西藏改则蛇绿岩中斜长花岗岩地球化学特征、锆石U-Pb年龄及构造意义

Geochemical characteristics and zircon U-Pb age of the plagiogranite in Gaize ophiolite of central Tibet and their tectonic significance

西藏改则蛇绿岩主要由地幔橄榄岩、均质辉长岩、玄武岩、玄武安山岩和斜长花岗岩组成。其中斜长花岗岩主要由石英、基性斜长石组成,SiO2含量较高,为72.18%～74.55%,Mg#均值为42,Na2O含量为1.30%～3.13%,K2O含量很低,为0.26%～0.67%,Na2O/K2O变化范围为3.64～8.23。斜长花岗岩和中基性岩(辉长岩、玄武岩和玄武安山岩)的元素地球化学特征表明,改则斜长花岗岩可能是由基性岩部分熔融形成的,并且斜长花岗岩富集Sr、Rb等大离子亲石元素,亏损Nb、Ta、Ti等高场强元素,具有岛弧型火山岩的特点,推测该斜长花岗岩形成于岛弧环境,是SSZ型蛇绿岩的组成单元。LA-ICPMS法测得斜长花岗岩中锆石U-Pb加权平均年龄为189.8±1.9Ma,表明班公湖-怒江缝合带改则地区在早侏罗世发生了俯冲作用,该区的俯冲消减时间要早于西段的班公湖地区,晚于东段丁青地区。

Located in central Tibet,the Gaize ophiolite is a key element within the middle part of the Bangong Co-Nujiang suture zone,marking the boundary between the Lhasa and Qiangtang blocks.It is a tectonic mélange consisting of numerous blocks of mantle peridotite,mafic lavas,isotropic gabbro and plagiogranite,in which,the genesis and tectonic setting of the plagiogranite is important in discussing the evolution of the Bangong Co-Nujiang suture zone.Based on detailed studies of field geological background and petrographical features,the authors selected some samples to analyze the whole-rock content of major elements,trace elements and rare earth elements and determine the ages of zircons separated from the plagiogranite by La-ICP-MS U-Pb method.The results show that the plagiogranite crops out as dykes intruding into gabbro,basalt and basaltic andesite with no thermal aureole along the boundary between the plagiogranite and associated lavas comprising basalt and basaltic andesite.The plagiogranite is mainly composed of quartz and plagioclase with granitice tex-ture.The content of SiO2 is high,varying in the range of 72.18%～74.55% with the Mg# of 42,and the content of Na2O and K2O is 1.30%～3.13% and 0.26%～0.67%,respectively,with high Na2O/K2O ratios ranging from 3.64 to 8.23.The chondrite normalized REE patterns of the plagiogranite are similar to those of the associated lavas with flat pattern and weak negative anomaly of Eu （δEuN=0.82～0.95）.La and Yb versus SiO2 correlations of the plagiogranite and associated lavas and isotropic gabbro reveal that the plagiogranite resulted from the magma remelting from the associated lavas.Like the associated lavas,the plagiogranite rocks have island arc affinity with HFSE depletion and LILE （Sr,Rb） enrichment,and Nb,Ta and Ti negative anomalies in chondrite-normalized plots indicate that these rocks might have originated in the suprasubduction zone setting.Thus,the plagiogranite and the associated lavas are considered to be members of the SSZ-type ophiolite.The grains of zircons separated from the plagiogranite are about 40～60 μm in length,with no residual old nuclear and metamorphic edge but the development of banded structure.The values of Th/U between 0.32 and 1.38 （higher than 0.1） suggest that they are magmatic zircons.The average age of zircons in the plagiogranite is 189.8±1.9 Ma,suggesting that the plagiogranite was formed in early Jurassic.The evidence of geochemistry and U-Pb age supports the hypothesis that the subduction occurred at the early Jurassic period in Gaize area within the middle part of the Bangong Co-Nujiang suture zone,earlier than the subduction in the Bangong Co area within the western part of the suture where the activity took place in mid-Jurassic time,but later than the subduction in the Dingqing area within the eastern part of the suture where the activity happened in late Triassic.Combined with previous studies,the authors believe that the Bangong Co-Nujiang Tethys subduction started from east to west during the late Triassic to Jurassic period.