Granulite

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

Evidence from ultra-high-pressure marbles for recycling of sediments into the mantle

Abstract: ROCKS of crustal origin metamorphosed at ultra-high pressures (P>2.5 GPa) have been described from several orogenic belts1–5. In the western Alps, for example, ultra-high pressure rocks originally equilibrated at about 750 °C and 3.5 GPa (ref. 1). During decompression, these rocks were cooled considerably and therefore did not pass through the granulite stability field. Here we describe ultra-high-pressure metasediments that have equilibrated at higher temperatures (> 1,100 °C) and followed a different exhumation path. A calcsilicate marble from the Bohemian massif contains clinopyroxenes with potassium-rich feldspar exsolutions. Potassium contents in the original clinopyroxenes indicate crystallization at pressures above 3–4 GPa (refs 6, 7). The high peak pressures and temperatures inferred for this rock, and its association with high-temperature peridotites and high-pressure granulites, suggest that carbonate sediments were subducted into the upper mantle, equilibrated at mantle conditions and were then emplaced in the crust along with mantle rocks. Our observations thus support suggestions based on less direct evidence (such as mass-balance considerations in orogenic belts8, and ocean-island basalt geochemistry9,10) that a limited amount of sediment must be recycled into the mantle.

Age and duration of ultrahigh‐temperature metamorphism in the Anápolis–Itauçu Complex, Southern Brasília Belt, central Brazil – constraints from U–Pb geochronology, mineral rare earth element chemistry and trace‐element thermometry

Abstract: Integrated petrographic and chemical analysis of zircon, garnet and rutile from ultrahigh-temperature (UHT) granulites in the Anapolis–Itaucu Complex, Brazil, is used to constrain the significance of zircon ID-TIMS U–Pb geochronological data. Chondrite-normalized rare earth element (REE) profiles of zircon cores have positive-sloping heavy-REE patterns, commonly inferred to be magmatic, whereas unambiguous metamorphic grains and overgrowths have flat to slightly negatively sloping heavy-REE patterns. However, in one sample, a core of zircon interpreted as having formed prior to garnet crystallization and a metamorphic zircon formed within microstructures involving garnet breakdown both display elevated heavy-REE (and Y) with positive-sloping patterns. DREE(zrc/grt) partition coefficients suggest an approximation to equilibrium between zircon and garnet cores, although progressive enrichment in heavy REE towards garnet rims occurs in two of the samples investigated. Titanium-in-zircon thermometry indicates zircon growth during both the prograde and post-peak evolution, but not at peak temperatures of the UHT metamorphism. By contrast, zirconium-in-rutile thermometry of inclusions armoured by garnet records crystallization temperatures, based on the upper end of the interquartile range of the data, of ∼890 to 870 °C and maximum temperature around 980 °C, indicating prograde and retrograde growth close to and after peak conditions. Rutile located in retrograde microstructures records crystallization temperatures of 850 to 820 °C. Rutile intergrown with ilmenite and included within orthopyroxene, which is associated with exsolved zircon, records temperatures ∼760 °C, consistent with Ti-in-zircon crystallization temperatures. ID-TIMS U–Pb geochronological data from two of the four samples investigated define upper intercept ages of 641.3 ± 8.4 Ma (MSWD 0.91) and 638.8 ± 2.5 Ma (MSWD 1.03) that correlate with periods of zircon growth along the prograde segment of the P–T path. Individual zircon U–Pb dates retrieved from all samples range from 649 to 634 Ma, indicating a maximum duration of c. 15 Myr for the UHT event. This period is interpreted as recording modest thickening of hot backarc lithosphere located behind the Arenopolis Arc at the edge of the Sao Francisco Craton consequent upon terminal collision of the Parana Block with the arc during the amalgamation of West Gondwana.

Early Cretaceous migmatitic mafic granulites from the Sabzevar range (NE Iran): implications for the closure of the Mesozoic peri‐Tethyan oceans in central Iran

Abstract: Terra Nova, 22, 26–34, 2010 Abstract The ophiolitic melange of the Sabzevar Range (northern Iran) is a remnant of the Mesozoic oceanic basins on the northern margin of the Neotethys that were consumed during the Arabia–Eurasia convergence history. Occurrence of km-scale, dismembered mafic HP granulitic slices is reported in this study. Granulites record an episode of amphibole-dehydratation melting and felsic (tonalite/throndhjemite) melt segregation at c. 1.1 GPa and 800 °C. In situ U(-Th)–Pb geochronology of zircon and titanite grains hosted in melt segregations points to an Early Cretaceous (Albian) age for the metamorphic climax. Results of this study (i) impose reconsideration of the current palaeotectonic models of the Neothetyan convergent margin during the Early Cretaceous and (ii) argue that punctuated events of subduction of short-lived back-arc oceanic basins accompanied the long-lasting history of the Neotethyan subduction in the region.