摘要
本文根据岩石组合、原岩建造、构造、岩浆事件,以及同位素年龄等资料的分析,认为内蒙大青山地区高级变质岩系由太古代和早元古代两个变质岩系组成。通过变质矿物组合的转变,讨论了它们的变质作用演化过程,太古代麻粒岩相变质与区域热流异常增高有关,早元古代高级变质起因于造山作用。它们是两次构造性质明显不同的麻粒岩事件。
General speaking, high-grade metamorphic rock series in Daqingshan area belongs to Archaean metamorphic cycle.
Recent study has discovered that the rock series consists of two parts, which are of distinct lithological characters, plutonic rock series, tectonic style and isotopic ages. These two kinds of rock series must separately belong to Archaean and Early Proterozoic eras.
The Archaean high-grade metamorphic rock series is composed of layered granulite, amphibolite-plagioclase gneiss, garnetiferous quartzite (or banded iron formation), which is identical to a volcanic-sedimentary formation. Its petrochemical and geochemical features are similar to Archaean deposits. Tectonic deformation caused the layered rock series to form a series of fold and ductile shear zones. The structural trend is N-S. Plutonic rock series consists of hypersthene-bearing quartz-diorite and plagioclase granite (enderbite or charnockite), which is identical to Archaean high-Al type tonalite-trondhjemite series. Main minerals are hypersthene, diopside, sillimanite, garnet. peak metamorphic P-T is about 760～870℃, 0.95～1.09GPa. Another tonalite-trondhjemite megmatic event in late Archaean (2441～2470Ma) had set the time for high-grade metamorphic rock series.
Early Proterozoic high-grade metamorphic rock series represents graphite-bearing gneiss, Al-rich gneiss, marbles and less granulites, as well as syntectonic potassium-rich granite.Lower metamorphic rocks belong to sedimentary clasolite-carbonate formation in intracontinental basin. This rock series is strongly deformed to form a series of recumbent or complex folds, ductile shear zone,and exhibits distinct E-W lineation, which is cross-cut to the Archaean rock association and structural trend.Major metamorphic minerals are cordierite, sillimanite, kyanite, garnet, hypersthene, diopside, hornblende and K-feldspar. Peak metamorphic P-T value is about 650～880℃, and 0.8～0.35GPa. The isotopic data indicate the time of metamorphism must be early Proterozoic era (1800～2360Ma).
The study of metamorphic dynamics shows different P-T change trend in these two kinds of rock formations. According to the change of mineral assemblages, a counter-clockwise P-T-t trajectory is constructed for the P-T evolution of Archaean granulite facies metamorphism. The main cause for granulite facies metamorphism in the Archaean is the abnormal change of heat flow in the mantle or lower crust.
The metamorphism of Early Proterozoic rocks can be also divided into three stages in accordance with mineral generations. P-T evolution of Early Proterozoic metamorphism shows a clockwise P-T-t path, which is similar to that of metamorphism in orogenic belts. That is related to the tectonic thickening, the heat flow change is caused by the perturbation of tectonism on thermal gradient in the crust.
出处
《岩石学报》
SCIE
EI
CAS
CSCD
北大核心
1991年第4期27-36,共10页
Acta Petrologica Sinica
基金
国家自然科学基金4870148
关键词
麻粒岩
变质
动力学
太古代
元古代
Archean granulite
Early Proterozoic granulite
Metamorphic dynamics
Daqingshan area
