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Most continental flood basalt (CFB) provinces of the world contain silicic (granitic and rhyolitic) rocks, which are of significant petrogenetic interest. These rocks can form by advanced fractional crystallization of basaltic magmas, crustal assimilation with fractional crystallization, partial melting of hydrothermally altered basaltic lava flows or intrusions, anatexis of old basement crust, or hybridization between basaltic and crustal melts. In the Deccan Traps CFB province of India, the Barda and Alech Hills, dominated by granophyre and rhyolite, respectively, form the largest silicic complexes. We present petrographic, mineral chemical, and whole-rock geochemical (major and trace element and Sr–Nd isotopic) data on rocks of both complexes, along with 40Ar–39Ar ages of 69.5–68.5 Ma on three Barda granophyres. Whereas silicic magmatism in the Deccan Traps typically postdates flood basalt eruptions, the Barda granophyre intrusions (and the Deccan basalt flows they intrude) significantly pre-date (by 3–4 My) the intense 66–65 Ma flood basalt phase forming the bulk of the province. A tholeiitic dyke cutting the Barda granophyres contains quartzite xenoliths, the first being reported from Saurashtra and probably representing Precambrian basement crust. However, geochemical–isotopic data show little involvement of ancient basement crust in the genesis of the Barda–Alech silicic rocks. We conclude that these rocks formed by advanced (70–75 %), nearly-closed system fractional crystallization of basaltic magmas in crustal magma chambers. The sheer size of each complex (tens of kilometres in diameter) indicates a very large mafic magma chamber, and a wide, pronounced, circular-shaped gravity high and magnetic anomaly mapped over these complexes is arguably the geophysical signature of this solidified magma chamber. The Barda and Alech complexes are important for understanding CFB-associated silicic magmatism, and anorogenic, intraplate silicic magmatism in general.

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.

Rapid northward drift of the Indian plate during Deccan volcanism assumes a gradual equatorward shallowing of the paleomagnetic inclinations amongst subsequently younger lava flows. Compilation of palaeomagnetic database and using 1062 statistically significant site mean directions from the Deccan Volcanic Province discovered an inclination anomaly of +10 degrees during the Deccan main phase eruptions (DE M within Chron C29r at 66.398–65.688 Ma). The anomaly represents northward tilt during C29r followed by its restoration in C29n (~65 Ma). This anomaly is explained here by the Indian lithospheric response to Réunion plume head during DE M . A sequence of coincident geodynamic instances including: (i) biostratigraphically constrained ‘the brief inland seaway’, (ii) development of a regional southward dip for the lava flows, (iii) major drop in sea level at the southern tip of peninsula, and (iv) accelerated spreading and convergence rates during C29r to 29n transitions; substantiate the effect of lithospheric tilt and its restoration. We present a plume-lithosphere evolutionary model to explain the anomaly and its wider implications over the Indian lithospheric plate.

Geochronological research on the Deccan traps, one of the larger continental flood basalt provinces, has been underway for over 50 years. Initial attempts by K–Ar dating, yielding scattered dates, was superseded by 40 Ar/ 39 Ar step-heating studies; however, several of these later attempts failed to follow the guidelines for what defines (statistically valid) plateau/isochron ages. It has been shown that working with altered samples can significantly hamper success; an alteration index, recovered from the argon isotopic data, uncovers which samples are altered. Over the last decade, high-precision argon ages narrowed the range of crystallisation ages of the bulk of tholeiitic material in this province to <1 Myr, straddling the Cretaceous–Palaeogene boundary. A group of researchers continue to insist that the duration of volcanism forming most of the composite Western Ghats section (CWGS) is 3–4 Myr. This dichotomous position is cleared up by critical assessment of all reports, including high-precision 40 Ar/ 39 Ar ages on plagioclase separates from flows, as well as U–Pb dating of single crystals of zircon, following chemical abrasion, from ash fall deposits between the lava flows, or high silica aggregations within the flows. It is shown that the duration of volcanism for the CWGS was limited to ~800 ka, starting at ~66.4 Ma. Lingering problems with the presentation and use of these high-precision ages hamper working out full details of the timing and duration of lavas, both within and between the formations of the CWGS. Lavas from the Mandla lobe, the Malwa plateau and the Rajahmundry traps, were also formed within this time period. Reports of argon ages of ~69 Ma for silicic volcanism in the Saurashtra area are shown to be incorrect; much of the silicic–alkaline volcanism in the Deccan is known to be coeval with the main phase of tholeiitic volcanism, based on both argon and U–Pb research. The bulk of volcanism in other continental flood basalt provinces (the Siberian traps, the Parana province and Columbia river basalt group) are now known to have been each confined to <1 Myr.

A ~3000 m long exposure of paleo-lava channel body within the Deccan volcanic province (India) has been investigated for mineral magnetism and anisotropy of magnetic susceptibility (AMS) to examine the lava flow-related fabrics. Distinct gradient from ferrimagnetic to antiferromagnetic mineralogy along the channel indicated progressive oxidative mixing. The AMS studies along and across the channel profiles decipher a combination of factors responsible for the resultant fabrics. Various dominating mechanisms including gravity settling, grain imbrications, viscous shear and competitive mineral readjustments penultimate to cooling appears to have imparted the final/resultant fabric. The orientation of principle susceptibility axes (K1-K2-K3) and petrographic observations define three major types of fabrics as: (I) mineral lineation, (II) crystal settling and (III) crystal-matrix-re-adjustment. The resultant of type I along with imbrications is generally observed along the axial part, while type II are developed during lava residency; and the opaque−non-opaque interlocking with groundmass resulted in type III fabrics. A three-stage model is produced to resolve the plausible sequence of fabric development during various stages of the lava flow formation. More detailed AMS studies on the Deccan lavas are envisaged to lead semi-quantitative information on fluid dynamical forces governed by slope gradients and gravity, dynamic thermal changes in viscosity, shear and the effects of paleotopography. Research Highlights Reporting magnetic fabrics and mineralogy in a systematically sampled lava channel. Interlocking of opaque and non-opaque grains penultimate to cooling dominate over crystal settling and viscous shear producing majority of resultant fabrics. Mineral magnetism recorded oxidative mixing as the channel progressed. AMS fabrics envisage influence of paleotopography and dynamic viscosity. The fabrics reflect dynamic setting amongst gravitational settling, temperature gradients, viscosity changes under decreasing accommodation spaces.

The western Indian volcanic rifted margin, and its large-scale tectonic feature called the Panvel flexure, formed at 62.5 Ma during the late stages of Deccan Traps flood volcanism. We present a geological account of late-stage (≤ 62.5 Ma) Deccan volcanism and plutonism in the relatively poorly studied Thane–Vasai region in the Panvel flexure zone. The study area shows west-dipping basaltic sequences up to hundreds of meters thick, overlain by pyroclastic deposits of various types. The volcanic units are intruded by gabbro plutons, and all these units are in turn intruded by dykes of varied compositions (including tholeiitic basalt, lamprophyre, and granophyre). There are also early tholeiitic dykes, some of which may be feeders to the basaltic sequence. We focus on the gabbro intrusions and provide extensive petrographic and mineral chemical data on them. The gabbros are tholeiitic, and of considerable interest in commonly containing interstitial silicic melts (granophyre or silicic glass). One of the intrusions, the Chena pluton, shows clear outcrop transitions from gabbro, and gabbro with interstitial granophyre, to transitional gabbro-granophyre, and then to a distinct upper zone of granophyre and microgranite. Granophyre is common in mafic intrusions in continental flood basalt provinces of the world (e.g., the Palisades Sill and the Skaergaard Intrusion), where its genesis is ascribed to mechanisms such as fractional crystallisation, liquid immiscibility, or crustal melting, typically based on geochemical data. The Chena gabbro outcrops are valuable in providing direct evidence for a fractional crystallisation origin of the granophyres (and silicic glasses) found in the Thane–Vasai gabbro intrusions, with which the mineral chemical compositions are also consistent.

A 2 to 4 °C warming episode, known as the Latest Maastrichtian warming event (LMWE), preceded the Cretaceous–Paleogene boundary (KPB) mass extinction at 66.05 ± 0.08 Ma and has been linked with the onset of voluminous Deccan Traps volcanism. Here, we use direct measurements of melt-inclusion CO₂ concentrations and trace-element proxies for CO₂ to test the hypothesis that early Deccan magmatism triggered this warming interval. We report CO₂ concentrations from NanoSIMS and Raman spectroscopic analyses of melt-inclusion glass and vapor bubbles hosted in magnesian olivines from pre-KPB Deccan primitive basalts. Reconstructed melt-inclusion CO₂ concentrations range up to 0.23 to 1.2 wt% CO₂ for lavas from the Saurashtra Peninsula and the Thakurvadi Formation in the Western Ghats region. Trace-element proxies for CO₂ concentration (Ba and Nb) yield estimates of initial melt concentrations of 0.4 to 1.3 wt% CO₂ prior to degassing. Our data imply carbon saturation and degassing of Deccan magmas initiated at high pressures near the Moho or in the lower crust. Furthermore, we find that the earliest Deccan magmas were more CO₂ rich, which we hypothesize facilitated more efficient flushing and outgassing from intrusive magmas. Based on carbon cycle modeling and estimates of preserved lava volumes for pre-KPB lavas, we find that volcanic CO₂ outgassing alone remains insufficient to account for the magnitude of the observed latest Maastrichtian warming. However, accounting for intrusive outgassing can reconcile early carbon-rich Deccan Traps outgassing with observed changes in climate and atmospheric pCO₂.

Clay minerals formed mainly by the alteration of volcanic glass are reported by many workers. The commonly formed minerals belonging to the smectite group especially nontronite and ferrosaponite are widely reported by many in basalts. However, most of the reports suggest the formation of ferrosaponite in marine conditions (Zhou et al., 1992; Chen et al., 1996) or by hydrothermal solutions (Shayan et. al., (1988)). Origin of ferrosaponite in continental flood basalts by a mechanism of alteration of volcanic glass to ferrosaponite is reported in the paper. The different stages involved in the alteration process are also highlighted. Copyright 2016 Geological Society of India

Fluid driven metasomatism and mass transfer from the earth's mantle have played an important role in the evolution of the lower continental crust in many geodynamically active areas. The epicentral region of the disastrous 1993 Killari earthquake (M 6.2), concealed below a thick suite of Deccan volcanics in central India, appear to be one such region. In connection with the study of seismotectonics of the earthquake prone Deccan volcanic region, we have carried out systematic and detailed geochemical and mineralogical investigation on core samples from the basement, obtained from the 617m deep KLR-1 borehole, drilled in the epicentral region of Killari. Our investigations indicate that the basement, concealed below 338m thick Deccan volcanics, is made up of CO2, Cl, FeO and CaO-rich, high density (2.82 g/cm3) - high velocity (avg. Vp: 6.2 km/s) moderately retrogressed upper amphibolite to granulite facies mid crustal rocks, which were subjected to pervasive Ca-metasomatism due to infiltration of mantle fluids. Granitic gneissic layer, typical of the upper crust, seems to be totally absent from this earthquake region. Chondrite normalized trace and rare earth elemental patterns display negative Eu anomalies together with LILE enrichment. Similarly, spider diagrams for incompatible elements show depletion in Zr, Hf, Y, Ta and Nb relative to the primitive mantle, indicating possible alterations of such relatively immobile elements at relatively high temperatures. Selective enrichment is also observed in transitional elements like Cu and Zn, indicating the possible role of chlorine in metal transport. The present study suggests that regional metasomatism beneath the Deccan Traps, which apparently alters the basic fabric of the rock during recrystallisation and makes it weak, may have a link with the nucleation of large earthquakes. Copyright 2014 Geological Society of India

The present paper completes a restudy of the main lava pile in the Deccan flood basalt province (trap) of India. Chenet et al. (2008) reported results from the upper third, and this paper reports the lower two thirds of the 3500‐m‐thick composite section. The methods employed are the same, i.e., combined use of petrology, volcanology, chemostratigraphy, morphology, K‐Ar absolute dating, study of sedimentary alteration horizons, and as the main correlation tool, analysis of detailed paleomagnetic remanence directions. The thickness and volume of the flood basalt province studied in this way are therefore tripled. A total of 169 sites from eight new sections are reported in this paper. Together with the results of Chenet et al. (2008), these data represent in total 70% of the 3500‐m combined section of the main Deccan traps province. This lava pile was erupted in some 30 major eruptive periods or single eruptive events (SEE), each with volumes ranging from 1000 to 20,000 km3 and 41 individual lava units with a typical volume of 1300 km3. Paleomagnetic analysis shows that some SEEs with thicknesses attaining 200 m were emplaced over distances in excess of 100 km (both likely underestimates, due to outcrop conditions) and up to 800 km. The total time of emission of all combined SEEs could have been (much) less than 10 ka, with most of the time recorded in a very small number of intervening alteration levels marking periods of volcanic quiescence (so‐called “big red boles”). The number of boles, thickness of the pulses, and morphology of the traps suggest that eruptive fluxes and volumes were larger in the older formations and slowed down with more and longer quiescence periods in the end. On the basis of geochronologic results published by Chenet et al. (2007) and paleontological results from Keller et al. (2008), we propose that volcanism occurred in three rather short, discrete phases or megapulses, an early one at ∼67.5 ± 1 Ma near the C30r/C30n transition and the two largest around 65 ± 1 Ma, one entirely within C29r just before the K‐T boundary, the other shortly afterward spanning the C29r/C29n reversal. We next estimate sulfur dioxide (likely a major agent of environmental stress) amounts and fluxes released by SEEs: they would have ranged from 5 to 100 Gt and 0.1 to 1 Gt/a, respectively, over durations possibly as short as 100 years for each SEE. The chemical input of the Chicxulub impact would have been on the same order as that of a very large single pulse. The impact, therefore, appears as important but incremental, neither the sole nor main cause of the Cretaceous‐Tertiary mass extinctions.