1. Geochemistry and geochronology of the Rajmahal Flood Basalt Province, northeastern India: Genetic links to Kerguelen hotspot activity.
- Author
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Baksi, Ajoy K
- Subjects
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FLOOD basalts , *GEOLOGICAL time scales , *GEOCHEMISTRY , *MAFIC rocks , *GEOCHRONOMETRY , *OLIVINE - Abstract
New geochemical and geochronological data lead to a fuller understanding of different aspects of the Rajmahal flood basalt province. Earlier work was on surface Rajmahal flows as well as from the nearby Bengal Traps. Samples from a ~300 m borehole near the northwestern corner of the Rajmahal lavas, analyzed herein, reveal the presence of carbonatitic (aillikitic) material. This small province, including the Bengal Traps, thus exhibits quartz and olivine tholeiites, andesitic basalts, alkali basalts as well as carbonatitic material. The Sylhet Traps, lying ~500 km to the east of the surface Rajmahal flows, show geochemical similarities, but were probably formed ~10 Myr after Rajmahal volcanism. Sr–Nd isotopic plots suggest that (a) not all lavas in the Rajmahal Traps were homogenized isotopically prior to eruption and (b) the Sylhet Traps show a much wider variety of isotopic values at eruption, possibly caused by alteration. Published and new 40Ar/39Ar ages for both the Rajmahal and other mafic rocks postulated to have been formed by the Kerguelen Hotspot in the Southern Indian Ocean, are critically examined; emphasis is placed on the freshness of the rocks dated. It is shown that a number of 'ages' for both the Rajmahal Province, and for rocks recovered from the Kerguelen Plateau area, should be rejected as invalid measures of the time of crystallization, as altered material was dated. For the surface Rajmahal material, valid ages span ~120 (quartz tholeiites) to ~116 Ma (andesitic basalts); alkali basalts and olivine tholeiites (Bengal Traps) as well as the aillikite were also formed within this time interval. The postulated genetic link to the Kerguelen Hotspot and basalts in the Southern Indian Ocean appears valid; in particular, a section of the southern Kerguelen Plateau (~58°S; 75°E) has strong geochemical and temporal links to the Rajmahal Traps. [ABSTRACT FROM AUTHOR]
- Published
- 2022
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