• Journal of the Mineralogical Society of Korea
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• v.25 no.4
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• pp.249-261
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• 2012

Phlogopite-bearing orthopyroxenite occurs in Andong ultramafic complex in a planar body of about 1 meter thick, and consists mostly of coarse subhedral to euhedral orthopyroxene crystals. Minor minerals are clinopyroxene, phlogopite, and plagioclase with trace chromian spinel, pentlandite, apatite, and zircon. Clinopyroxene occurs as either exolution lamella or interstitial fillings with phlogopite and plagioclase. Electron microprobe analysis showed that orthopyroxenes are entatite, while clinopyroxenes are diopside with little chemical variation through samples. Hydrous alteration resulted in the formation of talc, amphibole, and serpentine from orthopyroxene, clinopyroxene, and plagioclase, respectively. The orthopyroxenite was probably formed by the fractional crystallization of the ultramafic magma. Radiogenic dating of phlogopite and zircon of the orthopyroxenite would reveal the age of the Andong ultramafic complex.

• The Journal of the Petrological Society of Korea
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• v.25 no.3
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• pp.261-281
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• 2016

Our knowledge of the lithosphere beneath the Korean Peninsula has been improved through petrologic and geochemical studies of upper mantle xenoliths hosted by Quaternary intraplate alkali basalts from Jeju Island. The xenoliths are mostly spinel lherzolites, accompanied by subordinate harzburgite and pyroxenites. The mantle xenoliths represent residual mantle material showing textural and geochemical evidence for at least a three-stage evolution, fractional partial melting, recrystallization, and metasomatism. Their composition primarily controlled by early fractional melt extraction and porphyroclastic and mylonitic fabrics formed in a shear-dominated environment, which was subsequently modified by residual slab-derived fluids (or melts). Modal metasomatic products occur as both anhydrous phase(orthopyroxene) and hydrous phase (phlogopite). Late-stage orthopyroxene is more common than phlogopite. However, chemical equilibrium is evident between the primary and secondary orthopyroxene, implying that the duration of post-metasomatic high temperatures enabled complete resetting/reequilibration of the mineral compositions. The metasomatic enrichment pre-dates the host Jeju Quaternary magmatism, and a genetic relationship with the host magmas is considered unlikely. Following enrichment in the peridotite protolith in the mantle wedge, the upper mantle beneath proto-Jeju Island was transformed from a subarc environment to an intraplate environment. The Jeju peridotites, representing old subarc fragments, were subsequently transported to the surface, incorporated into ascending Quaternary intraplate alkali basalt. The result of this study implies that long term material transfer in the transformation of geotectonic setting from a subarc to intraplate may have played a significant role in the evolution of lithospheric mantle, resulting in the enriched mantle domains, such as EM I or EM II in the lithospheric mantle beneath East Asia.