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Rehanul Haq Siddiqui, Geoscience Advance Research Laboratories (Pakistan)
Muhammad Asif Khan, National Centre of Excellence in Geology (Pakistan)
Muhammad Qasim Jan, Vice Chancellar (Pakistan)
Masa Ogasawara, Geological Survey of Japan (Japan)
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The Plio-Pleistocene volcanic rocks, which are designated as Koh-e-Sultan Volcanic Group, occur in an east-west trending subduction related magmatic belt known as Chagai arc in the western part of Pakistan. The volcanism in this arc was initiated during the Late Cretaceous due to an intra-oceanic convergence in the southern Neo-Tethys, which intermittently continued up to the Quaternary period. The latest episode of volcanism in this arc was marked by an explosive activity represented by pumice deposits of 0.09 Ma.
Petrological studies of lava flows and lava fragments from various pyroclastic deposits show several varieties of andesites (57.45-63.44 wt.% SiO2) and dacites (63.55-65.53 wt.% SiO2), which include hypersthene, hornblende, lamprobolite andesites and hornblende biotite dacite. The main textures exhibited by these rocks include hypocrystalline, porphyritic, vitrophyric, subcumulophyric and subpilotaxitic. The phenocrysts generally range in size from < 0.3 mm to 5 mm and phenocryst and groundmass ratio is 40:60. The dacite is holocrystalline medium to coarse-grained and porphyritic in texture. The main phenocryst minerals are hornblende, hypersthene, plagioclase, biotite and quartz, which are also found in the groundmass. The phenocrysts in dacite range in size from 0.3 mm to 1 cm and the phenocryst and groundmass ratio is 45:55. The anorthite content in these rocks ranges from An15-An38. The magnetite, hematite and pyrite are main opaque minerals and apatite and rutile found as the accessories. The andesites are medium-K (0.18-1.28 wt.% K2O) and have low Mg # (47-64), and higher FeO (total) MgO (1.01-1.77) ratios, suggesting fractionated nature of the parent magma. The primordial mantle-normalised trace element patterns of andesites exhibit marked negative Nb anomalies with spikes generally on K, Sr and Ba. These geochemical signatures strongly confirm their island arc affinity that is also supported by their LREE enriched chondrite-normalised REE patterns. Their Zr/Y (5.07-9.08), Zr/Nb (17.75-28.75), Ti/V (24.22-33.91), La/Yb (9.07-11.87), Ta/Yb (0.11-1.18) and Th/Yb (1.69-2.25) ratios are consistent with calc alkaline rocks continental margin type island arcs. The Zr versus Zr/Y studies suggest that these volcanics are fractionated from 15% partially melted enriched mantle source. The andesitic rocks from Koh-e-Sultan are more enriched in LILE as compared with the analogous rocks in the surrounding satellite cones and plugs. The average andesite from the Chagai arc shows relatively more resembles in LILE, HFSE and REE with its counterpart in Zagros arc and less with Japan and Sunda arcs, whereas greatly differs in these elements with its counterpart in Andes arc.
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