Ignimbrite, Ash-Flow Tuff and Basic Volcanics of the Daling Formation of Sikkim Himalaya: Geotectonic Implications

Ignimbrite and ash-flow tuff's have been identified within the graywacke unit of the Daling turbidite sequence of the Sikkim Lesser Himalaya. These, together with basic flows constitute the basal part of Proterozoic Daling Formation. Geochemistry of these rocks indicates that the tuffs are derived from calc-alkaline volcanism while the basic flows have MORB characters. It is concluded that within the Daling ocean (ProtoTethys or Palaeo-Tethys) graywacke was deposited as a trench prism, the tuffs were the products of island-arc explosive volcanism, and the basic flows, of spreading in the back-arc basin. This island arc system disappeared due to tectonic activity which deformed (the first recognisable deformation) the Daling rocks. This event probably represents the Caledonian orogeny in the Himalayas.