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Papers by Ron Vernon
K-feldspar augen in felsic gneisses and mylonites—deformed phenocrysts or porphyroblasts?
Geologiska Föreningen i Stockholm Förhandlingar
Abstract A review of mesostructural and microstructural evidence favours the hypothesis that K-fe... more Abstract A review of mesostructural and microstructural evidence favours the hypothesis that K-feldspar augen and megacrysts in felsic gneisses and mylonites are generally, if not always, residual phenocrysts, rather than porphyroblasts that grew either during or after the deformation. In most examples, the augen appear to represent variably deformed megacrysts in former granitoids, as they commonly have similar distributions and shapes of inclusions and many of them show simple twinning; megacrysts in granitoids show abundant evidence of a phenocryst origin. Some augen may represent phenocrysts that grew in migmatite melt leucosomes and later were deformed, and others may have been phenocrysts in felsic volcanic or pyroclastic rocks, although megacrysts are uncommon in these rock-types. A porphyroblastic origin is opposed by the typical zonal distribution of inclusions and the common evidence of plastic deformation and partial recrystallization of the augen. The use of K-feldspar megacrysts as indicators...
Transactions of the Royal Society of Edinburgh: Earth Sciences
ABSTRACTSeveral mesosocopic structures are consistent with mechanical accumulation of crystals an... more ABSTRACTSeveral mesosocopic structures are consistent with mechanical accumulation of crystals and movement of melt in granite magmas, as well as compaction and shear of crystal-melt aggregates, concentrations of microgranitoid enclaves indented by megacrysts, and concentrations of crystals of the same mineral with different crystallisation histories. Evidence for crystal and enclave accumulation is shown clearly in mafic and silicic layered intrusions (MASLI-type granite plutons), for example, the Kameruka Granodiorite, Bega Batholith, south-eastern Australia.Crystal accumulations with interstitial liquid may become mobile in a magma chamber, owing to instabilities in the host magma caused by seismic and replenishment events or thermal and buoyancy variations. This remobilised material may intrude other parts of the chamber, as well as earlier-formed cumulates and even wall-rocks, as dykes, tubes, troughs and pipes. Marked concentrations of accessory and mafic minerals may also dev...
Microstructures of microgranitoid enclaves and the origin of S-type granitoids
Australian Journal of Earth Sciences
Arguments about the origin of microgranitoid enclaves (‘mafic inclusions’) in S-type granitoids p... more Arguments about the origin of microgranitoid enclaves (‘mafic inclusions’) in S-type granitoids partly revolve around the interpretation of microstructures. Microstructural evidence indicates that the enclaves are of igneous origin, rather than the metamorphic or partly melted metamorphic origin favoured by some proponents of the restite unmixing hypothesis for granitoid formation. Microgranitoid enclaves in S-type granitoids have no bearing on the restite unmixing hypothesis, because they are globules of hybrid magma incorporated into the magma chamber after the formation of the host magma. Microstructural and isotopic evidence indicates mixing between felsic S-type and mantle-derived enclave-forming magmas in the general magmatic system. However, the extent to which magma mixing contributes to the chemical composition of S-type granitoids remains controversial.
Relationships between microstructures and metamorphic assemblages
Tectonophysics
ABSTRACT
American Mineralogist
Flame perthite occurs locally in high-grade migmatitic gneisses of pelitic composition at Cooma, ... more Flame perthite occurs locally in high-grade migmatitic gneisses of pelitic composition at Cooma, SE Australia. It is developed best in leucosomes and rocks in which cordierite is largely unaltered. Such rocks are inferred to have been stronger than more-altered rocks, causing local stress concentrations sufficient to promote the nucleation of flames. The variability in the abundance of flames could be due to variability in stress concentrations in relation to orientation of the microperthite grains, if the Pryer-Robin model for flame perthite is applicable to these rocks. Possible sources of Na are (1) release of Na as cordierite and potassium feldspar were replaced by biotite-andalusite-quartz symplectite at upper amphibolite-facies conditions and (2) release of Na as microperthite was replaced by muscovite. Both these reactions appear to have occurred at amphibolite-facies conditions, because late fibrous sillimanite has replaced all minerals, including myrmekite, albite flames, and much of the muscovite; conceivably some muscovite that has not been replaced by sillimanite may have formed at greenschist-facies conditions. Alteration of plagioclase does not appear to have been a major source of Na for the flames, because plagioclase is absent from many of the Cooma metapelitic rock and, where present, is unaltered.
The structural and metamorphic geology of basement rocks in the McMurdo sound area, Antarctica
Journal of the Geological Society of Australia
... Many plagioclase grains are untwinned, simply twin-ned, or multiply twinned with only a few l... more ... Many plagioclase grains are untwinned, simply twin-ned, or multiply twinned with only a few lamellae, though a small proportion of grains has many thin, lenticular, lamellar twins indi-cative of a deformation origin (Vance, 1961; Vernon, 1965a). ...
Microgranitoid enclaves in granites—globules of hybrid magma quenched in a plutonic environment
Nature
Microgranitoid enclaves (autoliths, cognate xenoliths, mafic inclusions) are common in high-level... more Microgranitoid enclaves (autoliths, cognate xenoliths, mafic inclusions) are common in high-level granitoid plutons. They have relatively fine-grained igneous (microgranitoid) microstructures1, and many show evidence of having flowed in a magmatic condition2. These features counter interpretations that the enclaves represent transformed solid fragments of diverse wall rocks3-5 or restite6-11. An alternative interpretation is that the enclaves represent globules of mafic magma that have mingled (`commingled') and quenched in the granitoid host magma12-16. A detailed review of the literature, occurrence, morphology and composition of micro-granitoid enclaves strongly supports this view, hut indicates that the enclave magmas range in composition from mafic to felsic, and that magma-mixing may be involved in their formation17. I summarize here the main features of microgranitoid enclaves and briefly outline the possible processes leading to their incorporation as magma globules in the host granitoid.
Textures and Microstructures, 1983
Coarse-grained, deformed albite occurs in veins within a blueschist from the Cazadero region, Cal... more Coarse-grained, deformed albite occurs in veins within a blueschist from the Cazadero region, California. In some grains, deformation and recrystallization are concentrated in narrow shear zones less than 50 μm wide. We have examined the substructural progression across these zones by transmission electron microscopy (TEM), in an attempt to determine the details of the dynamic recrystallization mechanism. The misorientation across subgrain and recrystallized grain boundaries has been determined by analysis of electron diffraction patterns.Dynamic recrystallization apparently proceeded by the following stages: 1) the formation of a well-ordered substructure from a more tangled, cell-like array, 2) increasing misorientation between subgrains, 3) rapid growth of subgrains at a misorientation between 3° and 5° to produce new “grains” with straighter grain boundaries and lower internal dislocation densities and 4) continued deformation and rotation of the recrystallized grains with local...
Metamorphic Processes
ABSTRACT
Deformation and metamorphism of the Cooma Complex, southeastern Australia
ABSTRACT
Local, mid-crustal granulite facies metamorphism and melting: an example in the Mount Stafford area, central Australia
High-temperature Metamorphism and Crustal Anatexis, 1990
... area, to the immediate south-east of the Mount Stafford area, and in the south-east Reynolds ... more ... area, to the immediate south-east of the Mount Stafford area, and in the south-east Reynolds Range, to the south of the Mount Stafford area, have each been affected by two or three episodes of folding that have minor or no expression in the Mount Stafford rocks (Collins et al. ...
Tectonophysics, 1992
The Palaeozoic Lachlan Fold Belt (LFB) is a low-pressure metamorphic belt characterized by greens... more The Palaeozoic Lachlan Fold Belt (LFB) is a low-pressure metamorphic belt characterized by greenschist-facies rocks and infolded volcanic sequences, which indicate that the belt has not undergone substantial uplift at any stage. Porphyroblast/matrix relationships in one of the highest-grade metamorphic zones of the LFB, the Omeo Complex, indicate that the thermal peak was reached before, or was synchronous with, the earliest deformation, similar to that in high-grade metamorphic zones of the northern Arunta Inlier, central Australia. In both areas, the metamorphism is of low-P, high-T' type, centred on major granite intrusions as regional aureoles, and characterized by anticlockwise P-T-t-paths. However, in the outer (lower-grade) parts of the aureoles, peak metamo~hic conditions postdate the earliest deformation. This diachronous relationship indicates that (1) outward migration of a thermal front was centred on major granitoid intrusions (batholiths), (2) deformation propagated outward more rapidly than migration of the thermal front, and (3) plastic and ductile deformation began only after heating around the batholiths. Thus, orogeny was initiated after, and localized by, heat-focusing in the mid-crust associated with batholith emplacement. Therefore, the early deformation is a result of thermal softening: it is typicaily subho~zontai ductile shear at mid-crustal levels, but is characterized by upright to inclined folds at upper crustal levels. In the latter environment, the typically discordant "contact-aureole" plutons are often interpreted as post-tectonic granitoids, but they were part of the ongoing mid-crustal thermal perturbation that induced regional greenschist-facies metamorphism, which overprinted the early-formed, upright folds as it migrated to upper crustal levels. Orogeny migrated generaily eastward through the LFB. It began with the Early Silurian Benambran Orogeny, centred on the Wagga Metamorphic Belt (WMB), and terminated with the Carboniferous Kanimblan Orogeny, which was most intense in the Hill End Trough. Eastward migration of orogeny across central Victoria, part of the West LFB, began in the Ballarat-Bend&o Zone during the Early Devonian and terminated in the Melbourne Zone in the Middle Devonian. Orogeny was centred on the meridional hathoIiths of the LFB, each of which are considered to represent the transitory axis of an ancient magmatic arc. Stepwise, but generally eastward, arc migration caused the eastward migration of orogeny. A tectonic model, based on the modern southwest Pacific arc system, can be applied to the LFB. Following Cambrian intra-oceanic arc growth associated with west-dipping subduction, an Ordovician marginal sea developed, partly on stretched Cambrian crust, and was flooded by an extensive turbiditic wedge. Closure of the eastern part, the East LFB, began in the Early Silurian after a magmatic arc deveioped over the WMB, possibly associated with an east-dipping subduction zone, After re-establishment of west-dipping subduction in the Middle Silurian, absolute-motion retreat of the upper (Australian) plate, caused by oblique plate convergence, resulted in dextral transtension and the generation of Siluro-Devonian inter-arc and back-arc basins. Transient compression resulted in deformation along the magmatic arc and induced formation of a new outboard (eastward) arc system. Periodic compre~~n within the evolving arc and back-arc system resulted in a series of east-younging, east-verging, linear, Siluro-Devonian fold-and-thrust belts, which were localized adjacent to elongate batholiths that represent the relict arc systems. Local westward jumps in deformation and plutonism occurred. The most significant is the _ 600 km westward jump into the West LFB, caused by closure of the remainder of the passive Ordovician back-arc basin (Melbourne and Bendigo-Bal-Corre~~~e~~e for W.
Tectonophysics, 1994
The Adelaide, Lachlan and New England fold belts of eastern Australia record the continental grow... more The Adelaide, Lachlan and New England fold belts of eastern Australia record the continental growth of eastern Gondwanaland during the Palaeozoic. The New England Fold Belt (NEFB) represents a tectonic collage formed by subduction/accretion during the Late Paleozoic, but contains remnants of subduction-related rocks that date from the Cambrian. The Lachlan Fold Belt (LFB) developed inboard from the NEFB, initially as an amalgam of Proterozoic continental and Cambrian oceanic fragments that formed by rift and drift at the leading edge of eastern Gondwanaland. Convergent tectonism at N 500 Ma welded the fragments to the craton and formed the Adelaide fold belt. Oceanward dispersal of detritus across the LFB produced an overlap assemblage of quartzose Ordovician turbidites, and a new subduction zone developed at the eastern margin. The tectonic setting was similar to the modern Philippines plate of the western Pacific. Behind-the-arc, Silurian-Devonian convergent tectonics converted the 1700-2000~km-wide continental margin of the Proto-LFB into a-750~km-wide, thinskinned , fold-magmatic belt within 60 Ma. The driving force was delamination, which produced a ubiquitous basaltic underplate that generated: (1) regional low-P metamorphism; (2) local anticlockwise P-T-t paths and crustal-scale isobaric cooling; (3) voluminous syn-to late-tectonic granitoids, emplaced at rates approximately twice that of modern arcs; and (4) normal thickness crust, despite an average of N 60% shortening. Delamination occurred in two separate areas originally-1000 km apart, first in the eastern, then in the western part of the LFB. The LFB and its northern counterpart, the Thomson fold belt, represent approximately one-fifth of the Australian continent, and evolved from dispersed fragments into stable continental crust in N 300 Ma. Such rapid growth rates are typical of the 2.7-2.5 Ga and 1.9-1.7 Ga periods, when voluminous, widespread granitoids and large turbidite-dominated, low-P metamorphic belts were produced in settings that are not obviously subduction-related. Therefore, a similar process of rift-drift-delamination (RIDDEL tectonics) may have periodically operated throughout Earth history.
Inverted Regional Metamorphism in the Coaxially Refolded Tonga Formation: Evidence for Cretaceous Accretional Tectonics in the Cascades Crystalline Core
The Tonga Formation, on the westernmost boundary of the Cascades crystalline core, records Cretac... more The Tonga Formation, on the westernmost boundary of the Cascades crystalline core, records Cretaceous plutonism, contact to regional metamorphism, and multiple episodes of folding related to intense east-west contractional deformation. The Tonga Formation is exposed in a fault-bounded, north-south elongate tectonic domain that comprises pelite-psammite metasediments, which increase from greenschist to amphibolite grade (south to north). This metamorphic gradient is inverted relative to a major westward verging and downward facing fold system that dominates the internal architecture of the formation. Sedimentary structures are remarkably well-preserved in the Tonga Formation, which allowed for the determination of younging directions. Using these and bedding-cleavage relationships, detailed field mapping indicates a stratigraphically overturned section that forms a large-scale antiformal syncline (exposed in the northern and eastern domain) and related synformal anticline (southern a...
Co-axial refolding and inverted regional metamorphism in the Tonga formation: Cretaceous accretionary thrust tectonics in the Cascades crystalline core
Trabajos de Geología
... Now at: Shell International Exploration & Production, Inc., e-mail: Luke.Jensen@shell ...... more ... Now at: Shell International Exploration & Production, Inc., e-mail: Luke.Jensen@shell ... a consequence of tectonic, thrust-related thickening, followed by rapid exhumation of the exposed crustal section of 10 to 40 km paleodepth (eg Matzel, 2004; Patterson et al., 2004; Stowell et al ...
Guidebook - Arunta Block, central Australia, IGCP Project 235 - Metamorphism and Geodynamics. Field Conference on Granulite Facies Metamorpism, June 25-July 1, 1989
ABSTRACT Bas Hensen's mob
Beneath Our Feet
Chemical & Engineering News, 1956
ABSTRACT
Fabric Development in a Late-Hercynian Magmatic Strike-Slip Shear Zone in Southern Corsica: Indications of Melt-Supported Large-Scale Deformation Localization
The calc-alcaline granitoids of the Hercynian Corsica Batholith show a large-scale magmatic flow ... more The calc-alcaline granitoids of the Hercynian Corsica Batholith show a large-scale magmatic flow pattern, outlined by the alignment of large (mm-cm) euhedral feldspar crystals. The trend of the steep magmatic foliation is generally N-S in the northern part of the island, swings to approximately E-W orientation in the central part of the Batholith and back again to approximately N-S orientation
Microstructures of metamorphic rocks
A Practical Guide to Rock Microstructure, 2004
Introduction Once you have seen what the microstructures of sedimentary and igneous rocks look li... more Introduction Once you have seen what the microstructures of sedimentary and igneous rocks look like, you are in a good position to appreciate what happens to them when they are heated (Chapter 4) and deformed (Chapter 5) in Earth's crust. These processes can greatly alter the microstructure, producing no less beautiful, but very different grain and crystal shapes. Moreover, during heating, new minerals are produced. This chapter is concerned with microstructures formed in the solid state, mainly as a result of metamorphism, but also discusses exsolution, which, though a solid-state process and so appropriate to this chapter in that sense, also occurs during the slow cooling of minerals in igneous rocks The microstructure of a metamorphic rock is the end-product of a complex history that may involve sequences of complicated chemical reactions, and commonly also repeated deformation events. Although microscopic evidence is used to infer the metamorphic or deformation history, we should be careful not to try to extract too much information from it, and should remain aware of complexity and possible alternative interpretations. Evidence for metamorphism How do we know that rocks remain solid during metamorphism in Earth's crust? The evidence consists of residual sedimentary and igneous structures (Section 4.15), such as phenocrysts and amygdales (filled gas bubbles; Section 3.14.2) in metavolcanic rocks, fragmental grain shapes in metasandstones, distorted pebbles in metaconglomerates, and, less commonly, fossils replaced by metamorphic minerals.
A Practical Guide to Rock Microstructure
An understanding of the physical relationships of minerals and rocks is essential for making the ... more An understanding of the physical relationships of minerals and rocks is essential for making the most of detailed chemical and isotopic mineral analyses. Ron Vernon discusses the basic processes behind various rock microstructures using high-quality colour illustrations ...
K-feldspar augen in felsic gneisses and mylonites—deformed phenocrysts or porphyroblasts?
Geologiska Föreningen i Stockholm Förhandlingar
Abstract A review of mesostructural and microstructural evidence favours the hypothesis that K-fe... more Abstract A review of mesostructural and microstructural evidence favours the hypothesis that K-feldspar augen and megacrysts in felsic gneisses and mylonites are generally, if not always, residual phenocrysts, rather than porphyroblasts that grew either during or after the deformation. In most examples, the augen appear to represent variably deformed megacrysts in former granitoids, as they commonly have similar distributions and shapes of inclusions and many of them show simple twinning; megacrysts in granitoids show abundant evidence of a phenocryst origin. Some augen may represent phenocrysts that grew in migmatite melt leucosomes and later were deformed, and others may have been phenocrysts in felsic volcanic or pyroclastic rocks, although megacrysts are uncommon in these rock-types. A porphyroblastic origin is opposed by the typical zonal distribution of inclusions and the common evidence of plastic deformation and partial recrystallization of the augen. The use of K-feldspar megacrysts as indicators...
Transactions of the Royal Society of Edinburgh: Earth Sciences
ABSTRACTSeveral mesosocopic structures are consistent with mechanical accumulation of crystals an... more ABSTRACTSeveral mesosocopic structures are consistent with mechanical accumulation of crystals and movement of melt in granite magmas, as well as compaction and shear of crystal-melt aggregates, concentrations of microgranitoid enclaves indented by megacrysts, and concentrations of crystals of the same mineral with different crystallisation histories. Evidence for crystal and enclave accumulation is shown clearly in mafic and silicic layered intrusions (MASLI-type granite plutons), for example, the Kameruka Granodiorite, Bega Batholith, south-eastern Australia.Crystal accumulations with interstitial liquid may become mobile in a magma chamber, owing to instabilities in the host magma caused by seismic and replenishment events or thermal and buoyancy variations. This remobilised material may intrude other parts of the chamber, as well as earlier-formed cumulates and even wall-rocks, as dykes, tubes, troughs and pipes. Marked concentrations of accessory and mafic minerals may also dev...
Microstructures of microgranitoid enclaves and the origin of S-type granitoids
Australian Journal of Earth Sciences
Arguments about the origin of microgranitoid enclaves (‘mafic inclusions’) in S-type granitoids p... more Arguments about the origin of microgranitoid enclaves (‘mafic inclusions’) in S-type granitoids partly revolve around the interpretation of microstructures. Microstructural evidence indicates that the enclaves are of igneous origin, rather than the metamorphic or partly melted metamorphic origin favoured by some proponents of the restite unmixing hypothesis for granitoid formation. Microgranitoid enclaves in S-type granitoids have no bearing on the restite unmixing hypothesis, because they are globules of hybrid magma incorporated into the magma chamber after the formation of the host magma. Microstructural and isotopic evidence indicates mixing between felsic S-type and mantle-derived enclave-forming magmas in the general magmatic system. However, the extent to which magma mixing contributes to the chemical composition of S-type granitoids remains controversial.
Relationships between microstructures and metamorphic assemblages
Tectonophysics
ABSTRACT
American Mineralogist
Flame perthite occurs locally in high-grade migmatitic gneisses of pelitic composition at Cooma, ... more Flame perthite occurs locally in high-grade migmatitic gneisses of pelitic composition at Cooma, SE Australia. It is developed best in leucosomes and rocks in which cordierite is largely unaltered. Such rocks are inferred to have been stronger than more-altered rocks, causing local stress concentrations sufficient to promote the nucleation of flames. The variability in the abundance of flames could be due to variability in stress concentrations in relation to orientation of the microperthite grains, if the Pryer-Robin model for flame perthite is applicable to these rocks. Possible sources of Na are (1) release of Na as cordierite and potassium feldspar were replaced by biotite-andalusite-quartz symplectite at upper amphibolite-facies conditions and (2) release of Na as microperthite was replaced by muscovite. Both these reactions appear to have occurred at amphibolite-facies conditions, because late fibrous sillimanite has replaced all minerals, including myrmekite, albite flames, and much of the muscovite; conceivably some muscovite that has not been replaced by sillimanite may have formed at greenschist-facies conditions. Alteration of plagioclase does not appear to have been a major source of Na for the flames, because plagioclase is absent from many of the Cooma metapelitic rock and, where present, is unaltered.
The structural and metamorphic geology of basement rocks in the McMurdo sound area, Antarctica
Journal of the Geological Society of Australia
... Many plagioclase grains are untwinned, simply twin-ned, or multiply twinned with only a few l... more ... Many plagioclase grains are untwinned, simply twin-ned, or multiply twinned with only a few lamellae, though a small proportion of grains has many thin, lenticular, lamellar twins indi-cative of a deformation origin (Vance, 1961; Vernon, 1965a). ...
Microgranitoid enclaves in granites—globules of hybrid magma quenched in a plutonic environment
Nature
Microgranitoid enclaves (autoliths, cognate xenoliths, mafic inclusions) are common in high-level... more Microgranitoid enclaves (autoliths, cognate xenoliths, mafic inclusions) are common in high-level granitoid plutons. They have relatively fine-grained igneous (microgranitoid) microstructures1, and many show evidence of having flowed in a magmatic condition2. These features counter interpretations that the enclaves represent transformed solid fragments of diverse wall rocks3-5 or restite6-11. An alternative interpretation is that the enclaves represent globules of mafic magma that have mingled (`commingled') and quenched in the granitoid host magma12-16. A detailed review of the literature, occurrence, morphology and composition of micro-granitoid enclaves strongly supports this view, hut indicates that the enclave magmas range in composition from mafic to felsic, and that magma-mixing may be involved in their formation17. I summarize here the main features of microgranitoid enclaves and briefly outline the possible processes leading to their incorporation as magma globules in the host granitoid.
Textures and Microstructures, 1983
Coarse-grained, deformed albite occurs in veins within a blueschist from the Cazadero region, Cal... more Coarse-grained, deformed albite occurs in veins within a blueschist from the Cazadero region, California. In some grains, deformation and recrystallization are concentrated in narrow shear zones less than 50 μm wide. We have examined the substructural progression across these zones by transmission electron microscopy (TEM), in an attempt to determine the details of the dynamic recrystallization mechanism. The misorientation across subgrain and recrystallized grain boundaries has been determined by analysis of electron diffraction patterns.Dynamic recrystallization apparently proceeded by the following stages: 1) the formation of a well-ordered substructure from a more tangled, cell-like array, 2) increasing misorientation between subgrains, 3) rapid growth of subgrains at a misorientation between 3° and 5° to produce new “grains” with straighter grain boundaries and lower internal dislocation densities and 4) continued deformation and rotation of the recrystallized grains with local...
Metamorphic Processes
ABSTRACT
Deformation and metamorphism of the Cooma Complex, southeastern Australia
ABSTRACT
Local, mid-crustal granulite facies metamorphism and melting: an example in the Mount Stafford area, central Australia
High-temperature Metamorphism and Crustal Anatexis, 1990
... area, to the immediate south-east of the Mount Stafford area, and in the south-east Reynolds ... more ... area, to the immediate south-east of the Mount Stafford area, and in the south-east Reynolds Range, to the south of the Mount Stafford area, have each been affected by two or three episodes of folding that have minor or no expression in the Mount Stafford rocks (Collins et al. ...
Tectonophysics, 1992
The Palaeozoic Lachlan Fold Belt (LFB) is a low-pressure metamorphic belt characterized by greens... more The Palaeozoic Lachlan Fold Belt (LFB) is a low-pressure metamorphic belt characterized by greenschist-facies rocks and infolded volcanic sequences, which indicate that the belt has not undergone substantial uplift at any stage. Porphyroblast/matrix relationships in one of the highest-grade metamorphic zones of the LFB, the Omeo Complex, indicate that the thermal peak was reached before, or was synchronous with, the earliest deformation, similar to that in high-grade metamorphic zones of the northern Arunta Inlier, central Australia. In both areas, the metamorphism is of low-P, high-T' type, centred on major granite intrusions as regional aureoles, and characterized by anticlockwise P-T-t-paths. However, in the outer (lower-grade) parts of the aureoles, peak metamo~hic conditions postdate the earliest deformation. This diachronous relationship indicates that (1) outward migration of a thermal front was centred on major granitoid intrusions (batholiths), (2) deformation propagated outward more rapidly than migration of the thermal front, and (3) plastic and ductile deformation began only after heating around the batholiths. Thus, orogeny was initiated after, and localized by, heat-focusing in the mid-crust associated with batholith emplacement. Therefore, the early deformation is a result of thermal softening: it is typicaily subho~zontai ductile shear at mid-crustal levels, but is characterized by upright to inclined folds at upper crustal levels. In the latter environment, the typically discordant "contact-aureole" plutons are often interpreted as post-tectonic granitoids, but they were part of the ongoing mid-crustal thermal perturbation that induced regional greenschist-facies metamorphism, which overprinted the early-formed, upright folds as it migrated to upper crustal levels. Orogeny migrated generaily eastward through the LFB. It began with the Early Silurian Benambran Orogeny, centred on the Wagga Metamorphic Belt (WMB), and terminated with the Carboniferous Kanimblan Orogeny, which was most intense in the Hill End Trough. Eastward migration of orogeny across central Victoria, part of the West LFB, began in the Ballarat-Bend&o Zone during the Early Devonian and terminated in the Melbourne Zone in the Middle Devonian. Orogeny was centred on the meridional hathoIiths of the LFB, each of which are considered to represent the transitory axis of an ancient magmatic arc. Stepwise, but generally eastward, arc migration caused the eastward migration of orogeny. A tectonic model, based on the modern southwest Pacific arc system, can be applied to the LFB. Following Cambrian intra-oceanic arc growth associated with west-dipping subduction, an Ordovician marginal sea developed, partly on stretched Cambrian crust, and was flooded by an extensive turbiditic wedge. Closure of the eastern part, the East LFB, began in the Early Silurian after a magmatic arc deveioped over the WMB, possibly associated with an east-dipping subduction zone, After re-establishment of west-dipping subduction in the Middle Silurian, absolute-motion retreat of the upper (Australian) plate, caused by oblique plate convergence, resulted in dextral transtension and the generation of Siluro-Devonian inter-arc and back-arc basins. Transient compression resulted in deformation along the magmatic arc and induced formation of a new outboard (eastward) arc system. Periodic compre~~n within the evolving arc and back-arc system resulted in a series of east-younging, east-verging, linear, Siluro-Devonian fold-and-thrust belts, which were localized adjacent to elongate batholiths that represent the relict arc systems. Local westward jumps in deformation and plutonism occurred. The most significant is the _ 600 km westward jump into the West LFB, caused by closure of the remainder of the passive Ordovician back-arc basin (Melbourne and Bendigo-Bal-Corre~~~e~~e for W.
Tectonophysics, 1994
The Adelaide, Lachlan and New England fold belts of eastern Australia record the continental grow... more The Adelaide, Lachlan and New England fold belts of eastern Australia record the continental growth of eastern Gondwanaland during the Palaeozoic. The New England Fold Belt (NEFB) represents a tectonic collage formed by subduction/accretion during the Late Paleozoic, but contains remnants of subduction-related rocks that date from the Cambrian. The Lachlan Fold Belt (LFB) developed inboard from the NEFB, initially as an amalgam of Proterozoic continental and Cambrian oceanic fragments that formed by rift and drift at the leading edge of eastern Gondwanaland. Convergent tectonism at N 500 Ma welded the fragments to the craton and formed the Adelaide fold belt. Oceanward dispersal of detritus across the LFB produced an overlap assemblage of quartzose Ordovician turbidites, and a new subduction zone developed at the eastern margin. The tectonic setting was similar to the modern Philippines plate of the western Pacific. Behind-the-arc, Silurian-Devonian convergent tectonics converted the 1700-2000~km-wide continental margin of the Proto-LFB into a-750~km-wide, thinskinned , fold-magmatic belt within 60 Ma. The driving force was delamination, which produced a ubiquitous basaltic underplate that generated: (1) regional low-P metamorphism; (2) local anticlockwise P-T-t paths and crustal-scale isobaric cooling; (3) voluminous syn-to late-tectonic granitoids, emplaced at rates approximately twice that of modern arcs; and (4) normal thickness crust, despite an average of N 60% shortening. Delamination occurred in two separate areas originally-1000 km apart, first in the eastern, then in the western part of the LFB. The LFB and its northern counterpart, the Thomson fold belt, represent approximately one-fifth of the Australian continent, and evolved from dispersed fragments into stable continental crust in N 300 Ma. Such rapid growth rates are typical of the 2.7-2.5 Ga and 1.9-1.7 Ga periods, when voluminous, widespread granitoids and large turbidite-dominated, low-P metamorphic belts were produced in settings that are not obviously subduction-related. Therefore, a similar process of rift-drift-delamination (RIDDEL tectonics) may have periodically operated throughout Earth history.
Inverted Regional Metamorphism in the Coaxially Refolded Tonga Formation: Evidence for Cretaceous Accretional Tectonics in the Cascades Crystalline Core
The Tonga Formation, on the westernmost boundary of the Cascades crystalline core, records Cretac... more The Tonga Formation, on the westernmost boundary of the Cascades crystalline core, records Cretaceous plutonism, contact to regional metamorphism, and multiple episodes of folding related to intense east-west contractional deformation. The Tonga Formation is exposed in a fault-bounded, north-south elongate tectonic domain that comprises pelite-psammite metasediments, which increase from greenschist to amphibolite grade (south to north). This metamorphic gradient is inverted relative to a major westward verging and downward facing fold system that dominates the internal architecture of the formation. Sedimentary structures are remarkably well-preserved in the Tonga Formation, which allowed for the determination of younging directions. Using these and bedding-cleavage relationships, detailed field mapping indicates a stratigraphically overturned section that forms a large-scale antiformal syncline (exposed in the northern and eastern domain) and related synformal anticline (southern a...
Co-axial refolding and inverted regional metamorphism in the Tonga formation: Cretaceous accretionary thrust tectonics in the Cascades crystalline core
Trabajos de Geología
... Now at: Shell International Exploration & Production, Inc., e-mail: Luke.Jensen@shell ...... more ... Now at: Shell International Exploration & Production, Inc., e-mail: Luke.Jensen@shell ... a consequence of tectonic, thrust-related thickening, followed by rapid exhumation of the exposed crustal section of 10 to 40 km paleodepth (eg Matzel, 2004; Patterson et al., 2004; Stowell et al ...
Guidebook - Arunta Block, central Australia, IGCP Project 235 - Metamorphism and Geodynamics. Field Conference on Granulite Facies Metamorpism, June 25-July 1, 1989
ABSTRACT Bas Hensen's mob
Beneath Our Feet
Chemical & Engineering News, 1956
ABSTRACT
Fabric Development in a Late-Hercynian Magmatic Strike-Slip Shear Zone in Southern Corsica: Indications of Melt-Supported Large-Scale Deformation Localization
The calc-alcaline granitoids of the Hercynian Corsica Batholith show a large-scale magmatic flow ... more The calc-alcaline granitoids of the Hercynian Corsica Batholith show a large-scale magmatic flow pattern, outlined by the alignment of large (mm-cm) euhedral feldspar crystals. The trend of the steep magmatic foliation is generally N-S in the northern part of the island, swings to approximately E-W orientation in the central part of the Batholith and back again to approximately N-S orientation
Microstructures of metamorphic rocks
A Practical Guide to Rock Microstructure, 2004
Introduction Once you have seen what the microstructures of sedimentary and igneous rocks look li... more Introduction Once you have seen what the microstructures of sedimentary and igneous rocks look like, you are in a good position to appreciate what happens to them when they are heated (Chapter 4) and deformed (Chapter 5) in Earth's crust. These processes can greatly alter the microstructure, producing no less beautiful, but very different grain and crystal shapes. Moreover, during heating, new minerals are produced. This chapter is concerned with microstructures formed in the solid state, mainly as a result of metamorphism, but also discusses exsolution, which, though a solid-state process and so appropriate to this chapter in that sense, also occurs during the slow cooling of minerals in igneous rocks The microstructure of a metamorphic rock is the end-product of a complex history that may involve sequences of complicated chemical reactions, and commonly also repeated deformation events. Although microscopic evidence is used to infer the metamorphic or deformation history, we should be careful not to try to extract too much information from it, and should remain aware of complexity and possible alternative interpretations. Evidence for metamorphism How do we know that rocks remain solid during metamorphism in Earth's crust? The evidence consists of residual sedimentary and igneous structures (Section 4.15), such as phenocrysts and amygdales (filled gas bubbles; Section 3.14.2) in metavolcanic rocks, fragmental grain shapes in metasandstones, distorted pebbles in metaconglomerates, and, less commonly, fossils replaced by metamorphic minerals.
A Practical Guide to Rock Microstructure
An understanding of the physical relationships of minerals and rocks is essential for making the ... more An understanding of the physical relationships of minerals and rocks is essential for making the most of detailed chemical and isotopic mineral analyses. Ron Vernon discusses the basic processes behind various rock microstructures using high-quality colour illustrations ...