High-Nb hawaiite-mugearite and high-Mg calc-alkaline lavas from northeastern Iran: Oligo-Miocene melts from modified mantle wedge

Tertiary volcanic rocks in northwestern Firoozeh, Iran (the Meshkan triangular structural unit),constitute vast outcrops (up to 250 km2) of high-Mg basaltic andesites to dacites that areassociated with high-Nb hawaiites and mugearites. Whole-rock 40Ar/39Ar ages show a restrictedrange of 24.1 ± 0.4-22.9 ± 0.5 Ma for the volcanic rocks. The initial ratios of 87Sr/86Sr and143Nd/144Nd vary from 0.703800 to 0.704256 and 0.512681 to 0.512877, respectively, in thehigh-Mg basaltic andesites-dacites. High-Th contents (up to 11 ppm) and Sr/Y values (27-100)and the isotopic composition of the subalkaline high-Mg basaltic andesites-dacites indicatederivation from a mantle modified by slab and sediment partial melts. Evidence such as reversezoning and resorbed textures and high Ni and Cr contents in the evolved samples indicate thatmagma mixing with mafic melts and concurrent fractional crystallization lead to the compositionalevolution of this series. The high-Nb hawaiites and mugearites, by contrast, have a sodicalkaline affinity and are silica undersaturated; they are also enriched in Nb (up to 47 ppm) and awide range of incompatible trace elements, including LILE, LREE, and HFSE. Geochemistry and Sr-Nd isotopic compositions of the high-Nb hawaiites and mugearites suggest derivation from amantle source affected by lower degrees of slab melts. Post-orogenic slab break-off is suggestedto have prompted the asthenospheric upwelling that triggered partial melting in mantle metasomatizedby slab-derived melts.