Magmatic peridotites and pyroxenites, Andong Ultramafic Complex, Korea: Geochemical evidence for supra-subduction zone formation and extensive melt–rock interaction

The Andong Ultramafic Complex (AUC) mainly comprises peridotites (wehrlites ± plagioclase or spinel; or plagioclase + spinel) and related serpentinites with subordinate low-Al pyroxenites (clinopyroxenites, orthopyroxenites, and websterites). These rocks are compositionally similar to sub-continental lithospheric mantle peridotites and pyroxenites. Wehrlites formed predominantly by fractional crystallization processes within supra-subduction zone magmas and the pyroxenites are generally consistent with segregation and accumulation in similar magmas. Bulk rock ratios of Al2O3/SiO2 (0.01–0.03) and MgO/SiO2 (up to > 1) exhibited by the wehrlites and serpentinites indicate crystallization from a refractory source that underwent high degrees of melt extraction. Spinel chemistry confirms this and demonstrates that wehrlite and clinoproxenite protoliths underwent approximately 20–23% and 12–15% partial melting, respectively. Wehrlites and serpentinites also preserve evidence of extensive melt–peridotite interaction manifest as bulk rock SiO2-depletions and FeOt-enrichments relative to mantle residua as well as low Mg# (0.39–0.45) spinels with variable Ti contents but constant Cr# (0.42–0.47). These features are identical to those of ‘impregnated’ plagioclase-peridotites of abyssal and sub-continental environments and compositional trends in spinel space imply reaction between secondary, MORB-like melts saturated in olivine + clinopyroxene or olivine and a harzburgitic protolith. High olivine:pyroxene (~ 3:1) and clinopyroxene:orthopyroxene ratios of the wehrlites coupled with chemical data dictate that reactions entailed orthopyroxene dissolution and olivine recrystallization. All AUC rock types exhibit primitive mantle-normalized incompatible element signatures characterized by LILE-enrichments, high fluid-mobile/immobile element ratios (Sr/Nd, Ba/La and Pb/Ce ≫ 1) and prominent HFSE (Nb, Zr, and Ti) depletions indicative of generation in a sub-arc environment within a supra-subduction zone system. A candidate for the associated arc-system is the one responsible for nearby arc-related Jurassic granitoids. Southeast-directed thrusting along the Andong Fault System may account for subsequent emplacement of the AUC into the Gyeongsang Basin.