Structural and stratigraphic problems of the Argylla Formation near Mary Kathleen,Queensland

Rocks formerly mapped as Lower Proterozoic Argylla Formation near Mary Kathleen in northwest Queensland consist of parallel lenses of very deformed schist, quartzite and various types of metaporphyry. Completely intermingled with these are bands of amphibolite and lenses of the adjacent Wonga Granite. Finite ductile strains of 65–80% maximum shortening have been measured; these have produced transposition on both a mesoscopic and macroscopic scale. It is shown that the major stratigraphic boundaries are much shallower than the steeply dipping, constantly oriented unit boundaries. Intrusive relationships and structural criteria indicate that all the lithologies found within the mapped boundaries of the Argylla Formation are chronologically separate. This is confirmed by mapping in the much less deformed area near Winston Churchill Mine to the north. It is concluded that an original sedimentary pile was intruded first by dolerite dyke swarms and then by acid porphyry sills followed by granitic sill‐like instrusions of the Wonga Granite. Subsequent deformation and macroscopic transposition produced the present parallel‐layered structure.

These conclusions differ from currently accepted relationships of the Argylla Formation. Various possibilities are suggested; we favour the view that none of these rock types is equivalent to the defined Argylla Formation, and that the acid porphyries are younger than at Jeast some of the metasediments of the Mary Kathleen Group and may be a precursor of the Wonga Granite intrusion.