Re-evaluation of contact relationships between Ordovician volcanic belts and the quartz-rich turbidites of the Lachlan Orogen*

Abstract

Some published tectonic reconstructions of the eastern Lachlan Orogen in New South Wales have shown Ordovician volcanic and volcaniclastic rocks of the Macquarie Arc conformably overlying or interfingering with a coeval Ordovician quartz-rich turbidite sequence. Re-examination of key contacts between the volcanic and quartz-rich successions has found no evidence to support this interpretation, and suggests that the two packages are separate tectonostratigraphic terranes. The contacts between these two coeval successions are generally marked by major faults containing mylonites, cataclasites and, at some locations, fragments of mid-ocean ridge-type pillow basalt and chert. The quartz-rich turbidites are generally highly deformed and of higher metamorphic grade than the adjacent volcanics. At Oberon and Mudgee, contacts are faulted but there are no mylonites or significant differences in metamorphic grade. At Palmers Oaky and Black Springs, Silurian quartz-rich sandstones overlying the Ordovician volcanics have been mistakenly assigned to the Ordovician in previous studies. Throughout the Lachlan Orogen, there is no mixing of framework grains. Quartz-rich turbidite successions are dominated by quartz with lesser feldspar and rare tourmaline, zircon and monazite derived from recycled continental sources. In contrast, the volcaniclastic sandstones contain feldspar, clinopyroxene and lithic fragments derived from subduction-related clinopyroxene-phyric basalt and plagioclase-phyric andesite. Detrital-zircon populations also differ, with separate U/Pb age populations and almost no overlap. Comparison of the Ordovician sequences of the Lachlan Orogen with modern turbidites from continental- and arc-related sedimentary basins suggests that complete separation of sedimentary sources is only possible if the sandstones were deposited hundreds of kilometres apart, in separate tectonic environments. The two sequences were juxtaposed along major faults in the Late Ordovician or Early Silurian, probably when the Macquarie Arc collided with a thick Ordovician sedimentary wedge located on the Gondwanan continental margin.