I.M. Tyler

Tectonic evolution of Proterozoic Australia

Proterozoic Australia has long been interpreted as a single intact continent in which all tectonic and magmatic activity was intracratonic. This paper proposes an alternative hypothesis in which numerous fragments of continental crust were assembled by plate tectonic processes. The assembly was completed between 1300 and 1100 Ma when the crustal fragments were combined as an early component of the Rodinian supercontinent. Rifting and fragmentation of Archaean continents began in the late Archaean and continued into the Proterozoic. Passive margin deposits, such as those of the Hamersley Basin, accumulated on isolated fragments of Archaean crust. These numerous fragments were subsequently assembled into three cratons by ∼ 1830 Ma. A West Australian Craton was established by collision of the Archaean Pilbara and Yilgarn cratons, which were joined along the Capricorn Orogen. Similarly, a South Australian Craton developed by amalgamation of the proto-Gawler and proto-Curnamona cratons along the Kimban Orogen. A North Australian Craton appears to have formed by accretion of numerous crustal fragments, including the Kimberley, Pine Creek, Lucas, and Altjawarra cratons, with sutures marked by the King Leopold, Halls Creek, Tennant Creek and proto-Isan orogens. The southern margin of the North Australian Craton was the site of repeated terrane accretion and orogenic activity between ∼ 1880 Ma and 1400 Ma. This included an orogenic event at ∼ 1880 – 1850 Ma; the Strangways (1780 – 1730 Ma), Argilke (1680 – 1650 Ma), and Chewings (1620 – 1580 Ma) orogenies; and the intracratonic Anmatjira uplift (1500 – 1400 Ma). Intracratonic rifting at ∼ 1750 to 1710 Ma and ∼ 1640 to 1600 Ma produced the McArthur Basin and related minor basins, parts of which were deformed by the Isan Orogeny at ∼ 1600 and ∼ 1530 Ma. Rifting along the line of the Capricorn Orogen led to deposition in the overlying intracratonic Bangemall Basin between 1630 and 1300 Ma. Along the eastern margin of the South Australian Craton, the 1670 to 1600 Ma Olarian Orogeny marks interaction with now obscured continental crust to the east. Tectonic activity between 1300 and 1100 Ma led to the assembly of Proterozoic Australia as an early component of the supercontinent of Rodinia. This first involved the amalgamation of the West Australian and North Australian cratons, followed by collision with the South Australian Craton. The Centralian Superbasin developed over the junction of the North, South, and West Australian cratons between ∼ 830 and 750 Ma. Rifting to the east formed the “Adelaide Geosyncline” at ∼ 830 Ma. This was followed by the breakup of Rodinia, with the rifting apart of Laurentia and Gondwanaland along the eastern margin of Proterozoic Australia at ∼ 750 Ma, and the subsequent formation of the Palaeo-Pacific Ocean. After the breakup of Rodinia, a series of northeast-southwest compressional events followed by periods of relaxation, reflect the assembly of a new supercontinent. Old lines of weakness were reactivated, culminating in the intracratonic King Leopold, Paterson, Petermann Ranges, and Pinjarra orogenies between 620 and 540 Ma. Subsequent reactivation continued into the Phanerozoic, with the widespread eruption of continental flood basalts and the formation of intracratonic basins (540 – 530 Ma).

Tectonic implications of Palaeoproterozoic post-collisional, high-K felsic igneous rocks from the Kimberley region of northwestern Australia

Abstract Palaeoproterozoic high-K I-type granites, high-level porphyry intrusions, and felsic volcanic rocks of the Whitewater Volcanics dominate the Hooper and Lamboo Complexes in the Kimberley region of northwestern Australia. The granites, porphyries and volcanic rocks are gradational into each other in the field, and they have the same mineralogy, similar major and trace element abundances, and indistinguishable SHRIMP U–Pb zircon ages of 1865–1850 Ma. There is evidence of widespread mingling between the granites and coeval gabbros. Magma mixing may be important in the formation of some of the mafic granites, but most of the rocks probably formed from felsic parent magmas that underwent variable degrees of fractional crystallization. The felsic igneous rocks may have formed by partial melting of intermediate to felsic, calc-alkaline rocks along the southern and eastern margins of the Kimberley Craton, following accretion of various earlier Palaeoproterozoic terranes to the craton. Therefore, models for Palaeoproterozoic high-K granites in northern Australia that invoke intracratonic rifting of a stable Archaean craton may need to be revised.

Crustal architecture of the Capricorn Orogen, Western Australia and associated metallogeny

A 581 km vibroseis-source, deep seismic reflection survey was acquired through the Capricorn Orogen of Western Australia and, for the first time, provides an unprecedented view of the deep crustal architecture of the West Australian Craton. The survey has imaged three principal suture zones, as well as several other lithospheric-scale faults. The suture zones separate four seismically distinct tectonic blocks, which include the Pilbara Craton, the Bandee Seismic Province (a previously unrecognised tectonic block), the Glenburgh Terrane of the Gascoyne Province and the Narryer Terrane of the Yilgarn Craton. In the upper crust, the survey imaged numerous Proterozoic granite batholiths as well as the architecture of the Mesoproterozoic Edmund and Collier basins. These features were formed during the punctuated reworking of the craton by the reactivation of the major crustal structures. The location and setting of gold, base metal and rare earth element deposits across the orogen are closely linked to the major lithospheric-scale structures, highlighting their importance to fluid flow within mineral systems by the transport of fluid and energy direct from the mantle into the upper crust.

The relationship between tectonism and composition of granitoid magmas, Yarlarweelor Gneiss Complex, Western Australia

Abstract The northwestern edge of the Archaean Yilgarn Craton in Western Australia was intensely deformed and metamorphosed during the Palaeoproterozoic Capricorn Orogeny (D 1n and D 2n ) to form the Yarlarweelor Gneiss Complex. High-grade metamorphism and crustal thickening in the complex at ca. 1812 Ma during D 1n was accompanied by voluminous veins and sheets of I-type granite and pegmatite. Most of these rocks are leucocratic, and are characterised by low Rb, FeO T +MgO, Y, Zr, Th, U and total rare earth elements (∑REE), and high SiO 2 , Ba, K/Rb and Ba/Rb, and chemically resemble leucosomes from stromatic migmatites (metatexites). The granite and pegmatite formed during compression and represent low to moderate degree melts of late Archaean and Palaeoproterozoic granitic rocks. At ca. 1800 Ma, dextral strike-slip faulting during D 2n followed exhumation of the Yarlarweelor Gneiss Complex, and was accompanied by intrusion of dykes and steeply dipping sheets of I-type granites. These granites have high K 2 O/Na 2 O ratios, high Rb, Zr, ∑LREE, Y, Th and U contents, and low K/Rb ratios similar to high-K granite melts and diatexite migmatites derived by large-scale biotite dehydration melting of igneous rocks. The source of the syn-D 2n granites was Palaeoproterozoic meta-igneous rocks. The change in granite chemistry between ca. 1812 and ca. 1800 Ma reflects a change in the nature of melting and melt extraction, which were in turn controlled by the style of deformation.

Syntectonic granite in the southern margin of the Palaeoproterozoic Capricorn Orogen, Western Australia

Along the southern margin of the Capricorn Orogen the Narryer Terrane of the Archaean Yilgarn Craton was pervasively intruded by granite during ductile deformation and two stages of meta‐morphism at 1820–1800 Ma. Medium‐ to high‐grade metamorphism was closely followed by greenschist‐facies metamorphism, indicating a period of uplift which was coincident with dextral strike‐slip faulting.

Isotope and rare earth element evidence for a late Archaean terrane boundary in the southeastern Pilbara Craton, Western Australia

Abstract RbSr, SmNd and REE data have been obtained from granitic rocks exposed in Archaean granite-greenstone terrain in the southeastern part of the Pilbara Craton. Samples were collected from the Kurrana Batholith; the Cooninia, Billinooka, and Sylvania Inliers; and from the bottom of a borehole drilled through the Hamersley Basin succession in the Fortescue Valley. Almost all the samples have 87 Rb / 86 Sr 144 Nd and SmNd model ages (TDM) in the range 3.1 to 3.3 Ga. These characteristics, together with REE patterns, differ from published data for granitic rocks of the northern Pilbara granite-greenstone terrain that have similar geological histories. RbSr isotopic systematics have been variably disturbed, with data arrays on isochron plots all indicating ages younger than the accepted age of the base of the Hamersley Basin succession (2.76 Ga). This is attributed to pervasive burial metamorphism beneath the Hamersley Basin, and to later hydrothermal events. Some rock units appear to be younger than their northern Pilbara equivalents. A shear zone separating the Mosquito Creek Synclinorium from the Kurrana Batholith has the characteristics of a suture zone, and is postulated to be the boundary between two distinct terranes that joined to form the Pilbara Craton between 3.0 and 2.76 Ga.

Identifying mineral prospectivity using 3D magnetotelluric, potential field and geological data in the east Kimberley, Australia

Abstract An integrated interpretation of the east Kimberley, northern Western Australia was completed to determine mineral prospectivity, and was centred on a portion of a magnetotelluric (MT) survey conducted across the entire Kimberley Craton and surrounding orogens. A structural geophysical interpretation used potential field data, and was constrained by geological field observations, petrophysics, remote sensing and understanding of the tectonic history of the region. Potential field forward modelling located along the same survey traverse as the MT data allowed comparison between the two datasets and their interpretations revealing interesting features suggesting the presence of large-scale structures, the presence of mineralization deep in the crust, and where mineralization may be at or near the surface. The King River Fault is shown from both the MT inversion and potential field modelling as a crustal-scale, west-dipping structure, the footwall of which bounds the western side of a large resistive body. A conductive anomaly is also located on the hanging wall of the King River Fault. Our assessment suggests that graphitic rocks, most likely with some sulphide content, contribute to the strength of this anomaly, and highlights the potential of the east Kimberley to host graphite and base metal deposits.

Role of geochronology in our present-day understanding of the Proterozoic: an Australian perspective

Geochronology has proven pivotal to our present-day understanding of the dynamic processes that have shaped the Earth. The proliferation of high-precision geochronology worldwide in the last 15 years, in particular that generated by sensitive high-resolution ion microprobe (SHRIMP), has revolutionised the geological communitys view of the Proterozoic Eon, and the implicit assumptions that form the basis of most work conducted on rocks of that age. Although most of this dating has concentrated on U–Pb zircon ages in felsic rocks, an impressive record of Proterozoic crust-forming events and high-grade tectonic reworking has been identified. In some instances, the dating has also defined the duration of orogenic activity. Our present-day understanding is that: (i) many Proterozoic orogens mark ancient collision zones, and as a corollary, plate tectonics has operated throughout the Proterozoic; (ii) Proterozoic orogens were commonly the sites of repeated reworking or reactivation; (iii) the Proterozoic saw the assembly and dispersal of at least two supercontinents; (iv) the Proterozoic was punctuated by the activity of numerous large igneous provinces; and (v) Proterozoic basin formation and evolution were driven largely by plate-tectonic processes. Although there are large areas yet to be dated, there is no reason to suggest that the processes identified there will be any different. The primary question has evolved from ‘Did plate tectonics operate?’ to ‘What form did plate tectonics take?’ One of our major challenges is to extend the geochronological framework of the Proterozoic by routinely dating other uranium-bearing accessory phases, following the successful application of zircon geochronology.