东昆仑卡尔却卡地区中志留世A型花岗岩岩石成因及构造环境

Petrogenesis and tectonic setting of Middle Silurian A-type granite in Kaerqueka area,East Kunlun

东昆仑造山带西部卡尔却卡地区出露大量似斑状二长花岗岩,对其展开岩相特征、锆石U-Pb年龄和岩石地球化学研究,结果显示,似斑状二长花岗岩年龄为425±2.3 Ma,表明其形成于中志留世。岩石具有高SiO 2(69.0%~71.2%)和K 2 O(4.3%~5.54%)含量,富集大离子亲石元素(Rb、K、Th和U),亏损高场强元素(Nb、Ta和Ti),强烈的负Eu异常(Eu/Eu=0.40~0.51)特征。岩石较低的Cr、Ni含量(Cr=13.81×10^(-6)~32.9×10^(-6),Ni=5.48×106~12.05×10^(-6))、低的Ce/Pb(2.01~4.19)和Nb/U(4.1~7.57)值,表明该岩体来自上地壳的部分熔融。岩体高10000Ga/Al值(3.22~3.42)、Zr+Nb+Ce+Y元素含量(459×10^(-6)~656×10^(-6))和锆石饱和温度(827~881℃),表明其为A型花岗岩。构造图解和高的Y/Nb值(2.93~3.15)指示,该岩体为A 2型花岗岩,形成于碰撞后伸展背景。结合区域岩浆岩的时空分布,表明东昆仑造山带从中志留世开始进入碰撞后伸展阶段。

A lot of porphyritic monzogranites are outcropped in the Kaerqueka area,west of the East Kunlun orogenic belt.Zircon U-Pb age and geochemistry of the porphyritic monzonites were studied.Zircon U-Pb dating yields an age of 425±2.3 Ma,indicating Middle Silurian.The rocks are characterized by high contents of SiO 2(69.0%~71.2%)and K 2 O(4.3%~5.54%),enrichment of large ion lithophile element(Rb,K,Th and U),depletion of high field strength element(Nb,Ta and Ti),and strong Eu negative anomaly(Eu/Eu=0.40~0.51).Their lower content of Cr,Ni(Cr=13.81×10^(-6)~32.9×10^(-6),Ni=5.48×10^(-6)~12.05×10^(-6))and the lower Ce/Pb(2.01~4.19)and Nb/U(4.1~7.57)values indicate that they were originated from partial melting of the upper crust.The high ratio of 10000Ga/Al(3.22~3.42),high contents of Zr+Nb+Ce+Y(459×10^(-6)~656×10^(-6))and zircon saturation temperature(827~881℃)imply they belong to A-type granite.Tectonic diagram and high Y/Nb ratio(2.93~3.15)indicate that they are A 2 type granites,and were formed in the post-collision extension setting.Combined with the spatial and temporal distribution of magmatic rocks in the region,it is suggested that the East Kunlun Orogen entered the post-collision extension stage from the Middle Silurian period.