Orthopyroxene–sillimanite–sapphirine granulites from the Bamble granulite terrane, southern Norway

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.
Fe²⁺/(Fe²⁺+ 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 H₂O-poor conditions.     

Sapphirine parageneses from the Caraiba complex, Bahia, Brazil: the influence of Fe2+–Fe3+ distribution on the stability of sapphirine in natural assemblages

Abstract Sapphirine-bearing rocks occur in three conformable, metre-size lenses in intrusive quartzo-feldspathic orthogneisses in the Curaçà valley of the Archaean Caraiba complex of Brazil. In the lenses there are six different sapphirine-bearing rock types, which have the following phases (each containing phlogopite in addition): A: Sapphirine, orthopyroxene; B: Sapphirine, cordierite, orthopyroxene, spinel; C: Sapphirine, cordierite; D: Sapphirine, cordierite, orthopyroxene, quartz; E: Sapphirine, cordierite, orthopyroxene, sillimanite, quartz; F: Sapphirine, cordierite, K-feldspar, quartz. Neither sapphirine and quartz nor orthopyroxene and sillimanite have been found in contact, however. During mylonitization, introduction of silica into the three quartz-free rocks (which represent relict protolith material) gave rise to the three cordierite and quartz-bearing rocks. Stable parageneses in the more magnesian rocks were sapphirine–orthopyroxene and sapphirine–cordierite. In more iron-rich rocks, sapphirine–cordierite, sapphirine-cordierite–sillimanite, cordierite–sillimanite, sapphirine–cordierite–spinel–magnetite and quartz–cordierite–orthopyroxene were stable. The iron oxide content in sapphirine of the six rocks increases from an average of 2.0 to 10.5 wt % (total Fe as FeO) in the order: C,F–A,D–B,E. With increase in Fe there is an increase in recalculated Fe₂O₃ in sapphirine. The four rock types associated with the sapphirine-bearing lenses are: I: Orthopyroxene, cordierite, biotite, quartz, feldspar tonalitic to grandioritic gneiss; II: Biotite, quartz, feldspar gneiss; III: Orthopyroxene, clinopyroxene, hornblende, plagioclase meta-norite; IV: Biotite, orthopyroxene, quartz, feldspar, garnet, cordierite, sillimanite granulite gneiss. The stable parageneses in type IV are orthopyroxene–cordierite–quartz, garnet–sillimanite–quartz and garnet–cordierite–sillimanite. Geothermobarometry suggests that the associated host rocks equilibrated at 720–750°C and 5.5–6.5 kbar. Petrogenetic grids for the FMASH and FMAFSH (FeO–MgO–Al₂O₃–Fe₂O₃–SiO₂–H₂O) model systems indicate that sapphirine-bearing assemblages without garnet were stabilized by a high Fe³⁺ content and a high X_Mg= (Mg/ (Mg+Fe²⁺)) under these P–T conditions.     

Reaction history of garnet-sapphirine granulites and conditions of Archaean high-pressure granulite-facies metamorphism in the Central Limpopo Mobile Belt, Zimbabwe

ABSTRACT Sequential reaction textures in Archaean garnet-corundum-sapphirine granulites from the Central Zone of the Limpopo Belt document a progression from early, coarse-grained, high-pressure (P > 9.5 kbar) granulite-facies assemblages (M1) to late, low-pressure (P <6 kbar) granulite-facies sub-assemblages (M2). The stable M1 assemblage was garnet (57% pyrope; Mg/(Mg + Fe) = 62) + sapphirine + corundum + gedrite + phlogopite + rutile. Late-M1 boron-free kornerupine grew at the expense of garnet and corundum, and coexisted with garnet, sapphirine and gedrite. Partial or complete breakdown of coarse garnet and kornerupine during M2 resulted in the development of pseudomorphs and coronas consisting of fine-grained symplectic intergrowths of cordierite, gedrite and sapphirine (later, spinel). The majority of reaction textures can be explained in terms of a stable reaction sequence, and a model time-sequence of mineral facies can be constructed. When compared with a qualitative petrogenetic grid of (Fe, Mg)-discontinuous reactions in the FMASH multisystem sapphirine-garnet-corundum-spinel-cordierite-gedrite-kornerupine, the facies-sequence indicates decompression at essentially constant T assuming constant a(H₂O). Exhumation of M1 corundum inclusions during M2 breakdown of kornerupine resulted in production of metastable spinel by a disequilibrium reaction with gedrite. A second disequilibrium reaction of the spinel with cordierite produced sapphirine. The operation of such reaction while pressure was decreasing (the opposite dP from that implied by the texture if assumed to be the product of an equilibrium reaction) has serious implications for the use of reaction textures in the construction of P-T vectors. Garnet-biotite thermometry on garnet interiors and phlogopite inclusions in corundum yields temperatures of ca. 850°C for the M1 stage. A minimum late-M1 pressure of ca. 7 kbar is indicated by the former association of kornerupine and corundum. Relict M1 kyanites reported by other workers indicate a minumum early-M1 pressure of 9.5 kbar, implying metamorphism at depths of at least 33 km (probably 38km). The high-pressure granulite-facies metamorphism was followed by an almost isothermal pressure decrease of > 5 kbar, indicative of rapid uplift. The P-T path is interpreted as the product of a single metamorphic cycle which probably took place in response to tectonic thickening of the crust. Such a process contrasts with the extensional origin recently proposed for isobarically cooled granulite-facies terranes.     

Orthopyroxene-Corundum in Mg-Al-rich Granulites from the Oygarden Islands, East Antarctica

High-Mg–Al, silica-undersaturated metapelites from theOygarden Group of islands, East Antarctica, preserve clear evidencefor the stable coexistence of the assemblage orthopyroxene +corundum in natural rocks. The quartz-absent metapelite occursas pods and isolated layers within a high-strain zone relatedto deformation during the c. 0·93 Ga Rayner StructuralEpisode. Assemblages that include orthopyroxene, corundum, sapphirine,sillimanite, cordierite, garnet and kornerupine are developedacross a pre-existing compositional zoning, leading to contrastingmineral Fe–Mg ratios. The assemblage orthopyroxene–corundumis shown to exist in only a very restricted range of bulk compositionsand P–T histories. Simplified qualitative FMAS grids havebeen constructed for kornerupine-absent and -present systems,illustrating MAS terminations and divariant equilibria thathelp to describe the mineral assemblage and reaction history.Reaction textures that include coronas of sapphirine and sillimaniteseparating orthopyroxene and corundum, and symplectites of orthopyroxene+ sapphirine ± cordierite/plagioclase, orthopyroxene+ sillimanite ± cordierite/plagioclase and orthopyroxene+ sapphirine + sillimanite embaying garnet, imply a clockwiseP–T–t evolution. Conditions of P > 9–10kbar and T     

Sapphirine/kornenipine-bearing rocks and crustal uplift history of the Limpopo belt,Southern Africa

Sapphirine/kornerupine-bearing rocks occur within the anorthosites of the Messina layered intrusion in the Limpopo mobile belt of Zimbabwe. The X_Mg range of the major minerals is as follows: cordierite (0.98-0.93); enstatite (0.97-0.86); chlorite (0.98-0.92); phlogopite (0.98-0.90); sapphirine (0.98-0.86); kornerupine (0.94-0.88); gedrite (0.96-0.85); spinel (0.92-0.78). There are four rock types, the constituent minerals of which have different values, which decrease in the above mineral order; other minerals are corundum, sillimanite and relict kyanite. We recognise twenty reactions without phlogopite and nine reactions involving phlogopite. The textural relations and the plots of the microprobe data of coexisting minerals in the MgO-Al₂O₃-SiO₂-(H₂O) system are consistent with the following sequence of main reactions: (1) enstatite+corundum cordierite+sapphirine; (4) sapphirine+sillimanite cordierite+corundum; (8) kornerupine+corundum cordierite+sapphirine; (13) kornerupine cordierite+sapphirine+enstatite; (15) enstatite+spinel chlorite+sapphirine; (18) cordierite+sapphirine chlorite+corundum; (20) sapphirine chlorite+corundum+spinel. The early reactions are shown by coarse-grained reaction intergrowths, kornerupine and gedrite breakdown is shown by finer-grained symplectites, and the latest reactions by very fine-grained products in micro-fractures. These selected reactions illustrate a remarkably steep trajectory from thePT peak close to 10 kbar and 800° C to the minimum observable at 3.5–4.5 kbar and 700° C as indicated by the pure MASH system. Very rapid uplift took place under nearly isothermal conditions. The protolith of this material was possibly sedimentary, derived from altered volcanic rocks. The bulk composition is close to the composition of kornerupine or to a mixture of alunite, chlorite and pyrophyllite. These texturally and mineralogically complex rocks contain a wealth of relevant data for documenting crustal uplift history.     

Sapphirine bearing granulites from the Sipiwesk Lake area of the late Archean Pikwitonei granulite terrain,Manitoba, Canada

Sapphirine occurs in the orthopyroxene-cordierite and feldspar-sillimanite granulites in the Sipiwesk Lake area of the Pikwitonei granulite terrain, Manitoba (97°40W, 55°05N). The orthopyroxene-cordierite granulites have extremely high Al₂O₃ (24.5 wt%) and MgO (24.6 wt%) contents and contain sapphirine (up to 69.2 wt% Al₂O₃), aluminous orthopyroxene (up to 8.93 wt% Al₂O₃), cordierite, spinel, phlogopite, and corundum. Sapphirine forms coronas mantling spinel and corundum. Corona sapphirine is zoned and its composition varies through the substitution (Mg, Fe, Mn) Si=2 Al as a function of the phases with which it is in contact. Textural and chemical relationships of sapphirine with coexisting phases indicate that spinel + cordierite reacted to form orthopyroxene + sapphirine under conditions of increasing pressure. Moreover, decreasing core to rim variation of Al₂O₃ in orthopyroxene porphyroblasts suggests decreasing temperature during sapphirine formation. On the basis of experimentally determined P-T stability of the assemblage enstatite + sapphirine + cordierite, and the Al content of hypothetical Fe²⁺-free orthopyroxene associated with sapphirine and cordierite, metamorphic temperatures and pressures are estimated to be 860–890° C and 3.0–11.2 kbar.In the feldspar-sillimanite granulites, sapphirine occurs as a relict phase mantled by sillimanite and/or by successive coronas of sillimanite and garnet. These textural relations suggest the reaction sapphirine + garnet + quartz = orthopyroxene + sillimanite with decreasing temperature. Compositions of minerals in the assemblage garnet-orthopyroxene-sillimanite-plagioclase-quartz, indicate metamorphic P-T conditions of 780–880° C and 9±1 kb.The metamorphic conditions estimated in this study suggest that the sapphirine bearing granulites in the Sipiwesk Lake area represent Archean lower crustal rocks. Their formation might be related to the crustal thickening processes in this area as suggested by Hubregtse (1980) and Weber (1983).     

Epitaxial quartz inclusions in corundum from a sapphirine–garnet boudin,Bamble Sector,SE Norway: SiO2–Al2O3 miscibility at high P–T dry granulite facies conditions

An Al‐rich, SiO₂‐deficient sapphirine–garnet‐bearing rock occurs as a metapelitic boudin within granulite facies Proterozoic charnockitic gneisses and migmatites on the island of Hisøy, Bamble Sector, SE Norway. The boudin is made up of peraluminous sapphirine, garnet, corundum, spinel, orthopyroxene, sillimanite, cordierite, staurolite and biotite in a variety of assemblages. Thermobarometric calculations based on coexisting sapphirine–spinel, garnet–corundum–spinel–sillimanite, sapphirine–orthopyroxene, and garnet–orthopyroxene indicate peak‐metamorphic conditions near to 930 °C at 10 kbar. Corundum occurs as single 200 to 3000 micron sized skeletal crystal intergrowths in cores of optically continuous pristine garnet porphyroblasts. Quartz occurs as 5–60 micron‐sized euhedral to lobate inclusions in the corundum where it is in direct contact with the corundum with no evidence of a reaction texture. Some crystal inclusions exhibit growth zoning, which indicates that textural equilibrium was achieved. Electron Back‐Scatter Diffraction (EBSD) studies reveal that the quartz inclusions share a common c‐axis with the host corundum crystal. The origin of the quartz inclusions in corundum is enigmatic as recent experimental studies have confirmed the instability of quartz–corundum over geologically realistic P–T ranges. The combined EBSD and textural observations suggest the presence of a former silica‐bearing proto‐corundum, which underwent exsolution during post‐peak‐metamorphic uplift and cooling. Exsolution of quartz in corundum is probably confined to fluid‐absent conditions where phase transitions by coupled dissolution–precipitation mechanisms are prevented.     

Petrology of phlogopite-sapphirine-bearing Al-Mg granulites from Ihouhaouene, In Ouzzal, Hoggar, Algeria: an example of phlogopite stability at high temperature

Al-Mg granulites, with cordierite, garnet, sapphirine, orthopyroxene, sillimanite, spinel, phlogopite, K-feldspar, plagioclase and variable quartz from Ihouhaouene (In Ouzzal, Algeria), display a range of decompression textures involving the breakdown of orthopyroxene and sillimanite, and of garnet. The succession of parageneses suggests that the P–T–t evolution corresponds to decompression with cooling from peak conditions of about 950°C and 10 kbar. This decompression path is obtained from the paragenetic analysis in the FMAS system. However, according to current KFMASH grids, this P–T–t evolution should take place outside the stability field of phlogopite+quartz; yet this assemblage is probably stable during most of the P-T evolution, notably during peak metamorphism. This discrepancy is interpreted as the effect of the high content of F in phlogopite which should shift its stability limit towards higher temperature. The consequences of this shift on the phase relationships in the KFeMASH system are investigated and it is concluded that a topological inversion could exist in the F-bearing system.     

Metastable growth of corundum adjacent to quartz in a spinel-bearing quartzite from the Archaean Napier Complex, Antarctica

In a granulite-facies spinel-bearing quartzite, corundum, orthopyroxene and sapphirine (and rarely cordierite and sillimanite) form partial rims separating spinel from quartz. Textures indicate the reactions:
spinel + quartz = orthopyroxene + corundum, and
spinel + quartz = orthopyroxene + sapphirine.
Thus, corundum and sapphirine are produced by reactions involving quartz. The low Al-content of the orthopyroxene (0.5–2.8 wt %) and low values for Mg–Fe distribution coefficient for spinel–sapphirine and spinel–orthopyroxene reflect low-temperature conditions during formation of the reaction products. Absence of zoning in spinel and a constant Mg–Fe distribution coefficient for spinel–sapphirine and spinel–orthopyroxene, over a compositional range, indicate Mg–Fe equilibration. It is suggested that stable reactions such as spinel + quartz = cordierite or spinel + quartz = garnet + sillimanite were over-stepped and that metastable reactions give rise to the anomalous juxtaposition of corundum + quartz.     

Metamorphic History of Sapphirine-bearing and Related Magnesian Gneisses from Namaqualand, South Africa

Sapphirine occurs with cordierite, phlogopite, spinel, sillimanite,corundum, orthopyroxene, and gedrite in granulite facies Mg-and Al-rich paragneisses within the low P, high T NamaqualandMetamorphic Complex. The gneisses reveal a three-stage texturalhistory. Sapphirine appeared during a second stage of progrademineral growth which produced nodular structures and intergrowthsinvolving spinel, corundum, and sillimanite, pseudomorphingan earlier generation of coarse, amphibolite facies minerals.A third generation of coarse, cross-cutting, mainly hydrousminerals (gedrite, kornerupine, phlogopite) is sporadicallydeveloped. The wide variety of cofacial mineral assemblages allows thedelineation of the stable mineral associations of sapphirinein the system K₂O-MgO-FeO-Al₂O₃-SiO₂-H₂O under P-T conditionsindependently estimated at about 5 kb, 750–800 °C.The natural assemblages provide constraints which, taken togetherwith existing thermodynamic and experimental data, allow theestimation of P-T slopes of sapphirine equilibria. The mineraltextures thus indicate sapphirine growth under increasing T,decreasing a(H₂O), and constant or slightly increasing P. The preservation of prograde reaction textures during fine-grainedmineral growth probably results from the reduced importanceand/or more CO₂-rich composition of the metamorphic fluid undergranulite facies conditions in these refractory rocks. Aqueousfluids were locally reintroduced after the metamorphic peak.     

Modelling of mineral equilibria in ultrahigh-temperature metamorphic rocks from the Anápolis–Itauçu Complex, central Brazil

A new quantitative approach to constraining mineral equilibria in sapphirine‐bearing ultrahigh‐temperature (UHT) granulites through the use of pseudosections and compatibility diagrams is presented, using a recently published thermodynamic model for sapphirine. The approach is illustrated with an example from an UHT locality in the Anápolis–Itauçu Complex, central Brazil, where modelling of mineral equilibria indicates peak metamorphic conditions of about 9 kbar and 1000 °C. The early formed, coarse‐grained assemblage is garnet–orthopyroxene–sillimanite–quartz, which was subsequently modified following peak conditions. The retrograde pressure–temperature (P–T) path of this locality involves decompression across the FeO–MgO–Al₂O₃–SiO₂ (FMAS) univariant reaction orthopyroxene + sillimanite = garnet + sapphirine + quartz, resulting in the growth of sapphirine–quartz, followed by cooling and recrossing of this reaction. The resulting microstructures are modelled using compatibility diagrams, and pseudosections calculated for specific grain boundaries considered as chemical domains. The sequence of microstructures preserved in the rocks constrains a two‐stage isothermal decompression–isobaric cooling path. The stability of cordierite along the retrograde path is examined using a domainal approach and pseudosections for orthopyroxene–quartz and garnet–quartz grain boundaries. This analysis indicates that the presence or absence of cordierite may be explained by local variation in a_H2O. This study has important implications for thermobarometric studies of UHT granulites, mainly through showing that traditional FMAS petrogenetic grids based on experiments alone may overestimate P–T conditions. Such grids are effectively constant a_H2O sections in FMAS‐H₂O (FMASH), for which the corresponding a_H2O is commonly higher than that experienced by UHT granulites. A corollary of this dependence of mineral equilibria on a_H2O is that local variations in a_H2O may explain the formation of cordierite without significant changes in P–T conditions, particularly without marked decompression.     

Petrological evolution of a suite of spinel granulites from Vizianagram, Eastern Ghats Belt, India, and genesis of sapphirine-bearing assemblages

A suite of spinel–cordierite granulites from Viziangram, Eastern Ghats Belt, India preserve mineral assemblages and reaction textures indicative of peak metamorphic conditions of >1000 °C, >8<10 kbar, followed successively by near isobaric cooling (down to 750–800 °C), near isothermal decompression (to 4–5 kbar), and late hydration. P–T conditions of each stage are evaluated through a combination of petrogenetic grid approach and thermobarometry. Sapphirine is developed in sillimanite‐bearing acid pegmatite veins that intruded the spinel–cordierite granulite close to peak metamorphic conditions, and also in the host rock in immediate contact with the pegmatite. Both sillimanite and sapphirine in the pegmatite are considered to be magmatic phases. Field observations and textural characteristics suggest that Al‐metasomatism of the spinel–cordierite granulite due to the intrusion of pegmatite was responsible for sapphirine formation in the spinel granulite.     

Kornerupine in Mg-Al-rich gneisses from Namaqualand,South Africa: mineralogy and evidence for late-metamorphic fluid activity

Three kornerupine occurrences are reported in distinctive SiO₂-poor, MgO- and Al₂O₃-rich paragneisses from the Namaqualand Metamorphic Complex in South Africa. Kornerupine coexists stably with phlogopite, cordierite, orthopyroxene, gedrite, sapphirine, sillimanite and plagioclase and, in sapphirine-free rocks, with spinel and corundum. Tourmaline of a texturally older generation than kornerupine is commonly present in the same samples.Ten analysed kornerupines show a variation in total Fe as FeO from 1.8 to 10.9 weight per cent. B₂O₃ contents are estimated from x-ray data and a few spectrochemical analyses to range from 0.9 to 3.5 weight per cent. There is a strong inverse correlation between B³⁺ and Al³⁺. Total iron content has a strong and systematic effect on refractive index, colour and dispersion. Fe and Mg are systematically partitioned with the other minerals, and Mg/(Mg+Fe) ratios increase as follows: spinel 相似文献   

Gedrite–garnet–sillimanite-bearing granulites from Amessmessa area, south In Ouzzal, Hoggar, Algeria

Some granulites from the Amessmessa area (south In Ouzzal unit, Hoggar) contain the peak assemblage gedrite+garnet+sillimanite+quartz that was used to estimate the P–T conditions of metamorphism. The rocks developed symplectites and corona textures by the breakdown of the primary paragenesis to orthopyroxene, cordierite and spinel. The successive parageneses formed in separate microdomains according to a clockwise P–T path. Geothermometry, geobarometry and phase diagram calculations indicate that the textures formed by decompression and cooling from 7–9 kbar and 850–900°C to 3.5–4.5 kbar and 700–800°C. This P–T evolution is consistent with low to medium a_H2O, between 0.4 and 0.7, and is similar to the metamorphic conditions deduced in Al–Mg granulites from the north of In Ouzzal.     

An example of ultrahigh-temperature metamorphism: orthopyroxene–sillimanite–garnet, sapphirine–quartz and spinel–quartz parageneses in Al–Mg granulites from In Hihaou, In Ouzzal, Hoggar

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+Qtz→Opx+Sil, (2) Opx+Sil→Grt+Spr+Qtz, (3) Grt+Sil+Qtz→Crd, (4) Grt+Sil→Crd+Spr, (5) Grt+Sil+Spr→Crd+Spl, (6) Grt+Sil→Crd+Spl, (7) Grt+Crd+Sil→Spl+Qtz and (8) Grt+Sil→Spl+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.     

Petro-tectonic Imprints in the Sapphirine Granulites from Anantagiri, Eastern Ghats Mobile Belt, India

Sapphirine granulite occurring as lenses in charnockite at Anantagiri,Eastern Ghat, India, displays an array of minerals which developedunder different P-T-X conditions. Reaction textures in conjunctionwith mineral chemical data attest to several Fe-Mg continuousreactions, such as

1. spinel+rutile+quartz+MgFe_–1=sapphirine+ilmenite
2. cordierite=sapphirine+quartz+MgFe_–1
3. sapphirine+quartz=orthopyroxene+sillimanite+MgFe_–1
4. orthopyroxene+sapphirine+quartz=garnet+MgFe_–1
5. orthopyroxene+sillimanite=garnet+quartz+MgFe_–1
6. orthopyroxene+sillimanite+quartz+MgFe_–1=cordierite.

Calculated positions of the reaction curves in P-T space, togetherwith discrete P-T points obtained through geothermobarometryin sapphirine granulite and the closely associated charnockiteand mafic granulite, define an anticlockwise P-T trajectory.This comprises a high-T/P prograde metamorphic path which culminatedin a pressure regime of 8?3 kb above 950?C, a nearly isobariccooling (IBC) path (from 950?C, 8?3 kb, to 675?C, 7?5kb) anda terminal decompressive path (from 7?5 to 4?5 kb). Spinel,quartz, high-Mg cordierite, and sapphirine were stabilized duringthe prograde high-T/P metamorphism, followed by the developmentof orthopyroxene, sillimanite, and garnet during the IBC. Retrogradelow-Mg cordierite appeared as a consequence of decompressionin the sapphirine granulite. Deformational structures, reportedfrom the Eastern Ghat granulites, and the available geochronologicaldata indicate that prograde metamorphism could have occurredat 30001?00 and 2500?100 Ma during a compressive orogeny thatwas associated with high heat influx through mafic magmatism. IBC ensued from P_max and was thus a direct consequence of progrademetamorphism. However, in the absence of sufficient study onthe spatial variation in P-T paths and the strain historiesin relation to time, the linkage between IBC and isothermaldecompression (ITD) has remained obscure. A prolonged IBC followedby ITD could be the consequence of one extensional mechanismwhich had an insufficient acceleration at the early stage, orITD separately could be caused by an unrelated extensional tectonism.The complex cooled nearly isobarically from 2500 Ma. It sufferedrapid decompression accompanied by anorthosite and alkalinemagmatism at 1400–1000 Ma.     

Orthopyroxene–sillimanite–quartz assemblages: distribution, petrology, quantitative P–T–X constraints and P–T paths

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 X_Mg 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.     

Mg-Al Sapphirine- and Ca-Al Hibonite-bearing Granulite Xenoliths from the Chyulu Hills Volcanic Field, Kenya

Basanites of the Chyulu Hills (Kenya Rift) contain mafic Mg–Aland Ca–Al granulite xenoliths. Their protoliths are interpretedas troctolitic cumulates; however, the original mineral assemblageswere almost completely transformed by subsolidus reactions.Mg–Al granulites contain the minerals spinel, sapphirine,sillimanite, plagioclase, corundum, clinopyroxene, orthopyroxeneand garnet, whereas Ca–Al granulites are characterizedby hibonite, spinel, sapphirine, mullite, sillimanite, plagioclase,quartz, clinopyroxene, corundum, and garnet. In the Mg–Algranulites, the first generation of orthopyroxene and some spinelmay be of igneous origin. In the Ca–Al granulites, hibonite(and possibly some spinel) are the earliest, possibly igneous,minerals in the crystallization sequence. Most pyroxene, spineland corundum in Mg–Al and Ca–Al granulites formedby subsolidus reactions. The qualitative P–T path derivedfrom metamorphic reactions corresponds to subsolidus cooling,probably accompanied, or followed by, compression. Final equilibrationwas achieved at T 600–740°C and P <8 kbar, inthe stability field of sillimanite. The early coexistence ofcorundum and pyroxenes (± spinel), as well as the associationof sillimanite and sapphirine with clinopyroxene and the presenceof hibonite, makes both types of granulite rare. The Ca–Alhibonite-bearing granulites are unique. Both types enlarge thespectrum of known Ca–Al–Mg-rich granulites worldwide. KEY WORDS: granulite xenoliths; corundum; sapphirine; hibonite; Kenya Rift     

Equilibration and reaction in Archaean quartz-sapphirine granulite xenoliths from the Lace kimberlite pipe, South Africa

Ultrahigh-temperature quartz-sapphirine granulite xenoliths in the post-Karoo Lace kimberlite, South Africa, comprise mainly quartz, sapphirine, garnet and sillimanite, with rarer orthopyroxene, antiperthite, corundum and zinc-bearing spinel; constant accessories are rutile, graphite and sulphides. Comparison with assemblages in the experimentally determined FMAS and KFMASH grids indicates initial equilibration at >1040 °C and 9–11  kbar. Corona assemblages involving garnet, sillimanite and minor cordierite developed on a near-isobaric cooling P–T  path as both temperature and, to a lesser extent, pressures decreased. Garnet-orthopyroxene Fe-Mg exchange thermometers record temperatures of only 830–916 °C. These estimates do not indicate the peak metamorphic conditions but instead reflect the importance of post-peak Fe-Mg exchange during cooling. Correction of mineral Fe-Mg compositions for this exhange using a convergence approach of Fitzsimons & Harley (1994 ) leads to retrieved P–T  estimates from garnet-orthopyroxene thermobarometry ( c . 1000 °C and 10.5±0.7  kbar) that are consistent with the petrogenetic grid constraints. U-Pb dating of a single zircon grain gives an age of 2590±83  Ma, interpreted as the age of the metamorphic event. Protolith major and trace element chemistries of the xenoliths differ from sapphirine-quartzites typical of the Napier Complex (Antarctica) but are comparable to less siliceous, high Cr and Ni, sapphirine granulites reported from several ultrahigh temperature granulite terranes.     

Metamorphism and Isotopic Evolution of Granulites of Southern India: Reference to Neoproterozoic Crustal Evolution

Granulites are important component of high-grade metamorphic rocks reflecting intense conditions observed for crustal rocks in terms of temperature, and pressure. This review paper demonstrates how these high-grade granulites are critical to understanding the evolution of the lower continental crust with special reference to southern India. Geothermobarometric traverse across different granulite blocks in southern India shows wide ranging P-T conditions of metamorphism (700–1000 ° C, and 5–10 kbar). The sapphirine-, orthopyroxene-sillimanite, and spinel -bearing quartz-deficient granulites recognised from parts of southern granulite terrain (Ganguvarpatti, Kiranur, and Palani hill ranges etc.) show oriented sillimanite aggregates pseudomorph after course twinned kyanite, staurolite + kyanite assemblages, and corroded blebs of gedrite within orthopyroxene, suggesting a prograde stage of a clockwise P-T evolution. Evidence of ITD history comes from the textures in which an early Mg-rich garnet (X_Mg 52–60) with orthopyroxene (up to 10% Al₂O₃) involving sillimanite breakdown forming variety of symplectites having combinations of orthopyroxene, sapphirine, cordierite, and spinel. These spectacular reaction textures, and mineralogic sensors from the high-grade rocks establish a prograde clockwise P-T-t path with notable decompressive history (ITD) in the southern granulite terrain. The inferred P-T-t paths have been further integrated with the recent geochronological, and isotopic data to constrain the timing, and duration of metamorphism, emplacement of the magmatic protolith for characterising the evolution of the granulites, and their bearing on the geodynamic implications. Based on the emerging evidence for Neoproterozoic tectonothermal imprints in the southern granulite terrain, history of the assembly of dispersed fragments is also addressed within the East Gondwana framework.