Geochronology of early to middle Proterozoic fold belts in northern Australia: a review

Abstract

This geochronological review integrates geologically relevant isotopic data from the several early to middle Proterozoic fold belts of northern Australia. A number of firm chronological benchmarks, largely established by means of U-Pb zircon studies, now provide a time framework within which to better quantify the tectonostratigraphic history of many of these terranes. The apparent, worldwide dearth of crustal events in the earliest Proterozoic, between 2500 and 2000 Ma, was broken in northern Australia with the advent of a major crust-forming episode between 1880 and 1850 Ma ago. These rocks may have evolved from an initial mantle or lower crustal melting and fractionation process about 400–300 Ma earlier, that is evidenced from Sm-Nd model ages. The extent, involvement, and/or degree of assimilation of any pre-existing Archaean crust is small, but whether or not it is negligible or zero is not satisfactorily known. The 1880–1850 Ma event, the Barramundi Orogeny, is well represented in northern Australia, both lithologically and analytically, and the rocks that pre- and post-date it have a similar tectonic setting over very wide areas of the continent. Two consistent and widespread elements of this tectonism are regional metamorphism and deformation of the first cycle of supracrustals (Barramundi Orogeny), and crystallization of I-type, K-rich felsic magmas closely following and, in some cases, overlapping with deformational processes. In the Halls Creek Inlier, the ages of this rapid deep crustal to supracrustal tectonic transition cannot be distinguished within the range 1860-1850 Ma. This major yet short-lived tectonism is mirrored in other fold belts such as Pine Creek, Tennant Creek, and Mount Isa, in which U-Pb zircon ages give slightly older constraints of 1885–1870 Ma, 1920-1870 Ma and 188510 Ma, respectively. The ensuing felsic magmatism in these latter terranes continued for a few tens of millions of years, with the crystallization and emplacement of comagmatic volcanic/plutonic pairs throughout, in the interval 1870-1860 Ma. In the Georgetown and Arunta Inliers, direct evidence of this otherwise ubiquitous event, if present, is masked by younger high-grade metamorphism. In the latter, however, metamorphic crustal components having approximate protolith ages of this order can be inferred from their evolved isotopic signatures. The geochronological and geochemical coherency of the extensive early Proterozoic, ∼1870 Ma magmatism contrasts with that of post-1800 Ma magmatism, which is less extensive, generally anorogenic, commonly bimodal in character, and varies in age from one terrane to another. Several major sedimentary basins were being filled at this time. Major igneous events at 1790-1740 Ma, 1670 Ma and ∼1500 Ma are recognized. Elongate, sublinear volcanic and plutonic belts, 1790-1740 Ma old, are present at Mount Isa. In high-grade granitic gneisses of the Arunta Inlier, a few U-Pb zircon ages and extrapolated Rb-Sr protolith ages are coincident at about 1760-1750 Ma, indicating that this event may be important in the development of the ‘Division 2’ sequences of that fold belt. The platformal Hart Dolerite and Oenpelli Dolerite, two of the largest mafic intrusions in the world, were emplaced at 1762 ± 25 Ma and 1688 ± 13 Ma. Younger felsic plutonism and volcanism gave rise to rapakivi-type granites in the Mount Isa, Tennant Creek and The Granites-Tanami Inliers at ∼ 1670 Ma, a time close to that of the Aileron deformational/metamorphic event in central Australia. Further, at ∼ 1650 Ma, there is growing evidence of an unexpectedly coherent hydrothermal episode that reset or disturbed many Rb-Sr ages, gave rise to pegmatites, and was possibly associated with mineralization in the Davenport and eastern Arunta Inliers. Discrete middle Proterozoic deformational events, between 1610 and 1470 Ma, punctuate the tectonostratigraphic history in the Arunta, Mount Isa and Georgetown fold belts where they are best documented.