ABOR VOLCANICS: SLAB WINDOW VOLCANISM AT THE INDIA-ASIA COLLISION ZONE

The Abor Volcanics formation is a sequence of tholeiitic to alkaline basalts, exposed at the eastern Himalayan syntaxis in NE India. The volcanics are exposed in the tectonic gap between the Indus-Tsangpo suture (ITS) in the north and west and the Indo-Burman suture in the south.

Fossiliferous sedimentary deposits interbedded with the Abor volcanics indicate a continental to shallow platform environment of Paleogene age. The Abor volcanics formation occurs in a thrust bound window, named the Siang window, beneath the Mishmi Group of rocks which represent a Gondwanan terrane accreted to the Asian margin prior to the India-Asia collision. The exact continuation of ITS is unknown in this area with speculative positions varying from the Himalayan foothills to the north of Namche Barwa massif.

The Abor Volcanics formation is characterized by enrichment in LILE and LREE, low Ba/La ( <20) and La/Ta ratios (<20), low Zr/Nb ratio (8-12) and LREE enriched REE pattern indicating derivation from an OIB-like enriched mantle source. Different authors have related the genesis of Abor Volcanics to continental rifting, back-arc volcanism, continental flood basalt eruption or to foreland basin volcanism.

The proximity of Abor Volcanics to the Indo-Asian suture, it's Paleogene age (indicating eruption synchronous with the India-Asia collision) and it's OIB-like geochemical characteristics are indicative of slab-window related volcanism. It is proposed that the Abor Volcanics erupted along the India-Asia collision zone along a subducted ridge-transform junction at the eastern edge of the neo-Tethys.

Recognition of the Abor Volcanics formation as slab-window volcanics implies it's position on the overriding, Asian, plate and the continuation of ITS south of the volcanic rocks along the Brahmaputra valley and then to the already established suture zone along the Indo-Burman ranges. Also explained is the absence of subduction related calc-alkaline volcanism in the eastern syntaxis of the Himalaya.

Thus, the rocks exposed along the Siang window in of NE India represents a gondwanan terrane correlatable with the Lhasa block of Tibet and with the Central Belt of Myanmar. It is also suggested that the alkaline volcanics of Northern Myanmar (Taungthon, Mt Popa) and of Southern Tibet also represent similar slab-window volcanism.