The Deccan Volcanic Province (DVP), India: A Review
Authors: Krishnamurthy P. 
Published: August 2020

Geochemical data (major and minor oxides, trace elements including REE, and Sr, Nd, Pb, and O isotopes) have been obtained on a number of flow sequences and plutonic and volcanic complexes of the DVP by numerous groups since the early 1970's. Evaluation of these data has led to the classification of the basalts and other rock types, inferences on their mantle sources, parental magmas and the numerous magmatic differentiation and crustal contamination processes that have caused the observed diversity. The DVP is predominantly composed of quartz- and hypersthene-normative tholeiitic basalts in the plateau regions (Western Ghats and adjoining central and eastern parts (Malwa and Mandla)). However, along the ENE-WSW-trending Narmada-Tapi rift zones, the N-S to NNW-SSE-trending Western coastal tract, the Cambay rift zone, and the Saurashtra peninsula and Kutch regions, the DVP shows considerable diversity in terms of structures, presence of dyke swarms and dyke clusters, and intrusive and extrusive centres with diverse rock types.

Based on the geochemical and isotopic variations observed in the twelve different formations of basalts from the Western Ghats, it has been established that the least contaminated basalts among the Deccan Basalt Group lavas are represented by the Ambenali Formation of the Wai sub-group (c. 500 m thick), with εNd(t) = +8 to + 2, (87Sr/86Sr)t = 0.7040-0.7044 and (206Pb/204Pb)0 = 18.0 ± 0.5, average Ba/Zr = 0.3, and Zr/Nb = 14.4, indicating a depleted T-MORB-like mantle source. Slight enrichment in (87Sr/86Sr)t ratios (0.705), and εNd(t) = (+5 to −5) and depletion in (206Pb/204Pb)0 = 18.5–17.0 and δ18O = +6.2 to +8.3 ‰ as observed in the Mahabaleshwar Formation, that overlies the Ambenali Formation, indicate an enriched or metasomatised lithospheric mantle source. Such uncontaminated magmas appear to have been variably contaminated by a variety of crustal rocks (gneisses, shales, schists, amphibolites and granulites) as indicated in the εNd(t) vs. (87Sr/86Sr)t plots of all other eight formations that underlie these two formations. The flows of the Bushe Formation from the Western Ghats and one dyke from the Tapi rift zone represent the most crustally contaminated rock types with εNd(t) = −10 to −20.2 and (87Sr/86Sr)t = 0.713 - 0.72315 and very high (208Pb/204 Pb)0 = 41.4, (207Pb/204Pb)0 = 16.03 and (206Pb/204Pb)0 = 20.93. Combined Sr-Nd-Pb, TiO2, MgO, Zr/Y and primitive mantle - normalised plots of basalts from flow sequences that are far away (c. 400-700 km) from the Western Ghats (e.g. Toranmal, Mhow, Chikaldara, Jabalpur and others) indicate their chemical similarity to those of the Western Ghats, especially Poladpur and Ambenali formations, except for some differences in the Pb-isotope ratios. Such features suggest either lithological continuity of flows over long distances from a single eruptive source or their coeval eruption from multiple sources providing basalts of analogous geochemistry. The DVP provides a plethora of crustal contamination processes such as assimilation and quasi- equilibrium crystallization (AEC) in the MgO-rich samples of the Western Ghats (e.g. Bushe) during emplacement or ascent, and assimilation- fractionation crystallisation (AFC-type) in intrusive and/or volcanic complexes (e.g., Phenai Mata, Pavagadh, Mumbai Island) in crustal magma chambers of the refilled, tapped and fractionated (RTF)- type. Operation of such RTF-magma chamber processes within the Mahabaleshwar sequence (c. 1200 m) indicates the complexities introduced in the magmatic process and hence in geochemical interpretations of such thick flow sequences.

High- and low-pressure experimental petrological studies have led to petrogenetic models which indicate the production of primary melts of picritic compositions (c. 16% MgO), by 15 - 30% melting of an Fe-rich lherzolitic source at c. 2-3 GPa (c. 60-100 km depths). These melts evolved through olivine-fractionation near the Moho and then gabbroic fractionation within the shallow-intermediate crust (c. 6 km below the surface under c. 2 kb pressure) to produce the most dominant quartz- and hypersthene-normative tholeiitic basalts. In some rare cases (e.g., borehole sequence of Saurashtra, Pavagadh and others), the primary picritic liquids that formed at mantle depths, and the spinel-peridotie mantle-nodule- hosting melanephelinites from Kutch, have erupted without much modification. They occur spatially in close proximity to deep faults or rifts (e.g Narmada, Cambay, Kutch and others) which have apparently facilitated their rapid ascent and eruption without significant pause or modification during transport. εNd(t) vs. (87Sr/86Sr)t, chondrite- and primitive-mantle normalized variations in the picritic rocks and basalts of the DVP indicate several types of mantle sources such as transitional-midocean-ridge basalt (TMORB), Ocean Island basalt (OIB) /Reunion- type of peridotitic compositions either metasomatised or normal.

Geodynamic and plate-tectonic considerations during the emplacement of the DVP envisage both an asthenospheric- plume source (Reunion) and continental rift-related volcanism with eclogitic sources. The role of dual sources, capable of producing large volumes of basalts through near-total melting seem to provide the answer to DVP's enigma of production of large volumes of lava in very short time as observed in the Western Ghats and the contiguous plateau, and also the extreme diversity in rock types found in the western parts from peridotitic-sources.

Age data based on Ar-Ar, U-Pb, Re-Os isotopes, constrained by paleomagnetic data for the whole of DVP conforming to C30N-C29R-29N, indicate a protracted period of volcanism from 69.5 Ma (Upper Cretaceous) to 62 Ma (Palaeocene) including polychronous complexes (e.g. Mundwara, Sarnu-Dandali, Rajasthan). Based on precise U-Pb age data on zircons, it has been shown that the whole sequence of the Western Ghats with ten formations (c.1.8 km thick) erupted over a short period of time (&lt; 1 Ma). The most dominant volcanic phase, however, represented by the Wai Subgroup, consisting of the Poladpur, Ambenali and Mahabaleshwar formations (c. 1.1 km thick) contain an estimated volume of c. 439,000 km3 of lavas that erupted over a short span of c. 700, 000 years. The precise timing of such large eruptions with reference to the Cretaceous-Palaeogene (K-Pg) boundary with or without links to the Chicxulub meteorite impact are being debated vigorously. In addition, the quantity of gases released (Cl, F, CO2, SO2 and others) during such large eruptions of the DVP and their influence on the mass extinctions of biota including the dinosaurs appear to be closely linked.

Economic aspects of the DVP include deposits of hydrothermal fluorite and REE, Y, Nb, Ba and Sr mineralisatiom (e.g. Amba Dongar) and REE (e.g. Kamthai). Residual laterite and bauxite and fertile soils (e.g., Maharashtra, Madhya Pradesh and Gujarat) support the Al- industry and a robust agrarian sector. The DVP has also been a rich source for building materials. Indications for possible resources of native copper, PGE's and micro-diamonds have also been indicated.