40Ar/39Ar dating of the Malwa Plateau subprovince, Deccan Traps, India

Abstract:

Our systematic study of the spatial-temporal evolution of magmatism in the Deccan Traps Large Igneous Province provides new data from the Malwa Plateau subprovince (MP), which lies north of the comparatively well-studied Western Ghats. ⁴⁰Ar/³⁹Ar analysis was performed by incremental laser heating of multigrained plagioclase aliquots, in multiple (typically 4) experiments per sample. Achievable precision is strongly anticorrelated with Ca/K of the plagioclase, reaching ~0.1% (pooled plateau ages, 1 s.d. intralaboratory precision) for some samples with Ca/K< 80. Results have been obtained from samples between ~100-800 m elevation, spanning virtually the entire exposed stratigraphy of the MP. Most of the MP overlaps in age with the older Kalsubai and Lonavala subgroups (~66.3 to 66.0 Ma) of the Western Ghats, but the lowest (134m asl) lava dated is 66.83 ±0.07 Ma, significantly older than anything yet dated in the Western Ghats but indistinguishable from the result of Schöbel et al. (2014) for a stratigraphically low lava elsewhere in the MP. This is consistent with the consensus that the inception of volcanism progressed generally from North to South. Collectively, our data indicate a much slower mean lava accumulation rate for the basal MP, suggesting a progressive buildup of magma production. Our results are consistent with the age model presented by Eddy et al. (2019) based on U/Pb dating of zircons from presumed silicic ashes preserved in red boles between lava flows, except that our data span about twice their elevation range and they did not sample the lower portions of the section where we obtained the oldest ages.