Geochemistry, U-Pb, Hf isotopic characteristics and geological significance of Zhalaxiageyong trachydacite in Tuotuohe area, Qinghai

The Zhalaxiageyong lead-zinc-copper polymetallic deposit is a typical porphyry deposit of the Tuotuohe area. Whole-rock geochemical analyses,Zircon U-Pb dating and Hf isotope analysis are undertaken for the ore host trachydacite with the aim of constraining its petrogenesis,magma source and regional tectonic setting.LA-ICP-MS zircon U-Pb dating indicates that the trachydacite was formed in 32. 68 ± 0. 50 Ma( MSWD =1. 6),i. e.,Oligocene. The trachydacite is rich in potassium and poor in Mg#( 5. 10-9. 70),belonging to the peraluminous shoshonite series. The rocks are enriched in LILE( large ion lithophile elements) Rb,Ba,K and LREE,depleted in HFSE( high field strength elements) Nb,Ta,P,Ti,with high Sr and low Y and Yb,having the characteristics of the C type adakite. It is calculated that the initial εHf( t) of the zircons range from-0. 92 to 2. 07 and their two-stage Hf model ages T_(DM2) range from 978 Ma to 1 169 Ma. The magma source should be mainly the partially melt mafic rocks of the thickened Middle Neoproterozoic lower crust of the Northern Qiangtang massif with the addition of ancient aluminosilica material in the melting process. The rocks formed in the tectonic setting of delamination of lithosphere and extension of the thickened crust. During the period of 40-32 Ma,large-scale potassium rich alkaline magmatism occurred in this area. The porphyry metallogenesis is related to the magmatic activities in this period.

沱沱河地区扎拉夏各涌铅锌矿成矿时代及其地质意义：钾长花岗斑岩年代学和地球化学特征

三江成矿带北段是我国著名的“三江”铜多金属巨型成矿带的北延部分，构造上属特提斯巨型成矿域的重要组成部分。野外调查期间，对沱沱河矿区的扎拉夏各涌铅锌矿（化）点进行了年代学及岩石地球化学研究。通过高精度LA-ICP-MS 锆石微区原位U-Pb同位素测年，获得与铅锌矿（化）密切相关的钾长花岗斑岩^206Pb/ ^238U 年龄加权平均值为 35.96±0.61 Ma，确定铅锌矿（化）发生在古近纪始新世。地球化学分析表明，钾长花岗斑岩具有高SiO2、Al2O3、K2O， 低 TiO2、Na2O 的特征。在 TAS 岩石分类图解中样品均落入碱性系列的碱性花岗岩区，QAP分类图解中样品均落入碱性长石花岗岩区，K2O-Na2O 图解中样品投入过钾质系列区，A/NK-A/CNK 指数图解中样品投入过铝质区，为过铝质过钾质碱性系列花岗岩。稀土和微量元素地球化学特征显示，该岩体LREE、LILE 和部分HFSE 富集，REE 表现为LREE/HREE 明显分离的分配模式，强不相容大离子亲石元素Sr、K、Rb 、Ba、Th、U 和高场强元素La、 Ce、Nd 等相对富集，从多元素多方法图解中可以看出该斑岩体属于过铝质过钾质碱性—高钾钙碱性系列S 型花岗岩，形成于印度板块与亚洲板块的陆—陆碰撞，青藏高原快速隆升之后的后造山构造阶段的构造环境。

青海沱沱河地区扎拉夏格涌粗面岩锆石U-Pb年代学及地球化学特征

对青海沱沱河地区扎拉夏格涌粗面岩进行U--Pb年代学和地球化学特征研究,结果表明：扎拉夏格涌粗面岩锆石U--Pb年龄为32.39-34.06 Ma,加权平均年龄为（33.09±0.26）Ma,形成时代为渐新世;岩石属于高硅高钾准铝质-过铝质钾玄岩系列。稀土元素配分模式图呈右倾型,轻重稀土元素分馏明显[（La/Yb）N=19.96-34.00,平均值26.99],弱铕正异常（δEu=1.00-1.20,平均值1.08）;样品富集大离子亲石元素（LILE：Rb,Ba,K）,亏损高场强元素（HFSE：Nb,Ta,P,Ti）,高场强元素Zr,Th较富集。岩石具有C型埃达克岩特征,为新生镁铁质下地壳部分熔融产物。通过区域对比和总结前人研究成果,认为扎拉夏格涌粗面岩形成于新生代早期造山加厚之后的伸展阶段。