ISOTOPE AND RARE-EARTH ELEMENT EVIDENCE FOR A LATE ARCHEAN TERRANE BOUNDARY IN THE SOUTHEASTERN PILBARA CRATON, WESTERN AUSTRALIA

Rb-Sr, Sm-Nd and REE data have been obtained from granitic rocks exposed in Archaean granite-greenstone terrain in the southeastern part of the Pilbara Craton. Samples were collected from the Kurrana Batholith; the Cooninia, Billinooka, and Sylvania Inliers; and from the bottom of a borehole drilled through the Hamersley Basin succession in the Fortescue Valley. Almost all the samples have Rb-87/Sr-86 < 4.0, SM-147/Nd-144 < 0.12 and Sm-Nd model ages (T(DM)) in the range 3.1 to 3.3 Ga. These characteristics, together with REE patterns, differ from published data for granitic rocks of the northern Pilbara granite-greenstone terrain that have similar geological histories. Rb-Sr isotopic systematics have been variably disturbed, with data arrays on isochron plots all indicating ages younger than the accepted age of the base of the Hamersley Basin succession (2.76 Ga). This is attributed to pervasive burial metamorphism beneath the Hamersley Basin, and to later hydrothermal events. Some rock units appear to be younger than their northern Pilbara equivalents. A shear zone separating the Mosquito Creek Synclinorium from the Kurrana Batholith has the characteristics of a suture zone, and is postulated to be the boundary between two distinct terranes that joined to form the Pilbara Craton between 3.0 and 2.76 Ga.