希夏邦马峰淡色花岗岩岩石成因初步研究

A preliminary study on petrogenesis of the Shisha Pangma leucogranites

淡色花岗岩是研究碰撞造山带地球动力学过程的重要对象。本文对喜马拉雅造山带希夏邦马峰海拔5800m大本营附近的淡色花岗岩样品进行了独居石U-Th-Pb年代学和全岩地球化学初步分析。研究样品根据矿物组合可分为黑云母花岗岩和二云母花岗岩,二者地球化学特征以及变形程度都有所差异,全岩地球化学中弱变形的黑花岗岩表现出低Si和富Al、Mg、Fe、K、Ti的特征,铝饱和指数A/CNK为1.06;强变形和弱变形的二云母花岗岩均表现出高Si和贫Mg、Fe、K、Ti的特征,A/CNK为1.11~1.17。同黑云母花岗岩相比,二云母花岗岩稀土元素中Eu元素负异常明显,微量元素中富集Sn、U,亏损Ba、Sr、Zr、Hf、LREE等元素,暗示岩浆经历了斜长石、黑云母、锆石、独居石和磷灰石的分离结晶作用。黑云母花岗岩和二云母花岗岩中的独居石给出U-Th-Pb年龄为22.9±0.2Ma、22.7±0.5Ma和22.7±0.7Ma的一致结果,这一年龄略早于前人报道的结果,指示藏南拆离系活动时间在该区域与喜马拉雅中段具有一致性。本文认为希夏邦马淡色花岗岩岩浆作用与碰撞后伸展阶段发育的藏南拆离系活动具有同时性。

Leucogranite plays a vital role in studying geodynamic processes within collisional orogenic belts.This study conducts a preliminary geochronological and geochemical analysis of leucogranite samples from the Himalayan orogenic belt near the climbing base camp at the Mount Shisha Pangma s 5800m.The samples are categorized into biotite granite and two-mica granite based on mineral assemblages,differing in geochemical characteristics and deformation.Biotite granite exhibits low Si but is rich in Al,Mg,Fe,and K in whole-rock geochemistry,with an aluminum saturation index A/CNK of 1.06.Two-mica granite,on the other hand,shows high Si content but is deficient in Mg,Fe,K,and Ti characteristics with A/CNK of 1.11~1.17.Two-mica granite displays noticeable negative anomalies of Eu in rare earth elements compared to biotite granite,being enriched in Sn,U,and depleted in Ba,Sr,Zr,Hf,etc.This indicates crystallization of plagioclase,biotite,zircon,monazite,and apatite within the magma.Monazite in both types of granite yields consistent U-Th-Pb ages of 22.9±0.2Ma,22.7±0.5Ma,and 22.7±0.7Ma,slightly earlier than previously reported,suggesting the development of the South Tibetan Detachment System in the region in line with the central Himalayan orogen.The study proposes that the Shisha Pangma leucogranites represent magma active concurrently with the South Tibetan detachment during the post-collisional extension stage.