冈底斯曲水岩基斜长石嵌晶结构对岩浆补给过程的启示

Poikilitic texture of plagioclase in the Quxu batholith of the Gangdese belt,southern Tibet:Implications for magma recharge process

藏南冈底斯岩浆带经历了中生代俯冲-增生造山作用和新生代碰撞造山作用,是研究大陆地壳生长与演化的理想场所。位于冈底斯岩浆带中段的早始新世曲水岩基,主要由花岗岩、花岗闪长岩、闪长岩及辉长岩等组成。前人已经在年代学、地球化学等方面对曲水岩基进行了大量研究。然而,对于岩基的形成是否由岩浆补给控制及其详细过程的研究还较为薄弱。花岗闪长质岩体中暗色岩浆包体特殊的产出状态,如包体墙和弥散状分布的包体等,显示岩浆补给在岩基形成过程中发挥了重要作用。包体中发育斜长石主晶包裹角闪石等客晶的嵌晶结构。本文聚焦包体斜长石的嵌晶结构及客晶矿物(如角闪石),结合背散射图像、矿物能谱扫描与矿物电子探针分析,以追踪曲水岩基形成期间的岩浆补给过程。计算结果显示客晶角闪石的结晶温度与压力最高(783~853℃、0.23~0.45GPa),包体基质角闪石次之(781~808℃、0.21~0.31GPa),花岗闪长质寄主岩中角闪石最低(769~802℃、0.18~0.26GPa)。此外,发育嵌晶结构的斜长石具有明显的成分环带,其偏钠质的核部指示了晶粥体的存在。本研究初步构建了岩浆补给模型与嵌晶结构的形成模型。镁铁质岩浆沿长英质晶粥体下部裂隙上升并淬火结晶大量小颗粒角闪石,随后进入晶粥体上部贫晶熔体中破碎成为小的岩浆滴。岩浆滴捕掳晶粥体中的斜长石,继续生长的斜长石包裹结晶于深部的角闪石而形成嵌晶结构。

The Gangdese belt in southern Tibet is an excellent location for studying the growth and evolution of continental crust as it has undergone the Mesozoic subduction-accretionary orogeny and the Cenozoic collisional orogeny.The Early Eocene Quxu batholith,located at central part of the Gangdese belt,consists of granite,granodiorite,diorite,and gabbro,etc.Lots of chronologic and geochemical studies have been done for the Quxu batholith.However,there are still debates about whether the formation of the Quxu batholith is controlled by magma recharge.Besides,the detailed processes of magma recharge are unclear.The wide distribution of magmatic microgranular enclaves in the granodiorite such as enclave dikes and dispersed enclaves,indicates the important role of magma recharge during the formation of the Quxu batholith.Poikilitic texture,with amphibole embedded in plagioclase,is developed in enclaves.Here,we will focus on the poikilitic texture of plagioclase and the chadacrysts(e.g.,amphibole)and attempt to track the processes of magma recharge through backscattered-electron imaging,mineral energy spectrum scanning,and mineral electron probe analysis.Analytical results reveal that the crystallization temperature and pressure of the amphibole chadacrysts are the highest(783~853℃,0.23~0.45GPa),followed by the amphiboles in the groundmass of enclaves(781~808℃,0.21~0.31GPa),and then the amphiboles in the granodioritic host rocks(769~802℃,0.18~0.26GPa).In addition,plagioclase with poikilitic texture develops obvious discontinuous zoning,and the sodic core indicates the existence of crystal mush.Based on these observations,we establish a tentative model to explain the process of magma recharge and the formation of poikilitic texture.Mafic magma rises along the fractures in the lower part of felsic crystal mush and quenches,leading to the formation of numerous small grains of amphibole.Subsequently,as the mafic magma enters the upper part of crystal mush,it breaks up into small droplets and captures plagioclase crystals from the felsic crystal mush.The plagioclase xenocrysts enclose amphiboles crystallized at deep level and produces the poikilitic texture.