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Venkateshwarlu Mamilla , J. Mallikharjuna Rao , K. P. Sarma , J. J. Laskar , Nivarani Devi

Abstract

Palaeomagnetic and petrological results from a volcanic tuff in the least studied Purana basin of the Shillong Plateau, NE India are reported here for the first time. Haematite is the dominant remanent carrier mineral identified by the rock magnetic, optical and palaeomagnetic studies. Intensity decay curves during progressive thermal demagnetization attest its characteristic nature. Characteristic Remanent Magnetization (ChRM) directions reveal that the declinations are in southern hemisphere with negative, shallow inclinations (N=5, Dm=182°, Im= -2°, α95=8.37). The signatures of the Kerguelen hotspot-related volcanism allow us to suggest that the Shillong Tuff rocks may be considered originated from an interaction between the Kerguelen hotspot and the lithosphere of the northeastern margin of Greater India during early Cretaceous (Royer&Coffin 1992).

Keywords

Shillong Plateau, Volcanic Tuff, Palaeomagnetism, NE India

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J. Mallikharjuna Rao ¹, G. V. S. Poornachandra Rao ¹, K. P. Sarma ²

Affiliations
1 National Geophysical Research Institute, Uppal Road, Hyderabad - 500 007, IN
2 Department of Geology, Guwahati University, Guwahati, IN

Abstract

The Archaean gneissic basement of Shillong plateau has been traversed by number of mafic dyke swarms. At least two suites of dykes are identified in the region represented by Proterozoic Khasi greenstone related dolerites and younger Cretaceous dolerite dykes in addition to mafic alkaline dykes. The older Khasi greenstone dolerites are altered and have undergone low-grade metamorphism compared to fresh Cretaceous dykes, which are well exposed in the West Garo Hills region. All the Khasi greenstone dolerites are tholeiite in composition and range from basalt to basaltic andesite in composition and show olivine or quartz normative character. Most of the dykes show continental nature of emplacement with some overlapping oceanic tectonic setting of origin. Petrochemical study suggests that they were derived from picrites that subsequently undergone low- pressure fractionation. Palaeomagnetic study of the older Khasi greenstone related dolerites show a direction of magnetization of Dm=17, Im=+57 (α₉₅= 23.34; K=31.5; N=24) with a palaeolatitude of 29.7°N to the Indian subcontinent that clearly support the Proterozoic dyke/dyke swarm emplacement in the region. The magnetic carrier as inferred from K-T studies is in multi domain (MD) size and cation deficient (CD) domain states.

Keywords

Khasi Greenstone, Dykes, Mafic Magmatism, Shillong Plateau, Meghalaya.

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