Detrital chromian spinels in the Cretaceous Sindong Group, SE Korea: implications for tectonic emplacement of hydrated mantle peridotites

The Sindong Group (Aptian-Albian), distributed in southeastern Korea, is a fluvio-lacustrine sequence deposited in the NE-SW-trending Nakdong Trough formed in an active continental margin. Sediments were derived from the Yeongnam Massif located west and NW of the Nakdong Trough, where Early Jurassic arc magmatism began in the southern part of the Korean Peninsula. We report mineral chemistry of detrital chromian spinels of first-cycle origin occurring as an accessory mineral in arkose to feldspathic litharenites of the Sindong Group. Chromian spinel grains have Cr/(Cr + Al) atomic ratio between 0.36 and 0.94, Mg/(Mg + Fe2+) atomic ratio between 0.12 and 0.74, Fe3+/(Fe3+ + Al + Cr) value mostly <0.1, and <0.2 wt% TiO2; these values are consistent with spinel composition of forearc mantle peridotites, but are different from spinel composition of ultramafic rocks exposed in the southern Korean Peninsula. The results of this study indicate that forearc mantle peridotite or serpentinite was present in the Yeongnam Massif, and its tectonic emplacement is interpreted to have occurred by protrusion through the NE-SW-trending strike-slip fault zone developed in Middle Jurassic-Early Cretaceous time in the Yeongnam Massif and along the border with the adjacent tectonic province to the NW as a result of oblique convergence.