摘要
The exhumation mechanism of high-pressure ( HP) and ultrahigh-pressure ( UHP) metamorphic rocks formed by the subduction of oceanic crust is one of the primary uncertainties associated with the subduction factory. Based on a worldwide compilation of key information for oceanic eclogites including petrologic characteristics,peak P-T conditions,and exhumation P-T paths,this study reappraises the exhumation of oceanic eclogites,which have received much less attention than continental ones during the last two decades. The oceanic eclogites reported in typical oceanic subduction zones worldwide can be subdivided into three groups based on peak mineral assemblages,P-T conditions,and geothermal gradients as follows: ( 1) Coesite-bearing UHP lawsonite eclogites ( 2. 7 ～ 3. 2GPa,470 ～ 610℃ ,5 ～ 7℃ / km) ; ( 2) HP lawsonite eclogites ( 1. 7 ～ 2. 6GPa,360 ～ 620℃ ,5 ～ 8℃ / km) ; and ( 3) HP epidote eclogites ( 1. 5 ～ 2. 3GPa,540 ～ 630℃ ,7 ～ 12℃ / km) . Compared with HP and UHP eclogites in continental subduction-collision zones,oceanic eclogites have lower peak P-T conditions and contain more light hydrous minerals. Kyanite,a common dense mineral in continental eclogites,is extremely rare in natural oceanic eclogites. All the hypotheses for the exhumation of oceanic eclogites are based on the assumption that oceanic eclogites were denser than the surrounding mantle; therefore,their exhumation must be aided by low-density serpentinites or metasedimentary rocks. Thermodynamic modeling for MORB 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℃ / km,the density of subducted oceanic crust is always lower than that of the surrounding mantle at depths shallower than 110 ～ 120km ( P < 3. 3 ～ 3. 6GPa) . However,along a warm subduction P-T path with a geotherm of ～ 10℃ / km,the subducted oceanic crust becomes denser than the surrounding mantle at depths greater than 60km ( P > 1. 8GPa) . On the basis of natural observations and our calculations,we suggest that beyond depths around ～ 120km ( 1) oceanic eclogites are not light enough,( 2) there are no serpentinites to compensate the negative buoyancy of the oceanic crust,and ( 3) increasing the dip angle of subducted oceanic plate inhibits the exhumation of HP- UHP metasedimentary rocks and eclogites from greater depth. They may be the reasons for explaining the lack of oceanic eclogites returned from ultradeep mantle ( > 120km) to the Earth’s surface. The formation of kyanite through lawsonite breakdown reactions would hamper the exhumation of oceanic crust,that may be the reason for the fact that lawsonite-free kyanite-bearing eclogite is extremely scarce in the oceanic subduction zones worldwide.
