Alkaline rocks from the Deccan Large Igneous Province: Time–space distribution, petrology, geochemistry and economic aspects

Abstract

We present a comprehensive review on the alkaline rocks from the Deccan Large Igneous Province (DLIP) and discuss their (i) temporal and spatial association with the Deccan Traps, (ii) petrography, mineral- and whole-rock geochemistry (including radiogenic and stable isotopes) and geophysical aspects, and (iii) P–T data available on their entrained xenoliths. The alkaline rocks occur in seven sub-provinces, viz., (i) the Kachchh, (ii) the Saurashtra, (iii) the Gujarat Central and Chhotaudepur, (iv) the Mumbai–Trombay, (v) the Central Deccan, (vi) the Aravalli, and (vii) the Tethyan Himalayan, with the first five in association with the Deccan Traps. A diverse variety of silica under-saturated to over-saturated alkaline rocks with varied mineralogical and geochemical compositions have been reported from these sub-provinces. These include alkali basalt, basanite, carbonatite, ijolite, lamprophyre, leucite, melteigite, mugearite, nephelinite, nepheline syenite, orangeites, alkali pyroxenite, phonolite, tinguaite, etc. Available geochronological data on the Deccan alkaline rocks reveal a wide duration of the related magmatic activity (124–55 My), and suggest the presence of pre-, syn- and post-emplacement ages of the DLIP units. Alkaline rocks of the DLIP are hosted by discrete aged lithotypes in a variety of stratigraphic horizons, such as the Deccan Traps, Cretaceous Bagh beds, Jurassic sandstones, Triassic Shrinab sediments, Proterozoic Godhra Granite and unclassified gneisses. In a majority of the sub-provinces, intrusions of alkaline rocks are controlled by fractures, rift or lineament systems such as the Kutch rift, the Son–Narmada Tapti rift, etc. Their major mineralogy is dominated by pyroxene, feldspar, amphibole, mica, olivine, nepheline, leucite, sodalite and carbonate minerals whereas accessory and minor minerals include titanite, apatite, spinel, rutile, pyrite, chalcopyrite, epidote, zircon, pyrochlore, garnet, perovskite and other REE-bearing phases. Geochemical studies reveal their sodic to potassic nature, with distinct shoshonitic character for some alkaline rocks. Combined geochemical and isotopic studies highlight the role of mixed mantle sources ranging from spinel to garnet stability depths and involvement of the lower degrees of partial melting. Source modification by subduction and crustal contamination is evaluated. Geodynamic implications for the orogenic and anorogenic signatures found in various occurrences, depth of the lithosphere–asthenosphere boundary, and economic resources are also examined and future research directions are identified. © 2022, Indian Academy of Sciences.