The interplay of structure and metamorphism at Broken Hill, NSW, Australia

Abstract

Downward younging of stratigraphy in several large F2 fold structures in the Paleoproterozoic Willyama Supergroup in the Broken Hill area has previously been explained by very large-scale F1 nappe folds followed by relatively upright F2 and F3 folds. In three major structures, the Mt Robe–East Eldee Structure, the Paps Synform and the Bijerkerno Synform, the andalusite-sillimanite metamorphic isograd parallels stratigraphy for tens of kilometres. Given the conformability of these segments of isograd in plan view, it is reasonable to suggest that in two of the structures, sillimanite-grade rocks rest on andalusite-grade rocks. In all cases the younger stratigraphic units are at lower metamorphic grade than the older stratigraphic units, whether the stratigraphy is upright, as in the Bijerkerno Synform, or inverted, as in the Mt Robe-East Eldee Structure and the Paps Synform. Major bodies (mini-batholiths) of pegmatite are confined stratigraphically to below the Sundown Group, whether the stratigraphy is upright or overturned. In the D1 nappe model the andalusite-sillimanite isograd and the emplacement of pegmatite mini-batholiths must both predate the D1 nappe folding. There is evidence that andalusite preceded D1, but not sillimanite, which is a defining mineral in the first-generation schistosity (S1). The large-scale nappe model is also not compatible with calculated metamorphic pressures. An alternative model involving extensional D1, followed by D2 sheath folds, explains downward-facing metamorphic facies but is not viable. Large-scale fold structures in the Mt Robe Block can be explained by recumbent folding at a moderate scale, followed by inclined sheath folding. The downward-facing stratigraphy and metamorphic facies in the Mt Robe and Paps synforms are interpreted to lie on the underside of inclined F2 sheath-like folds. KEY POINTS In major fold structures in the Willyama Supergroup, upper stratigraphic units are at a lower metamorphic grade than lower stratigraphic units, whether the fold is upward-facing or downward-facing. In two large F2 folds sillimanite-grade rocks showing melting are situated above andalusite-grade rocks. Pegmatitic mini-batholiths show semi-regional stratigraphic control below the top of the Broken Hill Group, whether they are in upward or downward-facing fold limbs. A new structural model with D1 recumbent folding, followed by D2 sheath-like folding, is based on structures in the Mt Robe Block.