Coesite eclogite is associated with metagranitoid in a 50×100 m 2 outcrop within the regionally developed amphibolite-facies Su-Lu orthogneiss. Primary intrusive relationships between the metagranitoid and basic rocks and bulk-chemistry analyses show that together they represent a composite igneous body that has subsequently been strongly deformed and metamorphosed. The presence of rutile, sodie pyroxene, corona garnet, and possible pseudomorphs after coesite all suggest very high pressures of metamorphism in the metagranitoid. This is the first documented occurrence of ultrahigh-pressure (UHP) metagranitoid outside of the European Alps. The existence of UHP metagranitoid shows that low density of rocks does not necessarily prevent subduction to mantle depths. Even at peak metamorphic conditions the UHP composite igneous body reported here would have a bulk density less than the mantle. Buoyancy forces may, therefore, have been important in the early exhumation of this unit. Other outcrops of coesite eclogite in the Su-Lu region may also have been originally metamorphosed along with low-density granitoid rocks.

The exhumation mechanism of high-pressure (HP) and ultrahigh-pressure (UHP) eclogites formed by the subduction of oceanic crust (hereafter referred to as oceanic eclogites) is one of the primary uncertainties associated with the subduction factory. The phase relations and densities of eclogites with MORB compositions are modelled using thermodynamic calculations over a P-T range of 1-4 GPa and 400-800°C, respectively, in the NCKFMASHTO (Na 2 O-CaO-K 2 O-FeO-MgO-Al 2 O 3 -SiO 2 -H 2 O-TiO 2 -Fe 2 O 3 ) system. Our modelling suggests that the mineral assemblages, mineral proportions and density of oceanic crust subducted along a cold P-T path are quite different from those of crust subducted along a warm P-T path, and that the density of oceanic eclogites is largely controlled by the stability of low-density hydrous minerals, such as lawsonite, chlorite, glaucophane and talc. Along a cold subduction P-T path with a geotherm of~6°C km À1 , lawsonite is always present at 1.1 to >4.0 GPa, and chlorite, glaucophane and talc can be stable at pressures of up to 2.3, 2.6 and 3.6 GPa respectively. Along such a P-T path, the density of subducted oceanic crust is always lower than that of the surrounding mantle at depths shallower than 110-120 km (< 3.3-3.6 GPa). However, along a warm subduction P-T path with a geotherm of~10°C km À1 , the P-T path is outside the stability field of lawsonite, and the hydrous minerals of chlorite, epidote and amphibole break down completely into dry dense minerals at relatively lower pressures of 1.5, 1.85 and 1.9 GPa respectively. Along such a warm subduction P-T path, the subducted oceanic crust becomes denser than the surrounding mantle at depths >60 km (>1.8 GPa). Oceanic eclogites with high H 2 O content, oxygen fugacity, bulk-rock X Mg [ = MgO/(MgO + FeO)], X Al [ = Al 2 O 3 /(Al 2 O 3 + MgO + FeO)] and low X Ca [ = CaO/ (CaO + MgO + FeO + Na 2 O)] are likely suitable for exhumation, which is consistent with the bulkrock compositions of the natural oceanic eclogites on the Earth's surface. On the basis of natural observations and our calculations, it is suggested that beyond depths around 110-120 km oceanic eclogites are not light enough and/or there are no blueschists to compensate the negative buoyancy of the oceanic crust, therefore explaining the lack of oceanic eclogites returned from ultradeep mantle (>120 km) to the Earth's surface. The exhumed light-cold-hydrous oceanic eclogites may have decoupled from the top part of the sinking slab at shallow depths in the forearc region and are exhumed inside the serpentinized subduction channel, whereas the dense-hot-dry eclogites may be retained in the sinking slab and recycled into deeper mantle.

The Xugou garnet peridotite body of the southern Sulu ultrahigh-pressure (UHP) terrane is enclosed in felsic gneiss, bounded by faults, and consists of harzburgite and lenses of garnet clinopyroxenite and eclogite. The peridotite is composed of variable amounts of olivine (Fo 91 ), enstatite (En 92)93 ), garnet (Alm 20)23 Prp 53)58 Knr 6)9 Grs 12)18 ), diopside and rare chromite. The ultramafic protolith has a depleted residual mantle composition, indicated by a high-Mg number, very low CaO, Al 2 O 3 and total REE contents compared to primary mantle and other Sulu peridotites. Most garnet (Prp 44)58 ) clinopyroxenites are foliated. Except for rare kyanite-bearing eclogitic bands, most eclogites contain a simple assemblage of garnet (Alm 29)34 Prp 32)50 Grs 15)39 ) + omphacite (Jd 24)36 ) + minor rutile. Clinopyroxenite and eclogite exhibit LREE-depleted and LREE-enriched patterns, respectively, but both have flat HREE patterns. Normalized La, Sm and Yb contents indicate that both eclogite and garnet clinopyroxenite formed by high-pressure crystal accumulation (+ variable trapped melt) from melts resulting from two-stage partial melting of a mantle source.Recrystallized textures and P-T estimates of 780-870°C, 5-7 GPa and a metamorphic age of 231 ± 11 Ma indicate that both mafic and ultramafic protoliths experienced Triassic UHP metamorphism in the P-T forbidden zone with an extremely low thermal gradient (< 5°C km )1 ), and multistage retrograde recrystallization during exhumation. Develop of prehnite veins in clinopyroxenite, eclogite, felsic blocks and country rock gneiss, and replacements of eclogitic minerals by prehnite, albite, white mica, and K-feldspar indicate low-temperature metasomatism.