On ultrahigh-temperature crustal metamorphism

Ferric iron is present in all metamorphic rocks and has the ability to significantly affect their phase relations. However, the influence of ferric iron has commonly been ignored, or at least not been considered quantitatively, mainly because its abundance in rocks and minerals is not determined by routine analytical techniques. Mineral equilibria calculations that explicitly account for ferric iron can be used to examine its effect on the phase relations in rocks and, in principle, allow the estimation of the oxidation state of rocks. This is illustrated with calculated pseudosections in NCKFMASHTO for mafic and pelitic rock compositions. In addition, it is shown that ferric iron has the capacity to significantly increase the stability of the corundum + quartz assemblage, making it possible for this assemblage to exist at crustal P–T conditions in oxidized rocks of appropriate composition.

Influence of ferric iron on the stability of mineral assemblages

Although natural occurrences of corundum + quartz ± aluminosilicate are known, internally consistent thermodynamic databases suggest that they do not represent a stable assemblage. This observation has motivated two sets of experiments. In the first set, the equilibrium kyanite = corundum + quartz has been reversed (±5 °C; ±25 MPa) at 600 °C/320 MPa and 700 °C/535 MPa (externally heated cold-seal hydrothermal autoclaves); 800 °C/775 MPa (gas apparatus; NaCl and CaF2 furnace assemblies, non end-load piston-cylinder press); and at 900 °C/1075 MPa and 1000 °C/1325 MPa (CaF2 furnace assembly, non end-load piston-cylinder press). These reversals imply an enthalpy of formation from the elements for kyanite of −2594.75 kJ/mol. The slope of the equilibrium curve also confirms both volume and experimental C P data for kyanite. These reversals can serve as a useful calibration for the piston-cylinder press using NaCl furnace assemblies in the 700–800 °C and 500–1000 MPa range and indicate a friction correction for CaF2 furnace assemblies of 75–100 MPa over 800–1000 °C and 1000–1500 MPa.

The second set of experiments (1200 °C and 2000 MPa) investigated the growth of kyanite along corundum-quartz grain interfaces. In experiments where no fluid was present, except adsorbed H2O, kyanite did not nucleate and grow. In experiments with 2 wt% H2O, kyanite formed and grew preferentially in the pores surrounding the corundum grains parallel to the corundum-quartz interface and along quartz grain boundaries. Due to a large Δ V R, reaction halos around the corundum grains never become closed to fluid migration. This suggests that in nature, fluids are channeled to these reaction sites via porous reaction halos surrounding the corundum grains and indicates that, under such circumstances, formation of kyanite is self-promoting and probably goes to completion quickly. The stability of sillimanite relative to corundum + quartz is also discussed, from the standpoints of what is predicted by internally consistent mineral databases and what is observed in nature.

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Stability of corundum + quartz relative to kyanite and sillimanite at high temperature and pressure

Corundum+quartz-bearing assemblages occur in small lenses in granulite facies metapelites in Rayagada, north-central part of the Eastern Ghats Granulite Belt, India. Corundum porphyroblasts and quartz coexist with porphyroblastic almandine-rich garnet, hercynite spinel, ilmenite and magnetite. Corundum and quartz are separated by sillimanite or a composite corona consisting of sillimanite and garnet, whereas corundum shows sharp grain boundaries with spinel, ilmenite and magnetite. Porphyroblastic corundum contains prismatic sillimanite inclusions in which irregularly shaped quartz is enclosed. Two distinct reactions are inferred from the textural features: corundum+quartz=sillimanite and spinel+quartz=garnet+sillimanite. From the petrographical features, we infer that corundum–quartz–garnet–spinel was the peak metamorphic assemblage. Although large uncertainties exist regarding the positions of the respective reactions in P–T  space, from several published experimental results and theoretical calculations a peak metamorphic condition of 12 kbar and 1100 °C is estimated as the lower stability limit of the corundum–quartz assemblage. Decompression from the peak P–T  condition to c. 9 kbar, 950 °C is inferred.

A corundum–quartz assemblage from the Eastern Ghats Granulite Belt, India: evidence for high P–T metamorphism?

High-MgAl rocks occur as xenoliths (up to 2 m in diameter) in mafic granulites at a newly discovered locality near Anakapalle. Following an early phase of deformation, ultrahigh-temperature (UHT) metamorphism and near-isothermal decompression, the rocks were intruded in a lit-par-lit manner by felsic melts (charnockite), which caused local-scale metasomatism. A subsequent deformation produced isoclinal folds and the distinct gneissic foliation of the charnockite still at granulite facies conditions.



The sequence of multiphase reaction textures in the high-MgAl xenoliths reflects the changes of physico-chemical conditions during the polyphase evolution of the terrane; UHT metamorphism (stage 1, > 1000°C, c. 10 kbar) is documented by relics of extremely coarse grained domains with the assemblage orthopyroxene (opx)1 + garnet (grt)1 + sapphirine (spr)1 + spinel (spl)1 + rutile (rt). A subsequent phase of near-isothermal decompression in the order of 1–2 kbar (stage 2) resulted in extensive replacement of grt1 and opx1 megacrysts by lamellar (opx2 + spr2) symplectites. The intrusion of felsic melt (stage 3) led to the development of a narrow metasomatic black wall reaction zone (bt + sil + plg3 + opx2,3 + rt) at the immediate contact of the xenoliths and in melt infiltration zones to the partial replacement of (opx2 + spr2) symplectites by biotite and sillimanite and/or plg3, mainly at the expense of orthopyroxene, with concomitant coarsening of the intergrowth texture. The subsequent deformation (stage 4) further modified the symplectite textures through polygonization, recrystallization and grain-size coarsening. The deformation was followed by a period of cooling and decompression (stage 5, c. 800°C, 4–7 kbar) as indicated by local growth of late garnet (grt5) at the expense of (opx + spr + plg) domains at static conditions.



Recently published isotope data suggest that the multistage evolution of the high-MgAl granulites at Anakapalle followed a discontinuous P–T trajectory that may be related to heating of the crust through magmatic accretion culminating in deep-crustal UHT metamorphism at 1.4 Ga (stage 1), fast uplift of the UHT granulites into mid-crustal levels as a consequence of extensional tectonics (stage 2), emplacement of felsic magmas in the Grenvillian (at c. 1 Ga, stage 3) resulting in reheating of the crust to high–T conditions followed by a phase of compressional tectonics (stage 4) and a period of cooling to the stable geotherm (stage 5) still in the Grenvillian.

Multistage reaction textures in xenolithic high‐MgAl granulites at Anakapalle, Eastern Ghats Belt, India: examples of contact polymetamorphism and infiltration‐driven metasomatism

A corundum-bearing, spinel-sapphirine-garnet-sillimanite-quartz-(Fe-Ti)-oxide granulite from the Eastern Ghats belt, India, shows reaction textures that suggest formation of coronitic garnet and sillimanite around spinel, quartz and sapphirine. Mineral-chemical data, chemographic analysis and consideration of the relevant petrogenetic grid in the system FMAS indicate that the mineral reactions occurred in response to nearly isobaric cooling after the metamorphic peak.

Juergen Ehl

Papers

Reaction textures in a spinel-sapphirine granulite from the Eastern Ghats, India, and their implications