Recently, new geochronological data and geological observations including the discovery of garnet amphibolite, Ordovician micro-fossils, unconformable contact between the ophiolitic melange related flysch and the Devonian to Early Carboniferous terrestrial volcano-sedimentary strata provide critical evidences for Paleo-environmental reconstruction in the western Junggar (Xinjiang, China). Two different geological layers could be clearly identified: the Early Paleozoic metamorphic terrane mainly consists of ophiolitic melange and flysch, and the Late Paleozoic volcano-sedimentary units lying on the ophiolitic melange and flysch. Both of these layers were intruded by granitic to dioritic magma during the Late Carboniferous to Early Permian period. Based on these data, a new geological map for the western Junggar has been compiled, which is essential not only for better understanding of the geological evolution but also for exploration of mineral resources.
New petrological and geochemical data for lherzolite, harzburgite, and gabbros in the Darbut ophiolitic mélange of west Junggar are combined to constrain the geological evolution of the Darbut ophiolite. Lherzolite, consisting of olivine, orthopyroxene, clinopyroxene, and chrome–spinel with low Cr# values (34–39), is analogous to fertile abyssal peridotite. Harzburgite, composed of olivine, orthopyroxene, clinopyroxene, and chrome–spinel with relatively high Cr# values (48–55), is similar to the supra–subduction zone (SSZ) peridotite. Isotropic gabbro, characterized by a flat rare earth element (REE) pattern as well as low Nb/Yb and high Ti/V ratios, is comparable to mid–ocean ridge basalt (MORB). Hornblende gabbro, displaying relative enrichments of fluid-soluble elements and elevated Th/Yb ratios, is similar to that of fore–arc basalt. Geochemical modelling of partial melting suggests that lherzolite samples are compatible with their formation after relatively low-degree (11–16%), anhydrous dynamic melting of the primitive mantle, while harzburgite samples have undergone 5–10% secondary-stage partial melting based on the already 16% depleted primitive mantle. These data suggest that the Darbut ophiolite was generated in a forearc setting. The upwelling asthenosphere triggered by the subduction initiation of the Junggar oceanic lithosphere led to low-degree, anhydrous decompression melting, producing lherzolite as well as the MORB–like melts at Late Silurian period. Increasing slab–derived fluids influx, accompanied by the progressively sinking slab, largely enhanced the partial melting degrees of the depleted mantle, and formed refractory harzburgite.
Research Article| October 01, 2002 Carbon recycled into deep Earth: Evidence from dolomite dissociation in subduction-zone rocks Yongfeng Zhu; Yongfeng Zhu 1Institute of Geochemistry, School of Earth and Space Sciences, Peking University, Beijing 100871, China Search for other works by this author on: GSW Google Scholar Yoshihide Ogasawara Yoshihide Ogasawara 2Department of Earth Sciences, Waseda University, Tokyo 169-8050, Japan Search for other works by this author on: GSW Google Scholar Geology (2002) 30 (10): 947–950. https://doi.org/10.1130/0091-7613(2002)030<0947:CRIDEE>2.0.CO;2 Article history received: 14 Feb 2002 rev-recd: 06 Jun 2002 accepted: 17 Jun 2002 first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share MailTo Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation Yongfeng Zhu, Yoshihide Ogasawara; Carbon recycled into deep Earth: Evidence from dolomite dissociation in subduction-zone rocks. Geology 2002;; 30 (10): 947–950. doi: https://doi.org/10.1130/0091-7613(2002)030<0947:CRIDEE>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract The dolomite-dissociation textures documented here in rocks from the Kokchetav ultrahigh-pressure massif suggest that the experimentally expected dolomite dissociation happened in the subducted slabs represented by these rocks. Two reactions, magnesite = C + MgO + O2, and majoritic garnet + MgO + H2O = garnet + clinochlore, recorded in carbonate inclusions and the host majoritic garnet are responsible for generation of graphite and clinochlore during the exhumation. The dolomite dissociation indicates that carbonate materials were subducted to depths of >250 km below Earth's surface. Such deep subduction evidently brings abundant carbon and carbonate into deep Earth. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
Abstract We report two newly identified Ordovician ophiolite belts in west Junggar, NW China: Tajin–Tarbahatai–Kujibai–Honguleleng (TTKH) and Tangbale–Baijiantan–Baikouquan (TBB) ophiolitic belts. These two ophiolitic belts provide constraints for the Palaeozoic reconstruction of Central Asia and the geological evolution of this region. The TTKH and TBB ophiolitic belts are dismembered parts of different ophiolitic belts which represent relics of Ordovician oceanic floor; they subducted to the north under the Chingiz–Tarbahatai arc and to the south under the Junggar plate, respectively. The Baijiantan–Baikouquan ophiolite mélanges comprise the major part of the TBB. Flat rare Earth element (REE) patterns with positive Eu anomalies and insignificant depletion of high-field-strength elements (HFSE) relative to melts of primitive mantle suggest a mid-ocean-ridge basalt (MORB) origin for the metagabbro. Lherzolite samples define a Sm–Nd isotopic isochron with age of 474 Ma and ɛ Nd( t ) of +8.9. Lherzolite samples with positive ɛ Nd( t ) values of +8.8 to +9.1 and initial 87 Sr/ 86 Sr ratios of 0.7037–0.7040 are rather homogeneous in Sr–Nd isotopic composition, whereas metagabbro samples show wider Sr–Nd isotopic compositional ranges with ɛ Nd( t ) of +5.9 to +11.0. The Sm–Nd isotopic isochron age ( c. 380 Ma) for garnet amphibolite samples, consistent with a zircon U–Pb age ( c. 385 Ma) for metagabbro, represents a magmatic event prior to subduction. Thermodynamic calculations for garnet amphibolite yield a clockwise pressure–temperature path with peak metamorphic condition of c. 15 kbar and 520–560°C at 342 Ma, indicating a subduction-channel setting. The Rb–Sr isochron ages (335 Ma, 333 Ma) for metagabbro represent a metamorphic event during exhumation.
Hydrothermal gold deposits commonly form during protracted multistage ore-forming processes, however, discriminating among different ore-forming fluids and tracing their origin to unique or different repositories is a challenging task. The Huilvshan gold deposit in West Junggar (Xinjiang province, NW China), was affected by three hydrothermal stages and thus provides an opportunity to examine complex ore-forming processes. The deposit consists of gold-bearing quartz-sulfide veins and disseminated sulfides, hosted within Early Carboniferous basalts and tuffs. Three stages of hydrothermal pyrite (Py1, Py2, and Py3) were identified. Gold is only present in Py2 where it occurs as native gold inclusion and in invisible form. Anhedral Py1, disseminated in the altered basalt, is characterized by higher Ni (13.7 to 635 ppm) concentrations than Py2 and Py3. Euhedral-subhedral Py2 is the richest in Au (1.46 to 25.7 ppm) among three stages of pyrite. Py3 is subdivided into grains with zonal texture (Py3a) and irregular grains (Py3b). Both sub-types have high Sb (56.8 to 1599 ppm) and Tl (0.02 to 68.7 ppm) concentrations. In situ δ34S values of Py1 (-6.0 to 2.7 ‰) are similar to those of Py2 (-5.9 to 3.9 ‰), whereas Py3 has extremely negative δ34S values (-44.3 to-18.0 ‰). Ar-Ar ages of hydrothermal muscovite coexisting with Py2 indicate that the Huilvshan gold deposit formed at ∼300 Ma. The sulfur isotopic systematics of Py1 and Py2, close spatial and temporal relationship between the Huilvshan gold deposit and adjacent felsic intrusions, and paucity of regional metamorphic rocks in the region all suggest that hydrothermal fluids in stage I (Py1) and II (Py2) were derived from magmatism. On the other hand, stage III pyrite and associated sulfides more likely precipitated from a hydrothermal fluid that circulated in tuffs, based on the extremely negative δ34S values of Py3, similar to those measured from framboidal pyrite in the host tuffs. Therefore, textural, trace elemental and sulfur isotope data from pyrite suggest different sources for the hydrothermal fluids in the Huilvshan gold deposit, and that magmatic activity may have contributed the most to the main ore-forming stage. The genesis of this Huilvshan gold deposit could provide significant insights into the origin of other hydrothermal gold deposits that show multiple sulfide generations.
Gold deposits in the Taihang Mountains, northern China, mainly consist of quartz sulfide veins in granitoid plutons. This paper describes the geological setting of the gold deposits, and presents the results of microthermometric, Fourier transform infrared spectra, and stable isotope analyses of ore—forming fluids for the purpose of examining the characteristics of these fluids. The ore—forming fluid was of high temperature (up to 380°C) and high salinity (33–41 wt% NaCl equiv.), represented by type I inclusions (with daughter minerals). This fluid evolved to low salinity at low temperatures recorded in type II (liquid-rich) and III inclusions (vapor—rich). Primary type II inclusions coexist with type III inclusions in quartz. Type III inclusions have almost the same homogenization temperatures as type II inclusions. This probably reflects boiling. The secondary fluid inclusions homogenized at lower temperatures, and have lower salinities than primary inclusions. Based on microthermometric data, we propose that the high—temperature fluid that separated from residual magma corresponded to the ore—forming fluid represented by type I inclusions. This fluid mixed with meteoric water in the upper part of the granitic pluton and was diluted. The diluted fluid boiled, probably due to abrupt pressure decrease, and formed liquid—rich type II inclusions and vapor—rich type III inclusions. The deposition of sulfide minerals and gold probably occurred during boiling.
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