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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<strong>the</strong> nature <strong>of</strong> parent magma is controlled by <strong>the</strong>dip angle <strong>of</strong> <strong>the</strong> subducted plate (Tatsumi andEggens, 1995). Therefore, <strong>the</strong> NW-SE trend <strong>of</strong> <strong>the</strong><strong>Plio</strong>-Pleistocne <strong>volcanic</strong> belt is attributed to <strong>the</strong>occurrence <strong>of</strong> volcanism in <strong>the</strong> B segment moresouthward, closer to <strong>the</strong> hinge line due to itssteeper depth as compared to <strong>the</strong> C segment.8. ConclusionThe petrogenetic studies <strong>of</strong> <strong>Plio</strong>-<strong>Pleistocene</strong><strong>volcanic</strong> <strong>rocks</strong> suggest that <strong>the</strong>y are calc-alkalinein nature and formed in a continental margin-type<strong>arc</strong> environment. Higher concentration <strong>of</strong> mostHFSE, REE and greatly enhanced Zr/Y, Ti/V,Ta/Yb, Th/Yb, La/Yb and Th/Yb ratio relative toN-MORB indicate that <strong>the</strong> parent magma <strong>of</strong> <strong>the</strong>se<strong>rocks</strong> was derived <strong>from</strong> an enriched sub-<strong>arc</strong>peridotite mantle source. The <strong>Plio</strong>-<strong>Pleistocene</strong>andesites have low MgO wt. %, Mg # (50-56),and and contents <strong>of</strong> Co, Ni, but enhancedLILE/HFSE and LREE/HREE ratios, whichsuggests that <strong>the</strong> source magma <strong>of</strong> <strong>the</strong>se <strong>rocks</strong>was not derived directly <strong>from</strong> an upper mantleperidotite source, but instead had undergoneolivine fractionation en route to eruption. The Zrversus Zr/Y studies suggest that <strong>the</strong> parent magmawas generated by about 15 % partial melting <strong>of</strong> anenriched sub-<strong>arc</strong> mantle source that fractionatedin an upper level magma chamber. It is alsosuggested that <strong>the</strong> Koh-e-Sultan <strong>volcanic</strong>s weregenerated by relatively higher degree <strong>of</strong> partialmelting <strong>of</strong> a least enriched sub-<strong>arc</strong> mantle sourceand exhibit minimum fractionation as comparedto <strong>the</strong> <strong>volcanic</strong>s in Batal Koh, Dam Koh and Kohe-KohDalil volcanoes. The latter shows <strong>the</strong>lowest degree <strong>of</strong> partial melting <strong>of</strong> a mostenriched sub-<strong>arc</strong> mantle source that underwentmore fractionation than <strong>the</strong> rest. A comparison <strong>of</strong>average calc-alkaline andesite <strong>from</strong> <strong>Plio</strong>-<strong>Pleistocene</strong> <strong>volcanic</strong>s <strong>of</strong> <strong>the</strong> Chagai <strong>arc</strong> with itscounterparts in some famous continental marginand oceanic island <strong>arc</strong>s <strong>of</strong> <strong>the</strong> world showrelatively more resemblance in LILE, HFSE andREE with Zagros, less with Japan and Sunda <strong>arc</strong>s,and least with its counterpart in <strong>the</strong> Andes <strong>arc</strong>.AcknowledgementsWe are highly indebted to Dr. Imran AhmadKhan, Director General, Geological Survey <strong>of</strong>Pakistan (GSP), Mr. S. Hasan Gauhar, ex-DirectorGeneral, GSP, and Mr. Tahir Karim (Director) <strong>of</strong><strong>the</strong> same Department for <strong>the</strong>ir support during fieldand laboratory rese<strong>arc</strong>h.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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