Hetu C. Sheth

Pin-pricking the elephant: Evidence on the origin of the Ontong Java Plateau from Pb-Sr-Hf-Nd isotopic characteristics of ODP Leg 192 basalts

Abstract Age-corrected Pb, Sr and Nd isotope ratios for early Aptian basalt from four widely separated sites on the Ontong Java Plateau that were sampled during Ocean Drilling Program Leg 192 cluster within the small range reported for three earlier drill sites, for outcrops in the Solomon Islands, and for the Nauru and East Mariana basins. Hf isotope ratios also display only a small spread of values. A vitric tuff with εNd(t) = +4.5 that lies immediately above basement at Site 1183 represents the only probable example from Leg 192 of the Singgalo magma type, flows of which comprise the upper 46–750 m of sections in the Solomon Islands and at Leg 130 Site 807 on the northern flank of the plateau. All of the Leg 192 lavas, including the high-MgO (8–10 wt%) Kroenke-type basalts found at Sites 1185 and 1187, have εNd(t) between +5.8 and +6.5. They are isotopically indistinguishable from the abundant Kwaimbaita basalt type in the Solomon Islands, and at previous plateau, Nauru Basin and East Mariana Basin drill sites. The little-fractionated Kroenke-type flows thus indicate that the uniform isotopic signature of the more evolved Kwaimbaita-type basalt (with 5–8 wt% MgO) is not simply a result of homogenization of isotopically variable magmas in extensive magma chambers, but instead must reflect the signature of an inherently rather homogeneous (relative to the scale of melting) mantle source. In the context of a plume-head model, the Kwaimbaita-type magmas previously have been inferred to represent mantle derived largely from the plume source region. Our isotopic modelling suggests that such mantle could correspond to originally primitive mantle that experienced a rather minor fractionation event (e.g. a small amount of partial melting) approximately 3 Ga or earlier, and subsequently evolved in nearly closed-system fashion until being tapped by plateau magmatism in the early Aptian. These results are consistent with current models of a compositionally distinct lower mantle and a plume-head origin for the plateau. However, several other key aspects of the plateau are not easily explained by the plume-head model. The plateau also poses significant challenges for asteroid impact, Icelandic-type and plate separation (perisphere) models. At present, no simple model appears to account satisfactorily for all of the observed first-order features of the Ontong Java Plateau.

From Deccan to Réunion: No trace of a mantle plume

The widely accepted mantle plume model postulates that (1) the currently volcanically active Réunion Island in the Indian Ocean is fed by the narrow “tail” of a mantle plume that rises from the core-mantle boundary, (2) the Deccan continental flood basalt province of India originated from the “head” of the same plume during its early eruptive phase near the end of the Cretaceous, and (3) the Lakshadweep-Chagos Ridge, an important linear volcanic ridge in the Indian Ocean, is a product of the plume. It is not generally appreciated, however, that this “classic” case of a plume contradicts the plume model in many ways. For example, there is little petrological evidence as yet that the Deccan source was “abnormally hot,” and the short (~1.0–0.5 m.y.) duration claimed by some for the eruption of the Deccan is in conflict with recent Ar-Ar age data that suggest that the total duration was at least ~8 m.y. The Deccan continental flood basalts (CFB) were associated with the break-off of the Seychelles microcontinent from India. Geological and geophysical data from the Deccan provide no support for the plume model and arguably undermine it altogether. The interplay of several intersecting continental rift zones in India is apparently responsible for the roughly circular outcrop of the Deccan. The Lakshadweep-Chagos Ridge and the islands of Mauritius and Réunion are located along fracture zones, and the apparent systematic age progression along the ridge may be a result of southward crack propagation through the oceanic lithosphere. This idea avoids the problem of a 10 paleolatitude discrepancy which the plume model can solve only with the ad hoc inclusion of mantle roll. Published Ar-Ar age data for the LakshadweepChagos Ridge basalts have been seriously questioned, and geochemical data suggest that they likely represent postshield volcanism and so are unsuitable for hotspot-based plate reconstructions. “Enriched” isotopic ratios, such as values of Sr/Sr higher than those for normal mid-ocean ridge basalts, which have been observed in basalts of the ridge and the Mascarene Islands, may mark the involvement of delaminated enriched continental mantle instead of a plume. High values of He/He also do not represent a deep mantle component or plume. The three Mascarene islands (Mauritius, Réunion, and Rodrigues) are not related to the Deccan but reflect the recent (post-10 Ma) tectonic-magmatic development of the Africa Plate. I relate CFB volcanism to continental rifting, which often (but not always) evolves into full-fledged seafloor spreading. I ascribe the rifting itself not to mantle plume heads but to large-scale plate dynamics themselves, possibly aided by long-term thermal insulation beneath a supercontinent that may have surface effects similar to those predicted for “plume incubation” models. Nonplume plate tectonic models are capable of explaining the Deccan in all its greatness, and there is no trace of a mantle plume in this vast region.

40Ar–39Ar age of the St. Mary’s Islands volcanics, southern India: record of India–Madagascar break-up on the Indian subcontinent

Abstract The felsic volcanics (rhyolites and rhyodacites) of the St. Mary’s Islands (SMI), southern India (∼13°N), were originally interpreted as a distant outlier of the ∼65 Ma Deccan volcanic province of west–central India, comprising dominantly flood basalts. Later the SMI volcanics were dated at ∼93 Ma by the K–Ar technique. However, this K–Ar ‘age’ was dubious, being merely an average of five out of six widely varying dates and arbitrary data selectivity being involved in this averaging. Our first 40Ar–39Ar dating of the SMI volcanics yields excellent plateau and isochron ages, and their weighted mean isochron age is 85.6±0.9 Ma (2σ). Interestingly, the southern Indian Precambrian terrain is intruded by numerous mafic–doleritic dyke swarms ranging in age from Proterozoic to the latest Cretaceous (69–65 Ma, Deccan-related), and indeed, two regional dykes (a leucograbbro and a felsite) from the Kerala region of southwestern India remain previously dated at ∼85 Ma, but again with the K–Ar technique. However, this age for the SMI volcanics also corresponds excellently with 40Ar–39Ar ages of ∼89–85 Ma (weighted mean isochron age 87.6±1.2 Ma, 2σ: equivalent to 88.1±1.2 Ma corresponding to MMhb-1 age of 523.1±2.6 Ma) for the Madagascar flood basalt province. Together, therefore, the Madagascar flood basalt province, the SMI volcanics, and possibly the Kerala dykes could represent volcanic activity associated with the break-up of Greater India (India plus Seychelles) and Madagascar, thought to have occurred in the Upper Cretaceous at ∼88 Ma.

Geology and Geochemistry of the Sangamner Mafic Dike Swarm, Western Deccan Volcanic Province, India: Implications for Regional Stratigraphy

Numerous large, NE‐SW‐ to E‐W‐trending mafic dikes outcrop around Sangamner in the western Deccan Volcanic Province. This area is part of a broader region postulated to be a shieldlike feature and a major eruption center. A combination of field, geochemical, and isotopic (Sr and Nd) characteristics is used here to understand the relationship of this dike swarm with the associated lava flows and their position in the established Deccan stratigraphy. Many dikes are compositionally similar to the Khandala and Poladpur formations belonging to the Lonavala and Wai subgroups, respectively, while one dike is similar to the Ambenali Formation. One dike has a composition distinct from all other dikes in this area as well as from most stratigraphic units, although there are many similarities in composition with the Bushe Formation as well as the Boyhare Member of the Khandala Formation. While several dikes are geochemically similar to specific flows/members within certain formations, their isotopic composition is often different, sometimes significantly so. This implies either that there is a greater range in isotopic composition for those members than previously realized or that magmas with different isotopic compositions underwent broadly similar petrogenetic evolution leading to similarities in elemental composition. NE‐SW‐trending Poladpur‐ and/or Khandala‐like dikes are concentrated in the central part of the area; these dikes appear to represent a vent system that could have fed southern, western, or eastern exposures of these younger formations. It is also possible, however, that some or many of the dikes along this system were simply late‐stage intrusions of magmas representing the younger formations.

40Ar‐39Ar ages of Bombay trachytes: Evidence for a Palaeocene phase of Deccan volcanism

We present 40Ar-39Ar ages of 60.4±0.6 Ma and 61.8±0.6 Ma (2σ) for Deccan Trap trachytes from Manori and Saki Naka, Bombay, situated in the tectonized Panvel flexure zone along the western Indian rifted continental margin. These ages provide clear evidence that (i) these trachytes are of Palaeocene age and therefore substantially younger than the lower part of the main flood basalt sequence exposed in the Western Ghats, which precedes the K-T Boundary in age and (ii) the formation of the Panvel flexure along the west coast must have been subsequent to ∼60 Ma. Considering early alkaline Deccan rocks previously dated at ∼68.5 Ma, the total duration of Deccan volcanism was at least ∼8 MY.

A historical approach to continental flood basalt volcanism: insights into pre-volcanic rifting, sedimentation, and early alkaline magmatism

Abstract Continental flood basalts are widely thought to be produced from mantle plume heads. However, plume theories do not observe any role for lithospheric rifting before flood basalt events, and consider irrelevant the fact that most, if not all, continental flood basalts have erupted through deep rifts containing thick sedimentary sequences. At best, plume theories invoke selective capture of such deep rifts or lithospheric thinspots by rising mantle plume heads. However, the fact that CFBs of the world erupted through deep, ancient rift zones, and alternative dynamical considerations of flood basalt genesis, directly lead towards a new, historical approach to flood basalt emplacement. This approach takes cognizance of the basic unity of geological history and processes, satisfactorily explains pre-volcanic rifting, sedimentation, mantle metasomatism, and early, pre-tholeiite, enriched alkaline magmatism for tens of millions of years. Both incubating and impacting plume heads ought to lead to pre-volcanic lithospheric doming which is usually not observed in flood basalts. Continental or oceanic flood basalt events instead seem to be derived by convective partial melting during sudden lithospheric pull-apart (splitting) along pre-existing lithospheric discontinuities such as deep rifts or fracture zones.

Beyond Subduction and Plumes: A Unified Tectonic-Petrogenetic Model for the Mexican Volcanic Belt

The Mexican Volcanic Belt (MVB) is a major linear belt of Miocene to present-day volcanism in southern Mexico. Its origin has been controversial, although the majority opinion views it as a volcanic arc related to the subduction of the Cocos plate under the North American plate. Both calc-alkaline and alkaline volcanism characterize the belt; the latter has been previously cited as indicative of the role of a mantle plume. Here we present objections to these explanations, and conclude on the basis of geological, geochemical, and geophysical data that the MVB is unrelated to subduction or to a mantle plume, and is instead a rift-like structure experiencing active extension. Calc-alkaline or alkaline geochemistry of magmas is not useful for inferring tectonic setting, but reflects source parameters and petrogenetic processes. For the MVB, calc-alkaline geochemistry suggests crustal contamination, and the OIB-like geochemistry suggests an enriched mantle source. Our proposal of a heterogeneous mantle beneath the MVB comprising “normal” mantle and metasomatic, enriched veins, can explain the close association in space and time of calc-alkaline and alkaline volcanism throughout the belt.

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

Abstract 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 eNd(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 eNd(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.

Were the Deccan Flood Basalts Derived in Part from Ancient Oceanic Crust Within the Indian Continental Lithosphere

Abstract Deep mantle plumes supposedly incorporate deeply subducted eclogitized oceanic crust, and continental flood basalts (CFBs) are now thought by some to be derived from such eclogite-bearing peridotite plumes. Eclogite-peridotite mixtures have much lower solidi (and produce much greater melt fractions for a given temperature) than peridotite. Fe-rich (eclogite- or pyroxenite-bearing) sources have been inferred for many CFBs. However, plumes with considerable amounts of eclogite should have difficulty in upwelling owing to the high density of eclogite. Besides, CFBs are always located along pre-existing lithospheric structures (suture zones, edges of thick cratons) and commonly associated with lithospheric rifting and continental breakup. Indias major late Mesozoic CFB, the Deccan Traps, erupted through rift zones and a new continental margin that had developed along ancient suture zones traversing the subcontinent. Many Deccan basalts are too Fe-rich to have been in equilibrium with a peridotite mantle source, and have commonly been considered to be significantly fractionated derivatives of picritic liquids. However, it is possible to view them as relatively less evolved liquids derived from a source with extra fertility (i.e., an Fe-rich source). A new non-plume, plate tectonic model for Icelandic hotspot volcanism involves melting of a shallowly recycled slab of eclogitized Iapetus oceanic crust formerly trapped along the Caledonian suture. The model explains the geochemical-petrological characteristics of Icelandic basalts, and is consistent with passive upper mantle upwelling under Iceland inferred from recent seismic tomography. Based on the petrological and geochemical features of the Deccan flood basalts of the type section, in the Western Ghats, I propose that old, eclogitized oceanic crust trapped in the ancient Indian suture zones could have produced voluminous basaltic melts during the Deccan event.

Correlations between silicic volcanic rocks of the St Mary's Islands (southwestern India) and eastern Madagascar: implications for Late Cretaceous India–Madagascar reconstructions

Abstract: The St Marys Islands (southwestern India) expose silicic volcanic and sub-volcanic rocks (rhyolites and granophyric dacites) emplaced contemporaneously with the Cretaceous igneous province of Madagascar, roughly 88–90 Ma ago. The St Marys Islands rocks have phenocrysts of plagioclase, clinopyroxene, orthopyroxene and opaque oxide, moderate enrichment in the incompatible elements (e.g. Zr = 580–720 ppm, Nb = 43–53 ppm, La/Ybn = 6.9–7.2), relatively low initial 87Sr/86Sr (0.7052–0.7055) and near-chondritic initial 143Nd/144Nd (0.51248–0.51249). They have mineral chemical, whole-rock chemical and isotopic compositions very close to those of rhyolites exposed between Vatomandry–Ilaka and Mananjary in eastern Madagascar, and are distinctly different from rhyolites from other sectors of the Madagascan province. We therefore postulate that the St Marys and the Vatomandry–Ilaka–Mananjary silicic rock outcrops were adjacent before the Late Cretaceous rifting that split Madagascar from India. If so, they provide a valuable tool to check and aid traditional Cretaceous India–Madagascar reconstructions based on palaeomagnetism, matching Precambrian geological features, and geometric fitting of continental shelves. Supplementary material: Mineral analyses, mass-balance calculations and locality information are available at http://www.geolsoc.org.uk/SUP18332.