SHRIMP Zircon U-Pb Age and Geochemical Characteristics of Ordovician Gabbro from Altun, Xinjiang Province

The Ordovician gabbro was firstly discovered in Changshangou, Xinjiang province. The authors got for the first time the zircon U-Pb age [444.9±3.4 Ma (n=9, MSWD=1.83)] of gabbro, whose occurence may represent a magmatic event after the closure of south Altun ocean basin. Geochemical analysis show that the differentiation index(DI) is 35.26 to 44.76 and the solidification index(SI) is 20.24 to 31.43, which reflects a lower differentiation degree of magmatic crystallization; the values of Rittmann index(3.26-9.49) and A/CNK(0.75-0.82) indicates that the gabbro is quasi-aluminous calc-alkaline to alkaline series; the content of REE is relatively high(∑REE =237.39×10^-6-329.26×10^-6), the fractionation of LREE and HREE is obvious((La/Yb)_N=6.14-8.55), the REE patterns show right oblique feature and unobvious negative Eu anomaly. Trace elements show enrichment of Ba, Hf, Rb, Th, U; the value of Rb/Sr is from 0.04 to 0.47, clearly lower than the average of continental crust. Combined with regional geological background and R1-R2 diagram, the authors hold that the gabbro is mantle-derived intrusive rock resulted from the plate uplift stage from pre-collision to post-collision, controlled by deep and major faults on the south of Altun. It is considered that the gabbro was formed in an extensional tectonic environment, probably be a result of lately extensional event after the closure of south Altun ocean basin.