Granulite

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

Experimental generation of hybrid silicic melts by reaction of high‐Al basalt with metamorphic rocks

Abstract: Melting and crystallization experiments (T = 1000°C; P = 0.5, 0.7, 1.0, 1.2, and 1.5 GPa) were performed on two hybrid bulk compositions consisting of 50% anhydrous high-Al olivine tholeiite glass (HAOT) and 50% metamorphic rock (aluminosilicate-bearing metapelite and aluminosilicate-free biotite gneiss). The objective of the experiments was to determine phase equilibrium constraints attending bulk assimilation of crustal rocks by basaltic melts. Experiments generated 32–38 wt % of H2O-undersaturated granitic melt (SiO2 > 70 wt %). Thermal and phase equilibrium arguments show that production of this amount of granitic melt at the chosen experimental conditions is a plausible model for bulk assimilation in nature. The compositions of the melts and of the coexisting crystalline assemblages are affected to a comparable extent by pressure and by composition of the crustal source. Reaction of HAOT with biotite gneiss at P ≤ 1.0 GPa produces calc-alkaline melts in equilibrium with gabbronoritic cumulates (plag + opx + cpx). Reaction of HAOT with metapelite at P ≤ 0.7 GPa produces strongly peraluminous melts that resemble S-type granites from the Lachlan Fold Belt, in equilibrium with noritic cumulates (plag + opx ± spi ± gar). At P > 1.2 GPa, both source compositions produce strongly peraluminous leucocratic melts (<2 wt % FeO + MgO + TiO2) in equilibrium with garnet-rich and plagioclase-poor residues (opx + cpx + gar + plag from the biotite gneiss, gar + plag from the metapelite). The experiments show that a wide spectrum of high-SiO2 melts can be hybrids formed directly by reaction of basaltic melts with amphibolite-facies metamorphic rocks. Accumulation of the complementary mafic crystalline assemblages in the deep crust will generate granulites which are neither restitic nor the products of subsolidus dehydration. Mafic granulites and granitic melts formed by reaction of basaltic melts with metamorphic rocks will share isotopic and trace element signatures reflecting inheritance from both crustal and mantle sources.

The History of Granulite-Facies Metamorphism and Crustal Growth from Single Zircon U-Pb Geochronology: Namaqualand, South Africa

Abstract: The Namaqualand Metamorphic Complex is a well-exposed, INTRODUCTION Mesoproterozoic, low-pressure, amphibolite–granulite-facies terrane flanking the Archaean Kaapvaal Craton of southern Africa. Previous isotopic dating in the region suggests an ~150 my period of prograde granulite-facies metamorphism and episodic granite emplacement The mid-crustal granulite-facies problem in the mid-crust. In contrast, thermal modelling suggests that Granulite-facies terranes are rocks of the Earth’s lower suband superjacent magmatic accretion should not have exceeded and middle crust that equilibrated at high pressures (P ) 30 my in duration. This enigma is resolved by precise U–Pb zircon and temperatures (T ). Their petrology and geoSHRIMP dating of the major orthogneissic units of the region. chronology commonly preserve both prograde and retroThese data point to Kibaran crustal growth at 1220–1170 Ma, grade characteristics. Because these terranes reflect a which occurred on the margins of a Palaeoproterozoic (2000–1800 number of different crustal and tectonic processes, their Ma) continental nucleus. A later, distinct, orogenic episode, here origin is important in understanding the nature of contermed the Namaquan (time equivalent of the Grenvillian), involved tinental growth and crustal evolution. crustal thickening and magmatism at 1060–1030 Ma and was Granulite-facies rocks typically reflect P–T conditions responsible for, and coeval with, the peak of metamorphism. Lowof 6–9 kbar and 750–850°C and comprise anhydrous P granulite-facies metamorphism resulted from advective heating mineral assemblages that point to conditions of reduced and crustal thickening by magmatic accretion over a 30 my interval. water activity (Harley, 1989). In the lower crust some granulite-facies rocks are thought to be residues of partial melting that has moved melt and volatiles to higher crustal levels. Alternatively, granulite-facies rocks may form where mutually soluble CO2–H2O-rich fluids stream upwards through the crust, causing local reduction in volatile content and accompanying mineral phase changes along the fluid flow paths (Harley, 1989). In the

Chemical composition and fractionation of the continental crust

Abstract: A new estimate of the bulk continental crust is reported consisting of 57 percent lower crust (60% felsic and 40% mafic granulites) and 43 percent upper crust. The proportions of crustal units are based on petrological observations (Bohlen &Mezger, 1989). The estimate of a bulk composition is intermediate between andesite and tonalite and is higher in Si, K, Rb, Sr, Zr, Nb, Ba, LREE, Pb, Th concentrations and lower in Mg, Ca, Sc, Mn, Fe than the crustal abundances reported byTaylor &McLennan (1985). Equal chemical composition between the upper crust and the felsic part of the lower crust is attained in balance computations if one restores a fraction of 12.5 percent S-type granite from the upper into the lower crust. An example of water-undersaturated partial melting and separation of a fraction of about 35 percent granitic magma at the conversion from amphibolite-into granulite-facies metasediments has been balanced bySchnetger (1988) in the Ivrea area (N. Italy). The worldwide observed discrepancy between a larger negative Eu anomaly in the upper crust compared with the half as large positive anomaly of the lower crust increasing from the early Precambrian to present has been explained by recycling of Ca-rich restite into the upper mantle. The composition of the Archean crust (example: Greenland) does not differ systematically from the post-Archean crust.