Lower crustal xenoliths from Junan, Shandong province and their bearing on the nature of the lower crust beneath the North China Craton

摘要

Geochronological, petrological and geochemical studies were performed on the granulite xenoliths from a Late Cretaceous basaltic breccia dike in Junan, Shandong province, eastern China. These xenoliths show close similarities to the Nushan granulite xenoliths from the southern margin of the North China Craton (NCC) and the Archean granulite terrains in terms of mineralogy and bulk rock compositions, but are quite different from the Hanuoba mafic granulite xenoliths from the northern NCC. In-situ zircon U-Pb age and Hf isotopic analyses, together with geochemical data reveal that the protolith of these xenoliths was formed around 2.3 Ga ago, through assimilation-fractional crystallization of a mafic magma. P-T conditions of these xenoliths suggest that the lower crust beneath the Junan region reaches to a depth of 35 km, which agree well with the result deduced from various geophysical methods. The consistent petrological and seismic Moho depths, the observed velocity structure and calculated velocity of these xenoliths imply the absence of underplating induced crust-mantle transition zone, which was well formed in the northern NCC. Compared to 40-50 km depth of the lower crust in Early Jurassic, the lower crust beneath Junan extended to a depth of 30 km in Late Cretaceous, suggesting that the lower crust of NCC was significantly thinned during Late Mesozoic.