新疆金窝子片麻状花岗岩锆石U-Pb 年代学、地球化学特征及其地质意义

[摘 要]金窝子金矿是新疆东部重要的金矿床之一,其赋矿围岩为上泥盆统金窝子组火山碎屑岩-沉积岩和金窝子片麻状花岗岩。前人研究显示金窝子片麻状花岗岩形成时代为356～392Ma,变化范围较大,且与金窝子组底部有片麻状花岗岩砾石的地质事实不符,在一定程度上制约着金矿床成矿作用和成矿地质背景的研究。金窝子片麻状花岗岩准确时代的确定,是研究该金矿成矿规律和构造岩浆演化的基础和关键。因此,本文采用高精度LA-ICP-MS 方法对金窝子片麻状花岗岩进行锆石U-Pb 定年和Hf 同位素研究。研究结果表明,片麻状花岗岩岩浆结晶时代为429.1±2.7Ma,形成于中志留世,结合野外观察认为岩体时代应早于金窝子组地层,两者为不整合关系。锆石的ε_HF(t)值和Hf 同位素两阶段模式年龄分别介于-3. 5～+1.2 和2545Ma～3072Ma 之间。地球化学资料显示,片麻状花岗岩轻稀土(LREE)和大离子亲石元素( Rb、Th 和K) 相对富集,Nb、Ta 和Ti 等高场强元素相对亏损,A/ CNK 在1.13～1.14之间,具有高K 钙碱性系列岩石的特征和岛弧钙碱性岩浆活动的特征,该岩体的形成与南天山洋早古生代晚期的闭合以及后来塔里木板块和哈萨克斯坦板块的碰撞密切相关。

Zircon U-Pb Dating and Geochemical Characteristics of the Gneissic Granite in theJinwozi Gold Deposit, Xinjiang and Their Geological Implications

The Jinwozi gold deposit is one of the important gold deposits in eastern Xinjiang.It is hosted in the volcaniclastic-sedimentary rocks of the upper Devonian Jinwozi Formation and the Jinwozi gneissic granite.Previous studies suggest that the Jinwozi gneissic granite formed at 356 ~ 392 Ma.This large time variation does not accord with the geological information that there is gneissic granite gravel in the bottom of the Jinwozi Formation.This conclusion,to some extent,limits the further study on the gold mineralization and geological background in this area.Accurate age determination of the Jinwozi gneissic granite is critical for researching gold mineralization and tectno-magmatic evolution.In this work,U-Pb dating and Hf isotope studies of zircon grains from the Jinwozi gneissic granite have been conducted by the high-precision LA-ICP-MS method.The result shows that the gneissic granite magma crystallization age is 429.1 ± 2.7Ma,in the middle Silurian.The εHf(t) values of Zircon and Hf two-stage isotope model ages are-3.5-+1.2 and 2545~3072 Ma,respectively.Geochemical data of granite demonstrates that it is rich in LILE and depleted in HFSE(Nb,Ta,Ti).A/CNK ranges from 1.13 to 1.14,which has the characteristics of high-K calc-alkaline volcanic rock series.It is thus concluded that the formation and emplacement of the Jinwozi gneissic granite are closely related to the closure of the South Tianshan ocean in the early Paleozoic and later collision of the Tarim plate with the Kazakhstan plate.