西藏狮泉河地区仁温斜长花岗岩LA-ICP-MS锆石U-Pb年龄及其构造意义

摘要

s:The Renwen plagiogranite in Shiquan River area of Tibet consists of oligoclase, quartz, etc. The zircon U-Pb weighted average age measured by LA-ICP-MS is 162.5 ± 2.6Ma. The major elements are characterized by SiO2 content from 72.05% to 72.83%, Al2O3 content from 12.72%to 13.54%, MgO content from 1.19%to 1.24%, Mg# values from 35.73 to 42.27, Na2O content from 4.56%to 4.74%, low K2O content from 0.34%to 0.57%, and Na2O/K2O values from 8.32 to 13.41. The major element geo-chemistry of Renwen plagiogranite belongs to low potassium calc-alkaline series. Geochemical characteristics of plagiogranite and me-tabasite suggest that the Renwen plagiogranite may have been derived from partial melting of metabasite, which shows island-arc vol-canic characteristics, being enriched in LILE such as Rb, Sr, Ba and depleted in HFSE such as Nb, Ta, Ti. It is concluded that subduc-tion took place in Middle Jurassic in the Shiquanhe suture (the north branch).%西藏狮泉河地区仁温斜长花岗岩主要由更长石、石英等组成,SiO2含量为72.05%~72.83%,Al2O3含量12.72%~13.54%(<15%), MgO含量1.19%~1.24%,Mg#值35.73~42.27(<50),Na2O含量介于4.56%~4.74%之间,K2O含量较低,在0.34%~0.57%之间,Na2O/K2O值为8.32~13.41,属低钾钙碱性系列岩石.斜长花岗岩与变质基性岩的地球化学特征表明,仁温斜长花岗岩可能是变质基性岩部分熔融形成的,该岩石中Sr、Rb、Ba等大离子亲石元素富集,Nb、Ta、Ti等高场强元素明显亏损,具有岛弧型火山岩的特征.用LA-ICP-MS法测得斜长花岗岩锆石U-Pb年龄为162.5±2.6Ma,表明狮泉河结合带(北亚带)在中侏罗世已发生俯冲作用.