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Granulite
About: Granulite is a research topic. Over the lifetime, 6763 publications have been published within this topic receiving 268925 citations.
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TL;DR: In the Sudetes, seven distinct lithostratigraphic terranes exhibit a symmetric distribution as discussed by the authors, including the Central Sudetic terrane and the Sowie Gory terrane.
Abstract: In the Sudetes, seven distinct lithostratigraphic terranes exhibit a symmetric distribution A central region of basinal/oceanic and ophiolitic rocks, the Central Sudetic terrane is bordered, respectively to the northwest and southeast, by the sialic Saxothuringian and Moldanubian terranes These exhibit contrasting metasedimentary/metavolcanic successions and tectonic-metamorphic sequences, but both are characterized by Palaeozoic plutonism These are in turn bordered (again respectively to the northwest and southeast) by the Lusatian and Moravian terranes, which are also sialic, but contain Cadomian granitoids and represent rifted and now widely separated fragments of Gondwana Along the southwestern flank of the Sudetes, the Barrandian terrane, largely covered by younger sediments, extends to the southwestern margin of the Bohemian Massif The Sowie Gory terrane forms a klippe of high grade gneisses tectonically emplaced on top of low-grade, sheared ophiolites of the Central Sudetic terrane The Sowie Gory terrane exhibits a history of three distinct, probably multi-orogenic, regional metamorphic events: an early high-pressure granulite/eclogite metamorphism followed by medium- to low-pressure granulite, and in turn by amphibolite facies metamorphism All the terrane boundaries are complex zones of ductile to brittle shearing, modified by later brittle movements Some, such as the Leszczyniec shear zone, mark lines of old, pre-Variscan rift and suture zones, reactivated and overprinted during a series of Variscan ductile to brittle events of extensional shearing with related metamorphism and plutonism
89 citations
TL;DR: In this article, a clockwise P-T path was estimated and defined for the southern Beishan orogen, an eastern extension of the Tianshan orogen in the Gubaoquan area in China.
Abstract: Numerous lenses of eclogite occur in a belt of augen orthogneisses in the Gubaoquan area in the southern Beishan orogen, an eastern extension of the Tianshan orogen. With detailed petrological data and phase relations, modelled in the system NCFMASHTO with THERMOCALC, a quantitative P-T path was estimated and defined a clockwise P-T path that showed a near isothermal decompression from eclogite facies (>15.5 kbar, 700-800 � C, omphacite + garnet) to high-pressure granulite facies (12-14 kbar, 700-750 � C, clinopyroxene + sodic plagioclase symplectitic intergrowths around ompha- cite), low-pressure granulite facies (8-9.5 kbar, 700 � C, orthopyroxene + clinopyroxene + plagio- clase symplectites and coronas surrounding garnet) and amphibolite facies (5-7 kbar, 600-700 � C, hornblende + plagioclase symplectites). The major and trace elements and Sm-Nd isotopic data suggest that most of the Beishan eclogite samples had a protolith of oceanic crust with geochemical characteristics of an enriched or normal mid-ocean ridge basalt. The U-Pb dating of the Beishan eclogites indicates an Ordovician age of c. 467 Ma for the eclogite facies metamorphism. An 39 Ar ⁄ 40 Ar age of c. 430 Ma for biotite from the augen gneiss corresponds to the time of retrograde metamorphism. The combined data from geological setting, bulk composition, clockwise P-T path and geochronology support a model in which the Beishan eclogites started as oceanic crust in the Palaeoasian Ocean, which was subducted to eclogite depths in the Ordovician and exhumed in the Silurian. The eclogite-bearing gneiss belt marks the position of a high-pressure Ordovician suture zone, and the calculated clockwise P-T path defines the progression from subduction to exhumation.
89 citations
01 Jan 2003
TL;DR: In this article, a semi-quantitative thermo- dynamic modeling of this reaction is presented, which is hampered by the lack of a melt model and gives results that are only moderately successful in rationalizing the natural and experimental data.
Abstract: Orthopyroxene-free garnet + clinopyroxene + plagioclase ± quartz-bearing mineral assemblages represent the paragenetic link between plagioclase-free eclogite facies metabasites and orthopyroxene- bearing granulite facies metabasites. Although these assemblages are most commonly developed under P-T conditions consistent with high pressure granulite facies,they sometimes occur at lower grade in the amphibolite facies. Thus,these assemblages are characteristic but not definitive of high pressure granulite facies. Compositional factors favouring their development at amphibolite grade include Fe-rich mineral compositions,Ca-rich garnet and plagioclase,and Ti-poor hornblende. The generalized reaction that accounts for the prograde development of garnet + clinopyroxene + plagio- clase ± quartz from a hornblende + plagioclase + quartz-bearing (amphibolite) precursor is Hbl + Pl + Qtz ¼ Grt + Cpx + liquid or vapour,depending on whether the reaction occurs above or below the solidus. There are significant discrepancies between experimental and natural constraints on the P-T conditions of orthopyroxene-free garnet + clinopyroxene + plagioclase ± quartz-bearing mineral assemblages and therefore on the P-T position of this reaction. Semi-quantitative thermo- dynamic modelling of this reaction is hampered by the lack of a melt model and gives results that are only moderately successful in rationalizing the natural and experimental data.
89 citations
TL;DR: In this paper, the phase relationships within the FMAS system show that a continuum exists between the earlier prograde reaction textures and those of the later decompressive event, which is consistent with the observed reactions.
Abstract: Quartz Al–Mg granulites exposed at In Hihaou, In Ouzzal (NW Hoggar), preserve an unusual high-grade mineral association stable at temperatures up to 1050°C, involving the parageneses orthopyroxene–sillimanite–garnet–quartz, sapphirine–quartz and spinel–quartz. The phase relationships within the FMAS system show that a continuum exists between the earlier prograde reaction textures and those of the later decompressive event. The following mineral reactions involving sillimanite are deduced: (1) Grt+QtzOpx+Sil, (2) Opx+SilGrt+Spr+Qtz, (3) Grt+Sil+QtzCrd, (4) Grt+SilCrd+Spr, (5) Grt+Sil+SprCrd+Spl, (6) Grt+SilCrd+Spl, (7) Grt+Crd+SilSpl+Qtz and (8) Grt+SilSpl+Qtz. Minerals in quartz Al–Mg granulites display compositional variations consistent with the observed reactions. The Mg/(Mg+Fe2+) range of the main minerals is as follows: cordierite (0.81–0.97), sapphirine (0.77–0.88), orthopyroxene (0.65–0.81), garnet (0.33–0.64) and spinel (0.23–0.56). The reaction textures and the evolution of the mineral assemblages in the quartz Al–Mg granulites indicate a clockwise P–T trajectory characterized by peak conditions of at least 10 kbar and 1050°C, followed by decompression from 10 to 6 kbar at a temperature of at least 900°C.
89 citations
TL;DR: In this paper, the authors assess the development and preservation of peak granulite facies mineral assemblages such as orthopyroxene + sillimanite + quartz with reference to calculated phase diagrams.
Abstract: Granulite facies magnesian metapelites commonly preserve a wide array of mineral assemblages and reaction textures that are useful for deciphering the metamorphic evolution of a terrane. Quantitative pressure, temperature and bulk composition constraints on the development and preservation of characteristic peak granulite facies mineral assemblages such as orthopyroxene + sillimanite + quartz are assessed with reference to calculated phase diagrams. In NCKFMASH and its chemical subsystems, peak assemblages form mainly in high-variance fields, and most mineral assemblage changes reflect multivariant equilibria. The rarity of orthopyroxene-sillimanite-quartz-bearing assemblages in granulite facies rocks reflects the need for bulk rock XMg of greater than approximately 0.60-0.65, with pressures and temperatures exceeding c. 8 kbar and 850 � C, respectively. Cordierite coronas mantling peak minerals such as orthopyroxene, sillimanite and quartz have historically been used to infer isothermal decompression P-T paths in ultrahigh-temperature granulite facies terranes. However, a potentially wide range of P-T paths from a given peak metamorphic condition facilitate retrograde cordierite growth after orthopyroxene + sillimanite + quartz, indicating that an individual mineral reaction texture is unable to uniquely define a P-T vector. Therefore, the interpretation of P-T paths in high-grade rocks as isothermal decompression or isobaric cooling may be overly simplistic. Integration of quantitative data from different mineral reaction textures in rocks with varying bulk composition will provide the strongest constraints on a P-T path, and in turn on tectonic models derived from these paths.
89 citations