黄沙坪矿区花岗岩岩石地球化学、U-Pb年代学及Hf同位素制约

湘南坪宝地区地处南岭中段,黄沙坪铅锌多金属矿床位于坪宝成矿带南部,304岩体是未来深部找矿的远景地区,岩石地球化学特征显示其具有A型花岗岩的特点,与区域上的千里山岩体相似.根据微量元素判别图解进一步划分为A1型,反映为非造山板内拉张背景;岩石稀土配分型式具有特征的四分组效应(tetrad effect),这是岩浆高度演化晚期熔体与流体相互作用的结果.采用LA-ICPMS方法对花斑岩中锆石进行了微区U-Pb同位素及Hf同位素测试,结果显示成岩年龄为179.9 ±1.3 Ma(MSWD=1.9),属于燕山早期第一阶段岩浆侵入产物,成岩时代早于301岩体.Hf同位素显示地壳模式年龄为2220～2459 Ma,平均为2353 Ma,反映源区物质为新太古代至古元古代地壳物质.

Petrogenesis of the Granites in the Huangshaping Area: Constraints from Petrochemistry, Zircon U-Pb Chronology and Hf Isotope

The Huangshaping Pb-Zn-Fe-W-Mo deposit is located in the south part of the famous Huangshaping-Baoshan metallogenetic belt,which is closely related to the shallow-seated granite-porphyry.There are quartz porphyry,granite porphyry,and granophyre in the mining area.As the granite porphyry(301) is closely related to the skarn type Pb-Zn deposit,many research works have been carried out on it.On the contrary,no detailed study about the 304 porphyry has been done because much less ore bodies were found in its contact belt.In recent years,deep drilling projects display that there are veinlet-disseminated type copper mineralization within the 304 porphyry.Fresh samples were collected from the mining tunnels,and the analyses showed that the granites are of A1 type formed in extensive tectonic regime.The REE patterns of the granites show typical tetrad effect caused by highly evolved magmatic melt-fluid interaction.U-Pb dating and Hf isotope composition of zircons from the 304 porphyry indicate that it was intruded at 179.9±1.3 Ma(MSWD=1.9),which is earlier than the 301 porphyry,while the average crust Hf model ages are calculated to be 2220~2459 Ma.Combined with the geochemical characteristics,the granites are interpreted to be formed by partial melting of middle-lower crustal material and intruded in shallow crust along deep fractures.Accordingly,the 304 granophyre may have provided ore-forming material for the latter large scale metallogenic activity and was parental rock for individual copper ore body.