CONVENTIONAL U-PB DATING OF SINGLE FRAGMENTS OF ZIRCON FOR PETROGENETIC STUDIES OF PHANEROZOIC GRANITOIDS

The investigation of accessory zircon has become a primary tool of petrogenetic studies. Not only can zircons survive erosion and crustal melting and thus store information on the protolith, but zoned zircon crystals can also record discrete stages of growth that can be correlated with geological events. To extract precise age information it is necessary to isolate these zircon zones and analyze them separately. We demonstrate the feasibility of mechanically isolating single, geometrically localized, fragments of these zones from zircons of the > 125 mum size fraction. The single zircon grain is embedded in Canada balsam, placed between glass plates, frozen with liquid air and cracked. Fragmentation is monitored at 250-400 x magnification and recorded on a videotape that, run in reverse mode, allows reconstruction of the original position of the fragments. Using conventional U-Pb isotope dilution techniques we substantiate that single fragments of Phanerozoic age and weighing less than 1 mug can be dated with sufficient accuracy and precision. The fragments provide a spreading in the U-Pb ratios and allow resolution into the various age components. Applications on previously dated zircon populations from granitoids in the polyorogenic Caledonian mountain belt of East Greenland were successful. They confirm that morphological study combined with the U-Pb analysis of a few well-defined single fragments from one carefully selected zircon grain may eventually suffice to determine both the age and nature of geological events that contributed to the evolution of such rocks.