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Wu Changhua and Zhong Chnagting(Tianjin Institute of Geology and Mineral Resources, CAGS, Tianjin 300170)  
The amphibolite- to granulite-facies basemnt of North China platform has consistently been regarded by most of internal and external geologists as a unified Archean craton,consequently the granulite belt in the north part of the craton, extending for about 1000 km from Eastern Hebei west into Jin-Meng (northern Shanxi and Inner Mongolia), has been thought as a unified Archean one either, specially based on 3. 5 Ga±constrains for the amphibolite-facies rocks from Eastern Hebei Province. Taking the presence or absence of khondalite series as the mark, however, this belt can be divided into two segments, having Chicheng as the beundary.The western part (Jin-Meng high-grade region) differs from the eastern (Eastern Hebei terrain)by the developed khondalite and a series of other geological features (Table 1 ).Eastern Hebei terrain with nearly N - S structural trend can be still thought as an old continent that had widely experienced cratonization in the middle and late Archean, whereas Jin-Meng high-grade region is postulated in this paper as an orogenic belt by the collision of Archean Ordos blork on the west of the belt with Eastern Hebei continent. Because the age of khondalite is constrained to be 1. 8~1. 9 Ga (Fig. 1) the timing of collision must be Early Proterosoic that is the Luliang Movementfamous in China. Thus, it can be concluded that the North China craton was once separated during Archena rather than unified.Jin-Meng collision belt can be divided into three subbelts, named Daqingshan granulite,Liangcheng khondalite and Huai'an granulite from north to south, each of which trends NE-SW and parallels another, converging slightly towards NE (Fig. 2). It is generally thought that since the orogenic trend is perpendicular to the motional direction, the collisional orientation was believed to be NW - SE. This view, however, is open to questions because the high-grade region analogous to Mattauer's (1980) orogenic root or the lower structural level (Fig. 3). Thus, the NE trend symbolizes the motional direction of collision rather than the orogen trend line, and that nearly all the mineral lineations (Fig. 2,4) with subhorizontal dip identically trend NE-SW and sheath fold converges to NE (Fig. 5), suggests the collisional motion from SW towars NE. Late deformation event (probably Pan-African) resulted in the same lowr level folded into two ones outcropping on both sides of the Liangcheng khondalite.Jin-Meng high-grade region is mainly composed of Archean basement granulite-gneiss (TTG), Archean - early Proterozoic granulite, amphibolite and BIF (metavolcanics in ocean-arc setting) and Early Proterozoic khondalite (sediments in continental margin). These rocks in distinct setting were collided to form thickened lower crust by meas of subhorisontally and mutually overlapping shear slices, just as shuffling cards. This callisional lower crust is dominated by thick khondalite slice on the upper and reworked basernent slice below, forming a double layer crustal architecture. Between them develops a transitional sone that consists of mutual slices about several kilomters total thick of the three abore-menthad rocks, in which the khonalite slice reqressively decreases downwards both in quantity and in size. It is pointed out that the remnant ocean-arc volcanics between the continents have also been overlapped as the slices in the transitional zone except for the subducted oceanic crust, so that the irregular suture is difficult to be recognized.Because the old continent was probably very large during Early Proterozoic when the collision of Ordos continent with Eastern Hebei continent occurred, there certainly will be an old mountain chain or old collision belt that trends N - S in the central part of North China craton. To thesouth of Jin-Meng high-grade region and crossing Hengshan remnant block, the LuliangshanWutaishan - Taihangshan areas are the part of collisional belt extending to south. As Fig. 2, the khondalite series located in Baotou, Liangcheng, Datong and Lanxian were the sediments deposited in northern and easte
【Fund】: 国家自然科学基金!49472141
【CateGory Index】: P542
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