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Rutile
Terms modified by Rutile Selected AbstractsOrientation and Phase Relationships between Titania Films and Polycrystalline BaTiO3 Substrates as Determined by Electron Backscatter Diffraction MappingJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 9 2010Nina V. Burbure Titania films have been grown on polycrystalline BaTiO3 (BTO) substrates at 700°C by pulsed laser deposition. Electron backscatter diffraction (EBSD) was used to determine grain orientations in the substrate before growth, and the phase and orientation of the supported films after growth. All BaTiO3 grains within 26° of (001) were covered by anatase films with an orientation relationship of (001)Anatase||(001)BTO and [100]Anatase||[100]BTO. Rutile with a variety of orientations grew on BaTiO3 grains with orientations closer to (110) and (111). EBSD mapping provides an efficient means for determining phase and orientation relationships of films over all orientation parameters. [source] Flux growth of straw-like rutile monocrystalsCRYSTAL RESEARCH AND TECHNOLOGY, Issue 2 2008A. Bloise Abstract Millimetric straw-like rutile monocrystals were grown by the flux growth technique. A suitable mixture of flux (MoO3, V2O5, Li2CO3) and amorphous TiO2 gel was slowly cooled down to 750°C from 1250°C or 1350°C. The best yields of straw-like rutile were obtained with a nutrient/flux ratio and a cooling rate in the range 0.015-0.006 and 1.8-1.9 K h -1, respectively. The hollowed crystals were characterized by powder and single-crystal X-ray diffraction, scanning electron microscopy, microthermometry, and µ-Raman spectroscopy. As for skeletal crystal, the formation of axial canals in rutile is attributed to a lack of nutrient due to the viscosity of the melt and the high growth rate along [001]. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] Comparative Sol,Hydro(Solvo)thermal Synthesis of TiO2 NanocrystalsEUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 11 2006Xianfeng Yang Abstract Three small inorganic compounds [acetic acid (CH3COOH), nitric acid (HNO3), and hydrochloric acid (HCl)] andfive small organic compounds [n -butyl alcohol (CH3CH2CH2CH2OH), 1,2-propanediol (CH2OHCHOHCH3), propane-1,3-dicarboxylic acid (HOOCCH2COOH), butane-1,4-dicarboxylic acid (HOOCCH2CH2COOH), and ethylenediamine (H2NCH2CH2NH2)] with different donors (Cl,, NO3,, OH, COO,, and NH2) are used respectively and comparatively in different reaction media for tailoring the sol,hydrothermal synthesis of TiO2 nanocrystals at a specific reaction temperature and/or for different reaction times. Both anatase and rutile with a variety of nanostructures such as nanocubes, nanorods, nanoneedles, and their self-assembled nanospheres are selectively grown. Well-defined and crystalline anatase nanocrystals with a narrow size distribution are prepared in the presence of n -butyl alcohol. With the use of HCl, nanoneedles and their assembled nanospheres of phase-pure rutile are obtained under much less drastic conditions, even at 100 °C and for only two hours. The effects of the reaction media, synthesis temperatures, and aging times on the structures and shapes of the products are investigated in detail by powder X-ray diffraction (XRD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006) [source] Titanium Containing ,-MnO2 (TM) Hollow Spheres: One-Step Synthesis and Catalytic Activities in Li/Air Batteries and Oxidative Chemical ReactionsADVANCED FUNCTIONAL MATERIALS, Issue 19 2010Lei Jin Abstract Titanium containing ,-MnO2 octahedral molecular sieves having hollow sphere structures are successfully prepared for the first time using a one-step synthesis method. Titanium cations are used as structure-directing agents in the synthesis process. The assembly of the hollow spheres is carried out at the beginning of the process. Various techniques including XRD, N2 adsorption, SEM, EDX, RAMAN, TEM, XPS, and TGA are employed for the materials characterization. Ti is incorporated into the MnO2 framework in isolated sites, and TiO2 phases (anatase and rutile) are not observed. When introduced in medium-sized lithium-air batteries, the materials give very high specific capacity (up to 2.3 A h g,1). These materials are also catalytically tested in the oxidation of toluene with molecular oxygen at atmospheric pressure, showing significant oxidative catalytic activities in this difficult chemical reaction. [source] Syntheses, Li Insertion, and Photoactivity of Mesoporous Crystalline TiO2ADVANCED FUNCTIONAL MATERIALS, Issue 17 2009Wenbo Yue Abstract Ordered mesoporous rutile and anatase TiO2 samples are prepared using mesoporous silica SBA-15 as template and freshly synthesized titanium nitrate and titanium chloride solutions as precursors. The rutile material formed from the nitrate solution is monocrystalline and contains minimal amounts of Si with a Si:Ti ratio of 0.031(4), whereas the anatase material formed from the chloride solution comprises nanocrystals and contains a higher content of Si with a Si:Ti ratio of 0.18(3). It is found that control of temperature and selection of Ti-containing precursor play important roles in determining the crystal phase and crystallinity. A possible formation mechanism of porous crystalline TiO2 is suggested. Characterization of these porous materials is performed by XRD, HRTEM, and nitrogen adsorption/desorption. SBA-15-templated mesoporous rutile TiO2 exhibits a higher Li ion insertion capability than KIT-6-templated TiO2 due to its larger surface area. Likewise mesoporous anatase TiO2:SiO2 composite has a better photoactivity than bulk TiO2 or TiO2 -loaded SBA-15 for bleaching methylene blue. [source] Palaeomagnetic study of the El Quemado complex and Marifil formation, Patagonian Jurassic igneous province, ArgentinaGEOPHYSICAL JOURNAL INTERNATIONAL, Issue 3 2003Maria P. Iglesia Llanos SUMMARY The upper Jurassic El Quemado Complex was sampled at 36 sites from five localities in the cordilleran foothills of southern Patagonia between Lago Argentino and Lago Posadas,Sierra Colorada, and the middle Jurassic Marifil Formation at 12 sites in the Somuncurá Massif near Camarones. The main lithology was ignimbrite, with minor tuff and lava. Petrographical and SEM observation show that the El Quemado rocks suffered an intense, high-temperature alteration which resulted in transformation of most primary Ti-magnetite in pseudobrookite, rutile and minor Ti-haematite and Fe hydrated oxides. A similar, less pronounced alteration occurred in the Marifil rocks. 40Ar/39Ar dating of El Quemado was possible for one sample from Sierra Colorada and yielded an age of 156.5 ± 1.9 Ma. Magnetic mineralogy measurements (isothermal remanence, hysteresis loop, Curie balance) show that the remanent magnetization is dominated by PSD low-Ti magnetite, often associated to a minor high-coercive mineral (haematite). Secondary magnetization components are usually absent or weak at El Quemado sites, strong at Marifil. They were completely erased by thermal and AF demagnetization and a characteristic remanence (ChRM) stable up to temperatures higher than 550°C or peak-field values of 100 mT was successfully isolated. The virtual geomagnetic pole (VGP) from the Marifil Formation (83°S, 138°E) is in agreement with the literature data for Jurassic rocks from stable South America. The El Quemado VGPs fall in two groups. The localities to the north of latitude 48°S (Lago Posadas, Sierra Colorada) yield a VGP (81°S, 172°E) close to that of Marifil, whereas those south of latitude 49°S (Lago San Martín, Lago Argentino) show a highly elongated VGP distribution consistent with counter-clockwise block-rotation about vertical axes. These rotations were likely caused by thrust sheets which were rotating counter-clockwise at the same time they were advancing towards the foreland. The amount of rotation varies according to the location of the sampling sites in the thrust and fold belt. [source] Photocatalytic Carbon-Nanotube,TiO2 CompositesADVANCED MATERIALS, Issue 21 2009Karran Woan Abstract The literature and advances in photocatalysis based on the combination of titania (TiO2) and carbon nanotubes is presented. The semiconductor basis for photocatalysis is introduced for anatase and rutile. Furthermore, the proposed mechanisms of catalytic enhancement resulting from the pairing of the titania semiconductor with either metallic, semiconducting, or defect-rich carbon nanotubes (CNT) is discussed. Differences are apparent for the mixtures and chemically bonded CNT,TiO2 composites. The article then highlights the recent advances in the synthesis techniques for these composites and their photocatalytic reactions with organic, inorganic, and biological agents. Finally, various applications and challenges for these composite materials are reported. [source] High Lithium Electroactivity of Nanometer-Sized Rutile TiO2,ADVANCED MATERIALS, Issue 11 2006Y.-S. Hu Nanometer-sized rutile shows much higher electroactivity towards Li insertion than micrometer-sized rutile. Up to 0.8,mol of Li per mole of TiO2 can be inserted into nanometer-sized rutile at room temperature (see figure), which is able to reversibly accommodate Li up to Li0.5TiO2 with excellent capacity retention and high rate capability on cycling, rendering it a promising anode material for high-power lithium-ion batteries. [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] Provenance of sandstones from the Wakino Subgroup of the Lower Cretaceous Kanmon Group, northern Kyushu, JapanISLAND ARC, Issue 1 2000Daniel K. Asiedu Abstract The Wakino Subgroup is a lower stratigraphic unit of the Lower Cretaceous Kanmon Group. Previous studies on provenance of Wakino sediments have mainly concentrated on either petrography of major framework grains or bulk rock geochemistry of shales. This study addresses the provenance of the Wakino sandstones by integrating the petrographic, bulk rock geochemistry, and mineral chemistry approaches. The proportions of framework grains of the Wakino sandstones suggest derivation from either a single geologically heterogeneous source terrane or multiple source areas. Major source lithologies are granitic rocks and high-grade metamorphic rocks but notable amounts of detritus were also derived from felsic, intermediate and mafic volcanic rocks, older sedimentary rocks, and ophiolitic rocks. The heavy mineral assemblage include, in order of decreasing abundance: opaque minerals (ilmenite and magnetite with minor rutile), zircon, garnet, chromian spinel, aluminum silicate mineral (probably andalusite), rutile, epidote, tourmaline and pyroxene. Zircon morphology suggests its derivation from granitic rocks. Chemistry of chromian spinel indicates that the chromian spinel grains were derived from the ultramafic cumulate member of an ophiolite suite. Garnet and ilmenite chemistry suggests their derivation from metamorphic rocks of the epidote-amphibolite to upper amphibolite facies though other source rocks cannot be discounted entirely. Major and trace element data for the Wakino sediments suggest their derivation from igneous and/or metamorphic rocks of felsic composition. The major element compositions suggest that the type of tectonic environment was of an active continental margin. The trace element data indicate that the sediments were derived from crustal rocks with a minor contribution from mantle-derived rocks. The trace element data further suggest that recycled sedimentary rocks are not major contributors of detritus. It appears that the granitic and metamorphic rocks of the Precambrian Ryongnam Massif in South Korea were the major contributors of detritus to the Wakino basin. A minor but significant amount of detritus was derived from the basement rocks of the Akiyoshi and Sangun Terrane. The chromian spinel appears to have been derived from a missing terrane though the ultramafic rocks in the Ogcheon Belt cannot be discounted. [source] Collagen nanofilm immobilized on at surfaces by electrodeposition methodJOURNAL OF BIOMEDICAL MATERIALS RESEARCH, Issue 2 2009Xiudong Yang Abstract A simple electrodeposition method is presented for the preparing of collagen nanofilms (EAT) on anodic oxidized titanium surfaces (AT). The nanofilms were observed by scanning electron microscopy and atomic force microscopy. Functional TiOx layers with anionic groups of PO4, SO4 and OH were investigated on the AT surface by X-ray photoelectron spectroscopy; X-ray diffraction results indicated that the AT surface was composed mainly of anatase and rutile. The bioactive electrodeposited TiOx layers on the AT surface showed lower water contact angles and higher surface energy than pure titanium surfaces (CT) and displayed higher collagen molecule immobilization. © 2009 Wiley Periodicals, Inc. J Biomed Mater Res Part B: Appl Biomater, 2009 [source] Trace Elemental Analysis of Titanium Dioxide Pigments and Automotive White Paint Fragments for Forensic Examination Using High-Energy Synchrotron Radiation X-Ray Fluorescence Spectrometry,JOURNAL OF FORENSIC SCIENCES, Issue 3 2009Yoshinori Nishiwaki M.S. Abstract:, High-energy synchrotron radiation x-ray fluorescence spectrometry (SR-XRF) utilizing 116 keV x-rays was used to characterize titanium dioxide pigments (rutile) and automotive white paint fragments for forensic examination. The technique allowed analysis of K lines of 9 trace elements in 18 titanium dioxide pigments (rutile), and 10 trace elements in finish coat layers of seven automotive white paint fragments. High-field strength elements (HFSE) were found to strongly reflect the origin of the titanium dioxide (TiO2) pigments, and could be used as effective parameters for discrimination and classification of the pigments and paint fragments. A pairwise comparison of the finish coat layers of seven automotive white paint fragments was performed. The trace elements in the finish coat layers detected by the high-energy SR-XRF were especially effective for identification. By introducing the trace element information of primer and electrocoat layers, all the automotive white paint fragments could be discriminated by this technique. [source] Infrared Spectra of U.S. Automobile Original Finishes.JOURNAL OF FORENSIC SCIENCES, Issue 3 2006ABSTRACT: The identification, analysis, and occurrence in U.S. automobile original finishes (1974,1989) of Nickel Titanate (yellow) and Chrome Titanate (yellow,orange) are described in this report. The titanate pigments are based on the rutile (titanium dioxide) structure and there are only minor differences between the infrared absorptions of rutile and the titanates. Titanate pigment absorptions in paint spectra can thus be easily mistaken for those of rutile. Each of the titanates, however, contains two elements in addition to titanium that can serve to distinguish them using elemental analyses. Fourier transform infrared (4000,220 cm,1) and X-ray fluorescence instruments were used in combination for the in situ analysis of the titanates. In addition to titanium, nickel, and antimony, the three main detectable elements comprising Nickel Titanate, all of the commercial products of this pigment that were examined also contained impurities of zirconium, niobium, and usually lead. These elements were also detected in most of the monocoats in which Nickel Titanate was identified, as well as in the Chrome Titanate pigments, and the zirconium to niobium ratio was found to exhibit a wide variation. Nickel Titanate is a relatively common pigment that was identified in nearly three dozen U.S. automobile yellow nonmetallic monocoats (1974,1989), while Chrome Titanate appears to have been used in only a few yellow and orange nonmetallic monocoats. The use of the titanate pigments likely increased after this time period as they were replacements for lead chromate pigments (last used in a U.S. automobile original finish in the early 1990s), and are more amenable for use in basecoat/clearcoat finishes than in monocoats. Minor distortions of the infrared absorptions of rutile, anatase, and the titanates obtained using accessories with diamond windows were noted, and their origins are discussed. [source] Microfabric characteristics and rheological significance of ultra-high-pressure metamorphosed jadeite-quartzite and eclogite from Shuanghe, Dabie Mountains, ChinaJOURNAL OF METAMORPHIC GEOLOGY, Issue 2 2010L. WANG Abstract Quantitative analysis of the structural evolution of jadeite-quartzite, a rare ultra-high pressure (UHP) rock type from the Dabie Mountains of eastern China, sheds light on the formation and evolution of UHP orogenic belts worldwide. Geological mapping of the Shuanghe area, where jadeite-quartzites crop out, was carried out to determine the spatial relationships between different UHP rocks within this orogen. The deformation mechanisms of jadeite-quartzite, geodynamical parameters (stress, strain, strain rate), and microstructure including lattice preferred orientation (LPO) were determined from six jadeite-quartzite samples from the Shuanghe area. LPOs of clinopyroxene (jadeite and omphacite), garnet, rutile and quartz from these jadeite-quartzite samples are compared with those of three eclogites preserving different degrees of deformation from the Shuanghe area. Microstructural LPOs of jadeite, omphacite, garnet, rutile and quartz were determined using electron backscattered diffraction (EBSD) analysis. Quartz fabrics were largely recrystallized during late, low-grade stages of deformation, whereas garnet shows no strong LPO patterns. Rutile fabrics show a weak LS fabric along [001]. Jadeite and omphacite show the strongest eclogite facies LPO patterns, suggesting that they may provide important information about mantle deformation patterns and control the rheology of deeply subducted continental crust. Microstructural data show that the jadeite LPO patterns are similar to those of omphacite and vary between L- and S-types, which correlate with prolate and oblate grain shape fabrics (SPO); quartz LPOs are monoclinic. Microstructural analysis using TEM shows that the dominant slip systems of jadeite in one sample are (100)[001], (110)[001] and (1 1 0)1/2[110], while in another sample, no dislocations are observed. Abundant dislocations in quartz were accommodated by the dominant slip system (0001)[110], indicating basal glide and represents regional shearing during the exhumation process. This suggests that dislocation creep is the dominant fundamental deformation mechanism in jadeite under UHP conditions. The protoliths of jadeite-quartzite, metasedimentary rocks from the northern passive continental margin of the Yangtze craton, experienced the same deep subduction and were deformed under similar rheological conditions as other UHP eclogite, marble and paragneiss. Experimental UHP deformation of quartzo-feldspathic gneiss with a chemical composition similar to the bulk continental crust has shown that the formation of a jadeite,stishovite rock is associated with a density increase of the host rock similar to the eclogite conversion from basaltic protoliths. The resulting rock can be denser than the surrounding mantle pyrolite up to depths of 660 km (24 GPa). Thus, processes of deep continental subduction may be better-understood through understanding the rheology and mechanical behaviour of jadeite. Jadeite-quartzites such as those from the Shuanghe may be exhumed remnants of deeply-subducted slabs of continental crust, other parts of which subducted past the ,depth of no return', and remain in the deep mantle. [source] Fluid flow and Al transport during quartz-kyanite vein formation, Unst, Shetland Islands, ScotlandJOURNAL OF METAMORPHIC GEOLOGY, Issue 1 2010C. E. BUCHOLZ Abstract Quartz-kyanite veins, adjacent alteration selvages and surrounding ,precursor' wall rocks in the Dalradian Saxa Vord Pelite of Unst in the Shetland Islands (Scotland) were investigated to constrain the geochemical alteration and mobility of Al associated with channelized metamorphic fluid infiltration during the Caledonian Orogeny. Thirty-eight samples of veins, selvages and precursors were collected, examined using the petrographic microscope and electron microprobe, and geochemically analysed. With increasing grade, typical precursor mineral assemblages include, but are not limited to, chlorite+chloritoid, chlorite+chloritoid+kyanite, chlorite+chloritoid+staurolite and garnet+staurolite+kyanite+chloritoid. These assemblages coexist with quartz, white mica (muscovite, paragonite, margarite), and Fe-Ti oxides. The mineral assemblage of the selvages does not change noticeably with metamorphic grade, and consists of chloritoid, kyanite, chlorite, quartz, white mica and Fe-Ti oxides. Pseudosections for selvage and precursor bulk compositions indicate that the observed mineral assemblages were stable at regional metamorphic conditions of 550,600 °C and 0.8,1.1 GPa. A mass balance analysis was performed to assess the nature and magnitude of geochemical alteration that produced the selvages adjacent to the veins. On average, selvages lost about ,26% mass relative to precursors. Mass losses of Na, K, Ca, Rb, Sr, Cs, Ba and volatiles were ,30 to ,60% and resulted from the destruction of white mica. Si was depleted from most selvages and transported locally to adjacent veins; average selvage Si losses were about ,50%. Y and rare earth elements were added due to the growth of monazite in cracks cutting apatite. The mass balance analysis also suggests some addition of Ti occurred, consistent with the presence of rutile and hematite-ilmenite solid solutions in veins. No major losses of Al from selvages were observed, but Al was added in some cases. Consequently, the Al needed to precipitate vein kyanite was not derived locally from the selvages. Veins more than an order of magnitude thicker than those typically observed in the field would be necessary to accommodate the Na and K lost from the selvages during alteration. Therefore, regional transport of Na and K out of the local rock system is inferred. In addition, to account for the observed abundances of kyanite in the veins, large fluid-rock ratios (102,103 m3fluid m,3rock) and time-integrated fluid fluxes in excess of ,104 m3fluid m,2rock are required owing to the small concentrations of Al in aqueous fluids. It is concluded that the quartz-kyanite veins and their selvages were produced by regional-scale advective mass transfer by means of focused fluid flow along a thrust fault zone. The results of this study provide field evidence for considerable Al mass transport at greenschist to amphibolite facies metamorphic conditions, possibly as a result of elevated concentrations of Al in metamorphic fluids due to alkali-Al silicate complexing at high pressures. [source] Palaeoproterozoic high-pressure granulite overprint of the Archean continental crust: evidence for homogeneous crustal thickening (Man Rise, Ivory Coast)JOURNAL OF METAMORPHIC GEOLOGY, Issue 1 2010P. PITRA Abstract The character of mountain building processes in Palaeoproterozoic times is subject to much debate. Based on the discovery of high-pressure granulites in the Man Rise (Côte d'Ivoire), several authors have argued that Eburnean (Palaeoproterozoic) reworking of the Archean basement was achieved by modern-style thrust-dominated tectonics. A mafic granulite of the Kouibli area (Archean part of the Man Rise, western Ivory Coast) displays a primary assemblage (M1) containing garnet, diopsidic clinopyroxene, red-brown pargasitic amphibole, plagioclase (andesine), rutile, ilmenite and quartz. This assemblage is associated with a subvertical regional foliation. Symplectites that developed at the expense of the M1 assemblage contain orthopyroxene, clinopyroxene, plagioclase (bytownite), green pargasitic amphibole, ilmenite and magnetite (M2). Multiequilibrium thermobarometric calculations and P,T pseudosections calculated with thermocalc suggest granulite facies conditions of , 13 kbar, 850 °C and <7 kbar, 700,800 °C for M1 and M2, respectively. In agreement with the qualitative information obtained from reaction textures and chemical zoning of minerals, this suggests an evolution dominated by decompression accompanied by moderate cooling. A Sm,Nd garnet , whole-rock age of 2.03 Ga determined on this sample indicates that this evolution occurred during the Palaeoproterozoic. It is argued that from the geodynamic point of view the observed features are best explained by homogeneous thickening of the margin of the Archean craton, re-heated and softened due to the accretion of hot, juvenile Palaeoproterozoic crust, as well as coeval intrusion of juvenile magmas. Crustal shortening was mainly accommodated by transpressive shear zones and by lateral crustal spreading rather than large-scale thrust systems. [source] Petrology of coesite-bearing eclogite from Habutengsu Valley, western Tianshan, NW China and its tectonometamorphic implicationJOURNAL OF METAMORPHIC GEOLOGY, Issue 9 2009Z. LÜ Abstract Coesite inclusions in garnet have been found in eclogite boudins enclosed in coesite-bearing garnet micaschist in the Habutengsu Valley, Chinese western Tianshan, which are distinguished from their retrograde quartz by means of optical characteristics, CL imaging and Raman spectrum. The coesite-bearing eclogite is mainly composed of porphyroblastic garnet, omphacite, paragonite, glaucophane and barroisite, minor amounts of rutile and dotted (or banded) graphite. In addition to coesite and quartz, the zoned porphyroblastic garnet contains inclusions of omphacite, Na-Ca amphibole, calcite, albite, chlorite, rutile, ilmenite and graphite. Multi-phase inclusions (e.g. Czo + Pg ± Qtz, Grt II + Qtz and Chl + Pg) can be interpreted as breakdown products of former lawsonite and possibly chloritoid. Coesite occurs scattered within a compositionally homogenous but narrow domain of garnet (outer core), indicative of equilibrium at the UHP stage. The estimate by garnet-clinopyroxene thermometry yields peak temperatures of 420,520 °C at 2.7 GPa. Phase equilibrium calculations further constrain the P,T conditions for the UHP mineral assemblage Grt + Omp + Lws + Gln + Coe to 2.4,2.7 GPa and 470,510 °C. Modelled modal abundances of major minerals along a 5 °C km,1 geothermal gradient suggests two critical dehydration processes at ,430 and ,510 °C respectively. Computed garnet composition patterns are in good agreement with measured core-rim profiles. The petrological study of coesite-bearing eclogite in this paper provides insight into the metamorphic evolution in a cold subduction zone. Together with other reported localities of UHP rocks from the entire orogen of Chinese western Tianshan, it is concluded that the regional extent of UHP-LT metamorphism in Chinese western Tianshan is extensive and considerably larger than previously thought, although intensive retrogression has erased UHP-LT assemblages at most localities. [source] Phase-equilibrium constraints on titanite and rutile activities in mafic epidote amphibolites and geobarometry using titanite,rutile equilibriaJOURNAL OF METAMORPHIC GEOLOGY, Issue 7 2009P. KAPP Abstract Activities of titanite (Ttn, CaTiSiO5) and/or rutile (Rt, TiO2) phase components were calculated for 45 well-characterized natural titanite- or rutile-undersaturated epidote,amphibolites by using the equilibria: (i) 3 anorthite + 2 zoisite/clinozoisite + rutile + quartz = 3 anorthite + titanite + water (referred to as TZARS) and (ii) anorthite + 2 titanite = grossular + 2 rutile + quartz (referred to as GRATiS). In titanite-bearing and rutile-absent samples aRt is 0.75 ± 0.26. In titanite-absent, rutile-bearing samples aTtn is 0.89 ± 0.16. Mean values derived for aRt/aTtn are 0.92 ± 0.12 for rutile + titanite-bearing samples and 0.42 ± 0.27 for samples lacking both titanite and rutile. Use of these values with TZARS yields pressure estimates for epidote,amphibolites that differ on average by <0.5 kbar from those recorded by established mineral barometers, even where both titanite and rutile are lacking. Despite rather large uncertainties in the average values obtained for aRt, aTtn or aRt/aTtn, application of TZARS yields pressure estimates that agree with independent estimates to within ±0.5 kbar for titanite- and/or rutile-saturated samples, and to within ±0.8 kbar for samples that contain neither Ti-phase. The accuracy and precision of the TZARS barometer are comparable to that of many well-calibrated barometers. TZARS offers a much-needed barometer for mafic rocks metamorphosed at epidote-bearing amphibolite and blueschist facies conditions. In addition, the results provide a basis for application of other thermobarometers, such as Ti-in-zircon, where rutile activity is required as input. [source] The P,T path of the ultra-high pressure Lago Di Cignana and adjoining high-pressure meta-ophiolitic units: insights into the evolution of the subducting Tethyan slabJOURNAL OF METAMORPHIC GEOLOGY, Issue 3 2009C. GROPPO Abstract The Lago di Cignana ultra-high-pressure unit (LCU), which consists of coesite,eclogite facies metabasics and metasediments, preserves the most deeply subducted oceanic rocks worldwide. New constraints on the prograde and early retrograde evolution of this ultra-high pressure unit and adjoining units provide important insights into the evolution of the Piemontese,Ligurian palaeo-subduction zone, active in Paleocene,Eocene times. In the LCU, a first prograde metamorphic assemblage, consisting of omphacite + Ca-amphibole + epidote + rare biotite + ilmenite, formed during burial at estimated P < 1.7 GPa and 350 < T < 480 °C. Similar metamorphic conditions of 400 < T < 650 °C and 1.0 < P < 1.7 GPa have been estimated for the meta-ophiolitic rocks juxtaposed to the LCU. The prograde assemblage is partially re-equilibrated into the peak assemblage garnet + omphacite + Na-amphibole + lawsonite + coesite + rutile, whose conditions were estimated at 590 < T < 605 °C and P > 3.2 GPa. The prograde path was characterized by a gradual decrease in the thermal gradient from ,9,10 to ,5,6 °C km,1. This variation is interpreted as the evidence of an increase in the rate of subduction of the Piemonte,Ligurian oceanic slab in the Eocene. Accretion of the Piemontese oceanic rocks to the Alpine orogen and thermal relaxation were probably related to the arrival of more buoyant continental crust at the subduction zone. Subsequent deformation of the orogenic wedge is responsible for the present position of the LCU, sandwiched between two tectonic slices of meta-ophiolites, named the Lower and Upper Units, which experienced peak pressures of 2.7,2.8 and <2.4 GPa respectively. [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] Refining the P,T records of UHT crustal metamorphismJOURNAL OF METAMORPHIC GEOLOGY, Issue 2 2008S. L. HARLEY Abstract Ultra-high-temperature (UHT) metamorphism occurs when the continental crust is subjected to temperatures of greater than 900 °C at depths of 20,40 km. UHT metamorphism provides evidence that major tectonic processes may operate under thermal conditions more extreme than those generally produced in numerical models of orogenesis. Evidence for UHT metamorphism is recorded in mineral assemblages formed in magnesian pelites, supported by high-temperature indicators including mesoperthitic feldspar, aluminous orthopyroxene and high Zr contents in rutile. Recent theoretical, experimental and thermodynamic data set constraints on metamorphic phase equilibria in FMAS, KFMASH and more complex chemical systems have greatly improved quantification of the P,T conditions and paths of UHT metamorphic belts. However, despite these advances key issues that remain to be addressed include improving experimental constraints on the thermodynamic properties of sapphirine, quantifying the effects of oxidation state on sapphirine, orthopyroxene and spinel stabilities and quantifying the effects of H2O,CO2 in cordierite on phase equilibria and reaction texture analysis. These areas of uncertainty mean that UHT mineral assemblages must still be examined using theoretical and semi-quantitative approaches, such as P(,T),, sections, and conventional thermobarometry in concert with calculated phase equilibrium methods. In the cases of UHT terranes that preserve microtextural and mineral assemblage evidence for steep or ,near-isothermal' decompression P,T paths, the presence of H2O and CO2 in cordierite is critical to estimates of the P,T path slopes, the pressures at which reaction textures have formed and the impact of fluid infiltration. Many UHT terranes have evolved from peak P,T conditions of 8,11 kbar and 900,1030 °C to lower pressure conditions of 8 to 6 kbar whilst still at temperature in the range of 950 to 800 °C. These decompressional P,T paths, with characteristic dP/dT gradients of ,25 ± 10 bar °C,1, are similar in broad shape to those generated in deep-crustal channel flow models for the later stages of orogenic collapse, but lie at significantly higher temperatures for any specified pressure. This thermal gap presents a key challenge in the tectonic modelling of UHT metamorphism, with implications for the evolution of the crust, sub-crustal lithosphere and asthenospheric mantle during the development of hot orogens. [source] Ultrahigh-pressure eclogite transformed from mafic granulite in the Dabie orogen, east-central ChinaJOURNAL OF METAMORPHIC GEOLOGY, Issue 9 2007Y.-C. LIU Abstract Although ultrahigh-pressure (UHP) metamorphic rocks are present in many collisional orogenic belts, almost all exposed UHP metamorphic rocks are subducted upper or felsic lower continental crust with minor mafic boudins. Eclogites formed by subduction of mafic lower continental crust have not been identified yet. Here an eclogite occurrence that formed during subduction of the mafic lower continental crust in the Dabie orogen, east-central China is reported. At least four generations of metamorphic mineral assemblages can be discerned: (i) hypersthene + plagioclase ± garnet; (ii) omphacite + garnet + rutile + quartz; (iii) symplectite stage of garnet + diopside + hypersthene + ilmenite + plagioclase; (iv) amphibole + plagioclase + magnetite, which correspond to four metamorphic stages: (a) an early granulite facies, (b) eclogite facies, (c) retrograde metamorphism of high-pressure granulite facies and (d) retrograde metamorphism of amphibolite facies. Mineral inclusion assemblages and cathodoluminescence images show that zircon is characterized by distinctive domains of core and a thin overgrowth rim. The zircon core domains are classified into two types: the first is igneous with clear oscillatory zonation ± apatite and quartz inclusions; and the second is metamorphic containing a granulite facies mineral assemblage of garnet, hypersthene and plagioclase (andesine). The zircon rims contain garnet, omphacite and rutile inclusions, indicating a metamorphic overgrowth at eclogite facies. The almost identical ages of the two types of core domains (magmatic = 791 ± 9 Ma and granulite facies metamorphic zircon = 794 ± 10 Ma), and the Triassic age (212 ± 10 Ma) of eclogitic facies metamorphic overgrowth zircon rim are interpreted as indicating that the protolith of the eclogite is mafic granulite that originated from underplating of mantle-derived magma onto the base of continental crust during the Neoproterozoic (c. 800 Ma) and then subducted during the Triassic, experiencing UHP eclogite facies metamorphism at mantle depths. The new finding has two-fold significance: (i) voluminous mafic lower continental crust can increase the average density of subducted continental lithosphere, thus promoting its deep subduction; (ii) because of the current absence of mafic lower continental crust in the Dabie orogen, delamination or recycling of subducted mafic lower continental crust can be inferred as the geochemical cause for the mantle heterogeneity and the unusually evolved crustal composition. [source] Experimental Investigation of Eclogite Rheology and Its Fabrics at High Temperature and PressureJOURNAL OF METAMORPHIC GEOLOGY, Issue 2 2007J. ZHANG Abstract Eclogite plays an important role in mantle convection and geodynamics in subduction zones. An improved understanding of processes in the deeper levels of subduction zones and collision belts requires information on eclogite rheology. However, the deformation processes and associated fabrics in eclogite are not well understood. Incompatible views of deformation mechanism have been proposed for both garnet and omphacite. We present here deformation behaviour of eclogite at temperatures of 1027,1427 °C, confining pressures of 2.5,3.5 GPa, and strain rates of 1 × 10,5 s,1 to 5 × 10,4 s,1. We obtained a power-law creep for the high temperature and pressure deformation of a ,dry' eclogite (50 vol.% garnet, 40% omphacite and 10% quartz) with A = 103.3 ± 1.0, n = 3.5 ± 0.4, ,E =403 ± 30 KJ mol,1 and ,V = 27.2 cm3 mol,1. The two principal minerals of eclogite have greatly different strengths. Progressive increase of garnet results in a smooth increase in strength. Analysis by electron back-scattered diffraction shows that: (1) garnet displays pole figures with near random distributions of misorientation angle under both dry and wet conditions; (2) omphacite shows pronounced lattice preferred orientations (LPOs), suggesting a dominant dislocation creep mechanism. Further investigation into the water effects on eclogite show: (3) water content does not influence the style of omphacite fabric but increases slightly the fabric strength; (4) grain boundary processes dominate the deformation of garnet under high water fugacity or high shear-strain conditions, yielding a random LPO similar to that of non-deforming garnet, despite the strong shape preferred orientation (SPO) observed. {110} [001] slip may dominate the deformation of rutile. Quartz displays complicated and inconsistent LPOs in eclogite. These results are remarkably similar to observations from deformed eclogites in nature. [source] Petrogenesis of lawsonite and epidote eclogite and blueschist, Sivrihisar Massif, TurkeyJOURNAL OF METAMORPHIC GEOLOGY, Issue 9 2006P. B. DAVIS Abstract The Sivrihisar Massif, Turkey, is comprised of blueschist and eclogite facies metasedimentary and metabasaltic rocks. Abundant metre- to centimetre-scale eclogite pods occur in blueschist facies metabasalt, marble and quartz-rich rocks. Sivrihisar eclogite contains omphacite + garnet + phengite + rutile ± glaucophane ± quartz + lawsonite and/or epidote. Blueschists contain sodic amphibole + garnet + phengite + lawsonite and/or epidote ± omphacite ± quartz. Sivrihisar eclogite and blueschist have similar bulk composition, equivalent to NMORB, but record different P,T conditions: ,26 kbar, 500 °C (lawsonite eclogite); 18 kbar, 600 °C (epidote eclogite); 12 kbar, 380 °C (lawsonite blueschist); and 15,16 kbar, 480,500 °C (lawsonite-epidote blueschist). Pressures for the Sivrihisar lawsonite eclogite are among the highest reported for this rock type, which is rarely exposed at the Earth's surface. The distribution and textures of lawsonite ± epidote define P,T conditions and paths. For example, in some lawsonite-bearing rocks, epidote inclusions in garnet and partial replacement of matrix epidote by lawsonite suggest an anticlockwise P,T path. Other rocks contain no epidote as inclusions or as a matrix phase, and were metamorphosed entirely within the lawsonite stability field. Results of the P,T study and mapping of the distribution of blueschists and eclogites in the massif suggest that rocks recording different maximum P,T conditions were tectonically juxtaposed as kilometre-scale slices and associated high- P pods, although all shared the same exhumation path from ,9,11 kbar, 300,400 °C. Within the tectonic slices, alternating millimetre,centimetre-scale layers of eclogite and blueschist formed together at the same P,T conditions but represent different extents of prograde reaction controlled by strain partitioning or local variations in fO2 or other chemical factors. [source] Separate or shared metamorphic histories of eclogites and surrounding rocks?JOURNAL OF METAMORPHIC GEOLOGY, Issue 3 2006An example from the Bohemian Massif Abstract Eclogite boudins occur within an orthogneiss sheet enclosed in a Barrovian metapelite-dominated volcano-sedimentary sequence within the Velké Vrbno unit, NE Bohemian Massif. A metamorphic and lithological break defines the base of the eclogite-bearing orthogneiss nappe, with a structurally lower sequence without eclogite exposed in a tectonic window. The typical assemblage of the structurally upper metapelites is garnet,staurolite,kyanite,biotite,plagioclase,muscovite,quartz,ilmenite ± rutile ± silli-manite and prograde-zoned garnet includes chloritoid,chlorite,paragonite,margarite, staurolite,chlorite,paragonite,margarite and kyanite,chlorite,rutile. In pseudosection modelling in the system Na2O,CaO,K2O,FeO,MgO,Al2O3,SiO2,H2O (NCKFMASH) using THERMOCALC, the prograde path crosses the discontinuous reaction chloritoid + margarite = chlorite + garnet + staurolite,+,paragonite (with muscovite + quartz + H2O) at 9.5 kbar and 570 °C and the metamorphic peak is reached at 11 kbar and 640 °C. Decompression through about 7 kbar is indicated by sillimanite and biotite growing at the expense of garnet. In the tectonic window, the structurally lower metapelites (garnet,staurolite,biotite,muscovite,quartz ± plagioclase ± sillimanite ± kyanite) and amphibolites (garnet,amphibole,plagioclase ± epidote) indicate a metamorphic peak of 10 kbar at 620 °C and 11 kbar and 610,660 °C, respectively, that is consistent with the other metapelites. The eclogites are composed of garnet, omphacite relicts (jadeite = 33%) within plagioclase,clinopyroxene symplectites, epidote and late amphibole,plagioclase domains. Garnet commonly includes rutile,quartz,epidote ± clinopyroxene (jadeite = 43%) ± magnetite ± amphibole and its growth zoning is compatible in the pseudosection with burial under H2O-undersaturated conditions to 18 kbar and 680 °C. Plagioclase + amphibole replaces garnet within foliated boudin margins and results in the assemblage epidote,amphibole,plagioclase indicating that decompression occurred under decreasing temperature into garnet-free epidote,amphibolite facies conditions. The prograde path of eclogites and metapelites up to the metamorphic peak cannot be shared, being along different geothermal gradients, of about 11 and 17 °C km,1, respectively, to metamorphic pressure peaks that are 6,7 kbar apart. The eclogite,orthogneiss sheet docked with metapelites at about 11 kbar and 650 °C, and from this depth the exhumation of the pile is shared. [source] Calcite,graphite isotope thermometry in amphibolite facies marble, Bancroft, OntarioJOURNAL OF METAMORPHIC GEOLOGY, Issue 9 2005S. R. DUNN Abstract This study presents calcite,graphite carbon isotope fractionations for 32 samples from marble in the northern Elzevir terrane of the Central Metasedimentary Belt, Grenville Province, southern Ontario, Canada. These results are compared with temperatures calculated by calcite,dolomite thermometry (15 samples), garnet,biotite thermometry (four samples) and garnet,hornblende thermometry (three samples). ,cal-gr values vary regularly across the area from >6.5, in the south to 4.0, in the north, which corresponds to temperatures of 525 °C in the south to 650 °C in the north. Previous empirical calibration of the calcite,graphite thermometer agrees very well with calcite,dolomite, garnet,biotite and garnet,hornblende thermometry, whereas, theoretical calibrations compare less well with the independent thermometry. Isograds in marble based on the reactions rutile + calcite + quartz =titanite and tremolite + calcite + quartz = diopside, span temperatures of 525,600 °C and are consistent with calculated temperature,X(CO2) relations. Results of this study compare favourably with large-scale regional isotherms, however, local variation is greater than that revealed by large-scale sampling strategies. It remains unclear whether the temperature,,cal-gr relationship observed in natural materials below 650 °C represents equilibrium fractionations or not, but the regularity and consistency apparent in this study demonstrate its utility for thermometry in amphibolite facies marble. [source] Pro- and retrograde P,T evolution of granulites of the Beit Bridge Complex (Limpopo Belt, South Africa): constraints from quantitative phase diagrams and geotectonic implicationsJOURNAL OF METAMORPHIC GEOLOGY, Issue 2 2004A. Zeh Abstract Interpretations based on quantitative phase diagrams in the system CaO,Na2O,K2O,TiO2,MnO,FeO,MgO,Al2O3,SiO2,H2O indicate that mineral assemblages, zonations and microstructures observed in migmatitic rocks from the Beit Bridge Complex (Messina area, Limpopo Belt) formed along a clockwise P,T path. That path displays a prograde P,T increase from 600 °C/7.0 kbar to 780 °C/9,10 kbar (pressure peak) and 820 °C/8 kbar (thermal peak), followed by a P,T decrease to 600 °C/4 kbar. The data used to construct the P,T path were derived from three samples of migmatitic gneiss from a restricted area, each of which has a distinct bulk composition: (1) a K, Al-rich garnet,biotite,cordierite,sillimanite,K-feldspar,plagioclase,quartz,graphite gneiss (2) a K-poor, Al-rich garnet,biotite,staurolite,cordierite,kyanite,sillimanite,plagioclase,quartz,rutile gneiss, and (3) a K, Al-poor, Fe-rich garnet,orthopyroxene,biotite,chlorite,plagioclase,quartz,rutile,ilmenite gneiss. Preservation of continuous prograde garnet growth zonation demonstrates that the pro- and retrograde P,T evolution of the gneisses must have been rapid, occurring during a single orogenic cycle. These petrological findings in combination with existing geochronological and structural data show that granulite facies metamorphism of the Beit Bridge metasedimentary rocks resulted from an orogenic event during the Palaeoproterozoic (c. 2.0 Ga), caused by oblique collision between the Kaapvaal and Zimbabwe Cratons. Abbreviations follow Kretz (1983). [source] Crystallization environment of Kazakhstan microdiamond: evidence from nanometric inclusions and mineral associationsJOURNAL OF METAMORPHIC GEOLOGY, Issue 5 2003L. F. Dobrzhinetskaya Abstract Nanometric solid inclusions in diamond incorporated in garnet and zircon from felsic gneiss of the Kokchetav massif, Kazakhstan, have been examined utilizing electron microscopy and focused ion beam techniques. Host garnet and zircon contain numerous pockets of multiple inclusions, which consist of 1,3 diamond crystals intergrown with quartz, phengite, phlogopite, albite, K-feldspar, rutile, apatite, titanite, biotite, chlorite and graphite in various combinations. Recalculation of the average chemical composition of the entrapped fluid represented by multiple inclusion pockets indicates that such fluid contained a low wt% of SiO2, suggesting a relatively low-temperature fluid rather than a melt. Transmission electron microscopy revealed that the diamond contains abundant nanocrystalline inclusions of oxides, rare carbonates and silicates. Within the 15 diamond crystals studied, abundant inclusions were found of SiO2, TiO2, FexOy, Cr2O3, ZrSiO4, and single grains of ThxOy, BaSO4, MgCO3, FeCr2O4 and a stoichiometric Fe-rich pyroxene. The diversity of trace elements within inclusions of essentially the same stoichiometry suggests that the Kokchetav diamond crystallized from a fluid containing variable amounts of Si, Fe, Ti, Cr, Zr, Ba, Mg and Th and other minor components such as K, Na, P, S, Pb, Zn, Nb, Al, Ca, Cl. Most of the components in crystals included in diamond appear to have their origin in the subducted metasediments, but some of them probably originate from the mantle. It is concluded that Kokchetav diamond most likely crystallized from a COH-rich multicomponent supercritical fluid at a relatively low temperature (hence the apparently low content of rock-forming elements), and that the diversity of major and minor components suggests interactions between subducted metasediments and mantle components. [source] Eclogites from the south Tianshan, NW China: petrological characteristic and calculated mineral equilibria in the Na2O,CaO,FeO,MgO,Al2O3,SiO2,H2O systemJOURNAL OF METAMORPHIC GEOLOGY, Issue 2 2003C. J. Wei Abstract Eclogites from the south Tianshan, NW China are grouped into two types: glaucophane and hornblende eclogites, composed, respectively, of garnet + omphacite + glaucophane + paragonite + epidote + quartz and garnet + omphacite + hornblende (sensu lato) + paragonite + epidote + quartz, plus accessory rutile and ilmenite. These eclogites are diverse both in mineral composition and texture not only between the two types but also among the different selected samples within the glaucophane eclogite. Using thermocalc 3.1 and recent models of activity,composition relation for minerals, a P,T projection and a series of P,T pseudosections for specific samples of eclogite have been calculated in the system Na2O,CaO,FeO,MgO,Al2O3,SiO2,H2O (NCFMASH) with quartz and water taken to be in excess. On the basis of these phase diagrams, the phase relations and P,T conditions are well delineated. The three selected samples of glaucophane eclogite AK05, AK11 and AK17 are estimated to have peak P,T conditions, respectively, of 540,550 °C at c. 16 kbar, c. 560 °C at 15,17 kbar and c. 580 °C at 15,19 kbar, and two samples of hornblende eclogite AK10 and AK30 of 610,630 °C and 17,18 kbar. Together with H2O-content contours in the related P,T pseudosections and textural relations, both types of eclogite are inferred to show clockwise P,T paths, with the hornblende eclogite being transformed from the glaucophane eclogite assemblage dominantly through increasing temperature. [source] P,T evolution of glaucophane,omphacite bearing HP,LT rocks in the western Tianshan Orogen, NW China:new evidence for ,Alpine-type' tectonicsJOURNAL OF METAMORPHIC GEOLOGY, Issue 2 2002R. Klemd Abstract The late Palaeozoic western Tianshan high-pressure /low-temperature belt extends for about 200 km along the south-central Tianshan suture zone and is composed mainly of blueschist, eclogite and epidote amphibolite/greenschist facies rocks. P,T conditions of mafic garnet omphacite and garnet,omphacite blueschist, which are interlayered with eclogite, were investigated in order to establish an exhumation path for these high-pressure rocks. Maximum pressure conditions are represented by the assemblage garnet,omphacite,paragonite,phengite,glaucophane,quartz,rutile. Estimated maximum pressures range between 18 and 21 kbar at temperatures between 490 and 570 °C. Decompression caused the destabilization of omphacite, garnet and glaucophane to albite, Ca-amphibole and chlorite. The post-eclogite facies metamorphic conditions between 9 and 14 kbar at 480,570 °C suggest an almost isothermal decompression from eclogite to epidote,amphibolite facies conditions. Prograde growth zoning and mineral inclusions in garnet as well as post-eclogite facies conditions are evidence for a clockwise P,T path. Analysis of phase diagrams constrains the P,T path to more or less isothermal cooling which is well corroborated by the results of geothermobarometry and mineral textures. This implies that the high-pressure rocks from the western Tianshan Orogen formed in a tectonic regime similar to ,Alpine-type' tectonics. This contradicts previous models which favour ,Franciscan-type' tectonics for the southern Tianshan high-pressure rocks. [source] | ||||||||||||||||||||||