Petrology of corundum‐spinel‐sapphirine‐anorthite rocks (sakenites) from the type locality in southern Madagascar
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Abstract ‘Sakenites’ constitute a unique association of corundum‐, spinel‐ and sapphirine‐bearing anorthitic to phlogopitic rocks, first described in rocks from an exposure along the beds of the Sakena river to the NW of Ihosy, south Madagascar. The exposure has been revisited and subjected to a detailed petrological and geochemical study. The aluminous anorthitic rocks occur as boudinaged bands and lenses, closely associated with corundum‐, spinel‐ and sapphirine‐bearing phlogopitites, diverse calcsilicate rocks and marbles within a series of biotite‐sillimanite‐cordierite gneisses of the Ihosy granulite unit in the NW of the Pan‐African Bongolava‐Ranotsara shear zone. Bimineralic anorthite + corundum domains preserve the earliest record of a polyphasic evolutionary history that includes two distinct metasomatic episodes. Probable protoliths of these bimineralic rocks were kaolinite‐rich sediments or calcareous bauxites that were altered by Ca or Si infiltration‐metasomatism prior to or coeval with the development of the anorthite‐corundum assemblage. P–T pseudosection modelling of metapelitic gneisses suggests peak‐conditions around 800 °C and 6–7 kbar for the regional high‐grade metamorphism and deformation in the NW part of the Bongolava‐Ranotsara shear zone. The well‐annealed granoblastic‐polygonal textures indicate complete chemical and textural re‐equilibration of the foliated bimineralic rocks during this event. Subsequently, at somewhat lower P–T conditions (750–700 °C, 6 kbar), the influx of Mg‐, Si‐ and K‐bearing fluids into the anorthite‐corundum rocks caused significant metasomatic changes. In zones infiltrated by ‘primary’ potassic fluids, the bimineralic assemblage was completely replaced by phlogopite and Mg‐Al minerals, thereby producing corundum‐, spinel‐ and sapphirine‐bearing phlogopitites. Further advance of the resulting ‘residual’ Mg‐ and Si‐bearing fluids into anorthite‐corundum domains led to partial to complete replacement of corundum porphyroblasts by spinel, spinel + sapphirine or sapphirine, depending on the activities of the solutes. The static textures developed during this second metasomatic episode suggest fluid influx subsequent to intense ductile deformation in the Bongolava‐Ranotsara ductile shear zone c . 530–500 Ma ago.Keywords:
Anorthite
Sillimanite
Metasomatism
Corundum
Sillimanite
Cordierite
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An example of cordierite-bearing gneiss that is part of a high-grade gneiss-migmatite sequence is described from the Hatch Plain in the Read Mountains of the Shackleton Range, Antarctica, for the first time. The cordieritebearing rocks eonstitute the more melanosomic portions of the metatectic and migmatitic rocks that are associated with relict granulite facies rocks such as enderbitic granulite and enderbitic garnet granulite. The predominant mineral assemblage in the eordierite-bearing rocks is chemically homogeneous eordierite (X 0.61) and biotite (Xo 0.47), strongly zoned garnot (X 0.18-0.11), silli;llanite, K-feldspar (Or:'.94Ab5.,sAn06)' plagioclase (An28)~ and quartz. Inclusions of sillimanite and biotite relics in both gamet and cordierite indicate that garnet and cordierite were produced by the coupled, discontinuous reaction
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ABSTRACT Silica‐deficient sapphirine‐bearing rocks occur as an enclave within granulite facies Proterozoic gneisses and migmatites near Grimstad in the Bamble sector of south‐east Norway (Hasleholmen locality). The rocks contain peraluminous sapphirine, orthopyroxene, gedrite, anthophyllite, sillimanite, sapphirine, corundum, cordierite, spinel, quartz and biotite in a variety of assemblages. Feldspar is absent. Fe 2+ /(Fe 2+ + Mg) in the analysed minerals varies in the order: spinel > gedrite ≥ anthophyllite ≥ biotite > sapphirine>orthopyroxene > cordierite. Characteristic pseudomorph textures indicate coexistence of orthopyroxene and sillimanite during early stages of the reaction history. Assemblages containing orthopyroxene‐sillimanite‐sapphirine‐cordierite‐corundum developed during a high‐pressure phase of metamorphism and are consistent with equilibration pressures of about 9 kbar at temperatures of 750–800°C. Decompression towards medium‐pressure granulite facies generated various sapphirine‐bearing assemblages. The diagnostic assemblage of this stage is sapphirine‐cordierite. Sapphirine occurs in characteristic symplectite textures. The major mineralogical changes can be described by the discontinuous FMAS reaction: orthopyroxene + sillimanite → sapphirine + cordierite + corundum. The disequilibrium textures found in the Hasleholmen rocks are characteristic for reactions which have been in progress but then ceased before they run to completion. Textures such as reaction rims, symplectites, partial replacement, corrosion and dissolution of earlier minerals are characteristic of granulite facies rocks. They indicate that, despite relatively high temperatures (700–800° C), equilibrium domains were small and chemical communication and transport was hampered as a result of dry or H 2 O‐poor conditions.
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The Bakhuis belt, one of the major granulite-facies domains in the Guiana Shield, consists of a core of banded rocks of the charnockite suite, metamorphosed and migmatized under granulite-facies conditions. Pelitic gneiss intercalations locally show sapphirine-quartz and orthopyroxene-sillimanite-quartz assemblages, with up to 10% Al2O3 in the orthopyroxene. These assemblages point to ultrahigh-temperature (UHT) metamorphism. P-T conditions are difficult to deduce, because the assemblages contain Fe3+ in sapphirine and sillimanite, and do not contain coeval garnet, but spinel, Feand Fe-Ti oxides instead. P-T conditions for the peak UHT metamorphism are estimated to have been 950 °C and 8.5-9 kb. An assemblage of impure corundum associated with quartz may also have formed during the UHT metamorphism. Unravelling the assemblages indicates a counterclockwise P-T path from an early cordierite-sillimanite assemblage via a subsequent sapphirine-quartz assemblage to the peak metamorphic assemblage orthopyroxene-sillimanitequartz. Retrogressive assemblages show an isobaric to near isobaric cooling path for the UHT occurrence. Single zircon Pb-evaporation and whole rock Sm-Nd dating were carried out on the Bakhuis granulites. Three zircons from a garnet-sillimanite-bearing gneiss yielded an age of 2072 ± 4 Ma, but a fourth grain provided increasing ages up to 2.10 Ga. An enderbitic granulite gave zircon ages in the range of 2.15-2.09 Ga. Zircons from a garnet-bearing gneiss defined an age of 2055 ± 3 Ma, while those from a garnet-bearing pegmatite layer gave ages ranging between 2085 Ma and 2058 Ma. A mylonitic orthopyroxene-bearing granite (a true charnockite) furnished a Pb-Pb zircon age of 2065 ± 2 Ma this is the first indication that the high-grade metamorphism was (at least locally) associated with melting and production of magmatic charnockite. Zircons from two samples of a discordant recrystallized basic dyke yielded ages of 2060 ± 4 Ma and 2056 ± 4 Ma, respectively. A discordant sheared pegmatite vein provided an age of 2059 ± 3 Ma. In conclusion, the 2072–2055 Ma ages are interpreted as the age of granulite metamorphism in the Bakhuis Mountains, and ages older than 2.07 Ga, and up to 2.15 Ga, are considered to reflect an inherited component from the early Transamazonian protoliths of the granulite. TDM model ages for the Bakhuis samples range from 2.40 to 2.19 Ga. These data, together with positive to slightly 175 GEOLOGIE DE LA FRANCE, N° 2-3-4, 2003 GEOLOGY OF FRANCE AND SURROUNDING AREAS, N° 2-3-4, 2003 The Bakhuis ultrahigh-temperature granulite belt (Suriname): I. petrological and geochronological evidence for a counterclockwise P-T path at 2.07-2.05 Ga La ceinture de granulites d’ultra-haute temperature des Monts Bakhuis (Suriname) : I. mise en evidence d’un chemin P-T anti-horaire a 2,07-2,05 Ga Geologie de la France, 2003, n° 2-3-4, 175-205, 12 fig., 6 tabl. Mots cles : Roche metamorphique, Facies granulite, Condition pression temperature, Datation, Pb-Pb, Sm-Nd, Zircon, Roche totale, Paleoproterozoique, Suriname, Bouclier guyanais.
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Concordant ultramafic rocks are exposed in the lower crustal granulites of Sri Lanka. The ultramafic rocks at Rupaha had emplaced early. They had been subjected to deformation and granulite facies metamorphism at 850 o C at 9 kbar during the PanAfrican tectono-thermal episode. The results of thermometry of ultramafic rocks are consistent with those of geo-thermobarometry obtained from surrounding granulites. Fluids circulating in the deep crust had caused the formation of phlogopite blackwalls. Structure, texture and mineralogy of the blackwalls suggest that the K-metasomatism had taken place contemporaneous to the granulite facies metamorphism. The metasomatic reactions had started due to infiltration of K2O and SiO2 between ultramafic rocks and surrounding gneisses and diffusion of these elements between two wall rocks respectively. Carbonation and hydration had occurred in the blackwall rocks at the upper level of the crust on cooling together with surrounding rocks as indicated by textures of a partial retrogression to tremolite and dolomite, which formed through
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Abstract Corundum laths have been identified as exsolution products from both ilmenite and spinel in granulite facies rocks from central Australia. The exsolution is interpreted as having occurred during a decrease in the P‐T conditions under which the minerals initially crystallized. The ilmenite + corundum possibly indicate the existence of a former homogeneous iron‐högboemite. The spinel + corundum suggests that under conditions of high Ps‐T (granulite facies) and low fo2 there exists a field where aluminous spinel is stable.
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