Mantle refertilization at high-pressure conditions of continental subduction channel: Evidence from orogenic peridotites in the Sulu UHP terrane, eastern China

Abstract

Refertilization of depleted peridotites widely occurs in lithospheric mantle but is poorly understood in a continental subduction channel. The Sulu orogenic belt is a typical ultra-high pressure (UHP) terrane formed by continental subduction and collision. To understand the complex metasomatic processes on peridotites during continental subduction, we combined detailed petrographic and geochemical studies for fertile garnet lherzolites (Chijiadian) from the Sulu UHP terrane, as well as refractory peridotites (Xugou and Lijiatun) for comparison. The fertile garnet lherzolites (Chijiadian) are not residues of low-degree partial melting, but they formed from refertilization processes during peak-UHP metamorphic stages (garnet-facies) in a continental subduction channel, as reflected by abundant clinopyroxene (12 vol%) and garnet (12 vol%), low bulk-rock Mg# of 91 and high Al2O3 of 2.4–3.3 wt%. Garnet, clinopyroxene, and orthopyroxene display secondary poikilitic textures and often contain early phases of olivine, spinel, and garnet inclusions, indicating later precipitation of these minerals into depleted peridotite protoliths. The clinopyroxenes show enrichment of large ion lithophile elements (LILE) and high 87Sr/86Sr of 0.7061–0.7084. The estimated pressure-temperature are 720–770 °C at 4.4–4.8 GPa, consistent with previous constraints on UHP conditions. These features suggest strong refertilization processes at UP-UHP conditions (garnet-facies) in continental subduction channels. The metasomatic agents for refertilization would be basaltic-intermediate melts which incorporated continental materials. Similarly, Xugou harzburgites and Lijiatun dunites also record the subducted continental signatures with enrichment of LILE and high Sr isotopes on clinopyroxenes (87Sr/86Sr: 0.7050 to 0.7065). However, Xugou harzburgites and Lijiatun dunites overall do not display the same typical refertilization features as for Chijiadian, as reflected by low abundances of clinopyroxene and high Mg# of 92–93. Consequently, continental subduction could lead to diverse types of metasomatism of refractory peridotites, including refertilization at UP-UHP metamorphic stages in subduction channels, as in other tectonic settings.