Dehydration of Granulites from the North China Craton: Implications for Craton Restabilization

Abstract:

The water content and rheology of the lower continental crust can provide important clues toward a better understanding of geodynamic processes, such as plateau uplift, lower crust delamination and craton reactivation. We currently know little of such information for the north China craton which is also one of the oldest Archean cratons in the world that experienced dramatic reactivation during the Mesozoic. We present here comparison of water content and deformation microstructure in lower crustal rocks before and after craton reactivation revealed by Fourier Transform Infrared Spectroscopy (FTIR) and the electron backscattered diffraction (EBSD) techniques. Our results revealed a significant dehydration of constituent minerals of lower crustal rocks before and after craton reactivation. There is a significant amount of water in the Archean felsic granulite (135 ppm in plagioclase, 885 ppm in orthopyroxene and 50 ppm in quartz) and mafic granulite (290 ppm in plagioclase, 903 ppm in clinopyroxene and 1038 ppm in orthopyroxene) from the Manjingtou of the Huaian terrane. In contrast, the water contents in the lower crustal mafic granulites (277 ppm in plagioclase, 120 ppm in clinopyroxene and 24 ppm in orthopyroxene) and garnet pyroxenites (123 ppm in Cpx and 7 ppm in Opx) of the Mesozoic mantle xenoliths from the Damaping is far less. Water is known to have an unproportional significant effects on the rheology of crustal and mantle minerals. The EBSD results suggest that pyroxenes are the strain accommodation phases in ‘wet’ granulites comparing to plagioclase being the weakest phase in ‘dry’ granulites. These results provide direct evidence for not only a low viscosity Precambrian lower crust which might facilitate thickening and delamination before the reactivation of the North China Craton but also a strengthened restabilized lower crust after reactivation.