Petrogenesis and timing of volcanism in the Rajmahal flood basalt province, northeastern India

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doi: 10.1016/0009-2541(94)00124-Q
Authors:Baksi, Ajoy K.
Author Affiliations:Primary:
Louisiana State University, Department of Geology and Geophysics, Baton Rouge, LA, United States
Volume Title:Chemical Geology
Source:Chemical Geology, 121(1-4), p.73-90. Publisher: Elsevier, Amsterdam, Netherlands. ISSN: 0009-2541
Publication Date:1995
Note:In English. 64 refs.; illus., incl. 2 tables, geol. sketch map
Summary:A suite of rocks from the Rajmahal-Bengal-Sylhet Traps of northeastern India has been analyzed to ascertain the timing and duration of volcanism and elucidate their petrogenesis. 40Ar/39Ar step-heating studies identified specimens that suffered post-crystallization loss of 40Ar* and indicate the Rajmahal Province was extruded in ∼2 Ma around 117 Ma ago. Trace- and rare-earth-element data suggest the existence of three different types of magmas. Rajmahal quartz tholeiites were formed from primary melts, following considerable gabbroic fractionation. Bengal Trap olivine tholeiites represent lavas formed by large partial melting of mantle material, leaving garnet in the residue. Alkali basalts in the Bengal Traps appear to represent partial melts of mantle containing LILE-enriched sections, rather than very small (<2%) melts of a garnet lherzolite source. Whole-rock δ18O-values for slightly altered tholeiites fall in the range +5.9 to +6.6per mil, indicating mantle-derived melts that have suffered minor crustal contamination; two alkali basalts, formed following considerable crystal fractionation of primary magmas, yield values of ∼ +7.2per mil. Sr-Nd isotopic analyses show two different contamination trends, overlapping those observed in an earlier study of surface Rajmahal quartz tholeiites, with the most primitive material showing 87Sr/86Sr ∼0.70400, 143Nd/144Nd ∼0.51280 at 117 Ma ago. The Bengal Trap olivine tholeiites were formed following assimilation of high-87Sr/86Sr (granulitic?) material. The main contamination trend includes quartz tholeiites from the Rajmahal Traps and alkali basalts from the Bengal Traps. Tholeiites, showing considerable isotopic modification, suggest ingestion of a high-Sr component, unlikely to be upper-crustal material; for the alkali basalts, with high Sr (∼1000 ppm) and Nd (∼55 ppm) contents, incorporation of a few percent of "exotic" material (in the source region?) is indicated. Carbonatite is the probable contaminant, strengthening the postulated link between flood basalt volcanism and carbonatite-lamproites in this area. The occurrence of two lavas with reversed magnetic polarity, in association with the 40Ar/39Ar ages reported herein, suggests the ISEA reversed event is displayed in the lavas of the Rajmahal Traps.
Subjects:Absolute age; Alkali basalts; Alkaline earth metals; Alteration; Ar-40/Ar-39; Argon; Basalts; Carbonatites; Chemical composition; Cretaceous; Crystal fractionation; Dates; Flood basalts; Gabbros; Igneous rocks; Isotope ratios; Isotopes; Jurassic; Lamproite; Lamprophyres; Lava; Magma contamination; Melts; Mesozoic; Metals; Nd-144/Nd-143; Neodymium; Noble gases; Paleomagnetism; Plutonic rocks; Radioactive isotopes; Rajmahal Series; Rare earths; Reversals; Sr-87/Sr-86; Stable isotopes; Strontium; Tholeiite; Trace elements; Volcanic rocks; Volcanism; Asia; Bihar India; India; Indian Peninsula; Jharkhand India; Rajmahal India; Bengal Traps; Fractionation; Northeastern India; Sylhet Trap
Abstract Numbers:96M/638
Record ID:1995041121
Copyright Information:GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from CAPCAS, Elsevier Scientific Publishers, Amsterdam, Netherlands
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