Deccan Flood Basalt Province Research Articles

Strontium isotope systematics of Amba Dongar and Sung Valley carbonatite-alkaline complexes, India: evidence for liquid immiscibility, crustal contamination and long-lived Rb/Sr enriched mantle sources

The results of a Sr isotopic study of coexisting alkaline silicate rocks and carbonatites of two Cretaceous alkaline complexes of India, Amba Dongar (Deccan Flood Basalt Province) and Sung Valley (Rajmahal–Bengal–Sylhet Flood Basalt Province) are reported. The overlapping nature of initial Sr isotopic ratios of alkaline rocks and carbonatites of both the complexes is consistent with a magmatic differentiation model. Modelling of initial 87Sr/ 86Sr variation in alkaline rocks of Amba Dongar is consistent with a process of crustal assimilation by the parent magma undergoing simultaneous fractional crystallization of silicate rocks and silicate–carbonate melt immiscibility. A maximum of ∼5% crustal contamination has been estimated for the parent magma of Amba Dongar, the effect of which is not seen in the Sr isotope ratio of carbonatites generated by liquid immiscibility. A two point Rb–Sr isochron of the Sung Valley carbonatites, pyoxenite and a phlogopite from a carbonatite yielded an age of 106±11 Ma, which is identical to the 40Ar– 39Ar age of this complex. The same age for the carbonatites and the alkaline silicate rocks, similar initial Sr ratios and the higher Sr concentration in the former than the latter favour the hypothesis of liquid immiscibility for the generation of the Sung Valley. The higher initial 87Sr/ 86Sr ratio for these complexes than that of the Bulk Earth indicates their derivation from long-lived Rb/Sr-enriched sources.

Emplacement of Amba Dongar carbonatite-alkaline complex at Cretaceous/Tertiary boundary: Evidence from40Ar-39Ar chronology

40Ar- 39 Ar analyses of three fresh alkaline rock samples and a phlogopite separate from a carbonatite from Amba Dongar carbonatite-alkaline complex of the Deccan Flood Basalt Province, India, yield indistinguishable precise plateau ages of 64.8±0.6, 64.7±0.5, 65.5±0.8 and 65.3±0.6 Ma, giving a mean plateau age of 65.0 ± 0.3 Ma, which is the age of emplacement of this complex. This age implies contemporaneity of Amba Dongar with several other carbonatite-alkaline activities of Chhota Udaipur subprovince and is consistent with their Reunion-Deccan plume origin hypothesis. The emplacement of these complexes at 65 Ma makes them very significant in the ongoing debate on the K/T extinctions owing to their capacity to rapidly inject a substantial amount of CO 2 and SO 2 into the atmosphere.

Isotope and rare earth element chemistry of carbonatite–alkaline complexes of Deccan volcanic province: implications to magmatic and alteration processes

Results of different isotopic and trace element studies on three carbonatite–alkaline complexes (Amba Dongar, Mundwara and Sarnu-Dandali) of the Deccan flood basalt province, India, are presented. The Amba Dongar (Ambadungar) complex has been dated precisely to 65.0±0.3 Ma by the 40Ar– 39Ar method. The minimum initial Sr isotopic ratio of alkaline rocks of Amba Dongar is found to be same as that of the coexisting carbonatites, suggesting their derivation from a common parent magma, probably through liquid immiscibility. The rare earth element abundance in these rocks also supports the liquid immiscibility hypothesis. Further investigation revealed that the parent magma of this complex has been contaminated (∼5%) by the lower crustal material, which is clearly reflected in the initial 87Sr/ 86Sr variation of alkaline rocks but not in the carbonatites. Sr study also suggests that the mantle source of Amba Dongar like the other two complexes is a Rb/Sr enriched source. The temporal and spatial relationships of all the three complexes with the Deccan flood basalts support the hypothesis of reunion plume origin for these. Fractional crystallization and subsequent hydrothermal/meteoric alteration are found to have controlled the stable carbon and oxygen isotopic variations in carbonatites. This study suggests that all the complexes have been derived from isotopically average mantle except for a particular batch of parent magma at Amba Dongar, which appears to have incorporated recycled crustal carbon. In a plume origin scenario such incorporation indicates the entrainment of 13C-enriched subcontinental lithospheric mantle by the plume.

Evolution of carbonatite complexes of the Deccan flood basalt province: Stable carbon and oxygen isotopic constraints

The stable carbon and oxygen isotopic composition of carbonatites from three carbonatite‐alkaline complexes (Amba Dongar, Mundwara and Sarnu‐Dandali) of Deccan province, India, can be placed into two groups, primary and secondary. The primary variations indicate the mantle origin of these complexes. The source of these complexes, the Reunion plume head, was largely composed of a mantle having δ18O similar to that of mean upper mantle (5 to 8‰) but higher δ13C (>−5.5‰). The higher δ13C, particularly that of a batch of parent magma for Amba Dongar (−3.4‰), suggests the incorporation of recycled inorganic crustal carbon in these carbonatites. As in the plume model for generation of continental flood basalts and associated carbonatite‐alkaline complexes, the fluid‐rich rim of the plume head acts as the source for the carbonatites, so the above incorporation of crustal carbon could have been facilitated through the migration of CO2‐rich, 13C enriched fluids (derived from ancient subducted carbonates) into the Reunion plume head. The correlated variations of δ13C and δ18O in unaltered calcite carbonatites in all three complexes are consistent with the formation of these rocks by fractional crystallization from CO2‐rich carbonate magmas, derived from parent carbonated silicate magmas through liquid immiscibility. The extreme enrichment of δ13C and δ18O in some carbonatites and metasomatic rocks is a result of postcrystallization alteration process caused by CO2‐bearing aqueous fluids. In Amba Dongar a pure magmatic fluid and/or a magmatic‐hydrothermal fluid could have caused the alteration, whereas rocks of Mundwara and Sarnu‐Dandali appear to have been altered by meteoric‐hydrothermal fluids.

Elemental and Nd–Sr–Pb isotope geochemistry of flows and dikes from the Tapi rift, Deccan flood basalt province, India

The Deccan Traps are a large rift-associated continental flood basalt province in India, parts of which have been studied extensively in terms of geochemistry, palaeomagnetism and stratigraphy. However, the basalts of the Tapi rift in the central part of the province have been little-studied thus far. Two ENE–WSW-trending tectonic inliers of the Deccan basalts in this region, forming ridges rising from younger alluvium, are made up of basalt flows profusely intruded by basaltic dikes. Both of these ridges lie along a single lineament, although they are not physically continuous. The flows are aphyric, plagioclase-phyric and giant-plagioclase basalts, and the dikes are aphyric or plagioclase-phyric. We consider the two inliers to have been originally continuous, from the presence of bouldery remnants of a major dioritic gabbro dike along both. Samples of this dike from both ridges have previously yielded typical Deccan ages of 65.6±0.5 Ma and 65.6±0.6 Ma by the 40 Ar– 39 Ar incremental heating technique. Initial 87 Sr/ 86 Sr ratios and ε Nd( t) values, and present-day Pb isotopic ratios of most dikes indicate that they are isotopically similar to lavas of the Mahabaleshwar and Panhala Formations of the Western Ghats, about 450 km to the south. Their mantle-normalized trace element patterns have small Pb and Ba peaks. One dike has a strong Bushe Formation affinity and a Nd–Sr isotopic composition more extreme than that of any other Deccan rock yet sampled, with ε Nd( t)=−20.2 and ( 87 Sr/ 86 Sr) t=0.72315 . Its mantle-normalized element pattern shows large Pb, Th and U peaks and large Nb–Ta troughs. Its elemental and isotopic chemistry reflects substantial continental contamination. The flows cut by the Mahabaleshwar-type dikes are isotopically similar to the Poladpur Formation lavas of the Western Ghats. Their mantle-normalized element patterns show modest peaks at Rb, Ba and Pb and rather low Nb and Ta relative to La, indicating that they have been contaminated to intermediate degrees. The mantle-normalized element patterns of all the flows and dikes show enrichment in the light rare-earth elements, with small or no Eu anomalies. The entire flow-dike sequence is similar to the Wai Subgroup of the Western Ghats, in terms of its elemental and isotopic chemistry and stratigraphic relationships. Wai Subgroup-like lavas (i.e., some of the younger magma types originally identified from the southern part of the Western Ghats) are previously known from the central, northern and northeastern Deccan, and many have been thought to be far-travelled flows erupted in the southwestern Deccan. Although at least the dikes, and probably the giant plagioclase basalt flows of our study area, are locally generated and emplaced, our new data extend the known outcrop area of these widespread magma types substantially, and these magma types indeed appear to have a nearly province-wide distribution.

Flood basalts and large igneous provinces from deep mantle plumes: fact, fiction, and fallacy

Flood basalts, large igneous provinces and all intraplate magmatism are almost unanimously ascribed to features called mantle plumes in current geodynamics literature. In the starting plume model, flood basalts and any associated hotspot tracks have been ascribed to the melting of a large plume head and a narrow tail respectively, and the Deccan flood basalt province of India and the Laccadives–Maldives–Chagos–Réunion Island volcanic chain in the Indian Ocean are widely thought to be an excellent example supporting this model. However, the existing geodynamic, thermal, petrological, geochemical and geochronological constraints on Deccan volcanism do not require or support a mantle plume origin. A new interpretation made here rules out any role for the Réunion plume in Deccan volcanism, and even questions the notion that the Réunion `hotspot' is the manifestation of an underlying Réunion `plume'. The Laccadives–Réunion hotspot track is viewed here as caused by a southward-propagating fracture, based on the published radiometric age data, and the ultimate cause of Deccan volcanism is argued to be mega-scale lithospheric rifting. The appreciation of the great original areal extent of the Deccan, nearly as far as the southern tip of peninsular India, solves the hitherto unsolved problem of the cause of the uplift of the 1500-km-long Western Ghats escarpment, this cause being combined surface erosion and magmatic underplating. Hypothesized mantle plumes do not appear responsible for most large igneous provinces; instead, their very existence is questionable. No geological evidence of any kind — geochemical, petrological, thermal, topographic — requires mantle plumes. The self-contradictions and special pleading that proliferate in plume explanations are unjustified and unnecessary if one accepts, based on various grounds, that a globe-encircling enriched mantle layer (the `perisphere') may overlie a deeper depleted mantle, which is the source of MORB. The systematic geochemical variations along the Laccadive–Réunion chain can be explained by this model and are not consistent with a plume tail experiencing entrainment from surrounding MORB mantle, as in the current plume model. Hotspot tracks may not reflect plate motion above stationary deep mantle upwellings but the stress state of the lithosphere, as originally proposed decades ago. The locations of large igneous provinces, hotspots and hotspot tracks are strongly controlled by lithospheric architecture and history: they are not placed randomly on the globe, indifferent to the surface geology, neither are large igneous provinces sudden chance events within the vastness of geological time (which plume theories typically allege).

Indian Intraplate and Continental Margin Rifting, Lithospheric Extension, and Mantle Upwelling in Deccan Flood Basalt Volcanism near the K/T Boundary: Evidence from Mafic Dike Swarms

Mafic dikes and dike swarms associated with tholeiitic flows of the continental Deccan flood basalt province are studied to understand the timing of lithospheric extension, rifting, and rift activation related to the eruption of the Deccan basalts. Integrated geophysical, tectonic, and geochemical studies and and K-Ar ages of these mafic dikes and associated Deccan flows along the intraplate Narmada-Tapti rift and western continental margin rift point to flood basalt eruptions between 67 and 64 Ma (peaking at ~65 Ma) with concurrent lineament reactivation into rifts and dominant N-S and E-W extension of the Indian lithosphere. Correlation of Bouger gravity, shallow and deep seismic, and heat flow anomalies show penetration of elongated and discontinuous mafic bodies between 5 and 6 km deep along the central axis and flanks of the intraplate rift and a long, elongated, and anomalous mantle body into the upper crust along the continental margin rift with upward inflexion of the Moho toward the intersection of the two rifts. Crustal thinning along the rifts ranges between 8 and 25 km. Correlated geophysical and field-structural data show listric faulting and high concentrations of mafic dikes over rift-oriented geophysical anomalies. Field and geochemical relations, and age (67-64 Ma) similarities of many mafic dikes and basal flows, indicate their comagmatic nature and establish many rift-oriented mafic dikes as primary feeders. Geochemical and petrological evidence indicates that the majority of the lower Deccan tholeiites evolved in local and multiple magma chambers close to the surface up to a depth of 7 km, which is consistent with the geophysical evidence. An asthenospheric plume origin under a thinned lithosphere for the parental olivine-tholeiites is possible. Liquid line of descent calculations shown on clinopyroxene-olivine-silica and clinopyroxene-plagioclase-olivine pseudoternary plots and variation diagrams involving MgO and various oxides and elements suggest that the tholeiites of dikes and flows are related by fractional crystallization. The dikes and flows also show evidence of contamination (mainly crustal), which diminishes from the lower Deccan to the Poladpur and the Ambenali flows. Geochemical-stratigraphic relations indicate that the basal flows and dikes in Narmada-Tapti rift region and basal flows in the Gujarat region are older than the Igatpuri flows in the Western Margin rift region. A mantle plume model of laterally spreading magma with propagation of dikes from magma chambers and formation of magma chambers and eruptive centers along lithospheric weak trends is presented to explain copious and rapid eruptive activity over a widely spread area, crustal contamination during ascent, and magma evolution in shallow crustal chambers in the Deccan province.

Did Deccan volcanism pre-date the Cretaceous/Tertiary transition?

Nine basalt samples collected from the bottom to the top of a > 2.5 km thick composite section in the western margin of the Deccan Flood basalt province, India, yield 40Ar- 39Ar plateau ages between 67 and 62.5 Ma relative to an age of 520.4 Ma for the monitor standard MMhb-1. These ages are consistent with the stratigraphy and comparable to earlier results [1]. They indicate that the lower ∼ 2 km thick reversely magnetised lava sequence erupted within ∼ Ma close to 67 Ma ago and the eruption interval even for the exposed and surviving units of the Deccan was not less than 3 Ma. With the Cretaceous/Tertiary boundary as demarcated by Haiti tektites (65.2 ± 0.1 Ma) [2] and melt rock glasses from Chicxulub crater dated precisely at 64.98 ± 0.10 Ma [3] relative to MMhb-1 (age 520.4 Ma), our results on the thick reversely magnetised lava units imply their eruption before the Cretaceous/Tertiary boundary events by more than the 1.0 Ma.