湖南常宁水口山Pb-Zn矿区花岗闪长岩元素地球化学,LA-ICP-MS锆石U-Pb年龄和Hf同位素特征

Characterization of Element Geochemistry,LA-ICP-MS Zircon U-Pb Age,and Hf Isotope of Granodiorite in the Shuikoushan Deposit,Changning,Hunan Province

在野外地质工作的基础上,本文通过显微观察、岩石化学、锆石U-Pb定年、锆石原位Hf同位素分析研究,深入探讨水口山Pb—Zn矿区花岗闪长岩的岩石化学性质、形成时代和物质来源。研究表明:花岗闪长岩以高钾、富碱,富集轻稀土元素、大离子亲石元素(Rb、Th、U、La)和Pb,亏损重稀土元素、高场强元素(Nb、Ta、Ti)和Ba、Sr等元素为特征。锆石U-Pb加权平均年龄为156.0±1.0Ma(MSWD=0.33,2σ,N=20),属于燕山早期产物。锆石原位Hf同位素初始比值εHf(t)为-10.80^-8.71,平均-9.71;Hf同位素二阶段模式年龄TCDM为1.75~1.88Ga,平均1.82Ga,指示花岗闪长岩的岩浆源区主要来自古元古界下地壳物质。燕山早期湘南地区岩石圈处于全面的拉张—减薄期,导致地幔物质上涌,岩浆底侵,引发大规模的地壳熔融,熔融形成的岩浆沿深大断裂上侵,形成水口山Pb—Zn矿区花岗闪长岩。

Based on field geological investigation,this paper studied the whole-rock petrochemistry,LA-ICP-MS zircon U-Pb dating,and in-situ zircon Hf isotopic geochemistry,for the granodiorite in the Shuikoushan Pb-Zn deposit to better constrain its formation mechanism.The granodiorite study shows that it is characterized by high potassium,rich alkali,enrichment in light rare earth elements （LREEs） and large ion lithophiles elements （LILEs,Rb,Th,U） and Pb,deficited in heavy rare earth elements （HREEs） and high field strength elements （HFSFs,Nb,Ta,Ti） and Sr,Ba.LA-ICP-MS zircon U-Pb dating yielded weighted average age of 156.0 ± 1.0 Ma （MSWD =0.33,2σ,N =20）,which is belong to the early Yanshanian product.In-situ zircon Hf isotope analysis indicates that the granodiorite has low εHf（t） values （-10.78 to-8.71）,and old Hf model ages TCDM （1.75 to 1.88 Ga）.These isotopic compositions suggest that the granodiorite magma source mainly came from the lower crust in the Paleoproterozoic.During the early Yanshanian,the lithosphere in southeastern China was overall tensional and thinning,causing mantle upwelling and basaltic magma underplating.With a large-scale crustal substances melting,the magma derived from the lower crust intruded along the deep fault,and ultimately emplaced in the near curst surface,formed the granodiorite in the Shuikoushan Pb-Zn deposit.