GEOCHEMICAL CHARACTERISTICS OF MAJOR OPHIOLITE MASSIFS IN IRAN: TECTONIC IMPLICATIONS

We compare the Upper Cretaceous extrusive ophiolitic sequences and diabase sheeted dikes in eight ophiolites from Iran. Peridotites in the northwestern Khoy and western Kermanshah ophiolites are mainly clinopyroxene bearing lherzolite and harzburgite, contain large bodies of cumulate gabbro cut by individual diabase dikes, but have no diabasic sheeted dikes. The 1100 m thick Khoy extrusive sequence contains pillow lava and phyric to aphyric basaltic to basaltic-andesitic sheet flows. In comparison, the Fanuj ophiolite in Makran (southeast Iran) contains mainly harzburgite and clinopyroxene bearing harzburgite where layered cumulate gabbro is the thickest and diabasic sheeted dikes is the most extensive in Iran. The other ophiolitic massifs in Nosratabad (east Iran), north of Fariman, south of Fariman-Torbat-e Heydarieh, Sabzevar (north of Central Iranian microplate), and south of Dehshir (west of Central Iranian microplate) have harzburgitic peridotite, small bodies of cumulate gabbro, and diabasic sheeted dikes.

In these areas, wehrlitic intrusions cut across the plutonic ophiolitic sequence. Pillow lava and sheet flow in the extrusive sequence of the Khoy ophiolite have a T-MORB composition compared to those with T-MORB and E-MORB compositions in the Kermanshah ophiolite. Primitive mantle-normalized spider diagram patterns of the extrusive sequence and diabasic sheeted dikes of the Fanuj, Nosratabad, south of Fariman-Torbat-e Heydarieh, and south of Dehshir eastern ophiolites display clear Nb depletion and LILE enrichment compared to the HREE, and sporadic Ti and Ta depletion. These samples plot in the subduction zone field of immobile element-based tectonomagmatic ternary diagrams. The sheet flow and pillow lava at the base of the extrusive sequence of the Sabzevar ophiolite show P-MORB characteristics suggesting that mantle plumes or asthenospheric flow played a role in the magmatic evolution of the ophiolite. The diabasic sheeted dikes and the extrusive sequence of southeast and northeast Iranian ophiolites display the effect of fluids (or melt), that emanated from subducted slab, on the mantle wedge. The northwest and west Iranian Upper Cretaceous ophiolites have a predominantly MORB composition compared to the east and northeast ophiolites that show supra-subduction characteristics.