Paper No. 2
Presentation Time: 9:00 AM-6:30 PM
PETROLOGY, GEOCHEMISTRY AND THE TECTONIC SETTING OF THE BASALTIC PILLOW LAVAS IN THE BAFT OPHIOLITIC MéLANGE, SOUTHWEST OF KERMAN, IRAN
Pillow lavas make up the most significant volcanic section of the Baft ophiolitic mélange in southwest Kerman in Iran and occur as spherical and tubular forms. The diameter of the pillows varies between 0.5 to 1.5 meters and their interstitials are filled with calcite, chlorite, zeolite and hyaloclastite materials. The pillows are often very vesicular and the vesicles are filled with secondary minerals such as calcite, quartz, chlorite and epidote. The concentration of vesicles increases from core to margin. Among many textures, the phyric and aphyric textures stand out in the pillows and plagioclase phenocrysts show a flow texture with interstitial altered olivine and pyroxene at the margin of the phyric texture. Other textures include microlitic porphiritic, intersertal, intergranular and amygdaloidal. Petrographic studies indicate albite, augite, iddingsited olivine, chlorite, calcite, epidote, zeolite and iron oxides as other constituents in the pillows. Various crystal morphologies-as a result of quenching- can be observed in plagioclase and pyroxene phases. For example, plagioclase microlites show spherulitic, swallow-tail, rosette, belt-buckle and long-delicate spikes while pyroxens show dendritic, spheritic, cockscomb and parallel chains. Geochemical analysis of trace and rare earth elements suggests that the Baft pillow lavas are both depleted tholeiite (depleted in light rare earth elements (LREEs)) and transitional tholeiite (enriched in light rare earth elements (LREEs)). Depleted tholeiites resemble the mid-ocean ridge basalts (MORB) whereas the transitional tholeiites show the characteristics of both enriched mid-ocean ridge basalts (E-MORB) and within plate basalts (WPB). The transitional tholeiitic basalts are also enriched in LILE (eg., Cs, Rb, Ba, Th, K, Sr, Pb) an d deficient in HFSE (e.g., Nb, Ta, Hf, Zr, Ti ) relative to N-MORB . This, together with their REE patterns suggests that the transitional tholeiitic basalts have a composition similar to supra-subduction zone (SSZ) basalts which lead us to believe that the Baft basaltic pillow lavas have formed in a SSZ tectonic setting.