THE INFLUENCE OF PRECAMBRIAN SOURCE COMPONENTS ON THE U-PB ZIRCON AGE OF A PALEOZOIC GRANITE FROM NORTHERN VICTORIA LAND, ANTARCTICA

Zircons from an S-type granite in Antarctica originally analysed by conventional U-Pb procedures have been reanalysed by ion-microprobe. The new data show that the Proterozoic age previously derived for granitoid emplacement is not correct but reflects a meaningless value between the magmatic age of the granite (544±4 Ma) and that of the provenance of its source rocks. Subsequent growth of metamorphic zircon has compounded the complexity. A total of six different episodes of zircon growth are documented within this rock. These occurred at 469 ± 4 Ma, 544 ± 4 Ma, 1130 ± 50 Ma, about 2000 Ma, about 2500 Ma and 2825 ± 100 Ma. The data illustrate the ever-present danger of extrapolating isotopic data alignments to concordia, especially when they form a 'reverse' discordia, in which the data points more closely approach the younger part of the concordia than the upper part. Although not itself of Precambrian age, the granite, which forms part of the Wilson Plutonic Complex of northern Victoria Land, Antarctica, formed by melting of rocks of currently unknown depositional age, but which were apparently derived from a dominantly ∼ 1100 Ma provenance. © 1990.