Abstract Available geochronological information on Deccan indicates prolonged (started at 68.5 Ma) alkaline magmatism related to the Réunion mantle plume based on the 40 Ar/ 39 Ar ages from Sarnu-Dandali and Mundwara alkaline complexes. We studied in detail an alkaline lamprophyre, from the Sarnu-Dandali Complex, rich in groundmass (magmatic) as well as xenocrystic phlogopites and clinopyroxenes. 40 Ar/ 39 Ar age determinations of the phlogopites from this lamprophyre reveal two distinct ages of 65.44 ± 1.5 Ma and 68.17 ± 1 Ma. However, palaeomagnetic results show a VGP at 32.31° N and 298.52° E concordant with that of the Deccan Super Pole at 65.5 Ma and support the younger eruption age at c. 65.44 ± 1.5 Ma. Analysed phlogopites lack any signs of retention of excess radiogenic Ar and yield similar inverse isochron ages, which suggests that the older age of c. 68.17 ± 1 Ma belongs to the crystallization of xenocrystic phlogopite during mantle metasomatism. Trace element compositions support derivation of lamprophyre magma from an OIB-type enriched (metasomatized) mantle source with an involvement of phlogopite. This finding suggests that the pre-Deccan ages of c. 68–69 Ma reported previously may reflect the timing of metasomatism of the subcratonic lithospheric mantle during the separation of Greater-Seychelles from India at c. 68.5 Ma. The absence of pre-Deccan alkaline rocks therefore indicates the short duration (between 67–65 Ma) of alkaline as well as small-volume, volatile-rich magmatism directly related to the Réunion (Deccan) plume.

Abstract The Deccan Traps are of global interest for their possible links to Cretaceous-Tertiary (K-T) mass extinction event and global climate change. Radiometric dating of Deccan Trap lavas and intrusions have shown that bulk of the magmatic activities occurred 65 (±1) Ma. Isotopic similarity between Deccan and Reunion lavas have been used as an evidence to suggest that Deccan magmas were supplied by the Deccan-Reunion plume-head. Kutch is located in northwest India where, based on the trajectory that Indian plate took while crossing over the Deccan-Reunion plume, the earliest Deccan lavas would be expected to erupt. Contrary to this expectation, published 40 Ar/ 39 Ar dates of igneous activities suggested that Kutch volcanics are as old as the rest of the Deccan province, even with a small “tail” eruption occurring as late as ca. 61 Ma. Kutch’s geology is complicated by the fact that it is marked by old rifting events and significant neotectonic activities. Here we present four new 40 Ar/ 39 Ar ages of igneous intrusions from Kutch. Three such intrusions occur in the so-called ‘Island Belt’ that juts out of the great salt flats (Great Rann of Kutch) in northern Kutch. The fourth intrusion is a gabbroic body that occurs at Dhar Dongar in southern Kutch. Two of the Island Belt intrusions (Sadara Sill and Nir Wandh gabbro) give ages of ca. 75 Ma. A lamprophyre dike from the Island Belt gave an age of ca. 67 Ma. The fourth intrusion from Kutch Mainland yielded the youngest age found in Kutch of ca. 61 Ma. These ages, combined with those determined in previous studies (65-67 ±1 Ma), suggest the following: Earliest rift-related magmatism occurred 75 Ma, and is clearly of pre-Deccan age. The rest of the igneous activities occurred in two episodes – the most voluminous episode coincided with Deccan age (65-67 Ma) whereas a small volume igneous activity took place at ca. 61 Ma, following a 6 m.y. long hiatus. The 61 Ma episode appears to occur in many different locations within the Deccan. We suggest that the 75 Ma pre-Deccan rifting-magmatic event is a relict of magmatism that occurred during separation of Madagascar from India, which was caused by the Marion plume.

Abstract The northern part of the western continental margin of India formed due to the separation of the Seychelles from India at c. 63 Ma. This produced offshore tectonic elements such as the Gop Rift, the Saurashtra Volcanic Platform (SVP) and the Laxmi Ridge, as well as numerous seamounts, e.g. the Raman and Panikkar seamounts. The Laxmi Ridge and the Laxmi Basin have been studied using high-resolution 2D reflection seismic data and well data. Patch and pinnacle carbonate reefs, indicating shallow waters, are common in the north, whereas large, isolated platforms are usually noted in the south. Palaeo-depth estimates are made from well biostratigraphy. Subsidence studies of the SVP suggest that the burial history is consistent with the anomalously hot Réunion plume. We have performed a subsidence analysis south of the SVP on the Laxmi Ridge and Laxmi Basin. The sediment-unloaded basement depths, estimated using using flexural isostasy with effective elastic thicknesses of 10–40 km have been found to be 2000–4000 m in areas where carbonates exist. These carbonates indicate <200 m bathymetry at c. 65 Ma, and the subsidence discrepancy is thus due to thermal cooling or anomalous heating due to the Deccan plume. Patch and pinnacle reefs in the north suggests that either the rise in sea-level or the rate of subsidence of the basement were fast. The presence of large platforms in the south indicates otherwise. This is possibly due to a greater influence from the Indus Fan sediments towards the north. In addition, the Laxmi Ridge is a spreading centre that remained emergent near or above sea-level due to plume support, which was also greater in the south due to proximity to the plume. When the plume support discontinued, the ridge subsided quickly to present-day depths, which matches the subsidence expected for 60–70 Myr old oceanic crust. Supplementary material: A table is available at https://doi.org/10.6084/m9.figshare.c.3470751

Abstract The Early to Late Cretaceous Mundwara alkaline complex (comprising the Musala, Mer and Toa plugs) displays a broad spectrum of alkaline rocks closely associated in space and time with the Deccan large igneous province in NW India. Petrology and Nd–Sr isotopic data on the two youngest and altogether compositionally different lamprophyre dykes of the Mundwara alkaline complex are presented in this paper to understand their petrogenesis and also to constrain the magmatic processes responsible for generation of the rock spectrum in the complex (pyroxenite, picrite ankaramite, carbonatite, shonkinite, olivine gabbro, feldspathoidal and foid-free syenite). The two lamprophyre dykes occurring in the Mer and the Musala hills are referred to as basaltic camptonite I and camptonite II, respectively. The basaltic camptonite I is highly porphyritic and contains olivine, clinopyroxene and magnetite macrocrysts embedded within the groundmass of microphenocyrsts composed of clinopyroxene, phlogopite, magnetite and feldspar. Camptonite II, however, with a more or less similar texture, contains amphibole, biotite, magnetite and clinopyroxene within the microphenocrystic groundmass of amphibole, biotite, apatite and feldspar. Pyroxenes are chemically zoned and display corrosion of the cores, revealing that they are antecrysts developed during the early stages of magma evolution and later on inherited by more evolved magmas. The mineral chemistry and trace element composition of the lamprophyres reveal that fractional crystallization was a dominant process. Early segregation of olivine + Cr-rich clinopyroxene + Cr-spinel from a primary hydrous alkali basalt within a magmatic plumbing system is inferred, which led to the generation of basaltic camptonitic magma (M1) forming the Mer hill lamprophyre. Subsequently, progressive fractionation of pyroxene and Fe–Ti oxides from the basaltic camptonitic (M1) magma generated camptonitic (M2) magma forming the Musala hill lamprophyre. Both lamprophyre dykes on the Sr–Nd isotopic array reflect plume-type asthenospheric derivation, which largely corresponds to the Réunion plume and other alkaline rocks of the Deccan large igneous province. Our study brings out a complex sequence of processes such as crystal fractionation, accumulation and corrosion in the magmatic plumbing system involved in the generation of the Mundwara alkaline complex.

Abstract The Koyna borehole penetrated c. 1 km through the Deccan basalt units and into the cratonic basement beneath, thus providing a unique insight into the subsurface succession of the main Deccan province. Earlier studies focused on southwestern Deccan lava packages exposed in the Western Ghat escarpment, and resolved a well-constrained stratigraphy and key reference sections, but lacked supporting subsurface data. To construct the stratigraphy and correlate it with the main Deccan formations, we report flow-wise physical and chemical data of a c. 932 m-thick core. We document 37 lava-flow units and four lava-flow groups that have similar major-oxide contents. These groups fit into two of the recognized chemostratigraphic formations, and the transitional Poladpur–Ambenali lavas. In addition, data plots on Ba v. Sr; Ba v. Zr/Nb; Ba/Y v. Zr/Nb; and Ba, Sr, Ba/Y, Zr/Nb v. height bivariate diagrams confine them to the Poladpur and Ambenali formations. Lava flows match with the Khumbarli and Mahabaleshwar Ghat sections and Killari core. The granitoid basement–basalt and the Poladpur Formation v. Ambenali Formation contacts lie at −332.5 and c. 482 m above sea-level, respectively. Further, the new data endorse the southern overstepping of chemostratigraphic units and the asymmetry of the Deccan edifice due to the northward motion of the Indian Plate over the nascent Réunion plume (c. 67–64 Ma). For comparison, the oldest 66.4 Ma lava flow predates the Cretaceous–Paleogene boundary (KPB) (66.052 Ma) by <0.35 Ma, with much of the Wai Subgroup erupted syn-KPB or >0.55 Ma post-KPB; however, the restricted lava thickness at the contact between the Poladpur and Ambenali formations provides a reference point in the Deccan stratigraphy.

Abstract Understanding the Deccan Trap Large Igneous Province in western India is important for deciphering the India–Seychelles rifting mechanism. This book presents 13 studies that address the development of this province from diverse perspectives including field structural geology, geochemistry, analytical modelling, geomorphology and geophysics (e.g., palaeomagnetism, gravity and magnetic anomalies, and seismic imaging). Together, these papers indicate that the tectonics of Deccan is much more complicated than previously thought. Key findings include: the Deccan province can be divided into several blocks; the existence of a rift-induced palaeo-slope; constraints on the eruption period; rift–drift transition mechanisms determined for magma-rich systems; the tectonic role of the Deccan or Réunion plumes; sub-surface structures reported from boreholes; the delineation of the crust–mantle structure; the documentation of sub-surface tectonic boundaries; post-Deccan-Trap basin inversion; deformed dykes around Mumbai, and also from the eastern part of the Deccan Traps, documented in the field.