Petrochemistry and genesis of olivine basalts from small monogenetic parasitic cones of Bazman stratovolcano, Makran arc, southeastern Iran

Small monogenetic Quaternary parasitic cones around Bazman stratovolcano, located at the western edge of the Makran arc, southeastern Iran, erupted low-Ti subalkaline olivine basalts with MgO (3.8-8.6 wt.%) and Al2O3 (16.5-18.6 wt.%). Positive correlation of decreasing MgO. Ni and Cr indicates that formation of low MgO basalts involved limited crystal-liquid fractionation of olivine and clinopyroxene. the common phenocrysts. The basalts have variable Sr-87/Sr-86 (0.704177-0.705139) and Nd-143/Nd-144 (0.512689-0.512830) ratios, within the range of OIB-like intra-plate alkaline basalts erupted in eastern Iran north of the Makran arc. This, and the lack of correlation between Sr content and Sr-isotopic ratio, suggest that upper crustal contamination was not significant in their formation, consistent with the relatively thin crust (<= 40 km) in the area. Enrichment of large-ion-lithophile elements (LILE) relative to light rare-earth-elements (LREE; Ba/La = 9-25), and depletions in Nb relatively to LILE (Ba/Nb = 12-35; La/Nb = 0.8-2.1), are similar in most cases to other convergent plate boundary arc basalts, suggesting that the Bazman basalts formed by melting of subcontinental mantle modified by dehydration of subducted Oman Sea oceanic lithosphere. PI) isotopic ratios of the basalts define a linear trend above the Northern Hemisphere Line, consistent with their derivation from mantle contaminated by Pb derived from subducted sediment Trace element contents and ratios (La-N = 10-25; Yb-N = 3-6; (La/Yb)(N) = 3-8) suggest that these basalts formed as a result of low (similar to 10%) degrees of partial melting of subarc mantle modified only moderately by subducted components. Relatively low Ba/Nb<15, La/Nb<1.5 and Ba/La<15 ratios for some basalts confirm only limited contamination of the source of these samples, consistent with observations in other arcs that parasitic cones tap sources less affected by slab-derived fluids than the larger stratovolcanoes they surround. Comparison with Taftan volcano in southeastern Iran and Koh-e-Sultan in Pakistan. both further to the east along the Makran arc, indicate regional west-to-east geochemical variations, including increasing K2O, LILE, LREE, La/Yb, Ba/La and Sr-87/Sr-86. These changes result from an along-strike increase in both input of slab-derived fluids relative to the volume of mantle below the arc and decreasing degree of subarc mantle melting to the east. This is due to a reduction in the volume of subarc mantle below the eastern part of the Makran arc resulting from a combination of both increasing crustal thickness and decreasing angle of subduction to the east. (C) 2011 Elsevier B.V. All rights reserved.