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Paleoproterozoic gneisses of the Ellesmere-Devon crystalline terrane on southeastern Ellesmere Island are deformed by metre-scale, east-striking mylonite zones. The shear zones commonly offset pegmatitic dikes and represent the last episode of ductile deformation. Samples were dated by the [.sup.40Ar]/[.sup.39Ar] step-heating method to put an upper limit on the time of deformation. Biotite from one tonalitic protolith and five shear zones give geologically meaningful results. Clusters of unoriented biotite grains pseudomorph granulite-facies orthopyroxene in some of the weakly deformed gneisses, whereas the shape-preferred orientation of biotite defines the mylonitic fabric. The intrusive age of the tonalitic protolith is 1958 [+ or -] 12 Ma, based on previous U-Pb dating of zircon. [.sup.40Ar]/[.sup.39Ar] analysis of biotite from the same sample gave a plateau age of 1929 [+ or -] 23 Ma, which is interpreted as cooling from regional granulite facies metamorphism. Three nearby samples of mylonitic tonalite have [.sup.40Ar]/[.sup.39Ar] ages in the range of f [approximately equal to]1870-1840 Ma. Biotite from two granitic mylonites over 80 km away return high-resolution Ar spectra in the same range, implying that widespread ductile shearing occurred at [approximately equal to]1870-1840 Ma, or [approximately equal to]90 million years after cooling from regional metamorphism. Although the 2.0-1.9 Ga gneisses of southeastern Ellesmere Island correlate with the Inglefield Mobile Belt in North-West Greenland and the Thelon Tectonic Zone, the late shear zones are superimposed on that juvenile arc long after the 1.97 Ga Thelon orogeny. Key words: [.sup.40Ar]/[.sup.39Ar] dating, biotite, Ellesmere Island, mylonite, shear zone. Des gneiss paleoproterozoiques du terrane cristallin d'Ellesmere-Devon dans le sud-est de l' ile d'Ellesmere sont deformes par des zones metriques de mylonites de direction Est. Dans bien des cas, ces zones de cisaillement ont entraine le decalage de dykes de pegmatite et representent le dernier episode de deformation ductile. Des echantillons ont ete dates au [.sup.40Ar]/[.sup.39Ar] par la methode de chauffe par paliers afin d' etablir une limite d'age superieure pour la deformation. Des biotites provenant d'un protolite tonalitique et de cinq zones de cisaillement donnent des resultats coherents du point de vue geologique. Des amas de grains de biotite sans orientation privilegiee representent des pseudomorphes d'orthopyroxene du facies des granulites dans certains des gneiss faiblement deformes, alors que l'orientation privilegiee d'autres grains de biotite definit la fabrique mylonitique. L' age d'intrusion du protolite tonalitique est de 1958 [+ or -] 12 Ma, a la lumiere de resultats passes de datation U-Pb sur zircons. L'analyse [.sup.40Ar]/[.sup.39Ar] de biotite du meme echantillon produit un age de plateau de 1929 [+ or -] 23 Ma, qui est interprete comme etant l'age du refroidissement ayant suivi le metamorphisme regional au facies des granulites. Trois echantillons de tonalite mylonitique preleves a proximite ont donne des ages [.sup.40Ar]/[.sup.39Ar] dans l'intervalle de [approximately equal to]1870-1840 Ma. De la biotite de deux mylonites granitiques situees a plus de 80 km donne des spectres de Ar de haute resolution dans la meme fourchette d' ages, ce qui sous-entend qu'un cisaillement ductile repandu a eu lieu dans l'intervalle de [approximately equal to]1870-1840 Ma, ou [approximately equal to]90 millions d'annees apres le refroidissement qui a suivi le metamorphisme regional. Bien que les gneiss de 2,0-1,9 Ga du sud-est de l' ile d'Ellesmere soient correles a la ceinture mobile d' Inglefield du Groenland du Nord-Ouest et a la zone tectonique de Thelon, les zones de cisaillement tardives ont ete superposees sur cet arc juvenile bien apres l'orogenese de Thelon a 1,97 Ga. [Traduit par la Redaction] Mots-cles: datation [.sup.40Ar]/[.sup.39Ar], biotite, ile d'Ellesmere, mylonite, zone de cisaillement., Introduction The Laurentian craton is a collage of large Archean provinces separated by long, narrow, mostly Paleoproterozoic orogenic belts (Fig. 1; Hoffman 1988; Whitmeyer and Karlstrom 2007). The Thelon tectonic [...]

Faults and deformation zones are important features of Archean terranes because of their significance for structural evolution and the formation of large gold districts. In the Chibougamau area, northeastern portion of the Abitibi Subprovince, an Apogee Metal Earth seismic reflection survey identified an association between an exceptional shallow-dipping subsurface reflector zone and the Barlow Fault Zone visible at the surface. This fault zone is located adjacent to the Opatica Subprovince and shares many characteristics with well-documented major gold-bearing faults in the more gold-endowed southern portion of the Abitibi Subprovince. This study aimed to reconstruct the kinematic evolution of the Barlow Fault Zone, determine its position within the structural setting of this section of the Abitibi Subprovince, and evaluate its importance for gold potential in the northern part of the Chibougamau area. The fault zone can be divided into three segments that differ markedly in dip. Structural reconstruction and field observations are compatible with a reverse south-over-north movement related to a ductile north--south shortening event, which culminated with amphibolite metamorphism. A late normal movement is deduced at the final stage. Geothermobar-ometers indicate peak metamorphism conditions of 550 [+ or -] 50 [degrees]C and [+ or -] 6 1.2 kbar. Results from this study suggest that amphibolite facies metamorphism covers a much wider area within the Chibougamau region than previously documented. The Barlow Fault Zone shares similar geometric characteristics and evolutionary history with other gold-bearing structures in the Abitibi Sub-province, but ultimately it was unable to provide optimal conditions for channelling fluid and precipitating gold. High-grade metamorphism and the limited crustal extent of the fault likely prevented the tapping of deep metamorphic fluid and magma reservoirs. The Barlow Fault Zone is interpreted as a back-thrust fault that belongs to a more extensive south-dipping fault system encasing juxtaposed tectonic slivers. This system, with amphibolite facies metamorphism, is a defining feature of the northern portion of the Chibougamau area and developed during the accretion between the Opatica and Abitibi subprovinces. Key words: fault segmentation, Archean, Abitibi Subprovince, structural inheritance, metamorphic P-T conditions. Les failles et zones de deformation sont d'importants elements des terranes archeens en raison de leur role dans revolution structurale et dans la formation de grands districts auriferes. Dans la region de Chibougamau de la partie nord-est de la sous-province de l'Abitibi, un leve de sismique reflexion pour le projet Metal Earth du Fonds Apogee Canada a releve une association entre une zone exceptionnelle de reflecteurs souterrains faiblement inclines et la zone de la faille de Barlow exposee a la surface. Cette zone de faille est attenante a la sous-province d'Opatica et presente de nombreuses caracteristiques semblables a celles d'importantes failles auriferes bien documentees dans la portion meridionale plus riche en or de la sous-province de l'Abitibi. L'etude visait a reconstituer l'evolution cinematique de la zone de la faille de Barlow, determiner sa position dans le contexte structural de cette portion de la sous-province de l'Abitibi et evaluer son importance en ce qui concerne le potentiel aurifere de la partie nord de la region de Chibougamau. La zone de faille peut etre subdivisee en trois segments de pendages nettement distincts. La reconstitution structurale et les observations de terrain sont compatibles avec un deplacement inverse du bloc sud sur le bloc nord associe a un episode de raccourcissement ductile nord-sud, qui a culmine en un metamorphisme au facies des amphibolites. Un deplacement normal tardif est deduit pour la phase finale. Des geothermobarometres indiquent des conditions de pic du metamorphisme de 550 [+ or -] 50 [degrees]C et [+ or -] 6 1,2 kbar. Les resultats de l'etude donnent a penser que le metamorphisme au facies des amphibolites couvre un secteur beaucoup plus grand dans la region de Chibougamau que ce qui etait documente auparavant. Si la zone de la faille de Barlow presente des caracteristiques geometriques et une evolution semblables a celles d'autres structures auriferes dans la sous-province de l'Abitibi, elle n'a pas produit des conditions optimales pour la canalisation de fluides et la precipitation d'or. Des fluides metamorphiques et des magmas provenant de reservoirs profonds ne sont pas intervenus, vraisemblablement en raison du metamorphisme de forte intensite et de l'etendue limitee de la faille dans la croute. La zone de la faille Barlow est interpretee comme une faille de retrochevauchement faisant partie d'un systeme plus grand de failles a pendages vers le sud delimitant des lambeaux tectoniques juxtaposes. Ce systeme, accompagne d'un metamorphisme au facies des amphibolites, est un element essentiel de la partie nord de la region de Chibougamau et s'est developpe durant l'accretion entre les sous-provinces d'Opatica et de l'Abitibi. [Traduit par la Redaction] Mots-cles: segmentation de failles, Archeen, sous-province de l'Abitibi, heritage structural, conditions metamorphiques de P et T., Introduction Archean greenstone belts around the world, and specifically that of the Abitibi Subprovince (Canada), are characterized by faults and shear zones that have a significant role in the structural [...]

Abstract: The Omineca Belt between Nelson and Creston in southeastern British Columbia was affected by overlapping pulses of Mesozoic magmatism, metamorphism, and deformation. U--Pb geochronological data from zircon and monazite [...]

Abstract: The Hamadi gold deposit is located in North Sudan, and occurs in the Neoproterozoic metamorphic strata of the Arabian-Nubian Shield. Two types of gold mineralization can be discerned: gold-bearing [...]

Abstract: The Connemara region is a world-class example of a regional-scale, high-temperature metamorphic terrain. Its rock record documents formation of a bi-vergent orogenic wedge and associated calkalkaline magmatism in a [...]

Abstract: Back-arc extension in the Aegean, which was driven by slab rollback since 45 Ma, is described here for the first time in two stages. From Middle Eocene to Middle [...]

The Paleoproterozoic tectono-metamorphic evolution of the pre-Athabasca basement (~1.7 Ga) within the Wollaston- Mudjatik Transition Zone (WMTZ) (Saskatchewan, Canada) has been characterized using both exposed basement and drill cores from the Wolly-McClean exploration drilling project. The finite ductile strain pattern of the WMTZ results from the superposition of two tectono-metamorphic events M1-D1 and M2-D2. M1-D1 is associated with the development of a gently dipping foliation striking N90°-N100° and a southward decrease in peak pressures from up to 10 kbar (1 kbar=100 MPa) in the Cochrane River area down to 6 kbar in the Wolly-McClean exploration drilling project. The M2-D2 event is responsible for the main northeasterly trend of the WMTZ that developed in a sinistral transpressional tectonic regime during the final oblique collision of the Trans-Hudson Orogeny. Thermobarometric estimations on M2-D2 assemblages show that the studied area was re- equilibrated at about 4-5 kbar and 750-825°C. The basement has thus been affected by a differential isothermal decompression event between D1 and D2 that allowed the juxtaposition of the deepest northeastern domains and the Wolly-McClean exploration drilling project, at the same structural level. These results suggest that the basement exposed to the northeast of the Athabasca Basin is not an analog of the basement located beneath the eastern Athabasca Basin where uranium-enriched granitic pegmatites and granites are known. We also suggest that uranium-enriched melts produced during the early M1-D1 stage of partial melting in the deep crust were transferred to the midcrust, owing to D2 shear zones, where they have differentiated to produce uranium-bearing pegmatites. L'evolution tectono-metamorphique Paleoproterozoique du socle pre-Athabasca (~1.7 Ga) de la Zone de Transition Mudjatik-Wollaston (ZTMW) (Saskatchewan) a ete definie a partir du socle affleurant et des forages du projet Wolly- McClean. Le champ de deformation finie ductile resulte de la superposition de deux evenements tectono-metamorphique M1-D1 et M2-D2. M1-D1 est associe au developpement d'une foliation peu inclinee orientee N90°-N100° et a un pic de pression decroissant vers le sud avec plus de 10 kbar (1 kbar = 100 MPa) a la riviere Cochrane, jusqu'a 6 kbar dans la zone de forage du projet Wolly-McClean. L'evenement M2-D2 est responsable de la structuration majeure Nord-Est de la ZTMW qui s'est developpee dans un regime transpressif senestre au cours de la phase finale de la collision oblique Trans-Hudsonienne. Les estimations thermobarometriques sur les assemblages M2-D2 mettent en evidence un reequilibrage a 4-5 kbar et 750-825 °C. Le socle a donc ete affecte entre D1 et D2 par un episode de decompression isotherme, ramenant au meme niveau structural, la partie nord-est la plus profonde de la zone d'etude et la zone de forage Wolly-McClean. Ces resultats suggerent que le socle affleurant au Nord-Est du bassin de l'Athabasca n'est pas l'analogue du socle situe sous le bassin la ou les enrichissements en uranium sont connus. Nous suggerons egalement que les liquides silicates enrichis en uranium, et produits au cours de l'episode de fusion partielle M1-D1 dans la croute profonde, ont ete transferes dans la croute moyenne, par le biais des zones de cisaillement D2, et differencies pour former les pegmatites enrichies en uranium., Introduction The establishment of metallogenic models requires a thorough knowledge of the geological processes involved from source to ore deposit, including the formation of the elemental source and the mechanisms [...]

Geochronological studies in northern Wedel Jarlsberg Land, southwestern Svalbard (Norway), showed that the Tonian (c. 950 Ma) igneous rocks were subjected to metamorphism during the Torellian (c. 640 Ma) and early Caledonian (470-460 Ma) events. Predominant augen gneisses, derived from a Tonian protolith, are intercalated in that area, with schists comprising two distinct metamorphic mineral assemblages. The Ml (Torellian) assemblage containing garnet-I + quartz + plagioclase-I + biotite-I + muscovite-I was formed under amphibolite-facies conditions at c. 550-600 °C and 5-8 kbar (1 kbar = 100 MPa). The M2 (Caledonian) assemblage comprising garnet-II + quartz + plagioclase-II + biotite-II + muscovite-II + zoisite + chlorite crystallized at c. 500-550 °C and 9-12 kbar, corresponding to epidote-amphibolite facies conditions. The M2 mineral assemblage constitutes the pervasive Caledonian fabric of the schists that was subsequently reactivated in a left-lateral strike-slip shear regime. The subsequent c. 70° clockwise rotation of the original structure to its present position was caused by a large-scale passive rotation during the Paleogene Eurekan orogeny. The new pressure-temperature estimates suggest that metamorphic basement in the study area was consolidated during the Torellian middlegrade event and then overprinted by Caledonian moderate- to high-pressure subduction-related metamorphism. A following sinistral shear zone assembled the present structure of basement units. Our results pose a question about the possible extent of Torellian precursor to the Caledonian basement across the High Arctic and the scale of its subsequent involvement in early Caledonian subduction. In conjunction with previous studies, the results suggest a possible correlation between southwestern Spitsbergen and the Pearya Terrane in Ellesmere Island. Des etudes geochronologiques dans le nord du WedelJarlsberg Land, sud-ouest du Svalbard, montrent que les roches ignees du Tonien (~950 Ma) ont subi un metamorphisme durant les evenements du Torellien (~640 Ma) et du Caledonien precoce (470-460 Ma). Des gneiss oeilles predominants, provenant d'un protolithe du Tonien, sont, dans ce secteur, intercales avec des schistes comprenant deux assemblages mineralogiques metamorphiques distincts. L'assemblage M1 (Torellien) contenant du grenat-I + quartz + plagioclase-I + biotite-I + muscovite-I a ete forme sous des conditions du facies des amphibolites a ~550-600 °C et a une pression de 5-8 kbar (1 kbar = 100 MPa). L'assemblage M2 (Caledonien), comprenant du grenat-II + quartz + plagioclase-II + biotite-II + muscovite-II + zoisite + chlorite, a cristallise a ~500-550 °C et a une pression de 9-12 kbar, ce qui correspond a des conditions de facies des amphibolites a epidote. L'assemblage M2 constitue la fabrique envahissante caledonienne des schistes, laquelle a par la suite ete reactivee lors d'un regime de cisaillement de decrochement lateral senestre. La rotation subsequente d'environ 70 ° dans le sens horaire de la structure originale a sa position actuelle a ete causee par une rotation passive a grande echelle durant l'orogene de l'Eureka au Paleogene. Selon les nouvelles estimations de pression-temperature, le socle metamorphique dans le secteur a l'etude a ete consolide durant un metamorphisme intermediaire, au Torellien, et par la suite surimprime au Caledonien par un metamorphisme a pression moderee a elevee relie a une subduction. Une zone de cisaillement senestre a par la suite assemble la structure actuelle d'unites de socle. Nos resultats questionnent l'etendue possible du precurseur du Torellien au socle caledonien a travers le Haut-Arctique et l'echelle de son implication subsequente dans la subduction au Caledonien precoce. En conjonction avec des etudes anterieures, ils suggerent une correlation possible entre le Spitsbergen du sud-ouest et le terrane de Pearya dans l'ile d'Ellesmere. [Traduit par la Redaction], Introduction A classical view on the evolution of crystalline basement in Svalbard (Norway) is that early Paleozoic Caledonian deformation and metamorphism overprint the effects of earlier tectonothermal events dating back [...]

A complex array of faulted arc rocks and variably metamorphosed forearc accretionary complex rocks form a mappable arc-forearc boundary in southern Alaska known as the Border Ranges fault (BRF). We use detrital U-Pb zircon dating of metasedimentary rocks within the Knik River terrane in the western Chugach Mountains to show that a belt of Early Cretaceous amphibolite-facies metamorphic rocks along the BRF was formed when older melange rocks of the Chugach accretionary complex were reworked in a sinistral-oblique thrust reactivation of the BRF during a period of forearc plutonism. The metamorphic subterrane of the Knik River terrane has a maximum depositional age (MDA) of 156.5 ± 1.5 Ma and a detrital zircon age spectrum that is indistinguishable from the Potter Creek assemblage of the Chugach accretionary complex, supporting correlation of these units. These ages contrast strongly with new and existing data that show Triassic to earliest Jurassic detrital zircon ages from metamorphic screens in the plutonic subterrane of the Knik River terrane, a fragmented Early Jurassic plutonic assemblage generally interpreted as the basement of the Peninsular terrane. Based on these findings, we propose the following new terminology for the Knik River terrane: (1) 'Carpenter Creek metamorphic complex' for the Early Cretaceous 'metamorphic subterrane', (2) 'western Chugach trondhjemite suite' for the Early Cretaceous forearc plutons within the belt, (3) 'Friday Creek assemblage' for a transitional melange unit that contains blocks of the Carpenter Creek complex in a chert-argillite matrix, and (4) 'Knik River metamorphic complex' in reference to metamorphic rocks engulfed by Early Jurassic plutons of the Peninsular terrane that represent the roots of the Talkeetna arc. The correlation of the Carpenter Creek metamorphic complex with the Chugach melange indicates that the trace of the BRF lies ~1-5 km north of the map trace shown on geologic maps, although, like other segments of the BRF, this boundary is blurred by local complexities within the BRF system. Ductile deformation of the melange is sufficiently intense that few vestiges of its original melange fabric exist, suggesting the scarcity of rocks described as melange in the cores of many orogens may result from misidentification of rocks that have been intensely overprinted by younger, ductile deformation. Un reseau complexe de roches d'arc faillees et un complexe accretionnaire de roches d'avant-arc a metamorphisme variable forment une limite arc/avant-arc cartographiable dans le sud de l'Alaska connue sous le nom de faille de Border Ranges (BRF). Nous datons par U-Pb, sur des zircons detritiques, les roches metasedimentaires dans le terrane de Knik River dans l'ouest des monts Chugach afin de montrer qu'une ceinture de roches metamorphiques au facies des amphibolites (Cretace precoce) le long de la BRF s'est formee lorsque d'anciennes roches de melange du complexe accretionnaire de Chugach ont ete retravaillees dans une reactivation de chevauchement oblique senestre de la BRF durant une periode de plutonisme d'avant-arc. Le sousterrane metamorphique du terrane de Knik River a un age de deposition maximal de 156,5 ± 1,5 Ma et un spectre d'ages sur zircons detritiques qui ne peut etre distingue de celui de l'assemblage de Potter Creek du complexe accretionnaire de Chugach, supportant ainsi la correlation de ces unites. Ces ages tranchent fortement avec les donnees, nouvelles et existantes, qui montrent des ages du Trias au Jurassique le plus precoce, determines sur des zircons detritiques provenant d'ecrans metamorphiques dans le sous-terrane plutonique du terrane de Knik River, un assemblage plutonique (Jurassique precoce) fragmente generalement interprete comme etant le socle du terrane Peninsular. En se basant sur ces resultats, nous proposons une nouvelle terminologie pour le terrane de Knik River. Nous proposons les termes suivants : (1) le <> pour le <> datant du Cretace precoce; (2) la <> pour les plutons d'avant-arc (Cretace precoce) a l'interieur de la ceinture; (3) <> pour une unite de melange transitionnel qui contient des blocs du complexe de Carpenter Creek dans une matrice chert-argilite et (4) le << complexe metamorphique de Knik River>> qui refere aux roches metamorphiques enveloppees par des plutons (Jurassique precoce) du terrane Peninsular, lequel represente les racines de l'arc Talkeetna. La correlation entre le complexe metamorphique de Carpenter Creek et le melange de Chugach indique que la trace de la faille Border Ranges se trouve a ~1 a 5 km au nord du trace montre sur les cartes geologiques, bien que, tout comme d'autres segments de la faille Border Ranges, cette limite soit brouillee par des complexites locales dans le systeme de failles Border Ranges. La deformation ductile du melange a ete suffisamment intense qu'il existe peu de vestiges de la fabrique originale du melange, suggerant que la rarete des roches decrites comme melange dans les noyaux de plusieurs orogenes peut decouler d'une mauvaise identification des roches qui ont subi une surimpression intense par une deformation ductile plus recente. [Traduit par la Redaction], Introduction The arc-forearc boundary is one of the most poorly understood tectonic elements of convergent plate margins because of poor exposure. In most margins, the boundary is either underwater or [...]

The mineral wadalite (ideal and simplified formula: [Ca.sub.6][Al.sub.5][Si.sub.2][O.sub.16][Cl.sub.3]) has been discovered for the first time in a meteorite, specifically in coarse-grained, igneous type B calcium-aluminum-rich inclusions (CAIs) from the CV carbonaceous chondrite Allende. We report the results of electron microprobe, scanning electron microscopy, and transmission electron microscopy analyses of wadalite-bearing assemblages in the Allende CAIs and propose that wadalite formed by metamorphic reaction between akermanitic melilite and anorthite, likely mediated by chlorine-bearing fluids. Petrographic relationships support the likelihood of multistage alterations by fluids of different chemistries interspersed or coinciding with thermal metamorphic episodes on the Allende parent asteroid. Fluid involvement in metamorphism of Allende CAIs implies that these objects experienced open-system alteration after accretion into the CV chondrite parent asteroid, which may have resulted in disturbances of their oxygen- and magnesium-isotope systematics. Keywords: Wadalite, Allende, type B CAIs, aqueous alteration, fluid-mediated metamorphism DOI: 10.2138/am.2010.3296

We present a new method for Lu-Hf geochronology using lawsonite, an index mineral of high-pressure, low-temperature metamorphism. The method uses common cation exchange chromatography to purify Lu and Hf from the sample and analysis by muiticollector inductively coupled plasma--mass spectrometry. Lawsonite [sup.176]Lu/[sup.177]Hf ratios are elevated and are similar to other minerals commonly used in Lu-Hf geochronology such as garnet or apatite. We obtain an age of 145.5 [+ or -] 2.4 Ma for lawsonite-blueschist facies metamorphism at Ring Mountain within the Franciscan Complex, California, the lawsonite type locality. The age agrees well with published geochronology and thermochronology for the Franciscan Complex. The new method provides a powerful tool to investigate processes and time scales of blueschist to eclogite facies metamorphism, crust-mantle recycling via subduction, and the geodynamics of convergent margins. doi: 10.1130/G30292A.1

Aqueous fluids have a profound influence on the evolution of the crust, both as agents of chemical mass transfer and mineral reactions, and through modifying its rheology. This paper is particularly concerned with the composition and role of fluids through the crustal cycle of burial, metamorphism and uplift. Information now comes from both conventional geological studies and from geophysics, which has documented both the presence of fluid in active areas of the crust, and also its migration in response to faulting. This contrasts with the overall slow rate of some fluid processes: for example, devolatilization reactions are endothermic and limited by the rate of heat supply. There is no clear distinction between diagenetic and metamorphic fluids, and extensive equilibration with host rocks means that few chemical or isotopic characteristics survive to provide tracers for deep fluid origins. After the metamorphic peak, remaining pore fluid is consumed in hydration reactions resulting in strong, dry rock. Subsequently, infiltration may reintroduce fluids but these normally survive only briefly until consumed by hydration reactions. Metamorphic rocks are strongly overpressured during prograde metamorphism and therefore have a low permeability. Considerable doubt is therefore cast on some claims of widespread and pervasive large fluid fluxes accompanying metamorphism.

Natural diamond from three ultrahigh-pressure metamorphic (UHPM) terranes (Erzgebirge Massif, Germany; Kokchetav Massif, Northern Kazakhstan; Rhodope Metamorphic Province, Greece) and synthetic diamond from cutting and polishing materials (paste, spray, saw blade) were studied by means of optical microscopy and Raman microspectroscopy, to constitute a new petrographic and spectroscopic data set that might be a useful tool for identifying and characterizing metamorphic diamond. Several criteria are established for distinguishing natural microdiamond identified in a rock thin section from the externally introduced ones [i.e., diamond as residual particles (contaminants) from the cutting and polishing material] such as the diamond size, the presence of inclusions, coatings, or coexistent phases and two diamond Raman band parameters, i.e., the Raman shift and the full-width at half maximum height (FWHM). Keywords: Diamond, Raman, UHP metamorphism, Kokchetav, Erzgebirge, Rhodope Metamorphic Province

Orthopyroxene-clinopyroxene-plagioclase needles and symplectite along the cleavage planes and grain boundaries of fluorine-bearing titanian-ferroan pargasite from the Highland Complex, Sri Lanka, are interpreted as evidence for dehydration melting at ultrahigh-temperature conditions. High Ti (up to 0.4 pfu) and F ([X.sub.F] up to 0.56) content in pargasite extends its stability to higher temperatures, and the composition indicates the dehydration melting reaction may take place at ultrahigh-temperatures (~950 [degrees]C) at a pressure around 10 kbar, close to peak metamorphic conditions. The increase of Ti content close to the grain boundaries and cleavage planes in pargasite indicates titanium partitioning from the melt during dehydration melting enhanced the stability of the mineral toward ultrahigh-temperature conditions. The REE content in the pargasite shows a similar behavior to that of titanium. The cores with no breakdown assemblage consist of low and flat REE concentrations with respect to the high and Eu-depleted rim. Clinopyroxene in symplectite and needle-shaped lamellae within the pargasite porphyroblasts have similar REE patterns with slightly low-concentrations relative to that of pargasite. In the breakdown assemblage, LREEs are partitioned mainly into plagioclase while the HREEs are partitioned into orthopyroxene. The REE enrichment in the pargasite rims signals their relative partitioning between pargasite rims and melt. Modeling of the partitioning of Ti and REEs associated with pargasite breakdown demonstrates that its stability is greatly enhanced at UHT conditions. This investigation implies that the stability of hydrous minerals such as amphibole can be extended to UHT conditions, and expands our knowledge of metamorphism in the lower crust. Keywords: Pargasite, ultrahigh-temperature UHT, dehydration melting, titanium partitioning, fluorine, mineral REE, Highland Complex, Sri Lanka

An albite-omphacite rock, containing the three sodic pyroxenes of the jadeite-omphacite-diopside/ hedenbergite ternary system, has been investigated in detail to better characterize the phase relationships in this system. The rock is from a serpentinite melange in the Kurosegawa Zone of Central Kyushu, Japan, and exhibits an omphacite overgrowth over diopside/hedenbergite and the further overgrowth of jadeite over omphacite, indicative of the successive replacement ofpyroxenes during metamorphism. Partial replacement of omphacite by diopside/hedenbergite at the margins of K-feldspar veins suggests that all three pyroxenes were stable locally at the same stage of metamorphism. The peak temperature and pressure conditions are roughly estimated to be 350[degrees]C and 5-10.8 kbar. Partitioning of [Fe.sup.2+]-Mg between pyroxene pairs is consistent with a recent thermodynamic model, indicating the attainment of local equilibrium for pyroxene pairs. The observed compositional gap is also consistent with the miscibility gap reported in many previous studies. The compositional field of omphacite in the ternary jadeite-aegirine-diopside/hedenbergite diagram extends from the jadeite-diopside/hedenbergite line toward the aegirine-rich field, maintaining approximately constant the jadeite content, resulting in an asymmetric phase diagram. Keywords: Miscibility gap, jadeite, omphacite, celsian, hyalophane

The metamorphosed Boliden volcanic-hosted massive sulfide (VHMS) ore deposit, northern Sweden, is characterized by an abundance of massive arsenopyrite ore and unusually high gold enrichments (up to 300-600 g/t) in crosscutting veins and fracture zones. The ore textures observed record progressive recrystallization and porphyroblast growth during metamorphism. We conducted a systematic laser-ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) study on the gold distribution in fine-grained massive arsenopyrite and pyrite ore, partially recrystallized arsenopyrite ore, and both arsenopyrite and pyrite porphyroblasts found in gold-rich veins. The gold concentrations in massive arsenopyrite are highest (30-50 ppm on average) and systematically decrease to 0-2.7 ppm with increasing degree of recrystallization. By contrast, gold concentrations in pyrite ore are much lower and do not exceed 0.2 ppm. The LA-ICP-MS data, coupled with textural data, provide the first direct conclusive evidence that gold is progressively liberated from massive sulfide ores during metamorphism and reprecipitated in veins and other low-strain sites. We suggest that such upgrading is the principal mechanism responsible for significant gold enrichment observed in many metamorphosed VHMS deposits worldwide. Keywords: massive sulfides, arsenopyrite, metamorphic upgrading, invisible gold.

Located at an intermediate position in the External Rif nappe pile, the Temsamane units (northern Morocco) are characterized by an abnormally intense metamorphism and a penetrative ductile deformation. We present new metamorphic data showing that, in spite of their external position in the Rif, part of the Temsamane units underwent medium-pressure low-temperature (MP-LT) metamorphism (at c. 7-9 kbar and 330-430[degrees]C), possibly during the Oligocene. Structural data show that the exhumation of these units, during Middle to Late Miocene times, was characterized by an intense approximately east west stretching and by top-to-the-west shear senses. We tentatively propose two possible origins for the MP-LT Temsamane units: (1) an internal origin related to the subduction and the HP LT event recorded in the Internal Rif (Alboran Domain), or (2) an external origin, implying a second subduction system within the External Rif, parallel to and almost contemporaneous with that of the Alboran Domain. This tectonometamorphic evolution of the Temsamane units is set within the context of the External Rif evolution. At a larger scale, we show that the exhumation history of the Temsamane units, which strongly resembles that documented in the core of the internal Betics, is compatible with the westward slab retreat occurring during the Middle to Late Miocene in the Betic-Rif region.

Abstract: The Grand Forks complex, in the southern Omineca belt of British Columbia, is a fault-bounded tectonic window exposing Proterozoic sediments and associated mafic rocks metamorphosed to upper amphibolite to [...]

The Salmon River belt is composed of metamorphic tectonites with poorly dated Mesozoic and Paleozoic protoliths that lie between volcanic arc assemblages of northeastern Oregon and the western margin of the North American continent in west-central Idaho. To the west, the Salmon River belt overlies late Paleozoic and Mesozoic volcanic arc rocks of the Wallowa terrane on a shallowly east-dipping fault; to the east the belt is separated from North American rocks by the western Idaho shear zone. The Salmon River belt is divided into western and eastern assemblages composed of greenschist- and amphibolite-facies metamorphic tectonites, respectively, and is deformed in three and locally four generations of penetrative structures. First-generation structures are synmetamorphic, west-vergent, tight to isoclinal folds with a penetrative axial-planar cleavage and downdip stretching lineation. Second- and third-phase structures are postmetamorphic and consist of open to close, upright to slightly overturned folds with north-south and east-west trending penetrative axial-planar cleavages, respectively. Low-angle faults truncate metamorphic structures within the Salmon River belt and juxtapose rocks with different metamorphic grades. The faults formed together with second-generation structures and are deformed by third-generation folds. The age of metamorphic and postmetamorphic structures is constrained by deformed carbonate rocks of Late Triassic age and clastic rocks of late Early Jurassic age and by the ca. 145 Ma emplacement of an undeformed tonalitic pluton. Along the eastern margin of the Salmon River belt, structures are overprinted and transposed by a younger, steeply east-dipping mylonitic shear zone, the mid-Cretaceous western Idaho shear zone. Synmetamorphic structures within the Salmon River belt formed before amalgamation of the Wallowa terrane and the North American fringing-arc complex and developed as part of a regional belt of intra-arc metamorphic tectonites. The pre-Cretaceous docking of the Wallowa terrane is recorded within the Salmon River belt by development of postmetamorphic structures and predates formation of the western Idaho shear zone. The western Idaho shear zone formed after the amalgamation of the arc terranes and records the intraplate deformation associated with the collapse of the fringing-arc and back-arc-basin system. Keywords: structure, tectonics, Wallowa terrane, Salmon River belt, Cordilleran Arc, Mesozoic.

The ultrahigh-temperature (UHT) metamorphism of the Napier Complex is characterized by the presence of dry mineral assemblages, the stability of which requires anhydrous conditions. Typically, the presence of the index mineral orthopyroxene in more than one lithology indicates that [H.sub.2]O activities were substantially low. In this study, we investigate a suite of UHT rocks comprising quartzo-feldspathic garnet gneiss, sapphirine granulite, garnet-orthopyroxene gneiss, and magnetite-quartz gneiss from Tonagh Island. High A1 contents in orthopyroxene from sapphirine granulite, the presence of an equilibrium sapphirine-quartz assemblage, mesoperthite in quartzo-feldspathic garnet gneiss, and an inverted pigeonite-augite assemblage in magnetite-quartz gneiss indicate that the peak temperature conditions were higher than 1,000 [degrees]C. Petrology, mineral phase equilibria, and pressure-temperature computations presented in this study indicate that the Tonagh Island granulites experienced maximum P-T conditions of up to 9 kbar and 1,100 [degrees]C, which are comparable with previous P-T estimates for Tonagh and East Tonagh Islands. The textures and mineral reactions preserved by these UHT rocks are consistent with an isobaric cooling (IBC) history probably following an counterclockwise P-T path. We document the occurrence of very high-density C[O.sub.2]-rich fluid inclusions in the UHT rocks from Tonagh Island and characterize their nature, composition, and density from systematic petrographic and microthermometric studies. Our study shows the common presence of carbonic fluid inclusions entrapped within sapphirine, quartz, garnet and orthopyroxene. Analysed fluid inclusions in sapphirine, and some in garnet and quartz, were trapped during mineral growth at UHT conditions as `primary' inclusions. The melting temperatures of fluids in most cases lie in the range of -56.3 to -57.2 [degrees]C, close to the triple point for pure C[O.sub.2] (-56.6 [degrees]C). The only exceptions are fluid inclusions in magnetite-quartz gneiss, which show slight depression in their melting temperatures (-56.7 to -57.8 [degrees]C) suggesting traces of additional fluid species such as [N.sub.2] in the dominantly C[O.sub.2]-rich fluid. Homogenization of pure C[O.sub.2] inclusions in the quartzo-feldspathic garnet gneiss, sapphirine granulite, and garnet-orthopyroxene gneiss occurs into the liquid phase at temperatures in the range of-34.9 to + 4.2 [degrees]C. This translates into very high C[O.sub.2] densities in the range of 0.95-1.07 g/[cm.sup.3]. In the garnet-orthopyroxene gneiss, the composition and density of inclusions in the different minerals show systematic variation, with highest homogenization temperatures (lowest density) yielded by inclusions in garnet, as against inclusions with lowest homogenization (high density) in quartz. This could be a reflection of continued recrystallization of quartz with entrapment of late fluids along the IBC path. Very high-density C[O.sub.2] inclusions in sapphirine associated with quartz in the Tonagh Island rocks provide potential evidence for the involvement of C[O.sub.2]-rich fluids during extreme crustal temperatures associated with UHT metamorphism. The estimated C[O.sub.2] isochores for sapphirine granulite intersect the counterclockwise P-T trajectory of Tonagh Island rocks at around 6-9 kbar at 1,100 [degrees]C, which corresponds to the peak metamorphic conditions of this terrane derived from mineral phase equilibria, and the stability field of sapphirine + quartz. Therefore, we infer that C[O.sub.2] was the dominant fluid species present during the peak metamorphism in Tonagh Island, and interpret that the fluid inclusions preserve traces of the synmetamorphic fluid from the UHT event. The stability of anhydrous minerals, such as orthopyroxene, in the study area might have been achieved by the lowering of [H.sub.2]O activity through the influx of C[O.sub.2] at peak metamorphic conditions (> 1,100 [degrees]C). Our microthermometric data support a counterclockwise P-T path for the Napier Complex.