Zircon grains in rocks from the Yilgarn Craton record crust formation dating back to shortly afte... more Zircon grains in rocks from the Yilgarn Craton record crust formation dating back to shortly after the formation of the Earth. However, much of the evidence is cryptic and not apparent in the mapped geology. New Lu–Hf isotopic results, combined with existing Lu–Hf and Sm–Nd isotopic data, indicate five model-age probability peaks in the central and eastern Yilgarn Craton: at ca 4200, ca 3500, ca 3100, ca 2800 Ma and ca 2700 Ma. The ca 3100 Ma, ca 2800 Ma and ca 2700 Ma model-age peaks likely correspond to crust formation events. Evidence of the earlier peaks is not seen directly in the rock record, although zircon crystals in rocks of the Southern Cross Domain of the Youanmi Terrane show a long history of reworking pointing back to mantle extraction more than 4200 million years ago. The earliest peak is not recorded in the Eastern Goldfields Superterrane, indicating that crust formation in this region post-dated the earliest development of the Yilgarn Craton. Subsequent, broadly contemporaneous, episodes of mantle extraction and crustal reworking are indicated by the datasets for both the Eastern Goldfields Superterrane and the Southern Cross Domain. Magmas in the Eastern Goldfields Superterrane had a substantial juvenile input whereas those in the Southern Cross Domain recorded major reworking of older crust. The rock records for both the Eastern Goldfields Superterrane and the Southern Cross Domain share common elements of history after ca 2960 Ma. Both regions appear to have been subjected to major heating at ca 3100 Ma and ca 2800 Ma that resulted in the generation of juvenile crust in the east and reworking of older crust in the west. The ca 3500 Ma event is not readily evident in the rock record and may reflect a mixed age. However, the ca 3100 Ma and ca 2800 Ma events are recorded by both granite suites and greenstone successions across the craton. The ca 2700 Ma event is most evident in rocks from the Eastern Goldfields Superterrane.
The Shaw Granitoid Complex, one of the classic granitoid domes of the Archaean Pilbara Craton, We... more The Shaw Granitoid Complex, one of the classic granitoid domes of the Archaean Pilbara Craton, Western Australia, evolved from a plutonic complex into a ∼60 km diameter steep-sided dome during migmatisation of the deep crust. Consistent stratigraphic younging of greenstones away from the dome and underlying outward-dipping domical foliations indicate that the present exposure is a crustal section. The section passes downward through progressively older greenstones, into a near-concordant, high-level, sheeted granite complex, the ca. 3470 Ma North Shaw Suite (NSS), to a younger sill-dyke complex of leucogranite, the Pilga Leucogranite. Farther below, the leucogranite grades via diatexite and a network of leucogranitic dykes into the lowermost unit of migmatitic orthogneisses, which have protolith ages that overlap with the NSS. A semi-continuous, solid-state domal foliation extends downward from the greenstones, through the NSS into the migmatitic orthogneiss, but is less evident in the intervening Pilga Leucogranite. A set of NNW-trending upright folds with axial planar leucogranite dykes exist in both the Pilga Leucogranite and underlying orthogneisses, but not in the NSS or greenstones.
Field relations indicate that much of the solid-state, dome-concordant foliation developed in a sub-horizontal attitude before the leucogranite sheets were emplaced above the migmatitic orthogneisses. Emplacement occurred along a sub-horizontal high-strain zone, probably at a rheological boundary between the relatively rigid NSS and softer underlying migmatitic orthogneisses. The sheeted leucogranite complex and underlying orthogneisses were subsequently folded with later leucogranite magmas injected along the axial surfaces of the folds. These leucogranite magmas locally extended into the overlying deformed NSS, where they also acquired a weak domical fabric, suggesting syn-doming emplacement. Although presently estimated to be 3445–3410 Ma old, stratigraphic constraints suggest that the leucogranite and the dome could be as young as 3300 Ma. The contrasting strain patterns above and below the Pilga Leucogranite are not expected for horizontal tectonic models invoking thrusting, core-complex formation or cross-folding, but are similar to one observed for gravity-driven, vertical tectonic models. Thermal softening initiated deformation, and then deformation localised melts at a specific rheological interface. Once established, this interface controlled the locus of Pilga Leucogranite intrusion during dome evolution, demonstrating the interplay between deformation and magmatism.
In the Archaean Pilbara Craton of Western Australia, three zones of heterogeneous centimetre- to ... more In the Archaean Pilbara Craton of Western Australia, three zones of heterogeneous centimetre- to metre-scale sheeted granites are interpreted to represent high-level, syn-magmatic shear zones. Evidence for the syn-magmatic nature of the shear zones include imbricated and asymmetrically rotated metre-scale orthogneiss xenoliths that are enveloped by leucogranite sheets that show no significant internal strain. At another locality, granite sheets have a strong shape-preferred alignment of K-feldspar, suggesting magmatic flow, while the asymmetric recrystallisation of the grain boundaries indicates that non-coaxial deformation continued acting upon the sheets under sub-solidus conditions. Elsewhere, randomly oriented centimetre-wide leucogranite dykes are realigned at a shear zone boundary to form semi-continuous, layer-parallel sheets within a magma-dominated, dextral shear zone.
It is proposed that the granite sheets formed by the incremental injection of magmas into active shear zones. Magma was sheared during laminar flow to produce the sheets that are aligned sub-parallel to the shear zone boundary. Individual sheets are fed by individual dykes, with up to 1000s of discrete injections in an individual shear zone. The sheets often lack microstructural evidence for magmatic flow, either because the crystal content of the magma was too low to record internal strain, or because of later recrystallisation.
Structural elements of a syn-kinematic pluton in the Pilbara Craton of Western Australia reflect ... more Structural elements of a syn-kinematic pluton in the Pilbara Craton of Western Australia reflect evolving rheology during progressive crystallisation, representing a transition from magmatic, through high-temperature sub-solidus to low-temperature deformation. Magmatic elements include: (1) NNE-trending, sub-vertical, kilometre-scale folds (outlined by xenolith trains) that have a magmatic axial planar foliation defined by aligned K-feldspar, (2) cross-cutting, ENE-trending, sub-vertical shear zones intruded by numerous ≤metre-scale granite sheets, and (3) a narrow, concordant magmatic K-feldspar foliation in the granite host-rock adjacent to the shear zones. Microstructures, such as fractured feldspars healed by quartz that is continuous with matrix grains, further suggest that deformation occurred while the pluton was magmatic. High-temperature structural elements include ‘checkerboard’ sub-grain boundaries in quartz, whereas deformation at lower temperatures is indicated by kinked feldspar, recrystallised microcline and undulose quartz. A NNE-trending, sub-vertical sub-solidus foliation, defined by elongate quartz aggregates, overprints all the above mentioned features. Generation of these sequential structural elements correspond to the transition from Newtonian-fluid, through Bingham-type, to Newtonian-solid behaviour as the crystal fraction of the felsic magma increased. The orientation of the structures is consistent with a regional ESE–WNW shortening direction throughout magma crystallisation. This study indicates how evolving magma rheology and regional strain regimes control the micro- to macro-structural elements of a syn-tectonic pluton.
Crystallising magma;
Foliation;
Rheology;
Shearing
The Albany-Fraser Orogen is considered to be a response to Mesoproterozoic continent–continent co... more The Albany-Fraser Orogen is considered to be a response to Mesoproterozoic continent–continent collision between the combined North and West Australian Cratons and the combined East Antarctic and South Australian Cratons. However, the tectonic history of the orogen and its components remain enigmatic. Recently, the Kepa Kurl Booya Province has been defined as the crystalline basement of the orogen and divided into the Fraser, Biranup, and Nornalup Zones. New geochronology shows that the Biranup Zone includes 1710–1650 Ma granitic to gabbroic intrusions and is a substantial crustal component extending at least 1200 km along the southern and southeastern margins of the Yilgarn Craton. Previous models interpreted the Biranup Zone as an exotic terrane accreted to the Yilgarn Craton during Mesoproterozoic collision, but new data presented here indicate a strong link to the craton margin during the Paleoproterozoic.
Proterozoic magmatism commenced in the Biranup Zone at 1708 ± 15 Ma with metasyenogranite emplacement. This granite has ɛHf values of −10 to −8 and whole rock ɛNd of −15, consistent with a reworked Archean Yilgarn source. Volcaniclastic deposition in the Biranup Zone occurred at 1689 ± 6 Ma, and was rapidly followed by granitic intrusion at 1686 ± 8 Ma. Deformation during the Zanthus Event is constrained by 1676 ± 6 Ma folded migmatitic leucosomes and 1679 ± 6 Ma cross-cutting axial planar leucosomes. A younger suite of granitic and gabbroic rocks, which exhibit distinct mingling and hybridization textures, is dated at 1665 ± 4 Ma. Magmatism in the eastern Biranup Zone displays high-K, calc-alkaline chemistry and a trend towards more juvenile compositions from 1710 to 1650 Ma. Based on the rapidly evolving tectonomagmatic history, modification of the original Yilgarn-like source by juvenile material, and the geochemical evolution of the melts, a feasible tectonic scenario for the Biranup Zone is an arc to back-arc setting on the active Yilgarn Craton margin. Such a model is supported by the 2684 ± 11 Ma magmatic crystallization age of an isolated Archean fragment, which has clear Yilgarn affinity, within the Biranup Zone.
The region was subsequently compressed and tectonically dismembered during Stages I (1345–1260 Ma) and II (1215–1140 Ma) of the Albany-Fraser Orogeny. Stage I was dominated by voluminous mafic and granitic magmatism, represented by the Fraser Zone intrusions and the Recherche Supersuite. Two granites from the Fraser Zone, dated at 1298 ± 4 Ma, have ɛHf values overlapping Biranup Zone compositions, indicative of a reworked Biranup source. The Biranup Zone was dominated by granulite facies metamorphism during Stage II. Zircons from the northeastern edge of the Fraser Zone are overgrown by two generations of zircon rims. The earlier rims, at c. 1270 ± 11 Ma, are broken and overgrown by a low-uranium fracture-filling phase at 1193 ± 26 Ma. This indicates uplift and brittle deformation between Stages I and II.
New geological mapping and geochronology in the northeast Yilgarn Craton has changed our geologic... more New geological mapping and geochronology in the northeast Yilgarn Craton has changed our geological understanding of this region. The Yilgarn Craton had previously been divided into a series of terranes, with the easternmost Eastern Goldfields Superterrane separated from the Youanmi Terrane, which forms the core of the protocraton, by the Ida Fault zone. The Eastern Goldfields Superterrane was subdivided into the western Kalgoorlie, central Kurnalpi, and eastern Burtville terranes, with the latter, easternmost terrane the focus of the new field mapping and geochronology. Four main episodes of greenstone crustal growth have been recognised in the northeast Yilgarn Craton: ca 2970–2910 Ma, ca 2815–2800 Ma, 2775–2735 Ma, and ca 2715–2630 Ma. Rather than a single Burtville Terrane, as previously proposed, the distribution of greenstone magmatism reveals a previously unrecognised young (<2720 Ma) Yamarna Terrane in the northeast corner of the craton. The Yamarna Terrane is separated from the older (>2735 Ma) redefined Burtville Terrane by the Yamarna Shear Zone, which is now regarded as a terrane boundary. The correlation of lithologies and ages of magmatism in the northeast Yilgarn Craton with the rest of the craton indicates that the Burtville Terrane has affinities with the Youanmi Terrane that forms the nucleus of the craton, whereas the Yamarna Terrane has affinities with the Kalgoorlie Terrane in the west of the Eastern Goldfields Superterrane. The Burtville and Youanmi terranes shared a common history from ca 2970 Ma until ca 2720 Ma, when regional extension accommodated deposition of the Kambalda Sequence in the Kalgoorlie Terrane. It appears that extension also occurred along the Yamarna Shear Zone after ca 2720 Ma, accommodating the deposition of greenstones in the Yamarna Terrane. Like the Kalgoorlie and Kurnalpi Terranes, the Yamarna Terrane contains inherited zircon and local older rocks. This suggests that the ca 2720 Ma extension did not result in widespread rifting and the formation of extensive oceanic crust. Rather, there was thinning of older crust that extended right across the current Yilgarn Craton.
Models for the formation of the high-amplitude (minimum 15 km), long wavelength (120 km) granitoi... more Models for the formation of the high-amplitude (minimum 15 km), long wavelength (120 km) granitoid dome-and-greenstone syncline geometry of the Archaean East Pilbara Granite–Greenstone Terrane (EP) of the Pilbara Craton are controversial. Diapiric models ascribe most structural features to vertical re-organisation of an inverted crustal density profile created by autochthonous magmatic processes during punctuated episodes of partial convective overturn of the upper and middle crust. Alternatively, uniformitarian models interpret the granitoid-cored domes as oversteepened metamorphic core complexes (MCCs) that formed during periods of active extension between periods of regional Alpine-style thrusting.
A review of recent advances in lithostratigraphy and geochronology in the EP shows that the greenstone belts are composed everywhere of a coherent, upward-younging stratigraphy, thereby precluding significant thrusting in the formation of the density inversion that drove partial convective overturn. We present new geological evidence that the domes contain some, or all, of the predictive characteristic features of diapirs, including; chaotic internal geometries of domes with otherwise simple outlines, ring faults along dome margins and in flanking greenstone belts, mushroom-shaped fold flaps around the margins of some domes, and sedimentation in inter-diapir synclines. These data, combined with a thorough review of previously proposed core complex models, show that horizontal tectonic models are inadequate to explain the structural, geometric, geochronological, and metamorphic features of the EP. Rather, an integrated model of punctuated partial convective overturn of the upper and middle crust in response to dominantly magmatic processes is presented to explain the ca. 750 Myr history of the terrane.
Zircon grains in rocks from the Yilgarn Craton record crust formation dating back to shortly afte... more Zircon grains in rocks from the Yilgarn Craton record crust formation dating back to shortly after the formation of the Earth. However, much of the evidence is cryptic and not apparent in the mapped geology. New Lu–Hf isotopic results, combined with existing Lu–Hf and Sm–Nd isotopic data, indicate five model-age probability peaks in the central and eastern Yilgarn Craton: at ca 4200, ca 3500, ca 3100, ca 2800 Ma and ca 2700 Ma. The ca 3100 Ma, ca 2800 Ma and ca 2700 Ma model-age peaks likely correspond to crust formation events. Evidence of the earlier peaks is not seen directly in the rock record, although zircon crystals in rocks of the Southern Cross Domain of the Youanmi Terrane show a long history of reworking pointing back to mantle extraction more than 4200 million years ago. The earliest peak is not recorded in the Eastern Goldfields Superterrane, indicating that crust formation in this region post-dated the earliest development of the Yilgarn Craton. Subsequent, broadly contemporaneous, episodes of mantle extraction and crustal reworking are indicated by the datasets for both the Eastern Goldfields Superterrane and the Southern Cross Domain. Magmas in the Eastern Goldfields Superterrane had a substantial juvenile input whereas those in the Southern Cross Domain recorded major reworking of older crust. The rock records for both the Eastern Goldfields Superterrane and the Southern Cross Domain share common elements of history after ca 2960 Ma. Both regions appear to have been subjected to major heating at ca 3100 Ma and ca 2800 Ma that resulted in the generation of juvenile crust in the east and reworking of older crust in the west. The ca 3500 Ma event is not readily evident in the rock record and may reflect a mixed age. However, the ca 3100 Ma and ca 2800 Ma events are recorded by both granite suites and greenstone successions across the craton. The ca 2700 Ma event is most evident in rocks from the Eastern Goldfields Superterrane.
The Shaw Granitoid Complex, one of the classic granitoid domes of the Archaean Pilbara Craton, We... more The Shaw Granitoid Complex, one of the classic granitoid domes of the Archaean Pilbara Craton, Western Australia, evolved from a plutonic complex into a ∼60 km diameter steep-sided dome during migmatisation of the deep crust. Consistent stratigraphic younging of greenstones away from the dome and underlying outward-dipping domical foliations indicate that the present exposure is a crustal section. The section passes downward through progressively older greenstones, into a near-concordant, high-level, sheeted granite complex, the ca. 3470 Ma North Shaw Suite (NSS), to a younger sill-dyke complex of leucogranite, the Pilga Leucogranite. Farther below, the leucogranite grades via diatexite and a network of leucogranitic dykes into the lowermost unit of migmatitic orthogneisses, which have protolith ages that overlap with the NSS. A semi-continuous, solid-state domal foliation extends downward from the greenstones, through the NSS into the migmatitic orthogneiss, but is less evident in the intervening Pilga Leucogranite. A set of NNW-trending upright folds with axial planar leucogranite dykes exist in both the Pilga Leucogranite and underlying orthogneisses, but not in the NSS or greenstones.
Field relations indicate that much of the solid-state, dome-concordant foliation developed in a sub-horizontal attitude before the leucogranite sheets were emplaced above the migmatitic orthogneisses. Emplacement occurred along a sub-horizontal high-strain zone, probably at a rheological boundary between the relatively rigid NSS and softer underlying migmatitic orthogneisses. The sheeted leucogranite complex and underlying orthogneisses were subsequently folded with later leucogranite magmas injected along the axial surfaces of the folds. These leucogranite magmas locally extended into the overlying deformed NSS, where they also acquired a weak domical fabric, suggesting syn-doming emplacement. Although presently estimated to be 3445–3410 Ma old, stratigraphic constraints suggest that the leucogranite and the dome could be as young as 3300 Ma. The contrasting strain patterns above and below the Pilga Leucogranite are not expected for horizontal tectonic models invoking thrusting, core-complex formation or cross-folding, but are similar to one observed for gravity-driven, vertical tectonic models. Thermal softening initiated deformation, and then deformation localised melts at a specific rheological interface. Once established, this interface controlled the locus of Pilga Leucogranite intrusion during dome evolution, demonstrating the interplay between deformation and magmatism.
In the Archaean Pilbara Craton of Western Australia, three zones of heterogeneous centimetre- to ... more In the Archaean Pilbara Craton of Western Australia, three zones of heterogeneous centimetre- to metre-scale sheeted granites are interpreted to represent high-level, syn-magmatic shear zones. Evidence for the syn-magmatic nature of the shear zones include imbricated and asymmetrically rotated metre-scale orthogneiss xenoliths that are enveloped by leucogranite sheets that show no significant internal strain. At another locality, granite sheets have a strong shape-preferred alignment of K-feldspar, suggesting magmatic flow, while the asymmetric recrystallisation of the grain boundaries indicates that non-coaxial deformation continued acting upon the sheets under sub-solidus conditions. Elsewhere, randomly oriented centimetre-wide leucogranite dykes are realigned at a shear zone boundary to form semi-continuous, layer-parallel sheets within a magma-dominated, dextral shear zone.
It is proposed that the granite sheets formed by the incremental injection of magmas into active shear zones. Magma was sheared during laminar flow to produce the sheets that are aligned sub-parallel to the shear zone boundary. Individual sheets are fed by individual dykes, with up to 1000s of discrete injections in an individual shear zone. The sheets often lack microstructural evidence for magmatic flow, either because the crystal content of the magma was too low to record internal strain, or because of later recrystallisation.
Structural elements of a syn-kinematic pluton in the Pilbara Craton of Western Australia reflect ... more Structural elements of a syn-kinematic pluton in the Pilbara Craton of Western Australia reflect evolving rheology during progressive crystallisation, representing a transition from magmatic, through high-temperature sub-solidus to low-temperature deformation. Magmatic elements include: (1) NNE-trending, sub-vertical, kilometre-scale folds (outlined by xenolith trains) that have a magmatic axial planar foliation defined by aligned K-feldspar, (2) cross-cutting, ENE-trending, sub-vertical shear zones intruded by numerous ≤metre-scale granite sheets, and (3) a narrow, concordant magmatic K-feldspar foliation in the granite host-rock adjacent to the shear zones. Microstructures, such as fractured feldspars healed by quartz that is continuous with matrix grains, further suggest that deformation occurred while the pluton was magmatic. High-temperature structural elements include ‘checkerboard’ sub-grain boundaries in quartz, whereas deformation at lower temperatures is indicated by kinked feldspar, recrystallised microcline and undulose quartz. A NNE-trending, sub-vertical sub-solidus foliation, defined by elongate quartz aggregates, overprints all the above mentioned features. Generation of these sequential structural elements correspond to the transition from Newtonian-fluid, through Bingham-type, to Newtonian-solid behaviour as the crystal fraction of the felsic magma increased. The orientation of the structures is consistent with a regional ESE–WNW shortening direction throughout magma crystallisation. This study indicates how evolving magma rheology and regional strain regimes control the micro- to macro-structural elements of a syn-tectonic pluton.
Crystallising magma;
Foliation;
Rheology;
Shearing
The Albany-Fraser Orogen is considered to be a response to Mesoproterozoic continent–continent co... more The Albany-Fraser Orogen is considered to be a response to Mesoproterozoic continent–continent collision between the combined North and West Australian Cratons and the combined East Antarctic and South Australian Cratons. However, the tectonic history of the orogen and its components remain enigmatic. Recently, the Kepa Kurl Booya Province has been defined as the crystalline basement of the orogen and divided into the Fraser, Biranup, and Nornalup Zones. New geochronology shows that the Biranup Zone includes 1710–1650 Ma granitic to gabbroic intrusions and is a substantial crustal component extending at least 1200 km along the southern and southeastern margins of the Yilgarn Craton. Previous models interpreted the Biranup Zone as an exotic terrane accreted to the Yilgarn Craton during Mesoproterozoic collision, but new data presented here indicate a strong link to the craton margin during the Paleoproterozoic.
Proterozoic magmatism commenced in the Biranup Zone at 1708 ± 15 Ma with metasyenogranite emplacement. This granite has ɛHf values of −10 to −8 and whole rock ɛNd of −15, consistent with a reworked Archean Yilgarn source. Volcaniclastic deposition in the Biranup Zone occurred at 1689 ± 6 Ma, and was rapidly followed by granitic intrusion at 1686 ± 8 Ma. Deformation during the Zanthus Event is constrained by 1676 ± 6 Ma folded migmatitic leucosomes and 1679 ± 6 Ma cross-cutting axial planar leucosomes. A younger suite of granitic and gabbroic rocks, which exhibit distinct mingling and hybridization textures, is dated at 1665 ± 4 Ma. Magmatism in the eastern Biranup Zone displays high-K, calc-alkaline chemistry and a trend towards more juvenile compositions from 1710 to 1650 Ma. Based on the rapidly evolving tectonomagmatic history, modification of the original Yilgarn-like source by juvenile material, and the geochemical evolution of the melts, a feasible tectonic scenario for the Biranup Zone is an arc to back-arc setting on the active Yilgarn Craton margin. Such a model is supported by the 2684 ± 11 Ma magmatic crystallization age of an isolated Archean fragment, which has clear Yilgarn affinity, within the Biranup Zone.
The region was subsequently compressed and tectonically dismembered during Stages I (1345–1260 Ma) and II (1215–1140 Ma) of the Albany-Fraser Orogeny. Stage I was dominated by voluminous mafic and granitic magmatism, represented by the Fraser Zone intrusions and the Recherche Supersuite. Two granites from the Fraser Zone, dated at 1298 ± 4 Ma, have ɛHf values overlapping Biranup Zone compositions, indicative of a reworked Biranup source. The Biranup Zone was dominated by granulite facies metamorphism during Stage II. Zircons from the northeastern edge of the Fraser Zone are overgrown by two generations of zircon rims. The earlier rims, at c. 1270 ± 11 Ma, are broken and overgrown by a low-uranium fracture-filling phase at 1193 ± 26 Ma. This indicates uplift and brittle deformation between Stages I and II.
New geological mapping and geochronology in the northeast Yilgarn Craton has changed our geologic... more New geological mapping and geochronology in the northeast Yilgarn Craton has changed our geological understanding of this region. The Yilgarn Craton had previously been divided into a series of terranes, with the easternmost Eastern Goldfields Superterrane separated from the Youanmi Terrane, which forms the core of the protocraton, by the Ida Fault zone. The Eastern Goldfields Superterrane was subdivided into the western Kalgoorlie, central Kurnalpi, and eastern Burtville terranes, with the latter, easternmost terrane the focus of the new field mapping and geochronology. Four main episodes of greenstone crustal growth have been recognised in the northeast Yilgarn Craton: ca 2970–2910 Ma, ca 2815–2800 Ma, 2775–2735 Ma, and ca 2715–2630 Ma. Rather than a single Burtville Terrane, as previously proposed, the distribution of greenstone magmatism reveals a previously unrecognised young (<2720 Ma) Yamarna Terrane in the northeast corner of the craton. The Yamarna Terrane is separated from the older (>2735 Ma) redefined Burtville Terrane by the Yamarna Shear Zone, which is now regarded as a terrane boundary. The correlation of lithologies and ages of magmatism in the northeast Yilgarn Craton with the rest of the craton indicates that the Burtville Terrane has affinities with the Youanmi Terrane that forms the nucleus of the craton, whereas the Yamarna Terrane has affinities with the Kalgoorlie Terrane in the west of the Eastern Goldfields Superterrane. The Burtville and Youanmi terranes shared a common history from ca 2970 Ma until ca 2720 Ma, when regional extension accommodated deposition of the Kambalda Sequence in the Kalgoorlie Terrane. It appears that extension also occurred along the Yamarna Shear Zone after ca 2720 Ma, accommodating the deposition of greenstones in the Yamarna Terrane. Like the Kalgoorlie and Kurnalpi Terranes, the Yamarna Terrane contains inherited zircon and local older rocks. This suggests that the ca 2720 Ma extension did not result in widespread rifting and the formation of extensive oceanic crust. Rather, there was thinning of older crust that extended right across the current Yilgarn Craton.
Models for the formation of the high-amplitude (minimum 15 km), long wavelength (120 km) granitoi... more Models for the formation of the high-amplitude (minimum 15 km), long wavelength (120 km) granitoid dome-and-greenstone syncline geometry of the Archaean East Pilbara Granite–Greenstone Terrane (EP) of the Pilbara Craton are controversial. Diapiric models ascribe most structural features to vertical re-organisation of an inverted crustal density profile created by autochthonous magmatic processes during punctuated episodes of partial convective overturn of the upper and middle crust. Alternatively, uniformitarian models interpret the granitoid-cored domes as oversteepened metamorphic core complexes (MCCs) that formed during periods of active extension between periods of regional Alpine-style thrusting.
A review of recent advances in lithostratigraphy and geochronology in the EP shows that the greenstone belts are composed everywhere of a coherent, upward-younging stratigraphy, thereby precluding significant thrusting in the formation of the density inversion that drove partial convective overturn. We present new geological evidence that the domes contain some, or all, of the predictive characteristic features of diapirs, including; chaotic internal geometries of domes with otherwise simple outlines, ring faults along dome margins and in flanking greenstone belts, mushroom-shaped fold flaps around the margins of some domes, and sedimentation in inter-diapir synclines. These data, combined with a thorough review of previously proposed core complex models, show that horizontal tectonic models are inadequate to explain the structural, geometric, geochronological, and metamorphic features of the EP. Rather, an integrated model of punctuated partial convective overturn of the upper and middle crust in response to dominantly magmatic processes is presented to explain the ca. 750 Myr history of the terrane.
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Papers by Mark Pawley
Field relations indicate that much of the solid-state, dome-concordant foliation developed in a sub-horizontal attitude before the leucogranite sheets were emplaced above the migmatitic orthogneisses. Emplacement occurred along a sub-horizontal high-strain zone, probably at a rheological boundary between the relatively rigid NSS and softer underlying migmatitic orthogneisses. The sheeted leucogranite complex and underlying orthogneisses were subsequently folded with later leucogranite magmas injected along the axial surfaces of the folds. These leucogranite magmas locally extended into the overlying deformed NSS, where they also acquired a weak domical fabric, suggesting syn-doming emplacement. Although presently estimated to be 3445–3410 Ma old, stratigraphic constraints suggest that the leucogranite and the dome could be as young as 3300 Ma. The contrasting strain patterns above and below the Pilga Leucogranite are not expected for horizontal tectonic models invoking thrusting, core-complex formation or cross-folding, but are similar to one observed for gravity-driven, vertical tectonic models. Thermal softening initiated deformation, and then deformation localised melts at a specific rheological interface. Once established, this interface controlled the locus of Pilga Leucogranite intrusion during dome evolution, demonstrating the interplay between deformation and magmatism.
It is proposed that the granite sheets formed by the incremental injection of magmas into active shear zones. Magma was sheared during laminar flow to produce the sheets that are aligned sub-parallel to the shear zone boundary. Individual sheets are fed by individual dykes, with up to 1000s of discrete injections in an individual shear zone. The sheets often lack microstructural evidence for magmatic flow, either because the crystal content of the magma was too low to record internal strain, or because of later recrystallisation.
Crystallising magma;
Foliation;
Rheology;
Shearing
Proterozoic magmatism commenced in the Biranup Zone at 1708 ± 15 Ma with metasyenogranite emplacement. This granite has ɛHf values of −10 to −8 and whole rock ɛNd of −15, consistent with a reworked Archean Yilgarn source. Volcaniclastic deposition in the Biranup Zone occurred at 1689 ± 6 Ma, and was rapidly followed by granitic intrusion at 1686 ± 8 Ma. Deformation during the Zanthus Event is constrained by 1676 ± 6 Ma folded migmatitic leucosomes and 1679 ± 6 Ma cross-cutting axial planar leucosomes. A younger suite of granitic and gabbroic rocks, which exhibit distinct mingling and hybridization textures, is dated at 1665 ± 4 Ma. Magmatism in the eastern Biranup Zone displays high-K, calc-alkaline chemistry and a trend towards more juvenile compositions from 1710 to 1650 Ma. Based on the rapidly evolving tectonomagmatic history, modification of the original Yilgarn-like source by juvenile material, and the geochemical evolution of the melts, a feasible tectonic scenario for the Biranup Zone is an arc to back-arc setting on the active Yilgarn Craton margin. Such a model is supported by the 2684 ± 11 Ma magmatic crystallization age of an isolated Archean fragment, which has clear Yilgarn affinity, within the Biranup Zone.
The region was subsequently compressed and tectonically dismembered during Stages I (1345–1260 Ma) and II (1215–1140 Ma) of the Albany-Fraser Orogeny. Stage I was dominated by voluminous mafic and granitic magmatism, represented by the Fraser Zone intrusions and the Recherche Supersuite. Two granites from the Fraser Zone, dated at 1298 ± 4 Ma, have ɛHf values overlapping Biranup Zone compositions, indicative of a reworked Biranup source. The Biranup Zone was dominated by granulite facies metamorphism during Stage II. Zircons from the northeastern edge of the Fraser Zone are overgrown by two generations of zircon rims. The earlier rims, at c. 1270 ± 11 Ma, are broken and overgrown by a low-uranium fracture-filling phase at 1193 ± 26 Ma. This indicates uplift and brittle deformation between Stages I and II.
A review of recent advances in lithostratigraphy and geochronology in the EP shows that the greenstone belts are composed everywhere of a coherent, upward-younging stratigraphy, thereby precluding significant thrusting in the formation of the density inversion that drove partial convective overturn. We present new geological evidence that the domes contain some, or all, of the predictive characteristic features of diapirs, including; chaotic internal geometries of domes with otherwise simple outlines, ring faults along dome margins and in flanking greenstone belts, mushroom-shaped fold flaps around the margins of some domes, and sedimentation in inter-diapir synclines. These data, combined with a thorough review of previously proposed core complex models, show that horizontal tectonic models are inadequate to explain the structural, geometric, geochronological, and metamorphic features of the EP. Rather, an integrated model of punctuated partial convective overturn of the upper and middle crust in response to dominantly magmatic processes is presented to explain the ca. 750 Myr history of the terrane.
Maps by Mark Pawley
Books by Mark Pawley
Field relations indicate that much of the solid-state, dome-concordant foliation developed in a sub-horizontal attitude before the leucogranite sheets were emplaced above the migmatitic orthogneisses. Emplacement occurred along a sub-horizontal high-strain zone, probably at a rheological boundary between the relatively rigid NSS and softer underlying migmatitic orthogneisses. The sheeted leucogranite complex and underlying orthogneisses were subsequently folded with later leucogranite magmas injected along the axial surfaces of the folds. These leucogranite magmas locally extended into the overlying deformed NSS, where they also acquired a weak domical fabric, suggesting syn-doming emplacement. Although presently estimated to be 3445–3410 Ma old, stratigraphic constraints suggest that the leucogranite and the dome could be as young as 3300 Ma. The contrasting strain patterns above and below the Pilga Leucogranite are not expected for horizontal tectonic models invoking thrusting, core-complex formation or cross-folding, but are similar to one observed for gravity-driven, vertical tectonic models. Thermal softening initiated deformation, and then deformation localised melts at a specific rheological interface. Once established, this interface controlled the locus of Pilga Leucogranite intrusion during dome evolution, demonstrating the interplay between deformation and magmatism.
It is proposed that the granite sheets formed by the incremental injection of magmas into active shear zones. Magma was sheared during laminar flow to produce the sheets that are aligned sub-parallel to the shear zone boundary. Individual sheets are fed by individual dykes, with up to 1000s of discrete injections in an individual shear zone. The sheets often lack microstructural evidence for magmatic flow, either because the crystal content of the magma was too low to record internal strain, or because of later recrystallisation.
Crystallising magma;
Foliation;
Rheology;
Shearing
Proterozoic magmatism commenced in the Biranup Zone at 1708 ± 15 Ma with metasyenogranite emplacement. This granite has ɛHf values of −10 to −8 and whole rock ɛNd of −15, consistent with a reworked Archean Yilgarn source. Volcaniclastic deposition in the Biranup Zone occurred at 1689 ± 6 Ma, and was rapidly followed by granitic intrusion at 1686 ± 8 Ma. Deformation during the Zanthus Event is constrained by 1676 ± 6 Ma folded migmatitic leucosomes and 1679 ± 6 Ma cross-cutting axial planar leucosomes. A younger suite of granitic and gabbroic rocks, which exhibit distinct mingling and hybridization textures, is dated at 1665 ± 4 Ma. Magmatism in the eastern Biranup Zone displays high-K, calc-alkaline chemistry and a trend towards more juvenile compositions from 1710 to 1650 Ma. Based on the rapidly evolving tectonomagmatic history, modification of the original Yilgarn-like source by juvenile material, and the geochemical evolution of the melts, a feasible tectonic scenario for the Biranup Zone is an arc to back-arc setting on the active Yilgarn Craton margin. Such a model is supported by the 2684 ± 11 Ma magmatic crystallization age of an isolated Archean fragment, which has clear Yilgarn affinity, within the Biranup Zone.
The region was subsequently compressed and tectonically dismembered during Stages I (1345–1260 Ma) and II (1215–1140 Ma) of the Albany-Fraser Orogeny. Stage I was dominated by voluminous mafic and granitic magmatism, represented by the Fraser Zone intrusions and the Recherche Supersuite. Two granites from the Fraser Zone, dated at 1298 ± 4 Ma, have ɛHf values overlapping Biranup Zone compositions, indicative of a reworked Biranup source. The Biranup Zone was dominated by granulite facies metamorphism during Stage II. Zircons from the northeastern edge of the Fraser Zone are overgrown by two generations of zircon rims. The earlier rims, at c. 1270 ± 11 Ma, are broken and overgrown by a low-uranium fracture-filling phase at 1193 ± 26 Ma. This indicates uplift and brittle deformation between Stages I and II.
A review of recent advances in lithostratigraphy and geochronology in the EP shows that the greenstone belts are composed everywhere of a coherent, upward-younging stratigraphy, thereby precluding significant thrusting in the formation of the density inversion that drove partial convective overturn. We present new geological evidence that the domes contain some, or all, of the predictive characteristic features of diapirs, including; chaotic internal geometries of domes with otherwise simple outlines, ring faults along dome margins and in flanking greenstone belts, mushroom-shaped fold flaps around the margins of some domes, and sedimentation in inter-diapir synclines. These data, combined with a thorough review of previously proposed core complex models, show that horizontal tectonic models are inadequate to explain the structural, geometric, geochronological, and metamorphic features of the EP. Rather, an integrated model of punctuated partial convective overturn of the upper and middle crust in response to dominantly magmatic processes is presented to explain the ca. 750 Myr history of the terrane.