Abstract A mafic magmatic sequence of the Bhandara–Balaghat Granulite (BBG) Belt is represented by gabbroic rocks containing orthopyroxene (Opx)–clinopyroxene (Cpx)–plagioclase (Pl)–hornblende±quartz±garnet and showing tholeiitic affinity. These rocks are divided into two groups: (I) garnet-bearing; and (II) garnet-free. The garnet-bearing group is characterized by nearly flat REE patterns. In the multi-element plots, Sr, Zr and Ti show negative anomalies, indicating plagioclase, Ti-magnetite and apatite fractionation. The garnet-free rocks are geochemically subdivided into two subgroups: IIa and IIb. Subgroup IIa is marked by flat REE patterns; the LREE shows 20–30 times chondrite abundances and small positive Eu anomalies. Multi-element patterns show negative anomalies of Nb, P and Ti. Subgroup IIb is characterized by slightly enriched patterns; the LREE shows 10–60 times chondrite abundances. The REE patterns for the Subgroup IIb show moderately to highly fractionated LREE with flat HREE. Multi-element plots show negative anomalies in Nb, Ti and Zr. The Nd–Ce relationship suggests that mafic granulites of the BBGs are derived from higher degrees (Group I, c. 15–30%; Subgroup IIa, c. 20–40%; and Subgroup IIb, c. 18–35%) of partial melting of variably enriched mantle sources, followed by the evolution of the parental melt by fractional crystallization of Opx–Cpx–Pl. The geochemical signatures also suggest that the magma was further modified by crustal contamination during the course of its evolution. The Nd (T DM) model ages, which vary from 3.2 to 1.6 Ga, suggest a long-term evolution of the mafic granulites, possibly starting with overprinting of the isotope composition of their mantle source by crustal isotope signatures as a consequence of crustal recycling; evolving by emplacement and crystallization of the protolith at 2.7 Ga, as well as through later tectonotermal events up to granulite-facies metamorphism and exhumation of the BBG Belt during the collision of the Archaean Bundelkhand and Bastar cratons, and the formation of the Central Indian Tectonic Zone (CITZ) at 1.5 Ga.

The results of 50 years of geochronological work in the Limpopo Complex are reviewed. The data define three main age clusters. The oldest, at ca. 3.3 Ga, exists in the Central and Southern Marginal Zones and is defined by magmatic zircon dates. The second, with a genuine spread between 2.7 and 2.55 Ga, occurs in all three zones. It was a period of high-grade regional metamorphism with intense deformation and widespread anatexis, dated also mainly (but not exclusively) by zircon U-Pb. The third cluster is well constrained at 2.02 ± 0.02 Ga in the Central Zone by zircon overgrowths, sparse magmatic zircons, monazite, apatite, Sm-Nd and Lu-Hf garnet dating, Pb/Pb discrete phase and stepwise leaching dating of garnet and titanite, and hornblende Ar/Ar dating. The Paleoproterozoic dates from metamorphic minerals are particularly associated with zones of intense transcurrent shearing at high-grade metamorphism. In the Northern Marginal Zone this event is more protracted, from 2.08 to 1.94 Ga, and defined in medium- to low-grade shear zones. In the Southern Marginal Zone it is absent. The evidence for both Neoarchean and Paleoproterozoic mineral ages, both defining high-grade tectono-metamorphic events, is in part paradoxical and has led to controversies as to the age of a proposed collisional orogeny. Studying the mineral dates in their tectonic context leads to the conclusion that fluid access in deformation, rather than mere reheating, mainly caused their partial resetting in the Paleoproterozoic event. This allows the controversy to be resolved.

The major periods of metamorphism in the Central Zone (CZ) of the Limpopo Complex occurred at 2.0 Ga and in the time range between ca. 2.7 and ca. 2.55 Ga. We investigate intracrustal radioactivity as a possible heat source for the earlier of these episodes. Available airborne radiometric surveys that cover the South African part of the CZ, combined with rock analyses, yield 2.15 μg/g U, 12.3 μg/g Th, and 12,650 μg/g K as a weighted regional average. The corresponding heat production rate at 2.65 Ga is 2.6 μW × m −3 . A steady-state geotherm, calculated assuming uniform [U], [Th], and [K] throughout the crustal column and its thickening to 45 km during the ca. 2.65 Ga event (both arguable on the basis of peak metamorphic pressure-temperature [P-T] data), surpasses temperatures of the peak metamorphism at middle and lower crustal levels, which cluster around the fluid-absent biotite dehydration solidus. Intracrustal radioactivity thus provided a sufficient heat source to account for the metamorphism at ca. 2.65 Ga, and partial melting acted as a lower crustal thermostat. After crustal thickening, up to more than 100 m.y. (dependent on U, Th, and K concentrations) would be needed to approach a new steady state. Predicted regional variations thus account for the long duration of the ca. 2.65 Ga metamorphism. Lower crustal partial melting could have led to diapirism, yielding the steep structures in the CZ, which are not aligned to a regional fabric. Metamorphism ceased after crustal thinning to a normal 30 km. The metamorphic event at 2.0 Ga cannot be explained by this type of process.