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.

Keywords

Flood basalt / Rhyolite / Trachyte / Continental breakup / Volcanic rifted margin / Deccan Traps / Mumbai

Data availability

All relevant data have been provided in the body of this paper.

CRediT authorship contribution statement

Arunodaya Shekhar: Writing - original draft, Investigation, Formal analysis, Methodology, Funding acquisition. Hetu Sheth: Formal analysis, Investigation, Methodology, Funding acquisition, Writing - original draft. Anmol Naik: Funding acquisition, Conceptualization, Methodology, Writing - review & editing, Investigation, Formal analysis. B. Astha: Funding acquisition, Writing - review & editing, Methodology, Investigation, Formal analysis.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

Field work was supported by the research award project RI/0220-10000618-001 to Sheth from the Industrial Research and Consultancy Centre (IRCC), IIT Bombay. Shekhar and Astha were supported by Prime Minister's Research Fellowships (PMRF, File Nos. 1303100 and 1303103, respectively). Naik was initially supported by an IIT Bombay Institute Post-Doctoral Fellowship (File No. HR-1 (HRM-1)/Rect/33/2022/20003002) and subsequently by a Goa State Research Foundation Post-doctoral Fellowship (File No. PDF2024003). We express our sincere gratitude to Prof. N. Prabhakar for kindly granting us access to the WD-XRF spectrometry facility (SIP Project; WBS Code: IN/22-1111039E-01), the ICP-MS facility, and the SERB-funded EPMA National Facility (IRPHA Grant No. IR/S4/ESF-16/2009(G)) in the Department of Earth Sciences, IIT Bombay. We are grateful to Prof. Prabhakar, Dr. Trupti Chandrasekhar, Mr. Premkumar Verma and Mr. Javed Shaikh for their supervision and assistance during the various laboratory analyses. We thank Gerardo Aguirre-Díaz, Shane Cronin, and an anonymous reviewer for helpful comments on an earlier version of this work. The present manuscript was considerably improved by the constructive reviews of Matt Brueseke and an anonymous referee. The editorial handling of C. Manikyamba and M. Santosh is appreciated.

Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.gsf.2025.102106.

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