Geochronology of Willyama Supergroup metavolcanics, metasediments and contemporaneous intrusions, Broken Hill, Australia

Abstract

New SHRIMP U - Pb dating of the rocks from the Willyama Supergroup clarifies the geochronology of the lower to middle parts of the sequence and contemporaneous intrusions, and confirms that the prospective upper part of the Broken Hill Group extends into South Australia. In the Rantyga Group that constitutes the Redan geophysical zone in the southeastern part of the Broken Hill Domain, the relative stratigraphic positions of the Redan Gneiss (lowermost) and (newly named) Farmcote Gneiss (uppermost) are confirmed. An age of 1710 ± 4 Ma on a probable metavolcanic gneiss in the Redan Gneiss confirms this formation as the oldest dated unit in the Rantyga Group and in the Broken Hill Inlier. Metasediments in the Farmcote Gneiss contain approximately contemporaneous zircons dated at 1705 ± 5 Ma. The Rantyga Group is considered part of the Willyama Supergroup, rather than Archean or Early Paleoproterozoic basement as was suggested earlier. Previous attempts to date the Thackaringa Group concentrated on albitised metasediments, producing a range of detrital dates, with no definitely contemporaneous zircon. In the present study, three probable metavolcanic rocks from the Cues Formation yielded a consistent age of ca 1700 Ma, slightly younger than the underlying Alma Granite Gneiss (metagranite). The metagranite was intruded under very shallow cover. Three other concordant metagranites were dated. The 1685 ± 5 Ma Wondervale Well Granite Gneiss is indistinguishable in age from the 1683 ± 3 Ma Rasp Ridge Granite Gneiss. However, the Stephens Creek Granite Gneiss appears to be slightly older in that the stratigraphically lower part was dated at 1689 ± 5 Ma, while the upper part was dated at 1686 ± 4 Ma. Previous correlations of the lower and upper parts, with the Alma and Rasp Ridge Granite Gneisses, respectively, are therefore invalid. The 1695 ± 4 Ma Georges Bore Granite Gneiss correlates with neither the Alma Granite Gneiss nor the Rasp Ridge Granite Gneiss. All three granite gneisses dated in this study have intruded the upper Thackaringa Group and/or lower Broken Hill Group, indicating very shallow levels of emplacement, probably <1 km of current stratigraphic thickness. Very shallow emplacement of sill-like granites more or less coincides with deposition of Broken Hill-type mineralisation, and may have influenced ore formation. Samples from two critical parts of the Broken Hill Group were analysed. A sample of suspected Hores Gneiss from near Mingary in South Australia gives an interpreted age of 1684 ± 6 Ma, indistinguishable from Hores Gneiss in the type area north of Broken Hill. An age of 1693 ± 4 Ma on Potosi-type gneiss from the Parnell Formation north of Broken Hill confirms earlier dating from the same location. The new geochronological information shows that the time interval between the oldest stratigraphic unit (Redan Gneiss) and the top of the Broken Hill Group (Hores Gneiss) is only ∼25 million years, leaving little space for major time breaks. More specifically, the time interval between the middle Thackaringa Group (Cues Formation) and the lower Broken Hill Group (Ettlewood Calc-Silicate Member) is only ∼5 million years, suggesting continuous or near-continuous deposition.