Multiphase solid inclusions in UHP rocks (Su-Lu, China): Remnants of supercritical silicate-rich aqueous fluids released during continental subduction

Abstract

Primary multiphase solid (MS) inclusions without preserved fluid are found within peak minerals in kyanite quartzite ± topaz and kyanite–phengite–epidote eclogite from Donghai area (Su-Lu terrane). Typical mineral association in inclusions is: paragonite + muscovite + anhydrite ± corundum ± “alunite-type” sulphate ± zircon ± calcite ± chlorite ± SiO 2 ± barite ± pyrite ± apatite in quartzites, and paragonite + rutile + apatite ± amphibole ± Zn-staurolite ± magnetite ± plagioclase ± zircon ± pyrite ± “alunite-type” sulphate ± Zn–Mg–Fe–Al–Ti spinel in eclogites. On the basis of the fluid inclusion textures and of the daughter-phase assemblage, calculated fluid composition is as follows: in quartzites 24 wt.% SiO 2, 30 wt.% Al 2O 3, 9 wt.% CaO, 5 wt.% K 2O, 3 wt.% Na 2O, 11 wt.% SO 3, 18 wt.% H 2O, with traces of TiO 2, Fe 2O 3, FeO, MgO, BaO, P 2O 5, Cl −, F, and (CO 3) 2−, and in eclogites 26 wt.% SiO 2, 21 wt.% TiO 2, 20 wt.% Al 2O 3, 2 wt.% MgO, 4 wt.% FeO, 6 wt.% Fe 2O 3, 7 wt.% CaO, 3 wt.% Na 2O, 3 wt.% P 2O 5, 7 wt.% H 2O, 1 wt.% Cl, and traces of ZnO, MnO, K 2O, SO 3, and F. Originally trapped chloride-poor aqueous fluids contained very high amounts – in the order of tens of wt.% – of Si 4+, Al 3+, and Ti 4+. Solute species reflect the chemical composition of the host rocks: Mg 2+, Fe 2+, Ti 4+, P 5+, and Na +, are abundant in the fluids present in eclogites, while fluids are enriched in Al 3+, K +, Na +, (SO 4) 2−, (CO 3) 2− in quartzites. We propose that multiphase solid inclusions represent remnants of high-density supercritical silicate-rich aqueous fluids that were in equilibrium with peak minerals at UHP conditions. These fluids show characters which are transitional between aqueous fluids and silicate melts, and were probably produced by dehydration reactions of the host rocks during the latest stages of subduction.