K-Ar age of Ukra glauconites from the Kutch Basin, India

The present paper reports the K-Ar ages determined on glauconitic samples c ollected from the Ukra Member of the Mesozoic Bhuj Formation in two differe nt sections, one located on the Ghuneri-Ghaduli road near Katesar Mahadeo t emple and the other at the base bf the Ukra hill in the northwestern part o f the Kutch Mainland area. Three glauconite samples viz., Ukra(KT)-1, Ukra(KT)-4 and Ukra(UH)-3 have y ielded K-Ar ages of 107.9 +/- 3.4 Ma, 105.5 +/- 3.3 Ma and 103.5 +/- 3.4 Ma , respectively. The sample Ukra(KT)-1 treated with 0.5N HCl and analysed in duplicate has yielded a mean age of 104 +/- 2.3 Ma while the sample UkraKT -4 treated with 0.1N HCl has given an age of 106.5 +/- 3.3 Ma. The ages of the treated and untreated glauconites are indistinguishable within 2 sigma uncertainty with a mean of 105.2 +/- 1.3 Ma, which has been interpreted as the depositional age of the Ukra Member of the Mesozoic Bhuj Formation. The study has further indicated that mild acid treatment (up to 0.5N HCl) does not lead to any loss of radiogenic argon in the glauconites and can be hel pful in purification of the samples.