The geology and petrology of the Mullaley area of New South Wales

Abstract

Igneous rocks dominate over sedimentary rocks in the Mullaley area and are divided, on the basis of field relationships and petrochemistry, into six groups: Garrawilla Volcanics, Nombi Extrusives, Glenrowan Intrusives, Napperby Limburgite, Tambar Trachybasalt, and Bulga Complex. The area consists of a basement of Permian Black Jack Formation overlain by Triassic Digby and Napperby Beds. This is overlain by vesicular and non‐vesicular flows of alkali basalt, hawaiite, mugearite, and soda trachyte, and pyroclastic units of the Upper Triassic/Lower Jurassic Garrawilla Volcanics. Basanites of the Nombi Extrusives are of similar age but crop out sporadically over a larger area. The Glenrowan Intrusives are found as sills of alkali dolerite and microsyenodolerite intruded into the volcanics and the Triassic Beds. Sedimentary rocks of the Purlawaugh and Pilliga Beds overlie the volcanic sequence in the south and west, indicating that much of the Garrawilla sequence remained as an island or west‐jutting peninsula in a Jurassic lake(s). Renewed volcanic activity, probably in the Tertiary, produced the Bulga Complex. Endogenous lava domes and massive flows consist dominantly of phonolitic trachyte and phonolite, and form the bold landscape of the central portion of the area. The Napperby Limburgite and the Tambar Trachybasalt are isolated in field occurrence and perhaps age.