柴北缘西段绿梁山早古生代复式花岗岩体成因

Petrogenesis of the Lüliangshan Early Paleozoic Composite Granite Pluton in North Qaidam Tectonic Belt

为探讨柴北缘西段绿梁山复式岩体的成因及其与大陆深俯冲-折返过程的关系,对其开展了系统的岩石学、地球化学和同位素年代学研究.结果表明,该复式岩体由三期中酸性侵入岩构成,是柴北缘构造带早古生代陆壳深俯冲-折返到造山带垮塌过程的岩浆响应.其中,Ⅰ期为似斑状花岗闪长岩,规模较小,形成年龄为436~430 Ma,是陆壳深俯冲/碰撞过程中加厚镁铁质下地壳部分熔融的产物;Ⅱ期为英云闪长岩-正长花岗岩-二长花岗岩,年龄介于400~390 Ma,由折返的超高压榴辉岩和下地壳镁铁质岩石在地壳伸展背景下部分熔融形成;Ⅲ期二云母花岗岩为绿梁山复式岩体的主体,形成年龄为365~360 Ma,具强过铝质S型花岗岩特征,是造山带去根、垮塌阶段中、上地壳变泥质岩部分熔融的产物.

Magmatism plays an important role in rebuilding evolutionary history of orogenic belts.This paper presents a systematic study of petrology,geochemistry and geochronology on the Lüliangshan granite pluton at west segment of the North Qaidam ultrahigh pressure metamorphic belt.The results show that the Lüliangshan pluton is a composite granite pluton formed in three magma periods in response to continental crust subduction,exhumation and mountain collapse.The first period(436-430 Ma)formed mainly small scale porphyry granodiorite,which is partial melting products of thickened mafic lower crust during continental subduction.The second period(400-390 Ma)formed tonalite and minor synogranite and monzonitic granite,which are partial melting products of exhumed eclogite-bearing ultrahigh pressure metamorphic terrane and lower continental crust during post-collisional extension.The third period(365-360 Ma)formed large scale two-mica granite,with strong peraluminous S-type granite geochemical characteristics and is partial melting products of metapelite during orogen unrooting and collapse.