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

The SiO 2 contents in KSV have a wider range(58.35-64.83 wt. %), as compared to others BKV(57.45-59.64 wt. %), KDV (57.98-59.87 wt. %),DKV (61.15-63.16 wt. %). The TiO 2, Al 2 O 3,Fe 2 O 3, CaO and P 2 O 5 contents (Table 1) are withinthe proposed range of most orogenic andesites(Gill, 1981). The volcanics have lower but greatlyvariable (2.55-4.80) range for MgO wt. %, exceptone andesite sample from KSV shows highervalue (5.90) for MgO wt. %, which is little lowerthan high-Mg andesite value (> 6 wt. % MgO)proposed by Gill (1981). The KDV have lowerand narrow overlapping range for Na 2 O (3.72-3.87 wt. %) and K 2 O (1.09-1.18 wt. %). All othervolcanics have a wider and variable range (Table1). The K 2 O/Na 2 O ratios in all the volcanic arelow (0.18-0.31), which are much lower than thereported (0.60-1.00) ratios for commoncontinental margin type arcs. The KDV havenarrow and lower (1.42-1.1.48) FeOt/MgO ratiosas compared to all other volcanics, which have awider and higher overlapping range (1.01-2) forthis ratio. All the volcanics show a lower andnarrow (47-58) range for Mg # (100 × Mg/Mg +Fe 2+ ), which are consistent with most volcanicrocks of orogenic arcs (Gill, 1981).4.3. Major Element VariationsSiO 2 is used as fractionation index in variousHarker-type variation diagrams for majorelements (Fig. 7). In these diagrams, almost allthe samples from the area show sharp to scatterednegative correlation for Al 2 O 3, MgO, Fe 2 O 3, TiO 2,CaO and P 2 O 5 . The negative correlation for MgO,Fe 2 O 3, and TiO 2 are due to the early fractionationof pyroxene, hornblende and magnetite. Thesimilar trends for Al 2 O 3 and CaO are probably dueto fractionation of plagioclase. The P 2 O 5 exhibit ascattered negative correlation, due probably tofractionation of apatite. Na 2 O and K 2 O showscattered positive correlation with SiO 2 probablydue to the enrichment of these elements in theresidual phase.4.4. Trace Element VariationsMgO is used as fractionation index in variousHarker-type variation diagrams for trace elements(Fig. 8). The analysis show scattered negativecorrelation for Ba, and Rb (except BVC) due tothe enrichment of these elements in the residualphase. Ti, V, Sc, P, Co and Ni exhibit sharp toscattered positive correlation (except BVC) for allthe volcanics probably due to the partitioning ofthese elements in magnetite, apatite, hornblendeand pyroxene during differentiation. In all thediagrams BVC show nonsystematic behavior,probably due to limited MgO concentration.4.5. Trace element abundancesLarge Ion Lithophile Elements (LILE): Thevolcanics generally have higher abundances ofLILE as compared to high field strength elements(HFSE). The KDV volcanics generally havelower abundances and narrow range for Rb (16-18ppm), Sr (855-917 ppm) and Ba (398-410 ppm).The other volcanics have a wider and variablyhigher overlapping range for Rb (14-34 ppm), Sr(433-619 ppm) and higher for Ba (300-661 ppm),which are consistent with the calc-alkalinevolcanic rocks of orogenic arcs (Gill, 1981). Thand U, analyzed only in the KSV, range from 1.74to 3.02 ppm and 0.44 to 1.02 ppm, respectively.High Field Strength Elements (HFSE): The KSVvolcanics generally have lower abundances of Zr(69-99 ppm), Nb (3-4 ppm) and Y (11-16 ppm).The other volcanics have higher overlappingabundances for these elements. Zr, 92 to 113 ppm,Nb 4 to 7 ppm, and and Y 12 to 17 ppm,respectively, which suggest that KSV volcanicswere formed by relatively higher degree of partialmelting as compared to the other volcanics. Theconcentration of HFSE in these volcanics isconsistent with an enriched sub-arc mantleperidotite source and corresponds to calc-alkalinevolcanic rocks of orogenic arcs (Gill, 1981). Taand Hf were analyzed only in KSV, andrangefrom 0.18 to 0.24 ppm and 0.36 to 1.39 ppm,respectively.Compatible Elements: The KSV volcanics have avariable and higher overlapping range for Cr (27-261 ppm), Ni (13-68 ppm), Co (15-50 ppm), V(89-151 ppm), and Sc (15-27 ppm). The othervolcanics have lower but greatly variableoverlapping range of compatible elements: Cr(33-123 ppm), Ni (12-21 ppm), Co (12-36 ppm),V (84-201 ppm) and Sc (84-201 ppm). Thesevalues are much lower than the values proposed(Gill, 1981) for the volcanic rocks of orogenicarcs that are directly derived from the primarymantle source.14