Lava Tube System at Gunjale, Western Deccan Volcanic Province, India

Abstract

Lava tubes are common in pāhoehoe lavas and are important in understanding the emplacement of large volumes of lava, and their transportation over long distances in Continental Flood Basalt provinces of the world. The Deccan Volcanic Province is one of the largest continental flood basalt provinces in the world, with an aerial coverage of nearly 500,000 km 2. Given the very large spatial distribution of lava flows over a relatively small timescale in the Deccan Traps, the sinuous structure at Gunjale, Ahmednagar (Ahilya Nagar), Maharashtra holds special interest regarding its genesis and role in the geology of the area. Following field work done in August 2022 at Gunjale, we discovered it is a lava tube hosted within compound pāhoehoe flows characterised by prominent levees formed by accreditation of breccias and lava lobes. We carried out petrography, modal composition measurements, XRF and EPMA to build a comprehensive emplacement and cooling history of the tube. The northern levee is the preserved remnant of the lava flow composed of pāhoehoe sheets and lobes, and hence primarily microporphyritic. It displays highly oxidised characteristics suggesting subaerial emplacement and deuteric alteration. In contrast, the southern levee displays an accretion vesicular crust and breccia. The texture is exceptionally microporphyritic with large lanceolate opaque suggesting under cooling. The core of the tube consists of massive basalt and contains spherical to elongated plagioclase- clinopyroxene cognates. These cognates are extremely coarse grained, which implies they crystallised in a magma chamber elsewhere and then deposited via the magmatic plumbing system. Based on the clinopyroxene compositions, the northern levee record the primary emplacement temperatures of the lava tube and crystallisation at a higher temperature (800 to 1300 °C) compared to the southern levee (~500 to 1000 °C). The core underwent cooling induced fractional crystallisation and recorded a lower temperature (~700 to 1100 °C). The cognate also records similar temperatures of ~900 to 1100 °C suggesting a similar thermal regime as that of the core. The plagioclase compositions from the northern levee are more calcic and have higher temperature (~750 to 900 °C) in comparison to the southern levee (650 to 900 °C). The plagioclase from the lava tube core and cognate record significantly lower temperatures (650 to 750 °C). Magnetites form the core and cognates record a high temperature of 1500 to 1550 °C while the ilmenites form the core and cognates record slightly lower temperature of 1400 °C. It appears that the two leeves and the tube interior have different thermal regions and cooling histories. The tube formed initially as a preferred conduit within the lava flow field.