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Metamorphic Event (metamorphic + event)
Selected AbstractsStructural and metamorphic evolution of the phengite-bearing schists of the northern Adula Nappe (Central Alps, Switzerland)GEOLOGICAL JOURNAL, Issue 1 2008F. Zulbati Abstract Phengite-bearing schists of the northern Adula Nappe experienced a polymetamorphic and polycyclic evolution that was associated with five deformation episodes. Evidence of a pre-Alpine metamorphic event is preserved within garnet cores of some amphibole-bearing schists. The D1 and D2 deformation episodes are recorded by S1 and S2 foliations preserved only within metre-scale domains of low-D3 strain. S1 is a relict foliation. Blueschist-facies conditions at 565,±,10°C and 11.5,±,1.5,kbar were attained during D2 and were associated with the development of isoclinal folding and an S2 foliation. The D3 episode took place at 665,±,50°C and 11.5,±,2.1,kbar and was responsible for the development of a transpositive S3 foliation. The D4 episode took place at T,<,550,±,10°C and was associated with the development of a discrete S4 foliation and S-C structures. The D5 episode is recorded by sub-vertical metre-scale open folds or centimetre-scale kinks. The structural and metamorphic evolution described here indicates that the northern and central parts of the Adula Nappe were distinct continental crustal fragments and were brought together under amphibolite-facies conditions. Copyright © 2007 John Wiley & Sons, Ltd. [source] The syn-collisional Danac,obas, biotite leucogranite derived from the crustal thickening in central Anatolia (K,r,kkale), TurkeyGEOLOGICAL JOURNAL, Issue 5 2005bel Tatar Abstract The Behrekda, composite batholith, which crops out as a huge N,S-trending plutonic body in central Anatolia, Turkey, consists of five mappable granitoid units of Late Cretaceous age. They are (1) the S-type, peraluminous Danac,obas, biotite leucogranite, (2) the I-type, hybrid, metaluminous Konur K-feldspar megacrystic quartz monzonite, (3) the mafic A-type, alkaline Kizdede monzogabbro, (4) the felsic A-type, alkaline Hasandede quartz syenite/monzonite, and (5) the M-type, low-K tholeiitic Yeniköy tonalite. The S-type Danac,obas, biotite leucogranite constitutes the oldest intrusive unit in the mapped area. It has coarse- to medium-crystalline texture and consists of quartz, orthoclase and plagioclase, with variable amounts of biotite and accessory minerals, including apatite, zircon and opaque phases. K-Ar age dating of biotite separates, yields cooling ages of 69.1,±,1.42 and 71.5,±,1.45,Ma for the Danac,obas, biotite leucogranite. Major-element, trace-element, and rare-earth element geochemical data suggest an exclusively peraluminous, S-type, high-K calc-alkaline, upper crustal genesis for the Danac,obas, biotite leucogranite. This petrogenetic interpretation is also supported by oxygen-isotope data from quartz separates, with a mean value of 10.58,±,0.11 , of ,18OVSMOW value. The magma source of the Danac,obas, biotite leucogranite is proposed to have been a syn-collisional leucogranitic melt derived by anatexis of high-grade metasediments of the Central Anatolian Crystalline Complex during peak conditions of regional metamorphism. This metamorphic event was induced by crustal thickening which was a result of Late Cretaceous collision between the Eurasia and Tauride,Anatolide Platform along the ,zmir,Ankara,Erzincan Suture Zone in central Anatolia. Copyright © 2005 John Wiley & Sons, Ltd. [source] Geology of the summit limestone of Mount Qomolangma (Everest) and cooling history of the Yellow Band under the Qomolangma detachmentISLAND ARC, Issue 4 2005Harutaka Sakai Abstract Newly discovered peloidal limestone from the summit of Mount Qomolangma (Mount Everest) contains skeletal fragments of trilobites, ostracods and crinoids. They are small pebble-sized debris interbedded in micritic bedded limestone of the Qomolangma Formation, and are interpreted to have been derived from a bank margin and redeposited in peri-platform environments. An exposure of the Qomolangma detachment at the base of the first step (8520 m), on the northern slope of Mount Qomolangma was also found. Non-metamorphosed, strongly fractured Ordovician limestone is separated from underlying metamorphosed Yellow Band by a sharp fault with a breccia zone. The 40Ar,39Ar ages of muscovite from the Yellow Band show two-phase metamorphic events of approximately 33.3 and 24.5 Ma. The older age represents the peak of a Barrovian-type Eo-Himalayan metamorphic event and the younger age records a decompressional high-temperature Neo-Himalayan metamorphic event. A muscovite whole-rock 87Rb,86Sr isochron of the Yellow Band yielded 40.06 ± 0.81 Ma, which suggests a Pre-Himalayan metamorphism, probably caused by tectonic stacking of the Tibetan Tethys sediments in the leading margin of the Indian subcontinent. Zircon and apatite grains, separated from the Yellow Band, gave pooled fission-track ages of 14.4 ± 0.9 and 14.4 ± 1.4 Ma, respectively. These new chronologic data indicate rapid cooling of the hanging wall of the Qomolangma detachment from approximately 350°C to 130°C during a short period (15.5,14.4 Ma). [source] Polymetamorphism, zircon growth and retention of early assemblages through the dynamic evolution of a continental arc in Fiordland, New ZealandJOURNAL OF METAMORPHIC GEOLOGY, Issue 4 2009J. M. SCOTT Abstract The Marguerite Amphibolite and associated rocks in northern Fiordland, New Zealand, contain evidence for retention of Carboniferous metamorphic assemblages through Cretaceous collision of an arc, emplacement of large volumes of mafic magma, high- P metamorphism and then extensional exhumation. The amphibolite occurs as five dismembered aluminous meta-gabbroic xenoliths up to 2 km wide that are enclosed within meta-leucotonalite of the Lake Hankinson Complex. A first metamorphic event (M1) is manifest in the amphibolite as a pervasively lineated pargasite,anorthite,kyanite or corundum ± rutile assemblage, and as diffusion-zoned garnet in pelitic schist xenoliths within the amphibolite. Thin zones of metasomatically Al-enriched leucotonalite directly at the margins of each amphibolite xenolith indicate element redistribution during M1 and equilibration at 6.6 ± 0.8 kbar and 618 ± 25 °C. A second phase of recrystallization (M2) formed patchy and static margarite ± kyanite,staurolite,chlorite,plagioclase,epidote assemblages in the amphibolite, pseudomorphs of coronas in gabbronorite, and thin high-grossular garnet rims in the pelitic schists. Conditions of M2, 8.8 ± 0.6 kbar and 643 ± 27 °C, are recorded from the rims of garnet in the pelitic schists. Cathodoluminescence imaging and simultaneous acquisition of U-Th-Pb isotopes and trace elements by depth-profiling zircon grains from one pelitic schist reveals four stages of growth, two of which are metamorphic. The first metamorphic stage, dated as 340.2 ± 2.2 Ma, is correlated with M1 on the basis that the unusual zircon trace element compositions indicate growth from a metasomatic fluid derived from the surrounding amphibolite during penetrative deformation. A second phase of zircon overgrowth coupled with crosscutting relationships date M2 to between 119 and 117 Ma. The Early Carboniferous event has not previously been recognized in northern Fiordland, whereas the latter event, which has been identified in Early Cretaceous batholiths, their xenoliths, and rocks directly at batholith margins, is here shown to have also affected the country rock. However, the effects of M2 are fragmentary due to limited element mobility, lack of deformation, distance from a heat source and short residence time in the lower crust during peak P and T. It is possible that many parts of the Fiordland continental arc achieved high- P conditions in the Early Cretaceous but retain earlier metamorphic or igneous assemblages. [source] Metamorphic phase relations in orthopyroxene-bearing granitoids: implication for high-pressure metamorphism and prograde melting in the continental crustJOURNAL OF METAMORPHIC GEOLOGY, Issue 4 2009S. K. BHOWMIK Abstract In this work, the factors controlling the formation and preservation of high-pressure mineral assemblages in the metamorphosed orthopyroxene-bearing metagranitoids of the Sandmata Complex, Aravalli-Delhi Mobile Belt (ADMB), northwestern India have been modelled. The rocks range in composition from farsundite through quartz mangerite to opdalite, and with varying K2O, Ca/(Ca + Na)rock and FeOtot + MgO contents. A two stage metamorphic evolution has been recorded in these rocks. An early hydration event stabilized biotite with or without epidote at the expense of magmatic orthopyroxene and plagioclase. Subsequent high-pressure granulite facies metamorphism (,15 kbar, ,800 °C) of these hydrated rocks produced two rock types with contrasting mineralogy and textures. In the non-migmatitic metagranitoids, spectacular garnet ± K-feldspar ± quartz corona was formed around reacting biotite, plagioclase, quartz and/or pyroxene. In contrast, biotite ± epidote melting produced migmatites, containing porphyroblastic garnet incongruent solids and leucosomes. Applying NCKFMASHTO T,M(H2O) and P,T pseudosection modelling techniques, it is demonstrated that the differential response of these magmatic rocks to high-pressure metamorphism is primarily controlled by the scale of initial hydration. Rocks, which were pervasively hydrated, produced garnetiferous migmatites, while for limited hydration, the same metamorphism formed sub-solidus garnet-bearing coronae. Based on the sequence of mineral assemblage evolution and the mineral compositional zoning features in the two metagranitoids, a clockwise metamorphic P,T path is constrained for the high-pressure metamorphic event. The finding has major implications in formulating geodynamic model of crustal amalgamation in the ADMB. [source] Geochronological and petrological constraints on Palaeoproterozoic granulite facies metamorphism in southeastern margin of the North China CratonJOURNAL OF METAMORPHIC GEOLOGY, Issue 2 2009Y.-C. LIU Abstract In the southeastern margin of the North China Craton, high-pressure (HP) granulite facies meta-basic rocks exposed as bands or lenses in the Precambrian metamorphic basement (e.g. Bengbu) and as xenoliths in Mesozoic intrusions (e.g. Jiagou) are characterized by the assemblage garnet + clinopyroxene + plagioclase + quartz + rutile ± Ti-rich hornblende. Cathodoluminescence imaging and mineral inclusions reveal that most zircon from the three dated samples displays distinct core-mantle-rim structures. The cores show typical igneous zircon characteristics and give ages of 2.5,2.4 Ga, thus dating the protolith of the metabasites. The mantles formed at granulite facies conditions as evidenced by inclusions of the HP granulite mineral assemblage garnet + clinopyroxene + rutile + plagioclase + quartz ± hornblende and Ti-rich biotite and yield ages of 1839 ± 31, 1811 ± 19 and 1800 ± 15 Ma. An inclusion-free rim yields an age of 176 ± 2 Ma with the lower Th/U ratio of 0.02. The geochronological and preliminary petrological data of this study suggest that the lower crust beneath the southeastern margin of the North China Craton formed at 2.5,2.4 Ga and underwent HP granulite facies metamorphism at c. 1.8 Ga. This HT-HP metamorphic event may be ascribed to large-scale crustal heating and thickening related to mantle-derived magma underplating at the base of the lower crust, as evidenced by widespread extension, rifting and related mafic magma emplacement in the North China Craton during this period. The age of 176 ± 2 Ma most likely records the late amphibolite facies retrogression occurring during exhumation. [source] Fluid flow during exhumation of deeply subducted continental crust: zircon U-Pb age and O-isotope studies of a quartz vein within ultrahigh-pressure eclogiteJOURNAL OF METAMORPHIC GEOLOGY, Issue 2 2007Y.-F. ZHENG Abstract Quartz veins in high-pressure to ultrahigh-pressure metamorphic rocks witness channelized fluid flow that transports both mass and heat during collisional orogenesis. This flow can occur in the direction of changing temperature/pressure during subduction or exhumation. SHRIMP U-Pb dating of zircon from a kyanite-quartz vein within ultrahigh-pressure eclogite in the Dabie continental collision orogen yields two age groups at 212 ± 7 and 181 ± 13 Ma, which are similar to two groups of LA-ICPMS age at 210 ± 4 and 180 ± 5 Ma for the same sample. These ages are significantly younger than zircon U-Pb ages of 224 ± 2 Ma from the host eclogite. Thus the two age groups from the vein date two episodes of fluid flow involving zircon growth: the first due to decompression dehydration during exhumation, and the second due to heating dehydration in response to a cryptic thermal event after continental collision. Laser fluorination O-isotope analyses gave similar ,18O values for minerals from both vein and eclogite, indicating that the vein-forming fluid was internally derived. Synchronous cooling between the vein and eclogite is suggested by almost the same quartz,mineral fractionation values, with regularly decreasing temperatures that are in concordance with rates of O diffusion in the minerals. While the quartz veining was caused by decompression dehydration at 700,650 °C in a transition from ultrahigh-pressure to high-pressure eclogite-facies retrogression, the postcollisional fluid flow was retriggered by heating dehydration at ,500 °C without corresponding metamorphism. In either case, the kyanite,quartz vein formed later than the peak ultrahigh-pressure metamorphic event at the Middle Triassic, pointing to focused fluid flow during exhumation rather than subduction. The growth of metamorphic zircon in the eclogite appears to have depended on fluid availability, so that their occurrence is a type of geohygrometer besides geochronological applicability to dating of metamorphic events in orogenic cycles. [source] Characterization of polymetamorphism in the Austroalpine basement east of the Tauern Window using garnet isopleth thermobarometryJOURNAL OF METAMORPHIC GEOLOGY, Issue 6 2006F. GAIDIES Abstract Garnet in metapelites from the Wölz and Rappold Complexes of the Austroalpine basement east of the Tauern Window typically shows two distinct growth zones. A first garnet generation usually forms the cores of garnet porphyroblasts and is separated by a prominent microstructural and chemical discontinuity from a second garnet generation, which forms rims of variable width. Whereas the rims were formed during the Eo-Alpine metamorphic overprint, the garnet cores represent remnants of at least two pre-Eo-Alpine metamorphic events. The pressure and temperature estimates obtained from garnet isopleth thermobarometry applied to the first growth increments of the pre-Eo-Alpine garnet cores from the Wölz and Rappold Complexes cluster into two distinct domains: (i) in the Wölz Complex, incipient growth of the first-generation garnet occurred at 4 ± 0.5 kbar and 535 ± 20 °C, (ii) in the Rappold Complex, incipient growth of the oldest garnet cores took place at 5.3 ± 0.3 kbar and 525 ± 15 °C. The Eo-Alpine garnet generation started to grow at 6.5 ± 0.5 kbar and 540 ± 10 °C. According to radiometric dating, the low-pressure garnet from the Wölz complex was formed during a Permian metamorphic event. The first-generation garnet of the Rappold Complex is probably of Variscan age. [source] Metamorphism and microstructures along a high-temperature metamorphic field gradient: the north,eastern boundary of the Královský hvozd unit (Bohemian Massif, Czech Republic)JOURNAL OF METAMORPHIC GEOLOGY, Issue 4 2002D. Scheuvens Abstract A metamorphic field gradient has been investigated in the Moldanubian zone of the central European Variscides encompassing, from base to the top, a staurolite,kyanite zone, a muscovite,sillimanite zone, a K-feldspar,sillimanite zone, and a K-feldspar,cordierite zone, respectively. The observed reaction textures in the anatectic metapsammopelites of the higher grade zones are fully compatible with experimental data and petrogenetic grids that are based on fluid-absent melting reactions. From structural and microstructural observations it can be concluded that the boundary between the kyanite,staurolite zone and the muscovite- and K-feldspar,sillimanite zones coincides with an important switch in deformation mechanism(s). Besides minor syn-anatectic shearing (melt-enhanced deformation), microstructural criteria point (a) to a switch in deformation mechanism from rotation recrystallization (climb-accommodated dislocation creep) to prism Sub-ophiolite metamorphic rocks from NW Anatolia, TurkeyJOURNAL OF METAMORPHIC GEOLOGY, Issue 5 2000Önen The metamorphic rocks from near Kütahya in north-west Anatolia record different stages in the history of closure of the Neo-Tethyan ,zmir,Ankara,Erzincan ocean. Sub-ophiolite metamorphic rocks within the Tav,anl, zone are a tectonically composite sequence of quartz,mica schists, amphibole schists, amphibolites and garnet amphibolites. They show increasing metamorphic grade towards the base of the ophiolite. A first metamorphic event, typical of sub-ophiolite metamorphic sole rocks, was characterized by high-grade assemblages, and followed by retrograde metamorphism. A second event was marked by a medium-to high-pressure overprint of the first-stage metamorphic assemblages with assemblages indicating a transition between the blueschist and greenschist facies. The chemistry of the sub-ophiolite metamorphic rocks indicates an ocean island basalt origin, and Ar,Ar dating indicates a high temperature metamorphic event at 93±2 Ma. Counter-clockwise P,T,t paths recorded by the sub-ophiolite metamorphic rocks are interpreted to result from intra-oceanic thrusting during the closure of the ,zmir, Ankara,Erzincan ocean, initiating subduction, which formed the high-temperature assemblages. Further subduction then produced the widespread blueschists of the Tav,anl, zone during the Late Cretaceous. Later cold thrusting obducted the ophiolite (with the metamorphic sole welded to its base), ophiolitic melanges and blueschists onto the Anatolide passive margin in the latest Cretaceous. All these events pre-date the final Anatolide,Pontide continent,continent collision. [source] SHRIMP U-Pb Zircon Age of the Inishi Migmatite around the Kamioka Mining Area, Hida Metamorphic Complex, Central JapanRESOURCE GEOLOGY, Issue 1 2006Masatoshi Sakoda Abstract. SHRIMP U-Pb ages were determined on single zircons separated from the Inishi migmatite in the Kamioka mining area, Hida metamorphic complex, central Japan. Twenty one determinations were distributed within the age of 234.2±1.8 Ma, excluding one inner core of a grain. As the analyzed crystals were mostly euhedral igneous zircons, the age indicates the crystallization of zircons from granitic melt during the formation of Inishi migmatite. The age of ca. 234 Ma corresponded to the later stage of the major regional metamorphic event in the Hida complex, while the age of ca. 265 Ma determined in a grain suggested the inherited age of the earlier phase of the metamorphism. [source] Petrographic and SHRIMP Studies of Zircons from the Caledonian Xiongdian Eclogite, Northwestern Dabie MountainsACTA GEOLOGICA SINICA (ENGLISH EDITION), Issue 4 2000JIAN Ping Abstract The Xiongdian eclogite occurring in the Sujiahe tectonic mélange zone at Luoshan County, Henan Province, in the western Dabie Mountains, is typical high-pressure (HP)-ultrahigh-pressure (UHP) and medium-temperature eclogite. The occurrence, internal texture and surface characteristics of zircons in eclogite were studied rather systematically petrographically combined with the cathodoluminescence (CL) and scanning electron microscope (SEM) methods. Zircons are mainly hosted in garnet and other metamorphic minerals with sharp boundaries, have a multifaceted morphology and are homogeneous or exhibit a metamorphic growth texture in the interior, thus indicating that they are the product of metamorphism. SHRIMP analyses give zircon 206Pb/238U ages of 335 to 424 Ma and show a certain degree of radiogenic Pb loss; therefore it may be inferred that the age of 424±5 Ma represents the minimum age of a HP-UHP metamorphic age. From the above analyses coupled with previous Sm-Nd, 40Ar- 39Ar, U-Pb and 207Pb/206Pb age data, it is suggested that the peak metamorphic age of the Xiongdian eclogite should be between 424 and 480 Ma. This study further validate the view of the existence of a Caledonian HP-UHP metamorphic event in the western Dabie Mountains. [source] Geology of the summit limestone of Mount Qomolangma (Everest) and cooling history of the Yellow Band under the Qomolangma detachmentISLAND ARC, Issue 4 2005Harutaka Sakai Abstract Newly discovered peloidal limestone from the summit of Mount Qomolangma (Mount Everest) contains skeletal fragments of trilobites, ostracods and crinoids. They are small pebble-sized debris interbedded in micritic bedded limestone of the Qomolangma Formation, and are interpreted to have been derived from a bank margin and redeposited in peri-platform environments. An exposure of the Qomolangma detachment at the base of the first step (8520 m), on the northern slope of Mount Qomolangma was also found. Non-metamorphosed, strongly fractured Ordovician limestone is separated from underlying metamorphosed Yellow Band by a sharp fault with a breccia zone. The 40Ar,39Ar ages of muscovite from the Yellow Band show two-phase metamorphic events of approximately 33.3 and 24.5 Ma. The older age represents the peak of a Barrovian-type Eo-Himalayan metamorphic event and the younger age records a decompressional high-temperature Neo-Himalayan metamorphic event. A muscovite whole-rock 87Rb,86Sr isochron of the Yellow Band yielded 40.06 ± 0.81 Ma, which suggests a Pre-Himalayan metamorphism, probably caused by tectonic stacking of the Tibetan Tethys sediments in the leading margin of the Indian subcontinent. Zircon and apatite grains, separated from the Yellow Band, gave pooled fission-track ages of 14.4 ± 0.9 and 14.4 ± 1.4 Ma, respectively. These new chronologic data indicate rapid cooling of the hanging wall of the Qomolangma detachment from approximately 350°C to 130°C during a short period (15.5,14.4 Ma). [source] Three metamorphic events recorded in a single garnet: Integrated phase modelling, in situ LA-ICPMS and SIMS geochronology from the Moine Supergroup, NW ScotlandJOURNAL OF METAMORPHIC GEOLOGY, Issue 3 2010K. A. CUTTS Abstract In situ LA-ICP-MS monazite geochronology from a garnet-bearing diatexite within the Moine Supergroup (Glenfinnan Group) NW Scotland records three temporally distinct metamorphic events within a single garnet porphyroblast. The initial growth of garnet occurred in the interval c. 825,780 Ma, as recorded by monazite inclusions located in the garnet core. Modelled P,T conditions based on the preserved garnet core composition indicate an initially comparatively high geothermal gradient regime and peak conditions of ,650 °C and 7 kbar. Monazite within a compositionally distinct second shell of garnet has an age of 724 ± 6 Ma. This is indistinguishable from a SIMS age of 725 ± 4 Ma obtained from metamorphic zircon in the sample, which is interpreted to record the timing of migmatization. This second stage of garnet growth occurred on a P,T path from ,6 kbar and 650 °C rising to ,9 kbar and 700 °C, with the peak conditions associated with partial melting. A third garnet zone which forms the rim contains monazite with an age of 464 ± 3 Ma. Monazite in the surrounding matrix has an age of 462 ± 2 Ma. This corresponds well with a U,Pb SIMS zircon age of 463 ± 4 Ma obtained from a deformed pegmatite that was emplaced during widespread folding and reworking of the migmatite fabric. The P,T conditions associated with the final phase of garnet growth were ,7 kbar and 650 °C. The monazite ages coupled with the phase relations modelled from this multistage garnet indicate that it records two Neoproterozoic tectonothermal events as well as the widespread Ordovician Grampian event associated with Caledonian orogenesis. Thus, this single garnet records much of the Neoproterozoic to Ordovician thermal history in NW Scotland, and highlights the long history of porphyroblast growth that can be revealed by in situ isotopic dating and associated P,T modelling. This approach has the potential to reveal much of the thermal architecture of Neoproterozoic events within the Moine Supergroup, despite intense Caledonian reworking, if suitable textural and mineralogical relationships can be indentified elsewhere. [source] Granulite facies thermal aureoles and metastable amphibolite facies assemblages adjacent to the Western Fiordland Orthogneiss in southwest Fiordland, New ZealandJOURNAL OF METAMORPHIC GEOLOGY, Issue 5 2009A. H. ALLIBONE Abstract In southwest New Zealand, a suite of felsic diorite intrusions known as the Western Fiordland Orthogneiss (WFO) were emplaced into the mid to deep crust and partially recrystallized to high- P (12 kbar) granulite facies assemblages. This study focuses on the southern most pluton within the WFO suite (Malaspina Pluton) between Doubtful and Dusky sounds. New mapping shows intrusive contacts between the Malaspina Pluton and adjacent Palaeozoic metasedimentary country rocks with a thermal aureole ,200,1000 m wide adjacent to the Malaspina Pluton in the surrounding rocks. Thermobarometry on assemblages in the aureole indicates that the Malaspina Pluton intruded the adjacent amphibolite facies rocks while they were at depths of 10,14 kbar. Similar P,T conditions are recorded in high- P granulite facies assemblages developed locally throughout the Malaspina Pluton. Palaeozoic rocks more than ,200,1000 m from the Malaspina Pluton retain medium -P mid-amphibolite facies assemblages, despite having been subjected to pressures of 10,14 kbar for > 5 Myr. These observations contradict previous interpretations of the WFO Malaspina Pluton as the lower plate of a metamorphic core complex, everywhere separated from the metasedimentary rocks by a regional-scale extensional shear zone (Doubtful Sound Shear Zone). Slow reaction kinetics, lack of available H2O, lack of widespread penetrative deformation, and cooling of the Malaspina Pluton thermal anomaly within c. 3,4 Myr likely prevented recrystallization of mid amphibolite facies assemblages outside the thermal aureole. If not for the evidence within the thermal aureole, there would be little to suggest that gneissic rocks which underlie several 100 km2 of southwest New Zealand had experienced metamorphic pressures of 10,14 kbar. Similar high- P metamorphic events may therefore be more common than presently recognized. [source] False metamorphic events inferred from misinterpretation of microstructural evidence and P,T dataJOURNAL OF METAMORPHIC GEOLOGY, Issue 4 2008R. H. VERNON Abstract Geometrical relationships involving inclusions and partial inclusions in metamorphic microstructures can be inadequate for inferring an order of crystallization and hence a metamorphic reaction. Unique spatial and/or chemical relationships need to be defined for mineral inclusions, in the context of a reference paragenesis, commonly the matrix assemblage. Corona microstructures are reliable indicators of metamorphic reactions, but require considerable care when used to infer reactions or changes in P,T conditions, owing to kinetic problems, as well as to changes in the effective reaction volume during changes across relatively broad P,T stability fields of assemblages. Mineral equilibria models, most commonly implemented through P,T pseudosections, may allow the order in which different minerals become stable along a given P,T path to be inferred. However, the order in which two minerals become stable may be different from the order in which two grains of these minerals nucleate. Furthermore, such diagrams cannot make predictions about which minerals will form porphyroblasts and which minerals will form inclusions in porphyroblasts. An evaluation of three examples from the Australian Proterozoic shows that modelling, in combination with inclusion-host relationships, is a powerful tool for understanding the metamorphic evolution of a rock, but involves considerable uncertainty. [source] On the roles of deformation and fluid during rejuvenation of a polymetamorphic terrane: inferences on the geodynamic evolution of the Ruker Province, East AntarcticaJOURNAL OF METAMORPHIC GEOLOGY, Issue 8 2007G. PHILLIPS Abstract Evaluating pressure,temperature (P,T) conditions through mineral equilibria modelling within an amphibolite facies polymetamorphic terrane requires knowledge of the fluid content of the rocks. The Archean-Palaeoproterozoic basement rocks of the Ruker Province, East Antarctica, preserve evidence of three metamorphic events (M1,M3). Of particular interest is the M3 event, which is constrained to the early Palaeozoic (c. 550,480 Ma). Evaluation of the tectonic setting during this time is important because the Ruker Province is located within a critical region with respect to models of Gondwana assembly. Structural evidence of the early Palaeozoic event is preserved as large (up to ,500 m wide) high strain zones that cut the orthogneiss-metasedimentary basement (Tingey Complex) of the Ruker Province. Rocks within these zones have been thoroughly recrystallized and preserve a dominant shear fabric and M3 mineral assemblages that formed at P,T conditions of 4.0,5.2 kbar and 565,640 °C. Distal to these zones, rocks preserve more complex petrographic relationships with S1 and S2 foliations, being incompletely overgrown by M3 retrograde assemblages. We show that the mineral assemblages preserved during the M3 event are highly dependent on the availability of fluid H2O, which is strongly influenced by the structural setting (i.e. proximity to the high-strain zones). P,T structural and fluid flow constraints support a model of basin inversion during early Palaeozoic crustal rejuvenation in the Ruker Province. [source] Fluid flow during exhumation of deeply subducted continental crust: zircon U-Pb age and O-isotope studies of a quartz vein within ultrahigh-pressure eclogiteJOURNAL OF METAMORPHIC GEOLOGY, Issue 2 2007Y.-F. ZHENG Abstract Quartz veins in high-pressure to ultrahigh-pressure metamorphic rocks witness channelized fluid flow that transports both mass and heat during collisional orogenesis. This flow can occur in the direction of changing temperature/pressure during subduction or exhumation. SHRIMP U-Pb dating of zircon from a kyanite-quartz vein within ultrahigh-pressure eclogite in the Dabie continental collision orogen yields two age groups at 212 ± 7 and 181 ± 13 Ma, which are similar to two groups of LA-ICPMS age at 210 ± 4 and 180 ± 5 Ma for the same sample. These ages are significantly younger than zircon U-Pb ages of 224 ± 2 Ma from the host eclogite. Thus the two age groups from the vein date two episodes of fluid flow involving zircon growth: the first due to decompression dehydration during exhumation, and the second due to heating dehydration in response to a cryptic thermal event after continental collision. Laser fluorination O-isotope analyses gave similar ,18O values for minerals from both vein and eclogite, indicating that the vein-forming fluid was internally derived. Synchronous cooling between the vein and eclogite is suggested by almost the same quartz,mineral fractionation values, with regularly decreasing temperatures that are in concordance with rates of O diffusion in the minerals. While the quartz veining was caused by decompression dehydration at 700,650 °C in a transition from ultrahigh-pressure to high-pressure eclogite-facies retrogression, the postcollisional fluid flow was retriggered by heating dehydration at ,500 °C without corresponding metamorphism. In either case, the kyanite,quartz vein formed later than the peak ultrahigh-pressure metamorphic event at the Middle Triassic, pointing to focused fluid flow during exhumation rather than subduction. The growth of metamorphic zircon in the eclogite appears to have depended on fluid availability, so that their occurrence is a type of geohygrometer besides geochronological applicability to dating of metamorphic events in orogenic cycles. [source] Characterization of polymetamorphism in the Austroalpine basement east of the Tauern Window using garnet isopleth thermobarometryJOURNAL OF METAMORPHIC GEOLOGY, Issue 6 2006F. GAIDIES Abstract Garnet in metapelites from the Wölz and Rappold Complexes of the Austroalpine basement east of the Tauern Window typically shows two distinct growth zones. A first garnet generation usually forms the cores of garnet porphyroblasts and is separated by a prominent microstructural and chemical discontinuity from a second garnet generation, which forms rims of variable width. Whereas the rims were formed during the Eo-Alpine metamorphic overprint, the garnet cores represent remnants of at least two pre-Eo-Alpine metamorphic events. The pressure and temperature estimates obtained from garnet isopleth thermobarometry applied to the first growth increments of the pre-Eo-Alpine garnet cores from the Wölz and Rappold Complexes cluster into two distinct domains: (i) in the Wölz Complex, incipient growth of the first-generation garnet occurred at 4 ± 0.5 kbar and 535 ± 20 °C, (ii) in the Rappold Complex, incipient growth of the oldest garnet cores took place at 5.3 ± 0.3 kbar and 525 ± 15 °C. The Eo-Alpine garnet generation started to grow at 6.5 ± 0.5 kbar and 540 ± 10 °C. According to radiometric dating, the low-pressure garnet from the Wölz complex was formed during a Permian metamorphic event. The first-generation garnet of the Rappold Complex is probably of Variscan age. [source] Contrasting textural record of two distinct metamorphic events of similar P,T conditions and different durationsJOURNAL OF METAMORPHIC GEOLOGY, Issue 8 2005O. LEXA Abstract A structural, metamorphic and geochronological study of the Staré M,sto belt implies the existence of two distinct metamorphic events of similar peak P,T conditions (700,800 °C, 8,10 kbar) during the Cambro-Ordovician and the Carboniferous tectonometamorphic events. The hypothesis of two distinct periods of metamorphism was suggested on the basis of structural discordance between an undoubtedly Carboniferous granodiorite sill intrusion and earlier Cambro-Ordovician fabrics of a banded amphibolite complex. The analysis of crystal size distribution (CSD) shows high nucleation density (N0) and low average growth rate (Gt) for Carboniferous mylonitic metagabbros and mylonitic granodiorites. The parameter N0 decreases whereas the quantity Gt increases towards higher temperatures progressively approaching the values obtained from the Cambro-Ordovician banded amphibolite complex. The spatial distribution of amphibole and plagioclase shows intense mechanical mixing for lower-temperature mylonitic metagabbros. In high-temperature mylonites a strong aggregate distribution is developed. Cambro-Ordovician amphibolites unaffected by Carboniferous deformation show a regular to anticlustered spatial distribution resulting from heterogeneous nucleation of individual phases. This pattern, together with CSD, was subsequently modified by the grain growth and textural equilibration controlled by diffusive mass transfer during Carboniferous metamorphism. The differences between the observed textures of the amphibolites are interpreted to be a consequence of the different durations of the Carboniferous and Cambro-Ordovician thermal events. [source]
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