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The Banded Gneissic Complex of the Aravalli Craton of northwestern India comprises Palaeo‐ to Neoarchaean (3.3–2.7 Ga) gneisses, including tonalite–trondhjemite–granodiorite (TTG) gneisses, intruded by Neoarchaean (2.6–2.5 Ga) TTGs, sanukitoids, and potassic granites. The latter include the Malola granite that crops out near Bhilwara and forms the basement for the Mesoproterozoic Pur‐Banera mineralized belt. The granite has experienced three deformation events, represented by an S1 foliation and superimposed open folds and dextral shears. Textural evidence and thermobarometry indicate that prograde metamorphism initially forming actinolite and subsequently diopside, and culminating in the upper amphibolite facies, was followed by retrograde metamorphism in the greenschist facies with fluid influx, forming zoisite. Zircon geochronology yields a crystallization age of 2,538 ± 11 Ma and indicates a prominent metamorphic overprint in the region at 1,307 ± 9 Ma. The Malola granite has low Sr–Nd isotopic ratios, enrichments in fluid‐mobile elements (Cs, Rb, Ba, Th, and Pb), and depletions in alteration‐resistant elements (Nb, Ta, and Ti). These characteristics combined with mineralogical evidence suggest that the granite was derived by anatexis of older crystalline crust (possibly granulitic lower crust) in the same subduction zone setting inferred previously for other Neoarchean granitoids in this region. Geochemical and Sr–Nd–Hf isotopic similarities between the Malola granite and Neoarchaean granitoids of the Aravalli, Bundelkhand, Bastar, Dharwar, and Congo cratons indicate a major role for anatectic crustal reworking in their genesis. [ABSTRACT FROM AUTHOR]

Continental flood basalt (CFB) provinces such as the Deccan Traps show numerous inter-flow horizons called boles, most commonly red or brown. These have been variably interpreted as altered glassy bases of basalt flows, paleosols developed on exposed flow tops, altered volcanic ash beds, or local inter-flow sediments. We believe that each of these mechanisms may apply, singly or in combination, to specific red boles, and propose an additional mechanism, namely the weathering of flow-top and flow-bottom breccias. The breccias are inherently highly prone to weathering, and evidence for the mechanism is a close observed correspondence between breccia zones and red boles. The mechanism explains the absence of red boles in the compound pāhoehoe flows which characteristically lack breccias, and their abundance in rubbly pāhoehoe and 'a'a lava flows. The latter observation has been misinterpreted as indicating significant eruptive hiatuses and a low eruption frequency. Importantly, even all breccia-derived boles do not have the same significance: those derived from flow-top breccias are paleosols, formed by subaerial weathering during eruptive breaks. Those derived from flow-bottom breccias, thus not in contact with the atmosphere, were formed due to groundwater ingress along flow contacts and fractures. These boles have developed slowly over long time periods. Significant development of all bole types must also have occurred in post-volcanism time and is arguably ongoing even today in the exposed and unexposed parts of CFB sequences. Recognizing multiple and complex origins of superficially similar-looking red boles is of critical importance in CFB volcanology and flow stratigraphy, and a warning sign to straightforward application of mineral chemical, mineral magnetic, geochemical, and similar data on boles to interpreting paleoweathering, paleoclimates, and paleoenvironments. [ABSTRACT FROM AUTHOR]

The ENE‐WSW‐trending Nandurbar‐Dhule swarm is the best developed tholeiitic dyke swarm in the Deccan Traps. We obtained 40Ar/39Ar ages of 67.06 ± 0.60, 67.49 ± 0.89, and 63.43 ± 0.48 Ma (2σ internal errors) on three of its largest dykes (36–79 km long), indicating that swarm emplacement spanned ≥2.5 million years under regional crustal extension. Our Sr‐Nd‐Pb isotopic data, combined with previously available geochemical data, identify multiple magma injections in some dykes and also identify probable feeder dykes of some lavas in the lower Western Ghats sequence and in Saurashtra, each ~200 km away. Several dykes are compositionally distinct from hitherto analysed lavas; >50% of the analysed Nandurbar‐Dhule dykes are isotopically like the Mahabaleshwar and Panhala formations of the upper Western Ghats sequence, covering a very narrow isotopic range, but have the distinctive chemical signatures of the high‐TiO2 Kolhapur Unit of the southernmost Western Ghats. These dykes thus possess a unique combination of isotopic and chemical characteristics not hitherto known in Deccan tholeiites, cross‐combining features of different eruptive units in the Wai Subgroup of the Western Ghats. This new, "Nandurbar‐type" chemical‐isotopic flavour is however frequently observed in dykes, sills, and lavas in the Pachmarhi, Shahdol, and Mandla areas 450–600 km to the east, and in Deccan‐age dykes cutting through the Early Cretaceous Rajmahal Traps of eastern India. Varied geochemical evidence indicates that the northern and northeastern Deccan lava stratigraphy (such as the Pavagadh section and the Pachmarhi‐Shahdol‐Jabalpur‐Mandla areas) is largely independent of the Western Ghats lava stratigraphy. [ABSTRACT FROM AUTHOR]

The Sarnu-Dandali alkaline complex in Rajasthan, northwestern India, is considered to represent early, pre-flood basalt magmatism in the Deccan Traps province, based on a single 40 Ar/ 39 Ar age of 68.57 Ma. Rhyolites found in the complex are considered to be 750 Ma Malani basement. Our new 40 Ar/ 39 Ar ages of 88.9–86.8 Ma (for syenites, nephelinite, phonolite and rhyolite) and 66.3 ± 0.4 Ma (2σ, melanephelinite) provide clear evidence that whereas the complex has Deccan-age (66 Ma) components, it is dominantly an older (by ~ 20 million years) alkaline complex, with rhyolites included. Basalt is also known to underlie the Early Cretaceous Sarnu Sandstone. Sarnu-Dandali is thus a periodically rejuvenated alkaline igneous centre, active twice in the Late Cretaceous and also earlier. Many such centres with recurrent continental alkaline magmatism (sometimes over hundreds of millions of years) are known worldwide. The 88.9–86.8 Ma 40 Ar/ 39 Ar ages for Sarnu-Dandali rocks fully overlap with those for the Indo-Madagascar flood basalt province formed during continental breakup between India (plus Seychelles) and Madagascar. Recent 40 Ar/ 39 Ar work on the Mundwara alkaline complex in Rajasthan, 120 km southeast of Sarnu-Dandali, has also shown polychronous emplacement (over ≥ 45 million years), and 84–80 Ma ages obtained from Mundwara also arguably represent post-breakup stages of the Indo-Madagascar flood basalt volcanism. Remnants of the Indo-Madagascar province are known from several localities in southern India but hitherto unknown from northwestern India 2000 km away. Additional equivalents buried under the vast Deccan Traps are highly likely. [ABSTRACT FROM AUTHOR]

The Mundwara alkaline plutonic complex (Rajasthan, north-western India) is considered a part of the Late Cretaceous-Palaeogene Deccan Traps flood basalt province, based on geochronological data (mainly Ar/Ar, on whole rocks, biotite and hornblende). We have studied the petrology and mineral chemistry of some Mundwara mafic rocks containing mica and amphibole. Geothermobarometry indicates emplacement of the complex at middle to upper crustal levels. We have obtained new Ar/Ar ages of 80-84 Ma on biotite separates from mafic rocks and 102-110 Ma on whole-rock nepheline syenites. There is no evidence for excess Ar. The combined results show that some of the constituent intrusions of the Mundwara complex are of Deccan age, but others are older and unrelated to the Deccan Traps. The Mundwara alkaline complex is thus polychronous and similar to many alkaline complexes around the world that show recurrent magmatism, sometimes over hundreds of millions of years. The primary biotite and amphibole in Mundwara mafic rocks indicate hydrous parental magmas, derived from hydrated mantle peridotite at relatively low temperatures, thus ruling out a mantle plume. This hydration and metasomatism of the Rajasthan lithospheric mantle may have occurred during Jurassic subduction under Gondwanaland, or Precambrian subduction events. Low-degree decompression melting of this old, enriched lithospheric mantle, due to periodic diffuse lithospheric extension, gradually built the Mundwara complex from the Early Cretaceous to Palaeogene time. [ABSTRACT FROM AUTHOR]

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 Ar-Ar 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 Ar-Ar 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 Ar-Ar 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. [ABSTRACT FROM AUTHOR]

Abstract: The Saurashtra region in the northwestern part of the Deccan continental flood basalt province (India) is notable for its many volcano-plutonic complexes, compositional diversity, and the abundance of rhyolite and granophyre. In this first detailed study of the Chogat-Chamardi subvolcanic complex in eastern Saurashtra, we report microgranite and granophyre plutons, gabbro plutons cut by basaltic andesite dykes, as well as rhyolite plugs. Bulk-rock geochemical (including Nd–Sr isotopic) data suggest a strong contribution from ancient, Rb-rich basement crust to the silicic magmas. The mafic rocks are also crustally contaminated but less than the silicic rocks, in conformity with observations elsewhere (e.g., the Skye and Skaergaard granophyres). The Chogat-Chamardi silicic rocks have initial 87Sr/86Sr (for t =65Ma) as high as 0.72914, and initial ε Nd values as low as −13.9. Trends defined by the Chogat-Chamardi and other Deccan rhyolitic suites on plots of Sr concentration vs. Sr isotopic ratio are modeled with concurrent assimilation and fractional crystallization (AFC) processes involving a basaltic magma and granitic basement rocks. Considering both Nd and Sr isotopic variations, the Chogat-Chamardi silicic rocks notably require crustal end members very different from those inferred in most previous isotopic studies of Deccan rocks. [Copyright &y& Elsevier]

Barren Island, Andaman Sea, is India's only active volcano and is built of lava flows and volcaniclastic deposits of prehistoric through recent ages that are mainly of basalt and basaltic andesite compositions. Historic eruptions occurred during 1787-1832, and it erupted thrice recently in 1991, 1994-95 and 2005-06. A fourth impressive one that might have begun in late 2008 was in full swing in March 2009, with a well-developed lava flow that was entering the sea from a completely different route (north) than all the recent lava flows. We describe and illustrate this new eruption and discuss aspects of hazards, logistics and geotourism potential of this active volcano. [ABSTRACT FROM AUTHOR]

Abstract: Crustal or mantle xenoliths are not common in evolved, tholeiitic flood basalts that cover huge areas of the Precambrian shields. Yet, the occasional occurrences provide the most direct and unequivocal evidence on basement composition. Few xenolith occurrences are known from the Deccan Traps, India, and inferences about the Deccan basement have necessarily depended on geophysical studies and geochemistry of Deccan lavas and intrusions. Here, we report two basalt dykes (Rajmane and Talwade dykes) from the central Deccan Traps that are extremely rich in crustal xenoliths of great lithological variety (gneisses, quartzites, granite mylonite, felsic granulite, carbonate rock, tuff). Because the dykes are parallel and only 4 km apart, and only a few kilometres long, the xenoliths provide clear evidence for high small-scale lithological heterogeneity and strong tectonic deformation in the Precambrian Indian crust beneath. Measured 87Sr/86Sr ratios in the xenoliths range from 0.70935 (carbonate) to 0.78479 (granite mylonite). The Rajmane dyke sampled away from any of the xenoliths shows a present-day 87Sr/86Sr ratio of 0.70465 and initial (at 66 Ma) ratio of 0.70445. The dyke is subalkalic and fairly evolved (Mg No.=44.1) and broadly similar in its Sr-isotopic and elemental composition to some of the lavas of the Mahabaleshwar Formation. The xenoliths are comparable lithologically and geochemically to basement rocks from the Archaean Dharwar craton forming much of southern India. As several lines of evidence suggest, the Dharwar craton may extend at least 350–400 km north under the Deccan lava cover. This is significant for Precambrian crustal evolution of India besides continental reconstructions. [Copyright &y& Elsevier]

Abstract: The patterns of eruption and dispersal of flood basalt lavas on the surface, or as magmas in dykes and sills within the crust, determine the volcanological and stratigraphic development of flood basalt provinces. This is a geochemical and Sr-isotopic study of lavas of varied compositions that outcrop around Mount Pavagadh (829m), Deccan Traps, an important outlier north of the main basalt outcrop. Most of the ∼550-m thick exposed section at Pavagadh is made up of subalkalic basalts rich in the incompatible elements (particularly Nb, Ba, and Sr). Picrite and rhyolite–dacite flows also occur, the latter capping the sequence. The relatively high initial 87Sr/86Sr ratios (up to 0.7083) and chemical characteristics of the rhyolitic rocks of Pavagadh are consistent with a small but significant involvement of the granitic basement crust in their genesis. An assimilation–fractional crystallization (AFC) model involving the picrite lava and either a southern Indian or a western Indian granite as the contaminant explains the geochemical and Sr-isotopic variation in the basalts and the rhyolites quite well. A systematic comparison of the basaltic lavas (with binary plots, normalized multielement patterns, and discriminant function analysis) to the well-established lava stratigraphy of the Western Ghats, 400–500km to the south, precludes any chemical–genetic relationships between the two. Basalts exposed in sections closer to Pavagadh (∼150–200km), in the Toranmal, Navagam, and Barwani–Mhow areas, have several flows with some similar chemical characteristics. However, the Pavagadh sequence is significantly different from all of these sequences geochemically, petrogenetically, and in magnetic polarity, to be considered independently built. This result is significant in terms of eruptive models for the Deccan Traps, as it is increasingly apparent that there were separate but possibly coeval eruptive centers with their own distinctive chemistries developed in various areas of this vast province. [Copyright &y& Elsevier]

Elephanta Island near Mumbai is an important area for understanding the stratigraphic and structural framework of the Deccan flood basalt province in the tectonically disturbed Panvel flexure zone on the western Indian rifted margin. Elephanta exposes a west-dipping, 66–65 Ma sequence of tholeiitic lava flows and dykes. Geochemical correlations with the thick, horizontal, 66–65 Ma Western Ghats sequence to the east show that lava flows of the Khandala and Ambenali formations are present at Elephanta, with two lava flows probably being locally derived. The Elephanta tholeiites have experienced crystal fractionation and accumulation, particularly of olivine. They have εNd(t) ranging from +5.4 to −7.9 and (87Sr/86Sr)t from 0.70391 to 0.70784, with most tholeiites little contaminated by continental lithosphere, probably lower crust. Field and geochemical data indicate a normal fault along the central part of Elephanta with a 220 m downthrow, consistent with a domino-type block-faulted structure of Elephanta, and the surrounding area as previously known. Seventeen of the 20 analyzed Elephanta intrusions, striking ~N–S, belong to the Coastal dyke swarm of the western Deccan province. Several of these are probable feeders to the Ambenali Formation in the Western Ghats sequence, requiring reconsideration of the current view that the voluminous Wai Subgroup lavas of the Western Ghats were erupted without organized crustal extension. East–west-directed extensional strain was already active at 66–65 Ma along this future (62.5 Ma) rifted continental margin. A young (~62 Ma) ankaramite dyke on Elephanta Island is a probable feeder to the Powai ankaramite flow in the 62.5 Ma Mumbai sequence 20 km to the northwest. [ABSTRACT FROM AUTHOR]

Abstract: The Tavidar volcanic suite in western Rajasthan, India, comprises a group of lava flows (and subordinate pyroclastic deposits) of highly diverse compositions ranging from basalt through trachyte to rhyolite. We have dated five samples of the Tavidar volcanic rocks by the 40Ar–39Ar incremental heating technique. One trachyte and two rhyolite samples yield very good plateau, isochron and inverse isochron ages of 67–65Ma, typical of the Deccan Traps large igneous province. A subalkalic basalt and a basaltic trachyandesite yield saddle-shaped argon release spectra and show evidence for excess argon. Importantly, all five samples have very similar Nd–Sr isotopic ratios, and constitute a broadly cogenetic magmatic suite with the rhyolites possibly derived by closed-system fractional crystallization of trachytic magmas. The Tavidar rocks’ isotopic data overlap, or are very close to, those of the Mahabaleshwar and Panhala Formation basalts in the Western Ghats type section 700–800km to the southeast. We therefore infer that the Tavidar rocks, having initial Nd values of +3.2 to +0.7, have incorporated only small amounts of lower continental crust. The Tavidar volcanic suite attests to the great areal extent of the Deccan Traps, and reaffirms the great compositional diversity evident in the northwestern Deccan Traps. [Copyright &y& Elsevier]

• Pur-Banera, a supracrustal sequence near Bhilwara, Aravalli Craton. • Structural mapping and zircon–monazite geochronology. • Complementary age records from zircons and monazites. • Magmatism at 3.2, 2.5, 1.8 Ga; Sedimentation at 1.6–1.3 Ga; Deformation at 1.3, 1.05, 0.85 Ga. • Relationship to assembly and breakup of supercontinent Rodinia. The Banded Gneissic Complex (BGC) of the Aravalli Craton in Rajasthan, northwestern India, forms the Archaean basement to the major, extensively studied, Proterozoic-age metasedimentary sequences of the Aravalli and Delhi Supergroups. However, the poorly documented deformation history of the basement and other supracrustal sequences, and insufficient geochronological work, limit a holistic understanding of the evolution of the Aravalli Craton. Here we present structural analysis and zircon and monazite geochronology of the relatively poorly studied Pur-Banera supracrustal belt in the Bhilwara region of south-central Rajasthan. The belt comprises paragneisses, carbonates, calc-gneisses, banded iron formations, garnetiferous schists, and amphibolites, deposited over Aravalli Supergroup rocks and locally also granite. A single detrital zircon core from the Pur-Banera paragneisses yields a 207Pb/206Pb age of 3159 ± 8 Ma (1σ), indicating a Mesoarchaean protolith. Magmatic zircon grains from the granite, here termed the Malola Granite, provide a 207Pb/206Pb crystallisation age of 2538 ± 11 Ma (95% confidence). The Pur-Banera supracrustals have experienced three phases of deformation, D 1 –D 3 , each of which produced a tectonic fabric. Zircon and monazite geochronology provides complementary ages. The detrital zircons, derived from both magmatic and metamorphic protoliths, provide ages of 3.2–1.7 Ga. Monazite ages of 1.3–0.8 Ga record multiple post-depositional tectonothermal events. Textural and compositional evidence and U–Th–total Pb monazite ages indicate that D 1 deformation occurred at ca. 1.30 Ga as a result of NW–SE-directed shortening, forming upright folds on the primary foliation that resulted in a penetrative fabric in the gneisses and schists. Subsequent NNE–SSW-directed deformation (D 2) between 1.13 and 1.05 Ga produced WNW–ESE-trending vertical folds in the gneisses and crenulations in the schists. Asymmetrical folds developed due to N–S dextral shearing at 0.85–0.80 Ga constitute the last phase of deformation (D 3). The Pur-Banera supracrustal rocks probably represent a rift-fill sequence, of Mesoproterozoic age (thus temporally equivalent to the Delhi Supergroup), deposited on an Archaean and Palaeoproterozoic basement, and repeatedly reworked during 1.30–0.80 Ga. [ABSTRACT FROM AUTHOR]