The Geochemical Characters And Its Tectonic Implications Of Ophiolites In Alax Area, Inner Mongolia

Alxa massif, located in the middle of the Central Asian orogenic zone, at the intersectionof Tianshan orogenic zone and Xing-Meng orogenic zone, is a complex of several plates withseveral ophiolite zones and ophiolitic mélange zones. In the 1:250000 regional geologicalsurvey process, we have identified three ophiolite zones: Wulijishanhen zone, Bijiertaiaobaozone and Yabulaishan ophillite zone in Alxa area, Inner Mongolia.Wulijishanhen ophiolite, is a suit of cumulate complex composed of gabbro, hornblendepyroxenite, is PTG-type cumulate complex combined. As for geochemical facts Wulijishanhenophiolite is characterized by low∑REE, low fractionation of LREE and HREE, and mediumEu positive anormaly. Except the similarity between pyroxenite and komatiite, the distributioncharacteristics of REE of gabbro are accorded with the layered gabbro. Moreover the contentof HFSE Nb, Ta, Zr, Hf is correspond with that of N-MORB. Furthermore geochemicalfeatures of Wulijishanhen ophiolite indicate that gabbros are similar to N-MORB andpyroxenites to mantle rock. Also the MOR environment is considered as a mean situation tobuild up Wulijishanhen ophiolite. Finally according LA-ICP-MS dating results the age ofWulijishanhen ophiolite is assigned to in the late Early Triassic (248.37±0.83 Ma).Bijiertaiaobao ophiolite, Extremely dominated by amphibolic gabbros and amphibolites,with striking accumulation characteristics, is PPG type cumulate complex mafic rocks. All ofthese features indicate that Bijiertaiaobao ophiolite roots in differentiation of magma.Strikingly the characteristics of Wulijishanhen ophiolite show a certain similarity with thatBijiertaiaobao ophiolite as low∑REE, low fractionation of LREE and HREE, and medium Eupositive anomaly. Also the distribution characteristic of REE of gabbros is similar to thelayered gabbro. Similarly the distribution characteristics of REE of amphibolites are muchsimilar to II - inclusions, partial similarity with komatiite, which infers its origin related topartial melting of mantle rocks. The characters of trace element of Bijiertaiaobao ophiolite areas follows: firstly the content of LILE Li, Rb, Sr, Ba are correspond with that of OIB; secondlythe content of HFSE Nb, Ta, Zr, Hf are close to that of E-MORB; thirdly trace elementparameters are more similar to that of IAT than to that of N-MORB. Moreover Geochemicalcharacters indicate that the Bijiertaiaobao ophiolite formed in SSZ environment. FinallyLA-ICP-MS dating results show that the Bijiertaiaobao ophiolite belong to the MiddlePermian - Early Middle Permian (285.8±1.7 Ma and 266.84±0.95 Ma). Yabulaishan ophiolite is the cumulate complex composed of hornblendite, amphibolicpyroxenite, amphibolic gabbro and is regarded as PPG type cumulate complex mafic rocks. Asto the point of the distribution characteristics of REE of gabbros, it is regarded as consonantingwith the layered gabbro. Moreover the distribution characteristics of REE of ultrabasic rocksare similar to II–inclusions and komatiite. The characters of trace element of Yabulaishanophiolite include three parts: Firstly the content of HFSE Nb, Ta, Zr, Hf, U, Th is close to thatof E-MORB. Secondly the content of LILE Li, Rb, Sr, Ba is correspond with that of OIB.Thirdly the main similarity is showed between trace element parameters and that of IAT,except minor similar part to that of N-MORB. The geochemical characters infer that theYabulaishan ophiolite was formed in SSZ environment. Based on the LA-ICP-MS datingresults the formation of Bijiertaiaobao ophiolite should occur in the Early middle Permian(278.4±0.95 Ma).The study shows that Wulijishanhen ophiolite is the product of the collision ofKazakhstan-Junggar plate and Tarim plate, Bijiertaiaobao ophiolite provides closed evidenceof ancient Asian Ocean south branch, and Yabulaishan ophiolite is the outcome of thecollision between Tarim plate and Sino-Korean plate.