Petrogenesis of Plio-Pleistocene volcanic rocks from the Chagai arc, Balochistan, Pakistan
Keywords:
Plio-Pleistocene volcanics; Chagai arc; Andean-type calc-alkaline affinitiesAbstract
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 CenoTethys, 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 ± 0.01 Ma (K-Ar whole rock age of pumice). Petrological studies of lava flows show several varieties of andesites and dacites, which include hypersthene, hornblende, lamprobolite andesites and hornblende biotite dacite. 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 of 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 Plio-Pleistocene andesite from the Chagai arc shows relatively more resemblance in LILE, HFSE and REE with its counterpart in Zagros arc and less with Japan and Sunda arcs; it greatly differs in these elements with its counterpart in the Andes arc.
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