麻山杂岩的两种变质作用及其与花岗岩的关系

在黑龙江佳木斯地块麻山杂岩中识别出麻山杂岩中具有两种类型的变质作用:早期局部的麻粒岩相变质作用(M1)和伴随的无水深熔作用(>530Ma),即狭义的麻山群高级变质作用,变质岩多以残留块体形式散布于后期混合岩或花岗岩之中;麻粒岩相变质之后由于广泛的岩浆活动,造成了强烈的混合岩化作用(500Ma)和相应大范围的晚期角闪岩相变质作用(M2)。角闪岩相和麻粒岩相变质并非带状递进变化,而是在时空上均有差异的两种变质作用。混合岩化过程中的富水流体对早期"干"岩石进行了明显的退变质改造,从而造成高级变质岩变质结构的复杂性。此外,含水花岗岩的侵位对先期麻粒岩相变质成因锆石同位素体系有重置作用,使得早期的变质年龄难以测定。麻山杂岩的变质-花岗岩关系与东南极普里兹带有类似之处,在变形-变质之后迅速发生了构造体系的转换,出现了新的岩浆活动,只是花岗岩发育强度有所不同,反映了两地构造演化细节上的差异。西伯利亚古陆南缘及中亚造山带内部的一些微陆块发生了与冈瓦纳陆块内泛非事件类似的构造-热-岩浆事件,因此,中亚造山带内部的佳木斯地块及其他类似的微陆块与冈瓦纳古陆边缘活动带具有相似的构造性质。

Two types of metamorphism and their relationships with granites in the Mashan Complex

Two types of metamorphism have been discerned from the Mashan Complex in the Jiamusi Block, Heilongjiang Province, northeastern China. The local earlier granulite facies metamorphism (M₁) was accompanied by fluid-absent anatexis (>530Ma). The subsequent widespread magmatism was responsible for the intensive migmatization (500Ma) and corresponding late amphibolite facies metamorphism (M₂). The former is actually the high grade Mashan Group (s.s.) and the metamorphic rocks are dominated by residual massives or enclaves scattered in late migmatitic granites. The change from amphibolite to granulite facies is not transitional or progressive, suggesting the great difference of the two kinds of metamorphism. The migmatization was probably fluid-rich and made substantial retrograded alteration to the earlier "dry" metamorphics, accounting for the complicated textures of the rocks. In addition, the emplacement of the fluid-bearing granites may reset the U-Pb system of the zircons formed in earlier high-grade metamorphism and the real metamorphic ages could be hardly measured. For the metamorphism-granite sequence, the Mashan Complex is rather similar to that of the Prydz Bay belt of East Antarctica. In both areas the deformation-metamorphism were closely followed by granites. The difference is that the intensity of late granitization varies in the two areas. Along the southern margin of the Siberian Craton and some microcontinents in the Central Asian belt occurred some tectono-thermal-magmatic events similar to that around the Gondwana margin. That is to say, the Jiamusi Block and other analogues in the Central Asian belt are tectonically similar to the Pan-African belts in the Gondwana, though they may belong to different domains in ancient continent configuration.