皖南歙县新溪口岩体锆石U-Pb定年、地球化学特征及找矿前景

Zircon U-Pb geochronology and geochemistry of the Xinxikou pluton in Shexian County,Southern Anhui Province and its prospecting significance

皖南歙县新溪口岩体为中细粒少斑花岗岩,侵位时间为121.31±0.67 Ma;岩石具有高硅、富钾特征。SiO 2含量为75.97%~76.49%,K 2O含量为4.84%~5.28%,K 2O/Na 2O为1.61~2.05,A/CNK为1.05~1.06、A/NK为1.16~1.19,属弱过铝质花岗岩。稀土元素总量为(261.90~329.83)×10^-6,具有海鸥型右倾配分模式,(La/Yb)N为3.26~5.15,具有强烈的Eu负异常(δEu=0.018~0.063)。大离子亲石元素Rb富集,Ba、Sr亏损,高场强元素Th、U富集,Nb、Ti具有明显的负异常。岩体形成于燕山晚期早阶段古太平洋板块俯冲后拉张伸展构造环境,是岩石圈减薄引发软流圈地幔上涌,上覆地壳受热部分熔融并发生结晶分离作用形成的A 2型花岗岩。岩体出露点较多,剥蚀较浅,富含W、Sn、Bi等金属元素,这些元素通过云英岩化等高温气液蚀变扩散至围岩中,指示良好的找矿前景。

The Xinxikou pluton is mainly consists of medium-to fine-grained granite,was emplaced at 121.31±0.67 Ma.The granites have high SiO2(75.97%~76.49%)and K 2O(4.84%~5.28%)contents with K 2O/Na 2O ratios from 1.61 to 2.05,and show weakly peraluminous nature with A/CNK values from 1.05 to 1.06.The granites have high total REE contents ranging from 261.90 to 329.83 ppm.On the chondrite-normalized patterns,they all show markedly LREE-enriched patterns with(La/Yb)N ranging from 3.26 to 5.15 and significant troughs of Eu anomalies(δEu=0.018~0.063).Normalized to primitive mantle,the granites are enriched in large ion lithophile elements(e.g.Rb)and high field strength elements(e.g.Th and U)but depleted in Ba and Sr showing affinity to A2 granites.Together with the regional geology,the granites formed in a extensional environment,related to asthenospheric mantle upwelling triggered by roll-back of the Paleo-Pacific oceanic slab.The Xinxikou pluton is widely exposed and have a shallow denudation.These granites are rich in metallic elements(W,Sn,Bi and etc)which diffused into surrounding rocks by high temperature gas-liquid alteration such as greisenization,indicating a good prospect of ore exploration.