THE DISCOVERY AND IMPLICATION OF THE "SHIQIAO STRUCTURAL WINDOW" IN ULTRAHIGH PRESSURE METAMORPHOSED BELT IN NORTH JIANGSU,CHINA
Dong Shuwen; Zhang Yong; Huang Dezhi (Anhui Institute of Geology, Hefei)Wang Xiaofeng(Geomechanics Institute of CAGS, Beijing)Xue Lingjia (No. 6 Party of Geological Exploration of Jiangsu Province, Donghai County)
The terrain ranging from Northern Jiangsu prov. to South Shandong prov. (also called as Su-Lu terrain)has been determined as high-ultrahigh pressure metamorphosed belt which is the east-extensional part of the Qinling-Dabie collisional belt (Enami et al. 1988, Yang et al. 1989, Zhang et al. 1990, Hirajima et al. 1990, Zhao et al. 1992, Yang et al. 1993)with the root part being exposed. In the centre of the high-ultrahigh pressure metamorphosed rocks, where the Shiqiao town, Gangyu county, Jiangsu province lies, a suite of epimetamorphosed sedimentary rocks (flysch facies) are excavated, which are composed of fine-grained conglomerate, sandstone, siltstone and mudstone with clear sedimentary cyclothem, graded bedding primitive bedding S0)and slatycleavage. It is compared strongly with the surrounding gneiss called Donghai group(ArPt1). The sedimentary rocks have been recognized as acient basement companied with the high-ultrahigh-P belt (Shandong Geological Survey, 1982, Jiangsu Geological Survey, 1984). Until 1986, prof. Sun suggested that the rock is independent of the high grade of metamorphosed series,and a new" Shiqiao group"of Pt1 set up (based on 1901 Ma dated from zircon by U-Pb method), which lies on the Donghai group as a syncline.Now, we suggest that it belongs to the Sinian based on the micropalaeophytology.A new exposed section of "Shiqiao group"found last year shows that the sedimentary sequence(andalusite slat with graded bedding)is overlapped by the gneiss of the Donghai group. The contact plane where mylonite and lenses of lamprophyre(with foliations)are occurs with 165°/40°. In other place 10 km from Shiqiao town to eastsouth, a borehole met also"Shiqiao group"through Donghai group gneiss of about 70m. So,the "Shiqiao structural window"is determined, and its implications are listed as following : 1. proving that a sequence of sedimentary rocks occurred under the high-ultrahigh pressure metamorphosed belt and a large-scaled detachment developed in Su-Lu terrain; 2. "Shiqiao group"could be considered as sedimentary cover or basement of Yangtze craton, a subducting terrain, and indicating that the Su-Lu orogenic belt uplifts higher than the Debie Mount. ; 3. depending on the fact that the orogenic belt of Qinling-Dabie-Sulu exposed gently from west to east by step uplift controlled NNE faults, the authors infer that further to east, South Korea may uplift the most and all of orogenic products be eroded, as a result, no high-ultrahigh metamorphosed rocks remain but only the subducting terrain-Yangtze craton.