Petrogenesis of the Yeonhwa ultrapotassic intrusions in the Yeongnam Massif―Evidence for enrichment of the Triassic continental lithospheric mantle beneath the Korean peninsula

Abstract

The Triassic period is when abundant late-orogenic magmatism occurred in the Korean peninsula, but the characteristics of the lithospheric mantle at that time beneath the Yeongnam Massif have not been well-defined because of the scarcity of mafic intrusions. We report zircon U-Pb and phlogopite 40Ar/39Ar ages, petrography, and geochemistry of the Triassic (227.6 Ma) ultrapotassic igneous rocks in the Yeonhwa I Mine (Yeonhwa Ultrapotassic Intrusions, YUI) emplaced into the Taebaeksan Basin in the Yeongnam Massif. The YUI occur as two stocks with different lithologies: phlogopite-rich and amphibole-rich, where the former underwent carbonate melt interaction along the margin in contact with the carbonate formation. The two lithologies share similar compositional and isotopic characteristics; however, the phlogopite-rich YUI better retains its mantle-derived features, having idiomorphic high Mg# (up to 91.9) phlogopite and diopside phenocrysts and bulk concentra tions of high MgO (Mg# = molar 100 Mg/[Mg + Fe] > 70), Cr (208？1150 ppm), and Ni (48？219 ppm). The enrichment of the YUI in H2O (phlogopite-rich), silica contents (50.2？56.9 wt%), and large ion lithophile ele ments (LILE; K, Rb, Sr, Ba, Pb), together with depletion of high fields strength elements (HFSE; Ti, Zr, Nb, Hf, Ta), are typical of the ultrapotassic rocks in orogenic settings. The elemental and isotopic data imply that the YUI originated from partial melts of the phlogopite-bearing lithospheric mantle near the spinel-garnet transition zone. The extremely enriched isotopic ratios (87Sr/86Sr(t) = 0.70756 to 0.71227, εNd(t) = -8.82 to -11.0) suggest a highly enriched mantle source, possibly more enriched than the contemporary pelagic sediments. Considering that the Yeongnam Massif was in a subduction system during Permian Triassic, not a continental collisional belt, the enrichment was inherited from a combination of both young, subduction-related and ancient, pre-existing metasomes in the continental lithospheric mantle. Our data confirm the presence of the enriched, volatile-rich lithospheric mantle beneath the Triassic Yeongnam Massif. This enriched origin contrasts with earlier (c. 240？280 Ma) Yeongnam arc plutons that originated from the depleted asthenospheric mantle (εNd(t) ≥+2). Since the enrichment is also characteristic of most of the c. 220？240 Ma igneous rocks around the Korean peninsula, such lithospheric mantle lithology was widespread during the Late Triassic.