Comprehensive refertilization of the Archean–Paleoproterozoic lithospheric mantle beneath the northwestern North China Craton: Evidence from in situ Sr isotopes of the Siziwangqi peridotites

Abstract

Petrological observations and in situ major and trace element and Sr isotopic data for spinel-facies harzburgite and lherzolite xenoliths from the Siziwangqi region provide constraints on the nature and evolution of the lithospheric mantle beneath the northwestern North China Craton (NCC). These peridotites can be divided into high-Mg# peridotites (Fo > 91) and low-Mg# peridotites (Fo < 91), according to olivine forsterite (Fo) contents. The high-Mg# peridotites are harzburgites, which are characterized by high Mg# (91.0–91.8), as well as low CaO (0.41–1.17 wt%) and Al2O3 (0.62–2.03 wt%) contents. The clinopyroxenes have light rare earth element (LREE) depleted to slightly enriched patterns, and variable but dominantly radiogenic Sr isotopic compositions (0.7031–0.7053). These features indicate that the high-Mg# peridotites are remnants of Archean–Paleoproterozoic refractory lithospheric mantle, which then experienced variable refertilization. In contrast, the low-Mg# peridotites are mainly lherzolites and minor harzburgites, which have low Mg# (89.2–90.9), and high CaO (0.98–3.53 wt%) and Al2O3 (1.38–3.85 wt%) contents, relative to the high-Mg# peridotites. The clinopyroxenes in low-Mg# peridotites have similar REE patterns as those in the high-Mg# peridotites, and some grains show increasing LREE contents from core to rim. These clinopyroxenes have relatively depleted Sr isotopic compositions (0.7021–0.7045). The low-Mg# peridotites are interpreted to be the reaction products of high-Mg# peridotites and asthenospheric melts. The lithospheric mantle beneath the Siziwangqi region experienced an earlier metasomatic event related to subduction of Paleo-Asian oceanic materials, as evidenced by the relatively high 87Sr/86Sr ratios (up to 0.7053) and depletion of Rb, Ba, Pb, Nb and Ta in clinopyroxenes. The clinopyroxenes in both high- and low-Mg# peridotites have low (La/Yb)N and high Ti/Eu ratios, positive correlations between 87Sr/86Sr and Mg# and Ca/Al, negative correlations between 87Sr/86Sr and Na2O, TiO2, Ti/Eu, and Σ(middle REEs + heavy REEs), and decreasing 87Sr/86Sr ratios from core to rim. These observations imply that the lithospheric mantle beneath the Siziwangqi region experienced later multi-stage refertilization by asthenospheric melts. Therefore, we conclude that the lithospheric mantle beneath the northwestern NCC has been comprehensively refertilized by multiple stages of peridotite–melt reactions, similar to the northern margin of the NCC, which had a significant role in the transformation of the lithospheric mantle.