Giant (30 km-diameter) silicic caldera of K/Pg boundary age in the northwestern Deccan Traps: the Alech Hills, Saurashtra

• Published in: International journal of earth sciences : Geologische Rundschau 2022-02, Vol.111 (1), p.379-399
• Main Authors: Sheth, Hetu, Naik, Anmol, Sheikh, Janisar M., Kumar, Alok
• Format: Article
• Language: English
• Subjects: Basalt; Calderas; Dikes; Earth and Environmental Science; Earth Sciences; Feldspars; Floods; Geochemistry; Geology; Geophysics/Geodesy; Hills; Hydrothermal alteration; Lava; Lead isotopes; Mass extinctions; Mineral Resources; Original Paper; Rhyolite; Rhyolites; Ridges; Sedimentology; Structural Geology; Volcanic activity; Volcanism
• ISSN: 1437-3254; 1437-3262
• DOI: 10.1007/s00531-021-02119-4

Voluminous silicic (rhyolitic–dacitic) eruptive units overlie mafic lavas in many continental flood basalt (CFB) provinces of the world. These silicic units comprise lavas as well as pyroclastic deposits, including lava-like ignimbrites, and many were erupted from identified calderas. Silicic volcanic units are widespread in the northern and northwestern parts of the Deccan Traps CFB province of India, particularly the Saurashtra peninsula. In the Alech Hills in Saurashtra, whose geology is poorly known, basal mafic lavas are intruded by a ring dyke of granophyre with a diameter of 30 km. Inside this ring, silicic eruptive units are exposed. The lowest and most extensive unit is a grey, feldspar-phyric, ≥ 3.8 km 3 flood rhyolite lava. This is overlain successively by rheomorphic and lava-like ignimbrites, eutaxitic ignimbrites, and non-welded tuffs and breccias. Based on a combination of geological features, such as the large granophyre ring dyke, the effusive and explosive rhyolite volcanism, fault-controlled arcuate and annular ridges of the rhyolites, possible caldera-collapse mesobreccias, and widespread hydrothermal alteration, we recognize the Alech Hills as a giant (30 km-diameter, 660 km 2 ) effusive-explosive silicic caldera. A recently published CA-ID-TIMS 206 Pb/ 238 U date of 65.765 ± 0.018 Ma (2σ) for the granophyre ring dyke suggests that the Alech Hills caldera formed contemporaneously with the K/Pg boundary mass extinction, and may have significantly contributed to it.