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

Abstract

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 northern part of the craton has been recognized. Nearby this metamorphic terrane, some xenoliths of granulites in Cenozoic basalts are reported to represent the recent lower crust. Therefore, it is possible to establish a typical Precambrian continental lower crust cross-section in the North China craton to understand features of the lower crust.

This exposed cross-section of lower crust can be divided into five layers from south (lowermost crust) to north (upper-lower crust) by faults. Along the section northward, the metamorphic grade gradually decreases from high-pressure granulite facies, middle-pressure granulite facies, middle-low-pressure granulite facies to amphibolite facies. The metamorphic pressures of these five layers are, respectively, 12 to 14 kbar, 9 kbar, 7 to 8 kbar, 6 kbar and 5 kbar. The petrological compositions of the five layers show a change from gabbroic granulites, intermediate-felsic orthogneiss to metamorphosed supracrustal rocks. Geochemically, the lowermost crust (gabbro) and lower crust (intermediate-acid orthogneiss) are relatively poorer in Si and Al compared to the middle-lower crust and upper-lower crust. The cross-section demonstrates a depletion trend of heat-producing elements and some large ion lithophile elements, such as decreasing abundance of Th, U, K, Rb and Sr 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 inclusions.

Isotopic data of garnet-bearing mafic granulites, which represent the lowermost crust, indicate that the lower crust in this area formed in Neoarchean, and uplifted and exposed to surface in late Paleoproterozoic. However, the study of granulite xenoliths in this area seems to demonstrate that the recent lower crust and Precambrian lower crust have some similarity and also some difference in geochemistry of trace elements and Sr, Nd and Pb isotopes. It is possible that the Precambrian crust is unstable in late geological 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 is mainly linked with magma underplating process related to Cenozoic basaltic volcanics.