Tectonic significance of dykes in the Sarnu-Dandali alkaline complex, Rajasthan, northwestern Deccan Traps

AbstractWhether swarms of preferentially oriented dykes are controlled by regional stress fields, or passively exploit basement structural fabric, is a much debated question, with support for either scenario in individual case studies. The Sarnu-Dandali alkaline complex, near the northwestern limit of the Deccan Traps continental flood basalt province, contains mafic to felsic alkaline volcano-plutonic rocks and carbonatites. The complex is situated near the northern end of the 600 km long, NNW–SSE-trending Barmer-Cambay rift. Mafic enclave swarms in the syenites suggest synplutonic mafic dykes injected into a largely liquid felsic magma chamber. Later coherent dykes in the complex, of all compositions and sizes, dominantly strike NNW–SSE, parallel to the Barmer-Cambay rift. The rift formed during two distinct episodes of extension, NW–SE in the early Cretaceous and NE–SW in the late Cretaceous. Control of the southern Indian Dharwar structural fabric on the rift trend, as speculated previously, is untenable, whereas the regional Precambrian basement trends (Aravalli and Malani) run NE–SW and NNE–SSW. We therefore suggest that the small-scale Sarnu-Dandali dykes and the much larger-scale Barmer-Cambay rift were not controlled by basement structure, but related to contemporaneous, late Cretaceous regional ENE–WSW extension, for which there is varied independent evidence

Mantle and Crustal Contributions to the Mount Girnar Alkaline Plutonic Complex and the Circum-Girnar Mafic-Silicic Intrusions of Saurashtra, Northwestern Deccan Traps

Continental flood basalt (CFB) provinces, while dominated by tholeiitic basalts and basaltic andesites, often also contain alkaline mafic to felsic lavas and intrusions. The tholeiitic and alkaline magmas may reflect different degrees of partial melting of the same mantle source, or the alkaline magmas may be derived from metasomatised, incompatible element-enriched mantle sources. The tholeiitic and alkaline suites, even if closely associated spatially or temporally, require independent magmatic plumbing systems. In the Saurashtra region of the northwestern Deccan Traps CFB province, India, tholeiitic lavas have been intruded by the ~66 Ma Mount Girnar plutonic complex, which comprises olivine gabbros (often with cumulate textures), diorites, and monzonites, profusely intruded by dykes and veins of foid-bearing syenites and lamprophyres. In the region surrounding the complex the tholeiitic lavas have been intruded by a large (12 km- diameter) silicic ring dyke, as well as tholeiitic dykes and sills. The region thus provides an excellent opportunity to study potential petrogenetic relationships between tholeiitic, alkaline and silicic magmatism in a CFB province, evaluated here using field, petrographic, mineral chemical, and whole-rock geochemical (including Sr-Nd-Pb isotopic) data. Initial (at 65 Ma) Sr-Nd-Pb isotopic ratios of an olivine gabbro and diorites of the Girnar plutonic suite are in the ranges (87Sr/86Sr)t = 0.70499 to 0.70584, (143Nd/144Nd)t = 0.512675 to 0.512484 (εNdt = +2.4 to –1.4) and (206Pb/204Pb)t = 18.270-18.679. Foid-bearing syenites and lamprophyres have broadly similar isotopic ratios and marked enrichments in the most incompatible elements. Thermobarometric calculations indicate crystallisation of mineral phases in the Girnar plutonic suite at varied crustal pressures (0.02-0.9 GPa). Small but significant Sr-Nd-Pb isotopic variations within the plutonic suite rule out closed-system fractional crystallisation as a viable process, whereas a lack of correlation between isotopic ratio and degree of magmatic evolution (rock type) also negates any simple scheme of combined assimilation-fractional crystallisation (AFC). The circum-Girnar tholeiitic intrusions, hitherto practically unstudied, are low-Ti and moderately to fairly evolved (MgO = 8.0-3.9 wt.%); olivine gabbro and picrite dykes with cumulus olivine show higher MgO (10.1–15.7 wt.%), Ni (360–700 ppm) and Cr (410–1710 ppm) contents. The circum-Girnar tholeiitic intrusions have a large range of Sr-Nd-Pb isotopic ratios (e.g. εNdt = +4.2 to –18.7) indicating open-system processes. We infer that magmas of the alkaline Girnar plutonic suite were derived from enriched mantle, with only minor crustal residence or material input, possibly reflecting a very thin basement crust under the complex. In contrast, magmas forming the circum-Girnar tholeiitic intrusions were derived from depleted mantle (εNdt > +4.2) by high degrees of melting, and they experienced olivine fractionation or accumulation in crustal chambers and significant contamination by ancient granitic basement crust. These features probably reflect a much thicker crust surrounding the plutonic complex than directly under it. The circum-Girnar silicic ring dyke has Sr-Nd-Pb isotopic ratios suggesting an origin by anatexis of the basement crust. Based on a range of evidence, the tholeiitic and silicic circum-Girnar dykes and sills are petrogenetically and structurally unrelated to the alkaline Girnar plutonic suite

Mineralogy, geochemistry and 40Ar–39Ar geochronology of the Barda and Alech complexes, Saurashtra, northwestern Deccan Traps: early silicic magmas derived by flood basalt fractionation

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

Breccia-cored columnar rosettes in a rubbly pahoehoe lava flow, Elephanta Island, Deccan Traps, and a model for their origin

Rubbly pahoehoe lava flows are abundant in many continental flood basalts including the Deccan Traps. However, structures with radial joint columns surrounding cores of flow-top breccia (FTB), reported from some Deccan rubbly pahoehoe flows, are yet unknown from other basaltic provinces. A previous study of these Deccan “breccia-cored columnar rosettes” ruled out explanations such as volcanic vents and lava tubes, and showed that the radial joint columns had grown outwards from cold FTB inclusions incorporated into the hot molten interiors. How the highly vesicular (thus low-density) FTB blocks might have sunk into the flow interiors has remained a puzzle. Here we describe a new example of a Deccan rubbly pahoehoe flow with FTB-cored rosettes, from Elephanta Island in the Mumbai harbor. Noting that (1) thick rubbly pahoehoe flows probably form by rapid inflation (involving many lava injections into a largely molten advancing flow), and (2) such flows are transitional to ‘a’ā flows (which continuously shed their top clinker in front of them as they advance), we propose a model for the FTB-cored rosettes. We suggest that the Deccan flows under study were shedding some of their FTB in front of them as they advanced and, with high-eruption rate lava injection and inflation, frontal breakouts would incorporate this FTB rubble, with thickening of the flow carrying the rubble into the flow interior. This implies that, far from sinking into the molten interior, the FTB blocks may have been rising, until lava supply and inflation stopped, the flow began solidifying, and joint columns developed outward from each cold FTB inclusion as already inferred, forming the FTB-cored rosettes. Those rubbly pahoehoe flows which began recycling most of their FTB became the ‘a’ā flows of the Deccan

Recurrent Early Cretaceous, Indo-Madagascar (89–86 Ma) and Deccan (66 Ma) alkaline magmatism in the Sarnu-Dandali complex, Rajasthan: 40Ar/39Ar age evidence and geodynamic significance

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 40Ar/39Ar age of 68.57 Ma. Rhyolites found in the complex are considered to be 750MaMalani basement. Our new40Ar/39Ar ages of 88.9–86.8Ma (for syenites, nephelinite, phonolite and rhyolite) and 66.3±0.4Ma (2σ, melanephelinite) provide clear evidence thatwhereas 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 88.9–86.8Ma40Ar/39Ar 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 40Ar/39Ar 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 fromseveral localities in southern India but hitherto unknown from northwestern India 2000 km away. Additional equivalents buried under the vast Deccan Traps are highly likely

Basaltic thermal and material inputs in rhyolite petrogenesis: the Chhapariyali rhyolite dyke, Saurashtra, Deccan Traps

The Saurashtra peninsula in the northwestern Deccan Traps continental flood basalt province, India, contains notable concentrations of rhyolitic rocks. The Chhapariyali rhyolite dyke, part of the compositionally diverse Southeastern Saurashtra dyke swarm, intrudes basaltic lava flows. It shows vitrophyric portions, basaltic magma enclaves, gabbroic mineral assemblages (plagioclase + clinopyroxene ± Fe-Ti oxide aggregates), spectacular quench textures, and intense spherulitisation. Thermobarometric calculations on equilibrium mineral–wholerock (or glass) pairs indicate clinopyroxene crystallisation at 1120–890 ◦C overlapping with plagioclase crystallisation at 948–932 ◦C, and a pressure range of 3.6–0.1 kbar, indicating crystallisation during magma storage or ascent in the upper crust. Petrographic, mineral chemical, and whole-rock geochemical (including Sr-Nd isotopic) data suggest advanced fractional crystallisation of a mafic magma with considerable assimilation of ancient granitic crust, or anatexis of such crust, due to heating by a basaltic magma chamber. In either scenario, abundant gabbroic cumulates were left by the crystallising basaltic magmas. A new basalt magma recharging the chamber entrained the gabbroic crystal mush and formed enclaves by mingling with the rhyolite magma stored above. Continued basaltic recharge pushed the enclave-bearing, gabbroic cargo-laden rhyolite magma out of the chamber and into the basaltic lava flows via a dyke. Strong supercooling within cold basaltic rock, possibly aided by meteoric water ingress, led to the development of spectacular quench textures in the crystallising rhyolite, whereas extensive low-temperature alteration produced intense spherulitisation. This case study of the Chhapariyali dyke underscores significant thermal and material inputs from basalt in rhyolite petrogenesis