Petrogenesis of Plio-Pleistocene volcanic rocks from the chagai arc ...
Petrogenesis of Plio-Pleistocene volcanic rocks from the chagai arc ...
Petrogenesis of Plio-Pleistocene volcanic rocks from the chagai arc ...
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AKoh-e-Sultan <strong>volcanic</strong>sBatal Koh <strong>volcanic</strong>sDam Koh <strong>volcanic</strong>sKoh-e-Dalil <strong>volcanic</strong>sB5.010FeOt/MgO2.5TholeiiteZr/YContinental ArcOceanic ArcCalc-alkaline0.048 52 56 60 65SiO 2110 100 500ZrCDTh/Yb1010.10.01Oceanic Island ArcActive Continental MarginSCWFS = Subduction zone enrichmentC = Crustal contaminationW= Within plate enrichmentF = Fractional crystalizationD e p l e t e d m a n t l e s o u r c e E n r i c h e d m a n t l e s o u r c e0.01 0.1 1Ta/Yb10Z r / Y10TongaSource depletionOshimaSource enrichmentCarlsberg R.Alula fartak Tr.30%20%HawaiHawai15%15%Partial melting trendOpen system fractionationClosed system fractionationC 3Chondrite composition15 10 100 500Z rFig. 11 A to D. Various tectonomagmatic discrimination diagrams for <strong>the</strong> Pli-<strong>Pleistocene</strong> <strong>volcanic</strong> <strong>rocks</strong><strong>from</strong> <strong>the</strong> Chagai <strong>arc</strong> (see <strong>the</strong> text for details).7. DiscussionThe petrological and geochemical datapresented in this paper strongly suggest that <strong>the</strong><strong>Plio</strong>-<strong>Pleistocene</strong> <strong>volcanic</strong> <strong>rocks</strong> represent a calcalkalineassociation <strong>of</strong> continental margin-type <strong>arc</strong>and <strong>the</strong> parent magma <strong>of</strong> <strong>the</strong>se <strong>rocks</strong> wasfractionated <strong>from</strong> an enriched sub-<strong>arc</strong> mantlesource. The <strong>Plio</strong>-<strong>Pleistocene</strong> Koh-e-Sultanvolcanism and <strong>the</strong> synchronous volcanism in Iranare considered to be related with convergence <strong>of</strong>Arabian oceanic plate below <strong>the</strong> sou<strong>the</strong>rn margin<strong>of</strong> Afghan and Iran blocks, respectively (Jacob andQuittmeyer, 1979; Arthurton et al., 1979; Dykstraand Birnie, 1979: Dupuy and Dostal, 1978).Dykstra and Birnie (1979) have proposed asegmented Quaternary (now <strong>Plio</strong>-<strong>Pleistocene</strong>)subduction zone based on termination and <strong>of</strong>fset in<strong>the</strong> belt <strong>of</strong> <strong>the</strong>se <strong>volcanic</strong>s and distribution <strong>of</strong>earthquake epicenters in this region. Theyproposed four NS trending segments, designatedas A, B, C and D in Iranian and PakistaniBalochistan (Fig. 12), with a common hinge linenear <strong>the</strong> Makran coast. The two outer segments Aand D have shallow dips (10°- 20°) and are devoid<strong>of</strong> any <strong>volcanic</strong> activity, whereas <strong>the</strong> inner twosegments, B (38°-50°) and C (19°-20°), havesteeper dips and associated volcanism. Partialmelting in subducted oceanic crust usually takesplace at 110-173 km depth in <strong>the</strong> Beni<strong>of</strong> zone and21
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