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Authors
Affiliations
1 National Geophysical Research Institute, Hyderabad 500 007, IN
Source
Journal of Geological Society of India (Online archive from Vol 1 to Vol 78), Vol 23, No 6 (1982), Pagination: 267-276
Abstract
40Ar_39Ar incremental heating studies have been carried out on a set of Middle Cambrian (age) whole-rock samples, which, in an earlier study (Hayatsu and Carmichael, 1970), yielded a K-Ar isochron age of ∼400 Ma and an initial 40Ar_39Ar ratio of ∼500. Age spectrum plots reveal that almost all of the argon originates from fine-grained K-feldspar and plagioclase in these rocks; the argon clock in these minerals was almost totally reset by a deformational event (part of the Acadian Orogeny) which occurred ∼400 Ma ago. Plotting the results on an Isochron diagram suggests that the high initial argon ratios noted earlier are not due to isotopic homogenisation on a grainsize scale. Since feldspars are known to be susceptible to argon loss at temperatures of >250°C, mild deformation and/or metamorphism in suitable rocks can now be identified and dated by careful 40Ar_39Ar incremental heating work.
