Himalayan Geology, Vol. 24 (2), 2003, pp. 103-111, Printed in India

Palaeomagnetism and age of the Phe Volcanics from Zanskar Valley, Ladakh

G.V.S. POORNACHANDRA RAO, M. SOMESWARA RAO, K. J. PRASANNA LAKSHMI & B. SRINIVASA RAO
Palaeomagnetism Laboratory,National Geophysical Research Institute, Hyderabad - 500 007, India

The Phe Volcanics occur as a linear belt in the Zanskar Valley over the marine fossiliferous rocks of the Carboniferous period and are overlain by the Triassic sediments, thus assigning them an age of late Carboniferous to early Permian period. In the Kurgiakh valley, the base of the Phe Volcanics represent an important unconformity all along which the volcanics show a wide formational overlap assigning them an upper Palaeozoic period. Thus, there exists a dispute with regard to their age. With a view to resolve this controversy, oriented block samples were collected from four sites and subjected to palaeomagnetic investigation to infer their probable period of emplacement. The detailed pilot and blanket demagnetization studies using alternating field demagnetization technique revealed a ChRM vector in these rocks acquired at the time of their formation. The fold test, which accounts for the observed tectonic disturbances, has been judiciously used in determining the ChRM of the rocks under study. The corresponding Virtual Geomagnetic Pole (VGP) position computed using this ChRM groups well with VGPs of the upper Palaeozoic rock formations of the other tectonic units from the subcontinent. This agreement of VGP of the Phe volcanics assign an age of upper Carboniferous - lower Permian to it and imply an autochthonous nature of the three tectonics units namely the peninsula, the extra-peninsula and the Salt Range.