40Ar/39Ar dating of the Mumbai tholeiites and Panvel flexure: intense 62.5 Ma onshore–offshore Deccan magmatism during India-Laxmi Ridge–Seychelles breakup

Abstract

Mumbai, located on the western Indian continental margin, exposes Danian-age Deccan magmatic units of diverse compositions, dipping seaward due to the Panvel flexure. The Ghatkopar-Powai tholeiitic sequence contains seaward-dipping (thus pre-flexure) flows and subvertical (thus post-flexure) dykes. We present new Ar-40/Ar-39 ages of 62.4 +/- 0.7 and 62.4 +/- 0.3 Ma (2 sigma) on two flows, and 62.2 +/- 0.3, 62.8 +/- 0.3 and 61.8 +/- 0.2 Ma on three dykes, showing that this sequence is much younger than the main 66-65 Ma Deccan sequence in the Western Ghats escarpment. The mutually indistinguishable ages of the Ghatkopar-Powai tholeiites overlap with available Ar-40/Ar-39 ages of 62.6 +/- 0.6 and 62.9 +/- 0.2 Ma for the seaward-dipping Dongri rhyolite flow and 62.2 +/- 0.6 Ma for the Saki Naka trachyte intrusion, both from the uppermost Mumbai stratigraphy. The weighted mean of these eight Ar-40/Ar-39 ages is 62.4 +/- 0.1 Ma (2 SEM), relative to an MMhb-1 monitor age of 523.1 +/- 2.6Ma (2 sigma), and indicates essentially contemporaneous volcanism, intrusion and tectonic flexure. This age also coincides with the rift-to-drift transition of the Seychelles and Laxmi Ridge-India breakup and the emplacement of the Raman-Panikkar-Wadia seamount chain in the axial part of the Laxmi Basin. Pre-rift magmatism is seen in the 64.55 Ma Jogeshwari basalt in Mumbai and 63.5-63.0 Ma intrusions in the Seychelles. Post-rift magmatism is seen in the 60.8-60.9 Ma Manori trachyte and Gilbert Hill basalt intrusions in Mumbai and 60-61 Ma syenitic intrusions in the Seychelles. The Mumbai area thus preserves the pre-, syn- and post-rift onshore tectonomagmatic record of the breakup between the Seychelles and the Laxmi Ridge-India. Voluminous submarine volcanism forming the Raman, Panikkar and Wadia seamounts in the Laxmi Basin represents the offshore syn-rift magmatism.