内蒙古中西部宝音图群石榴角闪岩的地球化学、变质作用及年代学特征

宝音图群分布于内蒙古中西部狼山、图古日格和达茂旗一带，主要由云母片岩、石英片岩、石英岩、角闪岩和大理岩组成。在狼山格尔敖包沟和图古日格西南地区发育十分典型的石榴角闪岩，均以似层状或透镜状产自云母片岩中，但两地具有不同的矿物组合，前者主要以石榴石+角闪石+斜长石+含钛矿物（金红石和钛铁矿）为特征，后者主要以石榴石+角闪石+绿帘石+含钛矿物（金红石和钛铁矿）为特征。地球化学研究显示两个地区的石榴角闪岩原岩具有相似的化学组成，均为亚碱性玄武岩的拉斑系列。宝音图群的石榴角闪岩具有右倾的稀土配分模式，REE总量为83.31×10^-6~125.9×10^-6，（La/Yb）_N比值为2.17~6.48，δEu=0.87~0.98。Ta、Nb、Ti没有明显的负异常，其配分型式类似E-MORB特点。微量元素构造判别图解表明这些石榴角闪岩的原岩产于板内拉张环境中。本文通过模拟格尔敖包沟样品LS01和图古日格西南地区样品LS35的P-T视剖面图研究其变质作用及矿物演化过程，结果显示这两个样品均经历了早期进变质，峰期以及峰后近等温减压的顺时针型P-T轨迹。样品LS01的峰期温压条件为~11kbar/~735℃，模拟得到的峰期矿物组合可能为石榴石+角闪石+斜长石+黑云母+透辉石+金红石+熔体，P-M（H₂O）视剖面图显示，由于峰后演化过程中存在饱和流体渗透，导致峰期透辉石无法保留。样品LS35的峰期温度压力条件为~8kbar/675℃，其峰期组合为石榴石+角闪石+绿帘石+金红石。两个地区的石榴角闪岩的温度压力条件明显不同，推测宝音图群中呈透镜体或似层状产出的变质基性岩，存在着递增型的变质作用。锆石LA-ICP-MS U-Pb年代学的研究结果显示石榴角闪岩样品LS01的变质锆石年龄为394±8Ma。由此推测这些石榴角闪岩的原岩可能形成于华北北缘在新元古代发育的裂陷盆地，在泥盆纪中期，宝音图群作为华北克拉通的一部分，被卷入到与古亚洲洋闭合有关的造山过程，发育中压型递增变质作用。

Geochemistry, metamorphism and geochronology characteristics of the garnet amphibolites in the Baoyintu Group, central-western Inner Mongolia

The Baoyintu Group is distributed in the areas of Langshan, Tugurige and Damaoqi in the central-western Inner Mongolia. It is mainly composed of mica-schist, quartz schist, quartzite, amphibolite and marble. Garnet amphibolites developed in the Geraobao and Tugurige, occur as bedded or lenticular bodies within the mica-schist, and they are characterized by different mineral assemblages. The former mainly contains garnet, hornblende, plagioclase, rutile and ilmenite, and the latter mainly contains garnet, hornblende, epidote, rutile and ilmenite. Geochemical study shows that the garnet amphibolites are characterized by tholeiitic basalt series. The garnet amphibolites show right-dipping REE patterns with ∑REE=83.31×10-6~125.9×10-6, (La/Yb)N=2.17~6.48 and δEu=0.87~0.98, and they have no obvious negative anomaly in Ta, Nb and Ti, showing an affinity to enriched mid-ocean ridge basalts (E-MORB). Discrimination diagram indicates that the protolith of garnet amphibolites may have formed in an intra-continental rift setting. Phase equilibria modeling shows that samples LS01 from Geraobaogou and LS35 from Tugurige both experienced clockwise P-T paths including prograde, peak and near isothermal decompression stages. The peak P-T condition of sample LS01 is estimated to be ~11kbar/~735℃ with the peak mineral assemblage predicted to be garnet+hornblende+plagioclase+biotite+diopside+rutile+melt. However, the P-M (H2O) pseudosection shows that the diopside would not be retained due to the saturated fluid penetration during the post-peak evolution. The peak P-T condition of sample LS35 is estimated to be ~8kbar/675℃, and the peak mineral assemblage is predicted to be garnet+hornblende+epidote+rutile. The different peak P-T conditions of the garnet amphibolites are indicative of the progressive metamorphism in the Baoyintu Group. Zircon LA-ICP-MS U-Pb dating indicates that the metamorphic zircons in garnet amphibolite sample LS01 are aged at 394±8Ma. Consequently, it is inferred that the protolith of these garnet amphibolites may have formed in the Neoproterozoic rifting basins in the northern margin of North China Craton. In the Middle Devonian, the Baoyintu Group as a part of the North China Craton, was involved in the orogenic process associated with the closure of the Paleo-Asian Ocean and experience progressive metamorphism of medium P/T series.