Petrology and Chemistry of Metasomatic Blocks from Bawshir, Northeastern Oman

Abstract

Abstract Several types of Ca-rich, metasomatically altered rocks occur as blocks or reaction zones in a serpentinite-matrix mélange in the Bawshir-Al-Khuwair area, west of Ruwi in northeastern Oman. Type A rodingites contain mainly hydrogarnet, with smaller amounts of diopside, tremolite ± pumpellyite ± prehnite. Type B rocks contain more diopside ± tremolite and smaller amounts of hydrogarnet. Type C nephrites consist chiefly of tremolite and have a distinctive botryoidal texture. Quartzite blocks in this mélange were also affected by Ca-metasomatism, and contain variable amounts of tremolite, hydrogarnet, chlorite, and clinozoisite. Relatively immobile trace and rare-earth element contents of these rocks suggest that the protoliths of types A and B metasomatic rocks were ocean-island basalts, whereas type C nephrites represent metasomatized serpentinites. Isocon diagrams show that all metasomatic rocks formed by the addition of Ca ± Mg ± volatiles, and leaching of almost all other elements to variable degrees. Field and textural relations interpreted with an activity-corrected petrogenetic grid in the system CMASH suggest that metasomatism took place in stages over a range of P-T conditions, beginning at 220-310°C and 3-6 kbar, and ending at 250-310°C, P < 3.2 kbar. Diffusion of Ca2+ out of the serpentinizing peridotites into blocks of basalts and quartzites was the primary cause of early metasomatism, which overlapped with high P/T metamorphism of the same blocks. Infiltration of low-salinity aqueous fluids was partly responsible for the latest stages of metasomatism, which took place during exhumation, and continued after the final emplacement of the ophiolite.