Abstract
Rb-Sr isochron ages have been determined for two suites of Precambrian dikes in the Bidadi-Harohalli area of southeast Mysore State. Whole-rock samples of unmetamorphosed dolerites yield an age of 2420±246 (2σ) m.y., which is a minimum value for the intruded Peninsular Gneiss and Closepet Granite. The dolerite magma originated in the mantle, as indicated by the initial 87Sr/ 86Sr ratio of 0.7012±0.0010 (2σ). A suite of alkaline dikes, also referred to as felsite and feldspar porphyry dikes, has an age of 832±40 (2σ) m.y., which correlates with the intrusion of the Chamundi Hill Granite and the feldspar porphyry dikes near Srirangapatnam. One of the alkaline dikes has a K-Ar age of 810±25 m.y., indicating an absence of subsequent thermal events in the area.
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