Petrology, geochemistry and Re–Os isotopes of peridotite xenoliths from Yantai, Shandong Province: Evidence for Phanerozoic lithospheric mantle beneath eastern North China Craton

Abstract

Petrology, geochemistry and Re–Os isotopes of peridotite xenoliths from Yantai (Shandong Province) are reported in this paper with aims of constraining the age and evolution of the lithospheric mantle beneath eastern North China Craton. The Yantai xenoliths contain predominant harzburgites and subordinate lherzolites. Although their highly incompatible element compositions have been modified by metasomatism, the heavy rare earth element (REE) and Y contents in the Yantai peridotites are primarily governed by partial melting, which started in garnet stability field then continued in spinel stability field after breakdown of garnet to two pyroxenes and spinel. Such a polybaric melting produced a residual mantle in which degree of depletion decreases with depth. Os isotopic ratios of the most refractory peridotites (Al2O3<1.2wt.%) range from 0.117 to 0.126, yielding TRD model ages between 0.5 and 1.7Ga. This suggests co-existence of Phanerozoic and Proterozoic mantle beneath Shandong Province. Alternatively, the whole lithospheric mantle beneath Yantai was likely formed during the Phanerozoic, given the resemblance of their Os isotopic ratios with those of abyssal peridotites. The latter interpretation is consistent with the fact that all the studied samples plot along the oceanic trend in a plot of forsterite content in olivine versus olivine mode. It also gains further support from the contrasting εNd between the late Mesozoic lithospheric mantle and Cenozoic mantle beneath the region. The data presented in this study therefore argue for a complete replacement of the cratonic mantle by upwelling asthenosphere.