The Pigna Barney Ophiolitic Complex and Associated Basaltic Rocks, Northeastern New South Wales, Australia

Abstract

The Pigna Barney Ophiolitic Complex (PBOC) is a lithologically and chemically distinctive ultramafic-mafic melange which is largely confined to major fault zones in the Pigna Barney-Curricabark area, southern New England Orogen (NEO), northeastern New South Wales. For the most part, the PBOC lies along the arcuate, southern-most section of the Peel Fault System. This major steeply-dipping fault system marks the line of contact between a complex accretionary terrane (Zone B) lying to the east and north, and a significantly less-deformed volcanic arcrelated basin (Tamworth Belt) lying to the west and south. Fragments of the PBOC also lie within Zone B, especially along the Curricabark Fault Zone and along faulted contacts between the? pre-Devonian, largely hemipelagic Myra beds and Permian epiclastics of the Nambucca Association. The PBOC is everywhere in contact with adjacent stratigraphic associations, and non-tectonic contacts between its various component lithologies are exceedingly rare. The PBOC consists of: (i) a highly disrupted and incompletely preserved ophiolitic assemblage whose members consistently display atypically low abundances of "incompatible" elements (e.g. Ti, Zr, P); and (ii) a diverse range of igneous, sedimentary and metamorphic lithologies which generally occur as tectonic blocks in serpentinite-matrix melange (highly sheared tectonized harzburgite). In its type area the PBOC displays a crude internal ophiolitic stratigraphy which, at least on present orientation, consistently faces the Tamworth Belt to the southwest. Seven distinct lithological groups comprise the ophiolitic members of the PBOC. All groups are altered to varying degrees. In order of general structural position and inferred stratigraphic sequence these groups are: (1) depleted tectonized harzburgite - Fo92-91,Al₂0₃ < 1.2wt%, modal diopside < l%, the lowest in the sequence, followed by: (2) cumulate harzburgites and rare olivine orthopyroxenite cumulates - Fo92-90; (3) "plagioclase"-bearing peridotite cumulates - Fo88-84; (4) cumulate olivine (mela-)norites - Fo2087-80,An90-85; (5) cumulate quartz-bearing gabbros/gabbronorites - mg 77-61; (6) dolerites and quartz "dolerites" - mg 56-45; and (7) pillowed and massive basaltic extrusives and related shallow intrusives - mg 69-56.