东昆仑祁漫塔格地区晚三叠世正长花岗岩岩石成因及构造意义

Petrogenesis and tectonic implications of the Late Triassic syenogranite in Qimantag area, East Kunlun Mountains

青海省东昆仑祁漫塔格地区肯德可克矿区外围东部发育一正长花岗岩体,主要矿物组合为正长石(50%~60%)+石英(20%~30%)+斜长石(10%~20%)+黑云母(1%~5%)。其LA-ICP-MS锆石U-Pb加权平均年龄为217.9±1.7 Ma(MSWD=0.74,n=20),形成时代为晚三叠世,与祁漫塔格地区铁多金属矿床基本同时形成。岩石地球化学组成具有高硅(Si O2=74.53%~75.28%)、富碱(K2O+Na2O=8.81%~8.95%)、富铁贫镁(Fe OT/Mg O=18.02~31.48)的特征,并具强烈的负Eu异常(δEu=0.04~0.05),富集Rb、Th、U、K、Ga,亏损Sr、Ba、Ta、P、Ti,显示其为准铝质A型花岗岩。正长花岗岩锆石εHf(t)为2.0~12.4,平均6.4,显示其源区具有壳幔混合作用的特征,壳幔物质交换为区内铁多金属矿化提供了大量成矿物质。该正长花岗岩属A2型花岗岩,暗示其形成于造山后的伸展构造体制,反映了祁漫塔格地区晚华力西-印支期造山旋回于晚三叠世由造山后期转为伸展阶段。

The syenogranite in Qinghai Province is located on the east periphery of the Kendekeke deposit in the Qimantag area, East Kunlun Mountains. It consists of alkali feldspar （50% - 60% ）, quartz （20% ~ 30% ）, pla- gioclase （10% -20%）, and biotite and hornblende （1% -5%）. The results of LA-ICP-MS zircon U-Pb isotope dating reveal that the syenogranite was formed at 217. 9 ± 1.7 Ma （ MSWD = 0. 74, n = 20）, suggesting the Late Triassic and the same time as the metallogenic age of the iron-polymetallic deposit in Qimantag area. The syenogranite is characterized by high silicon （ SiO2 = 74. 62% - 75.28%）, high alkali （ K2O ＋ Na20 = 8.81± 8.95 %）, and fairly high ratio of FeOT/MgO （ 18.02 ± 31.48）. The trace element and REE compositions show the features of significant negative Eu anomalies （SEu = 0. 04 - 0. 05）, enrichment of Rb, Th, U, K, Ga, and depletion of Sr, Ba, Ta, P, Ti. The geochemical characteristics indicate that the syenogranite belongs to metaluminous- weakly peraluminous A-type granite. The ~Hf（t） values of syenogranite vary from 2.0 to 12.4 with the two-stage model ages being tDM2 of ] 126 ~458 Ma. It is held that the source region of syenogranite probably had a mixture with the mantle and ancient crust, which provided abundant materials for the iron-polymetallic mineralization in this area. In the discrimination diagrams of A-type granite, the syenogranite falls into A2-type granites, which suggests that it was formed in an extensional tectonic regime after the orogenic process. All these characteristics display a transition of the Late Variscan- Indosinian orogenic cycle from the epi-orogenic stage to the extensional tectonic regime in Qimantag area in the late Triassic.