Petrology and geochemistry of mafic dykes of Sylhet traps, Northeastern India, and their Kerguelen plume linkage

  • Bibhas SenEmail author
  • Tapan Pal
  • Kevilhoutuo Theunuo
Original Paper


We present here a new set of mineralogical and geochemical data from a cluster of east southeast-trending mafic dykes that intrude a lava flow of Sylhet traps. These tholeiitic dykes remained largely unattended in recent petrological and geochemical characterization of the Sylhet traps. This flood basalt province is believed to be a lava outpouring of Kerguelen plume active in the Northeastern India during the Cretaceous time along with that of the Rajmahal traps and Bengal basin. The studied dykes are generally thin (<3 m), high-angled to vertically dipping. They developed chilled margins along their contacts with the host basaltic lava and are apparently product of single magma injections. The dyke rock is porphyritic basalt with phenocrysts of bytwonite with or without Ca-rich augite set largely in subophitic to intergranular (and/or intersertal) groundmass comprising bytownite/labradorite, augite, titano-magnetite and glass. The major oxide and trace element variations in these dykes indicate that the basaltic magma underwent of fractional crystallization with concomitant assimilation. In spite, the immobile trace elements like Na, Zr, Y and rare earths of uncontaminated and little contaminated dyke samples (Nb/La >0.8) show an undoubted plume-derived character with ΔNb >0 (ΔNb indicates the source characteristics of a basalt sample by calculating excess or deficiency in Nb). These elements also led to identifying the possible genetic connection between these dykes and the Kerguelen plume having similar concentrations and variation patterns with the basalts derived from it. However, robust geochemical, geochronological and palaeomagnetic constraints for both the lava flows and the dykes of Sylhet traps are required to understand the genesis and evolution of igneous activity in this little known domain.


Geochemistry Mafic dykes Sylhet traps Kerguelen plume 



Authors are thankful to Mulkh Raj Jarngal, Additional Director General and Head, Geological Survey of India, North Eastern Region, Shillong for providing administrative support to carry out this work. We appreciate efforts of Chinchu Sv (in the EPMA laboratory, Faridabad), Debasis Banerjee (in the XRF laboratory, Chemical Division, Shillong), and Anindya Das (in the ICP–MS laboratory, Central Chemical Division, Kolkata) of the Geological Survey of India, for providing help in or analyzing our samples. Discussions with Amitava Kundu, Sujit Tripathy and Siva Shankar Kambhampati were fruitful. We also thank two anonymous reviewers, handling Associate Editor Francesco Stoppa and Editor-in-Chief Lutz Nasdala for their constructive comments that immensely improved the manuscript.

Supplementary material

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Authors and Affiliations

  1. 1.Geological Survey of India, Northeastern RegionShillongIndia

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