GEOCHEMISTRY OF PLIOCENE/QUATERNARY BASALTS ALONG NORTH TABRIZ FAULT, NW IRAN

Pliocene/Quaternary volcanism accompanied strike-slip related transtensional deformation along the Northern Tabriz Dextral Fault (NTDF) in the northwest Iran. These volcanic rocks are represented by tephrite, olivine basalts, basdalt, trachi-basalts, trachi-andesite. They can be divided into three different sub-groups on the basis of their mineralogy and geochemical features. Alkaline leucite-bearing basaltic rocks, are characterized with high ratios of K₂O/Na₂O high content LILE and low values of high field strength elements like Ti, Nb and Ta. They are display fractionated REE patterns and based on different discrimination diagrams show similarity with arc related magmas. The olivine basalt studied samples shows close similarity to OIB with high content of TiO₂, Nb, Ta, fractionated REE pattern and primitive mantle normalized spider diagrams. However, compared with a global average of OIB, they are display slightly higher large ion lithophile element (LILE) and lower high field strength element (HFSE) concentrations, features that resemble slightly the arc magmas and suggest that the source of the magmas may have been slightly contaminated by slab-derived fluids. Basaltic to trachy-basaltic rocks have simple mineralogical composition with plagioclase and clinopyroxene without feldspatoied, olivine and k-rich minerals like phlogopite. They are Na₂O rich and their REE chonderite normalized patterns are moderately fractionated. The large variation in the ratios of different incompatible trace elements like Zr/Nb, Zr/Ba and Ba/La also suggest that crustal contamination is affected the magmas during ascent of magma. Pliocene/Quaternary basaltic magmas along dextral North Tabriz Fault is erupted after formation andesitic to dominant dacitic volcanic rocks with adakitic geochemical characteristics rocks such as highly fractionate REE pattern, high Sr/Y ratio. Generation of adakitic rocks may be related to increased temperatures in the subduction zone due to mantle upwelling. This may have also given rise to melting of the subducted oceanic crust in the eclogite field where garnet is stable. Subsequent asthenospheric upwelling could have caused direct melting to produce the alkaline magmas, these melts having the chemical signature of the asthenosphere slightly metasomatised by slab fluids. Rupture of the continental lithosphere by strike-slip-related transtensional deformation might have caused decompressional partial melting of the asthenospheric mantle and generating alkali basalts in northern part of North Tabriz Fault. Probably continental lithosphere rupturing is occurred following of slab breakoff/delamination processes following closure of Neo-tethys and collision of Arabian plate with microplate of Iran.