华北太古宙两类灰色片麻岩及其地质意义

TWO TYPES OF ARCHAEAN GREY GNEISS IN NORTH CHINA AND THEIR GEOLOGICAL SIGNIFICANCE

灰色片麻岩是具太古宙特征而有构造意义的英云间长质侵入体。华北太古苗灰色片麻岩可分为两种类型:其一为遵化型。它与基性麻粒岩呈“互层”产出,形成双峰式混合岩伏建造。矿物成分复杂,稀土型式变化大,有正铕异常,属原地半原地深位混合岩化成因,熔融量大(30％±),作为早期深变质绿岩带根部的板底垫托而阻止了基性岩的沉没。其二为恒山型。有岩体形态和较大幅度的上升侵位,矿物成分简单,稀土型式变化小,常无铕异常,为深变质绿岩带俯冲下插并熔融侵位的产物,熔融量小(15%±)。本文试图通过灰色片麻岩的研究为古板块构造和早期地壳演化提供重要信息。

Grey gneisses are characteristically metamorphic tonalite-trondhjemite paleo-intru-sives with Archaean feature. Now people usually take them as the product of partial melting of high-grade metamorphic basic crustal rocks (greenstone belt) with a great thickness. Some important information about ancient plate tectonics and early crustal evolution may be obtained from grey gneisses.Through ten years of study, the author concludes that there are two types of Archaean grey gneisses in North China.The first one is the Zunhua type. This type of grey gneiss is interbeded with basic granulite, forming a bimodal migmatitic formation. It is characterized by complex mineral composition, highly variable REE patterns and positive anomaly of Eu. It originated from autochthonous or partly autochthonous deep-level migmatiza-tion with a relatively high degree of partial melting (30 % ±). As underplating magma, this type of grey gneiss at the root of the early high-grade metamorphic greenstone belt prevented basic rocks from subsiding.The second one is the Hengshan type, occurring as intrusives. This type of grey gneiss was emplaced after large-amplitude uplifting. It is characterized by rather simple mineral composition, very limited change of REE patterns and no anomaly of Eu. Model calculation suggests that it is a product of subduction and underthrusting of the high-grade metamorphic green belt and then melting (15%±) and emplacement. In northwest Hebei, from northeast to southwest the following tectonic units may be found. (1) Chongli high-grade metamorphic greenstone belt; (2) Dadonggou high-pressure granulite belt; (3) Hengshan grey gneiss complex belt. Thus it is reasonable to think that in the late Archaean, some evidence similar to that of the modern plate subduction zone had begun to appear in the Earth. However, owing to the high value of heat flow and small scale of convection in the Archaean, the tectonie plate was rather small, and magma that originated from the subduction zone and was marked by tonalitic magma, is obviously different from that of today.