500100Koh-e-Sultan <strong>volcanic</strong>sBatal Koh <strong>volcanic</strong>sDam Koh <strong>volcanic</strong>sKoh-e-Dalil <strong>volcanic</strong>sSample / Primordial Mantle101N-MORB0.1Rb Ba K Nb Sr P Zr Ti YFig. 9. Primordial mantle normalized spider diagram for <strong>the</strong> <strong>Plio</strong>-<strong>Pleistocene</strong> <strong>volcanic</strong> <strong>rocks</strong> <strong>from</strong> <strong>the</strong>Chagai <strong>arc</strong>. The half filled square pattern is for average N-MORB. Average N-MORB andnormalization values are after Sun and McDounough (1989).500100Koh-e-Sultan <strong>volcanic</strong>sBatal Koh <strong>volcanic</strong>sDam Koh <strong>volcanic</strong>sKoh-e-Dalil <strong>volcanic</strong>sSample / Chondrite10N-MORB1La Ce Nd Sm Eu Gd Er YbFig. 10. Chondrite normalized REE diagram for <strong>the</strong> <strong>Plio</strong>-<strong>Pleistocene</strong> <strong>volcanic</strong> <strong>rocks</strong> <strong>from</strong> <strong>the</strong> Chagai <strong>arc</strong>.The half filled square pattern is for average N-MORB. Average N-MORB values are after Sunand McDounough (1989), whereas normalization values are after Nakamura (1974).18
5.2. Tectonic SettingA number <strong>of</strong> plots and tectonomagmaticdiscrimination diagrams based on major, minor ortrace elements are designed to study <strong>the</strong> parentmagma and tectonic setting <strong>of</strong> <strong>the</strong> <strong>volcanic</strong> <strong>rocks</strong>.Diagrams based on major elements or large ionlithophile (LIL) elements must be used withcaution, as <strong>the</strong>se elements are more mobile duringpost-magmatic alteration or metamorphicprocesses as compared to high field elements(Pe<strong>arc</strong>e and Can, 1973; Floyed and Winchister,1975; Weaver and Tarney, 1981).The FeO/MgO ratios generally show highervalues in tholeiitic assemblages for a given SiO 2content (Miyashiro; 1974; Gill, 1981). The SiO 2versus FeO/MgO plots (Miyashiro, 1974) for all<strong>the</strong> samples reveal that most <strong>of</strong> <strong>the</strong> <strong>Plio</strong>-<strong>Pleistocene</strong> <strong>volcanic</strong> <strong>rocks</strong> are calc-alkaline innature (Fig. 11A). To ascertain whe<strong>the</strong>r <strong>the</strong>studied <strong>rocks</strong> were erupted in an oceanic island<strong>arc</strong> environment or in a continental margin-type<strong>arc</strong> settings, <strong>the</strong> sample are plotted in Zr versusZr/Y plot (after Pe<strong>arc</strong>e, 1983). In this diagram(Fig. 11B), all <strong>the</strong> sample plot in continentalmargin-type <strong>arc</strong> domain. The discriminationdiagram based on trace element ratios isconsidered more au<strong>the</strong>ntic. The Th/Yb versusTa/Yb plot <strong>of</strong> Pe<strong>arc</strong>e, (1983) is widely used, notonly for <strong>the</strong> determination <strong>of</strong> tectonicenvironment but also for <strong>the</strong> estimation <strong>of</strong> nature<strong>of</strong> source (enriched or depleted), crustalcontamination, within-plate enrichment andfractionation, etc. A plot <strong>of</strong> <strong>the</strong> KSV <strong>volcanic</strong>s inthis diagram (Fig. 11C) confirms <strong>the</strong>ir continentalmargin-type <strong>arc</strong> affinities. The plot <strong>of</strong> <strong>the</strong> analysesin this diagram fur<strong>the</strong>r suggests that <strong>the</strong> parentmagma <strong>of</strong> <strong>the</strong>se <strong>volcanic</strong>s was generated by anenriched mantle source.The spider patterns (Fig. 9), which exhibitpositive spikes generally on K and Sr and markednegative anomalies on Nb, fur<strong>the</strong>r confirm <strong>the</strong>island <strong>arc</strong> signatures <strong>of</strong> <strong>the</strong> <strong>rocks</strong> (Pe<strong>arc</strong>e, 1982;Wilson 1989; Saunders et al., 1991). The markednegative Nb anomalies are explained by retention<strong>of</strong> this element in <strong>the</strong> residual mantle peridotitesource during its partial melting. The positivespikes or enrichment <strong>of</strong> certain LIL elements aregenerally considered to have formed byincorporation <strong>of</strong> <strong>the</strong>se elements in <strong>the</strong> source <strong>from</strong><strong>the</strong> subducting slab (Pe<strong>arc</strong>e, 1982; Wilson, 1989).5.3. Nature <strong>of</strong> <strong>the</strong> Source <strong>of</strong> Parent MagmaThe Zr versus Zr/Y diagram (Fig. 11D)provides useful information about <strong>the</strong> nature <strong>of</strong>source, degree <strong>of</strong> partial melting andfractionation, etc. Plots <strong>of</strong> <strong>the</strong> analyses in thisdiagram indicate that <strong>the</strong>se <strong>rocks</strong> are fractionated<strong>from</strong> 15 % partially melted enriched mantlesource. The plots also indicate that <strong>the</strong> KSV<strong>volcanic</strong>s were fractionated <strong>from</strong> a least enrichedsource as compared to KDV, DKV and BKV<strong>volcanic</strong>s, which were developed in progressivelyincreasing enriched sources. This diagram fur<strong>the</strong>rsuggests that KSV <strong>volcanic</strong>s are least fractionatedas compared to o<strong>the</strong>r <strong>volcanic</strong>s, which show anoverlapping increase in fractionation.6. Comparison with o<strong>the</strong>r <strong>arc</strong>sIn Table 2, average trace element chemistry<strong>of</strong> <strong>the</strong> <strong>Plio</strong>-<strong>Pleistocene</strong> calc-alkaline <strong>volcanic</strong><strong>rocks</strong> <strong>of</strong> <strong>the</strong> Chagai <strong>arc</strong> is compared with averagecalc-alkaline <strong>rocks</strong> <strong>of</strong> continental margin type<strong>arc</strong>s, including Andes, Zagros, Sunda, and Japan<strong>arc</strong>s, and oceanic island <strong>arc</strong>s including MarianaSouth West Pacific <strong>arc</strong>s, Fiji and New Britain<strong>arc</strong>s. This comparison shows close affinities <strong>of</strong><strong>the</strong> <strong>Plio</strong>-<strong>Pleistocene</strong> <strong>volcanic</strong> <strong>rocks</strong> <strong>of</strong> <strong>the</strong> Chagai<strong>arc</strong> with continental margin type <strong>arc</strong>s ra<strong>the</strong>r thanthose <strong>of</strong> <strong>the</strong> oceanic <strong>arc</strong>s. The average andesite<strong>from</strong> <strong>the</strong> Chagai <strong>arc</strong> shows closer similarity inLILE, HFSE and REE and <strong>the</strong>ir ratios with itscounterpart in Zagros <strong>arc</strong>, and less with Japanand Sunda <strong>arc</strong>s It greatly differs in <strong>the</strong>seelements with its counterpart in Andes <strong>arc</strong> (Table2).19
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