The Precambrian Southern Granulite Terrane (SGT) of south India is well-known for the preservation of high- to ultrahigh-temperature (HT-UHT) granulites, prominently exposed in its central part forming a linear belt referred to as the Kambam UHT belt. This belt also hosts widespread occurrences of mafic granulites that are observed in close spatial association with the HT-UHT granulites. This study presents detailed petrology, geochemistry and geochronology of representative mafic granulites from the area to understand their petrogenesis and tectonic setting. The results demonstrate that mafic granulites are low- to medium-K tholeiites, with continental arc affinity, formed by the partial melting of a subduction-modified enriched mantle source. The composition of the parent mantle source is modelled with a spinel/garnet lherzolite contribution ratio between 100/0 and 70/30, suggesting the mixing of spinel and garnet bearing melts during asthenosphere upwelling. Zircon U–Pb geochronology of mafic granulites constrains their emplacement between 612 Ma and 625 Ma, that subsequently underwent metamorphism between 581 Ma and 531 Ma. This overlaps with the timing of HT-UHT metamorphism in the Kambam UHT belt bracketed between 593 and 532 Ma. Zircon Hf isotopic studies reveal parent magma generation from reworked melting sources involving Archean and Proterozoic components. These results propose an alternative heat source for the formation of HT-UHT granulites in the Kambam UHT belt which can be designated as a major terrane boundary within the SGT.

A mafic amphibole-bearing granulite with porphyroblastic garnet was investigated to evaluate: (1) the rare earth element (REE) partition among garnet, zircon, orthopyroxene and amphibole during the metamorphic evolution; (2) the significance of the REE distribution along lobes and bights of reabsorbed garnet rim; and (3) REE distribution coefficient values (DREE) suggestive of chemical equilibrium, assuming garnet as a reference. The results have been compared with those deriving from an intermediate granulite containing porphyroblastic garnet, without amphibole. Porphyroblastic garnet from both samples is rimmed by a continuous corona formed during post-peak decompression characterized by REE-enriched lobes and REE-poor bights. The amphiboles from corona have various REE abundances, reflecting a different dissolution rate of original garnet rim. The initial slow rate of garnet dissolution caused high REE concentration in the new garnet rim due to intra-crystalline diffusion, leading to the formation of REE-poorer amphiboles in corona. Subsequently, under an increasing geothermal gradient and fluid-present conditions, the faster dissolution of garnet determined the formation of bights and the transfer of REEs towards the corona. The timing of garnet growth and its dissolution were checked by U–Pb zircon ages. The zircons dated from 339 Ma to 303 Ma in two rock types combined with the garnet domains (core, outer core, rim) show similar distribution of patterns relative to heavy rare earth elements for zircon and garnet (DHREEzrn/grt), suggesting chemical equilibrium. Zircons dated at c. 300 Ma do not appear in equilibrium with REE-rich garnet lobes, and younger zircons (278 Ma) show a new equilibrium with REE-poor garnet bights. On this basis, the DHREEamph/grt values obtained in specific textural sites might be interpreted as suggestive of equilibrium under granulite conditions.

In the Proterozoic complex of the Schirmacher region of East Antarctica, a retrograde pressure–temperature (P–T) history has been inferred through quantitative geothermobarometry and fluid inclusion studies of the mafic granulites. Microthermometric investigations of the fluid phases trapped in quartz and garnet identified three types of inclusions, namely, earliest pure CO2 inclusions (0·987–1·057 g cm−3), CO2–H2O inclusions and aqueous inclusions.

The temperature and pressure of metamorphism have been estimated through different calibrations of geothermometers and geobarometers. The mineral reactions and compositional zoning in the minerals record P–T conditions from nearly 837 ± 26°C, 7·1±0·2 kbar to 652 ± 33°C, 5·9 ± 0·3 kbar. A good correlation between the fluid and mineral data is observed. The isochores typical of highdensity CO2 fluids fall well within the P–T box estimated by mineral thermobarometry. The abundance of primary CO2 inclusions in early metamorphic minerals (notably quartz and primary garnet) and the general correspondence between fluid and mineral P–T data indicate a ‘fluid-present’ carbonic regime for the high-grade metamorpism; however, from the present data largescale CO2 advection could not be envisaged. The subsequent stages involved a decrease in CO2 density, a progressive influx of hydrous fluids and the generation of retrograde amphibolite facies metamorphism in the area.

The estimated P–T conditions of the region suggest that the rocks were metamorphosed at a depth of 19–24 km, with a geothermal gradient of c. 3°5C km−1. The estimated P–T conditions of the rocks imply a clockwise P–T–t path with a gradual decrease in temperature of around 250°C and a decrease in pressure of around 1700 bar. They have a dP/dT gradient of ≈7 ± l bar °C−1, arguing for an isobaric cooling history of the terrane under normal thickened crust after the underplating of mantle-derived material.