GRANITE-GREENSTONE TERRANES IN THE PILBARA BLOCK, AUSTRALIA, AS COEVAL VOLCANO-PLUTONIC COMPLEXES - EVIDENCE FROM U-PB ZIRCON DATING OF THE MOUNT EDGAR BATHOLITH

The Mount Edgar Batholith in the Pilbara Block, Western Australia, consists, in part, of a gneiss complex that resembles other Archean high-grade gneiss terranes. The complex is extensively intruded by variably foliated granitoids which also intrude the surrounding greenstone succession, the Warrawoona Group. The contact between the greenstone belt and the gneiss complex is tectonic. Field evidence indicates that the banded gneisses are the oldest part of the batholith; zircon U-Pb ages for three samples are 3429 ± 13 Ma, ≥ 3443 ± 10 Ma and 3448 ± 8 Ma, all very similar to the age of zircons in the felsic volcanic rocks near the base of the greenstone succession (Pidgeon, 1978) [1]. Two granitoids, representing pre- and post-tectonic phases respectively, give zircon ages of 3304 ± 10 Ma and 3314 ± 13 Ma, thereby constraining the major structural events to be contemporary with granite magmatism, consistent with a diapiric model for the tectonic evolution of the batholith (Collins, 1989) [2]. These ages are similar to the zircon age for terminal volcanism in the Warrawoona Group (Pidgeon, 1984) [3], but are ∼ 100 Ma older than the precisely-defined Rb-Sr whole-rock ages obtained throughout the batholith (Collins and Gray, 1989) [4]. Batholith-wide resetting or late closure of Rb-Sr isotopic systems is implied, possibly associated with late fluid circulation. The temporal coincidence between plutonism and volcanism in the Mount Edgar region strongly suggests that the greenstone succession contains the extrusive equivalents of both the ∼ 3300 Ma granites and the ∼ 3440 Ma gneisses in the batholith. The isotopic evidence, combined with the structural data, indicates that the granites cannot be derived from the surrounding greenstone belt. Further, there is no compelling evidence for a sialic basement to the greenstone succession. © 1990.