Rock Probes And Seismic Methods To Constrain The Structure, Composition And Evolution Of The Deep Crust Beneath North China Block

Deep continental crust archives critical information on the bulk crustal composition as well as its formation. It is commonly the inherent zone for complex crust-mantle interaction. Unlike the shallow crust, the deep crust is generally inaccessible for direct geological sampling. Instead, knowledge of the deep crust can be obtained from a combination study of geophysical techniques and "lithoprobe" (deep crustal fragments entrained in magmatic outcrops). Wide-angle reflection/refraction seismic profiles have the highest resolution on crustal velocity structure and provide information on the averaged present-day nature of large deep crust domains; "lithoprobe", instead, give detailed information of locality-specific deep crustal sections through determining the physical and chemical properties, dating the formation and alteration ages and constraining the melting and equilibration depths of the xenoliths. Here, the two methodologies are integrated to constrain the crustal structure on the North China Block. The seismic crustal structure model of the block is compiled from all available seismic wide-angle reflection/refraction profiles and receiver function results. The model parameters include Moho depth, Vp velocity and thicknesses for four crustal layers (sedimentary cover, and the upper, middle and lower crust). The crust as a whole is thin (< 36 km) in the eastern part relative to the western part (38-44 km). In addition, the crust is shown to have a thick upper and middle crust and a relatively thin (typically <5 km) lower mafic crust; no spatial variations in average crustal Vp velocity and densities are observed. The average crustal Vp and density for the whole North China Block are 6.33-6.37 km/s and 2.72-2.78 g/cm(3), respectively. The overall absence of high P-wave velocity layer and the relatively low average Vp and density indicate that the block has an intermediate to felsic bulk crustal composition. In contrast, regional crustal xenoliths include mafic to felsic granulites and amphibolites and rare eclogite xenoliths (e.g., Xinyang and Xuhuai), indicating the presence of thick lower crust (e.g., >45 km) at that time. Meanwhile, mafic xenoliths in Hannuoba and Nushan show strong mafic underplating at the lower crust in the Meso-Cenozoic while no high-velocity layer is shown in the seismic crustal model. These inconsistences can be explained by the fact that the two methodologies tap the crustal information at different timeslices and dimensions and be complementary to understand the chemical and physical structure of the deep crust. Therefore, the combination study of petrological and geochemical of the crustal xenoliths and geophysical observations can provide essential information on making and destroying the deep continental crust through time.