ALBERT

Description: The dissociation of dolomite into magnesite and aragonite has been regarded as a useful indicator for ultrahigh-pressure (UHP) metamorphism. In this study we investigate an unusual texture involving magnesite and calcite intergrowths with dolomite relicts in a garnet-bearing dunite block from the Sulu UHP terrane, eastern China. The carbonate intergrowths typically occur as interstitial grains with low dihedral angles against surrounding olivines and have a dolomitic precursor composition. Our observations indicate that the carbonate intergrowths were initially inherited from the well-documented magnesite and aragonite assemblage after dolomite dissociation. The initial dolomite grains were likely to crystallize during the dolomitic melt metasomatism within the shallow lithospheric mantle. A series of experimental studies have well determined the equilibrium boundary of dolomite = magnesite + aragonite greater than 5 GPa along a wide temperature range, which provides direct evidence that the dunite block was subducted to depths greater than 150 km during the Triassic continental subduction. The preservation of magnesite and aragonite (now calcite) intergrowths without dolomite synthesis reaction during exhumation is probably due to the lack of fluid and rapid decompression from the peak stage to the calcite stability field. In this study, we suggest that dunite blocks from high-pressure and UHP terranes could have subducted to UHP conditions similar to garnet lherzolite and pyroxenite and were then entrained into slab slices rapidly en route to the surface.

Description: The replacement of rutile by Fe-Ti oxides is a common phenomenon during the retrogression of eclogites. Here, we report an unusual case regarding the replacement of Fe-Ti oxides by rutile during greenschist-facies metamorphic overprinting of veins in amphibolites (retrograded eclogites) from the Dabie ultrahigh-pressure (UHP) terrane, eastern China. The veins mainly consist of plagioclase, Fe-Ti oxides, and quartz, which crystallized from a Ti-rich amphibolite-facies fluid that formed during exhumation of the eclogites. Two types of textures involving the replacement of Fe-Ti oxides by rutile are recognized in the veins: (1) the first type is characterized by the development of rutile coronas (Rt-C) and other silicates (high-Fe epidote, muscovite, and chlorite) around the external boundaries of the Fe-Ti oxide grains, and (2) the second type is characterized by the formation of symplectitic intergrowths of rutile (Rt-S) and magnetite after exsolved hemo-ilmenite (H-Ilm) lamellae in the Fe-Ti oxides. The micro-textures, mineral assemblages, and Zr-in-rutile thermometry indicate that both replacement reactions involved mineral re-equilibration processes in the presence of an infiltrating fluid phase at ~476–515 °C, taking place by an interface-coupled dissolution-precipitation mechanism. Thermodynamic modeling reveals that both replacement reactions occurred during oxidation processes under relatively high-oxygen fugacity ( f O 2 ) conditions, approximately 2.5–4.5 log f O 2 units higher than the fayalite-magnetite-quartz (FMQ) reference buffer. In situ Sr isotopic analyses of epidote (Ep-C) coexisting with the Rt-C suggest that the infiltrating fluid involved in the greenschist-facies replacement reactions was externally derived from the surrounding granitic gneisses (the wall rocks of the amphibolites). Compared with the rutile in the UHP eclogites (Rt-E) and amphibolites (Rt-A), the Rt-C is characterized by distinctly lower contents of Nb (〈10 ppm) and Ta (〈2 ppm) and Nb/Ta ratios (〈10) and higher contents of Cr (〉340 ppm) and V (〉1580 ppm). These results provide a geochemical fingerprint for distinguishing the low-pressure (LP) rutile from relic high-grade phases in retrograded HP-UHP rocks. Our results reveal that rutile can form during LP retrograde stage in UHP rocks by high- f O 2 fluid-induced replacement reactions. The unusual replacements of Fe-Ti oxides by rutile-bearing assemblages during retrogression provide important constraints on fluid-mineral reactions and f O 2 variations in exhumed UHP slabs.