兴蒙造山带橄榄岩捕掳体中石榴石次变边对地幔组成转变的启示  

Implications on the compositional transformation of lithospheric mantle from garnet kelyphites in mantle peridotite xenoliths from the Xing an-Mongolia Orogenic Belt

作　　者：邹东雅 张宏福 ZOU DongYa;ZHANG HongFu(School of Earth Sciences,Zhejiang University,Hangzhou 310058,China)

机构地区：[1]浙江大学地球科学学院,杭州310058

出　　处：《岩石学报》2023年第1期104-118,共15页Acta Petrologica Sinica

基　　金：国家自然科学基金项目(41688103、42003026)资助.

摘　　要：地幔橄榄岩捕虏体中石榴石次变边的形成过程对理解地幔的构造演化和转变具有非常重要的意义。兴蒙造山带锡林浩特地区新生代玄武岩携带的石榴石橄榄岩捕虏体中的石榴石普遍发育冠冕状次变边结构。本文通过对石榴石及其次变边进行详细的岩相学和电子探针分析,探讨石榴石次变边的成因及其揭示的岩石圈地幔经历的深部过程。根据次变边矿物组成的不同,将其分为原始的次变边(R1和R2)和交代的次变边(MR1和MR2)。原始的次变边中,新鲜的石榴石由内向外依次被放射状且矿物颗粒较细的R1和粒状且矿物颗粒较粗的R2包围,且R1通常比R2宽。R1主要组成矿物为Opx+Sp+Melt1/Pl±Cpx,R2主要组成矿物为Opx+Sp+Cpx。与R2及橄榄岩捕虏体相比,R1的斜方辉石和单斜辉石具有较高的Al 2O 3含量和较低Mg#值及SiO 2含量。与橄榄岩捕虏体相比,R1和R2中的尖晶石均具有较低的Cr#值和较高的Mg#值。R1的斜长石为钙长石,熔体成分与斜长石相比具有偏高的MgO和FeO含量。计算的R1的全岩成分与新鲜的石榴石一致,是石榴石等化学分解的产物。R2的全岩成分比新鲜的石榴石具有偏高的MgO和偏低的SiO 2及Al 2O 3含量,是石榴石和橄榄石反应的产物。交代的次变边是由原始的次变边受到部分或完全的交代作用形成的。完全交代的次变边仍然保留原始次变边的双圈层结构,而未完全交代的次变边则仅在原始次变边的局部出现。交代的次变边中,矿物颗粒较细的核部(MR1)和矿物颗粒较粗的边部(MR2)主要矿物组成一致,皆为Ol+Cpx+Sp+Melt2。与原始的次变边相比,MR1和MR2中的橄榄石和单斜辉石均具有较高的Mg#值,单斜辉石同时具有较高Ca/Al比值(>8),尖晶石具有较高的Cr#值和较低的Mg#值,熔体较富SiO 2、Na 2O和K 2O含量。这些现象说明交代的次变边可能是碳酸盐熔/流体交代原始的次变边消耗斜方辉石生成橄榄石�Kelyphites around garnets in mantle peridotite xenoliths are vital to understand the tectonic evolution and transformation of the mantle.Abundant coronae kelyphites around garnets were found in the garnet peridotite xenoliths hosted by the Cenozoic basalt from the Xilinhot region of the Xing an-Mongolia Orogenic Belt(XMOB).Detail texture and mineral compositions of garnets and their kelyphites were analyzed to investigate the mantle processes beneath the XMOB.Kelyphites of garnets were classified into primitive and metasomatic types based on their different mineral assemblages.Fresh garnets were surrounded by two concentric coronae in the primitive kelyphites:the inner coronae(R1)with radial fine-grained minerals and the outer coronae(R2)with coarse-grained minerals,of which R1 is usually wider than R2.For the mineral assemblage,R1 is composed of Opx+Sp+Melt1/Pl±Cpx,while R2 mainly consists of Opx+Sp+Cpx.Orthopyroxene and clinopyroxene in the R1 show higher Al 2O 3 content and lower Mg#value and SiO 2 content than those in the R2 and peridotite xenoliths.Spinels in the R1 and R2 have similar lower Cr#and higher Mg#values than those in the peridotite xenoliths.Plagioclase in the R1 is anorthosite.Melts in the R1 show similar SiO 2 and CaO contents and slightly higher MgO and FeO contents than those of plagioclase.The bulk compositions of R1 are consistent with that of the fresh garnets,which indicates that the R1 was produced by isochemical breakdown of garnet.In contrast,the bulk compositions of R2 show slightly higher MgO and lower SiO 2 and Al 2O 3 contents than those in the fresh garnet,which were products of garnet-olivine reaction.The metasomatic kelyphites were formed by late metasomatism of the primitive kelyphites.Some complete metasomatic kelyphites retained the two concentric coronae textures of primitive kelyphites,while others were locally present in the primitive kelyphites.The inner corona(MR1)with fine-grained minerals and the outer corona(MR2)with coarse-grained minerals of the metasomatic kelyph

关 键 词：石榴石 次变边 橄榄岩捕虏体 兴蒙造山带 地幔转变 

分 类 号：P542.5[天文地球—构造地质学] P574.2[天文地球—地质学]