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Liang F.,Chinese Academy of Geological Sciences | Xu Z.,Chinese Academy of Geological Sciences | Zhao J.,Chinese Academy of Geological Sciences | Zhao J.,China University osciences
Acta Geologica Sinica | Year: 2014

We report the discovery of an in-situ natural moissanite as an inclusion in the Cr-spinel from the dunite envelope of a chromitite deposit in Luobusa ophiolite, Tibet. The moissanite occurs as a twin crystal interpenetrated by two quadrilateral signal crystals with sizes of 17 μm × 10 μm and 20 μm × 7 μm, respectively. The moissanite is green with parallel extinction. The absorption peaks in its Raman spectra are at 967-971 cm -1, 787-788 cm-1, and 766 cm-1. The absorption peaks in the infrared spectra are at 696 cm-1, 767 cm-1, 1450 cm-1, and 1551 cm-1, which are distinctly different from the peaks for synthetic silicon carbide. Moissanites have been documented to form in ultra-high pressure, high temperature, and extremely low fO 2 environments and their 13C-depleted compositions indicate a lower mantle origin. Combined with previous studies about other ultra-high pressure and highly reduced minerals in Luobusa ophiolite, the in-situ natural moissanite we found indicates a deep mantle origin of some materials in the mantle sequence of Luobusa ophiolite. Further, we proposed a transformation model to explain the transfer process of UHP materials from the deep mantle to ophiolite sequence and then to the supra-subduction zone environment. Interactions between the crown of the mantle plume and mid-ocean ridge are suggested to be the dominant mechanism. © 2014 Geological Society of China. Source


Dong H.,Chinese Academy of Geological Sciences | Xu Z.,Chinese Academy of Geological Sciences | Li Y.,Chinese Academy of Geological Sciences | Liu Z.,China University osciences
Acta Geologica Sinica | Year: 2014

The Motuo area is located in the east of the Eastern Himalayan Syntaxis. There outcrops a sequence of high-grade metamorphic rocks, such as metapelites. Petrology and mineralogy data suggest that these rocks have experienced three stages of metamorphism. The prograde metamorphic mineral assemblages (M 1) are mineral inclusions (biotite + plagioclase + quartz ± silUmanite ± Fe-Ti oxides) preserved in garnet porphyroblasts, and the peak metamorphic assemblages (M2) are represented by garnet with the lowest XSps values and the lowest XFe# ratios and the matrix minerals (plagioclase + quartz ± K-feldspar + biotite + muscovite + kyanite ± silUmanite), whereas the retrograde assemblages (M 3) are composed of biotite + plagioclase + quartz symplectites rimming the garnet porphyroblasts. Thermobarometric computation shows that the metamorphic conditions are 562-714°C at 7.3-7.4 kbar for the Mi stage, 661-800°C at 9.4-11.6 kbar for the M2 stage, and 579-713°C at 5.5-6.6 kbar for the M3 stage. These rocks are deciphered to have undergone metamorphism characterized by clockwise P-T paths involving nearly isothermal decompression (ITD) segments, which is inferred to be related to the collision of the India and Eurasia plates. © 2014 Geological Society of China. Source


Bao P.,Chinese Academy of Geological Sciences | Su L.,China University osciences | Wang J.,Chinese Academy of Geological Sciences | Zhai Q.,Chinese Academy of Geological Sciences
Acta Geologica Sinica | Year: 2014

The Zedang and Luobusa ophiolites are located in the eastern section of the Yalung Zangbo ophiolite belt, and they share similar geological tectonic setting and age. Thus, an understanding of their origins is very important for discussion of the evolution of the Eastern Tethys Ocean. There is no complete ophiolite assemblage in the Zedang ophiolite. The Zedang ophiolite is mainly composed of mantle peridotite and a suite of volcanic rocks as well as siliceous rocks, with some blocks of olivine-Pyroxenite. The mantle peridotite mainly consists of Cpx-harzburgite, harzburgite, some lherzolite, and some dunite. A suite of volcanic rocks is mainly composed of calc-Alkaline pyroclastic rocks and secondly of tholeiitic pillow lavas, basaltic andesites, and some boninitic rocks with a lower Ti02 content (Ti02 <0.6%). The pyroclastic rocks have a LREE-enriched REE pattern and a LILE-enriched (compared to HFSE) spider diagram, demonstrating an island-Arc origin. The tholeiitic volcanic rock has a LREE-Depleted REE pattern and a LILE-Depleted (compared to HFSE) spider diagram, indicative of an origin from MORB. The boninitic rock was generated from fore-Arc extension. The Luobusa ophiolite consists of mantle peridotite and mafic-Ultramafic cumulate units, without dike swarms and volcanic rocks. The mantle peridotite mainly consists of dunite, harzburgite with low-Opx (Opx < 25%), and harzburgite (Opx > 25%), which can be divided into two facies belts. The upper is a dunite-harzburgite (Opx < 25%) belt, containing many dunite lenses and a large-Scale chromite deposit with high Cr203; the lower is a harzburgite (Opx >25%) belt with small amounts of dunite and lherzolite. The Luobusa mantle peridotite exhibits a distinctive vertical zonation of partial melting with high melting in the upper unit and low melting in the lower. Many mantle peridotites are highly depleted, with a characteristic U-Shaped REE pattern peculiar to fore-Arc peridotite. The Luobusa cumulates are composed of wehrlite and olivine-Pyroxenite, of the P-P-G ophiolite series. This study indicates that the Luobusa ophiolite was formed in a fore-Arc basin environment on the basis of the occurrence of highly depleted mantle peridotite, a high-Cr203 chromite deposit, and cumulates of the P-P-G ophiolite series. We conclude that the evolution of the Eastern Tethys Ocean involved three stages: The initial ocean stage (formation of MORB volcanic rock and dikes), the fore-Arc extension stage (formation of high-Cr203 chromite deposits and P-P-G cumulates), and the island-Arc stage (formation ofcalc-Alkaline pyroclastic rocks). © 2014 Geological Society of China. Source


Hu P.-Y.,Jilin University | Li C.,Jilin University | Su L.,China University osciences | Li C.-B.,Jilin University | Yu H.,China University osciences
Geology in China | Year: 2010

Qiangtang basin located in northern Tibetan plateau is a key area in studying the evolution of the Paleo -Tethys Ocean and the boundary of Gondwana and Eurasia, and the age and properties of its basement directly determine the tectonic attribute of southern Qiangtang plate and the range of Gondwana. In this paper, LA-ICP-MS U-Pb dating was carried out for zircons from granitic gneiss inclusions (xenoliths) in Wugong Mountain area, central Qiangtang. The dating results show that granitic gneiss inclusions were formed in late Triassic (209.1 ±2.8Ma). This age is consistent with the existing research results of the convergence events of Gondwana and Eurasia in central Qiangtang, so the granitic gneiss inclusions are material records of the convergence events of Gondwana and Eurasia. Moreover, the dating results also indicate that granitic gneiss inclusions contain a age record of the end of Pan-African period (464.5±4.8Ma), which is the first age of PanAfrican movement found in Qiangtang area, and this age can be compared with ages of large amounts of early Paleozoic granitic rocks in Nujiang area and Baoshan area in western Yunnan, internal Indian plate and Himalayan orogenic belt, suggesting that southern Qiangtang plate has good affinity to Indian plate. All the results obtained by the authors can provide fresh evidence for the study of the properties of Qiangtang basement and the age of the convergence events of Gondwana and Eurasia. Source


Guo L.-J.,China University osciences | Guo L.-J.,Geological Survey Institute of Inner Mongolia | Hao J.-F.,China University osciences | Hao J.-F.,Geological Survey Institute of Inner Mongolia | And 7 more authors.
Geology in China | Year: 2012

The Abag Pb-Zn ore concentration area near the Sino-Mongolian border belongs to the ErenhotDong Ujimqin Banner copper -polymetallic ore belt. A series of Pb -Zn mesothermal deposits, such as Hadatetaolegai, Moruogeqing, Gaoerqi, Ganzhuershande and Azhahada, were recently found in this area by synthetic prospecting methods. The achievements acquired have established the Abag Pb-Zn ore concentration area as an important Pb -Zn resource district. This paper sums up features of geology, physical geography and geochemistry in the study area and describes characteristics of this kind of ore deposits with two typical Pb-Zn deposits of Gaoerqi and Hadatetaolegai as examples. Some conclusions have been reached from the angle of the spatial occurrence of the Pb-Zn ore body that this type of Pb-Zn deposits resulted from the joint action of the strata (Niqiuhe Formation) and structure location (faulted structure). In the end, this paper provides the line of thinking in search for this kind of Pb-Zn deposits according to their discovery processes and features. Source

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