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Widespread and abundant interstitial silicic glasses in tholeiitic dykes of the Konkan Plain, western Deccan Traps: implications for silicic magma genesis in continental flood basalt provinces

B. Astha1 , Hetu Sheth1, Anmol Naik1,2, Arunodaya Shekhar1

  • Affiliations: 1 Department of Earth Sciences, Indian Institute of Technology Bombay, Powai, Mumbai, 400076, Maharashtra, India; 2 School of Earth, Ocean and Atmospheric Sciences (SEOAS), Goa University, Taleigao Plateau, 403206, Goa, India 

  • Presentation type: Talk

  • Presentation time: Thursday 14:45 - 15:00, Room S150

  • Programme No: 1.9.3

  • Theme 1 > Session 9

Abstract

The petrogenesis of silicic magmas in continental flood basalt (CFB) provinces involves mechanisms ranging from fractional crystallisation to crustal anatexis. Silicic magmatism in the Deccan Traps of India, though minor overall, is concentrated along the western Indian rifted continental margin. We report widespread and abundant interstitial silicic glasses in tholeiitic basalt and basaltic andesite dykes exposed on the Konkan Plain, which were likely feeders to lavas forming part of the kilometers-thick Western Ghats volcanic sequence. Thermobarometric calculations for the dykes indicate co-crystallisation of clinopyroxene (1176--1115 ± 22 °C), olivine (1168--1135 ± 19 °C) and plagioclase (1181--1083 ± 14 °C), at pressures of 3.5--0.1 (± 1.5) kbar, suggesting crystallisation during magma ascent or storage in the uppermost crust.  The interstitial glasses are dominantly rhyolitic (less commonly trachytic and rarely dacitic), and contain microlites of highly evolved plagioclase, pyroxene and olivine. The textures, whole-rock geochemical and mineral chemical compositions of the tholeiitic dykes and silicic glasses, and mass balance calculations, suggest the derivation of these silicic melts by 67-75% closed-system fractional crystallisation of their host tholeiites. However, the residual silicic melts, despite their considerable volumes, were unable to segregate from their mafic hosts. We relate this to H2O loss on decompression during emplacement in the shallow crust, resulting in substantial undercooling, quenching, and trapping of the now-dry, extremely viscous silicic melts. This would explain well why the Western Ghats sequence notably lacks silicic extrusive or intrusive units, despite the widespread production and abundance of silicic melts at depth.