首页|Petrogenesis of the Yeonhwa ultrapotassic intrusions in the Yeongnam Massif—Evidence for enrichment of the Triassic continental lithospheric mantle beneath the Korean peninsula
Petrogenesis of the Yeonhwa ultrapotassic intrusions in the Yeongnam Massif—Evidence for enrichment of the Triassic continental lithospheric mantle beneath the Korean peninsula
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NSTL
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 formadon. 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 concentrations 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 H20 (phlogopite-rich), silica contents (50.2-56.9 wt%), and large ion lithophile elements (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 pardal melts of die phlogopite-bearing lithospheric mantle near the spinel-garnet transition zone. The extremely enriched isotopic ratios (87Sr/86Sr(t) = 0.70756 to 0.71227, eNd(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 mande (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 die Late Triassic.
NSTL
