ON THE CLOSURE TEMPERATURE FOR FISSION TRACKS IN ZIRCON.

The concept of closure temperature of a mineral and isotopic system (Dodson, 1973) is valuable in elucidating cooling and uplift studies, and in delineating temperature constraints for the interpretation of measured ages. Although apatite has a closure temperature well-defined at 100-120 degree C by both laboratory experiment and natural borehole studies, an effective closure temperature for the retention of FT in zircon is imprecisely known. New isotopic studies have been made on 19 orthogneiss samples (each 30 kg) along a 25 km profile through the Maggia Valley in Ticino, Southern Switzerland. Rb/Sr, K/Ar white mica and biotite results together with FT zircon and apatite dates obtained for each sample permit construction of cooling curves (extending the work of Wagner et al. , 1977) and the evaluation of a closure temperature for zircon by comparison of measured ages. From the 11 analyses completed, a mean value of 230 degree C is derived.