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R. W. Holt

Bio: R. W. Holt is an academic researcher. The author has contributed to research in topics: Granulite & Archean. The author has an hindex of 2, co-authored 2 publications receiving 577 citations.

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TL;DR: In this paper, it was shown that a large N-S strike-slip shear belt was formed by a plate tectonic and thermal event that purged the deep crust of H₂O to form granulites.
Abstract: About 3.4 Ga ago voluminous calc-alkaline magmas represented by the granitoid gneisses of southern India were emplaced into a now poorly preserved non-continental crust. Unstable ensialic basins, initiated at about 3.0 Ga, were filled with volcanic and sedimentary rocks up to about 2.6 Ga. This basement-cover sequence was deformed at the close of the Archaean, first by northward accretion and thickening of several crustal slabs formerly separated by prisms of stable shelf sediments. Structures produced by this episode were refolded and dislocated by large N-S strike-slip shear belts, to impart intense, steep planar fabrics to large volumes of the crust. Fluids rich in CO₂, possibly derived from sedimentary material driven beneath the crustal slabs by the thickening mechanism, purged the deep crust of H₂O to form granulites. Upward migration of hot H₂O-rich fluids encouraged partial melting at intermediate crustal levels. These late-Archaean tectonic and thermal events began soon after the close of basin f...

469 citations

Journal ArticleDOI
TL;DR: Geobarometry and geothermometry of garnet-bearing chamockitic and metapelitic assemblages from the high-grade terrain from South India indicate three fields of equilibration as mentioned in this paper.
Abstract: Geobarometry and geothermometry of garnet-bearing chamockitic and metapelitic assemblages from the high-grade terrain from South India indicate three fields of equilibration. High pressure granulites ($P = 8.3 \pm 1.0 kb, T = 760 \pm 40^{\circ}C$) are restricted to the charnockites and mafic granulites of the north slopes of the Nilgiris. These place a minimum constraint of ~30 km on the maximum thickness of late Archean crust. Medium pressure granulites ($P = 6.4 \pm 1.0 kb, T = 735 \pm 40^{\circ}C$) characterize the charnockites of the central Nilgiris and the Madras granulites to the east. Low pressure granulites ($P = 5.0 \pm 1.0 kb, T = 700 \pm 20^{\circ}C$) characterize the charnockites and metapelites south of the Bhavani shear zone, although migmatites from the Kodaikanal massif may reflect somewhat higher temperatures. All three granulite fields lie on a metamorphic geotherm (piezothermic array) which is strongly convex to the temperature axis and characteristic of convective heat transfer. The P...

140 citations


Cited by
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TL;DR: In this paper, a Pascal computer program, thermocalc, is described for various thermodynamic calculations using the thermodynamic dataset presented in earlier papers in this series (Holland & Powell, 1985; Powell & Holland, 1985).
Abstract: This paper provides methods and a description of a Pascal computer program, thermocalc, for various thermodynamic calculations using the thermodynamic dataset presented in earlier papers in this series (Holland & Powell, 1985; Powell & Holland, 1985). The dataset involves uncertainties on the thermodynamic parameters and therefore allows uncertainties to be calculated on results, for example in geothermometry and geobarometry. Recommendations are made for the uncertainties on activities to be used in calculations on rocks, particular emphasis being placed on preventing underestimates of these uncertainties at small mole fractions. Apposite examples of phase diagram and rock calculations are presented with ouput from thermocalc, demonstrating the utility of the program. Of the rock calculations, the most valuable are considered to be those involving simultaneous combination ‘least squares’of calculated conditions for a set of reactions applicable to a rock. This set of reactions involves the independent reactions which can be written between the end-members in the minerals in a rock and in the thermodynamic dataset. In contrast to an approach based on specific geothermometers and geobarometers, this approach maximizes the benefit of having an internally consistent thermodynamic dataset. thermocalc is available in IBM PC and Mac versions, from Roger Powell for A$25 or Tim Holland for £10 per version.

1,431 citations

01 Jan 1991
TL;DR: In this paper, an "internally consistent method" for performing thermobarometric calculations is described in which all possible equilibria implied by a given mineral assemblage are computed using an internally consistent set of thermodynamic data for end members and mixing propeilies.
Abstract: An "internally consistent method" (Powell 1985) for performing thermobarometric calculations is described in which all possible equilibria implied by a given mineral assemblage are computed using an internally consistent set of thermodynamic data for end members and mixing propeilies. Results are presented graphically and as best estimates determined by weighted averaging of the graphical results, Several examples illustrate that convergence of all equilibria in a single P-T region correlates well with textural and chemical indications of equilibrium, suggesting that this technique makes it possible to reasonably assess the state of equilibration of samples for systems in which the thermodynamic properties appear to be well described, One implication is that portions of P-T paths may be reconstructed from rocks that display disequilibrium textures but that pass this test for local equilibrium. The general applicability of this technique is presently limited by the accuracy of thermodynamic data, particularly for solid solutions, but should expand as these properties are refined in the future. This method facilitates the refinement process by (1) highlighting which minerals are most incompatible in each calculation and thus may be in need of refinement, (2) illustrating the sensitivity of results to different solution-models and offering the chance to correlate these differences with petrographic observations, and (3) providing a means to select "well-equilibrated" samples that can be used to refine thermodynamic properties in the absence of appropriate experimental data.

777 citations

Journal ArticleDOI
TL;DR: In this article, the authors present a model for calculating the amounts of melt that may be formed by fluid-absent breakdown of micas and amphiboles in common crustal rock types (pelitic, quartzofeldspathic, intermediate and mafic).

720 citations

Journal ArticleDOI
TL;DR: In this paper, the authors proposed a model for the near-coeval formation of the Archaean continental crust (dominantly tonalite-trondhjemite-granodiorite: TTG), which can be explained by melting the base of basaltic plateaux formed above mantle upwellings (plumes), leaving behind restites containing pyroxene, garnet, and rutile.

658 citations

Journal ArticleDOI
TL;DR: The abundance and scale of ultra-high-temperature (UHT) metamorphic belts from the Neoarchean to the Cambrian imply a significant change in geodynamics during the Neo-Archean Era, after which transient sites of high heat flow were available at intervals throughout this period of Earth evolution as mentioned in this paper.
Abstract: In general, Archean rocks exhibit rather ordinary moderate-P-high-T facies series metamorphism; neither blueschists nor any record of deep continental subduction and return are documented. However, the abundance and scale of ultrahigh-temperature (UHT) metamorphic belts from the Neoarchean to the Cambrian imply a significant change in geodynamics during the Neoarchean Era, after which transient sites of high heat flow were available at intervals throughout this period of Earth evolution. Many Neoproterozoic-Cambrian UHT metamorphic belts appear to have developed in settings analogous to modern backarcs that were closed and inverted during crustal aggregation and formation of the Gondwana supercontinent. If backarcs were the general setting for UHT metamorphism, then on a hotter Earth the cyclic formation of supercratons (in the Neoarchean Era) and supercontinents (in the Proterozoic Eon) required the destruction of oceans floored by thinner lithosphere that may have generated hotter backarcs than those as...

500 citations