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40Ar/39Ar geochronology and geochemistry of the Central Saurashtra mafic dyke swarm: insights into magmatic evolution, magma transport, and dyke-flow relationships in the northwestern Deccan Traps

Ciro Cucciniello1, Elena I. Demonterova2, Hetu Sheth3, Kanchan Pande3, Anjali Vijayan3
 
1. Dipartimento di Scienze della Terra, dell’Ambiente e delle Risorse (DiSTAR), Università di Napoli Federico II, via Mezzocannone 8, 80134 Napoli (Naples), Italy
2. Institute of the Earth’s Crust, Siberian Branch of the Russian Academy of Sciences, Irkutsk, Russia
3. Department of Earth Sciences, Indian Institute of Technology Bombay, Powai, Mumbai 400076 India
 
 
Bulletin of Volcanology 77:45
                                              
Abstract
The Central Saurashtra mafic dyke swarm in the northwestern Deccan Traps contains a few picrites, several subalkalic basalts and basaltic andesites, and an andesite. We have obtained precise 40Ar/39Ar ages of 65.6 ± 0.2 Ma, 66.6 ± 0.3 Ma, and 62.4 ± 0.3 Ma (2σ errors) for three of the dykes, indicating the emplacement of the swarm over several million years. Mineral chemical and whole-rock major and trace element and Sr-Nd isotopic data show that fractional crystallization and crystal accumulation were important processes. Except for two dykes (with εNdt values of –8.2 and –12.3), the magmas were only moderately contaminated by continental crust. The late-emplaced (62.4 Ma) basalt dyke (CSD11) has compositional characteristics (low La/Sm and Th/Nb, high εNdt of +4.3) suggesting little or no crustal contamination. Most dykes are low-Ti and a few high-Ti, and these contrasting Ti-types cannot be produced by fractional crystallization processes but require distinct parental magmas. Some dykes are compositionally homogeneous over tens of kilometers, whereas others are heterogeneous, partly because they were formed by multiple magma injections. The combined field and geochemical data establish the Sardhar dyke as ≥ 62 km long and the longest in Saurashtra, but this and the other Central Saurasthra dykes cannot have fed any of the hitherto studied lava flow sequences in Saurashtra, given their very distinct Sr-Nd isotopic compositions. As observed previously, high-Ti lavas and dykes only outcrop east-northeast of a line joining Rajkot and Palitana, probably because of underlying enriched mantle at ~65 Ma.
 
Keywords: Volcanism; Deccan Traps; flood basalt; dyke swarms; India; Saurashtra
 
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Didascalia figura
 
Photomicrographs (left column) and back-scattered electron (BSE) images (right column) of selected Central Saurashtra dykes. (a) Picrite CSD8, with olivine phenocrysts in a groundmass of olivine, clinopyroxene, plagioclase and opaque oxides. Parallel polars. Olivine phenocrysts in the BSE image (b) are zoned (light grey zones are Fe-rich) and contain small spinel inclusions. (c) Subalkalic basalt CSD17, with large clinopyroxene crystals completely or partially enclosing small plagioclase laths and subhedral or rounded olivine grains. Crossed polars. (d) Subalkalic basalt CSD16, with olivine phenocrysts set in a subophitic groundmass of clinopyroxene, plagioclase and opaque oxides. BSE image. (e) Subalkalic basalt CSD18, with subophitic to intergranular texture. Crossed polars. (f) Groundmass of tholeiitic basalt CSD18, with altered olivine, plagioclase, clinopyroxene, pigeonite, orthopyroxene, opaque oxides and quartz. BSE image. (g) Basaltic andesite CSD5, with clinopyroxene and plagioclase microphenocrysts in a groundmass (h, BSE image) of clinopyroxene, plagioclase, pigeonite, quartz and opaque oxides, parallel polars. Abbreviations: ol, olivine; pl, plagioclase; cpx, clinopyroxene; pg, pigeonite; opx, orthopyroxene; ox, opaque oxides; cr, chromiferous spinel; mt, magnetite; ilm, ilmenite; qz, quartz; afs, alkali feldspar.