青海牛鼻子梁镍矿勘查区闪长岩U-Pb年代学、地球化学及Hf同位素研究

U-Pb Dating,Geochemistry,and Hf Isotopic Compositions of Diorite from the Niubiziliang Ni-(Cu) Deposit in Qinghai Province

本文对牛鼻子梁镁铁质-超镁铁质杂岩体中产出的闪长岩进行了详细的U-Pb年代学、岩石地球化学和Hf同位素分析,以便对岩石的成因及南祁连造山带对柴达木盆地的俯冲效应予以研究。闪长岩中锆石LAICP-MS U-Pb定年结果表明,闪长岩的形成时代为361±3 Ma(MSWD=0.049),即晚泥盆世。岩石地球化学方面,SiO_2含量为53.44%~58.27%,Na_2O+K_2O为4.80%~6.39%,K_2O/Na_2O比值为0.33~0.97,显示出富钠的特征,岩石由钙碱性序列向高钾钙碱性序列过渡。微量元素原地地幔标准化蛛网图上显示相对富集大离子亲石元素(Rb、K)及轻稀土,相对亏损高场强元素(Ta、Nb、Zr),同时Th含量介于2.03×10^(-6)~6.57×10^(-6)之间,Th/Ta比值为2.73~12.82之间,显示为壳幔混染作用的产物,另外据闪长岩中发现的辉长岩包体也表明了壳幔岩浆的混合。闪长岩Hf同位素显示,岩石的ε_(Hf)(t)值和二阶段模式年龄(TDM2)分别介于±6.41~±9.36和714~878Ma之间。上述结果表明,岩体的原始岩浆是一种幔源物质为主,混合新生陆壳部分熔融的产物。综合区域岩石构造背景的研究,认为牛鼻子梁闪长岩岩体形成于南祁连向柴达木板块俯冲碰撞后伸展的环境。

The LA-ICP-MS U-Pb dating and petrogeochemical and Hf isotopic analysis were carried out in this study to understand petrogenesis of mafic-ultramafic composite pluton in Niubizishan and subduction effect of South Qilianshan on the Tarim basin. LA-ICP-MS U-Pb dating of zircon from diorite body shows that the diorite formed at Late Devonian （361±3 Ma, MSWD=0. 049）. Petrogeochemieal analysis suggests that the diorite has SiO2 content of 53.44%- 58.27%, Na2O＋K2O of 4.80% -6.39%, and K2O/Na2O ratio of 0.33-0.97, indicating the rock is enriched in Na and falls into the calc-alkaline to high-K calc- alkaline series. Primitive mantle-normalized trace element spider diagrams show relative enrichment of LILE （such as Rb and K） and LREE, relatively depletion of HFSE （Ta, Nb and Zr）, Th=2.03×10-6 - 6.57× 10 -6, Th/Ta=2.73- 12.82, all of which suggest they were the product of crust-mantle mixed magma. In addition, discovery of gabbro enclave in diorite also shows mixing of crustal and mantle magma. The Hf isotopic also shows that εHf（t） values of the diorite vary from ±6.41 to ±9.36, and twostage model ages （TDM2） vary from 714 to 878Ma. These characteristics suggest that the primary magma was derived from the depleted sources and remelting of Neoproterozoic juvenile crustal materials. Based on previous and present study of structural setting, it can be concluded that the diorite from the Niubiziliang intrusion formed in an extensional structure setting, which could be related to the subduction of the South Qilian Orogenic belt beneath the Qaidam block.