西秦岭天水地区晚三叠世柴家庄二长花岗岩及其暗色包体地球化学——岩石成因及岩浆混合作用

摘要

晚三叠世花岗岩类在秦岭-大别造山带西端广泛分布,其成因机制及地球动力学背景的研究对于反演华北、扬子两大板块沿秦岭-大别造山带在三叠纪时期的拼合历史具有重要意义.本文选择西秦岭天水地区柴家庄晚三叠世二长花岗岩及其中的暗色包体进行精细的岩石学和地球化学研究.暗色包体中普遍发育针状磷灰石及斜长石捕掳晶,暗示岩浆混合作用;暗色包体具有较低的SiO2(60.27％～60.38％)、高的Mg#(54～55)和Nb/Ta比值(14.8～16.6),表明其来源于富集岩石圈地幔的部分熔融作用;寄主二长花岗岩表现出典型埃达克岩的地球化学特征,其富集Sr、Ba,亏损Y和HREE,岩石的Sr/Y比值介于88～98之间,Y/Yb比值介于13～15之间,暗示源区有石榴石残余.结合前人的研究结果,提出柴家庄二长花岗岩可能为增厚的造山带下地壳在碰撞后伸展环境下发生部分熔融作用的产物,可能与晚三叠世时期秦岭造山带的板片断离作用有关.%The Late Triassic granitoid is widely distributed in the western Qinling-Dabie orogenic belt.The paper chooses the Late Triassic monzogranite and mafic enclaves from Chaijiazhuang pluton of western Qinling Mountains for detailed petrological and geochemical study.Acicular apatite and plagioclase megacrysts are developed in the mafic enclaves,suggesting the magmatic mixing process.The mafic enclaves are characterized by low SiO2 (60.27％-60.38％),high Mg# (54-55) and Nb/Ta ratio (14.8-16.6),which indicates that they were derived from the partial melting of subeontinental lithospheric mantle.Geochemically,the host monzogranite shows adakitic affinity with enrichment of Sr and Ba,depletion of Y and HREEs,high Sr/Y ratio (88-98) and Y/Yb ratio (13-15),hinting at the residue of garnet in the source area.Combining with previous researches,it is suggested that the monzogranite in the study area is the product of partial melting of thickened lower crust under the post-collision extension background caused by the Late Triassic subducting slab break-off in Western Qinling orogenic belt.