Granulite

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

Late Archaean crust-mantle interactions: geochemistry of LREE-enriched mantle derived magmas. Example of the Closepet batholith, southern India

Abstract: The Closepet batholith in South India is generally considered as a typical crustal granite emplaced 2.5 Ga ago and derived through partial melting of the surrounding Peninsular Gneisses (3.3 to 3.0 Ga). In the field, it appears as a composite batholith made up of at least two groups of intrusions. (a) An early SiO2-poor group (clinopyroxene quartz-monzonite and porphyritic phyritic monzogranite) is located in the central part of the batholith. These rocks display a narrow range in both initial 87Sr/86Sr (0.7017–0.7035) and ɛNd(−0.9to −4.1). (b) A later SiO2-rich group (equigranular grey and pink granites) is located along the interface between the SiO2-poor group and the Peninsular Gneisses. They progressively grade into migmatised Peninsular Gneisses, thus indicating their anatectic derivation. Their isotopic characteristics vary over a wide range (87Sr/86Sr ratios=0.7028–0.7336 and ɛNd values from-2.7 to-8.3, at 2.52 Ga). Field and geochronological evidence shows that the two groups are broadly contemporaneous (2.518–2.513 Ga) and mechanically mixed. This observation is supported by the chemical data that display well defined mixing trends in the ɛSr vs ɛNd and elemental variation diagrams. The continuous chemical variation of the two magmatic bodies is interpreted in terms of interaction and mixing of two unrelated end-members derived from different source regions (enriched peridotitic mantle and Peninsular Gneisses). It is proposed that the intrusion of mantle-derived magmas into mid-crustal levels occurred along a transcurrent shear zone; these magmas supplied additional heat and fluids that initiated anatexis of the surrounding crust. During this event, large-scale mixing occurred between mantle and crustal melts, thus generating the composite Closepet batholith. The mantle-derived magmatism is clearly associated with granulite facies metamorphism 2.51±0.01 Ga ago. Both are interpreted as resulting from a major crustal accretion event, possibly related to mantle plume activity.

Osumilite-sapphirine-quartz granulites from Enderby Land Antarctica — Mineral assemblages and reactions

Abstract: The pre-Cambrian granulites of Enderby Land Antarctica, contain coexisting spinel-quartz, sapphirine-quartz, hypersthene-sillimanite-quartz and osumilite on a regional extent. Osumilite is present in a variety of mineral assemblages, most of which are documented in granulites for the first time. The mineral assemblages, reactions and compositional zoning in minerals are discussed in terms of continuous and discontinuous reactions in response to changing conditions of metamorphism. The development of many of the mineral coronas can be explained by continuous rather than discontinuous reactions, due to the effects of Mg-Fe and (Mg,Fe)-2Al exchange equilibria with decreasing temperature. The highest P-T conditions of metamorphism (8–10 kb, 900 °–980 ° C, Ellis, in preparation) were beyond the stability limit of coexisting garnet-cordierite. Secondary cordierite has developed through a large number of mineral reactions in response to cooling of these granulites.

The composition and petrogenesis of the lower crust: A xenolith study

Abstract: Granulite facies lower crustal xenoliths from a single basaltic vent (Hill 32) in the McBride volcanic province, north Queensland, Australia, illustrate the extreme lithologic diversity of the deep crust. These xenoliths are dominantly mafic, but intermediate and felsic granulites represent a significant proportion (∼20%) of the xenolith population. All xenoliths have high-grade mineralogies and generally well-equilibrated textures, as well as decompression features which are indicative of derivation from deep crustal levels (0.7–1.0 GPa). Major and trace element chemistry for 12 xenoliths, chosen to span the observed lithologic range, are used to constrain the petrogenesis of these rocks. In the smaller, layered samples, metamorphic differentiation may lead to nonrepresentative sampling. However, in most instances such secondary processes can be identified and the original chemical characteristics delineated. The mafic xenoliths formed through a variety of processes, including crystallization of mafic magma, crystal accumulation from mafic and felsic magmas, and partial melting of intermediate compositions to yield a mafic residuum. The two analyzed intermediate xenoliths are metasediments, based upon high alumina to alkali ratios and rare earth element patterns, whereas the two felsic xenoliths have compositions similar to igneous rocks. Comparisons of noncumulate and/or restite xenoliths with unmetamorphosed rock types show that K and Rb are variably depleted whereas Th and U are strongly depleted, in all rock types. The large ion lithophile element depletion patterns for xenoliths are similar to those of rocks from granulite facies terrains, suggesting similar processes were operative. Suggestions of an anomalously Ba- and Sr-rich lower crust are not supported by the data. By using the observed lithologic proportions of xenoliths at Hill 32, and the chemical analyses presented here, a weighted mean composition of the lower crust can be obtained and compared with recently proposed lower crustal compositions. The weighted mean composition is mafic, and formed through a combination of basaltic underplating, crystal accumulation, tectonic underplating, and partial melting. No simple model of lower crust formation through basaltic or andesitic underplating nor intracrustal melting is sufficient to explain lower crust formation in Phanerozoic continental margin settings.

Origin and evolution of granulites in normal and thickened crusts

Abstract: Rocks buried in the upper part of a doubly thickened crust during orogeny can show prograde reactions with transitional features from lower grade rocks up to granulite facies. They can be exposed at the earth9s surface within this one erogenic cycle. For example, the granulites exposed in British Columbia have concordant U/Pb zircon ages of 65–85 Ma. In contrast, rocks buried in the lower part of a thickened crust will crystallize granulites that typically show only retrograde relations with younger, lower grade rocks. At the cessation of orogeny, such rocks will undergo near-isobaric cooling from the peak metamorphism and can have very long residence times at the base of a crust of normal thickness. These granulites require a second orogeny to uplift and expose them at the surface. As an example, the Archean granulites of Enderby Land, Antarctica, were metamorphosed at 3070 Ma at 1000 °C at 8–10-kbar pressure. They then followed at least a 400 °C isobaric cooling path. They also had a prolonged residence time of 2000 m.y. near the base of the crust before uplift. No doubt a continuum exists between these extremes, but it is proposed that many intermediate- and high-pressure granulites form in the lower parts of thickened crust and therefore require a two-stage cycle of tectonism to be uplifted and exposed at the earth9s surface. Although most granulites probably form in thickened crust at active plate margins, underplating of normal (30–40-km thickness) crust by mantle melts during extension can also produce granulites. These granulites would also undergo isobaric cooling and require a later orogeny to expose them. An example is the Paleozoic Lachlan Fold Belt of eastern Australia, which has undergone only minor uplift since the major ultrametamorphism at about 400 Ma. However, isotopic, petrologic, and geochemical differences are to be expected between these extremes of granulite formation.

The characteristics and tectonic-metallogenic significances of the adakites in Yanshan period from eastern China.

Abstract: Adakite is a suite of intermediate acid igneous rocks characterized by HREE depletion and no obvious negative Eu anomaly, indicating the derivation from very deep source with garnet in the residue A lot of Yanshanian intermediate acid magmatic rocks in eastern China have similar geochemical characteristics to the adakite, their formation environment however is unrelated to subduction process In this paper, adakite is divided into two types: one is O type adakite, which is characterized by Na enrichment and is related to subduction process; another is C type adakite, which is enriched in K (most of them are still enriched in Na, a few K enriched), is probably a product of partial melting of the lower crust granulite in the thickened crust (50km) resulted from underplating of basaltic magma The occurrence of C type adakites in eastern China is indicative to the explanation of the geological phenomenon of Yanshanian magmatism Because C type adakite can preserve some imprints of the lower crust, it in turn can trace the composition of the lower crust and discuss metallogenesis related to the lower crust and crust mantle process