The Primary Study Of Geochemistry And Tectonic Setting Of Ophiolite In Jiaomuru Area,Central Qiangtang,Tibet

It was reported that the mafic and utramafic rocks existed in central Qiangtang area, in 1980s, however, there are great argument for the formation of these rocks. On one hand, some scholars consider them to be the ophiolite which present the Paleo-tethys suture zone between Gondwana continent and Eurasia continent; on the other hand, some others think the rocks do not belong to the ophiolite but the rocks formed in the environment of within plate, because of without the representative rock assemblage of ophiolite. During our regional investigation of Mayigangri, a seris of chert, pillow basalt, gabbro, diabase and metaperidotite are discovered in Jiaomuri area, which is in high keep. On the basis of field and room study, we think them as ophiolite. Peridotites are few and consist of serpentinized two-pyroxene peridotite and altered olive pyroxenite, just like slivers, and olivines of perrdotite are all chrysolite. Gabbro dyke swarms are under the pillow basalt and cut it. The breadth of one dyke ranges from 10cm to 50cm, and large grain and fine-grained is clear. Besides there is cumulus olivine gabbro with stratification. The pillow basalt is kept well and the pillow are irregular and uneven, with flowing, twisting structure and chilled border. Skeletal texture of plagioclase in basalt is the result of the magma erupting under the water and cooling down quickly. Radiolarian silicalite and volcaniclastic rock overlie basalt, and Fusulinida fossils are discovered in the volcaniclastic rock, the age is middle permian. In addition, there is turbidite and Bouma sequence is kept well. The content of SiO₂ in peridotite ranges from 36.07% to 44.16%, and the one of MgO is between 19.79% and 35.18%, Mg″is 72%, all these characters accord with utramafic accumulative rocks'. The content of SiO2 in gabbro is between 38.64% and 52.22%, and the one of TiO2 ranges from 0.80% to 3.20%. The content of K2O (about 0.60%) and P2O5 (about 0.55%) is high. The content of SiO2 in basalt ranges from 43.87% to 54.25%, the one of TiO2 is medium (average 1.60%), and the one of P2O5 is high too (average 0.48%). Furthermore, the content of K2O is high (average 0.58%). These characters are consistent with what the MORB-like basalt of Sanjiang area have. According to REE and trace element analysis, we find that all the rocks in this area are characterized by light REE enrichment, the ∑REE is not high, and turns higher from peridotite to gabbro and basalt, most of the ratios of ∑LREE/∑HREE range from 2.02 to 2.41 and trace element spidegrams have the pattern of E-MORB. The LaN/SmN ratios of the basalt average 1.98, ∑LREE/∑HREE average 2.02 and LaN/YbN average 4.78, which accord with one of E-MORB (4.3⁶.8). Besides, the ratio of CeN/YbN average 4.05, SmN/YbN average 2.42, SmN/NdN average 0.79, and δEu is normal (average 0.97). Rb, Th and Ba are very enriched in basalt, and K, Nb and Ce are enriched too, on the contrary, Y and Yb delplete. These resemble the MORB-like basalt in Sanjiang area. Furthermore, the ratio of Ti/Zr are more than 60 (average 69.9), which accord with the MORB-like from the outer Mariana forearc, and the ratio of Zr/Y ranges from 3.95 to 11.03, averages 6.04, the ratio of Zr/Nb ranges from 4.98 to 17.47, averages 9.88, which are the character of enriched mantle. Basalt is the key for confirming tectonic setting of ophiolite. Most of the basalt from our study area are tholeiitic basalts, alkalic basalts are only few. Basalt, chert, volcaniclastic rock and few flysch facies siltite and silty microsandstone with Bouma sequence coexist, which represent deep marine environment. This is consistent with the radiolarian in chert which interbeds with basalt from adjacent area. In addition, there is Carbonatite –radiolarian chert-basalt assemblages of Permian in the study and adjacent area (for example northern Caimaer Co), which show the character of Ocean Island rocks. It is clear that there is a ocean in central Qiangtang area during Permian, and the place where ophiolite emplaced near the Ocean Island or accreted with it, just like Iceland area, superposition of expanding oceanic ridge and hotspot magma activity. Discrimination diagram is an important tool for geologists in determining the tectonic setting of basalt. From the F1-F2 and Ti+Cr-Ca+Na diagram for...