High-grade Terrains Research Articles

Fluids in Granulite Facies Metamorphism: A Comparative Oxygen Isotope Study on the South India and Adirondack High-Grade Terrains

Oxygen isotope compositions have been determined for samples from the South India Archean to Pro-terozoic and northwest Adirondack late-Proterozoic high-grade terrains. $\delta^{18}O$ values of charnockites and related orthogneiss protolith from the prograde amphibolite facies to granulite facies transition zone in South India range from 5.9 to 8.4‰. Charnockites, khondalites (sillimanite gneisses), and retrogressed gneisses from the interior of the granulite facies terrain average 2‰ higher than orthogneisses of the transition zone. Individual minerals in incipient charnockitic net veins, as at Kabbaldurga, Karnataka, and Ponmudi, Kerala, have $\delta^{18}O$ identical to minerals of their non-charnockitic host rocks. Quartz $\delta^{18}O$ values of the northwestern Adirondack granites and gneisses average 3-4‰ higher than those from the Indian rocks. The two terrains are thus isotopically distinct, which may reflect differences in premetamorphic history or different degrees of communication with external oxygen reservoirs during metamorphism. The low $\delta^{18}O$ of the South India terrain, including metasedimentary rocks, suggests the pervasive influence of a deep crustal or upper mantle igneous reservoir, whereas the elevated $\delta^{18}O$ of the Adirondack rocks suggests that the isotopic effects of near-surface, low-temperature processes in the protoliths survived subsequent fluid-deficient granulite-facies metamorphism. The near-identity of $\delta^{18}O$ of individual minerals from charnockitic veins and non-charnockitic host gneiss in the South India terrain indicates either that fluids causing charnockitic alteration were nearly in isotopic equilibrium with the country rocks or that oxygen isotopes were set in premetamorphic processes and little altered by incipient charnockite formation. Therefore we do not have direct isotopic evidence of fluid action in the origin of the much-discussed South Indian charnockitic net-veining. Isotopic temperatures are 100°C or more lower than peak metamorphic temperatures in both terrains, as inferred from mineralogic thermometry, indicating extensive postmetamorphic re-equilibration to lower temperatures, even in the fluid-deficient Adirondack terrain. An exception seems to be the quartz-garnet pair, which yields temperatures of 750° to 850°C for South India. Diffusion theory calculations show high closure temperatures in garnet, providing an explanation for the refractory behavior of this pair. It is possible that quartz-garnet will provide a useful oxygen isotope thermometer for granulites when favorable circumstances for postmetamorphic preservation are realized.

Isotopic Mapping of Age Provinces in Precambrian High-Grade Terrains: Sri Lanka

Nd model ages of amphibolite- and granulite-grade rocks in Sri Lanka form a simple regional pattern that broadly correlates with mappable geological units, and is in effect an isotopic map of the island's basement. The granulite-grade units of the Highland Group and Southwest Group have model ages of 2.2-3.0 Ga indicating derivation mainly from late Archean sources. They are bounded to the east and west by late Proterozoic gneisses of the Vijayan Complex with model ages of 1.1-2.0 Ga. The isotopic data identify three distinct crustal provinces and are not consistent with earlier suggestions that the Vijayan gneisses are retrograde equivalents of the Highland granulites. Sri Lanka is not a direct continuation of the Archean Dharwar Craton of southern India. Identification of "Vijayan-type" juvenile crustal terrains in other Gondwana fragments may play a key role in determining the precise attachment of southern India-Sri Lanka in eastern Gondwana.

Thermal and baric evolution of garnet granulites from Sri Lanka

Abstract Garnet granulites from Sri Lanka preserve textural and chemical evidence for prograde equilibration at temperatures of at least 700–750°C and pressures in the vicinity of 6–8 kbar. Associated strain patterns suggest prograde metamorphism occurred during and immediately following an episode of crustal thickening, with the prograde P–T conditions probably reflecting a combination of the conductive and advective transport of heat at the mid‐levels of tectonically thickened crust. The occurrence of prograde wollastonite provides evidence for internally buffered fluid compositions, or fluid absent conditions, during peak metamorphism and precludes pervasive advection of a CO2‐rich fluid. The advective heat component is therefore likely to have been provided by the transport of silicate melt. Intricate symplectitic textures record partial re‐equilibration of the garnet granulites to lower pressures (˜ 4–6 kbar) at high temperatures (600–750°C), and testify either to the erosional denudation of the overthick crust prior to significant cooling (i.e. quasi‐isothermal decompression) or to a subsequent static heating possibly of early Palaeozoic age (Pan‐African). The metamorphic history of the Sri Lankan granulites is compared with high grade terrains in the neighbouring fragments of Gondwana, with the emphasis on similarities with Proterozoic granulites of the East Antarctic craton.

Early proterozoic tectonic styles and associated mineral deposits of the North China Platform

The remarkable change in tectonic style from Archean granite-greenstone belts and high grade terrains to linear sedimentary basins marks the beginning of the Early Proterozoic in China. The different evolutionary histories of, and accumulations in, the Early Proterozoic basins result in different Archean-Early Proterozoic boundaries. Two time limits, 2.8 and 2.5 Ga, are significant for the boundary in eastern China. Early Proterozoic basins displaying different tectonic styles contain different mineral deposits. The Liaodong Peninsula type of Early Proterozoic basin contains the most abundant mineral resources and deposit types, including stratabound BFe and Pb-Zn deposits in the eugeosynclinal facies of the Liaojitite Suite, and pyrite, P and magnesite deposits in miogeosynclinal facies rocks. The magnesite deposits in Liaodong Peninsula are the largest in the world and were formed in a unique geological setting.

Early Precambrian Terrains: Evolution of the Lewisian and Comparable Precambrian High Grade Terrains . R. G. Park and J. Tarney, Eds. Published for the Geological Society by Blackwell Scientific, Palo Alto, CA, 1987. viii, 315 pp., illus. $80. Geological Society Special Publication no. 27. From a conference, Leicester, U.K., March 1985.

Early Precambrian Terrains: <i>Evolution of the Lewisian and Comparable Precambrian High Grade Terrains</i> . R. G. Park and J. Tarney, Eds. Published for the Geological Society by Blackwell Scientific, Palo Alto, CA, 1987. viii, 315 pp., illus. $80. Geological Society Special Publication no. 27. From a conference, Leicester, U.K., March 1985.

Early Precambrian Terrains: Evolution of the Lewisian and Comparable Precambrian High Grade Terrains . R. G. Park and J. Tarney, Eds. Published for the Geological Society by Blackwell Scientific, Palo Alto, CA, 1987. viii, 315 pp., illus. $80. Geological Society Special Publication no. 27. From a conference, Leicester, U.K., March 1985.

Early Precambrian Terrains: <i>Evolution of the Lewisian and Comparable Precambrian High Grade Terrains</i> . R. G. Park and J. Tarney, Eds. Published for the Geological Society by Blackwell Scientific, Palo Alto, CA, 1987. viii, 315 pp., illus. $80. Geological Society Special Publication no. 27. From a conference, Leicester, U.K., March 1985.

An open-system fractional crystallization model for very early continental crust formation

A model is discussed which seeks to explain the origin of Archaean tonalitic-trondhjemitic-granodioritic (TTG) crust by an open-system fractional crystallization mechanism. Following the assumption that in the pre- and early Archaean the melt which formed above sites of large-scale mantle upwelling was too voluminous and too hot to solidify, an almost global near-surface partially molten zone could form. The magma in this layer would converge above mantle sinks and undergo fractional crystallization. Recharged fractionating reservoirs would form at sites of convergence; the major element composition of the magma in such reservoirs would evolve towards one resembling that of TTG rocks. Provided that amphibole was a significant fractionating phase from such reservoirs, their incompatible as well as compatible trace element abundances would also become similar to those of TTG terrains. It is proposed that such terrains resulted from the consolidation of these recharged fractionating reservoirs. It is shown that the reservoirs would remain indistinguishable from the primitive mantle in terms of radiogenic isotopes, so that model ages cannot be greater than the age of their consolidation. Consideration of the main aspects of greenstone belts and Archaean high grade terrains indicates no contradictions with the model. The intrusion of late Archaean adamellites and granites is interpreted as an indirect effect of pressure-induced breakdown of amphibole in the cumulate underlying the TTG crust, which in turn resulted from crustal shortening. Cratonization is seen as the result of consolidation not only of the crust but also of the underlying cumulate as subcontinental lithosphere.

Platinum group elements in mafic-ultramafic rocks of the Western Gneiss Terrain, Western Australia

The New Norcia and the Yornup bodies are situated within the high-grade Western Gneiss Terrain of the Yilgarn Block. The New Norcia body consists of mafic and ultramafic rocks of gabbronoritic, olivine-gabbronoritic and harzburgitic composition respectively, metamorphosed to amphibolite, amphibolitic serpentinite and serpentinite.

Arrested charnockite formation in southern India and Sri Lanka

Arrested prograde charnockite formation in quartzofeldspathic gneisses is widespread in the high-grade terrains of southern India and Sri Lanka. Two major kinds of orthopyroxene-producing reactions are recognized. Breakdown of calcic amphibole by reaction with biotite and quartz in tonalitic/granitic “gray gneiss” produced the regional orthopyroxene isograd, manifest in charnockitic mottling and veining of “mixed-facies” exposures, as at Kabbal, Karnataka, and in the Kurunegala District of the Sri Lanka Central Highlands. Chemical and modal analyses of carefully chosen immediately-adjacent amphibole gneiss and charnockite pairs show that the orthopyroxene is produced by an open system reaction involving slight losses of CaO, MgO and FeO and gains of SiO2 and Na2O. Rb and Y are depleted in the charnockite. Another kind of charnockitization is found in paragneisses throughout the southern high-grade area, and involves the reaction of biotite and quartz±garnet to produce orthopyroxene and K-feldspar. Although charnockite formation along shears and other deformation zones at such localities as Ponmudi, Kerala is highly reminiscent of Kabbal, close pair analyses are not as suggestive of open-system behavior. This type of charnockite formation is found in granulite facies areas where no prograde amphibole-bearing gneisses exist and connotes a higher-grade reaction than that of the orthopyroxene isograd. Metamorphic conditions of both Kabbaltype and Ponmudi-type localities were 700°–800° C and 5–6 kbar. Lower P(H2O) in the Ponmudi-type metamorphism was probably the definitive factor. CO2-rich fluid inclusions in quartz from the Kabbaltype localities support the concept that this type of charnockite formation was driven by influx of CO2 from some deep-seated source. The open-system behavior and high oxidation states of the metamorphism are in accord with the CO2-streaming hypothesis. CO2-rich inclusions in graphitebearing charnockites of the Ponmudi type, however, commonly have low densities and compositions not predictable by vapor-mineral equilibrium calculations. These inclusions may have suffered post-metamorphic H2 leakage or some systematic contamination. Neither the close-pair analyses nor the fluid inclusions strongly suggest an influx of CO2 drove charnockite formation of the Ponmudi type. The possibility remains that orthopyroxene and CO2-rich fluids were produced by reaction of biotite with graphite without intervention of fluids of external origin. Further evidence, such as oxygen isotopes, is necessary to test the CO2-streaming hypothesis for the Ponmudi-type localities.

High-grade terrains in and around the Yilgarn Block of Western Australia

Summary Two kinds of high-grade terrain occur in the western part of the Archaean Yilgarn Block. One consists of repeatedly deformed and metamorphosed granites (3650 and 3500 Ma), layered basic rocks (3750 Ma), and siliceous metasedimentary rocks, which were metamorphosed to granulite facies at 3300 Ma. The other consists of much less deformed granites and some basic rocks which form deeper, granulite-facies equivalents of the adjacent high-level 3000–2600 Ma granite-greenstone terrain. The Yilgarn Block is bounded by Proterozoic belts of intense deformation and high-grade metamorphism. These belts were active between 2000 and 1600 Ma and comprise both reworked Archaean and early Proterozoic rocks. Two belts were mobile again and metamorphosed to granulite or amphibolite facies between 1300 and 1000 Ma, and one of these belts was also intruded by plutonic rocks, deformed and metamorphosed to granulite facies at about 650 Ma.

The Lewisian complex: a typical Precambrian high-grade terrain?

Summary The main structural, lithological and geochemical features of the Lewisian complex are summarized in relation to available geochronology and an attempt is made to integrate this information into an evolutionary model for the Lewisian from the Archaean through the Proterozoic. The phase of extensive crustal generation which formed the early Lewisian (Scourian) complex occurred at about 2900 Ma and was marked by the production of vast volumes of tonalite which, together with supracrustal components, were affected by strong horizontal thrusting and deformation. The deeper parts of the Lewisian crust were subject to (Badcallian) granulite facies metamorphism culminating at c. 2700 Ma. Major NW-SE shear zones developed shortly after this and resulted in the segmentation of the Lewisian crust and juxtaposition of different crustal levels. This marked a period of quite extensive retrogression of the high-grade gneisses, which continued (with further local shearing) during and after the emplacement of the Scourie dyke suite at c. 2400 Ma. Extension of the Lewisian crust occurred in the southern (Gairloch) area around 2000 Ma and extensive outpourings of mafic volcanics, with associated exhalative mineralization, took place accompanied by sedimentation in the developing basin to form the Loch Maree Group. Igneous activity related to the South Harris igneous complex may have occurred at this period on the Outer Hebrides. Laxfordian deformation and metamorphism began before 1900 Ma and resulted in major reactivation of the earlier shear zones between and within the juxtaposed crustal blocks. More pervasive deformation accompanied the closure of the Loch Maree volcano-sedimentary basin, with major overthrusting and isoclinal folding. Culmination of Laxfordian metamorphic activity occurred at c. 1900 Ma with migmatization and granite injection at Laxford and on Harris, and locally in the southern area. Shear deformation, increasingly more brittle and localized, continued to affect the Lewisian complex until 1400 Ma and probably until 1000 Ma. Interestingly, the main shear zones established at the end of the Archaean crust-forming episode at 2600 Ma continued to be the focus of tectonic, metamorphic and magmatic activity over the following 1500 million years.

An Overview of the Soutpansberg Sedimentary and Volcanic Rocks

Volcanic and sedimentary rocks occupy a faulted graben within the previously uplifted and eroded high-grade gneiss terrain of the Limpopo Mobile Belt. The rocks comprise the Soutpansberg Group and represent an important sequence of Proterozoic rocks. Their general geology and volcanology is summarised in this paper.

Rare earth element patterns in Archean high-grade metasediments and their tectonic significance

Metasediments from two contrasting types of Archean high-grade terrains are interpreted as being derived from distinct tectonic settings. The Kapuskasing Structural Zone, Canada, represents the deep roots of a typical greenstone belt, whereas the Limpopo Province, southern Africa and Western Gneiss Terrain, Australia, mainly consist of shelf sediments deposited on a granitic basement and then buried to the depths required for granulite fades metamorphism. Upper amphibolite to granulite fades paragneisses from the Kapuskasing Structural Zone have REE patterns similar to those of greenstone belt sediments, except where partial melting has occurred, forming restites with Eu enrichment and melts with Eu depletion. Except in this latter instance, metamorphism has not affected the primary REE patterns. REE patterns in Archean upper amphibolite to granulite facies metasediments from the central Limpopo Province and Western Gneiss Terrain show wide differences, ranging from patterns which resemble those in post-Archean terrigenous sediments, to typical Archean sedimentary rock patterns. The diversity in REE patterns for these shallow shelf metasediments is interpreted as resulting from derivation from local provenances. Samples with “post-Archean” patterns, displaying Eu depletion, are interpreted as being derived from K-rich granitic plutons which were portions of small, stable early Archean terrains, precursors of the widespread late Archean-Proterozoic episode of major cratonic development.

A new correlation of sapphirine granulites in the indo-antarctic metamorphic terrain: Late proterozoic dates from the eastern ghats province of India

Abstract Uranium-lead and thorium-lead ages were obtained from perrierite and zircons in two samples from the Eastern Ghats Province of South India, a sapphirine-bearing granulite from Anakapalle and a charnockite from Visakhapatnam. The isotopic data are concordant or close to concordant at 1 Ga, which is interpreted to be the age of granulite-facies metamorphism and charnockite plutonism in the vicinity of Anakapalle and Visakhapatnam, and provides the first report of a late Proterozoic granulite-facies event in the Eastern Ghats Province. In reassemblies of Gondwanaland based on DuToit's original proposal, the Eastern Ghats Province is juxtaposed against a high-grade terrain in East Antarctica affected by a granulite-facies event and charnockitic plutonic activity 800–1100 Ma ago. The ages reported here suggest that a portion of the Eastern Ghats Province represents an extension of the late Proterozoic terrain from Antarctica into India. The radiometric results on the Anakapalle rocks are the first evidence in South India for sapphirine formation during the late Proterozoic; other South Indian sapphirine localities appear to be Archaean in age. A review of sapphirine localities in the Indo-Antarctic metamorphic terrain indicates that sapphirine developed under a range of pressure-temperature conditions during the Archaean and Proterozoic. Evidence for a systematic change in peak metamorphic conditions with time is obscured by these regional variations. Characterization of the entire pressure-temperature path, in addition to estimates for the peak conditions, is needed to distinguish the Archaean and Proterozoic metamorphic regimes.

Structural Pattern in the Dharwar Craton

High-grade and low-grade supracrustal belts (Sargur and Dharwar) set in a matrix of gneisses dotted with granitic plutons of different ages make up the Dharwar craton. The overall structural pattern is one of subparallel linear belts and stringers whose regional trend changes from NW-SE in the northern part to N-S in the south. Both the high-grade and low-grade belts show three principal episodes of folding. However, the correlation between the deformation episodes in the Sargur and the Dharwar rocks is controversial, and the structural relations between them are not unequivocally established: parallelism of the first phase structures in the Sargur and the Dharwar rocks can be interpreted either as synchronous development or rotation of the earlier Sargur structures into parallelism with the younger Dharwar structures during later deformation. The extension of the supracrustal belts is parallel to the axial traces of the second deformation. Granite diapirism and basement uplift further modified the structural pattern in the Dharwar belts. In the Sargur Group the peak of metamorphism (upper amphibolite to lower granulite facies) was attained during the first deformation, which may be older than 3000-3100 Ma. There is no consensus regarding the age of the first deformation of the Dharwars; it is either coeval with the Sargur first deformation or younger. The closing stage of the second deformation in the Dharwars is no younger than 2500-2600 Ma. The climax of Dharwar metamorphism (greenschist or lower amphibolite facies) is post-tectonic with respect to the first deformation. The relation of the oldest component (3400 Ma) of the polyphase Peninsular Gneiss to the Sargurs is not established. Another major component was emplaced at 3000-3100 Ma, syn- to post-tectonic with respect to the first phase Sargur deformation. A third component was emplaced at 2500-2600 Ma at the late- to post-tectonic stage of the second phase Dharwar deformation. Ductile deformation style and polyphase migmatization, interspersed with folding episodes, characterize the Peninsular Gneiss. The gneisses exhibit a complicated deformation history: no comprehensive regional analysis is currently possible. The high-grade granulite terrain in southern Karnataka evolved at about 2500-2700 Ma under essentially static conditions. Much charnockite formation was definitely later than the formation of the gneissic foliation in the Peninsular Gneiss, and the structures in the granulites and in the low- to medium-grade terrains are essentially the same. It remains to be seen whether this is a mimetic preservation. It is possible that the high-grade terrain contains even older granulites, perhaps resulting from a metamorphism at 3000-3100 Ma ago. The youngest deformation features are the E-W shear systems in southern Karnataka and northern Tamil Nadu, which are probably Proterozoic in age and correlate with the N-S thrust system in the Eastern Ghats. These have caused retrogression of the granulites and development of mylonites and augen gneisses.

Editors' Preface

Previous articleNext article No AccessEditors' PrefaceRobert C. Newton, and Alfred T. AndersonRobert C. Newton Search for more articles by this author , and Alfred T. Anderson Search for more articles by this author PDFPDF PLUS Add to favoritesDownload CitationTrack CitationsPermissionsReprints Share onFacebookTwitterLinkedInRedditEmail SectionsMoreDetailsFiguresReferencesCited by The Journal of Geology Volume 94, Number 2Mar., 1986 Article DOIhttps://doi.org/10.1086/629018 Views: 3Total views on this site Citations: 2Citations are reported from Crossref Copyright 1986 The University of ChicagoPDF download Crossref reports the following articles citing this article:K. Naha, R. Srinivasan, S. Jayaram Structural evolution of the Peninsular Gneiss — an early Precambrian migmatitic complex from South India, Geologische Rundschau 79, no.11 (Feb 1990): 99–109.https://doi.org/10.1007/BF01830449R. C. Newton The Late Archean High-Grade Terrain of South India and the Deep Structure of the Dharwar Craton, (Jan 1990): 305–326.https://doi.org/10.1007/978-94-009-0675-4_12

The Southern High-Grade Margin of the Dharwar Craton

The amphibolite-facies to granulite-facies transition at the southern margin of the Dharwar craton has been studied in the Krishnagiri, Satnur-Halaguru, and Kushalnagar areas of southern India. In all three areas the transition appears to be a progressive metamorphic overprint on cratonal gneisses and their mafic and metasedimentary enclaves, without major structural interruption. In the Kabbal-Satnur-BR Hills section of Karnataka, a high-grade charnockite massif with pronounced Rb depletion is the culmination of an apparently continuous increase of metamorphic grade southward. In this and the Kushalnagar areas, increase of paleopressure from near 6 to near 8 kbar with increasing grade indicates a depth-zone relationship of the amphibolite and granulite facies. Incipient charnockite replacing Peninsular Gneiss first appears along N-S shears parallel to the regional grain of the craton. Low-P(), high vapors were instrumental in the creation of orthopyroxene. Introduction from a deep source, either a decarbonating mantle, basaltic underplate, or deeply buried sediments, was facilitated by the N-S deformation system. A deformed continental margin or infracontinental basin in the latest Archean is a plausible setting for the metamorphism. Great crustal thickening, perhaps with entrainment of shelf or basin sediments, was effected by overthrusting, the record of which may be preserved in an early isoclinal foliation and in fold-interference patterns in the southern cratonal margin. Subsequent transcurrent shearing facilitated outgas-sing of the deep crust and upper mantle, and rising vapors transported heat, , and K to middle levels of a thickened crust, which resulted in anatectic melting. Eventual uplift and erosion exposed a 6-8 kbar paleopressure surface at the southern cratonal fringe. The weakened and segmented continental shelf or platform south of the present craton was prone to later remobilization and retrogression, in the manner of a typical "mobile belt." This view of the relationship of the Dharwar craton to the southern high-grade terrain provides no support for the concept that granulite facies terrains of the general aspect of the South Indian massifs everywhere underlie the interior of the craton. The 2-3 kbar terrain of the cratonal interior was not greatly thickened by overthrusting nor magmatic underplating during the 2.6 Ga high-grade event that affected the southern cratonal margin. It is, however, possible that granulite-grade roots of an older metamorphic cycle underlie the cratonal interior.

The South India-Sri Lanka high-grade terrain as a possible deep-crust section

Summary Precambrian granulite-facies terrains are regarded as possible models for the deep crust. In the South India-Sri Lanka late Archaean-early Proterozoic terrain, there is a regional gradation from greenschist and amphibolite facies south through an orthopyroxene isograd to the Granulite Massifs of S. India and of Sri Lanka. The metamorphic gradation is unusually complete and continuous and may represent a partial Archaean crustal profile. Large areas of low-grade grey gneiss may have been formed after charnockite in a retrogression event or may have somehow escaped late Archaean granulite facies metamorphism. Palaeopressures indicate an increase from 5 kbar to more than 7 kbar in the Transition Zone, though various fault-bounded blocks in the high-grade massif area show different pressure ranges, indicative of different amounts of uplift. The highest pressures of above 9 kbar are for Rb- and U-depleted intermediate charnockites. Undepleted acid charnockites show pressures of 6.5–7.5 kbar, and supracrustal sequences show still lower pressures. The supracrustal-rich Central Highlands of Sri Lanka show pressures of 6 to 7.5 kbar, and most or all rocks are undepleted in Rb. Associated temperatures of all the Massif rocks are in the range 700°–800°C. This grossly-stratified Archaean crustal section is only the upper half of a thickened crust: there is no indication of what lies below the deepest exposed section, and no compelling reason that depleted intermediate charnockite continues down to the Mohorovicic Discontinuity. Another possibility is deep thrust-bounded repetitions of the section which is exposed at the surface.

Garnet-orthopyroxene-plagioclase-quartz barometry: refinement and application to the English River subprovince and the Minnesota River valley

Recent experimental, theoretical, and thermodynamic studies permit better calibration of two reactions for geobarometry: grossular+pyrope+quartz=anorthite+enstatite grossular+almandine+quartz=anorthite+ferrosilite If both reactions are applied using the same thermodynamic data and activity models they should yield the same pressure for a given garnet-pyroxene-plagioclase-quartz assemblage. Application to a variety of high-grade terrains generally yielded excellent results. However, poor results have been obtained for high-Fe rocks which can be traced to errors in activity models for garnet and/or pyroxene. Either a two-site ideal mixing model for orthopyroxene (cf. Wood and Banno 1973) underestimates enstatite activity for high-Fe orthopyroxenes or the Ganguly and Saxena (1984) model overestimates pyrope activity in low-Mg garnets.

Charnockite formation at Ponmudi in southern India

Charnockites, constituents of most Precambrian high-grade terrains1,2, are essential for understanding the evolution of the early continental crust. The arrested development of charnockite in shear veins at Kabbaldurga3, in the state of Karnataka in India, suggests flow of CO2-rich water-deficient fluids through deep-seated rocks as a mechanism of granulite grade metamorphism4,5. We report here an occurrence of arrested charnockite formation well south of Kabbaldurga, in the khondalite belt of southern Kerala, where rocks with amphibolite facies give way to the vast southern India-Sri Lanka charnockite terrain, indicating that metamorphism due to CO2-rich fluids (carbonic metamorphism) may have operated over a large area in southern India. If such localities prove to be widespread, the present level of exposure throughout much of the high-grade terrain probably does not extend far beyond an isofacial surface marking the boundary between upper and lower crust in the late Archaean.