A detailed study of the structure and petrology of the rocks bordering the Kabbaldurga-type charnockites provides important constraints on the origin of these charnockites. The structural elements register three phases of deformation and show a uniform pattern in the larger area, a pattern consistent with the regional structure of the Precambrian of Southern Karnataka. In the Kabbaldurga area, however, some of the earlier structures are poorly preserved. Yet there are vestiges of early folds described by banded/layered charnockites as in the neighbouring Kodamballi area, and a consistent development of dilatant structures which can be related to the kinematics of deformation in the larger terrain. At Kabbaldurga the pegmatitic charnockites occur as veins of diverse orientation; but they rarely follow the shear - generated structures.
The metamorphic reactions invoked by previous workers to explain in situ transformation of gneiss to charnockite were based on chemical similarity of some close pairs. But the petrographic and chemical variations in the pegmatitic charnockites and the Peninsular gneisses at Kabbaldurga quarry are compelling features which cannot be explained by the hypothesis of in situ transformation. We have argued, on the basis of rock and mineral chemistry, that derivation of the pegmatitic charnockites by dehydration melting in metabasites offers a better explanation. Pressure-temperature values (at least 850° to 900° C, 7 kbar) obtained by us for the granulites of this area, viewed against the results of experimental dehydration melting in basic rocks with hornblende and/or biotite, provide strong support for this model. In the field leucosomes within the basic granulites of Kabbaldurga are not uncommon. The compositions of the pegmatitic charnockites (tonalitic and granitic) match those of the melts produced in experiments. Further, the pattern of variation in the composition of hornblende and plagioclase in the basic granulites of the Kabbaldurga area is compatible with extraction of melts. This alternative model for the origin of the Kabbaldurga charnockites is petrologically feasible and does not require either in situ transformation or structurally controlled growth, which, incidentally, are not ubiquitous at Kabbaldurga 相似文献
The Menderes Massif is a major polymetamorphic complex in Western Turkey. The late Neoproterozoic basement consists of partially migmatized paragneisses and metapelites in association with orthogneiss intrusions. Pelitic granulite, paragneiss and orthopyroxene-bearing orthogneiss (charnockite) of the basement series form the main granulite-facies lithologies. Charnockitic metagranodiorite and metatonalite are magnesian in composition and show calc-alkalic to alkali-calcic affinities. Nd and Sr isotope systematics indicate homogeneous crustal contamination. The zircons in charnockites contain featureless overgrowth and rim textures representing metamorphic growth on magmatic cores and inherited grains. Charnockites yield crytallization age of ~590 Ma for protoliths and they record granulite-facies overprint at ~ 580 Ma. These data indicate that the Menderes Massif records late Neoproterozoic magmatic and granulite-facies metamorphic events. Furthermore, the basement rocks have been overprinted by Eocene Barrovian-type Alpine metamorphism at ~42 Ma. The geochronological data and inferred latest Neoproterozoic–early Cambrian palaeogeographic setting for the Menderes Massif to the north of present-day Arabia indicate that the granulite-facies metamorphism in the Menderes Massif can be attributed to the Kuunga Orogen (600–500 Ma) causing the final amalgamation processes for northern part of the Gondwana. 相似文献
The spinel–quartz-bearing Al–Fe granulites from Ihouhaouene (In Ouzzal, West Hoggar) have a migmatitic appearance with quartzo-feldspathic layers intercalated with restitic layers. These granulites are characterized by a hercynitic spinel–quartz assemblage typical of high grade terranes. The stability of the spinel–quartz assemblage is attributed to an elevation of temperature (from 800 to >1100 °C) at high pressures (10–11 kbar), followed by an isothermal decompression from 9 to 5 kbar, an evolution typical of the In Ouzzal clockwise P–T path. The Al–Fe granulites’ history can be subdivided into different successive crystallisation stages. During the first stage, the spinel–quartz assemblage formed, probably following a prograde event that also produced partial melting. During a second stage, the primary spinel–garnet–sillimanite–quartz paragenesis broke-down to give rise to the secondary assemblage. The metamorphic evolution and phase relations during this stage are shown in P–T–X pseudosections calculated for the simple FMASH system. These pseudosections show that the orthopyroxene–cordierite–spinel symplectite appeared during a high temperature decompression, as a product of destabilisation of garnet in sillimanite-free microdomains with high XMg values. At the same time, the spinel–quartz association broke-down into cordierite in Fe-rich microdomains. Average pressure and temperature estimates for the orthopyroxene–spinel–garnet–cordierite–quartz association are close to the thermal peak of metamorphism (1000 ± 116 °C at 6.3 ± 0.5 kbar). With decreasing temperatures garnet–sillimanite corona developed from the breakdown of the primary spinel–quartz assemblage in the Fe-rich microdomains, whereas cordierite–spinel formed at the expense of primary sillimanite and garnet in the Mg-rich microdomains. 相似文献
Abstract Orthopyroxene-bearing migmatites, exposed at the summit of Cone Peak in the Santa Lucia Range, California, offer an opportunity to explore potential links between granulite facies metamorphism and migmatite formation. Geothermobarometry indicates that the metamorphic temperatures and pressures were in the approximate ranges of 700–750° C and 7.0–7.5 kbar. The rocks at the summit comprise three domains: relatively coarse-grained, leucocratic veins; relatively fine-grained, biotite-enriched zones at the margins of the veins; and a biotite–hornblende-bearing host rock. Orthopyroxene is concentrated in the veins, which have also the highest ratio of anhydrous to hydrous minerals of the three rock types. The composition of the veins, together with their textures and modes, suggest that they formed through anatexis involving a dehydration-melting reaction which consumed hornblende and produced orthopyroxene. Variability in mineralogy and composition indicates that there was some local migration of magma along the veins before their final solidification. The biotite-enriched zones formed either by the concentration of residual biotite at the margins of the vein, or through the metasomatic conversion of hornblende (and/or pyroxene) to biotite, or by a combination of the two processes. Significant differences in the chemistry of the neosome (vein + biotite-enriched zone) and the host rock rule out simple dehydration melting in a local closed system. The model that explains best the mineralogical and chemical patterns involves triggering of melting by an influx of a low- a H2O mixed fluid which added K and Si to and removed Ca from the neosome. 相似文献
Abstract Scapolite, wollastonite, calcite, diopside, grossular-andradite garnet and sphene occur in calc-silicate rocks in the granulite terrain of the Arunta Block, central Australia. This assemblage buffers the CO2 activity at a low value, so that any coexisting fluid phase must be H2O rich and CO2 poor ( X co2 = 0.2-0.3). In contrast, the H2O activity in the surrounding felsic and mafic granulites was low. Thus fluid activities during granulite facies metamorphism were locally buffered in various rock units and fluid flow appears to have been restricted or fluid may have been absent. Late retrograde rims of garnet and garnet-quartz separate phases formed in the high-grade stage. Formation of these rims would have required either an influx of water-rich fluid or a decrease in pressure. Evidence from the surrounding granulites shows that in one locality, the calc-silicate rocks had undergone late isobaric hydration; in another locality, minor uplift had occurred soon after peak P-T conditions. In both, scapolite had partly broken down to plagioclase-calite. A calc silicate rock from the granulite terrain of Enderby Land, Antarctica, contains scapolite, wollastonite, calcite, diopside, quartz and sphene; this assemblage also indicates low CO2 activities. In this rock, wollastonite has broken down to calcite-quartz, to indicate isobaric cooling without influx of hydrous fluid. 相似文献