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

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Al₂O₃含量和较低Mg^#值及SiO₂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SiO₂及Al₂O₃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SiO₂、Na₂O和K₂O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和单斜辉石形成的，这与岩相学观察到的单斜辉石中包裹斜方辉石残余体一致。此外，同一样品中R1的平衡温度略高于R2的平衡温度，且二者均高于橄榄岩的平衡温度。因此，锡林浩特地区石榴石橄榄岩至少经历了两阶段的退变质作用：第一阶段为橄榄岩自石榴石相抬升至尖晶石相，且受到地幔上涌的加热作用，导致石榴石和橄榄石进行缓慢的反应形成R2；第二阶段是在连续减压且加热的背景下，第一阶段残余的石榴石发生快速等化学分解反应，形成R1。退变质作用之后，石榴石原始的次变边又经历了碳酸盐熔/流体的交代作用形成MR1和MR2，最终被寄主玄武岩携带至地表。所以，石榴石次变边的形成记录了新生代时期兴蒙造山带经历的广泛的地幔上涌和多次的地幔隆升，以及地幔交代作用，为研究深部地幔过程提供了重要证据。华北克拉通晚中生代时期经历了强烈的岩石圈伸展运动并伴随着软流圈的上涌，这些过程同样会造成岩石圈地幔的减压和加热，从而导致石榴石相橄榄岩向尖晶石相转变，这可能也是华北克拉通岩石圈地幔转变的机制之一。

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

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 Al2O3 content and lower Mg# value and SiO2 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 SiO2 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 SiO2 and Al2O3 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 kelyphites are made of Ol+Cpx+Sp+Melt2. Compared to R1 and R2, MR1 and MR2 display relatively higher Mg# values of olivine and clinopyroxene, higher Ca/Al ratios of clinopyroxene, higher Cr# and lower Mg# values of spinel, and higher Na2O and K2O contents of melt. These observations indicate that the metasomatic kelyphites were formed by a reaction between primitive kelyphites and carbonate melts/fluids. Olivine and clinopyroxene grew at the expense of orthopyroxene during the reaction, as evidenced by the residual orthopyroxene in clinopyroxene of the metasomatic kelyphites. The equilibrium temperature of the R1 in individual sample is marginally higher than that of the R2. In addition, the equilibrium temperatures of R1 and R2 are higher than that of peridotite xenoliths. Therefore, we suggest that the garnet peridotites from the Xilinhot region could have experienced two stages' retrograde metamorphism at least. The first stage could be triggered by the exhumation processes from garnet-facies to spinel-facies as heated by the asthenospheric upwelling, resulting in the formation of R2 by slow reaction of garnet-olivine. The second stage was associated with consecutive decompression and heating, which led to the fast isochemical breakdown of garnet and hence produced R1. The primitive kelyphites were metasomatized by the carbonate melts/fluids in different degree after its formation and carried by the host basalt finally. Therefore, kelyphites of garnet from mantle peridotite xenoliths recorded extensive mantle upwelling, multiple exhumation and metasomatism beneath the XMOB during the Cenozoic. The North China Craton experienced strong lithospheric extension and mantle upwelling during the Late Mesozoic, which led to the decompression and heating of the lithospheric mantle. These processes also caused the transformation of garnet peridotite to spinel peridotite, which could be one of the transformation mechanisms of lithospheric mantle beneath the North China Craton.