Abstract We propose a Precambrian megacraton (consisting of two or more ancient cratons), DHABASI in the Indian Shield, which includes the Dharwar, Bastar and Singhbhum cratons. This interpretation is mainly based on seven large igneous provinces (LIPs) that are identified in these three cratons over the age range of c. 3.35–1.77 Ga, a period of at least 1.6 Ga. The absence of any subsequent break-up of DHABASI since 1.77 Ga suggests that this megacraton has existed for the past 3.35 Ga. In addition to their use in recognizing this megacraton, these LIP events may also provide likely targets for Cu–Ni–Cr–Co–platinum group element deposits. We suggest that the megacraton DHABASI was an integral part of supercontinents/supercratons through Earth's history, and that it should be utilized as a distinct building block for palaeocontinental reconstructions rather than using the individual Dharwar, Bastar and Singhbhum cratons.

ABSTRACT Large igneous provinces and associated silicic magmatism can have a significant global climatic effect, so we explored the relationship between the large igneous province record and the ca. 580 Ma Gaskiers glaciation. The late Ediacaran glaciation exists on at least 14 different paleocontinental blocks, and assuming synchroneity, this Gaskiers glaciation was likely of short duration, with estimates ranging from 1.6 m.y. to 340 k.y. The Central Iapetus magmatic province event found in Laurentia, Baltica, and West Africa consists of multiple pulses in the range 620–520 Ma, with the ca. 580 Ma pulse particularly well developed in North Africa. Based on the age matches of 580–570 Ma Central Iapetus magmatic province pulses and the Gaskiers glaciation, and taking into consideration that there is no robust evidence for a major meteorite impact at the time of the Gaskiers onset, we propose that: (1) the initial silicic ca. 580 Ma pulse of the Ouarzazate event (Anti-Atlas of Morocco) helped to trigger the Gaskiers glaciation, and (2) global warming associated with the subsequent ca. 579–570 Ma continental flood basalts, marking the second stage of the Ouarzazate event, helped to end the ice age.

Abstract A transition from gabbro to eclogite has been investigated at Vinddøldalen in south-central Norway, with the aim to link reaction textures to metamorphic zircon growth and to obtain a direct U–Pb zircon age of the metamorphic process. In the different rocks of the transition zone zircon occurs as (I) igneous prismatic grains, (II) metamorphic polycrystalline rims and pseudomorphs after baddeleyite, and (III) as tiny (<10 µm) bead-like zircon grains. Textural relations suggest that type II zircon formed by breakdown of baddeleyite in the presence of silica, whereas Fe–Ti oxides were the main Zr source for the type III zircon. Subsolidus liberation of Zr and formation of bead zircon took place by oxyexsolution of titanomagnetite during fluid-assisted metamorphism, and by resorption of Fe–Ti oxide in rock domains that were completely recrystallized to eclogite. SIMS (secondary ion mass spectrometry) and TIMS (thermal ionization mass spectrometry) dating provides comparable U–Pb ages of magmatic zircon and baddeleyite. Baddeleyite (TIMS) yielded an age of 1457±11 Ma for the gabbro emplacement. Bead-type metamorphic zircon from eclogite gave 425±10 Ma (TIMS) dating the metamorphic transition from gabbro to eclogite in the upper basement of the Lower Allochthon in the south-central Scandinavian Caledonides.