Genesis of high-Al chromitites from Xiaosuihe peridotite (Central Jilin Province, NE China): Implication for initial subduction of the Paleo-Asian ocean

Abstract

This study presents whole-rock major- and trace-element data of serpentinites and mineral geochemistry of chromite/spinel from both serpentinites and associated podiform chromitites from the Xiaosuihe region in central Jilin Province, NE China, aimed to elucidate the genesis of serpentinites and podiform chromitites. The serpentinites exhibit low Al2O3/SiO2 ratios (0.01–0.02) and high Mg# (0.90–0.91), indicating their derivation from the fore-arc mantle wedge and protolith as harzburgite. Chromitite ores with semi-massive to disseminated textures occur as small pods. The chromite/spinel (Chr) present in both the podiform chromitite and serpentinite can be categorized into four distinct microstructural groups: (1) partially altered chromite, characterized by high-Al primary cores (Cr# = 0.44–0.49), surrounded by high-Cr porous rims; (2) porous chromite, featuring abundant pores enriched in Cr and Fe and depleted in Mg and Al, filled with chlorite; (3) zoned chromite consisting of high-Al cores surrounded by non-porous rims of Cr-magnetite (Cr-mag); and (4) homogeneous chromite, which is non-porous ferritchromite (Fe-Chr). The high-Al primary cores of the partly altered chromite are depleted in Cr but enriched in Mg, exhibiting MORB-like tholeiitic affinities. In contrast, the altered Chr displays low Mg# [Mg/(Mg + Fe2+)] along with elevated Cr# [Cr/(Cr + Al)] and Fe# [Fe3+/(Cr + Al + Fe3+)] values. Based on the above results, the high-Al cores of chromite within the chromitite originally formed through melt/fluid-rock interaction within the forearc mantle wedge. During the exhumation and post-magmatic processes, the reaction between high-Al Chr and olivine resulted in chlorite formation alongside secondary Chr enriched in Fe2+ and Cr content. Subsequently, the infiltration of hydrous fluid led to the development of Fe3+-rich Chr or Cr-mag along grain boundaries or fractures within the original chromite during the retrograde process. The Xiaosuihe serpentinized harzburgites, along with associated chromitites, represent the composition of peridotite in the forearc mantle wedge that could have formed during the initial subduction of an oceanic island arc abutting the northern North China Craton during the Cambrian. Subsequently, these rocks underwent retrograde metamorphism from amphibolite (eclogite) to greenschist facies during their exhumation.