2003 Volume 112 Issue 1 Pages 1-17
Zircon U-Pb ages were re-measured for Archean felsic intrusive and volcanic rocks of the greenstone belt in the North Pole area, Pilbara craton, Western Australia by using laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS). A concordant age for a single zircon grain from the North Pole adamellite is measured to be 3404 ± 60 Ma, much younger than previously reported zircon U-Pb discordant age 3459 ± 19 Ma from the same unit. Together with the oldest 207 Pb/206 Pb TIMS age 3418.3 ± 0.13 Ma previously reported, the present result suggests that adamellite intruded into the surrounding basaltic greenstones at ca. 3420 Ma.
We measured 16 zircon grains in total from 4 rhyolitic tuffs intercalated in the greenstones of the area, and most of them range within 3482-3576 Ma. The youngest age, i.e. 3482 Ma, is consistent with the previously reported concordant TIMS age, 3458 Ma, from the same unit. This suggests that rhyolitic volcanism occurred probably around 3460 Ma. A single zircon grain from rhyolite (lava) with an exceptionally older 207Pb/206Pb age, 3609 ± 31 Ma, may have been reworked from older units.
The present results indicate that the North Pole adamellite intruded into the basaltic greenstones at about 3420 Ma, while rhyolite erupted much earlier at 3458 Ma together with basalt. This newly recognized age gap of ca. 40 m.y. is consistent with the field observations on the intrusion of adamellite into basaltic greenstones, and with the interbedded association of rhyolite and basalt. Thus the previous model for a syngenetic and synchronous relationship between adamellite and rhyolite in the North Pole area is clearly rejected.
