西昆仑慕士塔格-公格尔印支期侵入岩岩石与锆石地球化学特征及研究意义

Chronology,Geochemistry,Hafnium Isotope Characteristics and Tectonic Implications of Muztag-Kongur Indosinian Intrusive Rocks

西昆仑造山带南侧的麻扎-康西瓦缝合带,是古特提斯洋闭合的位置。慕士塔格-公格尔作为昆仑山的主峰,紧邻该缝合带的东北侧分布,主要岩性为花岗闪长岩和黑云母二长花岗岩。作者系统研究了两种岩性的地球化学及年代学特征,探讨了岩石成因,反演了古特提斯洋的构造演化历史。岩体岩浆锆石LA-ICP-MS U-Pb测年结果显示,花岗闪长岩和黑云母二长花岗岩的成岩年龄分别为(213.0±0.5)^(215.4±0.9)Ma和(220.6±0.5)^(222.1±0.4)Ma,是晚三叠世岩浆活动的产物。两种岩性均为高硅(w(SiO2)>65%)、富碱(w(K2O+Na2O)>6%)、钙碱性-高钾钙碱性、准铝质(A/CNK<1),富集大离子亲石元素(LILE)和轻稀土元素(LREE),亏损高场强元素(HFSE)和重稀土(HREE)。微量元素组成特征、低锆石饱和温度及高分异指数显示慕士塔格-公格尔花岗岩体为高分异I型花岗岩。岩体锆石的εHf(t)值变化范围较小,为-4.46^-0.17,指示岩浆以壳源为主。综合研究表明,慕士塔格-公格尔花岗岩体可能是同碰撞造山环境下,老的下地壳受地幔热源影响部分熔融,形成的长英质壳源岩浆侵入地壳内部而冷却结晶形成。

Mazha-Kangxiwa suture zone（MKSZ）,located to the south of the western Kunlun orogenic belt,is the location where Paleo-Tethys ocean finally closed.As the main peaks of Kunlun Mountain ranges,Muztag-Kongur granitoids closely distributed to the northeast of MKSZ and composedmainly of granodiorite and monzonitic granites.Based on the geochemical characteristics and U-Pb dating of Muztag-Kongur granitoids,we discussed the petrogenesis of granitoids and the evolution history of Paleo-Tethys ocean.The zircon LA-ICP-MS U-Pb dating results show that the forming ages of granodiorite and monzonitic granite are at（213.0±0.5）-（215.4±0.9）Ma and（220.6±0.5）-（222.1±0.4）Ma respectively,which is the result of Late Triassic magmatism activities.Muztag-Kongur granitoids contain high silicon（SiO2〉65%）and total alkali（K2O＋Na2O〉6%）,and are characterized by high K calc-alkaline and metaluminous（A/CNK〈1）.They are enriched in LILE and LREE and depleted of HFSE and HREE.The contents of minor elements,low zircon saturation temperature,and high differentiation index suggest that Muztag-Kongur granitoids are highly fractionated I-type granites.The zircon Hf isotopic results show a narrow range ofεHf（t）=-4.46--0.17,suggesting that the magmas originated mainly from lower crust.The comprehensive studies indicate that Muztag-Kongur granitoids formed likely in a syn-collisional orogenic setting where old lower crust melted partially under the heating of uprising mantle magmas.These felsic magmas intruded into crust level and formed Muztag-Kongur granitoids.