GEOCHEMICAL CONSTRAINTS ON PETROGENESIS OF PLEISTOCENE BASALTIC LAVA FLOWS IN THE SHAHRE BABAK AREA, NW OF KERMAN, IRAN: IMPLICATIONS FOR THE EVOLUTION OF URMIEH-DOKHTAR MAGMATIC ASSEMBLAGE

Pleistocene basaltic lava flows, consist of trachybasalt and basaltic trachyandesite and cover an area of 250 Km^2, northeast of Shahre-Babak in southeastern Iran and are bounded by two north-south running faults which are deep fractures that reach deep down to the base of lithosphere.

The major and trace element chemistry indicates that the lavas are dominantly alkaline and mildly calc-alkaline in character. Variation diagrams of SiO₂ with major and trace elements are consistent with fractional crystallization process involving olivine, pyroxene, plagioclase, ± hornblende and Fe-Ti oxides. Both rock types appear to be on the same fractionation trend, implying similar origins, but different degrees of evolution. The MORB normalized incompatible trace element concentrations diagram for both rock types show enrichment in large ion lithophile elements (LILE; e.g., Sr, K, Rb and Ba) and light rare earth elements (LREE; e.g., Ce), but depletion in high field strength elements (HFSE; e.g., Ta, Nb, Ti, Zr, Hf and Y) and heavy rare earth elements (HREE; e.g. Yb). Their trace elements variations are similar with high LILE/HFSE ratios, suggesting the same magma source. According to major, minor and trace element concentrations, Shahre-Babak alkaline basaltic rocks show characteristic of subduction related (active) continental margins; OIB and within-plate tectonic environment. In connection with the Late Miocene collision between Arabia and Central Iran along the Zagros Suture Zone which led to final closure of NeoTethys, the Shahre-Babak mantle-derived alkaline basaltic lava flows are probably collision related (i.e., post-collisional) and their enrichment in LILE and LREE relative to Ta and Nb can be explained either by (a) the presence of a subduction component or the addition of an LILE-enriched, Nb-Ta poor fluid component to the mantle wedge or, (b) the crustal contamination of mantle-derived magmas during their ascent to the surface through assimilation and fractional crystallization (AFC) and or MASH (melting, assimilation, storage and homogenization).