Petrogenesis of Plio-Pleistocene volcanic rocks from the chagai arc ...

5.2. Tectonic SettingA number of plots and tectonomagmaticdiscrimination diagrams based on major, minor ortrace elements are designed to study the parentmagma and tectonic setting of the volcanic rocks.Diagrams based on major elements or large ionlithophile (LIL) elements must be used withcaution, as these elements are more mobile duringpost-magmatic alteration or metamorphicprocesses as compared to high field elements(Pearce 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 allthe samples reveal that most of the Plio-Pleistocene volcanic rocks are calc-alkaline innature (Fig. 11A). To ascertain whether thestudied rocks were erupted in an oceanic islandarc environment or in a continental margin-typearc settings, the sample are plotted in Zr versusZr/Y plot (after Pearce, 1983). In this diagram(Fig. 11B), all the sample plot in continentalmargin-type arc domain. The discriminationdiagram based on trace element ratios isconsidered more authentic. The Th/Yb versusTa/Yb plot of Pearce, (1983) is widely used, notonly for the determination of tectonicenvironment but also for the estimation of natureof source (enriched or depleted), crustalcontamination, within-plate enrichment andfractionation, etc. A plot of the KSV volcanics inthis diagram (Fig. 11C) confirms their continentalmargin-type arc affinities. The plot of the analysesin this diagram further suggests that the parentmagma of these volcanics was generated by anenriched mantle source.The spider patterns (Fig. 9), which exhibitpositive spikes generally on K and Sr and markednegative anomalies on Nb, further confirm theisland arc signatures of the rocks (Pearce, 1982;Wilson 1989; Saunders et al., 1991). The markednegative Nb anomalies are explained by retentionof this element in the residual mantle peridotitesource during its partial melting. The positivespikes or enrichment of certain LIL elements aregenerally considered to have formed byincorporation of these elements in the source fromthe subducting slab (Pearce, 1982; Wilson, 1989).5.3. Nature of the Source of Parent MagmaThe Zr versus Zr/Y diagram (Fig. 11D)provides useful information about the nature ofsource, degree of partial melting andfractionation, etc. Plots of the analyses in thisdiagram indicate that these rocks are fractionatedfrom 15 % partially melted enriched mantlesource. The plots also indicate that the KSVvolcanics were fractionated from a least enrichedsource as compared to KDV, DKV and BKVvolcanics, which were developed in progressivelyincreasing enriched sources. This diagram furthersuggests that KSV volcanics are least fractionatedas compared to other volcanics, which show anoverlapping increase in fractionation.6. Comparison with other arcsIn Table 2, average trace element chemistryof the Plio-Pleistocene calc-alkaline volcanicrocks of the Chagai arc is compared with averagecalc-alkaline rocks of continental margin typearcs, including Andes, Zagros, Sunda, and Japanarcs, and oceanic island arcs including MarianaSouth West Pacific arcs, Fiji and New Britainarcs. This comparison shows close affinities ofthe Plio-Pleistocene volcanic rocks of the Chagaiarc with continental margin type arcs rather thanthose of the oceanic arcs. The average andesitefrom the Chagai arc shows closer similarity inLILE, HFSE and REE and their ratios with itscounterpart in Zagros arc, and less with Japanand Sunda arcs It greatly differs in theseelements with its counterpart in Andes arc (Table2).19