The recognition of coesite and diamond as phases in the ultrahigh pressure (UHP) metamorphism of continental crust is of very important implications for the dynamics of plate tectonic processes at convengent margins. Occurence of coesite and micro-diamond in eclogites from the Dabie Mountains in East China has been interpreted to provide evidence for the subduction of continental materials to the mantle depth of greater than 100 km.
The metamorphic complexes in this region, together with those in the Qingling orogenic belt, mark the suture zone between the Sino-Korean and Yangtse cratons. Rocks of the Dabie suture zone range from ultrahigh pressure eclogite to greenschist facies. The UHP rocks are hosted in the Dabie Group which are predominantly quartzo-feldspathic gneisses with minor eclogites, marbles, and rare ultramafic rocks. The UHP eclogites are found as pods or bands, centimeters to tens of meters wide in the gneisses. In general, the paragenesis of the eclogites is garnet + omphacite + phengite + quartz + zoisite ± kyanite + rutile + apatite + zircon ± sphene.
A detailed study of oxygen and hydrogen isotopes has been carried out for mineral separates from the UHP eclogites. Samples were collected from Shuanghe in the eastern part of the Dabie Mountains and Qinglongshan in the western part of the Sulu terrane. The d**O values of single minerals in the Shuanghe eclogites are -0.9 to +7.0 for omphacite, -2.6 to +5.5 for garnet, -0.7 to +6.9 for phengite. The extremely light oxygen is found in the Qinglongshan eclogite, with d**O values being -9.2 to -9.3 for omphacite, -9.8 to -10.1 for garnet, -8.5 to -9.0 for kyanite, and -8.8 to -8.9 for phengite. Compared with other known unusually low d**O values in granulites, anorthosites, skarns and eclogites in the world, the Qinglongshan eclogite has the most negative d**O values yet reported for metamorphicsotopic geothermometry for quartz-mineral pairs yields a temperature range from 550 to 750°C, which is close to the petrological temperatures of
600 to 800°C derived from cation partitioning between
coexisting phases. This indicates that the oxygen isotope equilibrium has been preserved at the temperature of formation of the eclogites.
The sequence of 18O-enrichment in the eclogite minerals is concordant with the empirically and theoretically expected values for equilibrium fractionations. The d18O values of coexisting quartz and omphacite or garnet paris display good positive correlation arrays with solpe equal to unity, suggesting isotopic equilibrium between the eclogite minerals. The isotopic geothermometry for quartz-mineral pairs yields a temperature range from 550 to 750°C, which is close to the petrological temperatures of 600 to 800°C derived from cation partitioning between coexisting phases. This indicates that the oxygen isotope equilibrium has been preserved at the temperature of formation of the eclogites.
The shift in d**O to negative values in metamorphic rocks reflects significant exchange of oxygen isotopes between the rocks and a low **O fluid, with the fluid itself forming the isotopically light oxygen reservoir. Seawater could be a candidate for the d**O -depleted fluid. However, the mineral dD values of -55 to -83 are incompatible with a seawater-rock interaction. Ancient meteoric water is the most likely fluid which would have caused the eclogites in the Dabie Mountains to be anomalously depleted in **O.
It is unrealistic to assume a direct interaction of ancient meteoric water with the rocks during UHP metamorphism. The preservation of isotopic equilibrium fractionations between the eclogite minerals precludes the possibility of hydrothermal alteration in the stage of postmetamorphism. Therefore, it is more likely that the eclogite precursor (a kind of basaltic rocks) would have exchanged oxygen isotopes with ancient meteoric water to produce the unusually low d**O values.
The presence of coesite and micro-diamond in the
UHP eclogites from the Dabie Mountains requires 36 kbar ultrahigh pressure metamorphic conditions. Assuming
that the ultrahigh pressure was produced by the lithostatic pressure, a tectonic model for the formation of the diamond-bearing eclogites has been proposed in that the eclogite precursor would have been buried to a mantle depth of more than 100 km by subduction of continental crust. The preservation of the unusually low d**O values for the eclogites from the Dabie-Sulu terranes indicates a limited isotope exchange between the subducted rocks and the surrounding mantle materials at the mantle depth. This implies that the oxygen isotope exchange would be very sluggish even under very high mantle temperatures and pressures. However, the rate of isotopic exchange by oxygen diffusion and recrystallization at mantle depths could be sufficiently large to achieve isotopic reequilibration between the mantle materials and the eclogite minerals within the subducted rocks. Therefore, the formation of coesite and diamond in the UHP eclogites could be due to dynamic ultrahigh pressure produced by the continent-continent collision between the Sino-Korean and Yangtse cratons. In this regard, there would be no plate subduction to the mantle depth in the Dabie suture zone.