Granulite

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

Post-peak, fluid-mediated modification of granulite facies zircon and monazite in the Trivandrum Block, southern India

Abstract: The quarry at Kottavattom in the Trivandrum Block of southern India contains spectacular examples of fluid-assisted alteration of high-grade metamorphic rocks. Garnet-biotite gneiss has undergone a change in mineral assemblage to form submetre scale orthopyroxene-bearing patches, later retrogressed to form an amphibole-bearing lithology. These patches, often referred to as arrested or incipient charnockite, crosscut the original metamorphic foliation and are typically attributed to passage of a low aH2O fluid through the rock. Whilst this conversion is recognised as a late stage process, little detailed chronological work exists to link it temporally to metamorphism in the region. Zircon and monazite analysed from Kottavattom not only record metamorphism in the Trivandrum Block but also show internal, lobate textures crosscutting the original zoning, consistent with fluid-aided coupled dissolution-reprecipitation during formation of the orthopyroxene-bearing patches. High-grade metamorphism at the quarry occurred between the formation of metamorphic monazite at ~585 Ma and the growth of metamorphic zircon at ~523 Ma. The fluid-assisted alteration of the garnet-biotite gneiss is poorly recorded by altered zircon with only minimal resetting of the U–Pb system, whereas monazite has in some cases undergone complete U–Pb resetting and records an age for fluid infiltration at ~495 Ma. The fluid event therefore places the formation of the altered patches at least 25 Myr after the zircon crystallisation in the garnet-biotite gneiss. The most likely fluid composition causing the modification and U–Pb resetting of zircon and monazite is locally derived hypersaline brine.

Giant granulite terranes of northeastern Superior Province: the Ashuanipi complex and Minto block

Abstract: The Ashuanipi complex and Minto block of the Superior Province are large regions that have been classified as "high-grade gneiss" terranes on the basis of the presence of orthopyroxene-bearing units. Like the granite-greenstone and metasedimentary subprovinces of the southern Superior Province, the two terranes consist predominantly of intrusive rocks, but are distinguished by their primary magmatic orthopyroxene. Both "high-grade" and "gneiss" are misnomers because granulite-facies gneisses are only sparingly present and the regions consist dominantly of massive, unmetamorphosed plutonic rock. The Ashuanipi complex consists of a deformed, metamorphosed package of metasedimentary rocks and primitive, early tonalite cut by widespread orthopyroxene + garnet granodiorite (diatexite), as well as plutons of tonalite, granite, and syenite. Based on its lithological and chronological similarity and on-strike position, the complex appears to be the continuation of metasedimentary subprovinces such as the Quetico. Its evolution involved deposition of immature greywacke in an accretionary prism, early arc (tonalitic) magmatism and deformation, followed by widespread intracrustal magmatism in the period 2700-2670 Ma. Both metamorphic and igneous rocks record equilibration under granulite-facies conditions (700-835°C; 0.35-0.65 GPa; a,,,, -0.3) and indicate exposure levels of -20 km. The Minto block at the latitude of Leaf River consists of several north-northwest-trending domains of similar scale and diversity to the east-trending subprovinces of the southern Superior Province. The central Goudalie domain is dominantly amphibolite-facies tonalitic rocks including some with ages > 3 Ga, with small belts of volcanic and sedimentary origin. Lake Minto domain contains poorly preserved supracrustal remnants in a plutonic complex comprising hornblende granodiorite, clinopyroxene + orthopyroxene granodiorite, orthopyroxene-biotite diatexite, and granite. The hornblende granodiorite suite constitutes most of the Utsalik and Tikkerutuk domains and is thought to represent continental arc magmatism. On the basis of their distinct aeromagnetic and lithological character, two additional domains are evident north of the Leaf River area, the Inukjuak domain in the west and the Douglas Harbour domain in the east. The northerly grain of the Minto block appears to have been established in situ with respect to the easterly belts of the southern Superior Province (i.e., no large-scale block rotation) during the same interval of time (3.0-2.7 Ga). Modification of the tectonic framework for the Superior Province is required to explain Minto arc magmatism. In the interval -2730-2690 Ma ago, a continental magmatic arc built the Berens River and Bienville subprovinces and Minto block on the southern and eastern edges, respectively, of a northern protocratonic foundation. In the same period, primitive volcanic arcs and accretionary prisms developed outboard on oceanic crust and were accreted to form a southern tectonic regime.