Age of the barramundi orogeny in northern Australia by means of ion microprobe and conventional U-Pb zircon studies

Abstract

U-Pb zircon geochronological studies are used to quantify the tectonostratigraphic history of a major crust-forming event in early Proterozoic terranes of northern Australia. Magmatism and tectonic processes associated with this event, the Barramundi Orogeny, appear similar over very wide areas, about one third of the Australian continent. The Barramundi event marks the deformation, metamorphism and cratonization of the earliest Proterozoic ∼2.0−1.9 Ga basinal successions, and it was followed by and closely linked to extensive orogenic felsic magmatism. The Yaringa Metamorphics are an inlier of paragneiss and schist that pre-dates 1865 ± 3 Ma Leichhardt felsic magmatism and younger basin formation in the Mount Isa Inlier. From an ion microprobe U-Pb zircon study of massive and migmatitic gneiss, the time of Yaringa metamorphism is established at 1890±8 Ma. Recognized younger Proterozoic deformations in the interval 1610−1550 Ma are generally not evident in zircon U-Pb systems, but have totally reset local Rb-Sr whole-rock values. Conventional U-Pb work on multi-grain zircon samples of Yaringa gneiss gives a geologically meaningless age (2090±40 Ma) that is an artifact of complex zircon growth, part of which was initially inherited from a 2.55 to 2.2 Ga provenance, and the other major part of which was formed in situ during the 1890 Ma high-grade metamorphism. Predictably, for simple zircon systems in low-grade metavolcanic térranes, conventional and ion microprobe zircon ages are in close agreement. Dating of volcaniclastic sequences above and below the Barramundi tectonic break in the Pine Creek Inlier brackets the orogeny in this terrane to a similar time frame, 1885–1870 Ma. Some zircons which crystallized at this time form selvedges around older cores whose age of 2100-2000 Ma may be relics of an earlier deep crustal underplating. The uniform chemistry of post-Barramundi felsic magmatism, and the similar tectonic setting of the areally extensive Barramundi event, are mirrored by their temporal coherency. Cycles of geosynclinal deposition, burial, metamorphism and uplift took place during intervals of several million years or less, implying comparable rates for early Proterozoic orogenesis to those established for Tertiary and Quaternary settings.