麻山杂岩的变质-混合岩化作用和花岗质岩浆活动

Metamorphism,migmatization and granites of the Mashan Complex in Heilongjiang Province,Northeast China

在黑龙江佳木斯地块麻山杂岩中可分别有高级和中级变质作用部分,两处均可见混合岩化作用。通过混合岩浅色脉体或花岗质脉体中的长石自形晶、斜长石周围的钠长石净边、黑云母向角闪石的转化等现象表明混合岩化作用主要表现为深部岩浆的注入,而不仅是高级变质之后的近原地深熔作用所致,高级变质与相关的深熔作用所致混合岩化在区域上的分布是有限的,集中于西麻山的高级区;而注入式混合岩化是普遍的,其产出可遍布所有麻山杂岩的出露区,在麻粒岩相和角闪岩相部位均可出现。早期麻粒岩相变质与后期混合岩化作用应是相互独立的构造或热事件;注入式混合岩化引起了中级变质作用和高级区的退变质作用,注入混合岩化作用的时代约为500Ma;中级变质作用是注入混合岩化的结果,而不是混合岩化的原因。与注入混合岩相关的花岗岩虽然表现出一些S型花岗岩的特征,但根据矿物组合、地化性质的综合分析,更可能是富钾及钾长石斑晶的钙碱性花岗岩类,属于I型花岗岩,形成于挤压向引张转化的过程中。麻山杂岩的变质与混合岩化特征表明,以西伯利亚古陆为中心的南部边缘发生了与冈瓦纳陆块内泛非事件类似的构造活动,只不过这里的规模略小,在变质之后迅速发生了构造体系的转换,而形成大量花岗质岩浆活动。

Both high-grade and middle-grade metamorphisms are developed in the Mashan Complex in the Jiamusi Block,Heilongjiang Province,Northeast China.In the high-grade area occurs the migmatization related with anatexis,while intensive injection migmatization prevails in the whole Mashan Complex.As the presence of euhedral plagioclase,albite rim along earlier plagioclase,the transformation textures of biotite to hornblende in the leucocratic granitic veins or bodies.It is deduced that the migmatization was predominantly derived from the magmatic injection from the deep,rather than in-situ anatexis after high-grade metamorphism.The anatexis was confined to granulite metamorphism area while the injection migmatization were widespread.With the age of ca.500Ma,the injection migmatization was responsible for the amphibolite-facies metamorphism in the large area and retrograde grade metamorphism in the high-grade part.The middle-grade metamorphism was resulted from,not in,the injection migmatization.Although demonstrating some features of S-type granite,the synthetic analysis of mineral assemblages and geochemical characteristics show that the migmatization granite was basically I-type,or the KCG type of Barbarin(1999).The magmatism occurred in the transitional period from compression to extension.Both the metamorphism and migmatization features of the Mashan Complex suggest that some tectono-thermal activity similar to the Pan-african event in the Gondwana took place at the southern margin of the Siberian Craton,and the scale here may be smaller and there was a rapid shift of structural regime from metamorphism to tremendous magmatism activity