Mesoproterozoic tectonism in the northwest Gawler Craton: 40Ar/39Ar geochronology, geophysical interpretations and extrapolations

The Gawler Craton represents one of the largest blocks of Archaean-Proterozoic crystalline basement in Australia, but its tectonic history and mineral resource potential remain relatively unknown. We present new reconnaissance 40Ar/39Ar geochronological results which better define the thermal history of the northwest Gawler Craton. In particular, we constrain timing of tectonic activity along major structures identified in potential field data, and contrast thermal histories of crustal blocks separated by such structures. Our results are from the Fowler Subdomain (Teasdale, 1997) and the Christie and Wilgena Subdomains (Daly et al., 1998) and their bounding-structures. Sheared granite from the Coorabie Shear Zone (SZ), separating the Christie and Wilgena Subdomains, yields muscovite and biotite 40Ar/39Ar ages of ~1540 Ma. Potassium feldspar from the same sample yields a gently rising age spectrum, with ages spanning ~1450 Ma to ~1540 Ma, most likely reflecting slow cooling over this time interval. This contrasts with K-feldspar collected from the Wilgena Subdomain, southeast of the Coorabie SZ, which exhibits a flat age spectrum with an age of ~1545 Ma. Farther east in the Wilgena Subdomain, at the western edge of the Harris Greenstone Belt, hornblende from a metadolerite dyke yields an age of ~1700 Ma. Biotite from the Mt Christie area, west of the Coorabie SZ, preserves an age of ~1650 Ma. Two samples from the Tallacootra SZ, which separates the Fowler and Christie Subdomains, yield 40Ar/39Ar mica (muscovite and biotite) ages of ~1450 Ma, and a K-feldspar age spectrum which rises monotonically from ~1360 Ma to ~1450 Ma. Muscovite from Lake Ifould, northwest of the main trace of the Tallacootra SZ but possibly not in the Christie Subdomain, preserves slightly older ages of ~1480 Ma. Taken together, these results show distinct differences in thermal and tectonic history across the northwest Gawler Craton. Both the Wilgena and Christie Subdomains preserve Paleoproterozoic or early Mesoproterozoic cooling ages, while the Fowler Subdomain is distinctly younger but also contains internal diachroneity. Previous workers proposed that deformation in the Fowler Subdomain was a consequence of continental collision between the "Mawson Continent", including the Gawler Craton, and the Yilgarn Block (Daly et al., 1998). Speculative links have also been made to "Grenvillian" tectonic activity in the Albany-Fraser and Musgrave provinces to the west and north of the Gawler Craton (Teasdale, 1997; Foster & Ehlers, 1998). The results presented here provide no evidence for Grenvillian-age tectonism in the Fowler Subdomain, although it remains possible that the Karari Shear Zone, which cuts the Coorabie and Tallacootra SZ, could have been active in Grenvillian times. Cross-cutting structural relationships apparent in magnetic images suggest that the broadly E-W-trending Yerda SZ is cut by the Coorabie SZ, thus the Yerda structure is older than the 40Ar/39Ar ages of ~ 1545 Ma obtained from the Coorabie SZ.