甘肃北山大山头基性杂岩体SHRIMP锆石U-Pb年龄及其地质意义

大山头岩体是北山地区黑山铜镍硫化物成矿带中一个较大的基性杂岩体,其地球化学特征、成岩时代、岩浆源区性质和成岩背景等问题是研究岩浆铜镍硫化物矿床成岩、成矿过程必须解决的基础科学问题.笔者首次获得大山头杂岩体SHRIMP锆石U-Pb年龄为(359.3±5.7)Ma,确定了黑山成矿带超镁铁岩带的成岩期为泥盆纪末.地球化学特征显示大山头杂岩体的原生岩浆可能为高Mg玄武岩浆,其岩浆源区为富集岩石圈地幔,并且受到强烈的洋壳俯冲流体交代和地壳物质的同化混染.构造背景研究显示红柳河-牛圈子-洗肠井早古生代古缝合带的俯冲作用可能持续到泥盆纪末.大约在359.3±5.7Ma大山头—黑山一带洋壳俯冲作用依然影响着大陆边缘初始裂谷近内的岩浆活动.

Zircon SHRIMP U-Pb Dating of the Dashantou Basic Complex and Its Geological Significance in Beishan Area, Gansu Province

Dashantou rock is a large basic complex of Heishan Cu-Ni sulfide metallogenic belt in Beishan area,of which the geochemical characteristics,petrogenic age,nature of magma source,diagenesis setting are the basic scientific problems that must be solved in studying magma Cu-Ni sulfide deposit rock-forming and ore-forming processes.A SHRIMP zircon U-Pb age of (359.3±5.7)Ma has been first obtained in this paper,which determining diagenesis stages of Heishan metallogenic belt is in the Late Devonian.The rock geochemistry shows that primary magma of Dashantou complex may be high magnesian basaltic magma,of which the magma source belongs to the enriched mantle and has been strongly metasomasised by fluid formed in the subduction process of oceanic crust and contaminated by crustal material.The study on tectonic setting of Dashantou area suggests that the subduction of Hongliuhe-Niujuanzi-Xichangjing paleo-suture zone in the early Paleozoic continued to the end of the Devonian possibly.In about (359.3±5.7)Ma the oceanic crust subduction in Dashantou-Heishan still affected magmatism activity in initial rift of continental margin.