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

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the nature of parent magma is controlled by thedip angle of the subducted plate (Tatsumi andEggens, 1995). Therefore, the NW-SE trend of thePlio-Pleistocne volcanic belt is attributed to theoccurrence of volcanism in the B segment moresouthward, closer to the hinge line due to itssteeper depth as compared to the C segment.8. ConclusionThe petrogenetic studies of Plio-Pleistocenevolcanic rocks suggest that they are calc-alkalinein nature and formed in a continental margin-typearc environment. Higher concentration of mostHFSE, REE and greatly enhanced Zr/Y, Ti/V,Ta/Yb, Th/Yb, La/Yb and Th/Yb ratio relative toN-MORB indicate that the parent magma of theserocks was derived from an enriched sub-arcperidotite mantle source. The Plio-Pleistoceneandesites have low MgO wt. %, Mg # (50-56),and and contents of Co, Ni, but enhancedLILE/HFSE and LREE/HREE ratios, whichsuggests that the source magma of these rockswas not derived directly from an upper mantleperidotite source, but instead had undergoneolivine fractionation en route to eruption. The Zrversus Zr/Y studies suggest that the parent magmawas generated by about 15 % partial melting of anenriched sub-arc mantle source that fractionatedin an upper level magma chamber. It is alsosuggested that the Koh-e-Sultan volcanics weregenerated by relatively higher degree of partialmelting of a least enriched sub-arc mantle sourceand exhibit minimum fractionation as comparedto the volcanics in Batal Koh, Dam Koh and Kohe-KohDalil volcanoes. The latter shows thelowest degree of partial melting of a mostenriched sub-arc mantle source that underwentmore fractionation than the rest. A comparison ofaverage calc-alkaline andesite from Plio-Pleistocene volcanics of the Chagai arc with itscounterparts in some famous continental marginand oceanic island arcs of the world showrelatively more resemblance in LILE, HFSE andREE with Zagros, less with Japan and Sunda arcs,and least with its counterpart in the Andes arc.AcknowledgementsWe are highly indebted to Dr. Imran AhmadKhan, Director General, Geological Survey ofPakistan (GSP), Mr. S. Hasan Gauhar, ex-DirectorGeneral, GSP, and Mr. Tahir Karim (Director) ofthe same Department for their support during fieldand laboratory research.E X P L A N A T I O N SSegment A Dip = ~10Segment B Dip = 38-50Segment C Dip = 19-28Segment D Dip =
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Page 3 and 4: M APCHAGAI ARCFig. 1. Geological ma
Page 5 and 6: The Plio-Pleistocene volcanic rocks
Page 7 and 8: ABCDEFGHFig. 4. (A) a view of the n
Page 10 and 11: Table 1. Bulk chemistry of Plio-Ple
Page 12 and 13: Table 1. ContinuedElements Dam Koh
Page 14 and 15: The SiO 2 contents in KSV have a wi
Page 16 and 17: 6040R bB aKoh-e-Sultan volcanicsBat
Page 18 and 19: 500100Koh-e-Sultan volcanicsBatal K
Page 20 and 21: Table 2. A comparison of average tr
Page 24: Pearce, J.A., 1982. Trace elements

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