西藏冈底斯斑岩铜矿带埃达克质斑岩含矿性:源岩相变及深部过程约束  

Copperore potential of adakitic intrusives in Gangdese porphyry copper belt: Constrains from rock phase and deep melting process

作　　者：侯增谦[1] 孟祥金[1] 曲晓明[1] 高永丰[2] 

机构地区：[1]中国地质科学院矿产资源研究所,北京100037 [2]石家庄经济学院,河北石家庄050031

出　　处：《矿床地质》2005年第2期108-121,共14页Mineral Deposits

基　　金：国家基础研究规划 973项目 (2 0 0 2CB412 60 0 );国家自然科学杰出青年基金 (4 0 42 5 0 14 )联合资助

摘　　要：西藏冈底斯斑岩铜钼成矿系统(13.6～16 .9Ma)发育在印_亚大陆后碰撞地壳伸展环境。成矿前斑岩成岩年龄≥17Ma ,以花岗闪长斑岩为主,成矿期斑岩形成于14 .5～17.6Ma之间,以二长花岗斑岩和石英二长斑岩为主,成矿后斑岩为花岗斑岩,其成岩年龄为11.2Ma。3期斑岩均为高钾钙碱性或钾玄岩系列,地球化学上类似于玄武质下地壳部分熔融产生的埃达克质岩。成矿前斑岩具有最低的ΣREE(2 7×10 -6～4 5×10 -6)、wY(2 .9×10 -6～3.4×10 -6)和wSm/wYb(3.0～4 .9) ,最高的wZr/wSm值(5 0～118) ;成矿后斑岩具有最高的ΣREE (12 2×10 -6～197×10 -6)和wY(8.2×10 -6) ,中等的wSm/wYb(5 .9～6 .2 )和wZr/wSm值(34～4 4 ) ;成矿期斑岩总体处于两者之间,其Sr_Nd同位素组成与CordilleraBlanca埃达克质花岗岩类似。研究提出,来自深部的软流圈物质或亏损地幔物质与下地壳物质交换,不仅导致冈底斯加厚、下地壳熔融,而且提供了巨量金属供应。部分熔融首先从下地壳底部开始,逐渐向上部迁移。下地壳石榴石角闪岩部分熔融过程中,残留相由角闪石向石榴石大规模转变导致角闪石的大量分解,释放出大量流体,是冈底斯斑岩含矿性的主导因素。The Gangdese porphyry Cu-Mo system in southern Tibet (13.6～16.9 Ma) was generated during the post-collision crustal extension. The pre-ore porphyries mainly consist of granodiorite porphyries with ages older than 17 Ma, whereas the porphyries of the ore-forming stage are mostly composed of monzonitic granite and adamellite porphyries with zircon SHRIMP ages ranging between 14.5 and 17.6 Ma. The post-ore porphyries are comprised of granite porphyries with a young Ar/Ar age of 11.2 Ma. Geochemical studies indicate that these porphyries are of shoshonitic and high-K calc-alkaline series, and show geochemical affinity with adakites derived from partial melting of a thickened basaltic lower crust. The pre-ore porphyries have the lowest contents of ΣREE (27×10 -6 ～45×10 -6 ), Y(2.9×10 -6 ～3.4×10 -6 ) and the lowest Sm/Yb ratio (3.0～4.9), coupled with the highest Zr/Sm ratio (50～118). The post-ore porphyries have the highest contents of ΣREE (122×10 -6 ～197×10 -6 ) and Y (8.2 ×10 -6 ), and intermediate ratios of Sm/Yb (5.9～6.2) and Zr/Sm (34～44). The porphyries of the ore-forming stage are transitional between the pre- and post-ore porphyries in terms of Y content and Sm/Yb and Zr/Sm ratios, but their Sr-Nd isotopic signature is similar to that of the adakitic rocks from Cordillera Blanca. Major, trace elements and Sr-Nd geochemistry of these adakitic porphyries indicates that partial melting of a thickened lower crust beneath southern Tibet involves input of materials derived from the depleted mantle, which provides necessary heat and metals (Cu, Au) for the generation of Cu-bearing adakitic melts. Upward migration of the melt zone from basal amphibole eclogite to upper garnet amphibolite proceeds gradually with time, but the transition of residual phase from amphibole-bearing to garnet-bearing assemblages in the garnet amphibolite source during melting is the fundamental and important process for the formation of the fertile adakite and porphyry Cu systems in southe

关 键 词：地质学 源岩相变 深部过程 埃达克质斑岩 冈底斯斑岩铜矿 

分 类 号：P618.41[天文地球—矿床学]