Serpentinites and talc-carbonate rocks of El Ideid-El Sodmein District (ISD), central Eastern Desert, Egypt, contain variably altered chrome-spinels. Back-scattered electron images and electron microprobe analyses of chrome-spinels and associated silicates are made to evaluate their textural and compositional variations with metamorphism. In most cases the chrome-spinel crystals are concentrically zoned with unaltered cores through transitional zone of ferritchromit to Cr-magnetite toward the rims. In talc-carbonate rocks chrome-spinels are extensively altered to Cr-magnetite. Compared to cores, the metamorphic rims are enriched in Cr# (0.83–1.0 vs. 0.58–0.63 for rims and cores, respectively) and impoverished in Mg# (0.05–0.29 vs. 0.57–0.63), due to Mg–Fe and Al(Cr)–Fe3+ exchange with the surrounding silicates during regional metamorphism rather than serpentinization process. Textural and compositional features of the chrome-spinels suggest transitional greenschist-amphibolite up to lower amphibolite facies metamorphism (at 500–600 °C), which is isofacial with the country rocks. The common preservation of unaltered chrome-spinel cores in the serpentinites, contrary to talc-carbonate rocks, implies that full equilibration has not been attained due to small metamorphic fluid–rock ratio. Microprobe analyses profile across a concentrically zoned grain confirms the presence of two compositional (miscibility?) gaps; one between chrome-spinel core and ferritchromit zone; and another one between ferritchromit zone and Cr-magnetite outer rim.Chrome-spinel cores do not appear to have re-equilibrated completely with the metamorphic spinel rims and surrounding silicates, indicating relic magmatic composition not affected by metamorphism. Core compositions suggest an ophiolitic origin and derivation by high degrees of melting of reduced, depleted harzburgite to dunite mantle peridotites in an oceanic supra-subduction zone (marginal-basin) tectonic environment. 相似文献
The publication of Tong-Dzuy Thanh et al. calls for two remarks: The metamorphism in the “Kon Tum block” cannot be younger than Permian because the outcrop they describe is not metamorphic. Their very interesting discovery is a new piece of evidence for the exotic origin of the Dien Binh Series. 相似文献
Migmatitic rocks near Grenville, Quebec, preserve features indicative of reactions at the onset of granulite facies metamorphism. In this area, metapelites and metacarbonates of the classic Grenville Series are spatially associated with granitic gneiss and metabasite, and flank a Paleozoic, Fe-rich syenite stock. Near this intrusion, the metapelite is diatexitic and nearly devoid of biotite, indicating the involvement of biotite during melting in the contact aureole of this intrusion. Outside of the contact aureole, metapelites and associated rocks contain biotite and are metatexitic. These features suggest two episodes of migmatization, the earlier predating the syenite, the later, synchronous with this intrusion.
Hornblende-rich metabasites near the syenite contain a two-part neosome consisting of coarse-grained leucosome veins and patches that are enclosed by fine-grained, pyroxene-rich envelopes. Migmatization is attributed to dehydration melting in the presence of CO2-rich fluids possibly derived from nearby carbonate rocks prior to and/or during emplacement of the syenite. The occurrence of isolated mafic clots in the mesosome and rarity of melanosome seams on leucosomes suggest that some melts were mobile on an outcrop scale. These observations suggest that the leucosomes formed by the segregation of melts, which, coupled with CO2 flux, dehydrated the wallrock along narrow margins, forming the pyroxene-rich neosomes. Back-reaction with residual fluids led to the local scapolitization of plagioclase and the concomitant formation of coronal garnet on pyroxene in neosomes. Thermobarometry of corona structures within the contact aureole generates diffusional Mg-Fe blocking temperatures ( 550 °C at 5.5 kbar). Extrapolated up-temperature, P-sensitive equilibria for the coronas yield similar pressures (8–9 kbar) as texturally-equilibrated assemblages for which high temperatures ( 750 ± 50 °C; XCO2 = 0.90−0.95) were determined for rocks sampled inside and outside of the contact aureole. This suggests that the Grenville migmatites had not been substantially decompressed by the time that the syenite was emplaced. 相似文献