An exposed cross-section of early Precambrian continental lower crust in North China craton

Early Precambrian rocks are widespread distributed in northern China and - constitute the metamorphic crystalline basement of the North China craton. An oblique successive rock sequence of the continental lower crust in the n orthern part of the craton has been recognized, Nearby this metamorphic ter rane, some xenoliths of granulites in Cenozoic basalts are reported to repr esent the recent lower crust. Therefore, it is possible to establish a typi cal Precambrian continental lower crust cross-section in the North China cr aton to understand features of the lower crust. This exposed cross-section of lower crust can be divided into five layers f rom south (lowermost crust) to north (upper-lower crust) by faults. Along t he section northward, the metamorphic grade gradually decreases from high-p ressure granulite facies, middle-pressure granulite facies, middle-low-pres sure granulite facies to amphibolite facies. The metamorphic pressures of t hese five layers are, respectively, 12 to 14 kbar, 9 kbar, 7 to 8 kbar, 6 k bar and 5 kbar. The petrological compositions of the five layers show a cha nge from gabbroic granulites, intermediate-felsic orthogneiss to metamorpho sed supracrustal rocks. Geochemically, the lowermost crust (gabbro) and low er crust (intermediate-acid orthogneiss) are relatively poorer in Si and Al compared to the middle-lower crust and upper-lower crust. The cross-sectio n demonstrates a depletion trend of heat-producing elements and some large ion lithophile elements, such as decreasing abundance of Th, U, K, Rb and S r from the upper-lower crust to the lowermost crust. The rocks in lowermost crust and lower crust only contain CO fluid inclusions, whereas the rocks in the middle-lower crust and upper-lower crust usually contain H2O fluid i nclusions. Isotopic data of garnet-bearing mafic granulites, which represent the lower most crust, indicate that the lower crust in this area formed in Neoarchean , and uplifted and exposed to surface in late Paleoproterozoic. However, th e study of granulite xenoliths in this area seems to demonstrate that the r ecent lower crust and Precambrian lower crust have some similarity and also some difference in geochemistry of trace elements and Sr, Nd and Pb isotop es. It is possible that the Precambrian crust is unstable in late geologica l processes and replaced by recent lower crust, and some old materials were added to the recent lower crust. The formation of the recent lower crust i s mainly linked with magma underplating process related to Cenozoic basalti c volcanics. (C) 2001 Elsevier Science Ltd. All rights reserved.