北喜马拉雅佩枯花岗岩年代学、成因机制及其构造意义

北喜马拉雅花岗岩位于特提斯喜马拉雅的中部，对其研究不仅有助于认识和理解碰撞造山过程中地壳物质的熔融行为和机制, 而且对探讨部分熔融作用与相关构造的关系也具有重要意义.通过对北喜马拉雅佩枯花岗岩开展系统的LA-MC-ICP-MS锆石U-Pb年代学和地球化学研究，结果表明佩枯花岗岩的结晶时间较长，从23.9 Ma持续到16.5 Ma，并记录了22.3±0.6 Ma和17.3±0.3 Ma两期深熔作用.全岩地球化学分析结果显示，佩枯花岗岩具有高含量的SiO₂(71.87%~75.56%)、Al₂O₃(13.57%~15.49%)和K₂O(3.34%~4.59 %)，以及高的K₂O/Na₂O比值(1.02~1.39) 和A/CNK值(1.21~1.23)，属于高钾钙碱性过铝质花岗岩.岩石强烈富集大离子亲石元素Rb和放射性生热元素Th、U，亏损Ba、Nb、Sr、Zr等元素；轻重稀土元素分馏较强((La/Yb)_N=10.76~16.60)，几乎无或弱的负Eu异常(δEu=0.76~0.97).样品的(87Sr/86Sr)_i值和ε_Nd(t)值变化范围分别为0.736 184~0.741 258和-14.6~-14.3，与大喜马拉雅变质沉积岩的Sr-Nd同位素组成一致，表明其源岩可能为大喜马拉雅变质沉积岩.样品(87Sr/86Sr)_i值较低而Sr浓度较高，且随着Ba浓度的增加，Rb/Sr比值基本不变，与水致白云母部分熔融的特征和趋势一致，表明佩枯花岗岩是水致白云母部分熔融的产物，部分熔融作用可能与藏南拆离系的活动密切相关. 

Geochronology and Formation Mechanism of the Paiku Granite in the Northern Himalaya,and Its Tectonic Implications

Situated in the middle of Tethyan Himalaya, northern Himalayan granites not only better our understanding of melting behaviors and mechanism of the crust during the collisional orogenic processes, but also facilitate the investigation and evaluation of tectonic-magmatic evolution of the Himalayan orogen.In this paper, we present geochronological, geochemical and Sr-Nd isotopic data of the Paiku granites in the northern Himalaya.LA-MC-ICP-MS zircon U-Pb dating reveals that Paiku granites were crystallized from 23.9 Ma to 16.5 Ma and have experienced at least two episodes of anatexis at 22.3±0.6 Ma and 17.3±0.3 Ma, respectively. The age of 17.3±0.3 Ma probably represents the final crystallized timing.Bulk-rock major and trace elements analyses indicate that Paiku granites were characterized by high SiO₂(71.87% to 75.56%), Al₂O₃(13.57% to 15.49%), K₂O(3.34% to 4.59%), and high values of K₂O/Na₂O(1.02 to 1.39), A/CNK(1.21 to 1.23), and enrichment in Rb, Th, U, depletion in Ba, Nb, Sr, Zr and no or weak negative Eu anomalies ((La/Yb)_N=10.76-16.60), and strong fractionation between LREE and HREE (δEu=0.76-0.97). These features demonstrate that Paiku granites belong to high potassium Calc-alkaline and peraluminous granites. The values of (87Sr/86Sr)_i and ε_Nd(t) range from 0.736 184 to 0.741 258 and from -14.6 to -14.3, and can compare well with the metasedimentary rock in the Greater Himalaya Crystalline complex (GHC). It is inferred that the Paiku granites were generated from partial melting of the GHC metasedimentary rocks. The granites show relatively high Sr, but low Rb and Rb/Sr which are nearly constant relative to large variations in Ba concentrations. These features are concordant with the trend of fluxed muscovite partial melting. Based on above evidences, we suggest the Paiku granites were derived from fluxed partial melting of the GHC metasedimentary rock, possibly related closely with the structural activity along the South Tibet detachment system (STDS).