THE REDAN GEOPHYSICAL ZONE - PART OF THE WILLYAMA SUPERGROUP - BROKEN-HILL, AUSTRALIA

The Redan Geophysical Zone forms a regional magnetic high in contrast to the regional magnetic low defined by the main part of the Broken Hi ll Block. The magnetic rocks are interpreted to dip below the remainde r of the Broken Hill Block and there has been speculation that they ar e significantly older than the Early Proterozoic Willyama Supergroup. Evaluation of lithological mapping and aeromagnetic data permitted int erpretation of a stratigraphic sequence within the Redan Geophysical Z one, consisting of three new formations: the Redan Gneiss, Ednas Gneis s and Mulculca Formation, plus the Lady Brassey Formation, part of the Thackaringa Group. The rocks are considered to belong to the lower pa rt of the Willyama Supergroup and are not an older basement. Although the Redan Geophysical Zone contains some rock types not found elsewher e in the Broken Hill Block, there are some lithological similarities w ith the lower part of the Willyama Supergroup: an abundance of albite- rich rocks, the presence of quartz magnetite rocks with Cu and trace C o, and abundant amphibolite/basic granulite in the Lady Brassey Format ion' The boundary between the Redan Geophysical Zone and the remainder of the Broken Hill Block appears to be conformable, with no evidence of major faulting. Similarly no evidence of unconformities or major di splacement of stratigraphic boundaries has been found within the Redan Geophysical Zone. Structural history, fold style and orientation, and metamorphic grade within the Redan Geophysical Zone are similar to ad jacent areas of the Broken Hill Block. It is concluded that the Broken Hill Block contains no outcropping equivalent of the first cycle of s edimentary/igneous rocks recognized in the Early Proterozoic of northe rn Australia. Albite-quartz hornblende magnetite rocks unique to the R edan Geophysical Zone most likely comprised detritus derived directly from an intermediate volcanic suite. Some were altered considerably, w hile other rocks retained the dacite/andesite composition, except for the addition of Na, an increase in the oxidation state, and partial le aching of some of the more mobile elements. These modifications could have taken place in shallow alkaline evaporitic lakes. The Redan Geoph ysical Zone contains some of the elements of a foreland basin adjacent to a continental volcanic arc: a thick stratigraphic sequence, oxidiz ing evaporitic conditions, and intermediate volcanic detritus. The cha nge from intermediate-acid volcanism in the earliest formations, to bi modal acid/basic volcanism in the Thackaringa and Broken Hill Groups c ould correspond with a change from initial continental arc volcanism i nto bimodal rift volcanism. The case for the arc volcanism is weakened , however, by the relative scarcity of rocks with andesitic compositio ns and the lack of basaltic andesite compositions. The alternative is that the intermediate to acid volcanism represents only a variation on the later bimodal rift volcanism.