Potassium–Argon Ages from the Oldest Metamorphic Belt in India

Abstract

INTENSIVE structural and stratigraphic studies, supplemented by several potassium–argon ages, recently led Sarkar and Saha^1–3 to recognize two distinct orogenic belts in the Pre-Cambrian tract of Singhbhum and adjacent areas^4,5 in Eastern India (Fig. 1, inset). The orogeny which gave rise to the Iron Ore orogenic belt in the south—which includes the Iron Ore Series of low grade metasediments and metavolcanics—had its culmination in the emplacement of the granitic rocks (about 2,000 × 10⁶ yr) which constitute the northern part of the Singhbhum granitic complex (Fig. 1). The Singhbhum Orogenic belt in the north, which is thought to have a closing age of about 900 × 10⁶ yr and which consists mostly of high grade metasediments and basic rocks, truncates the Iron Ore Orogenic belt along a prominent arcuate thrust zone. Evidence for the existence of an older group—underlying the Iron Ore Series, the Older Metamorphic Series—of moderate to high grade metamorphic argillites, calcmagnesian metasediments and arenites was also recorded^2,3. Sarkar and Saha⁶ also suggested that there was old granitic activity associated with the Older Metamorphic Series about 3,000 × 10⁶ yr ago on the basis of a potassium–argon age of 3,035 × 10⁶ yr (ref. 7) from a coarse muscovite pegmatite near Joropokhar (22° 24′ 45″: 85° 45′ 10″).