] 2706 Ma, respectively). Sedimentation was followed by regional dip-slip-dominated ([D.sub.2]) deformation between ca. 2704 and 2671 Ma, coeval with extensive high-grade granulite facies metamorphism ([M.sub.2]) and anatexis in the Nemiscau subprovince from ca. 2697 to 2685 Ma, followed by exhumation between ca. 2677 and 2671 Ma. This [D.sub.2] event was followed by regional-scale dextral strike-slip shearing ([D.sub.3]) from ca. 2658 until 2621 Ma at amphibolite facies metamorphism ([M.sub.3]). The youngest deformation event, [D.sub.4], was accompanied by strain localization along brittle-to-ductile conjugated shear zones and waning crustal cooling from amphibolite to greenschist facies conditions at ca. 2598 Ma and younger. It is suggested that the Nemiscau, La Grande, and Opatica subprovinces represent a single composite terrane, and that their mutual boundaries do not correspond to 'collisional sutures' between different crustal blocks or microcontinents. The Nemiscau subprovince is interpreted as a sedimentary sequence unconformably overlying a ca. 2760-2756 Ma (and older) basement made up of volcanic and crosscutting TTG. In terms of tectonic evolution, the overall structural architecture and isotopic ages of the Nemiscau, La Grande, and Opatica subprovinces support a tectonic model in which the vertical transfer of crustal material occurred during the early stages ([D.sub.2]) of regional deformation, which evolved into a predominant lateral crustal flow of the ductile crust during later stages ([D.sub.3]-[D.sub.4]). A non-uniformitarian tectonic model for the Archean more adequately accounts for synchronous vertical and horizontal tectonism as preserved in the Nemiscau basin. Key words: Nemiscau subprovince, structures, U-Pb geochronology, tectonometamorphic evolution, Archean tectonism, 1. Introduction The Superior Province is the largest Archean craton in the world (Calvert and Ludden 1999). The southern part of the craton is characterized by an E-W linear arrangement [...]

The Bras d'Or terrane of central Cape Breton Island, Nova Scotia, Canada, contains a well-preserved record of the Ediacaran to early Cambrian evolution of Ganderia, a Gondwana-derived terrane in the northern Appalachian orogen. A complex assemblage of low- to high-grade metasedimentary rocks has varied detrital zircon signatures from laser ablation inductively coupled plasma mass spectrometry U-Pb zircon dating, but combining three or more samples yielded representative age spectra that support correlation of the low- and high-grade metasedimentary rocks throughout the Bras d'Or terrane and the corresponding Ganderian Brookville terrane of southern New Brunswick. In quartzite samples from the McMillan Flowage Formation in the northwestern Bras d'Or terrane, the youngest detrital zircons have ages >900 Ma, in contrast to previously studied psammitic and semipelitic samples from correlative units in the eastern Bras d'Or terrane in which youngest detrital ages are 620-600 Ma. Both quartzite and semipelitic samples from the McMillan Flowage Formation contain Neoproterozoic dates from zircon rims, which reflect metamorphic overgrowths during peak metamorphism at ca. 550 Ma, providing a robust age for peak metamorphism in the Bras d'Or terrane that supports similar, albeit sparse, ages reported previously from monazite and titanite samples. This metamorphism is coeval with the emplacement of voluminous dioritic to granitic plutons that occur throughout the Bras d'Or terrane and form in an Andean-type continental margin subduction zone. New U-Pb zircon ages presented here from plutons in the northern Bras d'Or terrane, combined with previously published ages, are consistent with subduction-related magmatism and associated metamorphism between ca. 575 and 540 Ma. Key words: U-Pb geochronology, Appalachian metamorphism, Bras d'Or terrane, Cape Breton Island, McMillan Flowage Formation, detrital zircon, plutonic ages, Introduction The Bras d'Or terrane of central Cape Breton Island, Nova Scotia (Fig. 1), consists of mainly Ediacaran and Cambrian metamorphic and plutonic rocks overlain unconformably by Carboniferous sedimentary rocks [...]

Lower Cretaceous marine sedimentary rocks, deposited in shallow shelf and hasin settings and unconformity-hound, are well exposed in southwest Alaska. Collections of Early Cretaceous fossils from across western Alaska show that similar and coeval Lower Cretaceous clastic rocks are widely distributed though only locally exposed. Volcanic rocks become an important part of the Lower Cretaceous sequence in the Yukon-Koyukuk hasin, where they have been interpreted to represent a mobile intra-oceanic island arc, the Koyukuk terrane, that collided with Arctic Alaska to form the Brooks Range orogen. The volcanic rocks are chemically unlike Aleutian arc rocks but share compositional characteristics with spatially related, mid-Cretaceous alkaline intrusive rocks. The volcanic-bearing sequence was also deposited on an angular unconformity, includes both shallow shelf and basin depositional settings, and is unconformably overlain by mid-Cretaceous clastic rocks. The volcanic rocks are therefore considered part of the Lower Cretaceous sequence now identified across western Alaska. In this interpretation, the Lower Cretaceous volcanic rocks are an initial expression of the mid-Cretaceous tectonic regime that included extensional exhumation and subsidence, crustal and upper mantle melting, and high-temperature metamorphism in the hinterland of the Brooks Range orogen. The Cretaceous heating that led to hinterland crust and upper mantle change may have been caused by deep mantle disturbances in a postsubduction setting. This interpretation has implications for the timing of contractional orogenesis, the location and nature of the related continental borderland, and the tectonic setting for development of the Anguyucham and related oceanic terranes. Key words: Koyukuk terrane, western Alaska, Cretaceous stratigraphy, Jurassic-Cretaceous tectonic history, Brooks Range orogen, Introduction A Lower Cretaceous (Valanginian and Hauterivian) marine clastic sequence is well exposed in southwest Alaska, where it unconformably overlies the Togiak and Goodnews terranes (Box 1985a, 1985b; Murphy 1987, [...]

The Halifax County complex (HCC) is a meta-ultramaflc/metamaflc body that crops out along the easternmost exposed part of the Carolina superterrane in northeastern North Carolina. Petrographic and mineral chemistry studies were undertaken to place some constraints on the evolution of the HCC. Halifax County Complex amphiboles are zoned, with hornblende cores and actinolitic rims. Feldspar minerals span the whole plagioclase- and potassium-feldspar spectrum. Evolved olivines (F[o.sub.69-75]) are primary, and pyroxenes plot in the enstatite, pigeonite, augite, and diopside fields. Low Ti[O.sub.2] ( Key words: Southern Appalachians, metamorphosed mafic/ultramafic rocks, ophiolite, mineral chemistry, 1. Introduction Terrestrial mafic and ultramafic rocks form in various tectonic settings and from a wide variety of magma types (e.g., Barnes and Roeder 2001; Dare et al. 2009). Examples [...]

The Grum deposit is one of five large clastic-dominated Zn-Pb-Ag deposits in the Anvil district of the Selwyn Basin, Yukon. Grum's mineralization occurs at the contact between the late Proterozoic to Cambrian Mount Mye Formation and the Cambrian to Early Ordovician Vangorda Formation. Paleomagnetic analyses of 113 specimens from 15 mineralized sites in the Grum open pit were undertaken to date Grum's mineralization. The analyses isolated a stable characteristic remanent magnetization (ChRM), mostly by thermal and then alternating field demagnetization. The main ChRM carrier is single- or pseudosingle-domain pyrrhotite with minor magnetite that give a paleopole at 67.8[degrees]N latitude and 317.4[degrees]E longitude (radius of the cone of 95% confidence, [A.sub.95] = 7.2[degrees]). The paleopole yields an age of 176 [+ or -] 12 Ma for the Grum deposit after a clockwise rotation and northward translation of the basin to best fit the North American apparent polar wander path. Thus, the Zn-Pb-Ag Grum mineralization significantly predates intrusion of the nearby mid-Cretaceous Anvil Batholith, but is coeval with widespread Early Jurassic metamorphism in the Selwyn Basin that correlates with the Early Jurassic collision of the Intermontane Belt terranes with the North America craton. Key words: Paleomagnetism, remagnetization, Zn-Pb-Ag mineralization, Grum deposit, Yukon Le gisement Grum fait partie de cinq grands gisements de Zn-Pb-Ag a predominance clastique dans le district d'Anvil du bassin de Selwyn (Yukon). La mineralisation a Grum se trouve au contact entre la Formation du mont Mye, d'age proterozoique tardif a cambrien, et la Formation de Vangorda, d'age cambrien a ordovicien precoce. Des analyses paleomagnetiques de 113 specimens provenant de 15 sites mineralises dans la fosse a Grum ont ete realisees afin de determiner l'age de la mineralisation. Les analyses ont permis d'isoler une magnetisation remanente caracteristique (MRC) stable, principalement par demagnetisation thermique, puis par champ alternatif. Le principal porteur de la MRC est de la pyrrhotite a domaine unique ou pseudo-unique avec un peu de magnetite, qui produisent un paleopole de latitude 67,8[degrees]N et de longitude 317,4[degrees]E (rayon du cone de l'intervalle de confiance de 95 %, [A.sub.95] = 7,2[degrees]). Le paleopole donne un age de 176[+ or -]12 Ma pour le gisement Grum apres une rotation horaire et une translation vers le nord du bassin, pour la meilleure concordance avec la trajectoire nord-americaine de derive apparente des poles. Ainsi, la mineralisation en Zn-Pb-Ag du gisement Grum est significativement plus ancienne que l'intrusion du batholithe d'Anvil, d'age cretace moyen, situe a proximite, mais est contemporaine du metamorphisme jurassique precoce repandu dans le bassin de Selwyn qui est correle avec la collision au Jurassique precoce des terranes du domaine intermontagneux avec le craton nord-americain. [Traduit par la Redaction] Mots-cles : paleomagnetisme, remagnetisation, mineralisation en Zn-P-Ag, gisement Grum, Yukon, 1. Introduction The Paleozoic Selwyn Basin in Yukon is dominated by marine sedimentary units containing black shales and cherts that host three substantial clastic-dominated (CD) Zn-Pb-Ag districts (Goodfellow and Lydon [...]

The Ellsworth belt is one of several fault-bounded blocks exposed along the southeastern coast of Maine that formed within Ganderia. New ID-TIMS U--Pb geochronological data integrated with field relationships provide additional insights into the timing of magmatism and deformation in the Ellsworth belt. The deformed Lamoine Granite was selected for U--Pb zircon analysis to: (i) establish the protolith age; (ii) provide direct temporal constraints on regional low-grade metamorphism and deformation; and (iii) elucidate relationships between the Ellsworth belt and coeval rocks elsewhere in the Appalachian orogen. The Lamoine Granite was emplaced within the Ellsworth Schist at 492 [+ or -] 1.7 Ma; this is the first unequivocal evidence for a Furongian magmatic event in the Ellsworth belt. The schistosity in the Lamoine Granite is parallel to the main fabric of the host Ellsworth Schist and provides a maximum estimate for timing of the regional metamorphic overprint. Widespread deformation in the Ellsworth belt where kinematic indicators indicate a top-to-northwest sense of shear is attributed to thrusting during which progressive horizontal shortening, caused crustal thickening and peak greenschist facies metamorphism. The Cambrian U--Pb age permits correlation of the Lamoine Granite with the Cameron Road Granite in the Annidale belt of New Brunswick where subduction-related magmas intruded the Penobscot arc-back-arc and were subsequently deformed during the Penobscot Orogeny. Key words: Appalachians, Ganderia, geochronology, granites, Maine, Cambrian. La ceinture d'Ellsworth figure parmi plusieurs blocs faillés exposés le long du littoral sud-est du Maine qui se sont formés au sein de la Gandérie. Des nouvelles données de géochronologie U--PbparID-TIMS jumelées à des relations de terrain jettent un nouvel éclairage sur la chronologie du magmatisme et de la déformation dans la ceinture d'Ellsworth. Le granite déformé de Lamoine a été sélectionné pour l'analyse U--Pb sur zircons afin (i) d'établir l'âge du protolite, (ii) d'obtenir de l'information temporelle directe sur le métamorphisme de faible degré et la déformation à l'échelle régionale et (iii) d'élucider les relations entre la ceinture d'Ellsworth et des roches de même âge ailleurs dans l'orogène des Appalaches. Le granite de Lamoinea étémis en placedans leschiste d'Ellsworth à 492 [+ or -] 1,7 Ma; il s'agit de la première preuve sans équivoque d'un épisode magmatique furongien dans la ceinture d'Ellsworth. La schistosité dans le granite de Lamoine est parallèle à la fabrique principale dans le schiste d'Ellsworth qu'il recoupe et fournit une limite supérieure estimée pour l'âge du métamorphisme régional surimposé. Une déformation ubiquiste dans la ceinture d'Ellsworth dont les indicateurs cinématiques témoignent d'un cisaillement avec déplacement du bloc supérieur vers le nord-ouest est attribuée à un chevauchement durant lequel un raccourcissement horizontal graduel s'est traduit par un épaississement crustal et un pic de métamorphisme au faciès des schistes verts. L'âge U--Pb cambrien permet de corréler le granite de Lamoine au granite de Cameron Road dans la ceinture d'Annidale du Nouveau-Brunswick, où des magmas associés à la subduction se sont mis en place dans l'arc-arrière-arc de Penobscot et ont ensuite été déformés durant l'orogenèse de Penobscot. [Traduit par la Rédaction] Mots-clés : Appalaches, Gandérie, géochronologie, granites, Maine, Cambrien., Preamble The Cambrian Ellsworth belt occupies the eastern portion of the Penobscot Bay inlier, coastal Maine (Fig. 1; Reusch et al. 2018). It remains an inadequately documented yet highly significant [...]

The Seve Nappe Complex (SNC) comprises continental rocks of Baltica that were subducted and exhumed during the Caledonian orogeny prior to collision with Laurentia. The tectonic history of the central SNC is investigated by applying in-situ zircon and monazite (Th-)U-Pb geochronology and trace element analysis to (ultra-)high pressure (UHP) paragneisses in the Avardo and Marsfjället gneisses. Zircons in the Avardo Gneiss exposed at Sippmikk creek exhibit xenocrystic cores with metamorphic rims. Cores show typical igneous REE profiles and were affected by partial Pb-loss. The rims have flat HREE profiles and are interpreted to have crystallized at 482.5 ± 3.7 Ma during biotite-dehydration melting and peritectic garnet growth. Monazites in the paragneiss are chemically homogeneous and record metamorphism at 420.6 ± 2.0 Ma. In the Marsfjället Gneiss exposed near Kittelfjäll, monazites exhibit complex zoning with cores enveloped by mantles and rims. The cores are interpreted to have crystallized at 481.6 ± 2.1 Ma, possibly during garnet resorption. The mantles and rims provide a dispersion of dates and are interpreted to have formed by melt-driven dissolution-reprecipitation of pre-existing monazites until 463.1 ± 1.8 Ma. Depletion of Y, HREE, and U in the mantles and rims compared to the cores record peritectic garnet and zircon growth. Altogether, the Avardo and Marsfjället gneisses show evidence of late Cambrian/early Ordovician partial melting (possibly in (U)HP conditions), Middle Ordovician (U)HP metamorphism, and late Silurian tectonism. These results indicate that the SNC underwent south-to-north oblique subduction in late Cambrian time, followed by progressive north-to-south exhumation to crustal levels prior to late Silurian continental collision., Author(s): Christopher J. Barnes [sup.1], Michal Bukala [sup.1], Riccardo Callegari [sup.2] [sup.3], Katarzyna Walczak [sup.2], Ellen Kooijman [sup.4], Melanie Kielman-Schmitt [sup.4], Jaroslaw Majka [sup.2] [sup.3] Author Affiliations: (1) grid.413454.3, 0000 [...]

The Flin Flon--Athapapuskow Lake area, situated in the Flin Flon Greenstone Belt, Manitoba, consists of ocean-floor and island-arc assemblages, deformed and metamorphosed during the Trans-Hudson Orogeny (~1.86-1.69 Ga). A new map of metamorphic mineral assemblages and isograds has been compiled that reveals a largely coherent regional metamorphic sequence increasing in metamorphic grade from prehnite-pumpellyite to amphibolite facies. Regional metamorphism postdates most of the deformation within the area, with the exception of the reactivation of major block-bounding faults. The regional prograde sequence has been subdivided into 10 metamorphic zones, separated by 9 isograds, that describe the transition from prehnite--pumpellyite to greenschist to amphibolite facies. The formation of contact metamorphic aureoles, predating regional metamorphism, record conditions up to amphibolite facies. Equilibrium phase diagrams for the island-arc (low-Mg) and ocean-floor (high-Mg) assemblages were calculated and allow for the evaluation of the modelling techniques and determination of pressure--temperature conditions. Discrepancies between the modelling predictions and natural observations occur due to (1) limitations in the thermodynamic models for some of the complex minerals (e.g., amphibole); and (2) metastable persistence of some minerals to higher grade due to sluggish reaction kinetics. Notwithstanding these discrepancies, the modelling suggests that metamorphosed mafic rocks in the Flin Flon--Athapapuskow Lake area reached about 430-480 [degrees]C and 3.0-4.5 kbar. Peak metamorphic conditions within contact aureoles that preceded regional metamorphism did not exceed 500 [degrees]C (at a pressure between 2.7 and 4.4 kbar). The metamorphic field gradient records a transition from 250-300 [degrees]C/1.5-2.3 kbar to 430-480 [degrees]C/3-4.5 kbar (100-150 [degrees]C/kbar), defining a geothermal gradient of approximately 25-31 [degrees]C/km. Key words: metamorphic petrology, greenstone belt, thermodynamic modelling, metamorphic field gradient. Le secteur de Flin Flon--lac Athapapusko, dans la ceinture de roches vertes de Flin Flon au Manitoba, est constitué d'assemblages du fond océanique et d'arc insulaire, déformés et métamorphosés durant l'orogenèse trans-hudsonienne (~1,86-1,69 Ga). Une nouvelle carte des assemblages minéraux et isogrades métamorphiques a été compilée et révèle une séquence métamorphique régionale généralement cohérente dans laquelle le degré de métamorphisme augmente du faciès à prehnite--pumpellyite à celui à amphibolites. Le métamorphisme régional a succédé à la majeure partie de la déformation dans la région, à l'exception de la réactivation d'importantes failles bordant des blocs. La séquence prograde régionale a été subdivisée en 10 zones métamorphiques séparées par 9 isogrades, qui décrivent la transition du faciès à prehnite--pumpellyite à celui à schistes verts, puis à celui à amphibolites. La formation d'auréoles de contact métamorphiques, qui a précédé le métamorphisme régional, témoigne de conditions allant jusqu'au faciès à amphibolites. Des diagrammes d'équilibre des phases pour les assemblages d'arc insulaire (faibles en Mg) et du fond marin (riches en Mg) ont été obtenus et permettent l'évaluation des techniques de modélisation et la détermination des conditions de pression et température. Des divergences entre les prédictions obtenues par modélisation et les observations se produisent en raison (1) des limites dans les modèles thermodynamiques pour certains minéraux complexes (p. ex. l'amphibole) et (2) la persistance métastable de certains minéraux à des degrés de métamorphisme élevés causée par une cinétique de réaction lente. En dépit de ces divergences, la modélisation donne à penser que des roches mafiques métamorphosées dans le secteur de Flin Flon--lac Athapapuskow ont atteint des températures d'environ 430-480 [degrees]C et des pressions de 3,0-4,5 kbar. Les conditions du pic du métamorphisme dans les auréoles de contact formées avant le métamorphisme régional n'ont pas dépassé 500 [degrees]C (à une pression entre 2,7 kbar et 4,4 kbar). Le gradient métamorphique de terrain reflète une progression de 250-300 [degrees]C/1,5-2,3 kbar à 430-480 [degrees]C/3-4,5 kbar (100-150 [degrees]C/kbar), ce qui définit un gradient géothermique d'environ 25-31 [degrees]C/km. [Traduit par la Rédaction] Mots-clés: pétrologie métamorphique, ceinture de roches vertes, modélisation thermodynamique, gradient métamorphique de terrain., Introduction The Flin Flon greenstone belt (FFB) is situated in west-central Manitoba and east-central Saskatchewan, Canada. It is 240 km wide and extends for about 40 km from the erosional [...]

The problem of the observed very rapid advection of heat into metamorphic thrust stacks is reviewed. Conductive models relying on the thermal relaxation of a thickened crust will not produce the observed Barrovian (medium temperature, medium pressure) assemblages within some short-lived orogens (e.g., western Ireland and Timor). Studies of the rate and timing of metamorphic mineral growth suggest that this is commonly faster than predicted by thermal relaxation. Barrovian assemblages are localised in some orogens (e.g., the Alps) but extensive in others (e.g., the Himalayas). Metamorphic mineral growth brackets deformation; consequently, slow growth is inconsistent with the rapid uplift of many orogens. Thus, no single mechanism can account for the development of Barrovian assemblages during collisional orogeny. The only mechanisms that can supply large amounts of heat for regional metamorphism quickly ( Key words: Barrovian metamorphism, thermal relaxation, advection, collisional orogeny. Le probleme de l'advection tres rapide de chaleur observee dans les empilements de nappes de charriage metamorphiques est passe en revue. Les modeles de conduction reposant sur la relaxation thermique d'une croute epaissie ne peuvent pas produire les assemblages barroviens (temperature et pression intermediaries) observes dans certains orogenes de courte duree (p. ex. ouest de l'Irlande, Timor). Des etudes du taux et du moment de la croissance de mineraux metamorphiques donnent a penser qu'elle est souvent plus rapide que ce que predit la relaxation thermique. Les assemblages barroviens sont localises dans certains orogenes (p. ex. les Alpes), mais de grande etendue dans d'autres (p. ex. Himalaya). La croissance des mineraux metamorphiques encadre la deformation, de sorte qu'une croissance lente ne concorde pas avec le soulevement rapide de nombreux orogenes. Ainsi, aucun mecanisme ne peut a lui seul expliquer le developpement d'assemblages barroviens durant les orogeneses de collision. Les seuls mecanismes pouvant fournir de grandes quantites de chaleur rapidement pour le metamorphisme regional (>). Le rechauffement visqueux et par frottement produit un rechauffement rapide local, mais non un rechauffement regional rapide. L'expansion arriere-arc ou toute forme d'expansion lithospherique accroit le gradient geothermique et le flux thermique, mais ne rechauffe pas les roches. Nous suggerons que l'advection magmatique de chaleur associee a l'amincissement lithospherique ou le charriage de type <> d'un arc/d'une ophiolite sont les principales sources de chaleur dans les orogenes de courte duree. [Traduit par la Redaction] Mots-cles: metamorphisme barrovien, relaxation thermique, advection, orogene de collision., Introduction A great problem in tectonics is the rapid advection of heat into supracrustal thrust stacks forming Barrovian metamorphic assemblages on the lower, refrigerated plate during collisional orogeny. The typical [...]

Infiltration-driven metamorphism has produced the widespread development of forsterite in the siliceous dolomites of the Alta, Utah contact aureole. SIMS (secondary ion mass spectrometry) [delta].sup.18O analyses show that in most of the middle to outer forsterite zone samples, forsterite, calcite and dolomite are homogenous in [delta].sup.18O at the grain-scale, but forsterite and calcite exhibit minor intergranular heterogeneities of < 1â° on the thin-section to hand sample-scales. In contrast, within ~ 300 m of the igneous contact (inner forsterite and periclase zones), grain-scale [delta].sup.18O heterogeneities in forsterite are as large as 3.0â°, but forsterite crystals do not exhibit systematic center-to-edge decreases in [delta].sup.18O due to skeletal growth and infilling. With one exception, forsterite, calcite and dolomite in all samples analyzed yield disequilibrium fractionations. The oxygen isotope disequilibrium commonly recorded among forsterite, calcite and dolomite may reflect the rapid growth of forsterite from increased reaction overstepping as temperature increased, or results from .sup.18O/.sup.16O depletion in the matrix carbonate minerals during continued fluid infiltration after forsterite crystallization was complete, during the initial stages of cooling in the forsterite zone. In a traverse across the boundary between forsterite- and periclase-bearing marble layers in the periclase zone, forsterite and calcite SIMS [delta].sup.18O profiles combined with the trend in forsterite textures indicate that these layers were not infiltrated simultaneously, nor continuously during both prograde reactions responsible for forsterite and periclase. Rather, the texture and SIMS data require infiltration and formation of forsterite first in the forsterite marble layer, followed by infiltration and formation of periclase in the periclase marble layer. As a consequence, the [delta].sup.18O profile recorded by forsterite formed earlier than the [delta].sup.18O profile recorded by calcite, but both profiles are prograde features-that is, both profiles formed in response to prograde reactions forming first forsterite and then periclase. These grain-scale [delta].sup.18O profiles further suggest that these layers experienced significant changes in permeability that are linked to their sequential infiltration-reaction history. The asynchronous growth of these index minerals at the outcrop-scale during prograde heating may be more common during contact metamorphism of carbonate lithologies than currently recognized., Author(s): Carl J. Beno [sup.1], John R. Bowman [sup.1], Kouki Kitajima [sup.2], John W. Valley [sup.2], Patrick C. Loury [sup.3], Lorraine M. Tapanila [sup.4] Author Affiliations: (1) grid.223827.e, 0000 0001 [...]

The distribution of ultrahigh-pressure metamorphism (UHP) at the scale of a mountain belt is of prime importance for deciphering its past subduction history. In the Western Alps, coesite has been recognized in the southern Dora-Maira Massif, in the lens-shaped Brossasco-Isasca Unit, but has not been found up to now in the other parts of the massif. We report the discovery of a new UHP unit in the northern Dora-Maira Massif (Western Alps), named Chasteiran Unit. It is only a few tens of metres thick and consists of graphite-rich, garnet-chloritoid micaschists, whose protoliths may be black shales of Silurian age. Garnet inclusions (chloritoid, rutile) and its growth zoning allow to precisely model the P-T evolution. Coesite crystals, which are pristine or partially transformed to palisade quartz occur as inclusions in the garnet outer cores. According to thermodynamic modelling, garnet displays a continuous record of growth during the prograde increase in P and T (25-27 kbar 470-500 °C) (stage 1), up to the coesite stability field (27-28 kbar 510-530 °C) (stage 2), as well as sub-isothermal decompression of about 10 kbar (down to 15 kbar 500-515 °C) (stage 3). The main regional, composite, foliation, marked by chloritoid and rutile, began to develop during this stage, and was then overprinted by chlorite-ilmenite (stage 4). The Chasteiran Unit is discontinuously exposed in the immediate hangingwall of the Pinerolo Unit, and it is located far away from, and without physical links to the classic UHP Brossasco-Isasca Unit. Moreover, it records a different, much colder, P-T evolution, showing that different slices were detached from the downgoing subduction slab. The Chasteiran Unit is the fourth and the coldest Alpine UHP unit known so far in the entire Alpine belt. Its P-T conditions are comparable to the ones of the Tian Shan coesite-chloritoid-bearing rocks., Author(s): Paola Manzotti [sup.1], Federica Schiavi [sup.2], Francesco Nosenzo [sup.1], Pavel Pitra [sup.3] [sup.4], Michel Ballèvre [sup.3] Author Affiliations: (1) grid.10548.38, 0000 0004 1936 9377, Department of Geological Sciences, Stockholm [...]

Pressure--temperature--time (P--T--t) estimates for the Montresor Belt, obtained using phase equilibria and geospeedometry modelling integrated with in situ U--Th--Pb monazite geochronology, shed new light on the tectonometamorphic effects of the Snowbird phase of the Trans-Hudson orogeny. Typical metapelitic assemblages of the lower Montresor group consist of white mica, biotite, plagioclase, quartz, and andalusite, which in some rocks is partly or completely pseudomorphed by white mica. The observed assemblages reflect peak P--T conditions centring at approximately 575 [degrees]C and 3 kbar. Rocks with high bulk Fe/Mg contents contain compositionally zoned garnet, permitting the addition of further constraints on the conditions of metamorphism in the Montresor Belt: Core compositions of earliest-grown garnets indicate initial garnet crystallization at approximately 535 [degrees]C and 2.3 kbar, suggesting a nearly isobaric P--T path of prograde metamorphism with a gradient of approximately 50 [degrees]C x [kbar.sup.-1]. Chemical age-dating of monazite inclusions in garnet yields ages of ca. 1870 [+ or -] 9 to 1837 [+ or -] 9 Ma. Retrograde, pseudomorphic andalusite replacement by white mica at approximately 540 [degrees]C is inferred to have been controlled by variations in bulk rock chemistry. Morphologically corroded and chemically heterogeneous monazite adjacent to white mica pseudomorphs suggests that andalusite replacement took place at ca. 1792 [+ or -] 10 Ma, possibly associated with extension and movement along the detachment fault separating the upper and lower Montresor groups. Simulations of diffusion across chlorite- and biotite-filled cracks in garnet assumed to be coeval with andalusite replacement suggest that the rocks have experienced the retrograde event for at least 20 My. Des estimations des conditions de pression-temperature-temps (P-T-t) pour la ceinture de Montresor, obtenues par modelisation de l'equilibre des phases et de la geospeedometrie integree a la geochronologie U-Th-Pb in situ sur monazite, jettent un nouvel eclairage sur les effets tectonometamorphiques de la phase de Snowbird de l'orogenese transhudsonienne. Les assemblages metapelitiques typiques du groupe de Montresor inferieur comprennent du mica blanc, de la biotite, du plagioclase, du quartz et de l'andalousite qui, dans certaines roches, est partiellement ou completement pseudomorphisee en mica blanc. Les assemblages observes refletent des conditions de P-T maximums autour de 575 [degre]C et 3 kbar. Les roches de rapports Fe/Mg eleves contiennent du grenat dont la composition presente une zonation, ce qui permet de delimiter encore mieux les conditions du metamorphisme dans la ceinture de Montresor : les compositions des noyaux des grenats les plus precoces indiquent que la cristallisation du grenat a commence autour de 535 [degre]C et 2,3 kbar, ce qui indiquerait une trajectoire P-T presque isobare du metamorphisme prograde suivant un gradient d'environ 50 [degre]C*[kbar.sup.-1]. La datation chimique d'inclusions de monazite dans le grenat donne des ages d'environ 1870 [+ ou -] 9 a 1837 [+ ou -] 9 Ma. Il est infere que le remplacement pseudomorphique retrograde de l'andalousite par du mica blanc autour de 540 [degrees]C etait contrôle par des variations de la composition globale de la roche. Des monazites de forme corrodee et de composition heterogene attenantes a des pseudomorphes de mica blanc indiquent que le remplacement de l'andalousite s'est produit vers 1792 [+ ou -] 10 Ma et etait possiblement associe a l'extension et au deplacement le long du decollement separant les groupes de Montresor superieur et inferieur. Des simulations de la diffusion a travers des fissures remplies de chlorite et de biotite dans le grenat presumees contemporaines au remplacement de l'andalousite indiqueraient que le metamorphisme retrograde subi par ces roches a dure au moins 20 millions d'annees. [Traduit par la Redaction], Introduction The Trans-Hudson orogeny was a continent-scale, Paleoproterozoic event characterized by widespread metamorphism, deformation, and plutonism at ca. 1.91-1.75 Ga, part of which is preserved in the Canadian Shield (Corrigan [...]

The Songjianghe deposit is a newly discovered altered gold deposit in the southeastern Jiapigou-Haigou Gold Metallogenic Belt (JHGMB) in southeastern Jilin Province of NE China. The host rocks were considered to be the Mesoproterozoic Seluohe Group, and the metallogenic epoch lacked accurate isotopic constraints. To determine the age and metallogenic setting of the deposit, we describe the geologic characteristics of the deposit and present the results of petrographic and geochronologic analyses of the host rocks and ores. The ore bodies are hosted within a suite of amphibolite facies metamorphic rocks superimposed by greenschist facies indicative of retrograde metamorphism. Zircon U--Pb dating results indicate that the host rocks belong to the Jiapigou Group that formed at the end of the Neoarchean (2543--2527 Ma). Subsequently, the rocks successively underwent metamorphism during the late Neoarchean (2521--2506 Ma), retrograde metamorphism caused by the closure of the Paleo-Asian Ocean during the late Permian to Early Triassic (262--250 Ma), and extension after the closure of the Paleo-Asian Ocean during the Late Triassic (231--210 Ma). Sericite [.sup.40]Ar/[.sup.39]Ar dating results suggest that the Songjianghe deposit formed during the Late Jurassic between 157 Ma and 156 Ma. By combining these new insights with those of previous studies, we propose that the Songjianghe deposit is a mesothermal gold deposit and that mineralization occurred during the extensional period in the intermittent stage that followed the first subduction of the Paleo-Pacific Plate. All the gold deposits in the JHGMB formed from the late Permian to Early Cretaceous by multi-stage mineralization events that corresponded temporally with the tectonic evolution of the Paleo-Asian Ocean and the episodic subduction of the Paleo-Pacific Plate. Key words: geochronology, sericite [.sup.40]Ar/[.sup.39]Ar, mesothermal gold deposit, Songjianghe, Northeast China. Le gisement de Songjianghe est un gisement aurifere altere decouvert recemment dans le sud-est de la ceinture metallogenique aurifere de Jiapigou-Haigou (CMAJH), dans le sud-est de la province de Jilin du nord-est de la Chine. Les roches hôtes etaient considerees comme constituant le Groupe mesoproterozoïque de Seluohe, et l'epoque metallogenique n'etait pas delimitee par des donnees isotopiques exactes. Afin de determiner l'age et le contexte metallogenique du gisement, nous en decrivons les caracteristiques geologiques et presentons les resultats d'analyses petrographiques et geochronologiques des roches hôtes et des minerais. Les corps mineralises se trouvent dans une serie de roches metamorphiques au facies des amphibolites sur lequel est surimpose un metamorphisme retrograde au facies des schistes verts. Les resultats de la datation U-Pb sur zircon indiquent que les roches hôtes appartiennent au Groupe de Jiapigou qui s'est forme a la fin du Neoarcheen (2543--2527 Ma). Par la suite, les roches ont successivement subi un metamorphisme durantle Neoarcheen tardif (2521--2506 Ma), un metamorphisme retrograde cause parlafermeture de l'ocean Paleoasiatique du Permien tardif au Trias precoce (262--250 Ma) et une extension apres la fermeture de l'ocean Paleoasiatique durant le Trias tardif (231-210 Ma). Les resultats de la datation [.sup.40]Ar/[.sup.39]Ar sur sericite donnent a penser que le gisement de Songjianghe s'est forme au Jurassique tardif, entre 157 Ma et 156 Ma. En combinant ces nouveaux renseignements aux resultats d'etudes anterieures, nous proposons que le gisement de Songjianghe est un gisement aurifere mesothermal et que la mineralisation s'est produite durant la periode d'extension dans l'intervalle qui a suivi la premiere subduction de la plaque paleo-Pacifique. Tous les gisements auriferes de la CMAJH se sont formes du Permien tardif au Cretace precoce par des episodes de mineralisation en plusieurs etapes qui correspondaient dans le temps avec l'evolution tectonique de l'ocean Paleoasiatique et la subduction episodique de la plaque paleo-Pacifique. [Traduit par la Redaction] Mots-cles: geochronologie, [.sup.40]Ar/[.sup.39]Ar sur sericite, gisement aurifere mesothermal, Songjianghe, nord-est de la Chine., 1. Introduction The Jiapigou-Haigou Gold Metallogenic Belt (JHGMB) is located in the NW-striking collage zone between the eastern section ofthe northern margin of the North China Craton (NCC) and the [...]

The Mae Ping shear zone (MPSZ) is one of the major ductile strike-slip systems associated with the Cenozoic extrusion tectonics in Southeast Asia. However, its sinistral shear lacks a robust temporal constraint. This study attempts to acquire the deformation timing by applying .sup.40Ar/.sup.39Ar thermochronology on a sheared pegmatitic leucogranite showing sinistral S-C fabrics with a thrust component. The contact of the leucogranite sub-paralleling to the major foliation in host gneiss indicates it could be a pre- to syn-shearing intrusion. Most minerals, including garnet, muscovite, K-feldspar, albite, and quartz, exhibit ductile to brittle deformation. Mineral microstructural analysis suggests a retrograde sinistral shear from > 600 to 250 °C. In situ .sup.40Ar/.sup.39Ar dating on muscovite yield ages mainly within 42-38 Ma with calculated closure temperatures of 435-330 °C. Fine-grained muscovite aggregates are slightly older than fish, implying that grain size reduction may not always reset .sup.40Ar/.sup.39Ar ages. The K-feldspar .sup.40Ar/.sup.39Ar step heating age spectrum with two segments of contiguous steps at 24.5 and 35.4 Ma may reflect the coexistence of high-T porphyroclast and low-T K-rich fine-grain recrystallizing at pressure shadows. The reconstructed cooling path and inferred deformation temperatures constrain a shear duration of 42-30 Ma for the MPSZ. The activation of the MPSZ before 42 Ma could be linked to the Eocene metamorphism within a transpressional regime triggering crustal thickening that may further induce the leucogranitic melt. This study also shows leucogranite can be a nice thermal history recorder for a shear zone regarding its petrogenesis and suitable mineral assemblage for thermochronology., Author(s): Yu-Ling Lin [sup.1], Tung-Yi Lee [sup.1], Ching-Hua Lo [sup.2], Sarah C. Sherlock [sup.3], Yoshiyuki Iizuka [sup.4], Tadashi Usuki [sup.2], Long Xiang Quek [sup.1], Punya Charusiri [sup.5] Author Affiliations: (1) [...]

The geometry and composition of deformation-related low-angle boundaries in naturally deformed olivine were characterized by electron backscattered diffraction (EBSD) and atom probe tomography (APT). EBSD data show the presence of discrete low-angle tilt boundaries, which formed by subgrain rotation recrystallisation associated with the (100)[001] slip system during fluid-catalysed metamorphism and deformation. APT analyses of these interfaces show the preferential segregation of olivine-derived trace elements (Ca, Al, Ti, P, Mn, Fe, Na and Co) to the low-angle boundaries. Boundaries with 2°), the interfaces become more ordered and linear enrichment of trace elements coincides with the orientation of dislocations inferred from the EBSD data. These boundaries show a systematic increase of trace element concentration with disorientation angle. Olivine-derived trace elements segregated to the low-angle boundaries are interpreted to be captured and travel with dislocations as they migrate to the subgrain boundary interfaces. However, the presence of exotic trace elements Cl and H, also enriched in the low-angle boundaries, likely reflect the contribution of an external fluid source during the fluid-present deformation. The observed compositional segregation of trace elements has significant implications for the deformation and transformation of olivine at mantle depth, the interpretation of geophysical data and the redistribution of elements deep in the Earth. The observation that similar features are widely recognised in manufactured materials, indicates that the segregation of trace elements to mineral interfaces is likely to be widespread., Author(s): Tommaso Tacchetto [sup.1] [sup.2], Steven M. Reddy [sup.1] [sup.2], David W. Saxey [sup.2], Denis Fougerouse [sup.1] [sup.2], William D. A. Rickard [sup.2], Chris Clark [sup.1] Author Affiliations: (1) grid.1032.0, [...]

The Khammam Schist Belt (KSB), southeastern India, represents a part of the collision zone that is sandwiched between the Eastern Dharwar Craton to the west and the Eastern Ghats Mobile Belt (EGMB) to the east. Quartzofeldspathic gneisses of the EGMB and garnet-kyanite metapelites of the KSB have been investigated to characterize P-T-t evolution of the Khammam region. Pseudosection modeling reveals that the melt-bearing quartzofeldspathic gneiss experienced peak and post-peak metamorphism at P-T conditions of 7.9-8.1 kbar/790-810 °C and ~ 7.0 kbar/740-750 °C, respectively. In contrast, metapelite witnessed peak metamorphism at 7.4-7.8 kbar/600-640 °C, followed by post-peak retrogression at 6.1-6.6 kbar/590-625 °C. U-Th-(total) Pb monazite ages from the gneiss constrain the peak and retrograde metamorphic episodes at 1.63-1.53 Ga and 1.48-1.38 Ga, while those in the metapelites were determined at 1.25-1.20 Ga and 1.18-1.10 Ga, respectively. These P-T-t estimates indicate that the KSB, Vinjamuru, and Ongole domains evolved distinctly during Late Paleoproterozoic-Late Mesoproterozoic. The younger ages (0.90-0.81 Ga) were ascribed to the formation of the Eastern Indian Tectonic Zone, implying its extension beyond the western margin of the EGMB. Besides, the distinct Neoarchean ages (2.79-2.45 Ga) are related to the Archean protolith of the quartzofeldspathic gneisses, which were likely derived from the Archean crust of the Napier-Rayner Complex. This tectonothermal restoration is new and characterizes the Khammam region as the hot and composite collision zone with protracted geological history. (250 words), Author(s): H. Hrushikesh [sup.1] [sup.2], Praveen C. Singh [sup.1], N. Prabhakar [sup.1], Sachin K. Thakur [sup.1], Bidisha Dey [sup.1] [sup.3] Author Affiliations: (1) grid.417971.d, 0000 0001 2198 7527, Department of [...]

The 156-138 Ma Chidliak kimberlites on the Eastern Hall peninsula (EHP) of Baffin Island entrained mantle xenoliths interpreted to have been a part of the Archean North Atlantic Craton (NAC) lithospheric mantle. We studied 19 Chidliak eclogite xenoliths that comprise 10 bimineralic, 5 rutile-bearing, 3 orthopyroxene-bearing and 1 kyanite-bearing eclogites. We report major and trace element compositions of the minerals, calculated bulk compositions, pressures and temperatures of the rock formation and model melt extraction from viable protoliths. The eclogite samples are classified into three groups of HREE-enriched, LREE-depleted and metasomatized based on their reconstructed whole-rock REE patterns. PT parameters of the eclogites were calculated by projecting garnet-clinopyroxene temperatures onto the local P-T arrays for 65 Chidliak peridotite xenoliths. All Chidliak eclogites are equilibrated in the diamond P-T field and cluster in two groups, low-temperature (n = 5, 840-990 °C at 4.1-5.0 GPa) and high-temperature (n = 11, T > 1320 °C at P > 7.0 GPa). The reconstructed Mg-rich major element bulk compositions and trace elements patterns are similar to Archean basalts from the North Atlantic and Superior cratons and the oceanic gabbros. The LREE-depleted Chidliak eclogites could be residues after 15-55% partial melting of Archean basalt at the eclogite facies of metamorphism that led to extraction of a tonalite-trondhjemite-granodiorite melt from the EHP. The HREE-depleted eclogites may have experienced a lower degree (95%. The former are more magnesian, less ferrous and calcic, contain more magnesian and less calcic garnets, and lower proportions of group C eclogites. The contrast may relate to the stronger NAC metasomatism by silicate-carbonate melt observed in Chidliak peridotitic mantle, or to the different formation ages of the eclogites beneath the two cratons., Author(s): Vedran Pobric [sup.1], Nester Korolev [sup.1] [sup.2], Maya Kopylova [sup.1] Author Affiliations: (1) grid.17091.3e, 0000 0001 2288 9830, Department of Earth, Ocean and Atmospheric Sciences, University of British Columbia, [...]

We report the results of kinetic experiments of graphitization at various pressures (0.5-8.0 GPa) and durations (1 s to 24 h) at 1200 °C. The natural carbonaceous material in sedimentary rocks from the Shimanto accretionary complex and the Hidaka metamorphic belt, Japan, underwent systematic changes in crystallinity and morphology with increasing pressure. To assess the pressure dependence of graphitization, we adopted three approaches to formulating the graphitization kinetics using a power law rate model, a Johnson-Mehl-Avrami-Kolmogorov model, and a superposition method. Activation volumes of - 21.7 ± 3.0 to - 45.7 ± 4.5 cm.sup.3 mol.sup.-1 and - 0.7 ± 0.2 to - 16.8 ± 1.8 cm.sup.3 mol.sup.-1 were obtained for pressures from 0.5 to 2.0 GPa and 2.0 to 8.0 GPa, respectively. Such large negative activation volumes might arise from structural modification and compression in the primary carbonaceous material. We applied the experimental data to the Arrhenius-type equation of graphitization, extrapolated to geological P-T-t conditions. Our model predicts that carbonaceous material undergoing metamorphism for ~ 10 Myr at pressures of 0.5-3.0 GPa will begin to crystallize at around 350-420 °C and transform fully to ordered graphite at around 450-600 °C, depending on the peak pressure. Thus, natural graphitization might proceed much more rapidly than previously estimated, owing to the large negative activation volumes for the reaction rate. This indicates that subducted carbonaceous materials will completely convert to fully ordered graphite by rapid recrystallization and metamorphic devolatilization before reaching sub-arc depths (< 100 km)., Author(s): Yoshihiro Nakamura [sup.1], Takashi Yoshino [sup.2], Madhusoodhan Satish-Kumar [sup.3] Author Affiliations: (1) grid.466781.a, 0000 0001 2222 3430, Geological Survey of Japan, National Institute of Advanced Industrial Science and Technology, [...]

This study presents boron (B) concentration and isotope data for white mica from (ultra)high-pressure (UHP), subduction-related metamorphic rocks from Lago di Cignana (Western Alps, Italy). These rocks are of specific geological interest, because they comprise the most deeply subducted rocks of oceanic origin worldwide. Boron geochemistry can track fluid-rock interaction during their metamorphic evolution and provide important insights into mass transfer processes in subduction zones. The highest B contents (up to 345 [mu]g/g B) occur in peak metamorphic phengite from a garnet-phengite quartzite. The B isotopic composition is variable ([delta].sup.11B = - 10.3 to - 3.6%) and correlates positively with B concentrations. Based on similar textures and major element mica composition, neither textural differences, prograde growth zoning, diffusion nor a retrograde overprint can explain this correlation. Modelling shows that B devolatilization during metamorphism can explain the general trend, but fails to account for the wide compositional and isotopic variability in a single, well-equilibrated sample. We, therefore, argue that this trend represents fluid-rock interaction during peak metamorphic conditions. This interpretation is supported by fluid-rock interaction modelling of boron leaching and boron addition that can successfully reproduce the observed spread in [delta].sup.11B and [B]. Taking into account the local availability of serpentinites as potential source rocks of the fluids, the temperatures reached during peak metamorphism that allow for serpentine dehydration, and the high positive [delta].sup.11B values ([delta].sup.11B = 20 ± 5) modelled for the fluids, an influx of serpentinite-derived fluid appears likely. Paragonite in lawsonite pseudomorphs in an eclogite and phengite from a retrogressed metabasite have B contents between 12 and 68 [mu]g/g and [delta].sup.11B values that cluster around 0% ([delta].sup.11B = - 5.0 to + 3.5). White mica in both samples is related to distinct stages of retrograde metamorphism during exhumation of the rocks. The variable B geochemistry can be successfully modelled as fluid-rock interaction with low-to-moderate (< 3) fluid/rock ratios, where mica equilibrates with a fluid into which B preferentially partitions, causing leaching of B from the rock. The metamorphic rocks from Lago di Cignana show variable retention of B in white mica during subduction-related metamorphism and exhumation. The variability in the B geochemical signature in white mica is significant and enhances our understanding of metamorphic processes and their role in element transfer in subduction zones., Author(s): Ralf Halama [sup.1], Matthias Konrad-Schmolke [sup.2], Jan C. M. De Hoog [sup.3] Author Affiliations: (1) 0000 0004 0415 6205, grid.9757.c, School of Geography, Geology and the Environment, Keele University, [...]

We applied interface-controlled kinetics, including interface-controlled Ostwald ripening and non-hydrostatic dissolution-precipitation, to metamorphic zircon and phengite growth in a high-pressure (high-P) metamorphic complex. This kinetic modeling yields the growth duration of metamorphic zircon, based on the assumption that dissolution and precipitation of zircon is much slower than that of phengite. The model was applied to zircon and phengite growth in the Nishisonogi unit of the high-P Nagasaki metamorphic complex, western Kyushu, Japan. Given that detrital zircons that are tens of microns in size remain after metamorphism, our model assumption is justified. Our results show that the duration of metamorphic zircon growth in an individual rock ranges from 10 to 20 Myr. In general, the duration of metamorphic zircon growth in the whole Nishisonogi unit is ca. 30 Myr. This prolonged duration implies that high-P metamorphism is sustained by continuous subduction of hydrated oceanic crust and overlying trench-fill sediments. The continuous subduction and accretion of these materials may supply the metamorphic fluid into the high-P metamorphic domain in the deeper part of the accretionary prism, thereby contributing to interface-controlled kinetics in the metamorphic complex., Author(s): Kazuhiro Miyazaki [sup.1], Kenshi Suga [sup.2], Yasushi Mori [sup.3], Hideki Iwano [sup.4], Koshi Yagi [sup.5], Miki Shigeno [sup.3], Tadao Nishiyama [sup.6], Tohru Danhara [sup.4], Takafumi Hirata [sup.7] Author Affiliations: [...]

Earth's mantle convection, which facilitates planetary heat loss, is manifested at the surface as present-day plate tectonics.sup.1. When plate tectonics emerged and how it has evolved through time are two of the most fundamental and challenging questions in Earth science.sup.1-4. Metamorphic rocks--rocks that have experienced solid-state mineral transformations due to changes in pressure (P) and temperature (T)--record periods of burial, heating, exhumation and cooling that reflect the tectonic environments in which they formed.sup.5,6. Changes in the global distribution of metamorphic (P, T) conditions in the continental crust through time might therefore reflect the secular evolution of Earth's tectonic processes. On modern Earth, convergent plate margins are characterized by metamorphic rocks that show a bimodal distribution of apparent thermal gradients (temperature change with depth; parameterized here as metamorphic T/P) in the form of paired metamorphic belts.sup.5, which is attributed to metamorphism near (low T/P) and away from (high T/P) subduction zones.sup.5,6. Here we show that Earth's modern plate tectonic regime has developed gradually with secular cooling of the mantle since the Neoarchaean era, 2.5 billion years ago. We evaluate the emergence of bimodal metamorphism (as a proxy for secular change in plate tectonics) using a statistical evaluation of the distributions of metamorphic T/P through time. We find that the distribution of metamorphic T/P has gradually become wider and more distinctly bimodal from the Neoarchaean era to the present day, and the average metamorphic T/P has decreased since the Palaeoproterozoic era. Our results contrast with studies that inferred an abrupt transition in tectonic style in the Neoproterozoic era (about 0.7 billion years ago.sup.1,7,8) or that suggested that modern plate tectonics has operated since the Palaeoproterozoic era (about two billion years ago.sup.9-12) at the latest. Variability in Earth's thermal gradients, recorded by metamorphic rocks through time, shows that Earth's modern plate tectonics developed gradually since the Neoarchaean era, three billion years ago., Author(s): Robert M. Holder [sup.1] [sup.2] , Daniel R. Viete [sup.1] , Michael Brown [sup.3] , Tim E. Johnson [sup.4] [sup.5] Author Affiliations: (1) Morton K. Blaustein Department of Earth [...]

Skarn garnets in the Mineral King roof pendant of the south-central Sierra Nevada within Sequoia National Park, California, USA reveal variable fluid chemistry with a significant component of meteoric water during metasomatism in the Early Cretaceous Sierra Nevada Batholith. We focus on andradite garnet associated with Pb-Zn mineralization in the White Chief Mine. Laser fluorination oxygen isotope analyses of [delta].sup.18O of garnet ([delta].sup.18O(Grt)) from sites along the skarn show a large range of values (- 8.8 to + 4.6â° VSMOW). Ion microprobe (SIMS) analyses elucidate that individual andradite crystals are strongly zoned in [delta].sup.18O(Grt) (up to 7â° of variation). Total rare-earth element concentrations ([summation]REE) across individual garnets show progressive depletion of skarn-forming fluids in these elements during garnet growth. These findings support a skarn model of earliest red high-[delta].sup.18O grandite garnet consistent with a magmatic-dominated equilibrium fluid ([delta].sup.18O(H.sub.2O) as high as [almost equal to] + 8â°). Later, green andradite crystallized in equilibrium with a low-[delta].sup.18O fluid indicating a significant influx of meteoric fluid ([delta].sup.18O(H.sub.2O) [almost equal to] - 6 to - 5â°), following a hiatus in garnet growth, associated with late-stage Pb-Zn mineralization. Latest orange overprint rims have higher [delta].sup.18O values ([delta].sup.18O(H.sub.2O) [almost equal to] 0-2â°), and depleted total REEs, suggesting influx of high-[delta].sup.18O, trace-element depleted fluid derived from regional metamorphism of the carbonate host. Remarkably, low [delta].sup.18O(Grt) values in the White Chief canyon skarn require a significant proportion of meteoric fluid available during > 400 °C andradite-forming metasomatism. Fluid flow was channelized at the pluton-wallrock contact, evidenced by the narrow extent of skarn., Author(s): J. Ryan-Davis [sup.1] [sup.2], J. S. Lackey [sup.2], M. Gevedon [sup.3], J. D. Barnes [sup.3], C-T. A. Lee [sup.4], K. Kitajima [sup.5], J. W. Valley [sup.5] Author Affiliations: (1) [...]

The Nolans Bore rare earth element (REE) deposit consists of a network of fluorapatite-bearing veins and breccias hosted within Proterozoic granulites of the Reynolds Range, Central Australia. Mineralisation is divided into three zones (north, central, and south-east), with the north and south-east zones consisting of massive REE-bearing fluorapatite veins, with minor brecciation and carbonate infill. The central zone is distinctively different in mineralogy and structure; it features extensive brecciation, a high allanite content, and a large, epidote-rich enveloping alteration zone. The central zone is a reworking of the original solid apatite veins that formed during the Chewings Orogeny at ca. 1525 Ma. These original apatite veins are thought to derive from phosphate-rich magmatic-hydrothermal fluid exsolved from as-yet unrecognised alkaline magmatic bodies at depth. We define four ore breccia types (BX1-4) in the central zone on the basis of detailed petrological and geochemical analysis of drillcore and thin sections. BX1 ore comprises fluorapatite with minor crackle brecciation with carbonate infill and resembles ore of the north and south-east zones. Breccia types BX2, BX3, and BX4 represent progressive stages of ore brecciation and development of calc-silicate mineral (amphibole, epidote, allanite, calcite) infill. Comparison of bulk ore sample geochemistry between breccia types indicates that REEs were not mobilised more than a few centimetres during hydrothermal alteration and brecciation. Instead, most of the REEs were partitioned from the original REE fluorapatite into newly formed allanite, REE-poor fluorapatite and minor REE carbonate in the breccias. Negative europium (Eu) anomalies in the breccia minerals are accounted for by a large positive Eu anomaly in epidote from the alteration zones surrounding the ore breccias. This observation provides a direct link between ore recrystallisation and brecciation, and the formation of the alteration halo in the surrounding host rocks. Where allanite and fluorapatite are texturally related, the fluorapatite is relatively depleted in the light rare earth elements (LREEs), whereas allanite is relatively LREE enriched, suggesting co-crystallisation. We tentatively date the BX1 ore stage to 1440 ± 80 Ma based on U-Pb dating of thorianite. Sm-Nd isotope isochrons derived from in situ isotope analysis of cognate apatite and allanite date the BX2 and BX3 events to ca. 400 Ma, while U-Pb dating of late-stage monazite from the BX4 ore stage returned an age of ca. 350 Ma. Therefore, formation of the central zone at Nolans Bore involved multiple alteration/brecciation events that collectively span over 1 billion years in duration. We suggest that the BX1-type veins and breccias were formed from REE-rich, saline (F- and Cl-bearing) fluids that infiltrated the granulite-grade host rocks in association with either shear activation events of the Redbank Shear Zone (1500-1400 Ma) or intrusion of late-stage pegmatites of the Mt Boothby area. BX2, BX3, and BX4 events record deformation and hydrothermal alteration associated with the Alice Springs Orogeny (400-350 Ma). These hydrothermal events occurred at temperatures of 450 to ~600 °C, due to inflow of highly acidic hydrous fluids derived from a magmatic source, or from mixing of meteoric and metamorphic fluids. Our data testify to the long and complex geological history of not only the Nolans Bore REE deposit, but also of the rocks of the eastern Reynolds Range, and demonstrate the great utility of using hydrothermally derived REE minerals to trace the timing of crustal deformation events and source of associated hydrothermal fluids. Electronic supplementary material The online version of this article (doi:10.1007/s00410-015-1168-x) contains supplementary material, which is available to authorized users., Author(s): Louise Schoneveld[sup.1] [sup.3] , Carl Spandler[sup.1] , Kelvin Hussey[sup.2] Author Affiliations: (1) Economic Geology Research Centre (EGRU), James Cook University, 4811, Townsville, QLD, Australia (2) Arafura Resources Ltd., Darwin, [...]

Integrated mapping, structural analysis, and U-Pb geochronology of the Committee Bay area, Nunavut, establish a record of Neoarchean crustal growth followed by penetrative Paleoproterozoic deformation. Supracrustal rocks include a lower ca. 2.73 Ga mafic-ultramafic volcanic-dominated sequence, a middle, economically significant 2.71 Ga intermediate volcanic-bearing sequence with intercalated sulphidized, gold-bearing iron formation, and an upper Une cartographie integree, une analyse structurale et de la geochronologie U-Pb de la region de la baie Committee, au Nunavut, ont permis de determiner la croissance de la croute au Neoarcheen, laquelle a ete suivie d'une deformation penetrante au Paleoproterozoique. Les roches supracrustales comprennent une sequence inferieure, vers 2,73 Ga, mafiqueultramafique dominee par des volcaniques, une sequence mediane, d'interet economique, vers 2,71 Ga, de roches volcaniques intermediaires avec interstratification d'une formation sulfurisee de fer aurifere et une sequence superieure clastique ± komatiite-quartzique,, Introduction Archean supracrustal belts are the most significant hosts of mineral resources within the Precambrian Canadian Shield. As such, their stratigraphic, structural, and chronological characteristics elucidate the context for interpreting [...]

Multiple pieces of geologic evidence suggest that interfaces between contrasted lithologies exert a strong control on the fate of volatiles in subduction zones. Here we present results from a contact between serpentinites and sediments, located in Corsica and metamorphosed in the blueschist facies during the alpine orogeny. It was shown previously that carbonates in the sediments have been reduced to graphitic carbonaceous material within a 5-10-cm-thick reaction zone at the contact with serpentinites. In an effort to investigate the mechanisms governing this unusual process, bulk rock geochemical analyses incorporating a statistical analysis of compositional data are presented. Observations show that the fate of C was decoupled from that of other elements such as O, H, and large-ion lithophile elements--e.g. K, Sr, Ba ..., As--that were extensively leached from the reaction zone. Notably, Na is strongly enriched in the reaction zone and structurally linked to pectolite. Reducing conditions, manifested by the depletion of O in the reaction zone compared to the bulk metasediment, were likely maintained by the presence of Fe(II) in the serpentinite. Moreover, thermodynamic calculations show that the low solubility of carbon in COH fluids at high-pressure and low-temperature conditions was the main driver for graphite precipitation synchronously with carbonate destabilization. This may have been kinetically favored by the presence of already existing graphitized carbonaceous material and phengite in the metasediment. Limited lateral flow might have contributed as well to the geochemical and petrological patterns observed in these rocks. Keywords Redox gradient * Lithological interface * Fluid-rock interaction * Graphite * Reducing fluids * Pectolite * Wollastonite * Metamorphism, Introduction Interface processes acting at different scales are a major focus of current geodynamic (e.g., Angiboust et al. 2012), geochemical (e.g., Ague 2000; Spandler et al. 2008), and petrological (e.g., [...]

The hydroxyl (O(4)) site composition of biotite can in principle be used to retrieve information about fluid composition during fluid-rock interaction; however, due to low F and Cl content, as well as difficulties involved with analyzing the H.sub.2O content using in situ techniques, measuring these species in biotite has remained an elusive goal. Here we present high-precision secondary ion mass spectrometry (SIMS) OH-F-Cl measurements from biotite within metapelites from the Western Adamello Tonalite (WAT) contact aureole, Northern Italy. Fluorine, chlorine and hydrogen are analyzed on the SIMS sequentially by peak-hopping at the same biotite spot; H.sub.2O, F and Cl content were measured with a precision (1[sigma]) [omicron][phi] 0.06 [omega][tau]%, 50 [alpha]ν[delta] 5 [pi][pi][micro], [rho][epsilon][sigma][pi][epsilon][chi][tau]ιÏ[epsilon][lambda][psi]. The compositions of isolated biotite crystals in andalusite are compared with that of biotite in the matrix, documenting that halogens and H.sub.2O behave refractory in biotite during the time scale of contact metamorphism. The H.sub.2O and halogen contents of biotite are mostly locked in during the prograde to peak formation of biotite, and are not reset during further heating or cooling, unless significant biotite recrystallization occurs. It also appears that both Ti content and X.sub.Mg of the biotite from the Western Adamello contact aureole were not significantly reset during cooling. The concentration of F and Cl does not vary systematically with metamorphic grade, which indicates that these species reflect initial compositions. No significant Rayleigh fractionation behavior was observed for these elements. H.sub.2O variations in the biotite from samples throughout the Western Adamello contact aureole suggest that Al-oxy substitution partially controls the variations in OH content through charge balance of the type R.sup.2+,VI + OH.sup.- = Al.sup.3+,VI + O.sup.2- + H.sub.2, while the Ti-oxy substitution does not seem to influence the O(4) site occupation. The main titanium substitution appears to be the Ti-vacancy ( [Formula omitted]) exchange. Variations in H.sub.2O and halogen concentrations in biotite define sub mm-scale areas of localized equilibration, even for biotite recrystallized during dehydration reactions that produced large amounts of fluid (chlorite or muscovite breakdown). Similar systematics were observed for Ti.sup.4+ and Al.sup.3+. These findings further support the increasing number of observations that kinetics control much of the mineralogical reactions occurring in contact aureoles, and hence care is advised in using equilibrium thermodynamics in this environment., Author(s): Guillaume Siron [sup.1] , Lukas Baumgartner [sup.1] , Anne-Sophie Bouvier [sup.1] Author Affiliations: (Aff1) 0000 0001 2165 4204, grid.9851.5, Institute of Earth Sciences, University of Lausanne, , CH1015, Lausanne, [...]

In this contribution, we address the vexed question of the extent to which microstructures in granitic rocks reflect their igneous histories or have been masked by later events. The previous works have tended to address the problem either using theoretical or modelling considerations, or by interpretation of observed microstructures. Here, we use an approach that integrates the theory of microstructural development and the results of experimental phase-equilibrium studies with direct observation of natural examples on a variety of scales. We show that the predictions of the theoretical and experimental approaches agree perfectly with the mesoscopic and microscopic evidence from granitic rocks themselves. Our conclusion is that although, in many cases, granitic rock microstructures have been modified by near-solidus reactions and crystallisation, in the absence of tectonic deformation the fundamental elements of their igneous heritage remain intact. This means that it is perfectly in order to infer aspects of crystallisation sequences, magmatic reactions, and magma flow through careful microstructural observations. Thus, our answer to the question of how deceptive granitic textures are is, in most instances, 'not very'. However, some undeformed plutons have undergone fluid-driven alteration, and others have been affected by contact metamorphism. Thus, each case should be examined on its own merits., Author(s): M. B. Holness [sup.1] , J. D. Clemens [sup.2] , R. H. Vernon [sup.3] Author Affiliations: (Aff1) 0000000121885934, grid.5335.0, Department of Earth Sciences, University of Cambridge, , Downing St, [...]

A Fe-Ti-rich garnet, clinopyroxene, and quartz eclogite sample from the 1.0 Ga Sveconorwegian orogen, SW Sweden, contains abundant quartz, rutile, and zircon in distinct micro-textural sites: garnet core, garnet rim, and matrix, constituting an ideal case for investigation of the behavior of Zr-in-rutile and Ti-in-quartz at high-pressure and temperature. A P-T path, peaking at 16.5-19 kbar and 850-900 °C, has been constrained independently for the same rock by pseudosection modelling; input pressures from this model were used for trace element geothermometry of each garnet micro-textural domain. Trace element thermo(baro)metry, based on in situ Secondary Ion Mass Spectrometry analyses of Ti contents in quartz and Zr contents in rutile, yields P-T estimates of progressive crystallization of quartz and rutile along the prograde metamorphic path. For inclusions in the garnet cores, Zr-in-rutile geothermometry yields 700-715 °C and Ti-in-quartz 620-640 °C at 7 kbar. For inclusions in the garnet rims, temperature estimates are 760-790 °C (Zr-in-rutile) and 740-920 °C (Ti-in-quartz) at 12-18 kbar. Finally, matrix rutile records 775-800 °C and locally ~ 900 °C, and quartz records temperatures up to 900 °C at 18 kbar. Ti-in-quartz estimates for the metamorphic peak (inclusions in the garnet rims and matrix) conform to the pseudosection, but appear too low for the early prograde stage (garnet cores), possibly due to lack of equilibrium at T < 700 °C. The pseudosection shows that rutile was produced by continuous ilmenite breakdown during the early stages of prograde metamorphism, a reaction that was completed at ~ 730 °C. Rutile grains in the garnet rims and the matrix grew subsequently larger by recrystallization of previously produced rutile. However, recrystallized rutile does not predominantly record peak temperatures, but instead yield 745-840 °C between 12 and 18 kbar. In the pseudosection, this temperature range broadly coincides with a stage during which (Ti-bearing) hornblende was consumed and clinopyroxene produced (i.e., dehydration); the Zr contents thus appear to reflect the last stage of efficient rutile recrystallization, catalysed by fluids released by the dehydration of hornblende preceding the metamorphic peak. Concurrently, combination of the isopleths for Ti content in quartz and Zr content in rutile (i.e. independent from pseudosection modelling) yields pressure and temperature conditions in almost perfect agreement with the P-T path as deduced from the pseudosection. The variation in Ti concentration in quartz is small regardless of crystal size, and the Ti-in-quartz geothermometer provides both precise and accurate peak temperatures of 875-920 °C, without a significant diffusional reequilibration. The lack of significant Ti diffusion in quartz is consistent with an inferred short residence time at high temperature. This study illustrates that Zr-in-rutile and Ti-in-quartz geothermobarometry can robustly constrain prograde P-T conditions and yield further insights into recrystallization processes at high temperature. The combination of these methods and integration of the results with pseudosection modelling is a versatile tool for investigating the petrologic history of high-grade rocks., Author(s): L. Tual [sup.1] [sup.2] , C. Möller [sup.1] , M. J. Whitehouse [sup.2] Author Affiliations: (Aff1) 0000 0001 0930 2361, grid.4514.4, Department of Geology, Lund University, , Sölvegatan 12, [...]

The Cretaceous Mount Daniel Complex (MDC) in northern Fiordland, New Zealand was emplaced as a 50 m-thick dyke and sheet complex into an active shear zone at the base of a Cordilleran magmatic arc. It was emplaced below the 20-25 km-thick, 125.3 ± 1.3 Ma old Western Fiordland Orthogneiss (WFO) and is characterized by metre-scale sheets of sodic, low and high Sr/Y diorites and granites. 119.3 ± 1.2 Ma old, pre-MDC lattice dykes and 117.4 ± 3.1 Ma late-MDC lattice dykes constrain the age of the MDC itself. Most dykes were isoclinally folded as they intruded, but crystallised within this deep-crustal, magma-transfer zone as the terrain cooled and was buried from 25 to 50 km (9-14 kbar), based on published P-T estimated from the surrounding country rocks. Zircon grains formed under these magmatic/granulite facies metamorphic conditions were initially characterized by conservatively assigning zircons with oscillatory zoning as igneous and featureless rims as metamorphic, representing 54% of the analysed grains. Further petrological assignment involved additional parameters such as age, morphology, Th/U ratios, REE patterns and Ti-in-zircon temperature estimates. Using this integrative approach, assignment of analysed grains to metamorphic or igneous groupings improved to 98%. A striking feature of the MDC is that only ~ 2% of all igneous zircon grains reflect emplacement, so that the zircon cargo was almost entirely inherited, even in dioritic magmas. Metamorphic zircons of MDC show a cooler temperature range of 740-640 °C, reflects the moderate ambient temperature of the lower crust during MDC emplacement. The MDC also provides a cautionary tale: in the absence of robust field and microstructural relations, the igneous-zoned zircon population at 122.1 ± 1.3 Ma, derived mostly from inherited zircons of the WFO, would be meaningless in terms of actual magmatic emplacement age of MDC, where the latter is further obscured by younger (ca. 114 Ma) metamorphic overgrowths. Thus, our integrative approach provides the opportunity to discriminate between igneous and metamorphic zircon within deep-crustal complexes. Also, without the tight field relations at Mt Daniel, the scatter beyond a statistically coherent group might be ascribed to the presence of 'antecrysts', but it is clear that the WFO solidified before the MDC was emplaced, and these older 'igneous' grains are inherited. The bimodal age range of inherited igneous grains, dominated by ~ 125 Ma and 350-320 Ma age clusters, indicate that the adjacent WFO and a Carboniferous metaigneous basement were the main sources of the MDC magmas. Mafic lenses, stretched and highly attenuated into wisps within the MDC and dominated by ~ 124 Ma inherited zircons, are considered to be entrained restitic material from the WFO. A comparison with lower- and upper-crustal, high Sr/Y metaluminous granites elsewhere in Fiordland shows that zircon inheritance is common in the deep crust, near the source region, but generally much less so in coeval, shallow magma chambers (plutons). This is consistent with previous modelling on rapid zircon dissolution rates and high Zr saturation concentrations in metaluminous magmas. Accordingly, unless unusual circumstances exist, such as MDC preservation in the deep crust, low temperatures of magma generation, or rapid emplacement and crystallization at higher structural levels, information on zircon inheritance in upper crustal, Cordilleran plutons is lost during zircon dissolution, along with information on the age, nature and variety of the source material. The observation that dioritic magmas can form at these low temperatures (< 750 °C) also suggests that the petrogenesis of mafic rocks in the arc root might need to be re-assessed., Author(s): Shrema Bhattacharya [sup.1] [sup.2] , A. I. S. Kemp [sup.1] [sup.3] , W. J. Collins [sup.4] Author Affiliations: (Aff1) 0000 0004 0474 1797, grid.1011.1, James Cook University, Geoscience, College [...]

High-pressure and ultrahigh-pressure (UHP) eclogite-bearing metamorphic assemblages in the North Tianshan of Kyrgyzstan are known from the Aktyuz and Makbal areas, where eclogites and garnet amphibolites are associated with continental rocks such as granitoid gneisses in Aktyuz and shallow-water clastic (passive margin?) metasediments in Makbal. We present the first Lu-Hf isotope data for an eclogite and two garnet amphibolite samples from the two metamorphic terranes which, combined with petrological analysis, tightly constrain the age of high-pressure metamorphism in the Kyrgyz North Tianshan. A fivepoint isochron for an Aktyuz eclogite sample provides a LuHf age of 474.3 ± 2.2 Ma, and a four-point isochron on a Makbal sample corresponds to 470.1 ± 2.5 Ma. A prograde, subduction-related path is inferred for both samples with peak P-T conditions ranging from 1.4 to 1.6 GPa and 610-620°C. A further Makbal sample provided a significantly older Lu-Hf age of 486 ± 5.4 Ma, most likely due to late alteration in the sample (late addition of unradiogenic Hf). We conclude that garnet growth in all three samples occurred around ca. 474 Ma and that these rocks likely experienced UHP metamorphism contemporaneously. Our results support previous geochronological evidence for an Early Ordovician collision belt in the North Tianshan and allow refinement of a tectonic model involving subduction of thinned continental crust to considerable depth along the margin of a small microcontinent. Keywords Lu-Hf * Mafic dike * Aktyuz * Makbal * Northern Tianshan * Kyrgyzstan, Introduction The Tianshan orogen of north-western China, Kyrgyzstan, Kazakhstan and Uzbekistan is part of the Central Asian Orogenic Belt (CAOB; Fig. 1) which evolved over some 800 Ma from the [...]

The chemical composition of fluid inclusions in quartz crystals from Alpine fissure veins was determined by combination of microthermometry, Raman spectroscopy, and LA-ICPMS analysis. The veins are hosted in carbonate-bearing, organic-rich, low-grade metamorphic metapelites of the Bundnerschiefer of the eastern Central Alps (Switzerland). This strongly deformed tectonic unit is interpreted as a partly subducted accretionary wedge, on the basis of widespread carpholite assemblages that were later overprinted by lower greenschist facies metamorphism. Veins and their host rocks from two locations were studied to compare several indicators for the conditions during metamorphism, including illite crystallinity, graphite thermometry, stability of mineral assemblages, chlorite thermometry, fluid inclusion solute thermometry, and fluid inclusion isochores. Fluid inclusions are aqueous two-phase with 3.7-4.0 wt% equivalent NaCl at Thusis and 1.6-1.7 wt% at Schiers. Reproducible concentrations of Li, Na, K, Rb, Cs, Mg, Ca, Sr, Ba, B, Al, Mn, Cu, Zn, Pb, As, Sb, Cl, Br, and S could be determined for 97 fluid inclusion assemblages. Fluid and mineral geothermometry consistently indicate temperatures of 320 ± 20°C for the host rocks at Thusis and of 250 ± 30°C at Schiers. Combining fluid inclusion isochores with independent geothermometers results in pressure estimates of 2.8-3.8 kbar for Thusis, and of 3.3-3.4 kbar for Schiers. Pressure-temperature estimates are confirmed by pseudosection modeling. Fluid compositions and petrological modeling consistently demonstrate that chemical fluid-rock equilibrium was attained during vein formation, indicating that the fluids originated locally by metamorphic dehydration during near-isothermal decompression in a rock-buffered system. Keywords Fluid inclusions * LA-ICPMS * Low-grade metamorphism * Accretionary wedge * Veins * Fluid-rock equilibrium * Graphitic metasediments, Introduction The chemical composition of metamorphic fluids is of fundamental importance for predicting mineral stability, evaluating the pressure-temperature history of rocks, and assessing rates and scales of mass transfer during [...]

The evolution of chlorite composition with temperature (and pressure) serves as basis to a number of chlorite chemical thermometers, for which the oxidation state of iron has been recognised as a recurrent issue, especially at low temperature (T). A new chlorite geothermometer that does not require prior [Fe.sup.3+] knowledge is formulated, calibrated on 161 analyses with well-constrained T data covering a wide range of geological contexts and tested here for low-T chlorites (T < 350°C and pressures below 4 kbar). The new solid-solution model used involves six end-member components (the Mg and Fe end-members of 'Al-free chlorite S', sudoite and amesite) and so accounts for all low-T chlorite compositions; ideal mixing on site is assumed, with an ordered cationic distribution in tetrahedral and octahedral sites. Applied to chlorite analyses from three distinct low-T environments for which independent T data are available (Gulf Coast, Texas; Saint Martin, Lesser Antilles; Toyoha, Hokkaido), the new pure-[Fe.sup.2+] thermometer performs at least as well as the recent models, which require an estimate of [Fe.sup.2+] content. This relief from the ferric iron issue, combined with the simple formulation of the semi-empirical approach, makes the present thermometer a very practical tool, well suited for, for example, the handling of large analytical datasets--provided it is used in the calibration range (T < 350°C, P < 4 kbar). Keywords Chlorite * Geothermometry * Diagenesis * Low-grade metamorphism * Solid solution, Introduction Chlorites are ubiquitous in most diagenetic and metamorphic rocks (Foster 1962; Cathelineau 1988; Laird 1988; De Caritat et al. 1993; Vidal et al. 2001). Their wide compositional variations, through [...]

Isotopic analyses of ancient mantle-derived magmatic rocks are used to trace the geochemical evolution of the Earth's mantle, but it is often difficult to determine their primary, initial isotope ratios due to the detrimental effects of metamorphism and secondary alteration. We present in situ analyses by LA-MC-ICPMS for the Pb isotopic compositions of igneous plagioclase ([An.sub.75-89]) megacrysts and the Hf isotopic compositions of BSE-imaged domains of zircon grains from two mantle-derived anorthosite complexes from south West Greenland, Fiskenaesset and Nunataarsuk, which represent two of the best-preserved Archean anorthosites in the world. In situ LA-ICPMS U-Pb geochronology of the zircon grains suggests that the minimum crystallization age of the Fiskenaesset complex is 2,936 ± 13 Ma (2σ, MSWD = 1.5) and the Nunataarsuk complex is 2,914 ± 6.9 Ma (2σ, MSWD = 2.0). Initial Hf isotopic compositions of zircon grains from both anorthosite complexes fall between depleted mantle and a less radiogenic crustal source with a total range up to 5 [ζ.sub.Hf] units. In terms of Pb isotopic compositions of plagioclase, both anorthosite complexes share a depleted mantle end member yet their Pb isotopic compositions diverge in opposite directions from this point: Fiskenaesset toward a high-μ, more radiogenic Pb, crustal composition and Nunataarsuk toward low-μ, less radiogenic Pb, crustal composition. By using Hf isotopes in zircon in conjunction with Pb isotopes in plagioclase, we are able to constrain both the timing of mantle extraction of the crustal end member and its composition. At Fiskenaesset, the depleted mantle melt interacted with an Eoarchean (~3,700 Ma) mafic crust with a maximum [sup.176]Lu/[sup.177]Hf ~0.028. At Nunataarsuk, the depleted mantle melt interacted with a Hadean (~4,200 Ma) mafic crust with a maximum [sup.176]Lu/[sup.177]Hf ~0.0315. Evidence from both anorthosite complexes provides support for the long-term survival of ancient mafic crusts that, although unidentified at the surface to date, could still be present within the Fiskenasset and Nunataarsuk regions. Keywords Archean * Anorthosites * LA-MC-ICPMS * Pb-Pb isotopes * Lu-Hf isotopes, Introduction Long-lived radiogenic isotope systems (e.g. Sm-Nd, Lu-Hf, Pb-Pb) are powerful tracers of geochemical processes. They have been used extensively to track complementary processes such as the geochemical depletion of [...]

The Golyamo Kamenyane serpentinite is a portion of a metaophiolite, located in the Upper High-Grade Unit of the metamorphic basement of the Eastern Rhodope Metamorphic Complex, SE Bulgaria. It consists of metaharzburgite and metadunite hosting layers of metagabbro and some chromitite bodies. All these lithologies were affected by ultrahigh-pressure (UHP) metamorphism and subsequent retrograde evolution during exhumation. Chromite from chromitites can be classified into four textural groups: (1) partly altered chromite, (2) porous chromite, (3) homogeneous chromite and (4) zoned chromite. Partly altered chromite shows unaltered, Al-rich cores with unit cell size of 8.255 Å and Cr# [Cr/(Cr + Al) atomic ratio] = 0.52-0.60, Mg# [Mg/(Mg + [Fe.sup.2+]) atomic ratio] = 0.65-0.70 and [Fe.sup.3+]/([Fe.sup.3+] + [Fe.sup.2+]) = 0.20-0.30, surrounded by porous chromite, with a cell size of 8.325 Å, [Fe.sup.3+]/([Fe.sup.3+] + [Fe.sup.2+]) < 0.20 and values of Cr# and Mg# evolving from 0.60 to 0.91 and 0.65-0.44, respectively, from core to rim. The chemical composition of porous chromite varies within the following ranges: Cr# = 0.93-0.96, Mg# = 0.48-0.35 and [Fe.sup.3+]/([Fe.sup.3+] + [Fe.sup.2+]) = 0.22-0.53. Its unit cell size is very constant (8.350 A). Most pores in porous and partly altered chromite are filled with chlorite, which also occurs between chromite grains. Homogeneous chromite has [Fe.sup.3+]/([Fe.sup.3+] + [Fe.sup.2+]) = 0.55-0.66, Cr# = 0.96-0.99, Mg# = 0.32-0.19 and a cell size of 8.385 Å. The cores of zoned chromite are similar to those of partially altered chromite, but the rims are identical to homogeneous chromite. Although chlorite predominates in the silicate matrix of homogeneous and zoned chromite, it coexists with some antigorite, talc and magnesiohornblende. Mineral data and thermodynamic modeling allow interpretation of the alteration patterns of chromite as the consequence of a two-stage process developed during retrograde metamorphic evolution coeval with fluid infiltration. During the first stage, chromite reacts in the presence of fluid with olivine to produce chlorite and Cr- and [Fe.sup.2+]-rich residual chromite (ferrous chromite) at ~700 to ~450°C. This dissolution-precipitation reaction involves continuous chromite mass loss resulting in the development of a porous texture. This stage takes place progressively on cooling under water-saturated and reducing conditions. The second stage mainly consists of the formation of homogeneous chromite with ferrian chromite composition by the addition of magnetite to the porous ferrous chromite during a late oxidizing hydrothermal event. Keywords Chromite * Ferrian chromite * HP-HT metamorphism * Retrograde evolution * Rhodope massif * CrMASH system, Introduction Experimental and empirical studies have shown that the chemical composition of chromite (hereafter this term denotes a mineral with wide but variable composition in the central region of the [...]

The Southern Marginal Zone of the Limpopo Complex is composed of granite-greenstone cratonic rocks reworked by a Neoarchean high-grade tectono-metamorphic event. Petrographic and mineral chemical characterization of an Al-Mg granulite from this zone is presented here. The granulite has a gneissic fabric with distinct Al-rich and Si-rich layers, with the former preserving the unusual lamellar (random and regular subparallel) intergrowths of corundum and symplectic intergrowth of spinel with orthopyroxene. The Al-rich layer preserves mineral assemblages such as rutile with orthopyroxene + sillimanite ± quartz, Al-rich orthopyroxene (~ 11 wt%), spinel + quartz, and corundum in possible equilibrium with quartz, while the Si-rich layer preserves antiperthites and orthopyroxene + sillimanite ± quartz, all considered diagnostic of ultrahigh-temperature metamorphism. Application of Al-in-opx thermometry, ternary feldspar thermometry and construction of suitable pressure-temperature phase diagrams, compositional and model proportion isopleth results indicate P-T conditions as high as ~1,050-1,100°C, and ~10-12 kbars for the Al-Mg granulite. Our report of ultrahigh-temperature conditions is significant considering that the very high temperature was reached during decompression of an otherwise high-pressure granulite complex (clockwise P-T path), whereas most other ultrahigh-temperature granulites are linked to magma underplating at the base of the crust (counterclockwise P-T path). Keywords Orthopyroxene + sillimanite ± quartz * High-Al orthopyroxene * Antiperthite * Spinel * quartz * Ultrahigh-temperature metamorphism * Al-Mg granulite * Clockwise P-T path * Southern Marginal Zone * Limpopo complex, Introduction Rocks that preserve mineral assemblages that are diagnostic of ultrahigh-temperature metamorphism, such as Mg-Al granulites with highly aluminous and magnesian bulk compositions, are volumetrically rare in nature and occur [...]

Byline: Ria Mukherjee (1), Sisir K. Mondal (1,2,3), Minik T. Rosing (2), Robert Frei (2,4) Keywords: Chromite; Ferritchromit; Komatiite; Boninite; Subduction zone; Archean greenstone belt; Dharwar Craton; India Abstract: The chromite deposits in the Archean Nuggihalli schist belt are part of a layered ultramafic--mafic sequence within the Western Dharwar Craton of the Indian shield. The 3.1-Ga ultramafic--mafic units occur as sill-like intrusions within the volcano-sedimentary sequences of the Nuggihalli greenstone belt that are surrounded by the tonalite--trondhjemite--granodiorite (TTG) suite of rocks. The entire succession is exposed in the Tagdur mining district. The succession has been divided into the lower and the upper ultramafic units, separated by a middle gabbro unit. The ultramafic units comprise of deformed massive chromitite bodies that are hosted within chromite-bearing serpentinites. The chromitite bodies occur in the form of pods and elongated lenses (~60--500 m by ~15 m). Detailed electron microprobe studies reveal intense compositional variability of the chromite grains in silicate-rich chromitite (~50% modal chromite) and serpentinite (~2% modal chromite) throughout the entire ultramafic sequence. However, the primary composition of chromite is preserved in the massive chromitites (~60--75% modal chromite) from the Byrapur and the Bhaktarhalli mining district of the Nuggihalli schist belt. These are characterized by high Cr-ratios (Cr/(Cr + Al) = 0.78--0.86) and moderate Mg-ratios (Mg/(Mg + Fe.sup.2+) = 0.38--0.58). The compositional variability occurs due to sub-solidus re-equilibration in the accessory chromite in the serpentinite (Mg-ratio = 0.01--0.38 Cr-ratio = 0.02--0.99) and in silicate-rich chromitite (Mg-ratio = 0.06--0.48 Cr-ratio = 0.60--0.99). In the massive chromitites, the sub-solidus re-equilibration for chromite is less or absent. However, the re-equilibration is prominent in the co-existing interstitial and included olivine (Fo.sub.96--98) and pyroxene grains (Mg-numbers = 97--99). Compositional variability on the scale of a single chromite grain occurs in the form of zoning, and it is common in the accessory chromite grains in serpentinite and in the altered grains in chromitite. In the zoned grains, the composition of the core is modified and the rim is ferritchromit. In general, ferritchromit occurs as irregular patches along the grain boundaries and fractures of the zoned grains. In this case, ferritchromit formation is not very extensive. This indicates a secondary low temperature hydrothermal origin of ferritchromit during serpentinization. In some occurrences, the ferritchromit rim is very well developed, and only a small relict core appears to remain in the chromite grain. However, complete alteration of the chromite grains to ferritchromit without any remnant core is also present. The regular, well-developed and continuous occurrence of ferritchromit rims around the chromite grain boundaries, the complete alteration of the chromite grains and the modification of the core composition indicate the alteration in the Nuggihalli schist belt to be intense, pervasive and affected by later low-grade metamorphism. The primary composition of chromite has been used to compute the nature of the parental melt. The parental melt calculations indicate derivation from a high-Mg komatiitic basalt that is similar to the composition of the komatiitic rocks reported from the greenstone sequences of the Western Dharwar Craton. Tectonic discrimination diagrams using the primary composition of chromites indicate a supra-subduction zone setting (SSZ) for the Archean chromitites of Nuggihalli and derivation from a boninitic magma. The composition of the komatiitic basalts resembles those of boninites that occur in subduction zones and back-arc rift settings. Formation of the massive chromitites in Nuggihalli may be due to magma mixing process involving hydrous high-Mg magmas or may be related to intrusions of chromite crystal laden magma however, there is little scope to test these models because the host rocks are highly altered, serpentinized and deformed. The present configurations of the chromitite bodies are related to the multistage deformation processes that are common in Archean greenstone belts. Author Affiliation: (1) Department of Geological Sciences, Jadavpur University, Kolkata, 700032, India (2) Nordic Center for Earth Evolution, Natural History Museum of Denmark, University of Copenhagen, Aster Voldgade 5-7, 1350, Copenhagen, Denmark (3) Department of Earth and Planetary Sciences, American Museum of Natural History, Central Park West@79th Street, New York, NY, 10024, USA (4) Institute of Geography and Geology, University of Copenhagen, Aster Voldgade 10, 1350, Copenhagen, Denmark Article History: Registration Date: 03/03/2010 Received Date: 06/10/2009 Accepted Date: 02/03/2010 Online Date: 20/03/2010 Article note: Communicated by T. L. Grove. Electronic supplementary material The online version of this article (doi: 10.1007/s00410-010-0511-5) contains supplementary material, which is available to authorized users.

Byline: Lyderic France (1,2,4), Juergen Koepke (2), Benoit Ildefonse (1), Sarah B. Cichy (2), Fabien Deschamps (3) Keywords: Mid-ocean ridge; Axial magma chamber; Hydrothermal system; Sheeted dike complex; Partial melting; Experimental petrology; Oceanic plagiogranite; Granoblastic dikes Abstract: In ophiolites and in present-day oceanic crust formed at fast spreading ridges, oceanic plagiogranites are commonly observed at, or close to the base of the sheeted dike complex. They can be produced either by differentiation of mafic melts, or by hydrous partial melting of the hydrothermally altered sheeted dikes. In addition, the hydrothermally altered base of the sheeted dike complex, which is often infiltrated by plagiogranitic veins, is usually recrystallized into granoblastic dikes that are commonly interpreted as a result of prograde granulitic metamorphism. To test the anatectic origin of oceanic plagiogranites, we performed melting experiments on a natural hydrothermally altered dike, under conditions that match those prevailing at the base of the sheeted dike complex. All generated melts are water saturated, transitional between tholeiitic and calc-alkaline, and match the compositions of oceanic plagiogranites observed close to the base of the sheeted dike complex. Newly crystallized clinopyroxene and plagioclase have compositions that are characteristic of the same minerals in granoblastic dikes. Published silicic melt compositions obtained in classical MORB fractionation experiments also broadly match the compositions of oceanic plagiogranites however, the compositions of the coexisting experimental minerals significantly deviate from those of the granoblastic dikes. Our results demonstrate that hydrous partial melting is a likely common process in the root zone of the sheeted dike complex, starting at temperatures exceeding 850degC. The newly formed melt can either crystallize to form oceanic plagiogranites or may be recycled within the melt lens resulting in hybridized and contaminated MORB melts. It represents the main MORB crustal contamination process. The residue after the partial melting event is represented by the granoblastic dikes. Our results support a model with a dynamic melt lens that has the potential to trigger hydrous partial melting reactions in the previously hydrothermally altered sheeted dikes. A new thermometer using the Al content of clinopyroxene is also elaborated. Author Affiliation: (1) Geosciences Montpellier, CNRS, Universite Montpellier 2, CC60, 34095, Montpellier Cedex 05, France (2) Institut fur Mineralogie, Leibniz Universitat Hannover, Callinstrasse 3, 30167, Hannover, Germany (3) LGCA UMR CNRS 5025, Universite Joseph-Fourier, BP 53, 38041, Grenoble cedex, France (4) Geosciences et Environnement Cergy, Universite Cergy-Pontoise, 5 mail Gay Lussac, Neuville sur Oise, 95031, Cergy-Pontoise cedex, France Article History: Registration Date: 10/02/2010 Received Date: 06/08/2009 Accepted Date: 10/02/2010 Online Date: 09/03/2010 Article note: Communicated by J. Hoefs. Electronic supplementary material The online version of this article (doi: 10.1007/s00410-010-0502-6) contains supplementary material, which is available to authorized users.

The Makalu leucogranite in the eastern Nepal Himalaya is a multiphase intrusion forming the structurally highest foliation-parallel sheets along the top of the Greater Himalayan Sequence. It is part of a chain of Miocene granites seen continuously along the length of the Himalaya and is composed of Grt + Tur + Ms [+ or -] Bt leucogranites but, unlike most other Himalayan granites, also locally contains coarse-grained cordierite. The cordierite-bearing leucogranite intrudes through and overlies lower sheets of 'normal' tourmaline granites and represents the most recent phase of magmatism. Cross-cutting feeder dykes channelled magma up from the source region within the sillimanite grade Barun gneiss to the upper sheet. Petrology shows evidence for muscovite dehydration melting (~700[degrees]C) in the upper part of the Barun gneiss of the Greater Himalayan Sequence, which retains biotite, indicating that melting temperatures did not exceed 800[degrees]C. Secondary cordierite around garnet in these gneisses and the presence of cordierite in leucogranites record the last low-pressure decompression phase of melting. P-T determinations detail peak sillimanite grade metamorphism at 713[degrees]C/5.9 kbar, with a secondary cordierite overprint at 618[degrees]C/2.1 kbar; this P-T transition lies wholly within the modeled melt field. Monazite, zircon, and xenotime geochronology links the metamorphism and the different leucogranites. The main phase of leucogranite production occurred from 24 to 21 Ma, while the most recent melting occurred in the cordierite leucogranite and the migmatitic Barun gneisses at 15.6 [+ or -] 0.2 and 16.0 [+ or -] 0.6 Ma, respectively. Pseudosections for the migmatitic Barun gneiss and cordierite leucogranite show conditions of final cordierite bearing melt crystallization at approximately 4 kbar and 700[degrees]C and two main phases of melting: one associated with muscovite dehydration melting and one associated with formation of cordierite. These data support the channel flow model for the Greater Himalaya where decompression melting was coeval with southward ductile extrusion of a partially molten layer of middle crust during the Early and Middle Miocene. doi: 10.1029/2009TC002533.

Byline: Ben Buse (1), John C. Schumacher (1), R. Stephen J. Sparks (1), Matthew Field (2) Keywords: Kimberlite; Hydrothermal metamorphism; Xenolith; Serpentinisation; Post-emplacement Abstract: Metamorphic assemblages within Karoo basalt xenoliths, found within volcaniclastic kimberlite of the B/K9 pipe, Damtshaa, Botswana, constrain conditions of kimberlite alteration. Bultfonteinite and chlorite partially replace the original augite-plagioclase assemblage, driven by the serpentinisation of the kimberlite creating strong chemical potential gradients for Si and Mg. Hydrogarnet and serpentine replace these earlier metamorphic assemblages as the deposits cool. The bultfonteinite (ideally [Ca.sub.2]SiO.sub.2[OH,F].sub.4) and hydrogarnet assemblages require a water-rich fluid containing F.sup.-, and imply hydrothermal alteration dominated by external fluids rather than autometamorphism from deuteric fluids. Bultfonteinite and hydrogarnet are estimated to form at temperatures of ca. 350--250degC, which are similar to those for serpentinisation. Alteration within the B/K9 kimberlite predominantly occurs between 250 and 400degC. We attribute these conditions to increased efficiency of mass transfer and chemical reactions below the critical point of water and a consequence of volume-increasing serpentinisation and metasomatic reactions that take place over this temperature range. A comparison of the B/K9 kimberlite with kimberlites from Venetia, South Africa suggests that the composition and mineralogy of included xenoliths affects the alteration assemblages within kimberlite deposits. Author Affiliation: (1) Department of Earth Sciences, University of Bristol, Bristol, UK (2) DiaKim Consulting Limited, Wookey Hole, Wells, UK Article History: Registration Date: 19/01/2010 Received Date: 11/08/2009 Accepted Date: 19/01/2010 Online Date: 10/02/2010 Article note: Communicated by M. W. Schmidt.