松潘—甘孜造山带万里城花岗岩及其岩浆包体的成因与地球动力学意义

Petrogenesis and geodynamic implications of the Wanlicheng granites and hosted magmatic enclaves in Songpan-Garze orogen: Evidence from petrography and geochemistry

松潘—甘孜造山带广泛分布着三叠纪花岗岩体，其成因对正确认识研究区花岗岩浆的动力学背景具有重要意义。地球化学分析表明，万里城岩体寄主花岗岩具有高的SiO2含量（69.43%~73.10%）和较高的全碱含量，具弱过铝质（A/CNK=1.01~1.12）特征，属于高钾钙碱性—钾玄岩系列I型花岗岩类。暗色微粒包体具较低的SiO2含量（52.85%~59.50%）和较高的Mg#值（45~63），为准铝质高钾钙碱性二长（闪长）岩。包体为典型的岩浆细粒结构，发育针状磷灰石、环带结构斜长石、瞳状石英、反鲍文序列的不平衡岩浆结构等。微量与稀土元素分析表明，包体起源于壳幔混合作用，是底侵的幔源玄武质岩浆与上覆壳源长英质岩浆混合的产物，混合的熔体经历了钛铁矿、黑云母等矿物的分离结晶，最终形成万里城暗色微粒包体。而寄主花岗岩则起源于纯的长英质陆壳，岩石具有较低的Mg#值（21~39）、中等的CaO/（MgO＋TFeO）值、较高的K2O/Na2O和（Na2O＋K2O）/（TFeO＋MgO＋TiO2）值等，指示源区主要为变杂砂岩类。综合区域地质资料，提出松潘—甘孜造山带内大规模花岗质岩体的形成主要受控于碰撞后伸展背景下的玄武质岩浆底侵加热。

Numerous Triassic granitic plutons are extensively developed in the Songpan-Garze orogen, and their petrogenesis is im-portant for understanding the geodynamic settings of these granitic magma. The host granites show high content of SiO 2 （69.43%~73.10%） and total alkaline and are weakly peraluminous rocks （A/CNK=1.01~1.12）, belonging to high-K calc-alkaline to shoshonit-ic I-type granites. MMEs show low content of SiO2 （52.85%~59.50%） and Mg# （45~63） and are metaluminous high-K calc-alkaline monzodiorite or monzonite. The MMEs exhibit typical magmatic texture, with acicular apatite, zoned plagioclase, pupil-like quartz and other disequilibrium textures. Trace and rare earth elements studies show that the MMEs were derived from the crust-mantle mixing, i.e., underplated basaltic magma mixed with the overlying crustal felsic magma, and the mixed melt experienced the fraction-al crystallization of ilmenite and biotite. The host granites were derived from pure felsic crust, characterized by low ratios of Mg # （21~39）, intermediate CaO/（MgO＋TFeO）, high ratios of K2O/Na2O and （Na2O＋K2O）/（TFeO＋MgO＋TiO2）, indicating that the source was dominated by metagraywacke. Combined with regional geological data, the authors hold that the large-scale granitic magmatism was controlled by basaltic magma underplating in a post-collisional extensional setting.