R. H. Vernon

A review of criteria for the identification of magmatic and tectonic foliations in granitoids

Abstract Foliations in granitoids can form by magmatic flow, ‘submagmatic flow’, high-temperature solid-state deformation and moderate- to low-temperature solid-state deformation. A review of previous work suggests that no single criterion can consistently distinguish foliations in granitoids formed by flow during ascent, diapiric emplacement and expansion, emplacement during regional deformation, or regional deformation post-dating emplacement. However, a magmatic origin is favoured for foliations defined by the alignment of igneous, commonly euhedral minerals, particularly where the foliation is parallel to internal or external pluton contacts. Foliations formed during expansion or ‘ballooning’ of diapirs may be strictly magmatic in origin, although some studies suggest that solid-state deformation also may occur. If so, we would hope to find evidence of deformation of crystal-melt systems, and that the solid-state deformation occurred at high temperatures. The inference of syntectonic foliations is most convincing where magmatic and high-temperature solid-state foliations are subparallel, these foliations are continuous with regionally developed foliations in the wall rocks, synkinematic porphyroblasts are present in the wallrocks, and igneous minerals have the same age as metamorphic minerals associated with the regional cleavage. A strictly tectonic origin for foliations in granitoids is favoured when the foliation is defined by metamorphic minerals, no alignment of igneous minerals occurs, the foliation is locally at high angles to pluton-wallrock contacts, and the foliation is continuous with a regionally developed cleavage.

Shape and microstructure of microgranitoid enclaves: Indicators of magma mingling and flow

Abstract At Tarandore Point, 328 km south of Sydney, New South Wales, Australia, two tonalite phases are in contact with a gabbroic diorite, all belonging to the Moruya Batholith. Lobate to crenulate contacts, pillowing of the more mafic phase in the more felsic phase, net-veining of the more mafic phase by the more felsic, local chilling of the more mafic phase against the more felsic, and an abundance of rounded to elongate microgranitoid enclaves all suggest mingling of magmas at the site of emplacement. Therefore, all phases were emplaced before the others solidified, and an order of intrusion cannot be determined. The variety of microgranitoid enclaves suggests that repeated magma mingling has occurred. Linear geochemical trends indicate either magma mixing or restite unmixing as a prime cause of variation in the members of the Moruya suite of granitoids. Direct evidence of mixing is not present, but evidence that the magmas were substantially liquid at the time of emplacement sugests that mixing is possible. Restite unmixing is unlikely to be the prime process, because near-minimum melts, which would occur in all granitoid phases according to that model, should mix easily, rather than mingling. In fact, mingling has occurred between tonalite phases of very similar composition, which would not be expected if the liquid parts of enclaves were minimum melts. Available experimental and theoretical data on mixing in both magma chambers and conduits cannot be applied directly to the occurrence at Tarandore Point, owing to inadequately constrained parameters, but mixing generally should be favoured by higher temperatures, which is consistent with our inference that relatively high proportions of melt were present in the granitoid phases at the time of emplacement. Conduit mixing may be reflected in a local zone of strong to extreme elongation of enclaves, in which the distortion has been accomplished by magmatic flow without solid-state deformation, as evidenced by strong dimensional preferred orientations, coupled with a lack of microstructural evidence of intragranular plastic strain. This suggests that elongate enclaves should not be used as indicators of solid-state deformation or as tectonic strain markers without microstructural evidence that suitably strong intracrystalline plastic deformation has taken place.

Bursting the bubble of ballooning plutons: A return to nested diapirs emplaced by multiple processes

A popular model for the emplacement of roughly spherical plutons is that of “ballooning” or in situ inflation of a magma chamber. In a common version of this model magma ascends until loss of heat or buoyancy causes the outermost magma to crystallize and cease ascent, while the hotter “tail” of magma continues to rise and expand the already crystallized outer margin. This expansion forms a concentric, gneissic to mylonitic foliation and flattening-type strain in the outer margin of the pluton by means of subsolidus deformation and pushes aside the surrounding country rock to form a dynamothermal aureole that postdates regional structures. Our reexamination of three supposedly ballooned plutons (Ardara, Ireland; Cannibal Creek, Australia; Papoose Flat, California) and evaluation of published descriptions of many others indicate that this model is largely incorrect. Deflections of country-rock structures, strains, and porphyroblast-matrix relationships indicate that only minor to moderate expansion (usually 30% or less) occurred during emplacement, that other emplacement mechanisms must have occurred, and that regional deformation continued during and after emplacement. Internal structures indicate that when magma chamber expansion did occur, it did so by flow of magma, that magmatic foliations and lineations formed late in the magma chamber evolution, that enclave shapes are neither good strain markers nor indicators of the magnitude of expansion, and that only minor internal subsolidus deformation results from emplacement. This study indicates that many plutons previously interpreted as post-tectonic ballooning plutons are better viewed as syntectonic, nested diapirs emplaced by a variety of country-rock material-transfer processes. This nested diapir model implies that magma ascent may occur by rise of large magma batches (instead of transport in dikes followed by ballooning), that magma chamber dynamics differ from that in the ballooning model, and that normally zoned plutons may form by intrusion of several pulses of magma rather than by in situ crystal fractionation from a single parent melt.

K-feldspar megacrysts in granites — Phenocrysts, not porphyroblasts

Abstract K-feldspar megacrysts in granitoid plutons have been interpreted as either phenocrysts or porphyroblasts. Most of the microstructural, mineralogical and chemical evidence (e.g., shape, alignment, concentration, Ba content, zoning, inclusions, and twinning) favours a phenocryst origin. The main features that have been used to support a porphyroblast origin are occurrence of megacrysts: (1) across aplite vein boundaries, (2) in country rocks, and (3) in or across boundaries of microgranitoid enclaves (mafic inclusions). However, these features can be explained by the phenocryst hypothesis. In particular, megacrysts in microgranitoid enclaves can be explained by growth or mixing in magma before a globule of that magma or a fragment of the resulting igneous rock was incorporated as an enclave. All available evidence favours or is consistent with a phenocryst origin for K-feldspar megacrysts in granitoid rocks and their enclaves. The large size of the megacrysts is evidently due to nucleation difficulties for K-feldspar in granitic melts. Though K-feldspar is commonly the last mineral to begin crystallizing in granitic magmas, abundant melt is still present at that stage, allowing sufficient space for the megacrysts to grow. The reason for the common lack of megacrysts in volcanic rocks may be that the phenocrysts do not grow large enough to be called “megacrysts” until the magma contains such a high proportion of crystals that it cannot erupt.

Igneous microstructures in migmatites

Clear differences between the microstructures of leucosomes and mesosomes occur in some nebulitic migmatites in the Proterozoic Arunta block, central Australia. The leucosomes show crystal faces of K-feldspar (microcline-microperthite), cordierite, andalusite, and plagio clase against quartz, indicating crystallization of a melt. In contrast, the mesosomes show predominantly polygonal grain shapes, modified by (001) crystal faces of biotite and rare crystal faces in some porphyroblasts of garnet, cordierite, and orthopyroxene, indicating re-crystallization in the solid state. The occurrence of abundant crystal faces in minerals such as feldspar, cordierite, and andalusite (which typically grow crystal faces in magmas, but generally not in metamorphic rocks) is evidence of former melt. Therefore, igneous microstructures can be preserved in leucosomes that have not undergone penetrative subsolidus deformation and recrystallization. The cores of many of the crystals of cordierite and K-feldspar have small, locally oriented inclusions similar to those in the same minerals in the mesosomes. These indicate that the cores remained solid during crystallization of the leucosome, which produced inclusion-free, euhedral rims. These crystals provide criteria for recognizing possible restite in granitoid magmas.

Large volumes of anhydrous pseudotachylyte in the Woodroffe Thrust, eastern Musgrave Ranges, Australia

A mylonitic thrust zone, at least 1.5 km thick, forms a sharp contact between granulite and amphibolite facies gneisses in the eastern Musgrave Ranges, central Australia. The thrust dips gently to the south and is interpreted as an extension of the Woodroffe Thrust, which was formed about 550 Ma ago. Mylonites at the base of the thrust grade upwards into ultramylonites, which pass abruptly into a pseudotachylyte-bearing zone approximately 1 km thick, containing approximately 4% of pseudotachylyte veining. The orientation of the veins appears to be random. Pseudotachylytes occur only in the granulite facies rocks, and their precursors are felsic pyroxene and/or garnet granofelses. Rotated blocks of ultramylonite are present in some of the pseudotachylytes, and some pseudotachylyte veins have been plastically deformed, suggesting nearly contemporaneous semiductile and brittle behaviour. The matrix of the pseudotachylyte shows spectacular examples of igneous quench microstructures, especially skeletal and dendritic crystals of plagioclase and feathery pyroxene dendrites. Also present are glass devitrification microstructures (spherulites), evidence of liquid flow, and partly melted residual grains with former glassy rims showing different optical properties from those of the surrounding isotropic material. These features confirm that the pseudotachylyte formed by melting in anhydrous conditions. The matrix of the pseudotachylyte veins is less siliceous than the host rocks, owing to non-equilibrium melting of pyroxene, garnet and plagioclase. The igneous assemblages of the melt, notably the crystallization of pigeonite, are consistent with rapid cooling from very high-temperature (>1000°C). Melting and quenching is probably due to very local, short-lived rises in temperature accompanied by dilation.

Grain-size reduction and foliation development in a deformed granitoid batholith

Abstract The predominantly peraluminous Wologorong Batholith in eastern New South Wales, Australia, has undergone heterogeneous deformation. It always has at least one gneissic foliation, but in strongly deformed zones it has two foliations. One of these is a gneissic foliation ( S 1 ) consisting of relatively discontinuous, fine-grained, micaceous and feldspathic folia anastomosing between feldspar relics and elongate aggregates of recrystallized quartz. The other foliation ( S 2 ) is more continuous than S 1 , and the folia are fine-grained and commonly laminated, as in mylonites. The two foliations could have formed siimultaneously or sequentially during a single deformation. S 2 formed oblique (up to 30°) to S 1 , in order to accommodate an imposed shear component of the strain in strongly deformed parts of the batholith. S 2 folia appear to have initiated on suitably oriented parts of anastomosing S 1 folia. Continued deformation locally reduced the angle between S 1 and S 2 and resulted in the development of mylonite zones. Grain-size reduction by recrystallization and/or neocrystallization occurred in all minerals during the deformation, but quartz appears to have attained a steady-state grain-size of around 0.15 mm, which was not reduced during more intense deformation. Deformation mechanisms are difficult to determine, but microfracturing associated with offsetting and microboudinage, intragranular slip, and solution-transfer of material may have operated in the earlier stages of deformation, and grain-boundary sliding and/or solution-transfer of material may have dominated the later stages. The material transfer may have been associated with chemical and mineralogical changes that accompanied neocrystallization.

Orogeny associated with anticlockwise P-T-t paths: Evidence from low-P, high-T metamorphic terranes in the Arunta inlier, central Australia

Low-pressure, high-temperature metamorphic terranes from the Proterozoic northern Arunta inlier, central Australia, are characterized by anticlockwise pressure-temperature-time paths. The thermal peak was reached before or during the earliest folding, and it was associated with abundant, repeated intrusion of granite sheets. We generalize our observations in the area to infer that low- P , high- T metamorphism is induced principally by focusing of mid-crustal heat from rapid, repeated granite sheet intrusion, which generates large metamorphic aureoles. Intrusion and associated metamorphism produce melting reactions, which increase the fluid pressure and thermally soften the mid-crust, resulting in accelerated, melt-enhanced deformation. Shortening is accommodated generally by thrusting in the mid-crust and by upright folding in the upper crust. Compression occurs during cooling of the terrane, as indicated by near-isobaric cooling paths and the preservation of upper-crust sections. The result is the production of discrete low- P , high- T terranes that are localized around abundant granitoids, show thin-skinned deformation, and have anticlockwise P - T -time paths. Continued granitoid intrusion produces posttectonic, discordant plutons at high crustal levels, within low- P metamorphic belts.

Palaeozoic arc growth, deformation and migration across the Lachlan Fold Belt, southeastern Australia

Abstract 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 anti-clockwise P-T-t-paths. However, in the outer (lower-grade) parts of the aureoles, peak metamorphic conditions post-date 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 typically subhorizontal 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 generally 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-Bendigo Zone during the Early Devonian and terminated in the Melbourne Zone in the Middle Devonian. Orogeny was centred on the meridional batholiths 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 developed 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 compression 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

Questions about myrmekite in deformed rocks

600 km westward jump into the West LFB, caused by closure of the remainder of the passive Ordovician back-arc basin (Melbourne and Bendigo-Ballarat zones). West-dipping subduction beneath the basin produced a low-pressure fold-thrust belt similar to the East LFB, though the granitoids have a more primitive isotopic character, indicating closure of thin, dominantly oceanic crust in the Early to Middle Devonian. Localized deformation, induced by thermal softening of the crust within magmatic arcs that have developed on earlier passive margins, is likely to produce anticlockwise P-7-t-paths and may be a typical feature of southwest Pacific-style tectonism.