西藏措勤尼雄矿田滚纠铁矿侵入岩LA-ICP-MS锆石U-Pb年龄与地球化学特征

Zircon LA-ICP-MS U-Pb dating and geochemistry of intrusive rocks from Gunjiu iron deposit in the Nixiong ore field, Coqen, Tibet

本文分析了尼雄矿田滚纠铁矿与成矿相关的花岗闪长岩和二长花岗岩主量元素、微量元素并对其进行锆石LA-ICP-MSU-Pb定年。结果显示,岩石富硅（SiO2平均分别66·76%和70·98%）、富碱（ALK平均为6·28%和7·70%）,铝饱和指数均小于1·1,属中高K钙碱性I型花岗岩。两者具相似的稀土元素球粒陨石标配分模式图和微量元素原始地幔蛛网图,∑REE较低（∑REE分别是130·6×10-6和134·3×10-6）,弱Eu负异常（δEu分别为0·84和0·88）,弱-中等轻重稀土分馏,（La/Yb）N分别平均8·07和6·72。Rb、Pb、Th等大离子亲石元素和稀土元素富集,Ba和U、Nb、Ta、P、Ti等高场强元素亏损,显示弧岩浆作用特点,指示岩石形成于弧后伸展环境,岩浆源区具壳幔混源的特征。LA-ICP-MS锆石U-Pb测年显示花岗闪长岩和二长花岗岩分别形成于113·6±1·2Ma和112·6±1·6Ma,属早白垩世晚期岩浆活动产物。结合区域地质资料,认为滚纠铁矿与成矿相关的花岗闪长岩和二长花岗岩因班公湖-怒江洋壳向南俯冲,并于-113Ma发生冈底斯陆壳和羌塘陆壳对接,已俯冲的班公湖-怒江洋壳发生板片回转,导软流圈地幔流体上涌,其所带来的巨大热能引发岩石圈地幔和上覆地壳发生部分熔融,形成以壳源为主的壳幔混源岩浆,在弧后拉张环境下上侵形成。

Whole-rock major and trace elements, zircon LA-ICP-MS U-Pb age of granodiorite and monzogranite from Gunjiu iron deposit in the Nixiong ore field, which are associated with the Fe mineralization, are analyzed in this paper. The major elements result show that both rocks are enriched in silica and alkali （with the average of SiO2 66.76 %, 70.98% and ALK 6.28%, 7.70%）. The aluminum saturation index are both less than 1.1. The rocks are belonged to medium-high K calc-alkaline I-type granite. There have similar chondrite-normalized REE patterns and primitive mantle normalized trace element spectrum, low ∑REE （130.6×10-6 and 134.3×10-6）and high δEu （0.84 and 0.88）, with （La/Yb）N value of 8.07 and 6.72. Both rocks are enriched in LILEs （Rb, K, Pb, Th） and LREE, depleted in Ba and HFSEs （U, Nb, Ta, P, Ti）. The geochemistry results show characteristic of arc volcanic rock; it also indicated that both rocks were formed from a mixed magma between the partial melting materials of the upper crust and asthenospheric mantle in the extensional environment. Zircon LA-ICP-MS U-Pb dating for both rocks yields the age of 113.6±1.2Ma and 112.6±1.6Ma, which indicate that there were formed in later of Lower Cretaceous. Combined with the data of regional geology, the authors have the opinion that the granodiorite and monzogranite of Gunjiu iron deposit are related to the southward subduction of Bangong Co-Nujiang Ocean crust. In about ~113Ma, the Gangdese and the Qiangtang continental crust start to collision. The subducted oceanic crust of Bangong Co-Nujiang occurred to rotary, which leading asthenospheric fluids upwelling. The tremendous heat of asthenospheric fluids caused partial melting of the lithospheric mantle and the overlying crustal and to formed mixed magma, which intruded above in the back-arc extensional condition and formed the granodiorite and monzogranite.