Muscovite Ages Research Articles

Hydrothermal bismuth mineralization of the Yucheon mine, South Korea: Oxygen and hydrogen isotope study

Hydrothermal bismuth ore deposit of the Yucheon mine, Kyongsang Basin, consists of quartz veins that filled fault-related fractures within Cretaceous sedimentary and Tertiary igneous rocks. Geologic setting and K−Ar age of muscovite (45.4±0.6 Ma) indicate that the ore mineralization was associated with calc-alkaline granitism during the Early Tertiary. The three phases of mineralization are: early phase of pyrite+arsenopyrite+pyrrhotite+sphalerite+chalcopyrite; middle phase of base-metal sulfides +bismuthinite+electrum+Bi-, Sb- and Te-bearing minerals; and late phase of fluorite+calcite in vugs. Based on fluid inclusion data and oxygen isotope geothermometry, the early mineralization phase was formed at higher temperatures (from 350° to 480°C) than the middle phase (230°–350°C). Such cooling was a result of progressive meteoric water mixing, likely due to further fracturing of veins. The main deposition of bismuth occurred as a result of cooling at temperatures between 250° and 350°C. The early to middle phases of mineralization occurred from high temperature fluid (up to 480°C) with the oxygen isotope composition and salinity of 5.5 to −1.4‰ and 0.7 to 18.4 wt.% eq. NaCl, respectively. The fluid experienced progressive meteoric water inundation toward the late mineralization (248−167°C; δ18Owater=−3.9 to −13.9‰; down to 1.9 wt.% eq. NaCl). The oxygen and hydrogen isotope data indicate the extensive evolution of hydrothermal fluids at Yucheon, characterized by the progressive increasing influx of an isotopically less-evolved (approaching the meteoric water composition) meteoric water into a deeply circulating highly-evolved meteoric water or a magmatic fluid.

Timing of normal faulting along the Indus Suture in Pakistan Himalaya and a case of major 231Pa/235U initial disequilibrium in zircon

We report age data by TIMS U–Pb, LA-ICP-MS and 39Ar–40Ar techniques for main magmatic events in the Lower Swat region of Pakistan, in order to constrain the tectonic evolution of the northwestern Himalaya. The pre-Himalayan history of the Indian continent is documented by single-zircon U–Pb results from the peraluminous Choga granite gneiss, which yielded a 468±5 Ma lower concordia intercept interpreted to approximate the time of magmatic emplacement. The presence of a well-defined 870±7 Ma inherited component (upper intercept) suggests a plutonic or volcanic protolith residing at unexposed levels of the Indian crust. Zircon data for the Swat granite gneiss from the northern part of the Loe Sar dome give an emplacement age of 267+6/−3 Ma, which is at variance with earlier correlations favoring an early Paleozoic origin. Subsequent to metamorphic overprint by the Himalayan orogeny, the Swat granite was intruded by late kinematic alkali-granite dykes. Single zircons from one of these dykes show reproducible 206Pb/238U data giving a precise mean age of 29.26±0.12 Ma, whereas the 207Pb/235U ages scatter between 34 and 81 Ma, pointing to huge and variable enrichment in 207Pb. The unsupported 207Pb can be explained by incorporation of 231Pa, an intermediate long-lived daughter nuclide in the 235U decay chain, in excess of the secular equilibrium ratio. LA-ICP-MS measurements confirm the presence of unsupported 207Pb, but do not show any correlation between the latter and other selected trace element concentrations in these zircons. A concordant Ar–Ar muscovite age of 28.4±1.1 Ma obtained for the same dyke postdates regional mica ‘cooling’ ages and indicates a lack of younger regional events capable of resetting the K–Ar system in muscovite. Because the dyke is pre- to synkinematic relative to normal faulting and related north-vergent folds, its emplacement age provides a maximum age for this event or reflects already ongoing local extension. A lower limit of 15 Ma has previously been established by apatite fission track analysis. The 29–15 Ma age bracket for normal faulting is coeval with extension along the South Tibetan Detachment System (STDS) on the north side of the High Himalaya. This suggests that the Indus Suture in Pakistan has acted as a western continuation of the STDS and that related faulting was roughly contemporaneous over most of the Himalaya.

Possible Jurassic age for part of Rakaia Terrane: Implications for tectonic development of the Torlesse accretionary prism

Greywacke sandstone and argillite beds comprising Rakaia Terrane (Torlesse Complex) in mid Canterbury, South Island, New Zealand, are widely regarded as Late Triassic (Norian) in age based on the occurrence of Torlessia trace fossils, Monotis, and other taxa. This paleontological age assignment is tested using published 40Ar/39Ar mica and U‐Pb zircon ages for these rocks and published and new zircon fission track (FT) ages. The youngest U‐Pb zircon ages in the Rakaia Terrane rocks in mid Canterbury are Norian, whereas 10–20% of the 40Ar/ 39Ar muscovite ages are younger than Norian. Numerical modelling of these mica ages shows that they cannot have originated from partial thermal overprinting in the Torlesse prism if the thermal maximum was short‐lived and early in the prism history (210–190 Ma), as commonly inferred for these rocks. The young component of mica ages could, however, be explained by extended residence (200–100 Ma) at 265–290°C in the prism. Early Jurassic (c. 189 Ma) zircon FT ages for sandstone beds from Arthur's Pass, the Rakaia valley, and the Hermitage (Mt Cook) are interpreted not to have experienced maximum temperatures above 210°C, and therefore cannot have been reduced as a result of partial annealing in the Torlesse prism. This is based on identification of a fossil Cretaceous, zircon FT, partial annealing zone in low‐grade schists to the west, and the characteristics of the age data. The Early Jurassic zircon FT ages and the young component of 40Ar/39Ar mica ages are regarded therefore as detrital ages reflecting cooling in the source area, and constrain the maximum depositional age of parts of the Rakaia Terrane in mid Canterbury. The zircon FT data also show the initiation (c. 100 Ma) of marked and widespread Late Cretaceous cooling of Rakaia Terrane throughout Canterbury, which is attributed to uplift and erosion of inboard parts of the Torlesse prism due to continuing subduction accretion at its toe. The critical wedge concept is proposed as a new framework for investigating the development of the Torlesse Complex. The Rakaia Terrane may have formed the core of an accretionary wedge imbricated against the New Zealand margin during the Middle or Late Jurassic. Late Jurassic nonmarine sediments (e.g., Clent Hills Formation) accumulated upon the inner parts of the prism as it enlarged, emerged, and continued to be imbricated. Exhumation of Otago Schist from c. 135 Ma may mark the development of a balance (steady state) between sediments entering the prism at the toe and material exiting at the inboard margin. The enlargement of the area of exhumation to all of Canterbury from c. 100 Ma may reflect a dynamic response to widening of the prism through the accretion of Cretaceous sediments. The model of a dynamic critical wedge may help to explain the various expressions of the Rangitata Orogeny.

Dating deformation and cooling in the Caledonian thrust nappes of north Sutherland, Scotland: insights from 40 Ar/ 39 Ar and Rb–Sr chronology

40 Ar/ 39 Ar and Rb–Sr mineral ages have been determined from various lithologies exposed in the Caledonian foreland and structurally overlying thrust nappes of north Sutherland, Scotland. Rb–Sr muscovite ages of c . 428, c . 421 and c . 413 Ma obtained from Moine Thrust Zone mylonites are interpreted to date closely regional thrusting during the Late Silurian to Early Devonian. 40 Ar/ 39 Ar muscovite ages within the lower parts of the Moine nappe are mostly anomalously old with respect to Rb–Sr analyses of muscovites from the same samples; it is likely that this discrepancy results from a component of extraneous or ‘excess’ argon. 40 Ar/ 39 Ar hornblende ages and Rb–Sr and 40 Ar/ 39 Ar muscovite ages obtained from structurally higher metamorphic units in the Caledonian thrust nappes generally range between c . 440 Ma and c . 410 Ma. These ages are interpreted to date cooling during and following ‘D 2 ’ regional thrusting and folding within internal sectors of the nappe sequence. A possible tectonic model involves the Silurian collision of Baltica with Scottish segments of Laurentia resulting in the Scandian orogeny and broadly coeval Moine Thrust Zone. D 2 structures were superimposed on structures and metamorphic fabrics formed during a regional Mid-Ordovician tectonothermal event dated previously at c . 470–460 Ma. Syn-D 2 temperatures were generally >600°C and sufficient to achieve more or less complete thermal rejuvenation of Rb–Sr and 40 Ar/ 39 Ar isotopic systems in muscovite and hornblende, even in areas of low D 2 strain.

Reconstruction of the Grampian episode in Scotland: its place in the Caledonian Orogeny

A study of the composition of detrital garnets from Ordovician siliciclastics from the Midland Valley and Southern Uplands terranes of Scotland reveals characteristics of the metamorphic sources very similar to the Grampian terrane plus a ‘lost’ blueschist–eclogite terrane. The radiometric ages of detrital muscovite from the Southern Uplands overlaps the ages of the metamorphic muscovite from the Grampian terrane. The depositional age of the earliest Midland Valley detritus is Llanvirn; in the Southern Uplands it is Caradoc. These observations support the hypothesis that flysch was formed when the Grampian terrane was suddenly uplifted into mountains which were immediately eroded into neighboring basins and trenches. Thus the Grampian, Midland Valley and Southern Uplands terranes were not exotic to each other in the Upper Ordovician. Therefore the main compressional orogeny (i.e. Peri-Laurentian island arc versus Laurentian continental margin collision) in the Grampian terrane in Scotland was post-Cambrian and began in the late Arenig. New radiometric age dates for Grampian terrane syn-metamorphic granites and gabbros and post-metamorphic granites in Scotland and Ireland plus mineral cooling ages support the case for a relatively short <15Ma catastrophic arc-continent collisional orogeny between ∼480 and ∼465Ma. Thus the Caledonian Orogeny is an example of one which was episodic with a short catastrophic collisional tectonometamorphic and magmatic (Grampian) episode followed by a long period of isostatic adjustment, decompression melting, erosion, subduction and batholith formation that lasted 70Ma. Orogeny stopped when Avalonia docked (i.e. the Scottish Scandian episode).

Tectonometamorphic evolution of the southern Quebec Appalachians: 40Ar/39Ar evidence for Middle Ordovician crustal thickening and Silurian–Early Devonian exhumation of the internal Humber zone

Research Article| January 01, 2001 Tectonometamorphic evolution of the southern Quebec Appalachians: 40Ar/39Ar evidence for Middle Ordovician crustal thickening and Silurian–Early Devonian exhumation of the internal Humber zone Sébastien Castonguay; Sébastien Castonguay 1Geological Survey of Canada, Centre Géoscientifique de Québec, C.P. 7500, Sainte-Foy, Quebec G1V 4C7, Canada Search for other works by this author on: GSW Google Scholar Gilles Ruffet; Gilles Ruffet 1Geological Survey of Canada, Centre Géoscientifique de Québec, C.P. 7500, Sainte-Foy, Quebec G1V 4C7, Canada Search for other works by this author on: GSW Google Scholar Alain Tremblay; Alain Tremblay 1Geological Survey of Canada, Centre Géoscientifique de Québec, C.P. 7500, Sainte-Foy, Quebec G1V 4C7, Canada Search for other works by this author on: GSW Google Scholar Gilbert Féraud Gilbert Féraud 4UMR-Géosciences Azur, Centre National de la Recherche Scientifique–Université de Nice–Sophia Antipolis, 06108 Nice Cedex 02, France Search for other works by this author on: GSW Google Scholar GSA Bulletin (2001) 113 (1): 144–160. https://doi.org/10.1130/0016-7606(2001)113<0144:TEOTSQ>2.0.CO;2 Article history received: 08 Jan 1999 rev-recd: 09 Feb 2000 accepted: 10 Feb 2000 first online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share MailTo Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation Sébastien Castonguay, Gilles Ruffet, Alain Tremblay, Gilbert Féraud; Tectonometamorphic evolution of the southern Quebec Appalachians: 40Ar/39Ar evidence for Middle Ordovician crustal thickening and Silurian–Early Devonian exhumation of the internal Humber zone. GSA Bulletin 2001;; 113 (1): 144–160. doi: https://doi.org/10.1130/0016-7606(2001)113<0144:TEOTSQ>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Published age data from the Humber zone of the southern Quebec Appalachians indicate that deformation and metamorphism occurred during Middle to Late Ordovician time. This paper presents the results of a regional-scale geochronological study of 70 amphibole and mica single grains by the 40Ar/39Ar laser step-heating method, which brings forward a more complete Middle Ordovician–Early Devonian tectonometamorphic evolution of the Quebec Appalachian Humber zone. Biotites have plateau ages varying between 420 and 411 Ma. Amphiboles are often compositionally rimmed and/or contain biotite inclusions. Their age spectra are generally disturbed, but may suggest the occurrence of two distinct events: Silurian (429–424 Ma) and Middle Ordovician (462–460 Ma). Two main groups of muscovite ages have been obtained: Middle Ordovician (469–461 Ma) and Silurian–Early Devonian (431–411 Ma), but Middle to Late Devonian (387–376 Ma) ages are also locally present. Middle Ordovician ages are interpreted to record crustal thickening by nappe emplacement during regional Taconian metamorphism. Silurian–Early Devonian ages are attributed to consequences of backthrusting and normal faulting. A statistical analysis of Silurian–Early Devonian muscovite age spectra reveals an irregular decrease in weighted apparent age maxima across the internal Humber zone. This age-decreasing trend is interpreted to result from the temporal (ca. 431–411 Ma) and possibly spatial (northwest to southeast) progression of deformation, recrystallization, and fluid circulation during tectonic exhumation of the internal Humber zone in southern Quebec. Tectonic models for the Northern Appalachian internal Humber zone should take into account the strong body of evidence for Silurian–Early Devonian hinterland-directed and extensional deformation as documented in southern Quebec. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

New U-Pb zircon ages for the Kuboos pluton in the Pan-African Gariep belt, South Africa: Cambrian mantle plume or far field collision effect?

The age of the Kuboos pluton, which forms part of a series of alkaline, apparently post-orogenic intrusive bodies along the Kuboos-Bremen line, was re-assessed using single zircon U-Pb isotope analyses. A new age of 507 ± 6 Ma obtained for the youngest intrusive phase, a monzogranite from the Kuboos composite pluton is clearly younger than the main phase of deformation in the Gariep Belt (545 ± 2 Ma). This age corresponds to a major thermal event, as reflected by Rb-Sr and Ar-Ar mineral ages and by coeval post-orogenic granite in the Damara Belt, and also with Ar-Ar ages of syn-tectonic muscovite, related to a late tectonic pulse in the Pan-African Gariep and Saldania Belts, their basement and corresponding foreland basins. While the earlier 545 Ma tectonothermal event is explained by the closure of the Adamastor Ocean between the Kalahari and the Rio de la Plata cratons, the younger 500 Ma tectonic pulse can be linked to subduction beneath the western margin of Gondwana. The alkaline magmatism, which is represented by the Kuboos pluton and which is contemporaneous with the latter tectonic event, can be explained either by a mantle plume underneath southern Africa or as a distant effect of collision along the Gondwana margin. The occurrence of a 1.1 - 1.2 Ga xenocrystic zircon in the Kuboos pluton hints at the external Gariep Belt being floored by Kibaran crust.

One-dimensional thermal modelling of Acadian metamorphism in southern Vermont, USA

One-dimensional thermal (1DT) modelling of an Acadian (Devonian) tectonothermal regime in southern Vermont, USA, used measured metamorphic pressures and temperatures and estimated metamorphic cooling ages based on published thermobarometric and geochronological studies to constrain thermal and tectonic input parameters. The area modelled lies within the Vermont Sequence of the Acadian orogen and includes: (i) a western domain containing garnet-grade pre-Silurian metasedimentary and metavolcanic rocks from the eastern flank of an Acadian composite dome structure (Rayponda–Sadawga Dome); and (ii) an eastern domain containing similar, but staurolite- or kyanite-grade, rocks from the western flank of a second dome structure (Athens Dome), approximately 10 km farther east. Using reasonable input parameters based on regional geological, petrological and geochronological constraints, the thermal modelling produced plausible P–T paths, and temperature–time (T –t) and pressure–time (P–t) curves. Information extracted from P–T –t modelling includes values of maximum temperature and pressure on the P–T paths, pressure at maximum temperature, predicted Ar closure ages for hornblende, muscovite and K-feldspar, and integrated exhumation and cooling rates for segments of the cooling history. The results from thermal modelling are consistent with independently obtained pressure, temperature and Ar cooling age data on regional metamorphism in southern Vermont. Modelling results provide some important bounding limits on the physical conditions during regional metamorphism, and indicate that the pressure contemporaneous with the attainment of peak temperature was probably as much as 2.5 kbar lower than the actual maximum pressure experienced by rocks along various particle paths. In addition, differences in peak metamorphic grade (garnet-grade versus staurolite-grade or kyanite-grade) and peak temperature for rocks initially loaded to similar crustal depths, differences in calculated exhumation rates, and differences in 40Ar/39Ar closure ages are likely to have been consequences of variations in the duration of isobaric heating (or ‘crustal residence periods’) and tectonic unroofing rates. Modelling results are consistent with a regional structural model that suggests west to east younging of specific Acadian deformational events, and therefore diachroneity of attainment of peak metamorphic conditions and subsequent 40Ar/39Ar closure during cooling. Modelling is consistent with the proposition that regional variations in timing and peak conditions of metamorphism are the result of the variable depths to which rocks were loaded by an eastward-thickening thrust-nappe pile rooted to the east (New Hampshire Sequence), as well as by diachronous structural processes within the lower plate rocks of the Vermont Sequence.

Emplacement of the Ardara pluton (Ireland): new constraints from magnetic fabrics, rock fabrics and age dating

The Ardara pluton as part of the Donegal batholith was intruded into Neoproterozoic metasediments and metadolerites at mid-crustal levels. The emplacement mechanism of the Ardara granite is very controversial, and mechanisms ranging from diapirism, ballooning and stoping followed by nested diapirism have been proposed. Magnetic fabrics, rock fabrics and K/Ar dating of micas are used here to constrain the emplacement history. The compositional zoning of the Ardara pluton is clearly reflected in the different bulk magnetic susceptibilities between the outer quartz monzodiorite and the central granodiorite, whereas the intervening tonalite is of intermediate nature. The magnetic carriers are characterized by the anisotropy of the magnetic susceptibility (AMS), thermomagnetic measurements and through high field analyses (HFA). The separation of the ferrimagnetic and paramagnetic contributions revealed that biotite and magnetite control the AMS in the quartz monzodiorite. Both minerals are oriented in such a way that their summed contribution is constructive and originates from the shape fabric of magnetite and the texture of biotite. Biotite is responsible mainly for the AMS in the tonalite and granodiorite. The magnetic foliation can be directly related to the macroscopic foliation and also to the D4 structures in the country rocks. The foliation is consistent with the geometry of the roughly circular shape and has a mostly steep to vertical dip. Towards the central granodiorite the magnetic foliation dies out, although plagioclase texture measurements indicate a weak magmatic shape fabric. With the exception of the tail, the Kmax axes (magnetic lineation) vary from steeply to gently plunging. The so-called lineation factor is approximately 1.01 and therefore points to a less significant axial symmetry. These observations coincide with strain estimates on mafic enclaves that show a very consistent pattern of K ∼0 flattening strain. Texture analyses of biotite and quartz additionally support the observations made by the strain analyses and the magnetic fabric data. Microstructural investigations give evidence that the fabrics are associated with the emplacement over a range of temperatures from truly magmatic to high-temperature solid-state conditions. The age of the intrusion is still under discussion, but a new cooling age was determined by K/Ar dating of biotite at 403.7±8 Ma corresponding to a temperature range between 450 and 300°C. For a mylonite along the southern contact between the Ardara pluton and the country rock a K/Ar muscovite age of 378.8±7 Ma indicates a minimum age for the shear zone when the Ardara pluton must have already been cooled down below 350±50°C.

Metallogeny of the southern Sierras Pampeanas, Argentina: geological, 40Ar– 39Ar dating and stable isotope evidence for Devonian Au, Ag–Pb–Zn and W ore formation

Early to mid-Paleozoic metamorphic and igneous rocks of the southern Sierras Pampeanas tectono-stratigraphic terrane, northwestern Argentina, host diverse metallic mineral deposits of Au, Ni–Cu, Ag–Pb–Zn, W, Li, Be, Nb, LREE and Th. Integration of deposit geology, 40Ar– 39Ar geochronology and stable isotope studies with regional geological mapping and U–Pb zircon dating, has led to the resolution of three principal metallogenic epochs. Ordovician intrusion-hosted Ni–Cu sulfide deposits and rare-metal-enriched pegmatites were succeeded by a province-wide metallogenic epoch and concurrent tectonic processes during the Devonian. This epoch was characterized by mesothermal lode Au deposits, high-level Ag–Pb–Zn veins, W±Cu-bearing veins and replacements, and various pegmatite- and granite-related lithophile element deposits. Finally, Miocene–Pliocene epithermal Au–Ag mineralization developed during Andean tectonic events. Sericitic alteration associated with mesothermal lode Au (±Cu±Ag) in the Sierra de Las Minas and Sierras de Córdoba yields Devonian 40Ar– 39Ar step heating ages in the ranges 382–393 and 351–378 Ma, respectively. These ages span the timing of Ag–Pb–Zn sulfide–quartz veining in the El Guaico district (386±4 Ma, sericite), and W-bearing quartz vein mineralization in the Aguas de Ramón and El Morro districts which yielded muscovite ages of 366±1 and 363±1 Ma, respectively. Oxygen isotope δ 18O SMOW compositions of the Au, Ag–Pb–Zn and W ore-forming fluids, calculated from quartz, white mica and chlorite isotope ratios, range from 5.2‰ to 10.8‰ ( n=32), whereas calculated δD SMOW compositions range from −61‰ in the La Rioja Au systems to −141‰ in the W veins ( n=9). Sulfide δ 34S values range from 1.5‰ to 10.0‰, averaging 6.5‰ ( n=15). The oxygen and hydrogen stable isotope data suggest contributions to the ore fluids of either D-depleted meteoric waters that have reacted extensively with metasedimentary rocks, or fluids equilibrated with degassed felsic magmas, or mixtures of both. This mineralization formed during and up to 20–30 Ma after widespread emplacement of high-level peraluminous to metaluminous granites, which yielded U–Pb zircon ages of 404±6–382±6 Ma. The Devonian Au deposits are localised in transcurrent and reverse fault and shear zones that may be related to collision of the Chilenia terrane with the western margin of Gondwana during the newly defined Achalian orogeny. These deposits are considered to be members of the mesothermal lode Au family of systems found worldwide.

The Kyonggi Shear Zone of the Central Korean Peninsula: Late Orogenic Imprint of the North and South China Collision.

The crustal-scale Kyonggi shear zone of central Korea has been identified as a major boundary between the Precambrian Kyonggi massif in the south and the Imjingang belt in the north. The latter is an eastward extension of the Qinling-Dabie-Sulu collisional belt of China. Field observations and microstructural analysis indicate that the extensional shear zone evolved from a deep crustal ductile regime to a shallow crustal brittle regime, associated with a rapid uplift of the Kyonggi massif following the Late Permian-Early Triassic collision between the Sino-Korean and Yangtze cratons. A Rb-Sr muscovite age (226+/-1.2 Ma) of the mylonite suggests that the extensional ductile shearing occurred during the Late Triassic.

Late Visean thermal event in the northern part of the French Massif Central: new 40Ar/39Ar and Rb–Sr isotopic constraints on the Hercynian syn-orogenic extension

he French Massif Central belongs to the central metamorphic domain of the European Hercynian Belt. 40Ar/39Ar mica and amphibole dates have been obtained on magmatic and metamorphic rocks from the northern part of the French Massif Central (Sioule and Brevenne-Violay series). The 336- to 337-Ma dates recorded by a granodiorite (the St. Gervais pluton) and its mylonitic border along the Ste Christine fault and the 337-5-Ma muscovite ages from the Brevenne series are consistent with the age of wrenching related to the late stage of the Hercynian compression. The remaining age spectra from the Sioule area provide ages ranging from 331 to 335 Ma that are interpreted as cooling ages related to rock exhumation after the main stages of metamorphism and deformation in the studied area. Late Visean dates widespread in North Massif Central correspond to a huge magmatic event characterized by the emplacement of red granites, granophyres and sub-aerial acidic volcanics known as the Tufs anthraciferes formation. Rb-Sr isotopic data from volcanic and hypovolcanic rocks from the Sioule and Montagne Bourbonnaise areas place constraints on the source of the magmas. Simple metapelite crustal melting or a major contribution from depleted mantle sources are ruled out, and instead, mantle-derived magmas were probably involved, at least as a heat source, during the Late Visean magmatism. Exhumation and cooling of metamorphic rocks below 300°C occurred contemporaneously with this Late Visean magmatism. In the geodynamic evolution of the Hercynian Belt, the Late Visean (ca. 335-330 Ma) magmatism corresponds to a major break marking the onset of post-collisional extension in the inner zone which had been thickened by collision in Latest Devonian-Earliest Carboniferous. This Late Visean extension in the inner zone which is synchronous to compression in the northern and southern forelands of the belt appears as a syn-orogenic process.

The Baltica-Gondwana suture in central Europe: evidence from K-Ar ages of detrital muscovites and biogeographical data

Abstract The Lysogory Unit, the Malopolska Massif and the Upper Silesian Massif in southern Poland are parts of a mosaic of contrasting crustal fragments separating the old Precambrian crust of the East European Platform (EEP) from the Phanerozoic mobile belts of western Europe. The geological histories of these blocks are markedly different. They have been regarded as integral parts of the palaeocontinent of Baltica (that is, the EEP), mostly because of presence of fossils typical for the Baltic realm, although geophysical and geological data and some faunal elements rather suggest linkages to the Peri-Gondwana plates. To provide additional constraints for the plate tectonic affinity of these blocks detrital muscovite grains extracted from Cambrian and Devonian clastic rocks were dated by the K-Ar method. The K-Ar cooling ages show a very complex provenance pattern for clastic material in Cambrian time. Combined with the biogeographical constraints, the new provenance data apparently show that the blocks of Lysogory, Malopolska, and Upper Silesia are in fact crustal fragments derived from the Gondwana margin, not displaced parts of the East European Craton. Thus, the Teisseyre-Tornquist Line (that is, the edge of the EEP) is the Baltica-Gondwana suture in central Europe. The combined data reveal an accretionary scenario in which the Malopolska Block was the first Gondwana-derived microplate that accreted to the margin of Baltica.

Acadian metamorphism in the Dunnage zone of southern Quebec, northern Appalachians: 40Ar/39Ar evidence for collision diachronism

Research Article| January 01, 2000 Acadian metamorphism in the Dunnage zone of southern Québec, northern Appalachians: 40Ar/39Ar evidence for collision diachronism Alain Tremblay; Alain Tremblay 1Centre Géoscientifique de Québec, INRS-Géoressources, 2535 Boulevard Laurier, Ste-Foy, Québec G1V 4C7, Canada Search for other works by this author on: GSW Google Scholar Gilles Ruffet; Gilles Ruffet 2UMR-Géosciences Azur, Centre National de Recherche Scientifique, Université de Nice-Sophia Antipolis, 06108 Nice Cedex 02, France Search for other works by this author on: GSW Google Scholar Sébastien Castonguay Sébastien Castonguay 1Centre Géoscientifique de Québec, INRS-Géoressources, 2535 Boulevard Laurier, Ste-Foy, Québec G1V 4C7, Canada Search for other works by this author on: GSW Google Scholar Author and Article Information Alain Tremblay 1Centre Géoscientifique de Québec, INRS-Géoressources, 2535 Boulevard Laurier, Ste-Foy, Québec G1V 4C7, Canada Gilles Ruffet 2UMR-Géosciences Azur, Centre National de Recherche Scientifique, Université de Nice-Sophia Antipolis, 06108 Nice Cedex 02, France Sébastien Castonguay 1Centre Géoscientifique de Québec, INRS-Géoressources, 2535 Boulevard Laurier, Ste-Foy, Québec G1V 4C7, Canada Publisher: Geological Society of America Received: 17 Aug 1998 Revision Received: 02 Mar 1999 Accepted: 12 Mar 1999 First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (2000) 112 (1): 136–146. https://doi.org/10.1130/0016-7606(2000)112<136:AMITDZ>2.0.CO;2 Article history Received: 17 Aug 1998 Revision Received: 02 Mar 1999 Accepted: 12 Mar 1999 First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Alain Tremblay, Gilles Ruffet, Sébastien Castonguay; Acadian metamorphism in the Dunnage zone of southern Québec, northern Appalachians: 40Ar/39Ar evidence for collision diachronism. GSA Bulletin 2000;; 112 (1): 136–146. doi: https://doi.org/10.1130/0016-7606(2000)112<136:AMITDZ>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract In New England, the Acadian orogen is divided into Western and Eastern metamorphic belts on the basis of contrasting pressure-temperature (P-T) paths, peak metamorphism conditions, and ages. Along-strike correlations of structures indicate that rock units affected by Devonian metamorphism and deformation in southern Québec belong to the Western Acadian belt, and share the same tectonic evolution as in New England. Muscovite 40Ar/39Ar ages obtained from greenschist-grade, Ordovician volcanic and plutonic rocks of the Ascot Complex in southern Québec indicate that Acadian metamorphism and deformation peaked ca. 380–375 Ma. Ordovician muscovite ages of ca. 462 Ma are locally well preserved in the granitic intrusion of the Ascot Complex, and provide constraints on the timing of island-arc plutonism in the Dunnage zone of southern Québec. Ordovician 40Ar/39Ar age spectra do not show important losses of radiogenic argon, indicating that Acadian metamorphism has been low grade, with temperatures below the Ar isotopic closure temperature proposed for muscovite. Acadian peak metamorphism is significantly younger, and P-T conditions of regional deformation have been lower in southern Québec than in amphibolite-grade rocks of the Western Acadian belt of New England. In the northern Appalachians, Acadian metamorphism and deformation are attributed to Devonian age crustal overthickening related to plate convergence. The comparison of Devonian tectonic fabrics in New England and southern Québec suggests that progression in the timing and P-T conditions of Acadian peak metamorphism in both areas are the result of the irregular geometry of the collision zone and of the northward migration of metamorphic and structural fronts along the orogen. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Acadian metamorphism in the Dunnage zone of southern Québec, northern Appalachians: 40Ar/39Ar evidence for collision diachronism

Research Article| January 01, 2000 Acadian metamorphism in the Dunnage zone of southern Québec, northern Appalachians: 40Ar/39Ar evidence for collision diachronism Alain Tremblay; Alain Tremblay 1Centre Géoscientifique de Québec, INRS-Géoressources, 2535 Boulevard Laurier, Ste-Foy, Québec G1V 4C7, Canada Search for other works by this author on: GSW Google Scholar Gilles Ruffet; Gilles Ruffet 2UMR-Géosciences Azur, Centre National de Recherche Scientifique, Université de Nice-Sophia Antipolis, 06108 Nice Cedex 02, France Search for other works by this author on: GSW Google Scholar Sébastien Castonguay Sébastien Castonguay 1Centre Géoscientifique de Québec, INRS-Géoressources, 2535 Boulevard Laurier, Ste-Foy, Québec G1V 4C7, Canada Search for other works by this author on: GSW Google Scholar GSA Bulletin (2000) 112 (1): 136–146. https://doi.org/10.1130/0016-7606(2000)112<136:AMITDZ>2.0.CO;2 Article history received: 17 Aug 1998 rev-recd: 02 Mar 1999 accepted: 12 Mar 1999 first online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Alain Tremblay, Gilles Ruffet, Sébastien Castonguay; Acadian metamorphism in the Dunnage zone of southern Québec, northern Appalachians: 40Ar/39Ar evidence for collision diachronism. GSA Bulletin 2000;; 112 (1): 136–146. doi: https://doi.org/10.1130/0016-7606(2000)112<136:AMITDZ>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract In New England, the Acadian orogen is divided into Western and Eastern metamorphic belts on the basis of contrasting pressure-temperature (P-T) paths, peak metamorphism conditions, and ages. Along-strike correlations of structures indicate that rock units affected by Devonian metamorphism and deformation in southern Québec belong to the Western Acadian belt, and share the same tectonic evolution as in New England. Muscovite 40Ar/39Ar ages obtained from greenschist-grade, Ordovician volcanic and plutonic rocks of the Ascot Complex in southern Québec indicate that Acadian metamorphism and deformation peaked ca. 380–375 Ma. Ordovician muscovite ages of ca. 462 Ma are locally well preserved in the granitic intrusion of the Ascot Complex, and provide constraints on the timing of island-arc plutonism in the Dunnage zone of southern Québec. Ordovician 40Ar/39Ar age spectra do not show important losses of radiogenic argon, indicating that Acadian metamorphism has been low grade, with temperatures below the Ar isotopic closure temperature proposed for muscovite. Acadian peak metamorphism is significantly younger, and P-T conditions of regional deformation have been lower in southern Québec than in amphibolite-grade rocks of the Western Acadian belt of New England. In the northern Appalachians, Acadian metamorphism and deformation are attributed to Devonian age crustal overthickening related to plate convergence. The comparison of Devonian tectonic fabrics in New England and southern Québec suggests that progression in the timing and P-T conditions of Acadian peak metamorphism in both areas are the result of the irregular geometry of the collision zone and of the northward migration of metamorphic and structural fronts along the orogen. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Age of crustal melting, emplacement and exhumation history of the Shivling leucogranite, Garhwal Himalaya

We report a U–Pb monazite age of 23.0±0.2 Ma for the Shivling leucogranite, a tourmaline+muscovite±biotite leucogranite at the top of the High Himalayan slab in the Garhwal Himalaya, north India. The Shivling–Bhagirathi leucogranite is a viscous near-minimum melt, emplaced as a foliation parallel laccolith via a dyke network not far from its source region. Prograde heating occurred soon after the India–Asia collision at c. 50 Ma up to melting at 23 Ma and high temperatures (&gt;550 °C) were maintained for at least 15 Ma after garnet growth. The leucogranite was emplaced at mid-crustal depths along the footwall of the Jhala fault, a large-scale low-angle normal fault, part of the South Tibetan Detachment system, above kyanite and sillimanite grade gneisses. The geometry of the leucogranite laccolith shows biaxial extension and boudinage both perpendicular (north-northeast–south-southwest) and parallel to the strike (west-northwest–east-southeast) of the mountain range. Unroofing occurred by underthrusting beneath the High Himalayan slab along the Main Central Thrust zone, progressively ‘jacking up’ the leucogranites, removal of material above by low-angle normal faulting, and erosion. Very rapid cooling at rates of 200–350 °C/Ma between 23–21 Ma immediately followed crystallization, as tectonic unroofing and erosion removed 24–28 km of overburden during this time. K–Ar muscovite ages are 22±1.0 Ma and fission track ages of zircons from &gt;5000 m on the North Ridge of Shivling are 14.2±2.1 and 8.8±1.2 Ma and apatites are 3.5±0.79 and 2.61±0.23 Ma. Slow steady state cooling at rates of 20–30 °C/Ma from 20–1 Ma shows that maximum erosion rates and unroofing of the leucogranite occurred during the early Miocene. This timing coincides with initiation of low-angle, north-dipping normal faulting along the South Tibetan Detachment system.

Laser 40Ar/39Ar thermochronology of Archean rocks in Yellowknife Domain, southwestern Slave Province: insights into the cooling history of an Archean granite-greenstone terrane

Laser 40Ar/39Ar ages for hornblende, muscovite, and biotite from Archean plutonic rocks of Yellowknife Domain, southwestern Slave Province, complement U-Pb ages from minerals with higher closure temperatures (zircon, monazite, titanite). Together, the data indicate that the plutonic suites of different ages (Defeat, Prosperous, Morose) coring three major thermal culminations in the domain have distinct Archean cooling histories until below approximately 250°C (biotite closure). The cooling trajectories for successive suites appear to document a decrease in cooling rate over the course of orogeny, possibly reflecting the build-up of heat within the orogen, coupled with a transition to lower rates of exhumation and (or) erosion. Greater structural relief over the Sleepy Dragon basement culmination may be responsible for slower cooling of the Morose Granite situated in its core. At or below the closure temperature of biotite the cooling curves converge with one another, documenting a transition from "localized" to "regional" cooling. The fact that cooling was locality specific until at least the biotite closure temperature suggests that, just as intrusion and related peak metamorphism were diachronous across the region, so was cooling to approximately 250°C. Uniform regional isotherms thus were not established until &gt;200 Ma after the youngest Archean plutonism. The variable disturbance of biotite Ar spectra, but generally not muscovite or hornblende, indicates that the Paleoproterozoic thermal disturbance of Archean rocks in Yellowknife Domain did not exceed 200-300°C.

40 Ar/ 39 Ar Thermochronology west of the Main Uralian fault, southern Urals, Russia

West of the Main Uralian fault, the main suture in the southern Urals, 40Ar/39Ar apparent ages of amphibole, muscovite and potassium feldspar are interpreted as cooling ages. A fast exhumation of the metamorphic complex of Kurtinsky during Upper Carboniferous time is indicated by the small age difference (15 Ma) between cogenetic amphibole and muscovite. Differentiated movement in the footwall of the Main Uralian fault along strike is indicated by the age difference of 70 Ma between the metamorphic complexes of Kurtinsky (north) and Maksyutov (south). No Upper Paleozoic (Uralian) medium- to high-temperature event is recorded in 40Ar/39Ar data from the metamorphic complex of Beloretzk (MCB). An amphibole age of 718±5 Ma and the occurrence of mafic intrusions might signal the break-up of Rodinia and therefore indicate the rifting period followed by the separate movement of the "Beloretzk terrane". Muscovite ages of approximately 550±5 Ma, the unique pre-Ordovician tectonometamorphic evolution of the MCB and the Late Vendian sedimentary history of the western Bashkirian Megaanticlinorium (BMA) imply the existence of a Neoproterozoic orogeny at the eastern margin of Baltica. This orogeny might have been initiated by the accretion of the "Beloretzk terrane". The metamorphic grade of the overlain Silurian shales and the K/Ar microcline ages from the "Beloretzk terrane" give evidence for a new thermal event at approximately 370 Ma. A microcline age of 530–550 Ma obtained for the Vendian conglomerate in the western BMA suggests that a maximum temperature of approximately 200°C was reached in Cambrian or Vendian times. An orthoclase age (590–630 Ma) of the Vendian Zigan flysch deposits might be inherited from the eastern source area, the Cadomian orogen. An orthoclase age (910–950 Ma) from the Riphean Zilmerdak conglomerate coincides with a documented decrease in the subsidence rate of the Upper Riphean basin.

Detrital and metamorphic 40Ar/39Ar ages from muscovite and whole-rock samples, Meguma Supergroup, southern Nova Scotia

40Ar/39Ar dates from eight muscovite separates and two whole-rock samples of slate, siltstone, and sandstone provide detrital, metamorphic, and post-metamorphic overprinting ages for the low-grade metamorphic rocks of the Meguma Supergroup in the Mahone Bay region of southern Nova Scotia. Variably reset detrital muscovite ages of 600-550 Ma agree with previously obtained U-Pb dates from detrital zircon. Separates of foliation-parallel muscovite yield precise 40Ar/39Ar ages of 395-388 Ma. Petrographic and metamorphic data indicate that these are metamorphic mica growth ages. The range overlaps with the younger limit of previously reported whole-rock ages (410-380 Ma), but is more tightly constrained. An apparent inconsistency between previous 40Ar/39Ar whole-rock ages as old as 410 Ma and regional deformation that affected sedimentary rocks as young as 385 Ma has been resolved by the younger ages reported here and by recent adjustments to the early Paleozoic time scale which suggest that the Lochkovian to Emsian Torbrook Group was deposited between 419 and 394 Ma. A muscovite plateau age of 376 Ma from one sample may be related to late-stage flexural slip deformation in the Ovens gold district.

Petrogenese kontrastierender Granitplutone in Westb�hmen (Tschechien)

Late-Variscan granitoid plutons in western Bohemia (Bor, Waidhaus-Rozvadov) have distinct petrographic, geochemical and isotopic features that suggest different magmatic evolutions. The Bor pluton comprises a suite of metaluminous tonalites and quartz diorites (Bor I), weakly peraluminous (monzo-)granites and granodiorites (Bor II) and medium-aluminous, late vein-forming leucomonzogranites (Bor III). The Waidhaus-Rozvadov pluton is strongly peraluminous, comprising a cordierite-biotite granitoid (CBG), the Rozvadov granite (ROG), the Barnau granite (BAG) and the subordinate, highly evolved Kreuzstein (Křižový kamen) granite (KG). Geochemical parameters and initial87Sr/86Sr ratios straddle the boundary between I- and S-type granites in the Bor pluton and are characteristic of purely S-type granites in the Waidhaus-Rozvadov pluton. The Bor II granitoids have been dated by the Rb-Sr whole-rock method at 341±17 Ma (ISr = 0.70724±0.00060). K-Ar biotite and muscovite ages of all units of the Bor pluton are mainly in the range 321-315 Ma. The K-Ar mineral ages are in good agreement with recently published U-Pb zircon data of these rocks. The different units of the Waidhaus-Rozvadov pluton have yielded less well-constrained Rb-Sr whole-rock ages, ranging from 313 to 300 Ma. However, the intrusion sequence is constrained by K-Ar muscovite ages (312-302 Ma), which define a systematic decrease towards the chemically more evolved granite types. Taken as a whole, it seems likely that the new radiometric ages characterize two temporally distinct periods of late-Variscan granitoid intrusion. The regional significance of these periods is emphasized by contemporaneous ages previously found in the adjacent northeastern Bavarian granitoids. The initial Sr and Nd isotope systematics indicate that the Bor and the WaidhausRozvadov plutons were derived from different source rocks. The Bor granitoids reflect the influence of less evolved crustal material which may have been similar to paragneisses of the Tepla-Barrandian region, including the Zone of ErbendorfVohenstraus (ZEV). The Waidhaus-Rozvadov granitoids probably resulted from anatexis of rocks resembling surrounding Moldanubian paragneisses or metapelites. In addition, the two plutons exhibit poorly defined, opposite trends of eNd(T) variation which are ascribed to assimilation processes.