内蒙古大青山地区石榴混合花岗质岩石地球化学特征及成因

Geochemical characteristics and origin of garnet migmatitic granites in Daqingshan area, Inner Mongolia

内蒙古大青山地区太古宙孔兹岩系在发生麻粒岩相变质作用的同时,榴云片麻岩岩组中的石榴黑云片麻岩在近水平剪切构造变形过程中发生部分熔融,形成了石榴混合花岗质岩石。现有证据表明,它们形成于新太古代晚期,在矿物组成、地球化学特征上,大体继承了石榴黑云片麻岩,而它们的结构和地球化学特征尤其是REE分布型式的变异又反映了其部分熔融和演化。尽管该石榴混合花岗质岩体规模小,但在矿物组成、结构和地球化学特征上显示出明显的不均一性,形成了高K2O低Na2O、CaO,稀土元素总量低,具正Eu异常和低K2O高Na2O、CaO,稀土元素总量高,具负Eu异常的两类石榴混合花岗质岩石。综合研究发现,这两类石榴混合花岗质岩石的形成与部分熔融及随后的流动过程中熔体与残留体的逐渐分离有关,前者残留体、残留矿物相极少,富长英质;而后者残留体和残留矿物相对较多,因而富镁铁质,并且控制REE行为的矿物相如石榴石和独居石、磷灰石、锆石等相对富集,从而造成了两类石榴混合花岗质岩石稀土元素分布型式的差异,与桑干地区成因相似、成熟度较高的古元古代花岗岩具有较大差别。

The garnet migmatitic granite （GMG） was formed by partial melting of Ga-Bi gneiss within Ga-Bi gneiss complex of Archaean khondalites in Daqingshan area, Inner Mongolia, in the process of subhorizontal shearing and granulite facies metamorphism. The GMG mainly inherited mineral composition and geochemical features from the Ga-Bi gneiss, while its difference from Ga-Bi gneiss in textures and geochemistry （especially REE） indicates the process of partial melting and evolution. In spite of its small bodies, GMG shows heterogeneity in mineral composition, textures and geochemistry, and can be divided into two types, of which the former has high K2O and low Na2O, CaO, showing apparently positive anomalies and Low ∑REE, whereas the latter has low K2O and high Na2O, CaO, assuming negative Eu anomalies and high ∑REE. It is suggested that the development of the two types of GMG is related to the gradual separation of partial melts from the solid residue during the partial melting and subsequent flowing. The relative lack in the former and enrichment in the latter of the restite and relic minerals, especially the REE behavior-controlling refractory minerals such as garnet, monazite, apatite and zircon, caused the difference in REE distribution of the two type.s of GMG. The available data suggest that the GMG was developed in Neoarchaean and is hence quite different from the Paleoproterozoic anatexis-derived garnet-bearing granite in Sanggan area.