Petrology and geochemistry of the Juzzak Sill, western Chagai arc (Pakistan): implications for petrogenesis and emplacement

详细信息    查看全文

• 作者：Rehanul Haq Siddiqui ; M. Qasim Jan ; M. Ishaq Kakar…
• 关键词：Petrogenesis ; Pyroxene ; andesite ; Juzzak Sill ; Western Pakistan
• 刊名：Arabian Journal of Geosciences
• 出版年：2016
• 出版时间：September 2016
• 年：2016
• 卷：9
• 期：13
• 全文大小：2,344 KB
• 刊物类别：Earth and Environmental Science
• 出版者：Springer Berlin Heidelberg
• ISSN：1866-7538
• 卷排序：9

文摘

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 (An_32–45) 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.