Formula omitted] laserprobe ages of metamorphic hornblendes from the Coongan Belt, Pilbara, Western Australia

Abstract

40Ar 39 Ar laser step-heating analyses of metamorphic amphiboles and a whole rock sample from metavolcanic rocks of the Archaean Coongan Greenstone Belt in the south-eastern Pilbara in Western Australia provide evidence that these rocks, with no previous age constraints, belong to the ca. 3460 Ma old Warrawoona Group. The western margin of the belt is formed by an east-dipping ductile shear zone; the eastern margin is undeformed and shows a narrow contact metamorphic zone. The Coongan Belt shows a metamorphic gradient from amphibolite facies at the western margin to greenschist facies in the centre of the belt. The rocks in the centre of the belt are transected by north-south striking retrograde shear zones, one of which has been constrained at ca. 2940 Ma and may be related to the ca. 2940 Ma intrusive event in the eastern Pilbara. Foliated granodiorites from the ductile shear zone along the western margin of the belt yield dates of ca. 3170 Ma, which are interpreted as the age of recrystallisation after deformation. Deformed blue-green hornblendes from the adjacent foliated amphibolites, however, give younger and inconsistent dates of ca. 3090 Ma and ca. 3020 Ma. These dates cannot be interpreted as meaningful ages and indicate an inhomogeneous distribution of argon in the shear zone. Age spectra from blue-green hornblendes from the western part of the Coongan Belt constrain the age of a regional metamorphic event at 3240 Ma, which compares well with the metamorphic hornblende ages of ca. 3200 Ma obtained in the Western Shaw Belt by Wijbrans and McDougall. The same event is indicated by complex age spectra of partially metamorphosed mafic volcanic rocks, which yield ca. 3260 Ma maximum ages in the higher temperature steps. The age spectrum of contact metamorphic hornblendes from the eastern margin indicates that the cooling age of the undeformed granite in the south-western Corunna Downs Batholith is ca. 3400 Ma, which is 70–90 Ma younger than the North Shaw granodiorites in the Shaw Batholith, but considerably older than previously assumed, that is ca. 3300 Ma. All obtained ages fall within the expected range of ca. 3.5-2.94 Ga, and generally increase from west to east through the Coongan Belt. The effects of excess argon on the age spectra seems to be rather considerable in some of the experiments.