Granulite

About: Granulite is a research topic. Over the lifetime, 6763 publications have been published within this topic receiving 268925 citations.

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.

Melting of the continental crust: Some thermal and petrological constraints on anatexis in continental collision zones and other tectonic settings

Abstract: Useful constraints on the depth-temperature range and degree of melting of lower crustal rocks have been deduced by combining the results from experimental petrology with plausible PTt paths calculated for continental collision zones. H2O is required to generate melts at the temperatures of crustal orogenesis. Our modeling has shown that residual free water from subsolidus dehydration reactions produces less than 0.5% granitic melt at the H2O-saturated solidus. Breakdown of hydrous minerals results in fluid-absent (dehydration)- melting. For average geotherms such melting of muscovite and biotite in metapelite will generate at most 25% granitic melt. Because of much higher solidus temperatures, fluid-absent melting of hornblende in metabasalt (amphibolite) will not occur in collision zones without augmented heat input from the mantle. Fluid-absent melting of epidote in metabasalt, and low Ca amphibole and biotite in metavolcanics, can produce up to 10% melt in continental collision zones. Minor melt production from dehydration-melting of chlorite, staurolite, chloritoid, or talc will occur at pressures greater than ∼0.6 GPa. The calculated paths envelope small regions or PT space, thus identifying which mineral reactions are important for crustal anatexis and at which PT conditions future experiments should be made. In a dynamically evolving crust during uplift, thermal buffering by melting reactions is a minor component of the heat bucket. Extension of thickened continental crust will not promote dehydration-melting without the incursion of mantle heat. Delamination of thickened eclogitic lower crust, or decompression of the asthenosphere in response to lithosphere thinning are the most promising mechanisms to induce extensive lower crustal melting including amphibolite.