Petrology of the dykes from the Waziristan Ophiolite, NW Pakistan

Abstract

The Waziristan Ophiolite is located in the suture zone between the Indian Plate to the east and Afghan Block to the west. It is highly dismembered and divisible into three main sheets or nappes, which from east to west are: the Vezhda Sar Nappe, entirely comprised of pillow basalts; the Boya Nappe, made up of ophiolitic melange with an intact section in its basal part; and the Datta Khel Nappe, consisting mainly of sheeted dykes with smaller proportions of other components. Faunal evidence suggests that the ophiolite is of Tithonian-Valanginian age. It was thrust over the Mesozoic shelf-slope sediments of the Indian Plate to the east during the Paleocene and is unconformably overlain by sedimentary rocks of Early to Middle Eocene age to the west. Beside the sheeted dykes, best exposed in the hanging wall of the Datta Khel Thrust ENE of Datta Khel, the ophiolite also contains isolated dykes. These are doleritic and basaltic in composition. The dykes contain high Na 2O contents and high FeO t/MgO and LILE/HFSE ratios, and low TiO 2 (<0.1 wt%) and K 2O contents. Non-depletion of Nb and high LILE/HFSE ratio negate, respectively, an island-arc or mid-ocean ridge setting for these dykes. Enrichment in the LILE suggests the involvement of a crustal component driven by fluids along the subduction zone. Several geochemical parameters suggest that the dykes of Waziristan Ophiolite have transitional characteristics between mid-ocean ridge basalt and island-arc tholeiite. It is therefore proposed that these dykes may have originated in a back-arc basin tectonic setting.