宝山花岗闪长斑岩的岩石成因:地球化学、锆石U-Pb年代学和Hf同位素制约

宝山矿床位于坪宝矿带的北端,产出与花岗闪长斑岩有关的铜钼铅锌银多金属矿床。对采自坑道内的新鲜岩脉进行岩石成分分析及锆石U-Pb和Hf同位素测试。锆石U-Pb定年结果显示:基质为细粒结构的斑岩成岩年龄为(180.5±2.0)Ma,基质为隐晶质的斑岩成岩年龄为(165.3±3.3)Ma,表明该区在燕山早期有多次(阶段)岩浆侵入活动,依据锆石Hf同位素组成计算的平均地壳模式年龄为1 709～1 951 Ma,在εHf(t)—t图解中,锆石点投在2.5 Ga平均地壳演化线附近,表明岩石源区为古老地壳。结合岩石地球化学特征,认为该区的花岗闪长岩是在燕山早期挤压背景下由中下元古界基底地层发生增厚熔融形成的。本区成矿时代对应于成岩时代,时限为160～180 Ma,其间岩浆的多阶段侵入带来充足的成矿物质,最终在地壳浅部层次形成多金属矿床。

Petrogenesis of granodiorite porphyry in Baoshan deposit: Constraints from geochemistry, zircon U-Pb chronology and Hf isotopes

Baoshan Cu-Mo-Pb-Zn-Ag deposit is located in the north of famous Huangshaping—Baoshan metallogenetic belt,whose formation is related closely with shallow-seated granodiorite-porphyry.Fresh samples were collected from mining tunnel,the whole rock chemical composition tests were carried out,meanwhile zircon grains were collected from granodiorite samples,and their U-Pb and Hf isotope composites were analyzed by La-ICPMS.The results of U-Pb dating display that one sample with fine-grain texture in the matrix has an age of(180.5±2.0) Ma,the other sample with cryptocrystalline texture in matrix has an age of(165.3±3.3) Ma,which implies that there is multi-stage magmatic intrusive activity in this area.According to Hf isotope compositions,the average crust model age is calculated to be 1 709-1 951 Ma,in the figure of εHf(t)—t,the points of zircon grains lie in the area neighbored to the evolution curve of 2.5 Ga average crust,suggesting that the origin of zircons comes from ancient crust.Combined with characteristics of petrogeochemistry,the granodiorites are interpreted to form in the background of crustal thickness increasing due to the regional tectonic compression.Many former studies show that the regional basement of this area is middle-lower proterozoic group,they melted to form magma which intruded to shallow position of crust to be granodiorite-porphyry along available fractures.According to former studies,the metallogenic time corresponds to the ages of two-stage magmatic rocks,the metallogenic time extends from 180 to 160 Ma,multi-stage magma activity provides abundant ore-forming material to form Baoshan multimetal ore deposit.