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.

A comprehensive study based on U–Pb and Hf isotope analyses of zircons from gneisses has been conducted along the western part (Babina area) of the E–W-trending Bundelkhand Tectonic Zone in the central part of the Archaean Bundelkhand Craton. 207Pb–206Pb zircon ages and Hf isotopic data indicate the existence of a felsic crust at ~ 3.59 Ga, followed by a second tectonothermal event at ~ 3.44 Ga, leading to calc-alkaline magmatism and subsequent crustal growth. The study hence suggests that crust formation in the Bundelkhand Craton occurred in a similar time-frame to that recorded from the Singhbhum and Bastar cratons of the North Indian Shield.

Ion microprobe (SIMS) dating shows that three plutonic rock bodies representative of the major, ∼15 000 km2, late tectonic (late-D3), plutonic rock series in the Central Iberian Zone of the Hercynian belt in west-central Iberia have indistinguishable zircon crystallization ages. Ledrada biotite granite and Colmenar cordierite-bearing biotite granite show gradual transitions in field appearance, petrography and chemistry and have statistically indistinguishable weighted average ages of 306.8 ± 1.9(2σ) Ma and 306.5 ± 1.5(2σ) Ma, respectively, which indicates that they originated during a single event involving a heterogeneous magma which notably varied in Al-content. The third rock body, Navahermosa meta-gabbronorite, has a weighted average zircon crystallization age of 305.6 ± 1.4(2σ) Ma, statistically indistinguishable (variance analysis, F-test, α = 0.05) from the granites. Zircon crystals in the gabbronoritic rock are anhedral, skeletal, millimetre-sized and partake in main magmatic textures, whereas the zircon grains in the granites are of more common appearance, much smaller, usually euhedral and enclosed in main magmatic crystals. As gestation times of granitic zircon, the time between zircon crystallization and magmatic intrusion, may be up to 5 Myr, the crystallization age of the gabbronoritic zircon may be the best estimate of the time of emplacement of the magmatic complex. Our study indicates co-existence of basic and silicic magmas in the Hercynian crustal section at c. 306 Ma, suggesting common genetic control. The two granitic rocks carry inherited zircon ranging from c. 1300 to 330 Ma, indicating that pre-Hercynian basement rocks of Proterozoic to Palaeozoic age contributed to the granitic magma.