Effusive and explosive silicic eruptions during India-Seychelles continental breakup: the 62.5 Ma Dongri-Uttan rhyolite sequence, Mumbai area, western Deccan Traps

Abstract

Large-scale Danian-age (post-K/Pg boundary) Deccan magmatism is well known from the Mumbai metropolitan area, located in the structurally complex Panvel flexure zone along the western Indian rifted continental margin. This compositionally diverse late-Deccan magmatic suite contains subaerial tholeiitic lavas and dykes typical of the main Deccan province, with many features atypical of the Deccan, such as spilitic pillow lavas, “intertrappean” sediments (often containing considerable volcanic ash), rhyolitic lavas and tuffs, gabbro-granophyre intrusions, and trachyte intrusions containing alkali basalt enclaves. Most of these units, previously dated at 62.5 Ma to 61 Ma, are contemporaneous with or slightly postdate the 62.5 Ma India-Seychelles continental breakup and Panvel flexure formation. In the Dongri-Uttan area, two samples of a >50-m-thick, columnar-jointed rhyolite from the Darkhan Quarry and from a section behind the current Uttan Sagari Police Station have previously been dated at 62.6 ± 0.6 Ma and 62.9 ± 0.2 Ma (⁴⁰Ar/³⁹Ar, 2σ errors). New exposures reveal that these two statistically indistinguishable ⁴⁰Ar/³⁹Ar ages correspond to two distinct rhyolite units, separated by well-bedded silicic ash. The columnar rhyolites are microcrystalline, composed of quartz and alkali feldspar, with rare small (1–2 mm), altered feldspar phenocrysts, and no recognisable relict vitroclasts. Given the westerly structural dip, most of their lateral extent is submerged under the Arabian Sea, and we consider them to be possible flood rhyolite lavas. We interpret the ash beds, composed of pumice clasts and glass shards, as a low-grade (nonwelded) vitric ash, derived from a possibly distal Plinian eruption and deposited by fallout. The lavas and ash are peraluminous rhyolites. The lavas are Sr-Ba-poor and Rb-Zr-Nb-rich, and show “seagull-shaped” rare earth element patterns with deep negative europium anomalies. These crystal-poor lavas are “hot-dry-reduced” rhyolites typical of intraplate, continental rift and rifted margin settings. The very different high-field strength element contents of the lavas and the ash indicate compositionally distinct magma batches. The 62.5 Ma Dongri-Uttan sequence provides clear evidence for rapid silicic eruptions of effusive and explosive nature, alternating with each other and sourced from distinct magma chambers and eruptive vents. A newly identified, highly feldspar-phyric trachyte intrusion marks the last phase of magmatic activity in the area, corresponding with late-stage trachyte-syenite intrusions exposed in coastal western India and the Seychelles, and shows that the Mumbai rhyolites and trachytes form a compositional continuum.