Various genetic models have been proposed for the supergiant Proterozoic Broken Hill Pb–Zn–Ag deposit largely based on geological and geochronological evidence. Here we present Zn, Cd and S isotope compositions as well as Zn/Cd ratios of sphalerite from Broken Hill and minor Broken Hill-type deposits (Australia) to help constrain these models but focus on syngenetic and magmatic–hydrothermal processes, since epigenetic models can be rejected because the orebodies were deformed and metamorphosed by the Olarian Orogeny. Values of δ34SVCDT, δ66ZnAA-ETH and δ114CdNIST SRM 3108 for sphalerite from Broken Hill range from +0.27 to +4.73 ‰, −1.15 to +0.46 ‰ and −0.48 to +0.01 ‰, respectively, while those for the smaller Broken Hill-type deposits range from −5.11 to +1.28 ‰, −0.97 to +0.10 ‰ and −1.02 to +2.59 ‰, respectively. By combining published S isotope data of sulfides from the Broken Hill district with those obtained here, the sources of sulfur via thermochemical sulfate reduction, bacterial sulfate reduction and a magmatic origin cannot be distinguished. However, when the S isotope compositions are considered along with the broad range of Cd and Zn isotope data for sphalerite, which are among the lightest and heaviest yet reported for a sulfide deposit, the isotopic datasets are consistent with low-temperature biogenic processes associated with syngenetic deposition of sulfides. Cadmium isotope compositions when coupled with Zn/Cd ratios of sphalerite have previously been used to classify Pb–Zn deposits, including low-temperature, high-temperature and exhalative ores. However, the Zn/Cd ratios of sphalerite from Broken Hill cannot be used for such classification purposes.

Monazite is a light rare-earth element (LREE)-bearing accessory phase common in felsic granitic rocks, and strongly influences LREE concentrations in granites and the chemistry of melts and residues formed during partial melting. Cheralite-rich (high-Th) monazite-(Ce) has been recorded as rims on partially dissolved cores in a suite of granulite-facies migmatites generated by disequilibrium melting. High-Th monazite has previously been recorded only in granitic-pegmatite occurrences. Subidiomorphic monazite-(Ce) from leucosome, melanosome and mesosome consists of complexly zoned cores (up to 300 µm in diameter) containing 5.17–9.87 wt.% ThO2, overgrown by essentially unzoned rims (containing up to 21.4 wt.% ThO2). Uranium contents are average for Th-rich monazite-(Ce) (0.22–3.17 wt.% UO2). Th enrichment (relative to LREE) in monazite-(Ce) rims is ascribed to the presence of fluorine-bearing melts (formed during the incongruent breakdown of biotite to produce a water-undersaturated melt), allowing the formation of REE-fluoride complexes in the melt, coupled with the increased charge balanced substitutions Th4+ + Si4+ ⇌ REE3+ + P5+ and Th4+ + Ca2+ ⇌ 2REE3+ into monazite-(Ce). Fractionation of Th and U (reflected by an increased in Th/U in rims relative to cores) may have occurred due to the removal by a CO2-rich fluid phase in the melt. These interpretations are consistent with elevated CO2 and F contents of granitic liquids produced during the water-undersaturated breakdown of biotite under granulite-facies conditions. Evidence for restricted monazite-(Ce)-melt equilibration and rapid melt removal is provided by the ubiquitous presence of partially embayed cores and unzoned rims.

Kornerupine-rich layers up to several centimetres thick with minor sillimanite, spinel, Fe oxide and ilmenite occur in a diopsidite in sillimanite-cordierite gneiss south of Beraketa (24°27′S, 46°48′E), southern Madagascar. Kornerupine, sillimanite, spinel and hematite grains up to 1 mm across have mutual polygonal boundaries indicating textural equilibrium at their crystallisation. Kornerupine has XMg 0.67–0.80 and 0.9 to 2.6 wt.% B2O3. Sillimanite contains up to 2.0 wt.% Fe2O3. Spinel is essentially (Mg,Fe2+) Al2O4 with an XMg range of 0.29–0.40 and exsolution lamellae of Fe oxide. Textural relations demonstrate two limited reactions, each confined to areas less than 500 µm across: (1) Kornerupine and spinel reacted along grain contacts to form very fine-grained tourmaline, corundum and chlorite. The replacing phases are symmetrically zoned with a central tourmaline and hematite, bordered by an aggregate of chlorite, tourmaline and corundum, followed outwards by a rim of chlorite against the kornerupine and spinel. (2) Within kornerupine grains, zoned, round aggregates consist of very fine-grained chlorite, tourmaline and corundum of different composition than in (1). They define the terminal reaction of kornerupine breakdown.

Geothermobarometry indicates that the early kornerupine-bearing assemblage was stable at 7.0 kbar and 700 °C. This P-T point lies close to the retrograde, nearly isothermal trajectory defined independently by nearby sapphirine-bearing assemblages. The fine-grained aggregates formed most likely during further cooling, or by increasing water fugacity.

Metanorite dykes intrude the Banded Gneiss Complex at various places in Rajasthan, N.W. India. They show neither chilled margins nor gradational contacts with the country rock amphibolite or granulite facies gneisses. They have ophitic to subophitic texture with strongly zoned subcalcic clinopyroxene and orthopyroxene, olivine and plagioclase, with subsidiary biotite. During slow cooling a series of reaction coronas developed with garnet forming round biotite, ilmenite and orthopyroxene; hornblende round pyroxenes and orthopyroxene, hornblende ± spinel round olivine, which may be totally replaced. It is inferred that the dykes crystallised from a tholeiitic magma at about 1100-1150 °C and were intruded during the waning stages of granulite facies metamorphism. The corona minerals grew at about 650–700 °C. A series of reactions to account for the development of the coronas is proposed using measured mineral compositions. Although these reactions do not balance for individual corona formation, metamorphism was probably isochemical with Ca, Na, K, Ti, Si and H2O only mobile on the scale of a thin section. Si and H2O were possibly mobile on a larger scale.

Porphyroblastic garnet-bearing metagabbros from the base of the lower crust section of the Serre (southern Italy) exhibit multi-stage dehydration and decompression after the Panafrican emplacement of their protoliths. The first dehydration event produced Am–Opx–Cpx–Pl–Grt as the peak assemblage. Two decompression stages are documented by: (1) coronas of Opx–Pl and Opx–Am, and symplectites of Opx–Am–Pl around clinopyroxene within the porphyroblastic garnet as well as in the matrix and (2) symplectites of Pl–Am–Opx–Grt having different textures around the porphyroblastic garnet. During the second decompression stage, a new local, somewhat intense, dehydration occurred and produced rims of Opx+Pl around the porphyroblastic amphibole, or lenses of Pl–Opx–Am–Spl±Bt between layers of dominant amphibole. A deformation stage separates older from younger reaction textures. The porphyroblastic garnet, its inclusions and the matrix are affected by fractures, which have been overgrown by coronas and symplectites around the porphyroblastic garnet and the amphibole of the matrix. Preferred P–T estimates are: ∼900 °C and ∼1.1 GPa at the metamorphic peak; ∼850 °C and 0.8–0.9 GPa during the formation of corona around clinopyroxene; 750–650 °C and 0.7–0.8 GPa during the formation of corona around garnet. All these textures formed under granulite-facies conditions. The subsequent metamorphic evolution consists of rehydration under amphibolite-facies conditions. The P–T–t path agrees with the path shown by the uppermost migmatites of the Serre section, and the P–T estimates at the top and the bottom of the section are consistent with the thickness (7–8 km) of the lower crustal segment. A contractional regime, which caused a crustal thickening of about 35 km, was followed by an extensional one producing significant crustal thinning; the change of tectonic regime probably occurred about 300 Ma ago when the emplacement of voluminous granitoids and the initial stages of exhumation of the lower crustal section had taken place.

The Mühlig-Hofmann- and Filchnerfjella in central Dronning Maud Land, Antarctica, consist of series of granitoid igneous rocks emplaced in granulite and upper amphibolite facies metamorphic rocks. The area has experienced high-temperature metamorphism followed by near-isothermal decompression, partial crustal melting, voluminous magmatism and extensional exhumation during the later phase of the late Neoproterozoic to Cambrian Pan-African event. Remnants of kyanite–garnet–ferritschermakite–rutile assemblages indicate an early higher-pressure metamorphism and crustal overthickening. The gneisses experienced peak granulite facies temperatures of 800–900 °C at intermediate pressures. Breakdown of garnet + sillimanite + spinel-bearing assemblages to cordierite shows subsequent re-equilibration to lower pressures. An E–W foliation dominating the gneisses illustrates transposition of migmatites and leucocratic melts which evolved during the near-isothermal decompression. Occurrence of extensional shear bands and shear zones evolving from the ductile partial melting stage through semiductile towards brittle conditions, shows that the uplift persisted towards brittle crustal conditions under tectonic W/SW-vergent extension. Late-orogenic Pan-African quartz syenites intruded after formation of the main gneiss fabric contain narrow semiductile to brittle shear zones, illustrating that the extensional exhumation continued also after their emplacement. The latest record of the Pan-African event is late-magmatic fluid infiltration around 350–400 °C and 2 kbar. At this stage the Pan-African crust had undergone 15–20 km exhumation from the peak granulite facies conditions. We conclude that the later phase of the Pan-African event in central Dronning Maud Land is characterized by a near-isothermal decompression P–T path and extensional structures indicating tectonic exhumation, which is most likely related to a late-orogenic collapsing phase of the Pan-African orogen.

Archaean zircon grains from the Oygarden Group of islands in Kemp Land, east Antarctica, record evidence for multiple episodes of recrystallisation, dissolution and growth from the Early to Middle Archaean to the Neoproterozoic. Zircon grains in layered felsic orthogneiss have an age of ∼3650 Ma, a minimum protolith age for this rock. These zircon grains were subsequently affected by a ∼3470 Ma Pb-loss event. Homogeneous felsic orthogneiss that cuts S1, but contains an intense S2 foliation, has disturbed ∼2780 Ma metamorphic zircon cores and rims that suggest a minimum age of ∼2780 Ma for the protolith to the orthogneiss. All zircon U-Pb data display considerable disturbance with further Pb-loss at ∼2400 Ma and ∼1600 Ma, and a major episode of isotopic resetting at ∼930 Ma. The highly disturbed data are related to complexly zoned zircon grains that developed through growth and modification during successive metamorphic events. Zircon cores with relic growth zoning patterns are inferred to have resulted from partial annealing or recrystallisation of older magmatic zircon. Highly luminescent zircon rims that embay cores along curved boundaries are interpreted to have formed through recrystallisation of zircon cores, and not new growth. The ages reported here confirm that central Kemp Land is composed of Archaean crust reworked during the Neoproterozoic Rayner Structural Episode.

In this feasibility study, Sm/Nd and 143Nd/144Nd ratios have been determined on garnets and coexisting mineral and whole-rock phases from a selection of amphibolite facies rocks, granulite facies rocks and an eclogite. In two micaceous schists and an eclogite the distribution coefficient of the Sm/Nd ratio in garnet relative to other phases is in the range 2-3, i.e. comparable to values for phenocrysts in igneous rocks. In two granulite facies rocks the garnets have Sm/Nd distribution coefficients relative to other phases in the range 13-20 and have yielded precise ages.

Garnet from a granulite facies shear zone from the Limpopo belt in southern Africa, of Archaean age, has yielded concordant duplicate ages of 1974±14 Ma and 1988±14 Ma. These results have been interpreted as relating to the last tectonic event in the region, long after the main gneiss-forming event. Garnet from a granulite facies xenolith from a Carboniferous vent in the Midland Valley of Scotland has yielded an age of 356±10 Ma; this is related to the onset of basic magmatism in the region.