Ron Vernon - Academia.edu (original) (raw)

Papers by Ron Vernon

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...

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.

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). ...

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...

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. ...

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...

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 ...

ABSTRACT Bas Hensen's mob

Chemical & Engineering News, 1956

ABSTRACT

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

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.

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 ...