Geochronology, petrogenesis and tectonic implications of rodingites from the Baixingtu ophiolite in central Inner Mongolia, southeastern Central Asian Orogenic Belt

Abstract

Rodingite, a metasomatic rock type related to the serpentinisation of ultramafic rocks, occurs as dykes or lenses in serpentinite of the ophiolitic mélange. The formation age, protolith and metamorphic context of the rodingites are crucial for evaluating the hydrothermal activity of the ancient ocean floor and the tectonic history of the ophiolite. This study presents particular research on metamorphic petrology, geochemistry and zircon U–Pb chronology of rodingites and their associated mafic–ultramafic rocks in the Baixingtu ophiolite, the middle segment of the Erenhot-Hegenshan ophiolite belt (EHOB), southeastern Central Asian Orogenic Belt. The mean metamorphic ages of rodingites are 345.8 ± 3.8 Ma, 339.9 ± 4.8 Ma, and 344.5 ± 9.2 Ma. According to the chlorite thermometer, the final mineral assemblages of rodingites formed at temperatures ranging from 114.99 °C to 351.10 °C. The high oxygen fugacity of nascent clinopyroxenes and the negative anomaly of Ce in adjacent serpentinites (δCe = 0.34–0.77) prove that rodingitisation occurs in shallow oceanic crust by the reaction of seawater with ultramafic rocks to produce Ca-rich fluids. Accordingly, the Baixingtu ophiolite was produced by an ocean floor metamorphism, whose rodingitisation occurred shortly after the formation of the oceanic crust. Combined with other ophiolite data from the EHOB, the Hegenshan Ocean was constantly generating new oceanic crust in the Early Carboniferous.