东昆仑东段香加南山花岗岩基斜长石成分组成与岩浆演化和混合作用

摘要

Granitic batholith in Xiangjiananshan in the eastern East Kunlun Orogenic Belt presents the distinct features of magma mixing.Plagioclase,as a main rock-forming mineral,is an effective tool to study petrogenesis,magma evolution and magma mixing.Petrography and mineral chemistry of plagioclases in host rock and its mMic microgranular enclaves (MMEs) in granitic batholith of Xiangjiananshan were studied through electron microporobe (EMPA).The results show that the plagioclases with oscillatory zonings(39～48,21～36) and plagioclase-bearing minerals(29～45,14～32) in host rock and the plagioclases xenocrystals (41 ～ 49,35 ～ 36,43 ～ 49,31 ～ 47,27 ～ 38) in MMEs have a serial geochemical evolution of anorthite-albite component value (An value),and part of plagioclase core has high An values (59～72) due to late fluid alteration.Most plagioclases in matrix of the MMEs have core-rim structures with An-value discontinuities from core (52,31) to rim (33 ～ 37,25).Minor plagioclases have lower An values (49)in cores due to alteration,with higher An values in cores(55～71)than in rim(46～49).In addition,some plagioclases in the matrix are jigsaw-like patches,with high An(66)in dark areas and low An(33～39)in light area.The plagioclase xenocrystals in MMEs have been divided into two categories according to mineral contents.The "clean" ones without minerals have apparent oscillatory zonings and polysynthetic twins,and a narrow fluctuated range of An-value(41～49,35～36).Whilst the "dirty" ones with minerals have an unsmooth fluctuation of An-values (43～49,31～47,27～38),with higher An-values in some testing points(78)and accretion rim(73)due to alteration.The studies above show that the normal evolutional sequence of plagioclases in the host rock represents the magmatic evolution of the host rocks with the magma evolving from mafie to feslie gradually.After the MMEs entered the host rock,the plagioclases in the MMEs would be eroded and crystallized due to decreasing of temperature,pressure and water saturation.Plagioclases xenocrystals in enclaves,which originated from host rock,vary in composition and structure due to change in temperature and pressure as well as growth at the late stage.The complex oscillatory zonings of plagioclases in granitic batholith of Xiangjiananshan and their MMEs are the results of injection of mantle-derived mafic magma into felsie magma.%东昆仑东段香加南山花岗岩基岩浆混合作用明显,斜长石作为主要造岩矿物,是研究岩石成因、示踪岩浆演化和岩浆混合过程的有效工具.对香加南山花岗岩基中寄主岩及暗色微粒包体中的斜长石进行岩相学和矿物化学研究.电子探针结果显示:寄主岩中正常环带斜长石(39～48,21～36)、包体中斜长石捕掳晶(41～49,35～36,43～49,31～47,27～38)和寄主岩矿物中包裹斜长石(29～45,14～32)具有演化的An值;部分寄主岩斜长石核部由于受到后期蚀变具有较高的An值(59～72).包体中基质斜长石大部分具核边结构,核部(52,31)和边部(33～37,25)An值存在间断;少量斜长石核部受到蚀变,An值较低(49),幔部(55～71) An值高于边部(46～49);部分包体中基质斜长石核部呈补丁状,暗色部分An值较高(66),浅色部分An值较低(33～39).包体中斜长石捕掳晶主要分为干净斜长石捕掳晶和含有矿物的斜长石捕掳晶两大类,干净斜长石环带明显或聚片双晶发育,An值变化范围较小(41～49,35～36);含有暗色矿物的斜长石捕掳晶An值整体也呈震荡变化(43～49,31～47,27～38),但由于受到蚀变,部分测点An值较高(78),少量斜长石具有高An值增生边(73).以上研究显示,结晶于寄主岩的斜长石正常演化序列反映寄主岩从演化早期到晚期,岩浆逐渐从偏基性向酸性转变;包体基质斜长石为包体进入寄主岩温度、压力和水饱和度降低导致斜长石受到熔蚀后继续结晶结果;包体中斜长石捕掳晶来自寄主岩,由于进入包体后温度和压力产生变化,以及后期生长,导致斜长石的成分和构造有所不同.香加南山花岚岩基及暗色微粒包体中斜长石的复杂环带为幔源镁铁质岩浆注入长英质岩浆混合作用的结果.