Browsing by Author "J. Amal Dev"

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Geochronology of the Vindhyan Supergroup: Implications for the closure of the Proterozoic basins of India
(Elsevier B.V., 2025) Bivin G. George; Jyotiranjan S. Ray; Milan Kumar Mahala; J. Amal Dev; Joseph Kallukalam Tomson; J. Haripriya; Alok Kumar
The sediments of the Vindhyan Supergroup, deposited in the largest Proterozoic basin of India, are known to have recorded changes in the seawater chemistry, changes in global climate, and one of the earliest fossil records of multicellular eukaryotes. These rocks, particularly the upper part of the sequence, are also home to some of the most controversial fossils of the Neoproterozoic lifeforms. Despite receiving enough attention, the evolutionary timeline of the upper part of the supergroup and the timing of the closure of the Vindhyan Basin remained uncertain. Issues related to the stratigraphic correlation within the basin, temporal relation with other Proterozoic basins of India, and validity of the evidence of global events reported from the basin remained unresolved. Here, we present results of Pb-Pb dating of carbonate formations and detrital zircon U-Pb dating of sandstone formations of the upper Vindhyans from the western and the eastern sectors of the basin to resolve some of the above chronological issues. Based on Pb-Pb isochron ages, we establish that the Bhander Limestone formation in the Son Valley sector (eastern) was deposited in two major periods: one at 874 ± 77 (2σ) Ma and the other at 768 ± 36 (2σ) Ma, with breaks in sedimentation in between. Based on the Pb-Pb isochron age of 871 ± 78 (2 σ) Ma, the Balwan Limestone, the topmost carbonate formation in the Rajasthan sector (western), is proposed to be laterally correlatable to the Bhander Limestone in the Son Valley. The Lakheri Limestone formation in Rajasthan is deemed to have no correlatable carbonate formation in the Son Valley. The maximum depositional ages of sandstone formations, determined from zircon U-Pb data, suggest that the top part of the Vindhyan Supergroup is younger than 830 Ma. Our results establish that the closure of the Vindhyan Basin happened sometime during the late Tonian. Therefore, these successions are unlikely to contain evidence of the Cryogenian glaciations or the Ediacaran life. This new age information puts the Vindhyan Basin in the league of at least two other Proterozoic basins of India (i.e., Marwar and Kaladgi), which, contrary to general belief, had remained active during the Neoproterozoic (< 1000 Ma). We did not find any evidence of coevolution of the Vindhyan Basin with other Proterozoic basins of India. © 2025 Elsevier B.V.
Glimmerite and melteigite xenoliths from the Early Cretaceous campto-tinguaite dyke, Nongchram Fault Zone (East Garo Hills), Shillong Plateau, North East India: Evidence for magma mixing and involvement of subducted as well as plume related mantle sources
(The Physiological Society of Japan, 2025) R. B. Ananth; Nittala Venkata Chalapathi Rao; Abhinay Sharma; J. Amal Dev; Tomson J. Kallukalam
We report the occurrence of glimmerite and melteigite xenoliths from a hybrid Early Cretaceous camptonite-tinguaite dyke from the Nongchram Fault Zone, East Garo Hills, Shillong Plateau, North East India. Glimmerite xenolith contains predominantly biotite with subordinate to minor amounts of pyroxene, ferro-dolomite, rutile, and ilmenite. The melteigite xenolith is dominated by pyroxene and contain pseudo-nepheline (altered to analcime) and rutile. The host campto-tinguaite dyke features a distinctive porphyritic-panidiomorphic and a tinguaitic texture with clinopyroxenes as macrocrysts, microcrysts and clots and amphibole as phenocrysts with accessory minerals such as apatite, magnetite, rutile, and ilmenite. In-situ trace element geochemistry of pyroxenes from the xenoliths and the host rock reveals significant enrichment in LILEs such as Ba and Sr, as well as LREEs like La and Ce, suggesting an enriched mantle source. The geothermobarometric data for pyroxenes from the (i) glimmerite and melteigite xenoliths and macrocrysts and (ii) microcrysts and clots from the campto-tinguaite suggest varying crystallization pressures and temperatures, indicating different depths of origin ranging from 23–96 km. The mineral composition, in-situ trace element data of pyroxenes as well as the geothermobarometric study of clinopyroxenes from both the host and the xenoliths, suggest involvement of multiple shallower magma chambers composed of the camptonitic and tinguaitic magmas generated during distinct pulses, thereby forming a complex magmatic plumbing system. The presence of orogenic geochemical signatures in the minerals of xenoliths and anorogenic geochemical signatures in the host campto-tinguaite pyroxenes imply a complex tectono-magmatic setting, with contributions from both the plume and subduction-modified mantle sources. © © 2025 The Geochemical Society of Japan.