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

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 melange 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(superscript #) 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.