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Petrology and geochemistry of the Juzzak Sill,western Chagai arc (Pakistan): implications for petrogenesis and emplacement
Authors:Rehanul?Haq?Siddiqui  M?Qasim?Jan  Email author" target="_blank">M?Ishaq?KakarEmail author  Ehsanullah?Kakar  Naghma?Haider
Institution:1.Engineering and Management Sciences,Balochistan University of Information Technology,Quetta,Pakistan;2.National Centre of Excellence in Geology,University of Peshawar, Pakistan & COMSTECH,Islamabad,Pakistan;3.Centre of Excellence in Mineralogy,University of Balochistan,Quetta,Pakistan;4.Geoscience Advance research Laboratories, Geological Survey of Pakistan,Islamabad,Pakistan
Abstract:The Juzzak Sill occurs in the western part of the east-west trending, subduction-related magmatic belt known as the Chagai arc. The sill is concordantly emplaced in the Paleocene Juzzak Formation and locally cross-cuts the Early to Middle Eocene Robat Limestone and Eocene Saindak Formation. The sill is a porphyritic pyroxene diorite that grades into a porphyritic andesite (60.12–61.57 wt% SiO2) along the chilled margins. It comprises phenocrysts of hypersthene and plagioclase (An32–45) in a medium- to fine-grained groundmass of these minerals, opaque oxide, and apatite. The rocks are high-K (2.37–2.86 wt% K2O) calc-alkaline with low Mg# (42–55), Cr (51–80 ppm), and Ni (22–30 ppm) contents. Mantle-normalized trace element patterns, exhibited by marked negative Nb anomalies and positive spikes for Sr, Rb, and Zr and are akin to island arc signatures. The relatively higher ratios of Zr/Y (3.57–6.58), Ti/V (46.05–54.36), Ta/Yb (0.14–0.15), and Th/Yb (2.56–2.65) and high 87Sr/86Sr ratio (0.70524) suggest the role of continental crust materials, thus implying continental margin-type arc affinity. The source diagnostic ratios including K/Ba, P/Zr, and La/Ce of Juzzak Sill andesite and Eocene andesite from the Chagai arc are more or less similar, but the former has a much higher K/Y and Ba/Y ratios, which suggests assimilations of the host sediments during intrusion.
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