VOLCANOLOGY AND TECTONO-PETROGENESIS OF QUATERNARY BASALTIC VOLCANOES IN THE GEHGHAN-E BALA REGION, SOUTHEASTERN IRAN: IMPLICATIONS FOR EVOLUTION OF THE MAKRAN SUBDUCTION ZONE

The Quaternary basaltic volcanoes of Gehghan area are located in Systan-Baluchestan province in southeastern Iran. The area is situated near the collision junction of three different structural zones of Central Iran, Lut Block, and Makran region. These strombolian type volcanoes which often show signs of phreatomagmatic eruption, are exposed along a SW-NE trending fault. The volcanic rocks consist mainly of baslatic scoria, cannon ball and spindle bombs which overlain andesite and basaltic andesitic rocks of Miocene-Pliocene and Plio-Quaternary age. Major element variations and petrographic observations provide evidence for fractionation assemblage of olivine, clinopyroxene and plagioclase, which are found as both phenocrysts and groundmass constituents, along with magnetite and biotite as accessory minerals. Different chemical discrimination diagrams of the basaltic rocks, and study of their eruptive environments as well as major, trace, and rare earth element analyses suggest that the Gehghan basalts are calc-alkaline in nature and have affinities with island-arc basalts. Their MORB normalized trace element patterns show enrichment in Th, Ce, P and Sm, whereas the other incompatible elements such as Ta, Nb, Zr, Hf, Ti, Y and Yb remain at low abundance. REE distributions and chondrite normalized patterns also show an enrichment in LREEs relative to HREEs, a pattern similar to calc-alkaline rocks of island arc settings. Thus, the Gehghan basalts show characteristic of subduction related magmatic rocks which formed by partial melting of the mantle wedge above the subducted lithosphere slab as a result of the subduction of the southern section of Neotethys under the Makran region.