Nature and evolution of lithospheric mantle beneath the Northeast China: Constraints from the Houtuancun mantle xenoliths

Mantle xenoliths entrained in a newly discovered Cenozoic basalt at Houtuancun within the Yilan-Yitong fault zone, situated beneath the Xing'an Mongolian Orogenic Belt in Northeast China, provide a unique opportunity to constrain the nature and evolution of the lithospheric mantle beneath this region. The mantle xenoliths are predominantly spinel lherzolite, with rare occurrences of harzburgite, wehrlite, and websterite. The Houtuancun spinel lherzolite and harzburgite are characterized by high forsterite contents in olivines (89.3-91.1), and their clinopyroxene rare earth element patterns range from slightly light rare earth element (LREE)-depleted to LREE-enriched, reflecting variable degrees of partial melt extraction (up to 20%) overprinted by later metasomatism. Despite this, the Fe isotopic composition of the Houtuancun lherzolite and harzburgite exhibits little variation (delta Fe-57/54 = - 0.05-0.09 parts per thousand), with an average of 0.02 +/- 0.09 parts per thousand (2SD, n = 9), close to the suggested delta Fe-57/54 value of 0.04 +/- 0.04 parts per thousand for the upper mantle, and there is no significant inter-mineral disequilibria. In contrast, the wehrlite has a slightly lower Mg# [=100 x Mg2+/(Mg2+ + Fe2+)] (88.4) and delta Fe-57/54 (-0.05 parts per thousand) compared to the spinel lherzolite and harzburgite, reflecting kinetic iron fractionation during melt-rock interaction. The websterite exhibits the lowest MgO (20.6-24.8 wt%), highest Al2O3 content (5.8-7.3 wt%), and the highest delta Fe-57/54 (0.25-0.28 parts per thousand) among the studied xenoliths, suggesting an episode of subduction-related silicate melt circulation through the upper mantle. The results demonstrate compositional heterogeneity of the mantle beneath the Xing'an Mongolian Orogenic Belt in Northeast China. The observed heterogeneity may result from a combination of varying degrees of melt extraction of the subcontinental lithospheric mantle and metasomatism by melts related to the subduction of the (Paleo)-Pacific plate.