麻山群孔兹岩系中的堇青片麻岩

CORDIERITE GNEISSES IN MASHAN KHONDALITE SERIES, NORTHEAST CHINA

黑龙江省东部麻山群中堇青片麻岩的主要矿物组合有石榴石+夕线石+堇青石+石英+钾长石+黑云母;石榴石+夕线石+堇青石+钾长石+尖晶石;堇青石+夕线石+斜长石;柱晶石+堇青石+斜长石+夕线石+石榴石+电气石;堇青石+紫苏辉石+石榴石+斜长石+黑云母+石英。这些含堇青石组合是在麻粒岩相条件下已存石榴石+夕线石+石英矿物组合分解反应的产物。其形成压力(0.45～0.5GPa)比麻粒岩相“高峰”压力明显低,而温度(800℃)相差不大。这意味着麻粒岩相主期区域变质作用后期,本区地壳有一个迅速的抬升过程。

Cordierite gneisses occur in the granulite region of Mashan khondalite series in eastern Heilongjiang Province, Northeast China. Detailed studies on outcrop, rock associations and their texture features reveal that they are regional metamorphic rocks in origin, but not concerned with the migmatization which is widespread in this terrain. Composition characters, mineral assemblages of representative rocks and formations of cordierite have been dealt with in this paper. Systematic researches on metamorphism indicate that the cordierite was formed at the expence of initial sillimanite, garnet and quartz which were stable under the peak metamorphic conditions. It is estimated that P-T condition of peak metamorphism of Mashan khondalite series was 800～850℃, 0. 73GPa, and the subsequent formation of cordierite was in the range of 800℃±, 0.45～0. 5GPa. This kind of typical near isothermal decompression P-T path indicates a rapid uplift of the crust during the late period of granulite facies metamorphism. Cordierite was the products at the early stage of the uplift event. Many cordierite gneisses in different granulite belts on the Earth have the same P-T path, which suggests that they are greatly significant for understanding the metamorphic evolution of these terrains and their subsequent uplift history.