Zhongposhanbei rock body lies in the middle belt of Beishan rift zone in Xinjiang and covers an area of 180 km(2) in shape of interconnected lopolith. It consists of hornblende gabbros, olivine pyroxenites, olivine gabbros, noritegabbros and anorthosites, these lithofacies belts extend in concentric ring form, and each other gradual transition. Zircon U-Pb age is 274 +/- 4 Ma. Evidences for greatly developmental fractional crystallization comes from the petrography, petrochemistry, crystal chemistry of petrogenetic minerals and REEs geochemistry. Olivine pyroxenites and olivine gabbros are mainly formed by assembly of early crystallized phases; olivine is original liquidus phase, and clinopyrixene is mainly fractional crystallized phase in quantity. Formation of anorthosites are originate from crystallization of residual magma. Primary magma formed these rocks should be high-Mg tholeiitic magma. Nd, Sr, Pb isotopic compositions and lithogeochemical characteristics fully imply the exchange of material between
The basaltic pillow lavas in the Liuyuan region of NW China, considered to be part of an ophiolitic suite, have been central to the models on tectonic setting, evolution and timing of the final closure of the Palaeo-Asian Ocean. New field evidence on the sedimentary units associated with the basalts reveals comparable sequences in the northern and southern flanks of the Liuyuan Volcanic Belt with coarse to fine sediments from periphery to the centre. The dacites and rhyolites formed coevally with the pillow basalts. The pillow basalts are interlayered with lacustrine sandstone, claystone and clayey lake deposits. Detrital zircons from these sediments yield zircon U-Pb ages of 291-285 Ma. Andesites, dacites and rhyolites from the basaltic sequence yield U-Pb ages of 280-277 Ma, similar to the 282-280 Ma ages of gabbros that intrude the pillow lavas. All these rocks cover the 460-440 Ma granite and greenschist basement and have been intruded by gabbros of c. 272 Ma age, with subsequent (230-227 Ma) north-south contractional thrusting and folding. The data from our study are incompatible with the existing models that consider the basalts as part of an ophiolitic suite. Along the northern continental margin of China from west to east, the Tarim, Dunhuang-Alxa and North China cratonic areas all show evidence for regional extension through rifting during early-middle Permian time. These rift features and basaltic eruptions occurred coevally with the assembly of various microcontinental blocks against the Siberian craton at c. 300-250 Ma, synchronous with amalgamation of the Central Asian Orogenic Belt (CAOB) on the northern side of the Liuyuan Rift. These events were also broadly synchronous with formation of the global supercontinent Pangea.
Exploration of the Hulu Cu-Ni-Co sulfide deposit in Eastern Tianshan, Central Asian Orogenic Belt, has detected horizons of sulfide mineralization in pyroxenite and peridotite units. SIMS U-Pb zircon ages of the gabbro-hosting Cu-Ni-Co sulfide deposit indicate that the Hulu intrusion was emplaced at 282.3 ± 1.2 Ma (95% confidence level, MSWD = 3.30, n = 13). Osmium isotopic data suggest that the Hulu intrusion and associated Cu-Ni mineralization were derived from crustally-contaminated mantle melts. The intrusions clearly show island-arc geochemical signatures, such as negative Nb, Ta, Zr and Ti anomalies and enrichment in LILE. These geochemical tracers indicate that the Hulu mafic-ultramafic intrusions, along with the Cu-Ni deposit, formed as a result of subduction of oceanic crust in the Early Permian. The arc-related geodynamic setting of the Hulu Cu-Ni deposit is unusual for magmatic sulfide deposits. This new type of ore deposit offers an alternative perspective for Cu and Ni exploration. [PUBLICATION ABSTRACT
Beishan Terrane, located in the northeast of the Tarim Block, in northwest China, has developed a 500-kin long and 100-km wide belt of Permian mafic-ultramafic intrusions. One of these mafic-ultramafic intrusions, the Xuanwoling Intrusion, is composed of dunite, troctolite, olivine gabbros, and gabbros, with cumulate texture and rhythmic layering. The crystallization sequence is olivine ＋ spinel ＋ plagio- clase→pyroxene, indicating that the crystallization pressure is lower than 0.5-0.8 GPa and that the intrusion has undergone variable degrees of crustal contamination, increasing from dunite to gabbros. The olivines found in the Xuanwoling Intrusion have high Fo values （up to 90）, suggesting a primary magma with a high composition of mg. It is likely that this high-rag magma was produced at extremely high temperatures （1,330-1,350℃）, and as a result, Nd-Sr isotopic compositions similar to oceanic island basalts are found in the Xuanwoling Intrusion, which we propose arose from the mantle plume.