Rapid developments in the scope and quality of internally consistent thermodynamic data sets arising from the incorporation of new experimental data and solid-solution models for minerals and melts involved in ultrahigh temperature (UHT) metamorphism now place petrologists in an excellent position to evaluate the detailed mineral assemblage evolutions of many UHT rocks and place these in quantitative frameworks using P-T diagrams and pseudosection approaches. These developments will soon be enhanced by extension of the methods to systems at high fO2, enabling a wider range of critical mineral assemblages to be considered and their UHT P-T evolutions, at present largely qualitatively constrained, to be determined quantitatively. Cordierite provides a useful monitor of the role and composition of any fluids and melts attending UHT metamorphism at <1000°C. Such fluids or melts remaining in communication with the mineral assemblages following UHT have a significant effect on preservation of the metamorphic record and may also strongly influence the ages recorded by geochronological systems, even to the extent of producing widespread post-UHT metamorphic zircon. Modelling of UHT metamorphism and P-T paths, whether IBC, ITD or hybrid ITD-IBC, in terms of specific and testable tectonic settings and processes is still problematic, with those models that can successfully duplicate the extraordinary temperatures attained in some UHT terrains generally being geologically unreasonable. Successful modelling of the thermal conditions and evolution of UHT terrains, constrained by quantitative P-T-t paths, is an important objective for future research that will inform our understanding of this intriguing aspect of the behaviour of the Earth system.
Corundum + quartz as an assemblage has been reported in a limited number of high to ultra-high grade metamorphic terrains. It is worth noting that rocks hosting this unusual assemblage have common features with respect to mineral assemblages, textures, metamorphic conditions and geological environments. In addition to corundum and quartz, these rocks are characterised by variable amounts of magnetite, spinel, ilmenite together with rutile and sillimanite/kyanite in Fe-rich rocks; and sapphirine, orthopyroxene, garnet and cordierite in more Mg-rich rocks. These rocks are characterised in general by (i) coarse-grained texture with inclusions of corundum and spinel in magnetite, (ii) sharp contact between corundum and quartz and/or separated by a thin rim of sillimanite or kyanite between these two minerals, (iii) presence of spinel + quartz, sapphirine + quartz in equilibrium as well as osumilite. Although there has been some discussions in the past concerning whether “corundum + quartz” assemblage represents a “thermodynamic” stable or metastable assemblage, it is clear that this assemblage observed mainly in the magnetite-ilmenite-spinel bearing rocks is “texturally” stable. On the basis of the available experimental and thermodynamic data as well as natural occurrences, we conclude that this assemblage is consistent with ultra-high temperature metamorphic environments (up to 1100°C at pressure of 7 to 12 kbars). Therefore, its presence in natural rocks should draw close attention from field metamorphic petrologists working on deep crustal processes.
Ultrahigh temperature (UHT) granulites in Tonagh Island, Napier Complex, East Antarctica record peak metamorphic pressure-temperature (P-T) conditions of up to 9 kbar and 1100°C. Sapphirine, garnet, orthopyroxene and quartz in these rocks contain very high density fluid inclusions with melting temperatures close to that of pure CO2 (−56.6°C) and homogenization temperatures down to −34.9°C translating into high CO2 densities (up to 1.07 g/cm3). The UHT granulites of Vizianagram in Eastern Ghats Belt, India which were subjected to extreme crustal metamorphism at >1000°C and 8-9 kbar also carry very high density (up to 1.15 g/cm3) pure CO2 inclusions in quartz adjacent to spinel rimmed by various coronas of sillimanite, orthopyroxene and garnet. Garnets in a granulite facies rock from Salem in southern India that equilibrated at peak P-T conditions of 740-800°C and 9-11 kbar carry the highest density (1.17 g/cm3) pure CO2 yet reported from continental crust. This rock also contains very high density CO2 inclusions in plagioclase and quartz. High density (0.998 g/cm3) pure CO2-rich fluid inclusions also occur abundantly within garnets from a mafic granulite in Ampitiya, central Highland Complex, Sri Lanka. The peak P-T conditions of metamorphism of the mafic granulite are around 10.6 kbar and 985°C with subsequent rapid isothermal decompression along a clock-wise path down to 5.5 kbar. In most of the cases above, the representative isochores for the CO2 inclusions either penetrate through or pass very close to the P-T windows defined from mineral phase equilibria indicating that the fluid inclusions were trapped at the time of peak or near post-peak metamorphism. We thus recognize a group of “ultrahigh density” (UHD) CO2-rich fluids that characterize UHT rocks and other granulites formed under extreme crustal metamorphism. These fluids contrast sharply with the lower density (generally <1.0 g/cm3) CO2 inclusions commonly reported from normal granulite facies rocks in various terrains. The presence of “synmetamorphic” UHD CO2 within various minerals is consistent with the low water activities predicted by the mineral assemblages in these rocks. Although the source of the CO2 is equivocal, mantle derived mafic magmas could have provided the heat and volatiles required for crustal metamorphism at extreme conditions displayed by these rocks.
Feldspathic leucosomes occur in an ultrahigh-temperature (UHT) metamorphosed garnet-bearing paragneiss/quartzite in the Napier Complex, East Antarctica. Coarse (200-400 μm) zircon grains occurring in the leucosomes display 3 textural domains: (I) dark-CL (cathodoluminescence) structured inner-core, (II) bright-CL structured outer-core, and (III) dark-CL structureless rim. Chemical compositions, especially chondrite-normalized REE patterns obtained by SIMS analysis, correlate with these three domains: the inner-core (domain-I) shows HREE-enrichment with Yb(n)/Gd(n) = 3.3, whereas the outer-core (II) and rim (III) have flat to relatively depleted HREE patterns with Yb(n)/Gd(n) = 0.7-0.8. Th/U ratios decrease from 3.2 in the zircon inner-core (I) to 1.2 in outer-core (II) and to 0.3 in the rim (III). Garnets near such zircon grains display two trace element compositional features. Firstly, high core Zr contents (300 ppm) decrease to 100 ppm within 50-100 μm of grain rims. Secondly, the HREE distribution between zircon inner-core (I) and garnet core is 2 at Gd (GdDZrn/Grt = 2), rising to 8 at Lu (LuDZrn/Grt = 8), whereas those defined from zircon outer-core (II) or rim (III) and garnet core or rim are much lower and generally below 1 for Gd thruogh to Lu (GdDZrn/Grt = 0.8-1.2; LuDZrn/Grt = 0.6-0.7). This marked change in the HREE distribution between zircon and garnet must reflect either a change in the minerals with which the zircon was growing or being modified, a change in the physical and chemical conditions of zircon growth, or a combination of the two. Based on comparisons with recent estimates of equilibrium zircon/garnet HREE distribution coefficients we infer that the inner-core (I) did not grow with the garnet that occurs in the paragneiss but grew within a garnet-absent melt that was then injected into the gneiss. The resulting leucosome then underwent wall-rock reaction with the enclosing garnet-bearing gneiss, causing a decrease in garnet to Zr contents to values approaching equilibrium with melt, and precipitating the zircon outer-core (II). Finally, the zircon rim (III) and later monazite formed in a HREE depleted environment. Melt injection, reaction and crystallization of the leucosomes took place within the time interval 2496-2471 Ma at the end of the UHT history of the Napier Complex.
High-pressure and high-temperature experiments were carried out at 9-13 kbar and 950-1200°C using a piston-cylinder apparatus, simulating the equilibrium conditions of a phlogopite-bearing orthopyroxene granulite from the Howard Hills, Napier Complex, East Antarctica. This granulite consists of orthopyroxene, sapphirine, spinel, alkali feldspar and F-rich phlogopite with minor amounts of plagioclase, rutile and quartz, and is garnet-free. The experiments clarified the equilibrium phase relations of the phlogopite-bearing orthopyroxene granulite under ultrahigh-temperature metamorphism conditions. The following mineral assemblages were obtained with decreasing temperature: orthopyroxene + spinel + liquid, orthopyroxene + phlogopite + spinel + liquid, orthopyroxene + phlogopite + sapphirine + spinel + rutile + liquid, orthopyroxene + alkali feldspar + phlogopite + sapphirine + spinel + rutile, orthopyroxene + alkali feldspar + plagioclase + phlogopite + sapphirine + spinel + rutile. Liquid was found in run products at 9 kbar and temperatures of ≥1100°C, and at pressures of 11-13 kbar and 1150°C. At higher pressure, garnet coexists with orthopyroxene, alkali feldspar, plagioclase, phlogopite, sapphirine, rutile and spinel. These experimental results constrain the P-T conditions of garnet-free, phlogopite-bearing orthopyroxene granulite from the Howard Hills.
Palaeoproterozoic ultra-high temperature (UHT) metamorphism of the Lewisian complex in South Harris and the North Atlantic regions were examined. The characteristic features of the UHT metamorphism in the South Harris can be summarized as a) mineral textures and constituent mineral associations are concordant with anti-clockwise metamorphic history, b) presence of subduction-related various precursory rocks lead to formation of various types of sapphirine-bearing rocks, and c) heat source that established UHT metamorpism in South Harris is the emplacement of igneous suite in the magmatic arc at c. 2.18-1.87 Ga. In the North Atlantic region, possible UHT metamorphism were reported from the Boothia-Somerset granulite terrane, Taltson-Thelon orogen and Kolvista-Umba zone during 1.9-2.0 Ga. Precise correlation between UHT in South Harris and these terranes are not clear, although magmatic arc or island arc are considered to be reasonable tectonic setting for the UHT metamorphism in the Palaeoproterozoic North Atlantic region.
We report ultrahigh-temperature (T > 950°C) peak metamorphic conditions for the Southern Marginal Zone of the Archean Limpopo Belt in South Africa. Pelitic granulites of the zone have a typical mineral assemblage of garnet + biotite + orthopyroxene + cordierite + quartz + K-feldspar + plagioclase ± sillimanite, to which the latest geothermobarometers are applicable. The revised P-T conditions were obtained from a ternary feldspar geothermometer using antiperthitic feldspars in leucocratic granulite (920-980°C), Al solubility in orthopyroxene in pelitic granulite (970-1020°C), and a revised garnet-orthopyroxene geothermometer corrected for retrograde Fe-Mg exchange (920-990°C). The estimated peak temperature of metamorphism is about 100°C higher than previously estimated based on conventional Fe-Mg exchange geothermometers (800-870°C), and indicates a high geothermal gradient of ∼35°C/km in Archean continental lower crust.
The Kontum massif in central Vietnam consists of low-grade schists and amphibolite- to granulite-facies metamorphic rocks, that have been intruded by S-type and I-type granites. This terrane was formerly considered to be composed mainly of Archean granulites (the Kannak Complex), Proterozoic amphibolite-facies metamorphic rocks (the Ngoc Linh Complex) and low-grade schists (the Kham Duc Complex). They were thought to be the basement of the Indochina Craton in south-east Asia. The Kannak Complex is dominated by pelitic-semipelitic gneisses metamorphosed under high- to ultrahigh-temperature (UHT) conditions into granulite-facies. A minor amount of mafic and calc-silicate rocks are also intercalated within the gneisses. The major types of ultrahigh-temperature pelitic metamorphic rocks in this complex are garnet-orthopyroxene-sillimanite-cordierite gneiss, orthopyroxene-bearing garnet-cordierite-silliamnite-biotite gneiss and garnet-orthopyroxene charnockitic gneiss. The highest-grade metamorphic condition is determined from garnet-orthopyroxene-sillimanite-cordierite gneiss, which indicates that multi-stage symplectite formation during retrograde stage started from isothermal decompression in UHT condition (1000°C<). Because of the high amount of pyrope (up to 59 mole%) in garnet and high-Al2O3 in orthopyroxene (up to 10 wt%) , these minerals were unstable in the P-T conditions during the retrograde stage. On the other hand, the newly found garnet-clinopyroxene-orthopyroxene granulites (eclogitic ultrahigh-temperature mafic granulite) from the Ngoc Linh Complex shows a series of changes in divariant assemblages from garnet-clinopyroxene-quartz to hornblende-quartz through clinopyroxene-orthopyroxene-plagioclase-(garnet). It was identified that these rocks were formed as a result of their metamorphic evolution of isothermal decompression followed by nearly isobaric cooling. The ultrahigh-temperature metamorphic rocks in the Kontum massif, which are exposed along the Dac To Kan shear zone, show a clockwise pressure-temperature path with the peak metamorphic condition of ca. 12 kbar and ca. 1050°C (M1 metamorphism). High-pressure M0 metamorphism (ca. 17 kbar<, ca. 1000°C) as part of the prograde metamorphism and low-pressure (but still ultrahigh-temperature) M2 metamorphism (9-10 kbar, ca. 1000°C) as part of the retrograde metamorphism during the clockwise pressure-temperature evolution are also recognized. The widely reported Permo-Triassic metamorphic event (ca. 240-260 Ma) from the ultrahigh-temperature metamorphic rocks would indicate a rapid metamorphic evolution from M0 stage to low-pressure and low-temperature retrograde stage (later M2 stage). The recently determined Sm-Nd internal isochron age of 670 Ma for the garnet amphibolite and monazite-CHIME age of ca. 480-500 Ma for the pelitic granulites from the Kannak Complex would also indicate that the Kontum massif had also undergone the Pan-African metamorphic event. The present results indicate that ultrahigh-temperature metamorphic rocks from the Kontum massif are remnants of previously metamorphosed rocks that might be derived from the Gondwana super continent and then re-metamorphosed by the Permo-Triassic metamorphism during the continents collision in eastern Asia.
Garnet-bearing mafic granulites from the Ngoc Linh Complex of the Kontum massif in central Vietnam occur as lenses or blocks within felsic gneisses. Some of these granulite blocks show some evidences for ultrahigh-temperature and high-pressure metamorphism. Samples studied here consist mainly of garnet (Alm34-48, Prp27-34, Grs18-24, Sps0-3), clinopyroxene (Mg# = 0.73-0.87) and quartz, which are relatively large grains. The garnet and clinopyroxene are surrounded by symplectites composed of orthopyroxene, plagioclase, magnetite and sometimes spinel. Therefore, the presumed equilibrium peak-pressure assemblage could be of garnet, clinopyroxene and quartz, which was stable at eclogite-facies conditions. Dual-stage generated symplectites involving orthopyroxene and plagioclase were identified among garnet, clinopyroxene and quartz. The first-generated symplectite is composed of Al- and Mg-poor orthopyroxene (Mg# = 0.61-0.68, Al2O3 = 1.5-2.6 wt%) and Na-rich plagioclase (An45-53). The second symplectite is composed of Al- and Mg-rich orthopyroxene (Mg# = 0.69-0.72, Al2O3 = 4.1-5.3 wt%), Na-poor plagioclase (An86-93), spinel (Mg# = 0.42-0.48) and magnetite. The compositional differences between the two symplectites can be interpreted to be due to the difference in metamorphic reaction stages. The first symplectite had been formed by the reaction between garnet, clinopyroxene and quartz, which occurred during the decompression process just after peak-pressure conditions. The second symplectite could be produced during the late breakdown of garnet. Temperature and pressure conditions around peak-pressure conditions are estimated from the garnet-clinopyroxene thermometries and from the reaction of jadeite + quartz = albite, respectively. The results indicated peak-pressure conditions as ca. 950°C and 1.6 GPa. The pressure-temperature conditions for the formations of first- and second-symplectites were 1000°C at 1.3 GPa and 850°C at 0.8 GPa, respectively. Whole mineral assemblages of the mafic granulites show that the metamorphic evolution took place under completely dry conditions. The textual observations as well as the pressure-temperature estimations indicate a near isothermal decompression process for this granulite. This evolution path is consistent with that of ultrahigh-temperature pelitic granulites from the Kannak Complex. Thus, the possibility of simultaneous ultrahigh-temperature metamorphic event in the Kontum massif can be argued.