In-situ trace element and Sr isotopic compositions of mantle xenoliths constrain two-stage metasomatism beneath the northern North China Craton

Article history: Received 29 March 2017 Accepted 27 July 2017 Available online 04 August 2017 Subduction and collision are the key processes triggering geochemical refertilization of the lithospheric mantle beneath cratons. However, the way that the subducted plate influences the cratonic lithospheric mantle remains unclear. Here, in-situ major and trace-element and Sr isotopic compositions of peridotite and pyroxenite xenoliths carried by the Dongbahao Cenozoic basalts, located close to the northern margin of North China Craton (NCC), were examined to investigate the effects of the subducted Paleo-Asian oceanic plate on the lithospheric mantle of the NCC. Based on petrographic and geochemical features, peridotites were subdivided into two types recording two-stage metasomatism. Clinopyroxene (Cpx) in both types of peridotites show chemical zoning. In those peridotiteswe refer to as Type 1 peridotites, Cpx exhibit uniform convex-upward rare earth element (REE) patterns but core-rim variations in Sr/Sr ratios (0.7065–0.7082 in the cores and 0.7043–0.7059 in the spongy rims), and have high (La/Yb)N ratios (N1.12) (N means normalized to chondrite), relatively low Ti/Eu ratios (b3756) and negative high field strength element (HFSE) (Nb, Ta, Zr, Hf and Ti) anomalies in the cores, indicating early-stage metasomatism by carbonatitic melts derived from the subducted sedimentary carbonate rocks. Cpx in the Type 2 peridotites have highly variable REE patterns (from light rare earth element (LREE)depleted to LREE-enriched) and feature zoned Sr isotopic compositions contrasting to those in Type 1, i.e., increasing Sr/Sr ratios from the cores (0.7020–0.7031) to the spongy rims (0.7035–0.7041). Accompanying variations of Sr/Sr ratios, Cpx in both types of peridotites display increasing Nb/La ratios from the cores to the spongy rims. In addition, Cpx in the Type 2 peridotites show remarkably increased (La/Yb)N, Ca/Al, Sm/Hf and Zr/Hf ratios but decreased Ti/Eu and Ti/Nb ratios from the cores to the spongy rims. These features imply a laterstage metasomatism by CO2-rich silicate melts derived from carbonated eclogites. Pyroxenites were also classified into two types. Both types of pyroxenites show higher Ni content in Cpx and orthopyroxene than peridotites at the sameMg# (=100 ∗Mg/(Mg+ Fe), atomic number) level. Their Cpx show high Ti/Eu, Ti/Sr ratios and similar Sr/Sr ratios (0.7039–0.7055) to the Cpx spongy rims in peridotites, suggesting that pyroxenites originated from silicate melt-peridotite reactions in the later-stage metasomatism. These observations collectively indicate that the lithospheric mantle beneath the northern NCC presents evidence for two distinct mantle metasomatic events. We propose that both were caused by the subduction of the Paleo-Asian oceanic plate,which could have contributed significantly to the transformation of the lithospheric mantle beneath the northern NCC. © 2017 Elsevier B.V. All rights reserved.