柴北缘察汗诺辉长岩成因:岩石地球化学、锆石U-Pb年代学与Lu-Hf同位素制约

柴北缘察汗诺地区广泛发育中生代辉长岩、闪长岩、花岗闪长岩和花岗岩。LA-ICP-MS锆石U-Pb测年结果表明, 辉长岩²⁰⁶ Pb/²³⁸ U加权平均年龄为246±0.7 Ma, 形成于中三叠世早期。辉长岩SiO₂(50.61%~54.41%)和全碱含量(2.36%~3.72%)低, 里特曼指数为0.54~1.12, 属于中钾钙碱性系列岩石; Mg^#(73~79)和Cr(342×10^-6~753×10^-6)、Ni(45.1×10^-6~145×10^-6)含量高, LREE/HREE为4.74~5.67, 富集U、Th等大离子亲石元素, 亏损Nb、Ta、Ti等高场强元素, 呈现出与典型俯冲作用密切相关的弧岩浆岩特征。锆石Lu-Hf同位素分析结果显示, 辉长岩ε_Hf(t)值为-2.24~+1.37, 研究表明察汗诺辉长岩岩浆源区因受俯冲板片流体交代而富集, 岩石主要起源于富集地幔的部分熔融。空间上, 察汗诺辉长岩与东昆仑布尔汗布达、鄂拉山以及西秦岭同仁-泽库等同时代的钙碱性岩浆岩, 共同构成了古特提斯洋向北俯冲的大陆边缘弧。

Petrogenesis of the Chahannuo gabbro on the northern margin of Qaidam Basin: Constraint from geochemistry,zircon U-Pb dating and Lu-Hf isotopes

The Mesozoic gabbro, diorite, granodiorite and granite are widespread at Chahannuo area in the north margin of Qaidam terrane. LA-ICP-MS zircon dating yields Middle Triassic age of 246±0.7 Ma for the Chahannuo gabbro. The gabbros have low SiO₂(50.6%~54.41%), total alkali contents(2.36%~3.72%), and low σ value caring from 0.54 to 1.12, indicating that they are belong to medium-K calc-alkaline series. They are characterized by elevated high MgO, Mg^#, Cr, Ni, enriched LREE, LILEs(U, Th, K), and depleted in HREE, HFSEs(Nb, Ta, Ti). The gabbro zircons ε_Hf(t)changed from -2.24 to +1.37. These indicated that gabbros were dominatly derived by the partial melting of enriched lithosphere which had been metasomatism the fluid from the subducted slab. The Chahannuo gabbro were contemporary with the calc-alkaline magmatism from Buerhanbuda in the East Kunlun orogen, Elashan and Tongren-Zeku area in the West Qinling orogeny. These calc-alkaline magmatism constituted a active continental margin arc formed by the northern subduction of Paleo-Tethys Ocean.