In general, Archean rocks exhibit rather ordinary moderate-P-high-T facies series metamorphism; neither blueschists nor any record of deep continental subduction and return are documented. However, the abundance and scale of ultrahigh-temperature (UHT) metamorphic belts from the Neoarchean to the Cambrian imply a significant change in geodynamics during the Neoarchean Era, after which transient sites of high heat flow were available at intervals throughout this period of Earth evolution. Many Neoproterozoic-Cambrian UHT metamorphic belts appear to have developed in settings analogous to modern backarcs that were closed and inverted during crustal aggregation and formation of the Gondwana supercontinent. If backarcs were the general setting for UHT metamorphism, then on a hotter Earth the cyclic formation of supercratons (in the Neoarchean Era) and supercontinents (in the Proterozoic Eon) required the destruction of oceans floored by thinner lithosphere that may have generated hotter backarcs than those as...

Metamorphic Conditions in Orogenic Belts: A Record of Secular Change

Discovery of sapphirine-bearing Mg–Al granulites in the North China Craton: Implications for Paleoproterozoic ultrahigh temperature metamorphism

Abstract Ultrahigh-temperature (UHT) crustal metamorphism is a division of medium-pressure granulite facies metamorphism where peak temperatures of 900–1100°C have been attained at pressures in the range 7–13 kbar. The key indicators of UHT conditions are mineral assemblages involving combinations of sapphirine, garnet, aluminous orthopyroxene, cordierite, sillimanite, spinel and quartz in pelites and quartzites. Experimentally constrained and calculated FMAS and KFMASH petrogenetic grids involving these phases and additional osumilite and melt indicate that sapphirine + quartz is stable only at >1040°C in reduced rocks, that osumilite is restricted to >900°C for pressures greater than 6 kbar and has an ultimate stability limit of 9 kbar in FMAS, and that the orthopyroxene + sillimanite + quartz assemblage is restricted to pressures greater than 8 kbar in KFMASH. These criteria, coupled with the grids isoplethed for the Mg/(Mg + Fe) of garnet and Al-content of orthopyroxene allow the peak pressure-temperature (P-T) conditions of several UHT occurrences to be defined and the post-peak P-T paths delineated. UHT conditions are seldom determined from slowly cooled granulites using conventional geothermometry principally because of the propensity of Fe-Mg exchange thermometry to only record closure temperatures of 700–850°C. However, pressure-convergence calculations for several granulites with UHT mineral assemblages yield back-calculated mineral compositions that are consistent with temperatures of 950–1000°C prior to post-peak Fe-Mg re-equilibration. The best compositional indicator of UHT conditions remains the preservation of high Al2O3 contents (8–12 wt%) in orthopyroxene coexisting with garnet, sillimanite or sapphirine. The P-T conditions and records preserved in the currently documented UHT localities and terranes are varied. Both types of post-peak P-T path isobaric cooling and isothermal decompression (ITD) are recorded from reaction textures in different UHT terranes, and several preserve very similar ITD histories that may reflect the final stage of collisional orogenesis. Although counter-clockwise and clockwise P-T paths have been proposed on the basis of textural observation for some terranes, the prograde P-T histories of most UHT areas are not known. Such information, and further experimental constraints on quartz-absent assemblages at UHT conditions, are of prime importance to interpret further this extreme form of crustal metamorphism.

On the occurrence and characterization of ultrahigh-temperature crustal metamorphism

On ultrahigh-temperature crustal metamorphism

Integrated textural and chemical characterisation of zircon is used to refine the U–Pb geochronology of the Archaean, ultra-high temperature Napier Complex, east Antarctica. Scanning electron microscope characterisation of zircon and the rare earth element compositions of zircon, garnet and orthopyroxene are integrated to place zircon growth in an assemblage context, thereby providing tighter constraints on the timing of magmatic and metamorphic events. Data indicate that magmatism occurred in the central and northern Napier Complex at ca. 2,990 Ma. A regional, relatively low-pressure metamorphic event occurred at ca. 2,850–2,840 Ma. Mineral REE data from garnet-bearing orthogneiss indicate that ca. 2,490–2,485 Ma U–Pb zircon ages provide an absolute minimum age for the ultrahigh temperature (UHT) foliation preserved in this rock. Internal zircon zoning relationships and estimated zircon-garnet DREE values from paragneiss suggest that an absolute minimum age of ultra-high temperature metamorphism is ca. 2,510 Ma, but that it is more likely to be older than ca. 2,545 Ma. We suggest that the high proportion of published zircon U–Pb data with ages between ca. 2,490–2,450 Ma reflects late, post-peak zircon growth and does not date the timing of peak UHT metamorphism.

An integrated microtextural and chemical approach to zircon geochronology: refining the Archaean history of the Napier Complex, east Antarctica

SAPPHIRINE and quartz have been found together in nature for the first time in granulite collected by I. R. McLeod, of the Australian Bureau of Mineral Resources, from a small nunatak (66° 57′ S, 51° 31′ E) about 42 miles east of Amundsen Bay, Enderby Land, Antarctica. This rock belongs to a gneiss–granulite–charnockite terrain (personal communication from I. R. McLeod), and was called blue quartzite in the field. In thin section it is seen to consist of rutilated quartz, cordierite-quartz intergrowths, sapphirine as grains up to 2 mm long, and accessory sillimanite, garnet, hypersthene, rutile, chlorite, zircon and biotite; the first three constituents make up about 98 per cent of the rock. Another granulite from the same nunatak contains no sapphirine, though it may formerly have done so; it consists of quartz, garnet, sillimanite, orthoclase microperthite with unusually low 2 V (35° to 40°), and accessory rutile, sericite and zircon.

W. B. Dallwitz

Papers

Co-existing Sapphirine and Quartz in Granulite from Enderby Land, Antarctica