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Peak Metamorphism (peak + metamorphism)
Selected AbstractsWhat Happened in the Trans-North China Orogen in the Period 2560-1850 Ma?ACTA GEOLOGICA SINICA (ENGLISH EDITION), Issue 6 2006Guochun ZHAO Abstract: The Trans-North China Orogen (TNCO) was a Paleoproterozic continent-continent collisional belt along which the Eastern and Western Blocks amalgamated to form a coherent North China Craton (NCC). Recent geological, structural, geochemical and isotopic data show that the orogen was a continental margin or Japan-type arc along the western margin of the Eastern Block, which was separated from the Western Block by an old ocean, with eastward-directed subduction of the oceanic lithosphere beneath the western margin of the Eastern Block. At 2550-2520 Ma, the deep subduction caused partial melting of the medium-lower crust, producing copious granitoid magma that was intruded into the upper levels of the crust to form granitoid plutons in the low- to medium-grade granite-greenstone terranes. At 2530-2520 Ma, subduction of the oceanic lithosphere caused partial melting of the mantle wedge, which led to underplating of mafic magma in the lower crust and widespread mafic and minor felsic volcanism in the arc, forming part of the greenstone assemblages. Extension driven by widespread mafic to felsic volcanism led to the development of back-arc and/or intra-arc basins in the orogen. At 2520-2475 Ma, the subduction caused further partial melting of the lower crust to form large amounts of tonalitic-trondhjemitic-granodioritic (TTG) magmatism. At this time following further extension of back-arc basins, episodic granitoid magmatism occurred, resulting in the emplacement of 2360 Ma, ,2250 Ma 2110,21760 Ma and ,2050 Ma granites in the orogen. Contemporary volcano-sedimentary rocks developed in the back-arc or intra-arc basins. At 2150-1920 Ma, the orogen underwent several extensional events, possibly due to subduction of an oceanic ridge, leading to emplacement of mafic dykes that were subsequently metamorphosed to amphibolites and medium- to high-pressure mafic granulites. At 1880-1820 Ma, the ocean between the Eastern and Western Blocks was completely consumed by subduction, and the closing of the ocean led to the continent-arc-continent collision, which caused large-scale thrusting and isoclinal folds and transported some of the rocks into the lower crustal levels or upper mantle to form granulites or eclogites. Peak metamorphism was followed by exhumation/uplift, resulting in widespread development of asymmetric folds and symplectic textures in the rocks. [source] Ultrahigh-pressure metamorphic records hidden in zircons from amphibolites in Sulu Terrane, eastern ChinaISLAND ARC, Issue 3 2003Fulai Liu Abstract The amphibolites occur sporadically as thin layers and blocks throughout the Sulu Terrane, eastern China. All analyzed amphibolite from outcrop and drill cores from prepilot drill hole CCSD-PP1 and CCSD-PP2, Chinese Continental Scientific Drilling Project in the Sulu Terrane, are retrograded eclogites overprinted by amphibolite-facies retrograde metamorphism, with characteristic mineral assemblages of amphibole + plagioclase + epidote ± quartz ± biotite ± ilmenite ± titanite. However, coesite and coesite-bearing ultrahigh-pressure (UHP) mineral assemblages are identified by Raman spectroscopy and electron microprobe analysis as inclusions in zircons separated from these amphibolites. In general, coesite and other UHP mineral inclusions are preserved in the cores and mantles of zircons, whereas quartz inclusions occur in the rims of the same zircons. The UHP mineral assemblages consist mainly of coesite + garnet + omphacite + rutile, coesite + garnet + omphacite, coesite + garnet + omphacite + phengite + rutile + apatite, coesite + omphacite + rutile and coesite + magnesite. Compositions of analyzed mineral inclusions are very similar to those of matrix minerals from Sulu eclogites. These UHP mineral inclusion assemblages yield temperatures of 631,780°C and pressures of ,2.8 × 103 MPa, representing the P,T conditions of peak metamorphism of these rocks, which are consistent with those (T = 642,726°C; P , 2.8 × 103 MPa) deduced from adjacent eclogites. These data indicate that the amphibolites are the retrogressive products of UHP eclogites. [source] Cooling and inferred exhumation history of the Ryoke metamorphic belt in the Yanai district, south-west Japan: Constraints from Rb,Sr and fission-track ages of gneissose granitoid and numerical modelingISLAND ARC, Issue 2 2001Takamoto Okudaira Abstract The Ryoke metamorphic belt in south-west Japan consists mainly of I-type granitoids and associated low-pressure/high-temperature metamorphic rocks. In the Yanai district, it has been divided into three structural units: northern, central and southern units. In this study, we measured the Rb,Sr whole-rock,mineral isochron ages and fission-track ages of the gneissose granodiorite in the central structural unit. Four Rb,Sr ages fall in a range of ca 89,87 Ma. The fission-track ages of zircon and apatite are 68.9 ± 2.6 Ma and 57.4 ± 2.5 Ma (1, error), respectively. Combining the newly obtained ages with previously reported (Th,)U,Pb ages from the same unit, thermochronologic study revealed two distinctive cooling stages; 1) a rapid cooling (> 40°C/Myr) for a period (~7 Myr) soon after the peak metamorphism (~ 95 Ma) and 2) the subsequent slow cooling stage (~ 5°C/Myr) after ca 88 Ma. The first rapid cooling stage corresponds to thermal relaxation of the intruded granodiorite magma and its associated metamorphic rocks, and to the uplift by a displacement along low-angle faults which initiated soon after the intrusion of the magma. Uplift by the later stage deformation having formed large-scale upright folds resulted in progress of the exhumation during the first stage. The average exhumation velocity of the stage is , 2 mm/yr. During the second stage, the rocks were not accompanied by ductile deformation and were exhumed with the rate of 0.1,0.2 mm/yr. The difference in the exhumation velocity between the first and second cooling stages resulted from the difference in the thickness of the crust and in the activity of ductile deformation between the early and later stages of the orogenesis. [source] P,T,t path of the Hercynian low-pressure rocks from the Mandatoriccio complex (Sila Massif, Calabria, Italy): new insights for crustal evolutionJOURNAL OF METAMORPHIC GEOLOGY, Issue 2 2010A. LANGONE Abstract The tectono-metamorphic evolution of the Hercynian intermediate,upper crust outcropping in eastern Sila (Calabria, Italy) has been reconstructed, integrating microstructural analysis, P,T pseudosections, mineral isopleths and geochronological data. The studied rocks belong to a nearly complete crustal section that comprises granulite facies metamorphic rocks at the base and granitoids in the intermediate levels. Clockwise P,T paths have been constrained for metapelites of the basal level of the intermediate,upper crust (Umbriatico area). These rocks show noticeable porphyroblastic textures documenting the progressive change from medium- P metamorphic assemblages (garnet- and staurolite-bearing assemblages) towards low- P/high -T metamorphic assemblages (fibrolite- and cordierite-bearing assemblages). Peak-metamorphic conditions of ,590 °C and 0.35 GPa are estimated by integrating microstructural observations with P,T pseudosections calculated for bulk-rock and reaction-domain compositions. The top level of the intermediate,upper crust (Campana area) recorded only the major heating phase at low- P (,550 °C and 0.25 GPa), as documented by the static growth of biotite spots and of cordierite and andalusite porphyroblasts in metapelites. In situ U,Th,Pb dating of monazite from schists containing low -P/high -T metamorphic assemblages gave a weighted mean U,Pb concordia age of 299 ± 3 Ma, which has been interpreted as the timing of peak metamorphism. In the framework of the whole Hercynian crustal section the peak of low -P/high -T metamorphism in the intermediate-to-upper crust took place concurrently with granulite facies metamorphism in the lower crust and with emplacement of the granitoids in the intermediate levels. In addition, decompression is a distinctive trait of the P,T evolution both in the lower and upper crust. It is proposed that post,collisional extension, together with exhumation, is the most suitable tectonic setting in which magmatic and metamorphic processes can be active simultaneously in different levels of the continental crust. [source] Exhumation during oblique transpression: The Feiran,Solaf region, EgyptJOURNAL OF METAMORPHIC GEOLOGY, Issue 6 2009T. S. ABU-ALAM Abstract The Feiran,Solaf metamorphic complex of Sinai, Egypt, is one of the highest grade metamorphic complexes of a series of basement domes that crop out throughout the Arabian-Nubian Shield. In the Eastern Desert of Egypt these basement domes have been interpreted as metamorphic core complexes exhumed in extensional settings. For the Feiran,Solaf complex an interpretation of the exhumation mechanism is difficult to obtain with structural arguments as all of its margins are obliterated by post-tectonic granites. Here, metamorphic methods are used to investigate its tectonic history and show that the complex was characterized by a single metamorphic cycle experiencing peak metamorphism at ,700,750 °C and 7,8 kbar and subsequent isothermal decompression to ,4,5 kbar, followed by near isobaric cooling to 450 °C. Correlation of this metamorphic evolution with the deformation history shows that peak metamorphism occurred prior to the compressive deformation phase D2, while the compressive D2 and D3 deformation occurred during the near isothermal decompression phase of the P,T loop. We interpret the concurrence of decompression of the P,T path and compression by structural shortening as evidence for the Najd fault system exhuming the complex in an oblique transpressive regime. However, final exhumation from ,15 km depth must have occurred due to an unrelated mechanism. [source] Synchronous peak Barrovian metamorphism driven by syn-orogenic magmatism and fluid flow in southern Connecticut, USAJOURNAL OF METAMORPHIC GEOLOGY, Issue 5 2008P. J. LANCASTER Abstract Recent work in Barrovian metamorphic terranes has found that rocks experience peak metamorphic temperatures across several grades at similar times. This result is inconsistent with most geodynamic models of crustal over-thickening and conductive heating, wherein rocks which reach different metamorphic grades generally reach peak temperatures at different times. Instead, the presence of additional sources of heat and/or focusing mechanisms for heat transport, such as magmatic intrusions and/or advection by metamorphic fluids, may have contributed to the contemporaneous development of several different metamorphic zones. Here, we test the hypothesis of temporally focussed heating for the Wepawaug Schist, a Barrovian terrane in Connecticut, USA, using Sm,Nd ages of prograde garnet growth and U,Pb zircon crystallization ages of associated igneous rocks. Peak temperature in the biotite,garnet zone was dated (via Sm,Nd on garnet) at 378.9 ± 1.6 Ma (2,), whereas peak temperature in the highest grade staurolite,kyanite zone was dated (via Sm,Nd on garnet rims) at 379.9 ± 6.8 Ma (2,). These garnet ages suggest that peak metamorphism was pene-contemporaneous (within error) across these metamorphic grades. Ion microprobe U,Pb ages for zircon from igneous rocks hosted by the metapelites also indicate a period of syn-metamorphic peak igneous activity at 380.6 ± 4.7 Ma (2,), indistinguishable from the peak ages recorded by garnet. A 388.6 ± 2.1 Ma (2,) garnet core age from the staurolite,kyanite zone indicates an earlier episode of growth (coincident with ages from texturally early zircon and a previously published monazite age) along the prograde regional metamorphic T,t path. The timing of peak metamorphism and igneous activity, as well as the occurrence of extensive syn-metamorphic quartz vein systems and pegmatites, best supports the hypothesis that advective heating driven by magmas and fluids focussed major mineral growth into two distinct episodes: the first at c. 389 Ma, and the second, corresponding to the regionally synchronous peak metamorphism, at c. 380 Ma. [source] Regional variation in exhumation and strain rate of the high-pressure Sambagawa metamorphic rocks in central Shikoku, south-west JapanJOURNAL OF METAMORPHIC GEOLOGY, Issue 7 2002K. Yagi Abstract Regional variation in the P,T path of the Sambagawa metamorphic rocks, central Shikoku, Japan has been inferred from compositional zoning of metamorphic amphibole. Rocks constituting the northern part (Saruta River area) exhibit a hairpin type P,T path, where winchite/actinolite grew at the prograde stage, the peak metamorphism was recorded by the growth of barroisite to hornblende and sodic amphibole to winchite/actinolite grew at the retrograde stage. In the southern part (Asemi River area), rocks exhibit a clockwise type P,T path, where barroisite to hornblende core is rimmed by winchite to actinolite. The difference in P,T path could suggest a faster exhumation rate (i.e. more rapid decompression) in the southern than in the northern part. On the other hand, physical conditions of deformation during the exhumation stage have been independently inferred from microstructures in deformed quartz. Recrystallized quartz grains in rocks from the low-grade (chlorite and garnet) zones are much more stretched in the southern part (aspect ratio , 4.0) than in the northern part (aspect ratio< 4.0), indicating a higher strain rate in the former than in the latter. These facts may indicate that the exhumation and strain rates are correlated (i.e. the exhumation rate increases with increasing the strain rate). The difference in the exhumation rate inferred from amphibole zoning between the northern and southern parts could be explained by an extensional model involving normal faulting, where the lower plate can be exhumed faster than the upper plate due to the displacement along the fault. Furthermore, the model may explain the positive correlation between the exhumation and strain rates, because the lower plate tended to support more stress than the upper plate. [source] Geochemical constraints of the eclogite and granulite facies metamorphism as recognized in the Raobazhai complex from North Dabie Shan, ChinaJOURNAL OF METAMORPHIC GEOLOGY, Issue 1 2001Y. L. Xiao Abstract A combined study of major and trace elements, fluid inclusions and oxygen isotopes has been carried out on garnet pyroxenite from the Raobazhai complex in the North Dabie Terrane (NDT). Well-preserved compositional zoning with Na decreasing and Ca and Mg increasing from the core to rim of pyroxene in the garnet pyroxenite indicates eclogite facies metamorphism at the peak metamorphic stage and subsequent granulite facies metamorphism during uplift. A P,T path with substantial heating (from c. 750 to 900 °C) after the maximum pressure reveals a different uplift history compared with most other eclogites in the South Dabie Terrane (SDT). Fluid inclusion data can be correlated with the metamorphic grade: the fluid regime during the peak metamorphism (eclogite facies) was dominated by N2 -bearing NaCl-rich solutions, whereas it changed into CO2 -dominated fluids during the granulite facies retrograde metamorphism. At a late retrograde metamorphic stage, probably after amphibolite facies metamorphism, some external low-salinity fluids were involved. In situ UV-laser oxygen isotope analysis was undertaken on a 7 mm garnet, and impure pyroxene, amphibole and plagioclase. The nearly homogeneous oxygen isotopic composition (,18OVSMOW = c. 6.7,) in the garnet porphyroblast indicates closed fluid system conditions during garnet growth. However, isotopic fractionations between retrograde phases (amphibole and plagioclase) and garnet show an oxygen isotopic disequilibrium, indicating retrograde fluid,rock interactions. Unusual MORB-like rare earth element (REE) patterns for whole rock of the garnet pyroxenite contrast with most ultra-high-pressure (UHP) eclogites in the Dabie-Sulu area. However, the age-corrected initial ,Nd(t) is ,,2.9, which indicates that the protolith of the garnet pyroxenite was derived from an enriched mantle rather than from a MORB source. Combined with the present data of oxygen isotopic compositions and the characteristic N2 content in the fluid inclusions, we suggest that the protolith of the garnet pyroxenite from Raobazhai formed in an enriched mantle fragment, which has been exposed to the surface prior to the Triassic metamorphism. [source] Petrology and P,T path of the Fuping mafic granulites: implications for tectonic evolution of the central zone of the North China cratonJOURNAL OF METAMORPHIC GEOLOGY, Issue 4 2000G. C. Zhao The Fuping Complex and the adjoining Wutai and Hengshan Complexes are located in the central zone of the North China craton. The dominant rock types in the Fuping Complex are high-grade tonalitic,trondhjemitic,granodioritic (TTG) gneisses, with minor amounts of mafic granulites, syntectonic granitic rocks and supracrustal rocks. The petrological evidence from the mafic granulites indicates three stages of metamorphic evolution. The M1 stage is represented by garnet porphyroblasts and matrix plagioclase, quartz, orthopyroxene, clinopyroxene and hornblende. Orthopyroxene+plagioclase symplectites and clinopyroxene+plagioclase±orthopyroxene coronas formed in response to decompression during M2 following the peak metamorphism at M1. Hornblende+plagioclase symplectites formed as a result of further isobaric cooling and retrograde metamorphism during M3. The P,T estimates using TWQ thermobarometry are: 900,950 °C and 8.0,8.5 kbar for the peak assemblage (M1), based on the core compositions of garnet, matrix pyroxene and plagioclase; 700,800 °C and 6.0,7.0 kbar for the pyroxene+plagioclase symplectites or coronas (M2); and 550,650 °C and 5.3,6.3 kbar for the hornblende+plagioclase symplectites (M3), based on garnet rim and corresponding symplectic mineral compositions. These P,T estimates define a clockwise P,T path involving near-isothermal decompression for the Fuping Complex, similar to the P,T path estimated for the metapelitic gneisses. The inferred P,T path suggests that the Fuping Complex underwent initial crustal thickening, subsequent exhumation, and finally cooling and retrogression. This tectonothermal path is similar to P,T paths inferred for the Wutai and Hengshan Complexes and other tectonic units in the central zone of the North China craton, but different from anti-clockwise P,T paths estimated for the basement rocks in the eastern and western zones of the craton. Based on lithological, structural, metamorphic and geochronological data, the eastern and western zones of the craton are considered to represent two different Archean to Paleoproterozoic continental blocks that amalgamated along the central zone at the end of Paleoproterozoic. The P,T paths of the Fuping Complex and other tectonic units in the central zone record the collision between the eastern and western zones that led to the final assembly of the North China craton at c. 1800 Ma. [source] Petrochemical constraints for dual origin of garnet peridotites from the Dabie-Sulu UHP terrane, eastern-central ChinaJOURNAL OF METAMORPHIC GEOLOGY, Issue 2 2000Zhang Garnet peridotites occur as lenses, blocks or layers within granulite,amphibolite facies gneiss in the Dabie-Sulu ultra-high-pressure (UHP) terrane and contain coesite-bearing eclogite. Two distinct types of garnet peridotite were identified based on mode of occurrence and petrochemical characteristics. Type A mantle-derived peridotites originated from either: (1) the mantle wedge above a subduction zone, (2) the footwall mantle of the subducted slab, or (3) were ancient mantle fragments emplaced at crustal depths prior to UHP metamorphism, whereas type B crustal peridotite and pyroxenite are a portion of mafic,ultramafic complexes that were intruded into the continental crust as magmas prior to subduction. Most type A peridotites were derived from a depleted mantle and exhibit petrochemical characteristics of mantle rocks; however, Sr and Nd isotope compositions of some peridotites have been modified by crustal contamination during subduction and/or exhumation. Type B peridotite and pyroxenite show cumulate structure, and some have experienced crustal metasomatism and contamination documented by high 87Sr/86Sr ratios (0.707,0.708), low ,Nd(t) values (,6 to ,9) and low ,18O values of minerals (+2.92 to +4.52). Garnet peridotites of both types experienced multi-stage recrystallization; some of them record prograde histories. High- P,T estimates (760,970 °C and 4.0,6.5±0.2 GPa) of peak metamorphism indicate that both mantle-derived and crustal ultramafic rocks were subducted to profound depths >100 km (the deepest may be ,180,200 km) and experienced UHP metamorphism in a subduction zone with an extremely low geothermal gradient of <5 °C km,1. [source] | ||||||||||