班公湖-怒江成矿带西段材玛花岗岩体岩石地球化学及年代学

Geochemistry and geochronology of the Caima granites in the western part of the Bangong Lake-Nujiang metallogenic zone,Xizang

材玛花岗岩体为班公湖-怒江成矿带西段日土-多不杂岩浆弧带的成矿岩体之一。对材玛岩体的中粒黑云二长花岗岩进行锆石LA-ICP-MS U-Pb同位素测试,结果为165.1±1.5Ma(n=17,MSWD=0.86);全岩Rb-Sr同位素年龄为163.5±2Ma(n=5),材玛岩体的形成年龄为163～165Ma(中侏罗世)。材玛岩体属于高钾钙碱性系列,ΣREE=109.5～225.2(10-6),LREE富集,LREE/HREE=2.37～7.77,并伴随Eu的亏损。微量元素特征表现为着强烈的Ba、Nb、P、Ti亏损和Th、U、Pb富集,以及Zr的弱亏损。材玛岩体为班-怒带向北俯冲作用的产物,为岛弧型岩浆岩,物质来源为俯冲带之上的地幔部分熔融,并有地壳物质混熔。

The caima granites are delineated as important metallogenic rock masses in the Rutog-Dobza magmatic arc zone in the western part of the Bangong Lake-Nujiang metallogenic zone,Xizang.The zircon LA-ICP-MS and U-Pb isotopic values of the medium-grained biotite adamellite were determined for the Caima granites,indicating an age of 165.1 ± 1.5 Ma（n=17,MSWD=0.86）.The whole-rock Rb-Sr isotopic ages are dated at 163.5± 2 Ma（n=5）.The analytical results suggest that the ages of the Caima granites should be traced back to the Middle Jurassic（163 to 165 Ma） rather than the formerly dated the Early Cretaceous.The Caima granites belonging to the high-K calc-alkaline series have the following characteristics： the enrichment of LREE,ΣREE=109.5×10-6–225.2×10-6,LREE/HREE ratios=2.37-7.77,depletion of Eu,highly depletion of Ba,Nb,P and Ti,enrichment of Th,U and Pb,and slightly depletion of Zr.It is inferred that the Caima granites may be the products of the northward subduction of the Bangong Lake-Nujiang oceanic crust and originated from partly melting of the mantle above the subduction zone and the melting and mixing of the crustal matter.