Late Mesozoic mafic magmatism from the North China Block: constraints on chemical and isotopic heterogeneity of the subcontinental lithospheric mantle

Abstract

AbstractAvailable major, trace element and Sr–Nd isotope data for the late Mesozoic mafic rocks in the eastern North China Block (NCB) show chemical and isotopic differences between rocks from different tectonic units. Such differences are interpreted as signatures inherited from the melted mantle sources, which had experienced distinctive enrichment processes during lithospheric evolution. The subcontinental lithospheric mantle beneath the NCB interior is characterized by long-term light REE (LREE) enrichment and EM1-like Sr–Nd isotopic signatures. Such a lithospheric mantle is mainly composed of chemically refractory peridotites that are common in cratonic regions. In contrast to that of the NCB interior, beneath the northern part of the NCB a relatively chemically fertile mantle was enriched in large ion lithophile elements and LREE and depleted in Nb–Ta and Th–U. It has higher 87Sr/86Sr(i) and εNd(t) than that of the interior of the block, and is interpreted to have been modified by recycled lower continental crust components related to the palaeo-Asian Ocean subduction. The lithospheric mantle beneath the southern NCB has the highest 87Sr/86Sr(i) and the lowest εNd(t), and is chemically transitional between the interior and northern part of the block. Formation of such an enriched lithospheric mantle was closely associated with modification from the subducted Yangtze lower–middle crust during Triassic collision between the North China and Yangtze Blocks. A lithospheric extension–thinning model is proposed to explain the petrogenesis of these late Mesozoic mafic rocks in the eastern North China Block. This process was amplified by effects from surrounding plate interactions, including the rapid northward movement of the palaeo-Pacific Ocean, compressional forces from the Siberian plate, the Tethyan tectonic belt and possibly the Indo-China Block. The resultant forces triggered lithospheric extension, asthenospheric upwelling, and decompressional melting of the enriched mantle sources.