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The Early Jurassic Butcher Ridge Igneous Complex (BRIC) in the Transantarctic Mountains contains abundant and variably hydrated silicic glass which has the potential to preserve a rich paleoclimate record. Here we present Fourier Transform Infrared Spectroscopic data that indicates BRIC glasses contain up to ~8 wt.% molecular water (H2Om), and low (

Petrological modeling and geochronology (Lu--Hf garnet, U--Pb monazite, zircon, and titanite) for seven mid-amphibolite to granulite-facies metamorphic rocks in the Thelon tectonic zone (Ttz) provide new insights into the evolution of this orogen. A Grt-Sp-Sil diatexite records 2.01-2.00 Ga garnet and monazite growth during >830 [degrees]C contact metamorphism associated with voluminous convergent margin plutonism. The most widespread, 1.96-1.90 Ga metamorphism is associated with clockwise pressure (P)--temperature (T) paths, indicating it was driven by crustal thickening. Earlier (1.96-1.92 Ga) and lower temperature (630 [degrees]C-730 [degrees]C; 6-8 kbar) metamorphism in the eastern Slave craton contrast with ~860 [degrees]C (7.5 kbar), 1.91-1.90 Ga diatexites in the central, Paleoproterozoic plutonic rock-dominated, core of the Ttz. These differences are interpreted to reflect lower vs. upper plate settings, respectively. A contribution of mantle heat is suggested by the counter-clockwise P-T path and 885 [degrees]C (5.7 kbar) conditions recorded by diatexite associated with c. 1.9 Ga peraluminous leucogranite that dominates a corridor along the Ttz--Rae craton boundary. A tectonic model is proposed wherein the Ttz evolved at an accretionary margin after rifting of Ttz microcontinent (mTtz) of the Rae craton at c. 2.2-2.1 Ga. Voluminous plutonism at 2.01-1.98 Ga formed via east-dipping subduction under mTtz. Following 1.97 Ga collision of the Slave craton and mTtz, west-dipping subduction generated 1.96-1.95 Ga plutonism in the composite mTtz-Slave upper plate. Collision of Rae craton at 1.95-1.94 Ga led to crustal thickening, widespread 1.91-1.90 Ga high-grade metamorphism and extensive crustal melting in the central Ttz. Key words: Thelon tectonic zone, Lu--Hf garnet geochronology, U--Pb accessory mineral geochronology, quantitative metamorphic petrology, thermobarometry, Introduction The Late Paleoproterozoic is marked by pan-continental orogenic belts that stitched together Earth's Archean cratons. Laurentia--one of the largest and longest lived continental collages--was in large part constructed from [...]

The recently recognized Sanagak Lake shear zone (SLsz) is a 165 km long, southwest striking corridor of high-strain rocks that transects the southern portion of Boothia Peninsula, Nunavut. This zone records pervasive deformation ([??]) at conditions of ~0.52 GPa and ~700 [degrees]C, and localized deformation ([??]) at [greater than or equal to]0.5 GPa and 300-500 [degrees]C that preserve left lateral and right lateral senses of movement, respectively. Neocrystallized [??] titanite in a hornblende-bearing granodiorite yields an age of 1804 [+ or -] 6 Ma, interpreted to be the timing of [??]. The timing of [??] is loosely bracketed by [.sup.40]Ar/[.sup.39]Ar hornblende (1814 [+ or -] 3 Ma) and biotite (1743 [+ or -] 1 Ma) cooling ages since the deformation temperature falls between the estimated closure temperature of these minerals. Similar rock types and metamorphic conditions on either side of the shear zone rule out the SLsz as a terrane boundary. Rather, strain localization may have been triggered by thermal softening related to the emplacement of a northeast-trending belt of high-temperature granites south of the shear zone between 1840 and 1820 Ma. Deformation and metamorphism at ca. 1.81 Ga south of Boothia Peninsula and in the central Rae (Committee Bay belt) have been attributed to the Superior Province colliding with the southeastern margin of the Rae craton, such that the SLsz may too have formed in response to far-field stresses derived from this collision. The absence of ca. 1.81 Ga tectonic fabrics north of the shear zone indicates that the SLsz marks the northwestern extent of mid-crustal, Trans-Hudson related tectonometamorphism. Key words: Churchill Province, shear zone, Trans-Hudson Orogeny, titanite geochronology, thermobarometry, quartz CPO La zone de cisaillement du lac Sanagak (zcLS) récemment découverte est un couloir de roches hautement déformées d'orientation sud-ouest de 165 km de long qui traverse la partie sud de la presqu'île de Boothia (Nunavut). Cette zone conserve le registre d'une déformation répandue (DSL1) à des conditions de ~0,52 GPa et ~700[grados]C et d'une déformation localisée (DSL2) à [mayor que o igual a] 0,5 GPa et 300-500 [grados]C, préservant respectivement des sens de déplacement dextre et senestre. De la titanite DSL1 néocristallisée dans une granodiorite à hornblende donne un âge de 1804 [+ ó -] 6 Ma interprété comme étant l'âge de DSL1. L'âge de DSL2 s'établit généralement entre les âges de refroidissement [.sup.40]Ar/[.sup.39]Ar de la hornblende (1814 [+ ó -] 3 Ma) et de la biotite (1743 [+ ó -] 1 Ma), étant donné que la température de la déformation se situe entre les températures de fermeture estimées de ces deux minéraux. Des types de roches et conditions métamorphiques semblables de part et d'autre de la zone de cisaillement excluent l'interprétation de la zcLS comme étant la limite d'un terrane. La localisation de la déformation pourrait plutôt avoir été déclenchée par l'amollissement thermique associé à la mise en place d'une ceinture de granites de haute température d'orientation nord-est au sud de la zone de cisaillement entre 1840 Ma et 1820 Ma. La déformation et le métamorphisme vers 1,81 Ga au sud de la presqu'île de Boothia et dans le centre du craton de Rae (ceinture de la baie Comité) ont été attribués à la collision de la province du Supérieur avec la marge sud-est du craton de Rae, la zcLS pouvant également s'être formée en réponse à des contraintes de champ lointain découlant de cette collision. L'absence de fabriques tectoniques d'environ 1,81 Ga au nord de la zcLS indique que cette dernière marque la limite nord-ouest du tectonométamorphisme médio-crustal associé à l'orogène trans-hudsonien. [Traduit par la Rédaction] Mots-clés : province de Churchill, zone de cisaillement, orogène trans-hudsonien, géochronologie sur titanite, thermobarométrie, OCP du quartz, Introduction The Churchill Province is one of the largest and most complex pieces of Archean crust on Earth (e.g., Hoffman 1988; Davis et al. 2006; Berman 2010). The province has [...]

The Purcell anticlinorium (PA) and the Kootenay arc (KA) are two adjacent Mesozoic tectonic domains in the southeastern Omineca belt of the Canadian Cordillera. Mesoproterozoic rocks of the Belt-Purcell Supergroup (BPSG) core the PA and have open, upward-facing structures. Deeper levels of the BPSG (Aldridge Formation) preserve a Mesoproterozoic phyllitic cleavage ([S.sub.1]) and biotite zone mineral assemblages. The KA, on the west flank of the PA, has a younger Neoproterozoic through Paleozoic stratigraphy. The central part of the KA, the focus of this study, experienced Jurassic recumbent folding and greenschist facies metamorphism that was overprinted by Early Cretaceous ductile deformation ([S.sub.2] schistosity) and Barrovian metamorphism. The interface between the PA and KA is taken as the easternmost occurrence of the [S.sub.2] schistosity. K-Ar and [.sup.40]Ar/[.sup.39]Ar mica dates in the PA range from 1318 to 240 Ma and show no consistent spatial distribution. They are interpreted to reflect partial resetting of Ar following initial cooling in the Mesoproterozoic, indicating that the PA was never sufciently buried or heated during younger tectonism to uniformly reset the mica cooling ages. In the KA, [.sup.40]Ar/[.sup.39]Ar dates range from 139 to 55 Ma and reflect previously documented periods of Mesozoic and Cenozoic metamorphism, magmatism, and exhumation. The PA--KA interface occurs above a west-dipping ramp in the Laurentian basement. The thick Mesoproterozoic sequence of turbidites and sills in the PA may have acted as a rigid buttress against which the more ductile rocks of the central KA 'piled up' during Early Cretaceous compressional orogenesis. Key words: structure, metamorphism, thermochronology, orogenesis, Purcell anticlinorium, Kootenay arc, Omineca belt, Canadian Cordillera L'anticlinorium de Purcell (AP) et l'arc de Kootenay (AK) sont deux domaines tectoniques mésozoïques attenants dans le sud-est du domaine d'Omineca de la Cordillère canadienne. Des roches mésoprotérozoïques du Supergroupe de Belt-Purcell (SGBP) forment le noyau de l'AP et présentent des structures évasées ouvertes vers le haut. Des niveaux plus profonds du SGBP (Formation d'Aldridge) conservent un clivage phylladique mésoprotérozoïque ([S.sub.1]) et des assemblages minéraux de la zone à biotite. L'AK, sur le flanc ouest de l'AP, présente une stratigraphie plus jeune, de néoprotérozoïque à paléozoïque. La partie centrale de l'AK, l'objet de la présente étude, est caractérisée par des plis couchés et un métamorphisme au faciès des schistes verts d'âge jurassique sur lesquels sont surimposés une déformation ductile (schistosité [S.sub.2]) et un métamorphisme barrovien d'âge crétacé précoce. L'interface entre l'AP et l'AK est considéré être représentée par la présence la plus à l'est de la schistosité S2. Les âges K-Ar et [.sup.40]Ar/[.sup.39]Ar sur micas de roches de l'AP vont de 1318 à 240 Ma et ne montrent pas de répartition spatiale cohérente. Ils sont interprétés comme reflétant la réinitialisation partielle de l'Ar après un refroidissement initial au Mésoprotérozoïque, indiquant que l'AP n'a jamais été enseveli ni chaufé sufsamment durant des épisodes tectoniques subséquents pour réinitialiser uniformément les âges de refroidissement des micas. Dans l'AK, les âges [.sup.40]Ar/[.sup.39]Ar vont de 139 à 55 Ma et reflètent des périodes déjà documentées de métamorphisme, de magmatisme et d'exhumation mésozoïques et cénozoïques. L'interface AP--AK se trouve au-dessus d'une rampe à inclinaison vers l'ouest dans le socle laurentien. L'épaisse séquence mésoprotérozoïque de turbidites et de filons-couches dans l'AP pourrait avoir agi comme contrefort rigide contre lequel les roches plus ductiles de l'AK central se sont << empilées >> durant l'activité orogénique en compression au Crétacé précoce. [Traduit par la Rédaction] Mots-clés : structure, métamorphisme, thermochronologie, orogenèse, anticlinorium de Purcell, arc de Kootenay, domaine d'Omineca, Cordillère canadienne, Introduction The southern Omineca belt of the Canadian Cordillera (Fig. 1) straddles the tectonic interface between autochthonous sedimentary rocks of the ancestral Laurentian margin, and allochthonous volcanic and sedimentary rocks [...]

Subduction zone processes are challenging to study because of the rarity of good exposures and the complexity of rock relationships within accretionary prisms. We report the results of field mapping and petrographic, geochemical, and geochronological analyses of the McHugh Complex accretionary prism melange in south-central Alaska that was recently exposed due to retreat of the Nelchina Glacier. Our new mapping and analyses of the melange, as well as adjacent Talkeetna arc intrusives, suggests that the previously mapped trace of the Border Ranges fault should shift northward in this location. Detailed petrographic analysis places this melange exposure with the Potter Creek assemblage of the McHugh Complex. Blocks of pillow lavas within the melange have both mid-ocean ridge basalt and intra-plate geochemical affinities, attesting to the complex relations of subduction-zone inputs in an alternating erosive--accretionary margin. A new zircon U-Pb age and geochemical analyses of a set of felsic dikes that cross-cut the accretionary sequence provide constraints on the regional tectonic evolution, including near-trench plutonism associated with the migration of a subducting spreading ridge along the southern Alaska margin during the Paleocene--Eocene. The McHugh section and cross-cutting dikes in this location are pervasively hydrothermally altered, which we attribute to elevated temperatures related to ridge subduction. Late-stage motion along the Border Ranges fault system, which is also recorded in the area, may also have contributed to the widespread alteration. Our data indicate that the Talkeetna volcanic arc and associated accretionary prism sediments were in their current configuration by 55 Ma. Key words: McHugh Complex, melange, Chugach terrane, southern Alaska, Talkeetna arc. Les processus associes aux zones de subduction sont difficiles a etudier en raison de la rarete de bons affleurements et de la complexite des relations entre les roches au sein de prismes d'accretion. Nous faisons etat des resultats de travaux de cartographie de terrain et d'analyses petrographiques, geochimiques et geochronologiques du melange de prisme d'accretion du complexe de McHugh, dans le centre-sud de l'Alaska, recemment expose par le retrait du glacier Nelchina. La nouvelle cartographie et les nouvelles analyses du melange, ainsi que des roches intrusives attenantes de l'arc de Talkeetna, donnent a penser que la trace cartographiee anterieurement de la faille des Border Ranges devrait se deplacer vers le nord en cet endroit. L'analyse petrographique detaillee place cette exposition du melange avec l'assemblage de Potter Creek du complexe de McHugh. Des blocs de lave a coussins dans le melange ont des affinites geochimiques avec du basalte de dorsale medio-oceanique et intraplaque, ce qui temoigne des relations complexes des apports de zone de subduction dans une marge alternant entre l'erosion et l'accretion. Un nouvel age U-Pb sur zircons et des analyses geochimiques d'un ensemble de dykes felsiques recoupant la sequence d'accretion permettent de preciser l'evolution tectonique regionale, dont du plutonisme proximal a une fosse associe a la migration d'une dorsale d'expansion en subduction le long de la marge sud de l'Alaska au Paleocene--Eocene. La coupe de McHugh et les dykes discordants dans cette localite temoignent d'une alteration hydrothermale penetrante que nous attribuons a des temperatures elevees associees a la subduction d'une dorsale. Un deplacement tardif le long du systeme de la faille des Border Ranges, qui est egalement preserve dans la region, pourrait aussi avoir contribue a l'alteration repandue. Nos donnees indiquent que la configuration actuelle de l'arc volcanique de Talkeetna et des sediments de prisme d'accretion associes etait deja etablie a 55 Ma. [Traduit par la Redaction] Mots-cles: complexe de McHugh, melange, terrane de Chugach, sud de l'Alaska, arc de Talkeetna., Introduction Arc--forearc relationships are difficult to study but are key to understanding processes that contribute to continental growth. Active margins are offshore and underwater, potentially buried by sediment. Ancient accretionary [...]

Garnet is a common rock‐forming mineral that occurs in a variety of rock types and over a wide range of pressure (P)–temperature (T) conditions in the Earth's lithosphere. Because garnet is considered a high‐strength mineral stable across an extensive range of conditions (1–25 GPa, <300–2000°C), it is generally accepted that garnets can retain their microstructures and chemical composition during deformation and metamorphism. Therefore, garnet is commonly used as a geothermobarometer and geochronometer to provide P–T and timing constraints on tectonic events. Herein, we study garnet from an eclogite facies mylonite (central Australia) to investigate the mechanisms of element mobility during high‐strain deformation under relatively dry, lower crustal conditions. Electron backscatter diffraction (EBSD) and electron channelling contrast imaging (ECCI) reveal evidence of crystal plasticity associated with brittle deformation in the form of heterogeneous misorientation patterns and low‐angle grain boundaries developed over length scales of 20–50 μm in the rims of garnet porphyroclasts. Atom probe tomography (APT) analysis of a low‐angle grain boundary within a highly strained portion of a clast shows Ca enrichment and Mg depletion along dislocations, whereas APT data along the rim of a mostly undeformed clast reveal a homogeneous distribution of garnet major components in the specimen matrix with the exception of Ca, Fe and Mg enrichment within a healed microfracture. The above‐mentioned results suggest that under relatively dry conditions, crystal plasticity enhances bulk element mobility via pipe diffusion, highlighting the importance of deformation‐induced microstructures on element mobility, with important implications for the robust and reliable use of garnet as a petrological tool. [ABSTRACT FROM AUTHOR]

The small volume, Mesozoic alkali gabbro intrusions of the Budgell Harbour and Dildo Pond stocks and associated alkaline lamprophyre dykes in Notre Dame Bay in Newfoundland are an example of onshore magmatism that may be associated with North Atlantic Ocean opening. Chemical abrasion isotope dilution thermal ionization mass spectrometry U–Pb dating of zircon from Budgell Harbour Stock drill core samples yielded a weighted average 206Pb/238U age of 147.9 ± 0.5 Ma (95% confidence interval, mean square weighted deviation = 0.10). Five 40Ar/39Ar laser step-heating, single phlogopite grain analyses from the Budgell Harbour and Dildo Pond stocks, plus a lamprophyre dyke, yielded ages ranging from 146.3 ± 0.2 to 149.5 ± 0.5 Ma. The data demonstrate a ca. 148 Ma (Jurassic and Tithonian) alkaline magmatic event in Newfoundland, contemporaneous with rifting and offshore basin formation. These new age data and published determinations from magmatic rocks on conjugate margins and adjacent regions reveal long-lived episodic magmatism in the embryonic North Atlantic rift environment. We propose a new model whereby magmatism occurred in Notre Dame Bay at the convergence of older crustal-scale faults and localized Moho depth variations that may have triggered lithospheric mantle melting through isothermal, distal, and edge-driven upwelling related to regional extension. [ABSTRACT FROM AUTHOR]

The Puy-les-Vignes W deposit, located in the northwestern French Massif Central (FMC), is a rare occurrence of a wolframite-mineralized hydrothermal breccia pipe hosted in high-grade metamorphic gneisses. We present an integrated study of this deposit aiming to characterize the ore-forming hydrothermal system in link with the Variscan late-orogenic evolution of the FMC. Based on a set of representative samples from the host rocks and mineralization, we describe a detailed paragenetic sequence and we provide the major and trace element geochemistry of the granitic rocks and W–Nb–Ta–Sn–Ti oxide minerals, in situ U/Pb and 40Ar/39Ar geochronology, and a fluid inclusion study of quartz and wolframite. We demonstrate that the formation of this W-mineralized breccia pipe results from a multistage development related to four major episodes during the late Carboniferous. The first episode corresponds to the emplacement of an unexposed peraluminous granite at ca. 324 Ma, which generated microgranite dykes exposed at the present-day surface. The second episode is the formation of the quartz-supported breccia pipe and wolframite mineralization at ca. 318 Ma at a paleodepth of 7 km. The mineralizing fluids have a H2O–NaCl–CO2–CH4–N2 composition, a moderate-salinity ( 400 °C) during lithostatic to hydrostatic pressure variations caused by hydrofracturing of the host rocks. Wolframite deposition is interpreted to result from a W-rich intermediate-density magmatic fluid that exsolved from an evolved leucogranite and interacted with volatile-rich metasedimentary country rocks and/or possibly mixed with low-salinity metamorphic fluids of deep origin. The third episode corresponds to magmatic-hydrothermal Nb–Ta mineralization overprinting the W-mineralized system interpreted to be related to the intrusion at ca. 311 Ma of a rare-metal granite, which is part of a regional peraluminous rare-metal magmatism during the 315–310 Ma period. Finally, the last episode corresponds to disseminated Bi ± Au–Ag mineralization emplaced at ca. 300 Ma, which shares similar mineralogical features with late Carboniferous orogenic gold deposits in the FMC. The Puy-les-Vignes W deposit records, therefore, a multistage and long-lived development that extends over a timespan of 25 million years in a regional setting dominated by protracted peraluminous magmatism and high-temperature and low-pressure metamorphism. Although the local environment of ore deposition is atypical, our results show that the mineral assemblages, alteration styles, and fluid characteristics of the Puy-les-Vignes breccia pipe are similar to those of other peri-granitic W deposits in the FMC.

In situ laser ablation and single‐grain fusion 40Ar/39Ar geochronological techniques were directly compared using white mica from nine metasedimentary rocks from the Vaimok Lens of the Seve Nappe Complex (SNC) in the Scandinavian Caledonides. Seven of the rocks are from the eclogite‐bearing Grapesvare nappe within the lens that is defined by D2 structures (S2 and F2), which were formed during exhumation following late Cambrian/Early Ordovician ultra‐high pressure metamorphism. Two other rocks were obtained from 'Scandian' shear zones that delimit the nappes within the lens. The shear zones were active during terminal collision of Baltica and Laurentia in the Silurian to Devonian. The rocks exhibit variable deformation intensities and degrees of strain localization, expressed in particular by white mica. The in situ laser ablation and single‐grain fusion 40Ar/39Ar dates both span from the late Cambrian to Middle Devonian. Results of both techniques generally show decreasing dates with increasing bulk deformation intensity and successive structural generations (i.e., D2 then Scandian structures). Furthermore, several discrepancies are evident when comparing the results of the two techniques for the same rocks, indicating the 40Ar/39Ar dates are not solely governed by bulk deformation intensities and structural generations. Instead, the discrepancies demonstrate the additional influence of white mica strain localization, which is illuminated by the different analytical volumes of the techniques. Thus, the 40Ar/39Ar datasets are altogether deciphered as a function of bulk deformation intensity and degree of strain localization that affected the overall white mica volume. The former controls the gross 40Ar loss from the overall volume and the latter dictates the variability of 40Ar loss within the volume. Exploiting the interplay of these two phenomena for the Vaimok Lens rocks with in situ laser ablation allows for the broad span of 40Ar/39Ar dates to be contextualized into a sequence of tectonic events: (1) cooling at 474 ± 3 Ma, (2) pre‐collision deformation at 447 ± 2 Ma and (3) activation of crustal‐scale shear zones in the SNC related to continental collision at 431 ± 3 Ma and 411 ± 3 Ma. [ABSTRACT FROM AUTHOR]

Abstract: The Thelon Basin located in Nunavut, Canada, shares many similarities with the U-producing Athabasca Basin in Saskatchewan. The Kiggavik project area, located near the northeastern edge of the Thelon [...]