Widespread rheomorphic and lava-like silicic ignimbrites overlying flood basalts in the northwestern and northern Deccan Traps

Abstract

The physical volcanology of widespread rhyolites and dacites overlying flood basalt lavas in the northwestern and northern parts of the 65.5-Ma Deccan Traps, India, is poorly known. We show that these are not lava flows but high-grade (welded and rheomorphic) to extremely high-grade (lava-like) ignimbrites deposited by pyroclastic density currents. Evidence for this interpretation includes the following: (i) flat, planar contacts with the underlying basalts and absence of basal brecciated zones; (ii) pervasive millimeter-to-centimeter-scale flow banding; (iii) parataxitic textures indicating very high degrees of compaction and welding; (iv) strong rheomorphic deformation, producing folds of diverse geometries and sizes showing overprinting relationships, with common tight-isoclinal and sheath (eye) folds that are sometimes refolded into F2 and even F3 folds; (v) curvilinear fold hinges with elongation lineations varying from transverse to subparallel to the fold axes; (vi) local boudinage and autobrecciation in flow-banded glassy tuffs; (vii) common and abundant spherulites and lithophysae; (viii) intimate association of tuff and vitrophyre at millimeter to tens-of-meter scales; and (ix) vitroclastic textures showing highly flattened and stretched glass shards and pumice fiamme wrapping around phenocrysts. Whereas rheomorphic and lava-like ignimbrites dominate, some non-welded and welded (eutaxitic) ignimbrites also occur. Voluminous rhyolites and dacites in many flood basalt provinces of the world are recognized as high-grade to extremely high-grade ignimbrites, and the Deccan is no exception. This Deccan explosive silicic volcanism may have significantly contributed to the K/Pg boundary mass extinctions, if contemporaneity can be established by future geochronological work.