Destruction of the lower crust beneath the North China Craton recorded by granulite and pyroxenite xenoliths
The lower crust beneath the North China Craton(NCC) was transformed during the craton destruction in the Mesozoic, however, the transformation processes are yet to be fully understood. Compositional and geochronological variations of granulite and pyroxenite xenoliths provided insights into the nature of the lower crust before and after the craton destruction.In this study, we summarized the latest results of geochemistry and zircon geochronology coupled with Hf-O isotopes from granulite and pyroxenite xenoliths hosted by Phanerozoic igneous rocks in NCC. Comparing previous studies on the granulite terranes and adakitic rocks of NCC, we aim to discuss the destruction processes of lower crust beneath the NCC. The granulite and pyroxenite xenoliths of NCC were divided into two and three groups, respectively, based on the differences of geochemical features. Group I granulite xenoliths from the NCC have silicic-basic compositions, with metamorphic ferrosilite. The Group I granulite xenoliths show relatively lower Mg# values of pyroxenes and whole-rock than that of the Group II granulite xenoliths,and enrichments of light rare earth elements and Sr-Nd isotopic compositions. Their zircons display Archean-Phanerozoic ages with three peaks of Neoarchean, Paleoproterozoic, and Mesozoic. Generally, Group I granulite xenoliths show close affinities to the granulite terranes of the NCC in terms of the major and trace elements and Sr-Nd isotopic compositions, with a consistent Archean-Proterozoic evolutionary history. However, Group I granulite xenoliths have abundant Phanerozoic zircons with variable Hf isotopic compositions from depleted to enriched, which could be formed by modifications of magma underplating.Therefore, Group I granulite xenoliths represent the modified ancient lower crust beneath the NCC. The Group II granulite and Group III pyroxenite xenoliths from the NCC have similar geochemical features and are basic in compositions, with metamorphic to magmatic orthopyroxenes. The Group II granulite and Group III pyroxenite xenoliths usually show higher MgO and lower incompatible elements compositions in minerals and bulk rocks than that in the granulite terranes and Group I granulite xenoliths, but their Sr-Nd isotopic compositions fall into the fields of granulite terranes and group I granulite xenoliths. Zircons from the Group II granulite and Group III pyroxenite xenoliths are predominantly Phanerozoic with subordinate ArcheanProterozoic ages, and the Hf-O isotopic compositions of zircons are similar to those in the Group I granulite xenoliths.Additionally, the trace element compositions of Group II granulite and Group III pyroxenite xenoliths are complementary to those of the adakitic rocks from the NCC. Furthermore, the similar Sr-Nd and zircon Hf isotopic compositions among Group II granulite and Group III pyroxenite xenoliths and adakitic rocks indicate that they are cognate. Therefore, we suggest that the Group II granulite and Group III pyroxenite xenoliths could be restites left after partial melting of the ancient basic lower crust that produced voluminous adakitic rocks. In contrast, Group I and II pyroxenite xenoliths from the NCC have cumulate and reaction origins, respectively. The Group I and II pyroxenite xenoliths commonly have magmatic enstatite and show higher Mg#values and depleted Sr-Nd isotopic compositions of minerals and bulk rocks relative to that in the granulite and Group III pyroxenite xenoliths. Formation of voluminous Group I pyroxenite cumulates in the crust-mantle transition zones implies extensive magma underplating beneath the NCC during the Mesozoic-Cenozoic, which also provided exotic materials and heat for the reworking of the ancient lower crust. Therefore, the destruction of the lower crust beneath the NCC could result from continuous modifications and remelting of the ancient lower crust triggered by magma underplating. These processes led to not only the transformations of some ancient basic lower crust into granulite and pyroxenite restites but also the compositional modifications of the ancient lower crust. Consequently, the lower crust beneath the NCC showed downward rejuvenation, similar to the lithospheric mantle.