Capricorn Orogeny Research Articles

Intracontinental reworking in the Capricorn Orogen, Western Australia: the 1680 – 1620 Ma Mangaroon Orogeny*

Structures and metamorphic mineral assemblages throughout much of the Proterozoic Capricorn Orogen have been attributed to the 1830 – 1780 Ma Capricorn Orogeny. However, in the northern part of the Gascoyne Complex at the exposed western end of the orogen, precursor sediments to pelitic gneiss and metamorphosed feldspathic sandstone were deposited after 1680 ± 13 Ma. The sediments were first deformed and intruded by voluminous granites during intracontinental reworking (the Mangaroon Orogeny) between 1680 Ma and 1620 Ma. The Mangaroon Orogeny comprises a subhorizontal gneissic layering and peak metamorphic conditions of upper amphibolite facies (D1m/M1m) followed by upright, macroscopic southeast-trending folds and an associated subvertical foliation formed at greenschist facies (D2m/M2m). Schlieric biotite – muscovite granodiorite dated at 1677 ± 5 Ma represents an anatectic melt that intruded during D2m. Cross-cutting biotite and biotite – muscovite monzogranites mainly crystallised at ca 1680 – 1660 Ma, but granite plutons were emplaced across the Gascoyne Complex until 1620 Ma. Our findings invalidate correlation of these metasedimentary rocks with lower grade rocks of the upper Wyloo Group along the northern margin of the orogen, and necessitate a reassessment of tectonic activity in the orogen attributed to the Capricorn Orogeny.

Oceanic plateau accretion onto the northwestern margin of the Yilgarn Craton, Western Australia: implications for a mantle plume event at ca. 2.0 Ga

The 2.0 Ga Bryah Basin (Group) is one of a series of Palaeoproterozoic basins in the eastern Capricorn Orogen, a major tectonic unit between the Archaean Pilbara and Yilgarn Cratons of Western Australia. The Narracoota Formation forms the bulk of the Bryah Group and consists of ultramafic and mafic schist and metabasaltic hyaloclastites, sheeted dykes and a layered mafic-ultramafic complex, collectively referred to as metabasites, because they are generally metamorphosed to greenschist facies. Chemically these rocks correspond to komatiites, high-Mg basalts and tholeiites. Previous interpretations have purported that these rocks represent rift-related volcanics, ophiolites and mid-ocean ridge settings. In this contribution, I show that the Narracoota Formation metabasites were likely part of an ancient oceanic plateau, created by a 2.0 Ga mantle plume. Geochemical data (eg REE patterns) indicate that the metabasites are identical to modern oceanic plateaux. The presence of orogenic gold lodes further supports the oceanic plateau model. During the 1.96 Ga Glenburgh Orogeny, which brought together an Andean-style magmatic arc (southern Gascoyne Complex) and the northwestern margin of the Yilgarn Craton, the Narracoota oceanic plateau was accreted onto the Yilgarn Craton. The 1.8 Ga Capricorn Orogeny further deformed the Narracoota metabasites, resulting in the present day structural configuration.

Deformation in a complex crustal-scale shear zone: Errabiddy Shear Zone, Western Australia

Abstract Detailed mapping of four areas representing different geological units with varying formation histories within the crustal-scale Errabiddy Shear Zone shows an apparently simple temporal progression from foliation and mineral lineation development to folding and then to brittle deformation across the shear zone. However, in detail the structural evolution of the shear zone shows considerable complexity. The dominant foliation throughout the shear zone was formed in the upper greenschist to amphibolite facies during the 2000–1960 Ma Glenburgh Orogeny, which involved the accretion of the Archaean to Palaeoproterozoic Glenburgh Terrane onto the Archaean Yilgarn Craton and the subsequent formation of the Errabiddy Shear Zone. Orthorhombic kinematic indicators formed during the Glenburgh Orogeny as did the widespread mineral lineation. These fabrics were overprinted by a greenschist facies deformation and metamorphic event during the 1830–1780 Ma Capricorn Orogeny. During the Capricorn Orogeny mineral lineation development was rare, and mostly took place in high-Capricorn strain zones in areas where a pre-existing Glenburgh-aged mineral lineation was present. Such mineral lineations trend parallel to Capricorn-aged fold hinges. Regardless of the presence or absence of Capricornaged mineral lineations, dextral strike-slip kinematics and simple shear indicated by delta and sigma porphyroclasts, and displacement along detachment faults, are prevalent close to discrete shear zone boundaries, within the Errabiddy Shear Zone. However, between shear zone boundaries flattening and coaxial strain dominated during the Capricorn Orogeny. This difference in Capricorn Orogeny kinematics throughout the shear zone is caused by strain partitioning — although progressive deformation throughout the shear zone with dextral strike-slip faults overprinting older structures formed by pure shear also took place. These results suggest that analyses of small parts of shear zones may not give the complete history of an evolving transpressional shear zone because of the presence of strain partitioning and strain localization over time.

Assembling and reactivating the Proterozoic Capricorn Orogen: lithotectonic elements, orogenies, and significance

Abstract The Capricorn Orogen was initiated during Palaeoproterozoic suturing events that brought together the Archaean Yilgarn and Pilbara cratons to form the West Australian Craton. The orogen comprises Palaeoproterozoic plutonic and medium- to high-grade metamorphic rocks of the Gascoyne Complex, a series of Palaeoproterozoic volcano-sedimentary and sedimentary basins, including the Ashburton, Blair, Yerrida, Bryah, Padbury and Earaheedy basins, and the deformed margins of the Pilbara and Yilgarn cratons. Major pulses of deformation and metamorphism took place during the ca. 2200 Ma Ophthalmian Orogeny, the 2000–1960 Ma Glenburgh Orogeny, the 1830–1780 Ma Capricorn Orogeny, and a more localized unnamed event at the end of the Palaeoproterozoic (∼1670–1620 Ma). The orogen has been the site of repeated intracratonic reactivation with renewed basin formation, magmatism and orogeny during the Mesoproterozoic and the Neoproterozoic. The exhumed Palaeoproterozoic components of the orogen are unconformably overlain by the intracratonic Mesoproterozoic Edmund and Collier Basins (Bangemall Supergroup), which have been deformed during the Neoproterozoic Edmundian Orogeny. The Ophthalmian and Glenburgh orogenies affected the northern and southern margins of the orogen, respectively. The former resulted in the development of a foreland basin along the Pilbara Craton margin, whereas the latter is associated with accretion of an allochthonous element, the Glenburgh Terrane of the Gascoyne Complex, to the northwestern margin of the Yilgarn Craton. Effects of the Capricorn Orogeny extended across the entire orogen and provide a younger limit for amalgamation of the Yilgarn and Pilbara cratons. A number of basins developed along both the northern and southern margin of the orogen during this event, and were filled with sediment eroded from the Yilgarn Craton, the Gascoyne Complex, and a presently unexposed early Palaeoproterozoic terrane. The Capricorn Orogeny corresponds with a major phase of continental collision both in Australia and world wide, which appears to mark a major phase of supercontinent assembly. Mineralization within the orogen includes a wide variety of deposit types that can be related to collisional settings, including the world-class iron orebodies of the Hamersley Basin.

High-strain zone deformation in the southern Capricorn Orogen, Western Australia: kinematics and age constraints

Abstract Detailed structural mapping of a the Kerba Fault, a high-strain zone along the contact between the Archaean to Palaeoproterozoic Yarlarweelor Gneiss Complex (partly derived from the Yilgarn Craton) and the Palaeoproterozoic volcano-sedimentary rocks of the Bryah Group, indicate a temporal progression from foliation development to folding and finally brittle deformation. Structural data from the penetrative S1 foliation indicate a superficially simple geometry and kinematic history with a single, pervasive, greenschist facies foliation forming during dextral strike slip shearing. However, systematic spatial variations in the magnitude and orientations of mineral elongation lineations indicate a more complex deformation history involving a combination of general transpressive dextral shear and subsequent reactivation. This deformation has previously been considered to develop between 1830 and 1780 Ma ago during the Capricorn Orogeny. Relative age relationships around the Kerba Fault indicate that the high-strain deformation postdates the intrusion of the Kerba Granite at 1808±6 Ma. Analysis of available age data indicates that the minimum age of deformation is poorly constrained. However, evidence from regionally similar structures and geochronological data indicate that deformation associated with high-strain zones in the southern Capricorn Orogen may be Mesoproterozoic in age—significantly younger than the Capricorn Orogeny.

Metallogeny in the Capricorn Orogen, Western Australia, the result of multiple ore-forming processes

The Capricorn Orogen includes metamorphosed tectonic units, intrusive igneous rocks, volcano-sedimentary and sedimentary basins situated between the Pilbara and Yilgarn cratons. These include the Gascoyne Complex, Ashburton, Blair, Bresnahan, Yerrida, Bryah, Padbury, and Earaheedy basins, and Edmund and Collier (former Bangemall) basins. Mineral deposits and occurrences in these units comprise stratabound base metals, including the giant Abra Pb–Cu–Ba (Edmund Basin) and Magellan Pb (Earaheedy Basin), and numerous hydrothermal veins and precious and/or base metal lodes. Some of the more important orogenic precious metal lodes are those of the Peak Hill group in the Bryah-Padbury basins and the Egerton field in the Gascoyne Complex. The metallogeny of the Capricorn Orogen reflects syn- to post-Capricorn orogenic events, linked to either protracted convergence tectonics between the Pilbara and Yilgarn cratons, or to a two-stage convergence history, one during the Capricorn Orogeny (ca. 1.83 Ga; Pilbara–Yilgarn convergence), the other during the Yapungku orogeny (ca. 1.79; North Australian–West Australian convergence). The metallogeny of the Capricorn Orogen was further sustained by continental mafic magmatism at about 1.45 and 1.07 Ga, during which heat advection may have caused widespread hydrothermal circulation, particularly in the Edmund, Collier, Earaheedy basins and adjacent tectonic units. An interesting, but as yet unexplained, aspect of the Capricorn metallogeny is the regional Pb–Ba enrichment. A conceptual model that attempts to explain the billion-year-long history of the metallogeny in the Capricorn Orogen is proposed. Mineral exploration in the Capricorn Orogen is somewhat limited and not as intensive as is in the brown-fields of the Archaean terranes of Western Australia.

Proterozoic evolution and tectonic setting of the northwest Paterson Orogen, Western Australia

Abstract The Proterozoic igneous, deformation and metamorphic histories of the Palaeoproterozoic Rudall Complex in the northwestern Paterson Orogen can be linked to those of the Arunta Inlier in central Australia, and in part with the Capricorn Orogen in central Western Australia. The similarities in deformation and metamorphic histories for these widely separated regions indicate a Palaeoproterozoic continent–continent collisional event between the Palaeoproterozoic West Australian and North Australian cratons between c. 1830 and 1765 Ma. In the Paterson Orogen this Palaeoproterozoic collisional event resulted in the Yapungku Orogeny, which included thrust stacking of clastic sedimentary and volcanic rocks, deposition of the protoliths for the c. 1790 Ma siliciclastic paragneiss succession contemporaneous with granitic intrusion, and metamorphism up to granulite facies. During this 65-million-year period, the Arunta Inlier and Capricorn Orogen were deformed, metamorphosed at medium to high grades and intruded by granitoids during the Strangways Orogeny in the Arunta Inlier and the Capricorn Orogeny in the Capricorn Orogen. The Neoproterozoic Tarcunyah, Throssell and Lamil groups are clastic sedimentary sequences that were deposited after 1070 Ma in the northwestern Paterson Orogen, and deformed by the Miles Orogeny before 678 Ma. The Miles Orogeny produced a northwesterly trending fold and fault system of tight to isoclinal upright and overturned folds and thrust faults. The orogeny may have been coincident with the c. 750–720 Ma Areyonga tectonic movement affecting the Arunta Inlier and the lower Neoproterozoic part of the Amadeus Basin in central Australia. At c. 550 Ma the Paterson Orogeny, which is most likely equivalent to the Petermann Orogeny in the Musgrave Complex of central Australia, deformed the northwestern Paterson Orogen and was preceded by local intrusion of granites. The similarities of styles and timing of deformation in the northwestern Paterson Orogen, Arunta Inlier and Capricorn Orogen indicate that these three regions were probably linked during most of the Proterozoic.

Geology and tectonic evolution of Palaeoproterozoic basins of the eastern Capricorn Orogen, Western Australia

Abstract The Palaeoproterozoic Yerrida, Bryah, Padbury, and Earaheedy Basins extend for about 700 km along the northern margin of the Yilgarn Craton and are part of the Capricorn Orogen. The basins developed between ca. 2.2 and 1.8 Ga, recording periods of sedimentation, volcanism, rifting, accretion and passive-margin tectonism. The oldest is the 2.17 Ga Yerrida Basin, which began as an intracontinental sag within which siliciclastics and evaporites accumulated. At about 2.0 Ga convergence of the Yilgarn Craton and the Glenburgh terrane closed an oceanic arm, resulting in the accretion of oceanic crust and plateaux onto the Yilgarn margin and the western side of the Yerrida sag basin. This accretion produced the Narracoota Formation, which together with the clastic and chemical (banded iron-formation) sedimentation that followed, forms the Bryah Basin. Continuing collision resulted in the establishment, around 1.96 Ga, of the Padbury foreland basin, developed on top of the Bryah Basin rocks. The ca. 1.83 Ga convergence and collision of the Pilbara and Yilgarn Cratons (Capricorn Orogeny), resulted in deformation of the Bryah–Padbury Basins and orogenic uplift with coarse clastic sedimentation accompanied by the eruption of flood basalts into the Yerrida Basin. To the east of the collision zone, and possibly postdating collision, a passive margin was present. Sedimentation along it consisted of clastic and chemical sediments (granular iron-formation), which form the Earaheedy Basin. During the ca. 1.79 Ga Yapungku Orogeny, due to the convergence of the North Australian and Western Australian cratons, the northern margin of the Earaheedy Basin was deformed (Stanley Fold Belt). This last deformation event concludes the Palaeoproterozoic history of the eastern Capricorn Orogen basins.

Palaeoproterozoic crustal accretion and collision in the southern Capricorn Orogen: the Glenburgh Orogeny

The Capricorn Orogen in central Western Australia records the Palaeoproterozoic collision of the Archaean Pilbara and Yilgarn Cratons. Until recently only one orogenic event was thought to be the cause of this collision, the 1830–1780 Ma Capricorn Orogeny. However, recent work has uncovered an older event, the Glenburgh Orogeny that occurred between 2000 and 1960 Ma. The Glenburgh Orogeny reflects the collision of a late Archaean to Palaeoproterozoic microcontinent (the Glenburgh Terrane) with the Archaean Yilgarn Craton and is therefore tectonically distinct as well as significantly older than the widespread 1900–1800 Ma tectonothermal events recorded in northern Australia. The Glenburgh Terrane preserves a different history from either the Yilgarn or Pilbara Cratons. Granitic gneiss protoliths dated at ca. 2550 Ma were intruded by widespread granite magmatism dated at 2005–1970 Ma, accompanied by high-grade metamorphism and deformation throughout the terrane. At ca. 1960 Ma silicic granite of the Bertibubba Supersuite intruded the northern margin of the Yilgarn Craton along the Errabiddy Shear Zone, a crustal-scale shear zone that today marks the contact of the Glenburgh Terrane and Yilgarn Craton. At ca. 1950 Ma silicic dykes intruded the southernmost part of the Glenburgh Terrane, marking the end of the Glenburgh Orogeny. East of the Glenburgh Terrane the Glenburgh Orogeny resulted in the cessation of mafic volcanism in the Bryah Basin, and the basin’s eventual closure. Siliciclastic, carbonate and chemical sedimentary rocks were deposited in the Padbury Basin that formed a retro-arc foreland basin on top of the Bryah Basin, and probably records the later stages of the Glenburgh Orogeny collision.

Veins and hydrothermal fluid flow in the Mt. Whaleback Iron Ore District, eastern Hamersley Province, Western Australia

Abstract With the advent of recent hydrothermal models for genesis of giant microplaty hematite ore deposits of the Hamersley Province, we have undertaken regional scale analysis of veins and rocks surrounding the Mt. Whaleback deposit with an aim to characterize the nature of fluid flow before, during and after ore formation. At least five groups of veins formed in the Brockman Iron Formation in deformational events associated with the Paleoproterozoic interaction between the Yilgarn and Pilbara cratons. In a ∼600 km 2 area around the Mt. Whaleback microplaty hematite orebody, the five main veining events most likely correspond to the collisional (D 2 ) and extensional collapse (D 3 ) phases of the ca. ∼2300–2200 Ma Ophthalmian Orogeny, with one or two later phases probably associated with the waning stages of the ca. 1700–1650 Ma Capricorn Orogeny (D 4 ) and/or later Proterozoic events. The abundance of veins associated with D 2 contraction and D 3 extensional fabrics, and the composition (up to 15 wt.% NaCl equivalent) and pressure-corrected homogenisation temperatures of primary fluid inclusions (200–400 °C), indicate that the temperature, salinity and likely volume of fluid reached a peak during the last stages of the Ophthalmian Orogeny and the first stages of post-Ophthalmian extension. Paragenesis of veins relative to the deformation history, and localization of highest vein densities in the Dales Gorge Member, provides circumstantial evidence that the peak of hydrothermal activity in the district was synchronous with the main phase of microplaty hematite ore formation at Mt. Whaleback. We propose that the veins record part of the fluid flow system related to the genesis of this giant ore deposit.

Revised geochronology of magmatism in the western Capricorn Orogen at 1805–1785 Ma: Diachroneity of the Pilbara‐Yilgarn collision

The June Hill Volcanics occupy a central stratigraphic position in the tectonic evolution of the Capricorn Orogeny that occurred along the southwestern margin of the Pilbara Craton in Palaeoproterozoic time. The volcanic rocks have been interpreted alternatively as due to continental rifting, foreland‐basin development, or backarc extension. Previous multigrain zircon U–Pb results from these rocks, as well as from an unnamed rhyolitic porphyry that is interstratified in turbidites of the overlying Ashburton Formation, are influenced by the combined effects of Pb loss and xenocrystic inheritance. We report new SHRIMP zircon ages of 1799 ± 8 and 1786 ± 11 Ma, respectively from the June Hill Volcanics and the unnamed porphyry, both of which have been deformed during the Capricorn Orogeny. Close similarity between these ages and results from various late‐stage Capricorn granitoids (including our new SHRIMP result of 1795 ± 8 Ma for the Minnie Creek granite) imply rapid development of the western Ashburton Trough, accumulating and deforming ∼10 km of turbiditic sediments in about 10 million years. Comparison of our new June Hill age with previous SHRIMP results from the central Ashburton Trough implies that foreland‐basin development of the Capricorn Orogeny was significantly diachronous, younging westward during oblique Pilbara‐Yilgarn convergence.

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

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 2O/Na 2O 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.

Did global tectonics drive early biosphere evolution? Carbon isotope record from 2.6 to 1.9 Ga carbonates of Western Australian basins

The δ 13C carb record of well preserved carbonates in outcrop and core is here examined from the 2.6 to 1.9 Ga old basins of Western Australia. These data, which are constrained by a well defined stratigraphic and tectonic framework, and by U–Pb zircon ages, provide an insight into the variables coincident with the evolution of an oxidative atmosphere and the evolution of the early biosphere. In the latest Archaean (ca. 2.6 Ga) the secular δ 13C carb curve is flat much like that seen in the later Palaeoproterozoic basins of Northern Australia (<1.8 Ga). This implies that photosynthesis was a major component of the biosphere at that time and that the carbon mass balance was stable. In the early Palaeoproterozoic, beginning after 2.5 Ga and continuing until at least 1.9 Ga, the δ 13C carb the curve is much more dynamic, with significant positive and negative excursions, including a major positive excursion (+9‰ pdb) close to 2.2 Ga. These excursions can be correlated with the Lomagundi event identified in Africa, Europe and North America. Previously published studies of the overlying Meso- to Palaeoproterozoic Bangemall basin and of 1.8–1.5 Ga old basins in northern Australia suggest that the δ 13C carb curve became relatively monotonic again after ca. 1.8 Ga and remained so for most of the following Mesoproterozoic. Comparisons with data from other ancient cratons, especially Africa, suggest that the secular carbon curve may be even more complex than presently understood and probably comparable to the major excursions seen in the Neoproterozoic. When the Western Australian data are placed in their stratigraphic and tectonic framework we find that the monotonic latest-Archaean curve coincides with a tectonically quiescent period in which carbonates formed in an basinal setting on a craton surrounded by passive margins. The data are consistent with an earth in which the carbon mass balance was in equilibrium. The δ 13C carb curve began to oscillate following the onset of glaciation as the Pilbara and Yilgarn Cratons began to converge during the Capricorn Orogeny suggesting periods of rapid carbon burial during continental dispersal. However, the major positive excursion is preserved in carbonates from back-arc basins formed as the ocean closed and subduction began. Since similar tectonic processes can be recognised, not only in Northern Australia but also on other early cratons, it can be argued that the carbon excursions relate to supercontinent cycles and to major periods of mantle overturn and superplume development. We explain this coincidence of carbon isotopic excursions and tectonism by the sequestration of carbon during ocean closure with organic-rich passive margin sediments containing isotopically light carbon subducted into and stored in the lower crust and mantle. The global ocean thereby became enriched in isotopically heavy carbon, releasing oxygen to the atmosphere. A second stepwise increase in atmospheric oxygen in the Neoproterozoic may also have been connected with the assembly of Rodinia. This second event has been associated with the development of multicellular life and the evolutionary ‘Big Bang’. Between the two events the carbon cycle, and to some extent the biosphere, appear to have entered a period of prolonged evolutionary stasis. This implies that the evolution of both the atmosphere, and the biosphere, may have been driven forward by planetary evolution, implying that biospheric evolution has largely been driven by the dynamo of earth's tectonism and its long term survival depends upon these endogenic (thermal) energy resources. If this is so it has fundamental implications, not only for life on earth, but for the more general problems surrounding the likelihood of life having evolved on other planetary bodies. Small planets with insufficient endogenic energy resources to sustain the crust/mantle interactions of plate tectonics (such as Mars) seem to us unlikely to have allowed evolution beyond single-celled life forms. Once a planet's energy resources are expended the biosphere would most likely enter a prolonged stasis and ultimately face extinction.

Petrochemistry and Genesis of Magnetite Ores from Addatigala and Devipatnam, Eastern Ghats Belt, South India

With the advent of recent hydrothermal models for genesis of giant microplaty hematite ore deposits of the Hamersley Province, we have undertaken regional scale analysis of veins and rocks surrounding the Mt. Whaleback deposit with an aim to characterize the nature of fluid flow before, during and after ore formation. At least five groups of veins formed in the Brockman Iron Formation in deformational events associated with the Paleoproterozoic interaction between the Yilgarn and Pilbara cratons. In a ∼600 km2 area around the Mt. Whaleback microplaty hematite orebody, the five main veining events most likely correspond to the collisional (D2) and extensional collapse (D3) phases of the ca. ∼2300–2200 Ma Ophthalmian Orogeny, with one or two later phases probably associated with the waning stages of the ca. 1700–1650 Ma Capricorn Orogeny (D4) and/or later Proterozoic events. The abundance of veins associated with D2 contraction and D3 extensional fabrics, and the composition (up to 15 wt.% NaCl equivalent) and pressure-corrected homogenisation temperatures of primary fluid inclusions (200–400 °C), indicate that the temperature, salinity and likely volume of fluid reached a peak during the last stages of the Ophthalmian Orogeny and the first stages of post-Ophthalmian extension. Paragenesis of veins relative to the deformation history, and localization of highest vein densities in the Dales Gorge Member, provides circumstantial evidence that the peak of hydrothermal activity in the district was synchronous with the main phase of microplaty hematite ore formation at Mt. Whaleback. We propose that the veins record part of the fluid flow system related to the genesis of this giant ore deposit.

Three Palaeoproterozoic basins—Yerrida, Bryah and Padbury—Capricorn Orogen, Western Australia

The Palaeoproterozoic Yerrida, Bryah and Padbury Basins record periods of sedimentation and magmatism along the northern margin of the Archaean Yilgarn Craton. Each basin is characterised by distinct stratigraphy, igneous activity, structural and metamorphic history and mineral deposit types. The oldest of these basins, the Yerrida Basin (ca 2200 Ma) is floored by rocks of the Archaean Yilgarn Craton. Important features of this basin are the presence of evaporites and continental flood basalts. The ca 2000 Ma Bryah Basin developed on the northern margin of the Yilgarn Craton during backarc sea‐floor spreading and rifting, the result of which was the emplacement of voluminous mafic and ultramafic volcanic rocks. During the waning stages of the Bryah Basin this mafic to ultramafic volcanism gave way to deposition of clastic and chemical sedimentary rocks. At a later stage, the Padbury Basin developed as a retroarc foreland basin on top of the Bryah Basin in a fold‐and‐thrust belt. This resulted from either the collision of the Pilbara and Yilgarn Cratons (Capricorn Orogeny) or the ca 2000 Ma westward collision of the southern part of the Gascoyne Complex and the Yilgarn Craton (Glenburgh Orogeny). During the Capricorn Orogeny the Bryah Group was thrust to the southeast, over the Yerrida Group. Important mineral deposits are contained in the Yerrida, Bryah and Padbury Basins. In the Yerrida Basin a large Pb–carbonate deposit (Magellan) and black shale‐hosted gossans containing anomalous abundances of Ba, Cu, Zn and Pd are present. The Pb–carbonate deposit is hosted by the upper units of the Juderina Formation, and the lower unit of the unconformably overlying Earaheedy Group. The Bryah and Padbury Basins contain orogenic gold, copper‐gold volcanogenic massive sulfides, manganese and iron ore. The origin of the gold mineralisation is probably related to tectonothermal activity during the Capricorn Orogeny at ca 1800 Ma.

Proterozoic biotite Rb–Sr dates in the northwestern part of the Yilgarn Craton, Western Australia

Rb–Sr dating of biotite in the northwestern corner of the Yilgarn Craton identified four areas with distinctive age ranges. Biotite in the northwestern area, which includes the Narryer Terrane and part of the Murchison Terrane, yields reset Rb–Sr dates of ca 1650 Ma. In the western area, along the margin of the craton, biotite has been reset to 629 Ma. Eastward of these areas, mainly in the Murchison Terrane, the modal biotite date is near 2450 Ma, though because of a skewed distribution the mean date is closer to 2300 Ma. Dates in a transition zone between the western and eastern areas range broadly between 2000 and 1000 Ma, averaging about 1775 Ma. The western area and the transition zone are continuous with analogous areas south of the limits of the present study. The 1650 Ma dates in the northwestern area are probably related to plutonic and tectonic activity of similar age in the Gascoyne Province to the north. They may represent cooling after thermal resetting during tectonic loading by southward thrust‐stacking of slices of Narryer Terrane and allochthonous Palaeoproterozoic volcanic arc and backarc rocks during the Capricorn Orogeny. This episode of crustal shortening resulted from the collision of the Yilgarn and Pilbara Cratons to form the West Australian Craton. The dates reflect cooling associated with subsequent erosion‐induced rebound. The 2450 Ma biotite dates of the eastern area are similar to biotite dates found over most of the Yilgarn Craton and represent a background upon which the later dates have been superimposed. The origin of dates in the western area is unknown but may be related to an associated dolerite dyke swarm or to possible thrusting from the west. There is some evidence of minor later intrusion of felsic hypabyssal rock between 2000 and 2200 Ma and localised shearing in the Narryer area at about 1350 to 1400 Ma. One small area near Yalgoo with biotite Rb–Sr dates near 2200 Ma may be cogenetic with the Muggamurra Swarm of dolerite dykes.

Early tectonic history of the Marymia Inlier and correlation with the Archaean Yilgarn Craton, Western Australia

The Archaean granite‐greenstone rocks of the Marymia Inlier outcrop within Proterozoic rocks forming the Capricorn Orogen. Five major deformation events are recognised in the rocks of the Plutonic Well and Baumgarten greenstone belts. The first two events were Late Archaean and synchronous with major epithermal gold mineralisation in the belts. Palaeoproterozoic extensional faulting was probably related to the early stages of the Capricorn Orogeny. The fourth event records a compressional phase of the Capricorn Orogeny associated with greenschist‐facies metamorphism, whereas the last major event involved wrench faulting associated with minor folding. The Archaean tectonic history, rock types and timing of mineralisation strongly suggest that the Marymia Inlier is part of the Yilgarn Craton, and that each of the provinces in the craton experienced the same geological history since 2.72 Ga. The inlier is now interpreted to include two components; one is the eastern or northern extension of either the Narryer Terrane, Murchison Province or Southern Cross Province, and the other is the northwestern extension of the Eastern Goldfields Province. The Jenkin Fault, which was active in Proterozoic times, separates these two components.

Structural-metamorphic evolution of the Palaeoproterozoic Bryah and Padbury Groups during the Capricorn orogeny, Western Australia

Volcano-sedimentary successions of the Bryah and Padbury Groups were deposited and deformed along the northern margin of Archaean Yilgarn Craton, Western Australia, during the ca 2.0-1.8 Ga Capricorn Orogeny. This orogeny is interpreted to record convergence and collision of the Yilgarn Craton with the Pilbara Craton to the north. The Bryah Group with its extensive mafic and ultramafic volcanic rocks represents a short-lived proto-oceanic backarc basin whereas the sedimentary Padbury Group records the filling in of a retro-arc foreland basin. The two groups were deformed as a single package and are now everywhere in faulted contact with older Palaeoproterozoic rocks or underlying Archaean granite-greenstone. Four events are recognized in a progressive compressional deformation regime. During D1 subhorizontal high strain shear zones and tectonic interleaving developed between the volcanosedimentary succession and reworked Archaean crustal fragments that were underthrust or underplated during initial basin closure. Movement directions were mainly north-to-south. D2 NS shortening resulted in upright EW folds dominated by two regional, basement-cored, anticlines, which are interpreted as fault bend folds above a crustal scale frontal thrust ramp. Uplifted deeper crustal rocks are now represented by zones of higher grade metamorphic assemblages in the core of these anticlines. D3 folds are upright, NS striking and restricted to a zone (depression) between the two basement-cored anticlines (or domes). D2 and D3 folds are largely mutually exclusive with little or no evidence for overprinting, and are regarded as contemporaneous. The D3 structures are interpreted to have formed above lateral ramps in the crustal scale thrust fault during continued NS shortening. D4 is a minor event of NNE-SSW compression and did not influence the regional geometry. This deformation history records the closure and deformation of marginal back-arc and foreland basins.

High pressure amphibolite-granulite facies metamorphism in the Paleoproterozoic Rudall Complex, central Western Australia

The Rudall Complex forms the basement of the Paleoproterozoic to Neoproterozoic Paterson Orogen, which was deformed during at least three major orogenies. The complex comprises a sequence of metamorphosed clastic and chemical sediments, and basalts, deposited in a marginal basin environment, prior to extensive felsic intrusion. Deformation of the sequence during the first “Yapungku” orogeny between 2000 and 1760 Ma, included extensive thrust stacking from ENE to WSW. The thermal peak of the associated high-pressure metamorphism was late- to post-tectonic with respect to this deformation. Thermobarometry on amphibolites and mafic granulites that contain various combinations of the minerals hornblende, plagioclase, quartz, garnet, othopyroxene and clinopyroxene, indicates that temperatures peaked at around 800°C, while pressures reached 1200 MPa. The late timing of peak temperatures together with the presence, in some amphibolites, of amphibole-plagioclase symplectitic coronas around garnet, indicates a steeply decompressive clockwise P-T-t path. Peak metamorphic pressures indicate that the crust was locally thickened by at least 40 km, during an event that showed similarities to Phanerozoic collisional tectonics. It is suggested that this deformational and metamorphic event records the collision between the Pilbara Craton and a continent to the northeast. The Paleoproterozoic evolution of Australia has previously been attributed to ensialic processes characterised by low-pressure metamorphism and anti-clockwise P-T-t paths. The metamorphic history of the Rudall Complex is not consistent with this view, and the documented high-pressure metamorphism is broadly synchronous with a medium-pressure metamorphic event in central Australia (the Strangways Orogeny), and possibly also in northwestern Australia (the Capricorn Orogeny). Collisional plate-tectonic processes played a significant role in the Paleoproterozoic evolution of northern Australia.

Proterozoic tectonic reworking in the Rudall complex, Western Australia

Metasedimentary gneisses and schists in the northern part of the Rudall Complex are divided into the Yandagooge and Tjingkulatjatjarra Formations. The Yandagooge Formation occurs as inclusions in, and is intimately interlayered with, a composite orthogneiss, which is the dominant rock type of the Rudall Complex. The Tjingkulatjatjarra Formation is not disrupted by the orthogneiss, and is interpreted as being younger than both the orthogneiss and Yandagooge Formation, possibly forming a cover sequence to these rocks. The Yandagooge Formation and orthogneiss record a tectonic event, D1, that pre‐dates the Tjingkulatjatjarra Formation, but all rock units were intensely deformed and metamorphosed during the Watrara Orogeny, which comprises events D2–3 and began at ∼ 1330 Ma. The recognition of marker horizons in the metasedimentary rocks enables regional structural analysis, and the identification of mesoscopic to macroscopic recumbent folds produced during D2, which were redeformed by upright D3 folding. Metapelitic assemblages produced during D2 include kyanite + staurolite + biotite and garnet + staurolite + biotite, and suggest that the Watrara Orogeny was a consequence of the burial of the Rudall Complex to depths of ∼ 20 km. These Barrovian‐style assemblages are unusual for the mid‐Proterozoic orogenic belts of Australia, and the Watrara Orogeny could be equivalent to the Capricorn Orogeny, which was consequent to the juxtaposition of the Pilbara and Yilgarn Cratons. Unconformably overlying low‐grade Late Proterozoic cover rocks, together with the basement rocks, were mildly affected by a regional folding and thrusting event during the Paterson Orogeny at ∼ 700 Ma.