5:15 PM - 6:30 PM
[BPT05-P02] U–Pb dating of zircon grains from the North Pole Adamellite in the eastern Pilbara Craton
Keywords:North Pole area, Paleoarchean adamellite, U-Pb zircon dating, Concordia age
In the North Pole area, mafic-ultramafic greenstones are capped by bedded cherts, and include zircon-bearing tuffaceous chert layers and felsic lavas. These successions were regionally uplifted by later adamellite intrusion. This study focuses on the adamellite to determine the crystallization age based on U-Pb geochronology of zircon. We separated and handpicked more than 1000 zircon grains from two adamellites (95NP207 and 96NP208). These zircon grains have euhedral shapes, and also exhibit oscillatory zoning under cathodoluminescence observation. On the other hands, most of the zircon rims are enriched in non-formula elements such as Ca and Al, which indicates that the zircons partially experienced metamictization. In this study, non-metamictized domain in oscillatory zoned zircon was selected for in-situ U-Pb analysis, and the U-Pb ratio was measured with LA-MC-ICP-MS at University of Kyoto.
7 and 4 zircons were plotted on the Tera-Wasserburg concordia curve within their analytical errors from 95NP207 and 96NP208, respectively. Moreover these concordant zircons have low contents of common lead (204Pb/206Pb values < 0.0005). Based on weight mean 207Pb/206Pb ages of the concordant zircons, 95NP207 and 96NP208 were respectively dated at 3486 ± 52 Ma and 3449 ± 17 Ma. The larger error of the former data was attributed to two older zircons of 3523 ± 17 Ma and 3567 ± 14 Ma. We concluded that the adamellite intrusion had occurred before 3449 ± 17 Ma, and that the intrusive age gave the minimum depositional age of the greenstone-chert successions in the North Pole area.