Exsolution of Garnet and Clinopyroxene from High-Al Pyroxenes in Xugou Peridotite, Eastern China

摘要

Serpentinized massif peridotite in the Xugou, Su-Lu ultrahigh-pressure (UHP) metamorphic belt, eastern China, preserves texturally old (porphyroclastic) ortho- and clinopyroxene with up to two generations of lamellae of garnet, clinopyroxene and Mg-chromite. Their crystallographic orientation with respect to the host pyroxene is consistent with an origin by solid-state exsolution. Comparison of integrated mineral chemistry with simplified and natural chemical datasets suggests that both aluminous precursor pyroxenes were in equilibrium at a minimum pressure of ∼4 GPa and within a temperature range of about 1300–1500°C. Steep isopleths of Ca in orthopyroxene imply that exsolution occurred during cooling. Al diffusion modelling suggests growth of widely spaced lamellae in orthopyroxene down to about 900°C. Integrated Al contents between wide lamellae record a minimum of 4 GPa pressure during cooling. Compositionally uniform exsolved minerals were formed at 4·3 ± 0·3 GPa and 730 ± 30°C and reflect a cratonic geotherm with about 33 mW m−2 surface heat flow. The peridotite matrix mineral assemblage of olivine + orthopyroxene ± garnet ± Mg-chromite ± clinopyroxene ± phlogopite records strain-induced recrystallization that partially to completely replaced precursor porphyroclasts. The recrystallized minerals lack lamellar exsolution. Recrystallized orthopyroxene, with Al₂O₃ at 0·13 wt %, indicates conditions of 5·5 ± 0·3 GPa and 760 ± 30°C, which are higher-grade metamorphic conditions than those preserved in the chemically equilibrated exsolution microstructures. Both estimates overlap with the range reported for the Early Mesozoic UHP metamorphism in the region (4·0–6·7 GPa and 760–970°C). Major element melt models applied to previously published Xugou peridotite data suggest high degrees of melt extraction (30–35 %) in the garnet peridotite stability field (3–4·5 GPa) until garnet and clinopyroxene exhaustion. Coincidence in pressure and in the order of temperature of equilibration of precursor pyroxenes and peridotite melting implies that peridotite formation occurred at ∼135 km depth in the subcontinental lithospheric mantle (SCLM) beneath the Archaean North China Craton. Subsequent refertilization, mineral exsolution and chemical re-equilibration during long-term cooling in the SCLM occurred prior to deformation and incorporation of the mantle fragment into the continental crust during UHP metamorphism at a minimum depth of 170 km. Because the Xugou precursor pyroxenes and peridotite formed at depths greater than the regional SCLM (c. 90 km), we infer that the orogenic peridotite massif formed part of the former hanging wall of the Archaean SCLM, which delaminated after the Late Mesozoic.