Melt-rock interactions during the early stages of rapid exhumation of a deeply subducted continental slab

Abstract

Polyphase inclusions of K-feldspar + quartz + sphene were founded in both omphacite and garnet from the Sulu ultra-high pressure eclogites. These inclusions have clear cut boundaries with their host and display irregular internal textures. Each of the fifteen identified inclusions is unique in its shape and modal contents of K-feldspar (Kf; 18-85%), quartz (Qtz; 8- 78%) and sphene (Sph; 11-38%). Reconstructed bulk compositions have SiO_2 ranging from 56.6 to 91.0 wt%, Al_2O_3 from 3.4 to 15.4 wt%, K_2O from 3.0 to 14.2 wt%, CaO from 0.4 to 10.8 wt%, and TiO_2 from 0.5 to 12.5 wt%. We interpret these inclusions as products of reaction between K-rich haplogranitic melt and quartz (coesite) aggregates, generating a range of compositions according to the degree of progress of this reaction. The hybrid melts subsequently migrated into a mafic assemblage and were trapped during dynamic recrystallization of the major eclogite minerals. This hypothesis implies that either a melt rich in K and Si or a supercritical fluid was present during the subduction and exhumation of the Sulu UHP slab, at a stage preceding the end of eclogite facies conditions. The fluid phase must furthermore have been sufficiently mobile to pass from its host lithology through both silica-saturated and mafic regions of the continental slab. Although the initial fluid may have transported only fluid-mobile (LILE) elements, evidently the final hybrid melt carried enough Ti to crystallize sphene and hence transported also fluid-immobile (HFSE) elements during the exhumation of the continental slab. Furthermore, if the mobile phase began as a hydrous fluid, by the time the hybrid melt or cumulate minerals from the hybrid melt were trapped, water activity had reached low enough values to avoid the precipitation of any hydrous minerals.

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