members The sandstones of the Ridge and Athleta of Chari Formation （Callovian-Oxfordian） exposed at Jara have been analyzed for their petrographical and geochemical studies. Texturally, these sandstones are medium t...members The sandstones of the Ridge and Athleta of Chari Formation （Callovian-Oxfordian） exposed at Jara have been analyzed for their petrographical and geochemical studies. Texturally, these sandstones are medium to coarse grained, poorly to well sorted, sub-an- gular to sub-rounded, and show low to medium sphericity. These sandstones were derived from a mixed provenance including granites, granite-gneisses, low and high-grade metamorphic, and some basic rocks of Aravalli range and Nagarparkar massif. The petrofacies analysis reveals that these sandstones belong to the continental block and recycled orogen tectonic regime. The studied sandstones are modified by paleoclimate, distance of transport, and diagenesis. Mineralogically and geochemically, sandstones are classified as quartzarenite, subarkose, arkose, sublithic arenite, and wacke, respectively. The A-CN-K ternary plot and CIA, CIW, PIA, and ICV values suggest that the similar source rocks suffered moderate to high chemical weathering under a hot-humid climate in an acidic envi- ronment with higher Pco2. Generally good to strong cor- relations between AI2O3 and other oxides in these sediments indicate clay mineral control. The K2O/Na2O versus SiO2 diagram indicates that the studied samples occupy passive margin fields but the SiO2/A12O3 versus K2O/Na20 plot suggests that the Athleta Sandstone and Ridge Sandstone fall within the passive margin field, while Ridge Shale falls within the active continental margin field.展开更多
Eclogites and omphacite-bearing blueschists have been newly found in the eastern segment of the southwest Tianshan orogenic belt, Xinjiang, northwest China. After detailed petrological study, three samples including o...Eclogites and omphacite-bearing blueschists have been newly found in the eastern segment of the southwest Tianshan orogenic belt, Xinjiang, northwest China. After detailed petrological study, three samples including one fresh eclogite TK003, one blueschist sample TK026-8 and one retrograded eclogite TK027, were selected for phase equilibrium modeling under NC（K）MnFMASHO （N20-CaO-K20-MnO-FeO-MgO-A1203-SiO2-H：O-O） system, by thermocalc 3.33 software. Composition analyses of garnets in these three samples show typical growth zoning with Xpy and Xgrs increasing, Xspss decreasing from core to rim. Pseudosection modeling of the garnet zonation reflects that the eclogites and blueschist experienced a similar P-T evo- lution trajectory, with a near iso-baric heating in the early stage, and reached eclogite facies metamorphic field with peak P-T regime of 480-515~C, 2.00-2.30 GPa. Subsequently the rocks experienced an early iso-thermal decompression retrograde stage with P-T conditions of 515-519~C, 1.78-1.93 GPa. Variations of mineralogy and modes of these rocks are probably due to different retrograde paths as a consequence of different bulk-rock composition, as well as a variation in fluid activity during exhumation. P-T calculation and a peak geothermal gradient of 6-7~C/km indicate HP rocks in the Kekesu Valley experienced cold subducted eclogite facies metamorphism. Thus a huge oceanic subduction eclogite facies metamorphic belt in southwest Tianshan has been recognized, extending from the Kekesu Valley in the east to the Muzhaerte Valley in the west for nearly 200 kin. However, UHP evidence has not been found in the Kekesu terrane, perhaps because the slab in east part of southwest Tianshan did not subduct into such a great depth.展开更多
文摘members The sandstones of the Ridge and Athleta of Chari Formation （Callovian-Oxfordian） exposed at Jara have been analyzed for their petrographical and geochemical studies. Texturally, these sandstones are medium to coarse grained, poorly to well sorted, sub-an- gular to sub-rounded, and show low to medium sphericity. These sandstones were derived from a mixed provenance including granites, granite-gneisses, low and high-grade metamorphic, and some basic rocks of Aravalli range and Nagarparkar massif. The petrofacies analysis reveals that these sandstones belong to the continental block and recycled orogen tectonic regime. The studied sandstones are modified by paleoclimate, distance of transport, and diagenesis. Mineralogically and geochemically, sandstones are classified as quartzarenite, subarkose, arkose, sublithic arenite, and wacke, respectively. The A-CN-K ternary plot and CIA, CIW, PIA, and ICV values suggest that the similar source rocks suffered moderate to high chemical weathering under a hot-humid climate in an acidic envi- ronment with higher Pco2. Generally good to strong cor- relations between AI2O3 and other oxides in these sediments indicate clay mineral control. The K2O/Na2O versus SiO2 diagram indicates that the studied samples occupy passive margin fields but the SiO2/A12O3 versus K2O/Na20 plot suggests that the Athleta Sandstone and Ridge Sandstone fall within the passive margin field, while Ridge Shale falls within the active continental margin field.
基金supported by the Major State Basic Research Development Program(Grant No.2009CB825007)the National Natural Science Foundation of China(Grant Nos.41121062&41272069)
文摘Eclogites and omphacite-bearing blueschists have been newly found in the eastern segment of the southwest Tianshan orogenic belt, Xinjiang, northwest China. After detailed petrological study, three samples including one fresh eclogite TK003, one blueschist sample TK026-8 and one retrograded eclogite TK027, were selected for phase equilibrium modeling under NC（K）MnFMASHO （N20-CaO-K20-MnO-FeO-MgO-A1203-SiO2-H：O-O） system, by thermocalc 3.33 software. Composition analyses of garnets in these three samples show typical growth zoning with Xpy and Xgrs increasing, Xspss decreasing from core to rim. Pseudosection modeling of the garnet zonation reflects that the eclogites and blueschist experienced a similar P-T evo- lution trajectory, with a near iso-baric heating in the early stage, and reached eclogite facies metamorphic field with peak P-T regime of 480-515~C, 2.00-2.30 GPa. Subsequently the rocks experienced an early iso-thermal decompression retrograde stage with P-T conditions of 515-519~C, 1.78-1.93 GPa. Variations of mineralogy and modes of these rocks are probably due to different retrograde paths as a consequence of different bulk-rock composition, as well as a variation in fluid activity during exhumation. P-T calculation and a peak geothermal gradient of 6-7~C/km indicate HP rocks in the Kekesu Valley experienced cold subducted eclogite facies metamorphism. Thus a huge oceanic subduction eclogite facies metamorphic belt in southwest Tianshan has been recognized, extending from the Kekesu Valley in the east to the Muzhaerte Valley in the west for nearly 200 kin. However, UHP evidence has not been found in the Kekesu terrane, perhaps because the slab in east part of southwest Tianshan did not subduct into such a great depth.
