藏南库局淡色花岗岩锆石U-Pb年龄、岩石地球化学特征及地质意义

Zircon U-Pb Chronology,Petrogeochemistry and Their Geological Implication for the Kuju Leucogranite in South Tibet

库局淡色花岗岩大地构造上位于喜马拉雅特提斯造山带中东部,淡色花岗岩呈不规则状、近椭球状侵位于古生代热拉岩组中。其中白云母二长花岗岩LA-ICP-MS锆石U-Pb年龄为21±0.3Ma(MSWD=4.3),时代为中新世。岩石具有高SiO2、富Al2O3与K2O,贫CaO、MgO、TiO的特征。A/CNK=1.03~1.15(平均1.13),3属于高钾钙碱性系列过铝质岩石。稀土元素球粒陨石标准化分布曲线总体显示向右倾斜的轻稀土富集Eu亏损型。大离子亲石元素Rb、Th、U、K富集,高场强元素Nb、Ta、Zr、Ti呈现低谷负异常,显示具同碰撞S型花岗岩特征,成岩岩浆源自地壳。结合最新资料及本文研究成果,初步认为库局淡色花岗岩形成于STDS启动之后的伸展减薄背景下,为印度板块和欧亚大陆碰撞过程中地壳物质部分熔融的产物。

The Kuju leucogranite(monzogranite)intrusive occurs as irregular and nearly ellipsoidal form in the middle and eastern parts of the Himalayan Tethys orogenic belt and was emplaced in the Paleozoic Rela Formation.The monzogranite yields LA-ICP-MS Zircon U-Pb age values of 21±0.3Ma(MSWD=4.3).The rock is rich in SiO2,Al2O3 and K2O,and poor in CaO,MgO and TiO2 with A/CNK ratio of 1.03~1.15(at an average of 1.13),belonging to the high-potassium-calc-alkalic peraluminous rock.The chondrite-normalized rare-earth element pattern shows LREE enrichment and depletion in Eu.The large ion lithophile elements Rb,Th,U and K are enriched,and the high field-strength elements Nb,Ta,Zr and Ti exhibit trough negative anomalies,showing the characteristics of syn-collision S-type granites and crust origin.It is preliminarily believed that the Kuju leucogranite was emplaced under the background of stretching and thinning after the start of STDS,being the product of partial melting of crustal material during the collision of the Indian Plate with the Eurasia Plate.