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The Panvel flexure is a 150-km long tectonic structure, comprising prominently seaward-dipping Deccan flood basalts, on the western Indian rifted margin. Given the active tectonic faulting beneath the Panvel flexure zone inferred from microseismicity, better structural understanding of the region is needed. The geology of Elephanta Island in the Mumbai harbour, famous for the ca. mid-6th century A.D. Hindu rock-cut caves in Deccan basalt (a UNESCO World Heritage site) is poorly known. We describe a previously unreported but well-exposed fault zone on Elephanta Island, consisting of two large faults dipping steeply east–southeast and producing easterly downthrows. Well-developed slickensides and structural measurements indicate oblique slip on both faults. The Elephanta Island fault zone may be the northern extension of the Alibag–Uran fault zone previously described. This and two other known regional faults (Nhava–Sheva and Belpada faults) indicate a progressively eastward step-faulted structure of the Panvel flexure, with the important result that the individual movements were not simply downdip but also oblique-slip and locally even rotational (as at Uran). An interesting problem is the normal faulting, block tectonics and rifting of this region of the crust for which seismological data indicate a normal thickness (up to 41.3 km). A model of asymmetric rifting by simple shear may explain this observation and the consistently landward dips of the rifted margin faults.

The Sindreth Group exposed near Sirohi in southern Rajasthan, western India, is a volcanosedimentary sequence. Zircons from Sindreth rhyolite lavas and tuffs have yielded U–Pb crystallization ages of ~768–761 Ma, suggesting that the Sindreth Group is a part of the Malani magmatic event. Earlier 40Ar–39Ar studies of other Malani volcanic and plutonic rocks yielded disturbed argon release spectra, ascribed to a ~550 Ma thermal event possibly related to the Pan-African orogeny. To test and confirm this possibility, we dated two whole-rock and three feldspar separate samples of the Sindreth volcanics by the 40Ar–39Ar method. All samples yield disturbed argon release spectra suggesting radiogenic argon loss and with plateau segments at 550 Ma or 490 Ma. We interpret these as events of argon loss at 550–490 Ma related to an Ediacaran–Cambrian thermal event, possibly related to the Malagasy orogeny. The combined older and new 40Ar–39Ar results are significant in showing that whereas Ediacaran–Cambrian magmatic and metamorphic events are well known from many parts of India, they left thermal imprints in much of Trans-Aravalli Rajasthan as well. The overall evidence is consistent with a model of multiphase assembly of Gondwanaland from separate continental landmasses.

The St Mary's 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 Mary's 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 Mary's 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. [ABSTRACT FROM AUTHOR]

Abstract: In the Deccan region of western India ferricrete duricrusts, usually described as laterites, cap some basalt summits east of the Western Ghats escarpment, basalts of the low-lying Konkan Plain to its west, as well as some sizeable isolated basalt plateaus rising from the Plain. The duricrusts are iron-cemented saprolite with vermiform hollows, but apart from that have little in common with the common descriptions of laterite. The classical laterite profile is not present. In particular there are no pisolitic concretions, no or minimal development of concretionary crust, and the pallid zone, commonly assumed to be typical of laterites, is absent. A relatively thin, non-indurated saprolite usually lies between the duricrust and fresh basalt. The duricrust resembles the classical laterite of Angadippuram in Kerala (southwestern India), but is much harder. The High Deccan duricrusts capping the basalt summits in the Western Ghats have been interpreted as residuals from a continuous (but now largely destroyed) laterite blanket that represents in situ transformation of the uppermost lavas, and thereby as marking the original top of the lava pile. But the unusual pattern of the duricrusts on the map and other evidence suggest instead that the duricrusts formed along a palaeoriver system, and are now in inverted relief. The two interpretations lead to different tectonic histories. Duricrust formation involved lateral material input besides vertical elemental exchange. We may have reached the stage when the very concepts of laterite and lateritization are hindering progress in regolith research.

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. India's 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.

Abstract: Rootless cones, also (erroneously) called pseudocraters, form due to explosions that ensue when a lava flow enters a surface water body, ice, or wet ground. They do not represent primary vents connected by vertical conduits to a subsurface magma source. Rootless cones in Iceland are well studied. Cones on Mars, morphologically very similar to Icelandic rootless cones, have also been suggested to be rootless cones formed by explosive interaction between surface lava flows and ground ice. We report here a group of gentle cones containing nearly circular craters from Mount Pavagadh, Deccan volcanic province, and suggest that they are rootless cones. They are very similar morphologically to the rootless cones of the type locality of Mývatn in northeastern Iceland. A group of three phreatomagmatic craters was reported in 1998 from near Jabalpur in the northeastern Deccan, and these were suggested to be eroded cinder cones. A recent geophysical study of the Jabalpur craters does not support the possibility that they are located over volcanic vents. They could also be rootless cones. Many more probably exist in the Deccan, and volcanological studies of the Deccan are clearly of value in understanding planetary basaltic volcanism.

Kilauea volcano in the Hawaiian Islands, Pacific Ocean, is the world’s most active volcano. Observations of active lava flows of Kilauea have a great relevance to studies of older, extinct volcanic systems, such as those found in India.

The "calc-alkaline" series of rocks was originally defined in the early 1930s on the basis of the "alkali-lime index" on a combined SiO2 versus (Na2O + K2O) and SiO2 versus CaO plot. The usage of the term has evolved considerably since, and today it is used variably for the subalkalic basaltandesite-dacite-rhyolite suite, or any rock suite containing andesite, or island-arc rocks, or rocks with high ratios of large-ion-lithophile elements (LILE) to high-field-strength elements (HFSE), or simply rocks with negative HFSE anomalies (e.g., Nb-Ta) in primitive mantle-normalized multielement diagrams. Although such variable usage is normal in science, the use of certain geochemical variation diagrams to define and depict the calc-alkaline series is not strictly appropriate. Two of these widely used diagrams are the total alkalies-silica (TAS) diagram and the (Na2O + K2O)-FeO*-MgO (AFM) triangular diagram, neither of which has calcium as one of the plotting parameters. The TAS diagram can be used to depict "alkaline," "subalkaline," and probably "transitional" rocks, but not "calc-alkaline" or "high-alumina" rocks. Care must be taken while using such diagrams for rock classification, inasmuch as some of them are inherently unsuitable and several together may classify even a single rock into widely different associations. "Association" or "suite" are probably better terms than "series," because they imply neither comagmatic relationships nor linear trends. The calc-alkaline suite of rocks is abundant along destructive plate margins, but calc-alkaline geochemistry is not a 100% foolproof indicator of subduction processes, inasmuch as calc-alkaline rocks are also known from regions undergoing extension, such as the Basin and Range province and the Gulf of California. Therefore, caution must be exercised in interpreting ancient terrains with complicated geology and calc-alkaline rocks as former subduction zones. Not all orogenic andesites are calc-alkaline, and not all calc-alkaline andesites are orogenic.

A petrologically diverse suite of Deccan Trap rocks, comprising basalts, trachytes, rhyolites, and various alkaline rocks, was studied in the Bombay area along the western Indian rifted continental margin. A previous petrogenetic study of these trachytes and rhyolites suggested derivation through fractional crystallization of basaltic magmas, or partial melting of basaltic material at depth, without involvement of continental crust. A Rb-Sr isochron age for the rhyolites was also calculated, but this isochron in our view is fictitious and represents a mixing line instead. Although some of the evolved liquids may have formed by fractional crystallization of basaltic magmas or partial melting of earlyformed basalts, we show that Rb/Sr-87Sr/86Sr variations in the Bombay trachyte-rhyolite suite are consistent with a combined assimilation-fractional crystallization (AFC) process involving a parental trachyte magma (Sr 225 ppm, 87Sr/86Sr = 0.7040) and an Archaean basement gneiss (Sr 145 ppm, 87Sr/86Sr = 0.7690). Although the required amount of crustal contamination is small (0.27-3.15%), the effects are sufficiently large to make any isochron age determination unviable. The role of simple or complex mixing/contamination processes must be evaluated and rejected before age significance is ascribed to linear arrays in parent-daughter isotopic ratio plots.

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.

A randomly oriented dike swarm in the Western Ghats region has been postulated to be the feeder dike swarm of the ∼2 km thick sequence exposed in that region of the Deccan province, and interpreted as evidence for the lack of crustal extension before this major flood basalt event. An enormous, central shield volcano has also been postulated in the same region based on flow stratigraphic studies and the randomly oriented dikes. These interpretations are subject to numerous objections and the lack of crustal extension before Deccan volcanism is not supported by presently available data. Rift zones of the province and the western Indian continental margin remain highly probable source areas for large volumes of the Deccan lavas.

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 outcroptransitions 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.