TECTONIC UNITS OF THE ARCHEAN CRYSTALLINE BASEMENT IN EASTERN HEBEI

From the angle of geological events and on the basis of the genetic types of the rocks, metamorphic grades, deformation and tectonic styles, features of regional geophysical fields, isotopic geochronology and mineralization, the Archean crystalline basement in eastern Hebei may be divided into the following tectonic units. High-pressure charnochite-gneiss terrain The terrain consists mainly of the sodian charnockite-gneiss series and is tectonically manifested by a series of brachy or oval folds. It has undergone granulite-facies metamorphism twice. The types and origin of the gneissosity are complex;it is mainly represented by deformed planar structure. The supracrustal rocks occupy a small proportion. Low-pressure charnockite-gneiss terrain It is made up mainly of thepotassie charnockite-gneiss series, which forms a large oval structure in terms of space. It has been subjected to the early-stage high amphibolite-facies and latestage granulite-facies metamorphism. In this terrain planar structures formed by diapiric emplacement and tectonic deformation of the rock body have been both preserved. The supracrustal rocks represent a relatively large proportion. The terrain contains BIF type deposits of important econmic value. TTG complex terrain This terrain is composed of TTG granite complex, which forms an imcomplete dome. It has undergone amphibolite-facies metamorphism. The rocks in the center of the dome are massive, and gneissosity is well developed at its edges. Tectono-magmatic complex terrain The rock types are very complex, includig granite—gneiss, granulite, TTG complex, tectonite, stratified supracrustal rocks and retrogressive metamorphic rocks which mix together. In terms of space, they form a large linear arcuate body. This terrain has gone through metamorphism at least three times. The Mesozoic witnessed strong superimposition of magmatic activities. Gold mineralization is prevalent Study has indicates that the high-pressure and low, pressure charnockitegneiss terrains were formed contemporaneously at different depths or pressures in the Archean crust of eastern Hebei. The tectono-magmatic complex terrain is a long-active linear gigantic tectonic mobile zone superimposed on the high-pressure charnockite-gneiss terrain. The TTG complex terrain is a granite complex dome formed at the relatively shallow level of the crust at the end of the Archean.