Ultrahigh-Pressure (60 Kbar) Magnesite-Bearing Garnet Peridotites from Northeastern Jiangsu, China

The very high P/T metamorphic belt in China's Shandong and fiangsu provinces, which has drawn much attention because of its numerous coesite-bearing eclogites, also contains little-studied massifs of garnet peridotites. Three samples, a chromite-bearing garnet peridotite (DZ01), a spinel-free garnet peridotite (DZ02) and a garnet websterite (DG01) from northeastern Jiangsu are studied here. Olivine compositions in peridotites are restricted to FO₉₀₋₉₂. The spinel phase in DZ01 is mostly Fe-rich chromite, with increasing magnetite toward its rim. Garnets are pyropic; their rims are relatively enriched in almandine. Orthopyroxene is extremely poor in Al, reflecting the very high P conditions. Clinopyroxene is Cr-rich, Al-poor diopside. Magnesite and phlogopite in DZ01 are believed to have formed in the mantle but somewhat later than the other primary phases. Magnesite, with very low Sr, Ba, and K contents, displays breakdown reactions producing dolomite, calcite, and periclase. Titanoclinohumite and pentlandite have been observed. Amphibole and chlorite occur as secondary phases. Geothermobarometry yields extremely high P of equilibration for these rocks (65-45 kbar). The P-T paths followed by the two garnet peridotites were considerably lower in T than the normal continental geotherm. Evidence against partial melting during uplift of the peridotites is clear. Metasomatism and oxidation of the mantle source by a H₂O-bearing fluid are demonstrated. Oxygen fugacity in DZ01 evolves from +0.45 to + 1.37 log units relative to the FMQ buffer, with decreasing P and T. These results suggest that the garnet peridotites were derived from a mantle wedge overlying a subducted slab.