Formation and evolution of lithospheric mantle beneath the Yilan-Yitong fault zone, Northeast China: Constraints from the Shangzhi peridotite xenoliths

Abstract

Despite numerous studies, the formation and evolution of the lithospheric mantle beneath the Yilan-Yitong fault zone (YYFZ), Northeast China, are still poorly constrained. Here, we use in-situ trace elements and Sr isotopes of clinopyroxene, as well as whole-rock platinum group elements and Re-Os isotopes of the Shangzhi peridotite xenoliths (including spinel lherzolite and harzburgite) within the YYFZ to address the above important issues. Clinopyroxenes in one harzburgite characterized by high Zr/Hf (38.0–59.9), Ca/Al (9.3–13.4), and low Ti/Eu ratios (118–334) can be attributed to carbonatite metasomatism. Clinopyroxenes in some samples that is selectively enriched in light rare earth element exhibits lower Zr/Hf (17.0–46.9), Ca/Al (4.0–7.6), but higher Ti/Eu ratios (2219–4538), which may have been metasomatized by a carbonated silicate melt. The Sr contents and 87Sr/86Sr ratios for these clinopyroxene grains from the Shangzhi peridotites increase gradually from core to rim, suggesting the addition of radiogenic Sr from the metasomatic melts. Such metasomatic melts are likely derived from subducted carbonate-bearing oceanic crustal components. The 187Os/188Os ratios of two groups of the Shangzhi peridotites are positively correlated with Al2O3, forming two nearly parallel lines, which indicate that the lithospheric mantle beneath the Shangzhi area was formed by partial melting of a fertile mantle precursor with variable 187Os/188Os during the Mesoproterozoic time (∼1.4 Ga). The dominance of Mesoproterozoic mantle peridotites in the Shangzhi area suggests that asthenosphere upwelling and lithospheric mantle replacement are not common beneath the YYFZ.