Origin of megacrystic felsic gneisses at Broken Hill

Three sheet‐like bodies of felsic gneiss containing abundant K‐feldspar megacrysts (megacrystic felsic gneiss, augen gneiss or granite gneiss) surrounding the Broken Hill Line of Lode in western New South Wales, Australia, are inferred to be pre‐ to syn‐D₁ granitoids. We interpret the Feral gneiss to be a pre‐ to early syn‐D₁ intrusion, as it contains S₁ as its earliest foliation. However, it has no magmatic flow foliation. The Alma Gneiss, and the megacrystic portions of the Rasp Ridge Gneiss, northwest of the Line of Lode, both contain S₁ parallel to a magmatic flow foliation, and are interpreted as having been magmatic during D₁. Therefore, the Alma and Rasp Ridge Gneisses may have been intruded during D₁, probably just after the Feral gneiss, as the Alma Gneiss intrudes the Feral gneiss. S₁ in the augen gneisses and the wall rocks is defined by biotite, sillimanite, garnet and ribbon quartz, and indicates that high‐grade metamorphic conditions accompanied D₁. Evidence suggesting that these rocks were originally granitoids includes: (i) the Alma Gneiss transecting and intricately intruding the Feral gneiss, the contacts being transected by S₁; (ii) euhedral to subhedral K‐feldspar porphyroclasts (former phenocrysts), especially those with concentrically arranged inclusions; (iii) microgranitoid enclaves, particularly where megacrystic and relatively large; (iv) aplite dykes (most common in plutonic rocks and therefore reliable indicators); (v) metasedimentary xenoliths; (vi) magmatic flow foliations overprinted by parallel tectonic foliations; and (vii) chemical affinities with undoubted Australian Proterozoic granitoids. Therefore, felsic gneisses at Broken Hill should not be used for stratigraphic correlation, unless they can be definitely determined to be of volcanic flow or tuffaceous origin. The inferred intrusion of granitoids early in the tectonic history of the Broken Hill Block suggests that they may have contributed to the metamorphic and/or hydrothermal heat, and may have helped concentrate metals to form orebodies.

Keywords:

• augen structure
• Broken Hill
• granite gneiss
• potash feldspar
• Proterozoic
• sheet intrusions
• syntectonics