Petrology of the Yugu peridotites in the Gyeonggi Massif, South Korea: Implications for its origin and hydration process

Abstract

Peridotites exposed in the Yugu area in the Gyeonggi Massif, South Korea, near the boundary with the Okcheon Belt, exhibit mylonitic to strongly porphyroclastic textures, and are mostly spinel lherzolites. Subordinate dunites, harzburgites, and websterites are associated with the lherzolites. Amphiboles, often zoned from hornblende in the core to tremolite in the rim, are found only as neoblasts. Porphyroclasts have recorded equilibrium temperatures of about 1000°C, whereas neoblasts denote lower temperatures, about 800°C. Olivines are Fo 90-91 in lherzolites and Fo 91 in a dunite and a harzburgite. The Cr# (= Cr/(Cr + Al) atomic ratio) of spinels varies together with the Fo of olivines, being from 0.1 to 0.3 in lherzolites and around 0.5 in the dunite and harzburgite. The Na 2 O content of clinopyroxene porphyroclasts is relatively low, around 0.3 to 0.5 wt% in the most fertile lherzolite. The Yugu peridotites are similar in porphyroclast mineral chemistry not to continental spinel peridotites but to sub-arc or abyssal peridotites. Textural and mineralogical characteristics indicate the successive cooling with hydration from the upper mantle to crustal conditions for the Yugu peridotites. Almost all clinopyroxenes and amphiboles show the same U-shaped rare earth element (REE) patterns although the level is up to ten times higher for the latter. The hydration was associated with enrichment in light REE, resulting from either a slab-derived fluid or a fluid circulating in the crust. The mantle-wedge or abyssal peridotites were emplaced into the continental crust as the Yugu peridotite body during collision of continents to form a high-pressure metamorphic belt in the Gyeonggi Massif. The peridotites from the Gyeonggi Massif exhibit lower-pressure equilibration than peridotites, with or without garnets, from the Dabie-Sulu Collision Belt, China, which is possibly a westward extension of the Gyeonggi Massif.

Diopsidites from a Neoproterozoic–Cambrian suture in southern India

“…Arai et al . (2008), in a recent study on Yugu peridotite in South Korea, demonstrated that shearing/suturing and water supply linked with cooling led to the production of anhydrous high-temperature pyroxene porphyroclasts and lower temperature neoclasts of hydrated amphiboles. The basic processes associated with the formation of diopsidites from PCSZ are similar to those discussed above; particularly the trace and rare earth elements features are comparable with those of diopsidite dykes in Oman ophiolites.…”

“…The diopsides are rich in Sr and LREEs, and depleted in Ti, Nb, Ta and Zr. Arai et al (2008), in a recent study on Yugu peridotite in South Korea, demonstrated that shearing/suturing and water supply linked with cooling led to the production of anhydrous hightemperature pyroxene porphyroclasts and lower temperature neoclasts of hydrated amphiboles. The early crystallization of mica, amphibole and titanite particularly rich in HFSEs might reduce their concentrations in the residual melt.…”

Petrochemistry of Ultramafic Rock in Baula - Pomalaa Ophiolite Complex, Southeast Sulawesi, Indonesia

“…Another source could also be the decomposition of water from previous serpentine formations. These processes likely occurred during the uplift of the ophiolite from the upper mantle to the surface (Arai et al, 2008). The absence of plagioclase in ultramafic rocks indicates that the uplift process of these ultramafic rocks occurred rapidly (Maulana et al, 2015).…”

Compositional diversity in peridotites as result of a multi-process history: The Pacific-derived Santa Elena ophiolite, northwest Costa Rica

“…In this reaction between the percolating melts and the harzburgite matrix, an "enriched component" would be added at the base of the mantle wedge in a SSZ context and the initial LILE-LREE-enriched melts would then percolate up through the porous mantle on the way to the surface (e.g. Parkinson and Pearce, 1998;Arai et al, 2008). En-route, the melt would react with relatively "unmetasomatized" mantle and might lose incompatible elements to the peridotite matrix or wall rocks along their way.…”