R. T. Wightman

Bio: R. T. Wightman is an academic researcher. The author has contributed to research in topics: Archean & Precambrian. The author has an hindex of 1, co-authored 1 publications receiving 438 citations.

Precambrian Tectonics and Crustal Evolution in South India

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...

Pressure-Temperature-Time Paths and a Tectonic Model for the Evolution of Granulites

Abstract: Petrologic studies and application of well-calibrated mineralogic thermometers and barometers reveal several important features common to many granulite terrains: (1) "Peak" metamorphic pressure and temperature conditions cluster around values of $$7.5 \pm 1 kbar$$ and $$800 \pm 50°C$$, implying average geothermal gradients of 30-35°C/km. (2) In comparison with upper amphibolite facies rocks, especially in paired, amphibolite-granulite terrains, it is evident that granulites are distinguished from amphibolites by the former's higher metamorphic temperatures but not higher pressure. (3) Initial cooling of granulite terrains from "peak" conditions is nearly isobaric. For garnet closure temperatures of $$600 \pm 50°C$$, the dP/dT retrograde paths inferred from compositionally zoned garnet rims are 2-8 b/°C in most granulites. (4) Primary, coarse sillimanite is found in most granulites, and retrograde kyanite is known in a few. These data suggest that granulites form as a result of anomalous thermal gradients...

Precambrian Continental Crust of India and Its Evolution

Abstract: The Indian Precambrian continental crust exhibits a variety of geological features fashioned at different times by different geotectonic processes. The bulk of this crust was formed prior to 2600 m.y. ago and remobilized at least twice between 2600-2000 m.y. ago (early Proterozoic Mobile Belt, EPMB) and 2000-1500 m.y. ago (middle Proterozoic Mobile Belt MPMB). Three early Precambrian nucleii: Karnataka (KN), Jeypore-Bastar (JBN), and Singhbhum (SN) appear to have survived in the craton and are characterized by low-grade supracrustals and tonalitic trondhjemite gneisses, formed 3800-2600 m.y. ago. The EPMB event involved sedimentation, amphibolite-granulite facies metamorphism, and $$CO_{2}-K$$ metasomatism and produced amphibolite facies rocks and K-granites in the north, and charnockite and other granulite facies rocks in the south. Gold sporadically distributed in the supracrustal rocks of the craton was remobilized during the EPMB event. K-granites form a garland around the central Dharwar craton, sugg...

Age and tectonic evolution of Neoproterozoic ductile shear zones in the Southern Granulite Terrain of India, with implications for Gondwana studies

Abstract: [1] The high-grade rocks of the Southern Granulite Terrain (SGT) of Peninsular India are bounded to the north by the Archean Dharwar Craton. Another high-grade terrane, the Mesoproterozoic Eastern Ghats, occurs to the northeast of the SGT. The tectonic relationship between these crustal domains is complex. We present new geochronological and structural data that indicate a continuation of the Dharwar Craton into the Southern Granulite Terrain as far south as a newly identified Neoproterozoic shear zone, here named the Karur-Kamban-Painavu-Trichur Shear Zone (KKPTSZ). South of the KKPTSZ, Mesoproterozoic dates of the SGT are similar to those recorded in the Eastern Ghats, and the two domains may have been conterminous. Thirty-three new U/Pb/Th single zircon and monazite dates of samples from six structural transects across the regional shear zones indicate that the SGT has experienced at least seven thermo-tectonic events at 2.5 Ga, ∼2.0 Ga, ∼1.6 Ga, ∼1.0 Ga, ∼800 Ma, ∼600 Ma, and ∼550 Ma, and two distinct episodes of metasomatism/charnockitization between 2.50–2.53 and between 0.55–0.53 Ga. Deformation along a number of major shear zones in the SGT is Neoproterozoic to earliest Paleozoic in age, with an early phase (D2) concentrated between 700–800 Ma, and a later phase (D3) between 550 and 600 Ma. Major charnockitization (530–550 Ma) post dates D3, and is, in turn, overprinted by granitization, retrogression, and uplift between 525 and 480 Ma. The KKPTSZ, active between 560 and 570 Ma, is either a terrane boundary, or a tectonized decollement between cover and Archean basement rocks represented by predominantly paragneisses to the south and orthogneisses to the north, respectively. Other regional Neoproterozoic shear zones do not appear to separate allochthonous terranes as previously suggested on the basis of Nd model ages and Rb/Sr biotite/whole rock dates. The Neoproterozoic-Cambrian tectonothermal history of the SGT and Eastern Ghats is similar to that recorded in parts of Madagascar, East Africa, and Antarctica, and is used to reconstruct parts of central Gondwana, here named the Deccan Continent, with more robust confidence.