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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)
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The major part of the Chagai arc occurs in the western part of Pakistan. A small part of it also extends towards north in Afghanistan and west in Iran. This arc terrain is about 500 km long, 150 km wide and trends in EW direction. It is convex towards south and is terminated by the Chaman and Harirud fault zones in the east and west, respectively. Several episodes of volcanism occurred during Late Cretaceous to Quaternary. The Cretaceous and Paleocene volcanic episodes are dominated by basaltic-andesites with minor basalts and andesites with their volcaniclastic counterparts, whereas Eocene onwards volcanism is generally dominated by andesites with minor dacites and their volcaniclastic equivalents. Petrochemical studies show that the Late Cretaceous lava flows of Raskoh arc and the Late Cretaceous to Paleocene lavas of Chagai arc belong to the tholeiite series, whereas Eocene and younger lava flows belong to the calc-alkaline series. Substantial amounts of Miocene shoshonites are also found towards west-central part of the Chagai arc. The arc terrane does not show any petrogenetic zonation of tholeiitic, calc-alkaline and alkaline series across the arc as reported for the Japan type arcs. The primordial mantle-normalized trace element patterns of all the lava flows from the Chagai arc exhibit marked negative Nb anomalies with spikes generally on K, Ba, Rb and Sr, which strongly confirm their island arc signatures. The trace element ratios including Ti/V, Zr/Y, Ce/Yb, La/Yb, Ta/Yb and Th/Yb of Chagai arc terrane generally show progressive increase towards younger volcanics, whereas Ti/Zr and Zr/Nb ratios exhibit similar decrease. These ratios in Late Cretaceous to Eocene lava flows are consistent with oceanic island arcs, whereas in Oligocene and younger lava flows these ratios are more consistent with continental margin type arcs. The Zr versus Zr/Y and Ta/Yb versus Th/Yb studies also reveal that Late Cretaceous to Paleocene volcanics exhibit oceanic island arc character, the Eocene volcanics exhibit transitional character, representing overlapping chemistry of both oceanic and continental margin arcs, while Oligocene onwards all the volcanics show typical continental margin (Andean) type arc affinities. The LILE/HFSE, LREE/HREE, 88Sr/86Sr ratios and ΣREE generally show progressive overlapping increase towards younger volcanics. The LILE/HFSE, LREE/HREE ratios and ΣREE are much higher in Oligocene and Miocene volcanics are much higher than their Quaternary counterparts. This suggests that during Oligocene-Miocene period, comparatively more LILE and LREE enriched fluids from the subducting slab were added to the sub-arc mantle source. The studies farther suggested that arc magmatism in Kohistan-Ladakh, Kandhar, Chagai-Raskoh, Zagros arc, Semail, Bela, Muslim Bagh and Waziristan back-arc terrenes were probably started in a single segmented convergence zone, which developed in the Ceno Tethys during the Early Cretaceous.
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