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The response of garnet and zircon to prograde amphibolite-facies metamorphism in late Proterozoic mica schists from the Scottish Highlands has been investigated. Spatial analysis of zircon populations using scanning electron microscopy was undertaken in Dalradian Schists that have undergone a sequence of prograde garnet growth and localised breakdown reactions involving coupled dissolution–reprecipitation. Fluid availability and matrix permeability strongly control this metamorphic response and different generations of garnet contain radically different populations of metamorphic micro-zircon and associated changes in the detrital zircon population. Micro-zircon abundance increases during garnet growth, whereas that of detrital zircon decreases. The mineralogy of the matrix influences zircon abundance in porphyroblast phases, where garnet overgrows a micaceous matrix zircon-rich garnet forms and where it overgrows a quartzofeldspathic matrix the result is zircon-poor garnet. Following garnet growth, micro-zircon abundance decreases at each stage of the prograde reaction history, with sillimanite-zone schists containing the lowest abundance, suggesting micro-zircons are texturally less stable at staurolite- and sillimanite-grade metamorphism. Micro-zircons are distributed evenly across host minerals in the matrix, with the exception of retrograde chlorite where micro-zircons are absent due to fluids removing Zr before new zircon can precipitate. There is an overall decrease in the mode of zircon at each stage of the reaction history, indicating that zircon is a highly reactive phase during amphibolite-facies metamorphism and is very sensitive to individual prograde and retrograde reactions.

We present petrographic and structural analyses of a basement-hosted border fault in the East African Rift. Understanding the mechanical evolution and fluid-rock interaction of rift-flank faults is integral to developing models of fluid flow in the crust, where hydraulic connections may occur between basement faults and basin sediments. The Bwamba Fault forms the flank of the Rwenzori Mountains Horst in western Uganda, and has locally reactivated older mylonitic fabrics in the basement gneisses. The fault core features discrete mineralised and veined units. Shear fabrics and fault scarp striations indicate predominately normal kinematics, with minor strike-slip faulting and fabrics. Transient brittle failure was accompanied by two phases of fluid ingress, associated with veining and mineralisation. The first was localised and strongly influenced by host lithology. The second involved widespread Fe-oxide and jarosite mineralisation. The latter signals the onset of a hydraulic connection between Fe- and S-rich sedimentary rocks in the adjacent Semliki Rift Basin and the Bwamba Fault, involving co-seismic and or post-seismic fluid injection into the fault at ca. 150–200 °C, and 2.5–3 km depth. Such evolving permeability connections between basin sediments and basement faults are important for local hydrocarbon and geothermal systems, and may be typical of active rifts.

Chemical zoning in the outer few 10s of microns of garnet porphyroblasts has been investigated to assess the scale of chemical equilibrium with matrix minerals in a pelitic schist. Garnet porphyroblasts from the Late Proterozoic amphibolite‐facies regional metamorphic mica schists from Glen Roy in the Scottish Highlands contain typical prograde growth zoning patterns. Edge compositions have been measured via a combination of analysis of traverses across the planar edges of porphyroblast surfaces coupled to X‐ray mapping of small areas within polished thin sections at the immediate edge of the porphyroblasts. These approaches reveal local variation in garnet composition, especially of grossular (Ca) and almandine (Fe) components, with a range at the edge from 16 mol% grs, across distances of less then 50 mm. This small‐scale patchy compositional zoning is as much variation as the core‐rim compositional zoning across the whole of a 3 mm porphyroblast. Ca and Fe heterogeneity occurs on a scale suggesting a combination of inefficient diffusive exchange across grain boundaries during prograde growth and the evolving microtopography of the porphyroblast surface control garnet composition. The latter creates haloes of compositional zoning adjacent to some inclusions, which typically extend from the inclusion towards the porphyroblast edge during further growth. The lack of a consistent equilibrium composition at the garnet edge is also apparent in the internal zoning of the porphyroblast and so processes occurring during entrapment of some mineral inclusions have a profound influence on the overall chemical zoning. Garnet compositions and associated zoning patterns are widely used by petrologists to reconstruct P‐T‐t paths for crustal rocks. The evidence of extremely localized (10‐50 mm scale) equilibrium during growth further undermines these approaches.

Rock Around the University (RAU) is a teaching resource made up of 16 large (~2.5m) blocks of “local” Scottish rock which have been transplanted and orientated into carefully planned locations and elevations between the buildings of the University of Glasgow to look like natural exposures. RAU mimics a real-life fieldwork experience, on-campus, with the aim of enhancing the learning experience of undergraduate geoscience students. RAU allows progressive, reflective, and effective on-campus outdoor training of a wide-range of geological field skills and concepts, including: the description, analysis and measurements of rock features and structures; geological mapping; the use of structure contours to predict geological boundaries in terrains lacking abundant exposures; construction of cross-sections; and, the interpretation and reconstruction of 3D structure and geological history. Students visit the RAU exposures both during timetabled supervised ‘lab’ sessions and in their own time, providing an authentic fieldwork experience in a controlled location where key geological skills can be developed at the optimal rate for individual students. Being located on the campus means that there are no travel or expenses for students, fewer timetabling issues, and fewer general logistical complications and natural complexities than in remote fieldwork locations. In addition, students benefit from receiving ‘instant’ on-site feedback from staff on the challenges, problems and pedagogic issues that they encounter.RAU allows us to introduce rigorous field-based teaching at an early stage in geoscience courses and to stimulate and encourage reflective learning. Students locate, analyse and synthesise information in the field to provide effective solutions to problems and use RAU as a self-directed learning experience where they build confidence while working independently in a familiar environment. Hence the students reinforce their field skills before experiencing independent work in remote areas. In effect RAU uses the campus as a sustainable geoscience teaching resource. Experiences with all levels of undergraduate students over the eight years since RAU was established at the University of Glasgow have demonstrated that this on-campus resource is an ideal complement to the traditional programme of fieldwork classes. Students are much better prepared for their first major residential fieldwork having completed the RAU programme, and are much more confident in their field skills. RAU has allowed us to address more effectively the disconnect between laboratory and fieldwork skills, and remote fieldwork classes are now more focussed on the application, rather than the development, of field skills. RAU has also had the effect of enhancing the awareness of geoscience among the entire University community, due to the presence of students carrying out fieldwork on campus. Rock around the University is also used in recruitment and outreach, and is open to schools, amateur geoscientists, and anyone interested in Earth history. Printed leaflets are available and more information is available at https://www.gla.ac.uk/schools/ges/community/rockaround/ .

Metamorphic equilibration requires chemical communication between minerals and may be inhibited through sluggish volume diffusion and or slow rates of dissolution in a fluid phase. Relatively slow diffusion and the perceived robust nature of chemical growth zoning may preclude garnet porphyroblasts from readily participating in low-temperature amphibolite facies metamorphic reactions. Garnet is widely assumed to be a reactant in staurolite-isograd reactions, and the evidence for this has been assessed in the Late Proterozoic Dalradian pelitic schists of the Scottish Highlands. The 3D imaging of garnet porphyroblasts in staurolite-bearing schists reveals a good crystal shape and little evidence of marginal dissolution; however, there is also lack of evidence for the involvement of either chlorite or chloritoid in the reaction. Staurolite forms directly adjacent to the garnet, and its nucleation is strongly associated with deformation of the muscovite-rich fabrics around the porphyroblasts. “Cloudy” fluid inclusion-rich garnet forms in both marginal and internal parts of the garnet porphyroblast and is linked both to the production of staurolite and to the introduction of abundant quartz inclusions within the garnet. Such cloudy garnet typically has a Mg-rich, Mn-poor composition and is interpreted to have formed during a coupled dissolution–reprecipitation process, triggered by a local influx of fluid. All garnet in the muscovite-bearing schists present in this area is potentially reactive, irrespective of the garnet composition, but very few of the schists contain staurolite. The staurolite-producing reaction appears to be substantially overstepped during the relatively high-pressure Barrovian regional metamorphism reflecting the limited permeability of the schists in peak metamorphic conditions. Fluid influx and hence reaction progress appear to be strongly controlled by subtle differences in deformation history. The remaining garnet fails to achieve chemical equilibrium during the reaction creating distinctive patchy compositional zoning. Such zoning in metamorphic garnet created during coupled dissolution–reprecipitation reactions may be difficult to recognize in higher grade pelites due to subsequent diffusive re-equilibration. Fundamental assumptions about metamorphic processes are questioned by the lack of chemical equilibrium during this reaction and the restricted permeability of the regional metamorphic pelitic schists. In addition, the partial loss of prograde chemical and textural information from the garnet porphyroblasts cautions against their routine use as a reliable monitor of metamorphic history. However, the partial re-equilibration of the porphyroblasts during coupled dissolution–reprecipitation opens possibilities of mapping reaction progress in garnet as a means of assessing fluid access during peak metamorphic conditions.

An investigation of the morphology of zircon in clasts and matrix of a greenschist facies metaconglomerate from Shetland has revealed a history of alteration of radiation-damaged grains, partial dissolution and growth of new zircon. These processes are linked to the generation of chemically modified dark BSE zircon that is spatially related to fractures generated during radiation damage; embayments and rounding of zircon margins; and late overgrowths of original grains. These late modifications of zircon are all linked to the presence of fluids and so zircon morphology is used to track fluid behaviour in different lithologies in the metaconglomerate. Alteration is unrelated to clast margins and radically different in various clast types. This reflects a difference in permeability and suggests that deformation strongly controls fluid influx into quartzite, whereas zircon alteration in granite is associated with a restricted permeability reflecting the more limited response to deformation events.

Zircon megacrysts are locally abundant in 1–40 cm-thick orthopyroxenite veins within peridotite host rocks in the Archaean Lewisian gneiss complex from NW Scotland. The veins formed by metasomatic interaction between the ultramafic host and Si-rich melts are derived from partial melting of the adjacent granulite-facies orthogneisses. The interaction produced abundant orthopyroxene and, within the thicker veins, phlogopite, pargasite and feldspathic bearing assemblages. Two generations of zircon are present with up to 1 cm megacrystic zircon and a later smaller equant population located around the megacryst margins. Patterns of zoning, rare earth element abundance and oxygen isotopic compositions indicate that the megacrysts crystallized from crustal melts, whereas the equant zircon represents new neocryst growth and partial replacement of the megacryst zircon within the ultramafic host. Both zircon types have U–Pb ages of ca. 2464 Ma, broadly contemporaneous with granulite-facies events in the adjacent gneisses. Zircon megacrysts locally form > 10% of the assemblage and may be associated to zones of localized nucleation or physically concentrated during movement of the siliceous melts. Their unusual size is linked to the suppression of zircon nucleation and increased Zr solubility in the Si-undersaturated melts. The metasomatism between crustal melts and peridotite may represent an analog for processes in the mantle wedge above subducting slabs. As such, the crystallization of abundant zircon in ultramafic host rocks has implications for geochemistry of melts generated in the mantle and the widely reported depletion of high field strength elements in arc magmas.

Calcite pseudomorphs have replaced euhedral ikaite (CaCO 3 .6H 2 O) porphyroblasts in Dalradian calcareous slates and metadolostones of western Scotland, with a volume decrease of at least 47%. Porphyroblast–fabric relationships indicate that the initial growth of ikaite post-dates a penetrative tectonic fabric developed during upright folding. This is the first reported occurrence of metamorphic ikaite porphyroblasts and points towards growth within the slates during an ultra-low-temperature metamorphism with an exceptionally low geothermal gradient. This event is associated with the penetration of long-lived and extreme permafrost deep into subaerially exposed bedrock during Neoproterozoic glaciation. The presence of the well-preserved pseudomorphs within the Easdale slates of the Argyll Group implies that a Neoproterozoic orogenic unconformity exists above the stratigraphic position of these rocks.

Zircon in greenschist-facies Dalradian metasedimentary rocks of Scotland is characterized by two types: large detrital grains and small euhedral metamorphic grains. The former are concentrated in psammites and contain abundant evidence of dissolution and reaction with metamorphic fluids. The smallest metamorphic microzircons ( 2 ) represent >65% of zircon grains, are clustered within pelites and form in aligned arrays that cut across the bedding. The combination of textural characteristics with the spatial distribution shows that the growth of microzircon occurred in a small-scale crack network that developed after folding, reflecting precipitation from locally derived metamorphic fluids. Zircons within psammites in the hinge of an antiform show an enhanced metamorphic response suggesting focusing of fluids by the fold. Metamorphic zircon may be successfully used to map detailed fluid pathways, providing a unique approach to monitoring permeability structure in the crust. The scarcity of unmodified detrital zircon, which forms Supplementary material: The location data ( x , y coordinates), sizes and shape descriptors for the zircon grains are available at www.geolsoc.org.uk/SUP18605.

A textural and geochemical investigation of titanite from the Ross of Mull Granite, Scotland, reveals that zoning in trace elements strongly correlates with field evidence of magma-mixing processes within the pluton. Diorite enclaves contain a range of textures indicative of variable states of interaction with the host granite and suggest that a number of discrete mixing events have occurred in the pluton, both in situ and prior to emplacement at the present level. Single titanites within both the enclaves and the host are compositionally zoned and provide a detailed record of magmatic processes. Zoning includes concentric fine-scale oscillatory types and major discontinuities linked to dissolution events associated with changes in titanite stability. The zones reflect growth during changing melt chemistry and correlate with variations in rare earth element and high field strength element contents. Geothermometry using the Zr content of titanite indicates that titanite crystallization occurs at temperatures of c. 750–700°C. Titanites respond to changes in melt composition, oxygen fugacity and temperature caused by mixing and the introduction of fresh diorite magma into the granite host. Mixing processes recorded by the titanites include complete mixing of crystal-free melts, crystal transfer between partially crystallized magmas, melt segregation and transfer from crystalline enclaves and late-stage diffusive exchange. Titanite chemistry responds to local activity adjacent to sites of mixing but is also influenced by more distal magma-mixing events. In this respect titanite is capable of revealing many otherwise hidden events within the magma chamber.

Zircon textures, chemistry and microstructures have been characterized in situ within Carboniferous sandstones from the Midland Valley of Scotland using back-scattered electron and cathodoluminescence images, electron backscatter diffraction techniques and chemical analyses. The study of polished thin sections reveals a variety of zircon types including unmodified detrital zircon, zircon outgrowths and different forms of modified zircon that formed in low-temperature conditions within the sedimentary rocks. These rocks have only experienced temperatures of

Apatite grain boundaries on fractured rock surfaces have been examined in an amphibolite facies regional metamorphic granite gneiss from the central Swiss Alps. The morphology of apatite has been characterized using a scanning electron microscope and matched to surface textures in adjoining silicates. Apatites show a wide variety of different surface features ranging from planar crystal faces, to small-scale ridges and dimples, to extensive irregular pitting. Many of these features form in response to the periodic infiltration of fluids along open grain boundaries during the cooling history of the gneiss. Apatite shows evidence of both dissolution and re-precipitation that is controlled by the nature of the grain boundary, the structure of the adjoining silicate phase and the alteration of the host rock. Fracturing occurs in a range of retrograde conditions and is common both within the apatite and along grain boundaries. This coupled to the evidence of fluid interaction with mineral surfaces suggests that extensive permeable networks may be typical of cooling crystalline basement rocks. Grain boundary textures have the potential to reveal a unique record of fluid infiltration in the crust that would be very difficult to decipher using traditional petrographic methods.

Outgrowths of zircon and xenotime are abundant on detrital zircon in a greenschist facies regional metamorphic slate from the Scottish Highlands. Back-scattered electron images reveal that ca. 3-μm wide zircon outgrowths are porous, inclusion-rich and contain fine-grained intergrowths with xenotime. Focused ion beam milling, transmission electron microscopy and low-voltage scanning transmission electron microscopy show that the outgrowth microstructure is complex, composed of mis-orientated nano-crystalline zircon and a poorly crystalline zircon–xenotime complex. Well-defined micron-sized xenotime is also present within the outgrowth. Micro-textures show that the outgrowth developed in a polyphase history during cooling and exhumation. Accommodation space for the outgrowth is probably generated by a combination of thermal decompaction and the migration of defects in adjacent quartz driven by the force of crystallisation. Zircon may be unique in its ability to record textural evidence of events during uplift and exhumation that are not recorded in major silicate phases.

Zircon in greenschist-facies metasedimentary rocks from the Scottish Highlands displays a range of complex textures that reflect low-temperature alteration of original detrital grains. In situ back-scattered electron, cathodoluminescence, electron backscatter diffraction and chemical analyses show that altered zircon is porous, weakly luminescent, enriched in non-formula elements such as Al and Fe, and is associated with fractures within the host zircon. The low-temperature zircon appears to be nano-crystalline and to replace U-rich zircon via modification of whole grains or selective alteration of parts of grains, and is linked to the development of zircon outgrowths. The altered zircon is also associated with epitaxial xenotime outgrowths and inclusions. Low-temperature zircon is abundant in slates and other mica-rich samples and its formation is linked to a dissolution–reprecipitation mechanism. Zircon within quartz-rich host rocks typically shows evidence of deformation and the resulting fractures enhance its dissolution, creating rounded embayed morphologies. In contrast, zircon from phyllosilicate-rich rocks contains more new low-temperature growth. Zircon alters during both prograde and retrograde metamorphic events and its development is controlled by both the progressive accumulation of radiation damage in the host grain and the access of metamorphic fluids to the metamict zircon.

Apatite fission-track analysis of rocks from the Sierra Nevada basement, southern Spain reveals a detailed record of Neogene denudation. This is combined with evidence of the accumulation of clastic sedimentary rocks in the adjacent Granada Basin to provide a comprehensive record of the denudation of the emerging mountain block. Fission-track ages of c . 9–10 and 4 Ma obtained from the basement reveal periods of rapid cooling linked to tectonic denudation of the metamorphic core. The first major pulse of erosion from the Sierra Nevada produced proximal conglomerates in the marine basin and is constrained at c . 7 Ma using Sr-isotope stratigraphy. No major signal of rapid cooling is preserved by the fission-track systems in the basement block associated with this event. Apatite fission-track data from the conglomerates show young ages and short track lengths indicative of reheating. Modelling of the data suggests that heating occurred at around 4 Ma and is linked to movement of hot fluids through the basin. The ability of thermochronometers to record erosional events during the early stages of orogenesis is critically examined and we suggest that in young rising mountain blocks surface processes are incapable of keeping pace with rock uplift, and denudation principally occurs via tectonic processes.

The sizes, distributions and shapes of zircon grains within variably deformed granite gneiss from the western Alps have been studied. Zircon shows numerous indicators of a metamorphic response in both the host gneiss and a 5 cm wide continuous ductile shear zone, within which the zircon grain sizes range from 50 μm. However, the very fine grain sizes are virtually absent from grain boundaries. Within this zone, zircons consistently have more rounded and embayed margins, which are interpreted as evidence of dissolution in response to fluid influx during shearing. Zircons are preferentially located near metamorphic muscovite in both the host gneiss and the shear zone and tend to show the poorest crystal shape, indicating that fluids linked to the formation and presence of muscovite may enhance both the crystallization of zircon and its subsequent dissolution. Larger zircon crystals typically show a brittle response to deformation when adjacent to phyllosilicates, with fractures consistently perpendicular to the (001) mica cleavage. The variety of metamorphic behaviour observed for zircon indicates that it may be highly reactive in sub-solidus mid-crustal metamorphic environments.

The size and distribution of zircon within a garnet-mica-schist from the Scottish Highlands were assessed using scanning electron microscopy. The study reveals that abundant 0.2–3.0 μm sized zircon is preferentially concentrated within garnet and biotite porphyroblasts. The micro-zircon has grown during regional metamorphism and represents >90% of the total number of zircon in the schist. It is texturally distinct from a few larger detrital zircon grains in the schist that commonly preserve evidence of dissolution, and more rarely, small metamorphic outgrowths. The sequential incorporation of zircon in porphyroblasts allows prograde changes in the morphology of the zircon population to be identified. Zircon is reactive and soluble, and responds to medium-grade metamorphism in a series of dissolution and crystallization events, linked to possible changes in fluid composition. Deformation also has a significant influence on the distribution of zircon, allowing inclusions previously trapped within biotite to react. About 8 × 106 micro-zircon occur as inclusions within a typical individual 5-mm garnet porphyroblast and their presence must be considered prior to trace-element or isotopic analysis of such metamorphic phases.

A combination of numerical analysis and 10Be concentrations measured in sediment samples from the high-relief Torrente catchment, southern Spain, allows us to investigate the sampling requirements for determining erosion rates using cosmogenic nuclides in high-relief, landslide-dominated terrain. We use simple modelling to quantify the effect of particle spalling and/or landsliding on erosion rates determined using a cosmogenic in-situ produced isotope. Analytical results show that the cosmogenic nuclide concentration of a surface experiencing regular detachment of a grain or block may be considered to be in steady state, and ‘in-situ’ erosion rates estimated, when an appropriate number of spatially independent samples are amalgamated. We present equations that enable calculation of the number of bedrock samples that must be amalgamated for the estimation of mean erosion rates on an outcrop experiencing regular detachment of a grain or chip of thickness L every T years. Our findings confirm that mean catchment erosion rates may be reliably estimated from 10Be concentrations in fluvial sediment in high-relief rapidly eroding terrain. These catchment-wide integrated erosion rates can be calculated where erosion is primarily accomplished through shallow ( 3 m) landslides are the dominant mode of erosion only minimum erosion rates can be determined. Lastly, we present erosion rate measurements from the Torrente catchment that reveal variation of two orders of magnitude (0·03–1·6 m ka−1) quantifying the high degree of spatial variation in erosion rates expected within rapidly uplifting catchments. Copyright © 2006 John Wiley & Sons, Ltd.

Synopsis A U–Pb monazite age of 474 ± 2 Ma has been determined for a weakly foliated granite boulder > 1 m in diameter from the Corsewall conglomerate, SW Scotland. The clast contains xenoliths and partly assimilated xenoliths of regionally metamorphosed rock from which the monazites have been derived. The age constrains the original emplacement of the pluton to between 474 ± 2 and c. 458 Ma, the age of the deposit in which the boulder is found. This age confirms an earlier Rb–Sr muscovite Ordovician age for boulders of this type and contradicts a pre-existing age of 1265 Ma used to suggest a source in Newfoundland and a post-Caradoc major lateral displacement of the Southern Uplands. The monazite age reaffirms that a basement-arc complex once existed north of the Southern Uplands and was the principal contributor of sediment to Ordovician–Silurian basins of southern Scotland. A source for the metamorphic detritus in the Dalradian block is unlikely, as the boulders would have been transported for > 150 km through zones of actively accreting terranes and marginal basins. The possibility of Archaean crust within this basement complex is further suggested by the Sm–Nd systematics of the granite boulders at Corsewall Point.

Apatites from the Shap Granite, northern England, are strongly zoned, reflecting multiple generations of growth and dissolution. Such chemical zoning is most readily displayed in cathodoluminescence images and correlates well with trace element variation determined using LA-ICP-MS analyses. The zoned apatites provide a detailed record of the changing scales of permeability during progressive crystallisation within the magma chamber. Early periods of apatite growth are preserved within cores and represent both early growth within a magma chamber dominated by vigorous mixing processes and inherited grains with significantly different chemistries. The main phase of apatite growth within the magma was strongly controlled by the presence of adjacent biotite phenocrysts and is characterised by fine scale oscillatory zoning, followed by the growth of a thin rim of relatively uniform composition. The chemical evolution of the later phases of apatite growth and the stratigraphy of the zoning appear to record late stage crystallisation within progressively more isolated interstitial melt pockets.

The early Proterozoic rapakivi suite in the Ketilidian mobile belt of South Greenland is bimodal with an abundance of mafic plutons which mingled and hybridised with the rapakivi granites. Basic rocks lacking signs of hybridisation and with mineralogy and bulk compositions apparently unaffected by proximity to the granites are enriched in large ion lithophile element (LILE) relative to high field strength element (HFSE) and have moderate Nb depletion. These are characteristics common to basic rocks found in back-arc basins. This tectono-magmatic environment supports a model for the late Ketilidian of crustal extension, probably caused by slab roll-back and is in keeping with the low-pressure, high-temperature metamorphic conditions which prevailed when the rapakivi suite was emplaced. Nd and Sr isotopes confirm the juvenile character of the Ketilidian crust and rapakivi granites. Hybrid mafic rocks have low eNd compared to depleted mantle, while apparently unhybridised examples have also undergone some isotopic modification.

The stratigraphical and structural continuity of the Late Proterozoic Dalradian rocks of the Scottish Highlands is re-examined in the light of new U–Pb zircon ages on the tuffs belonging to the Tayvallich Volcanic Formation (601 ± 4 Ma), and on the late Grampian ‘Newer Gabbros’ (470 ± 9 Ma) of Insch and Morven–Cabrach in Aberdeenshire. These age data, together with the existing 590 ± 2 Ma age for the Ben Vuirich Granite, provide key radiometric constraints on the evolution of the Dalradian block, and the implications arising from these ages are critically assessed. Three main conclusions are drawn. (1) The entire Caledonian orogeny, although short-lived, is unlikely to have affected sediments of Arenig age and a break probably occurs between those Dalradian sediments of late Proterozoic ( (2) A period of crustal thickening probably affected some Dalradian rocks prior to 590 Ma. Such an event is indicated by both the polymetamorphic histories of the lower parts of the Dalradian pile and the contact metamorphic assemblages within the aureole of the Ben Vuirich Granite, which are incompatible with sedimentary thicknesses. (3) Age constraints on global Late Proterozoic glacial activity also suggest that the Dalradian stratigraphy is broken into discrete smaller units. Models involving continuous deposition of Dalradian sediments from pre-750 Ma to 470 Ma are rejected.

Prograde metamorphic reactions involving the growth of phyllosilicates and accompanying cleavage development have been investigated in Dalradian metasediments from the biotite zone of eastern Scotland. Crystallization of muscovite within the psammites of the Southern Highland Group is linked to the replacement of plagioclase porphyroclasts. This reaction is triggered by significant alkali metasomatism during active deformation and plays an important role in the formation of a prominent spaced cleavage within the psammites. The Si content of most of these early-formed muscovites is partially buffered by the quartz content of the rock, although close to the Highland Boundary Fault, evidence of greater influence from externally derived fluids on muscovite compositions is preserved. Locally higher fluid fluxes adjacent to the fault are also indicated by a relatively high δ18O(SMOW) signature in the rocks. The biotite-producing reaction in these greenschist-facies rocks is linked to the later production of a celadonite-poor muscovite which formed as overgrowths around pre-existing white micas. This reaction is sensitive to the initial composition of muscovite and preferentially occurs in quartz-rich metasediments containing a celadonite-rich muscovite. A systematic increase in the progress of the biotite-producing reaction northwards across the biotite zone confirms the presence of high geothermal gradients along the southern margin of the Dalradian block, adjacent to the Highland Boundary Fault.

The rapakivi granites of South Greenland were emplaced into the Ketilidian orogenic belt at about 1.74 Ga during an episode of extensional tectonics. Petrographically, they can be subdivided into a roughly subequal black facies and white facies, distinguished by the colour (turbidity) of the alkali feldspars and by the associated mineralogy which is more hydrous (i.e., amphibole and biotite compared to olivine and pyroxene) in the white facies. The oxygen isotope composition of these rocks is notably homogeneous; from thirteen outcrops sampled over an area extending some 200 km×100 km all alkali feldspars have an average δ 18 O of 10.2±0.4‰. This lack of variation is consistent with the consensus petrogenetic model which invokes a very well mixed two-component protolith; the high abundance of 18 O (mean whole rock δ 18 O =9.4‰) argues that the major contribution (perhaps 90%), although originally mantle-derived, had undergone 18 O enrichment in a low temperature sedimentary cycle. There is no oxygen isotopic distinction between alkali feldspars in the black and white facies, which is argued to be a consequence of the relatively anhydrous nature of the melt. Hydrogen isotope compositions vary widely—by 30‰—but are coherent on a hand specimen scale, as demonstrated by approximately equal δD of biotite and alkali feldspar. It is not clear at which stage the D/H heterogeneity was introduced.

Plagioclase-rich reaction zones occur around numerous aluminous crustal xenoliths within a suite of Palaeogene sub-volcanic basic sheets on the Isle of Mull, NW Scotland. The xenoliths consist of a glassy core, containing mullite needles, generated from the melting of pelitic source rocks. Thick plagioclase mantles grew at the interface between the aluminous liquid and the enclosing basaltic magma and provide a high-level analogue for the petrogenesis of Proterozoic massif-type anorthosites. Similar interactions between mantle-derived basic magmas ponded at the base of the crust and relatively Al-rich lower crustal lithologies would result in the precipitation of large volumes of plagioclase. Anorthosite massifs were then emplaced at higher crustal levels as crystal-rich mushes within relatively juvenile Proterozoic crust. The model negates the need to crystallize large volumes of mafic minerals prior to the production of plagioclase-saturated liquids, and also accounts for the significant influence of crustal sources on the isotopic signatures of all members of the anorthosite suite.

Radiometric age determinations on syn-orogenic intrusive rocks coupled with evidence from field relationships, indicate that metamorphism and ductile deformation was taking place hi the Scottish metamorphic blocks coeval with extension on the Hebridean foreland. Consequently metamorphic rocks now found east and south of the Moine Thrust are regarded as substantially allochthonous, being located within the region where there was formerly a Cambrian-Early Ordovician passive margin sequence and its attendant oceanic crust. We regard these blocks as displaced terranes which acquired their metamorphic and structural signatures outside of the present Hebridean foreland regime. This may have occurred initially at the tune of megacontinent splitting and later on the destructive margin which was initiated hi Laurentia.

The biotite isograd reaction in Cambro–Ordovician pelites from the Garonne dome in the Central Pyrenees involves the production of biotite at the expense of chlorite, and the gradual reduction in the celadonite (Si) content of individual white micas. The mineral assemblages in the biotite-bearing rocks in the Melles area retain abundant evidence of chemical disequilibrium, due to the sluggish nature of major element diffusion within the white micas. The progress of the isograd reaction is strongly controlled by the progressive development of regional Variscan fabrics, and chemical exchange in the white micas in these metamorphic conditions is only possible during active deformation. Chemical resetting of the white micas takes place via the development of compositional zoning in the deforming micas; this probably occurs as a consequence of the introduction of defects and dislocations, causing more efficient diffusion within parts of individual grains. In the absence of deformation, this biotite isograd reaction would take place at significantly higher temperature and be controlled by the relatively high closure temperature of major element diffusion in white micas. Thus, assigning thermal significance to such continuous isograd reactions is impossible without independent constraints: kinetic factors such as deformation may be the dominant influence in many cases, not the thermodynamic controls.

Synopsis Na-biotites have been found within pelitic schists in lower amphibolite facies Dalradian metasediments. These micas are optically very similar to normal K-biotites which they partially replace, although the Na-biotites do contain a more prominent 001 cleavage. The formation of the Na-biotites is linked to the local infiltration of a Na-rich fluid during retrogression and was dependent on late deformation opening fluid pathways within existing biotite porphyroblasts. Micas of such unusual compositions (up to 3 wt% Na 2 O) have not previously been reported from pelitic rocks.

Textural, chemical and Sr isotopic studies of feldspars from the subsolvus Shap granite, northern England, demonstrate that a number of magma mixing events have dominated the evolution of this pluton. K-feldspar megacrysts are phenocrysts formed in the magma chamber. They contain a number of Ba-rich zones that developed during periods of slight dissolution and regrowth linked to the hybridization of the granite by the intrusion of basic magmas. Diorite enclaves represent the relicts of these magmas and these also contain K-feldspar megacrysts, which show evidence of major dissolution. They are xenocrysts picked up from the host granite and incorporated in the basic magma. Increasing H 2 O contents during fractional crystallization caused a late switch from growth of megacrysts to finer-grained K-feldspars in the matrix. The chemically and isotopically zoned K-feldspar megacrysts preserve an exceptional record of the evolution of the magma, and the zones also had a significant influence on the development of exsolution microtextures during cooling.

The Buchan area of Dalradian rocks represents a distinct tectonothermal domain within the Scottish Highlands, and the Portsoy-Duchray Hill lineament on the western margin of this block is one of the best examples of a long-lived domain boundary. Rb-Sr mica cooling ages, from metamorphic rocks from across the trace of this lineament, document a prolonged Ordovician cooling history. Delayed, but then relatively rapid, cooling to the west of the Portsoy-Duchray Hill lineament is recognized whereas to the east the age data point to a high heat flow regime lasting more than 50 Ma. The isotopic ages suggest that c. 460 Ma was a time of active tectonism, resulting in final cooling of the low-grade rocks and differential uplift and rapid cooling of deeper structural levels to the west. The radiometric ages do not support previous tectonic models for the region that have invoked thrust thickening along the line of the Portsoy-Duchray Hill lineament and a new model for the metamorphism is proposed. The overprinting of andalusite-bearing assemblages by kyanite to the west of the Portsoy-Duchray Hill lineament is linked to pressure increases associated with the intrusion of the Newer Gabbros and represents a magmatic loading related to the same tectonothermal event that generated the low-pressure metamorphism, rather than a separate tectonic episode.

Crustal xenoliths occur in a variety of Carboniferous and Tertiary alkali basaltic rocks across Scotland and provide indications of the nature of the deep crust beneath the major lithotectonic zones from the Archean Lewisian of the far NW to the Paleozoic terranes in the SE. Sr, Nd, and Pb isotopic and representative chemical compositions are presented for 35 mafic xenoliths from the different age and tectonic zones of Scotland and for 21 felsic, gneissose xenoliths, from the Midland Valley and Southern Uplands in the south. The coexistence of mafic granulite and spinel lherzolite xenoliths at several of these localities, together with seismic velocity data, provide evidence that the mafic xenoliths, and possibly some of the felsic xenoliths, may be derived from 20–30 km depth. The absence of garnet from the mafic assemblages suggests that in most areas the crust was probably not much more than 30 km thick in the Permo-Carboniferous. The extremely variable Al2O3 (up to 27 wt %) and MgO contents at any one location, together with positive Eu and Sr anomalies in chondrite- and mantle-normalized diagrams, provide evidence that many of the mafic xenoliths represent cumulates from basaltic magmas. The Loch Roag mafic xenoliths, erupted through the depleted high-grade metamorphic Lewisian Complex, are especially rich in K (average 1.3 % K2O) and Rb (average 54 ppm) with K/Rb extending as low as 100. The K/U ratios for the mafic xenoliths are high, ranging to over 100,000 at Loch Roag and in the SW Midland Valley. The relative concentrations of U, K, Rb, Pb, Nd, Sm, Eu and Sr in most of the mafic Xenoliths, while displaying regional characteristics, display intra-site variations that are consistent with crystal-liquid fractionation. There is little evidence of U or Rb depletion via fluid or melt extraction during metamorphism. The concentrations of U in the mafic xenoliths display as much intrasite uniformity as the rare earth elements and correlate with Zr concentrations. U/Zr ratios are similar to those found in ocean island basalts. The mafic xenoliths display a marked decrease in ϵNd and 206Pb/ 204Pb with increasing age of the upper crust from SE to NW, as also observed in the ∼400 Ma Caledonian granites. Sm-Nd crustal residence ages for the Loch Roag suite are particularly variable ranging from 1.6 to 3.6 Ga. However, these isotopic compositions and model ages appear to be, in part, the product of crustal assimilation by the igneous precursors to the mafic xenoliths. Model ages, mineral ages and scattered whole rock isochrons indicate that many of the precursors to these Xenoliths were in fact formed and metamorphosed during the late Proterozoic and Paleozoic and provide evidence of widespread deep crustal magma reservoirs, underplating, crustal assimilation and arc accretion at these times. Nd and Sr isotopic data for coexisting pyroxenite xenoliths at Loch Roag indicate that they may represent contaminated ultramafic cumulates that were cogenetic with the precursors to the mafic xenoliths. The Pb isotopic compositions of the Midland Valley mafic and felsic xenoliths are similar and relatively uniform despite large variations in Nd and Sr isotopic compositions. This is explicable if the crust in this region was relatively uniform and similar in composition to the mantle and/or if crustal Pb dominated in arc environments. The average major element composition of the Scottish mafic xenolith suite is similar to that reported from other regions such as the Sierra Nevada. This is taken as evidence that the processes of crystal fractionation and formation of mafic and ultramafic cumulates during underplating are relatively consistent between different regions. Average U/Pb ratios of mafic and felsic xenoliths are effectively identical (0.068 and 0.064, respectively). The average Sm/Nd ratio (0.20) for the entire suite is lower and average K/U ratio (27,000) higher than estimates of average lower crust based on Queensland mafic xenoliths. These compositions as well as those of basement rocks, sedimentary supracrustal rocks and granites provide evidence of systematic chemical differences between the Scottish and Australian lithosphere. The compositional features of the Paleozoic Scottish crust such as low Sm/Nd and high K/U may have been sustained in part by recycling of Archean and early Proterozoic lithosphere through sedimentary processes, subduction and arc volcanism. The average Sm-Nd crustal residence age and Pb isotopic composition of the mafic xenoliths at Loch Roag is very similar to that of the host Lewisian. Conservative mass balance models imply an average ratio of Lewisian-derived Nd to parent magma Nd in the xenoliths of about 2. Such large degrees of contamination indicate that although underplating may be significant process in reconstituting Archean lithosphere, it does not necessarily result in substantial changes to the average age of the crust.

The thermal history of the Hebridean Igneous Province has been determined through the application of low-temperature thermochronology to the four central complexes in the province. The zircon (U–Th)/He age (59.4 ± 3.3 Ma, 1σ, n = 18) and the ages from each complex (60.7 ± 2.3 Ma for Skye; 58.0 ± 0.4 Ma for Mull; 55.9 ± 3.2 Ma for Ardnamurchan; 55.6 ± 2.7 Ma for Rum) are indistinguishable from the crystallization ages. Apatite fission-track ages (61.2–57.2 Ma, mean of 59.3 ± 3.4 Ma) from the major plutonic units also overlap crystallization ages, implying that on a regional scale the Hebridean Igneous Province cooled rapidly to near-surface temperatures immediately after emplacement. However, apatite fission-track ages and track lengths and apatite (U–Th)/He ages from some small-volume intrusions in the Skye and Rum central complexes identify localized mid-Eocene (45–47 Ma) cooling. Forward and inverse modelling suggests a discrete heating–cooling event at c . 47 Ma, which may have been caused by structurally controlled localized advection of heat above shallow emplacement. This is the first suggestion of Eocene magmatism in the Hebridean Igneous Province.

White micas (phengites) in the metasediments of the Scottish Dalradian display a large range of compositions within single samples. The variations in the composition of these phengites are strongly controlled by their structural age, with early fabrics containing a paragonite-poor, celadonite-rich phengite whereas in later fabrics the micas are generally paragonite-rich and celadonite-poor. Retrograde phengite growth, identified using back scattered electron imaging, occurs as celadonite-rich rims on micas within all existing fabrics and appears to be preferentially developed along existing white mica-plagioclase grain boundaries. The presence of these chemically distinct phengite populations within single samples implies that chemical exchange between the individual micas was inefficient. It is proposed that diffusion-controlled exchange reactions in phengites have relatively high closure temperatures below which major element exchange is effectively impossible. This closed system behaviour of micas questions the ease with which phengites may equilibrate with other phases during prograde greenschist and lower amphibolite facies metamorphism. Many of the chemical variations preserved in phengites from such metamorphic rocks may reflect deformation/recrystallization controlled equilibria.

Synopsis Xenoliths within a late-Caledonian lamprophyre dyke intruded into Dalradian metasediments reveal the presence at depth of Dalradian rocks of relatively high regional metamorphic grade and probable Highland Border Complex rocks. The xenolith suite is largely composed of fragments of cognate cumulate rocks together with metamorphosed country rocks both associated with a deep magma chamber. The latter formed close to the Highland Boundary Fault, which dips to the north beneath a thickened Dalradian pile. Pyrope-rich garnets within the xcnocryst assemblage may indicate that the lamprophyre magma originated in high pressure rocks within the mantle.

The development of rapakivi texture in feldspars from the Ketilidian granitoids of south Greenland has been investigated using Sr, O and H isotopes. A low temperature signature is found in the Sr and O data which seemingly contradicts some textural features that point to a magmatic origin of the plagioclase mantles around the K-feldspar ovoids. An origin for these mantles involving exsolution from an original alkali feldspar solid solution is proposed, which involves growth of mantles over a range of conditions determined by the mobility of the exsolving sodic feldspar. This mobility may be enhanced at high temperatures in the presence of melts or increased fluid pressures and at lower temperatures by the processes responsible for the transformation of K-feldspar to microcline. Rapakivi granites with both white and dark green feldspar occur in south Greenland but show no major isotopic differences, although the dark alkali feldspars contain significantly more fluid. Equivalent fluids in the white alkali feldspars may have escaped during plagioclase exsolution.

Sm–Nd and K–Ar isotopic determinations on an amphibolite from the Highland Border Complex yield Cambrian ages. Collisional events which might have generated the amphibolite appear incompatible with the passive margin character of Laurentia at that time. A possible explanation is that the part of the Highland Border Complex containing the amphibolite and associated rock units was generated outside Laurentia.

The southernmost part of the early Proterozoic Ketilidian mobile belt (1·9–1·7 Ga) of southern Greenland comprises a thick series of largely flat-lying migmatitic metasediments and metavolcanics of upper amphibolite to lower granulite facies. Regional tectonics are extensional and there is little structural evidence for earlier crustal thickening. Several episodes of granite magmatism are recorded, culminating in the late- to post-orogenic rapakivi granites, which are associated with with a coeval suite of norites. These were emplaced at ca. 1740 Ma, roughly synchronously with the peak of regional metamorphism (2–3·5 kbar, 650–800°C). The rapakivi granites have been emplaced into major crustal-scale extensional shear zones and have ramp-flat geometries. ‘Black’ and ‘white’ rapakivi granites are found. The mineralogy of the black granites is dominated by orthopyroxene–amphibole assemblages with rare fayalite. In the white granites, biotite and amphibole are the only Fe–Mg silicates. All Fe–Mg silicates have high Fe/(Fe+Mg) ratios. In terms of bulk chemistry, the suite is bimodal. Both norites and granites are strongly fractionated and have Fe/(Fe+Mg) ratios of 0·50–0·65 (norites) and 0·65–0·90 (granites). REE modelling shows that the granites can be derived from 40 per cent partial melting of a metasedimentary protolith. The sequence of magmatic events in this region reflects the progressive metamorphism and dehydration melting of juvenile sediments in an extending basin. The heat source for melting may have come from the underplating of olivine tholeiite melts generated during lithospheric extension.

ONE of the principal ways in which continental crust grows is by the incorporation, from deeper within the Earth, of large volumes of granitic magma. A 'space problem' exists1 as to how these magmas are accommodated in the crust. Traditionally two main emplacement mechanisms have been emphasized: 'forceful' intrusion, whereby buoyancy-driven magmas physically push the crust aside, creating granitic diapirs and balloons; and 'passive' emplacement characterized by replacive mechanisms such as cauldron subsidence and stoping. Although more recent work2,3 has demonstrated that space for granites may be created within bends and offsets of large transcurrent faults, the simple view of either forceful or passive still fails to account for the intrusive mechanisms of many granites. Here we report the discovery of a new igneous intrusion mechanism. In the superbly exposed Proterozoic continental crust of South Greenland we have observed that rapakivi granite was intruded as large-scale sheets along ductile extensional shear zones that were active during emplacement. In such a process the space problem seems to be simply resolved.

[1] We integrated optically stimulated luminescence dating and 10Be cosmogenic nuclide measurements to quantify short-to-medium-term (102–104 years) catchment dynamics and response to active tectonics. In the 27 km2 Rio Torrente catchment, Sierra Nevada, southern Spain, rapid base-level fall has triggered knickpoint migration up both trunk and tributary channels, resulting in two distinct geomorphic zones: (1) a steep lower catchment with concordant rates of hillslope erosion and channel incision over both short (100 years: 5–8 mm yr−1) and medium (12 ka: 5 ± 1 mm yr−1) timescales and (2) a low-relief soil-mantled upland topography with uniformly low bedrock and hillslope erosion rates (0.05 ± 0.02 mm yr−1). The uniformity of erosion in this upland surface indicates that catchment topography was previously in steady state. Rapid river incision below the channel knickpoints has resulted in the development of steep landslide-dominated hillslopes that are essentially “tracking” the incising channel. The magnitude of base-level fall required to generate these steep hillslopes is >100 m; at least 50 m of this base-level fall occurred during the past ∼21 ka. These steep hillslopes are eroding back into the low-angled upland surface at a much slower rate than the channel knickpoint. Consequently, the trunk channel knickpoint has already reached the catchment headwaters while hillslopes continue to adjust to the new base level, indicating that the channel profile will regain equilibrium form long before hillslopes. Thus hillslopes are the limiting factor for the duration of landscape transience in this small mountain catchment.

[1] The denudation history of the rapidly uplifting western part of the Spanish Sierra Nevada was assessed using apatite fission track (AFT) ages and 10Be analyses of bedrock and fluvial sediments. Major contrasts in the denudation history are recorded within the 27 km2 Rio Torrente catchment. Upland areas are characterized by low-relief, low slope angles, and locally the preservation of shallow marine sediments, which have experienced 2 km of denudation since circa 4 Ma. The minimum denudation rates of 0.4 mm yr−1 derived from AFT also contrast with the slow medium-term (104 years) erosion rates (0.044 ± 0.015 mm yr−1) estimated from 10Be measurements at high elevations. The local medium-long-term contrasts in denudation rates within the high Sierra Nevada indicate that much of the unroofing occurs by tectonic denudation on flat-lying detachments. In lower elevation parts of the catchment, rapid river incision coupled to rock uplift has produced ∼1.6 km of relief, implying that the rivers and adjacent hillslopes close to the edge of the orogen are sensitive to normal-fault-driven changes in base level. However, these changes are not transmitted into the low-relief slowly eroding upland areas. Thus the core of the mountain range continues to increase in elevation until the limits of crustal strength are reached and denudation is initiated along planes of structural weakness. We propose that this form of tectonic denudation provides an effective limit to relief in young orogens.

[1] First-order topographical features at high-elevation passive margins have the potential to store information about the early stages of continental rifting, which are usually lost in the sediment record. In this study we present a new combination of apatite fission track and (U-Th)/He analyses from two coast-perpendicular traverses to decipher the time of formation and evolution of the escarpment at the southeastern Australian margin. A combination of inverse and forward modeling of the apatite fission track (AFT) and He data indicates that the coast experienced a rapid denudational event starting at 110 ± 10 Ma, at least 15 Myr before seafloor spreading (85 Ma), in agreement with the hypothesis that rifting at magma-poor margins evolves slowly. Thus the enhanced denudation is not related to the synbreakup base level drop, but it may have been related to a thermally driven transient surface uplift. Considerations on isostatic flexure and rigidity of the lithosphere indicate that unless significant synrift brittle movements are assumed to have occurred along margin-parallel faults that are no more visible, the coast must have experienced a higher geothermal gradient than normal. The evolution of the rest of the coastal plain may have taken up to ∼90 Myr, in agreement with numerical models of passive margins evolution. The (U-Th)/He ages across the coastal plain are consistent with an in-place-excavation scenario of a high rift shoulder, and they rule out the possibility of a constant retreat of the escarpment. AFT + He data from the plateau indicate that the highlands remained stable throughout continental breakup, experiencing rates of erosion of 5–10 m/Myr since, at least, 200 Ma.

Fluids play a key role in the transfer of both heat and mass within the crust and in the attainment of chemical equilibrium in metamorphic rocks, controlling both reaction rates and deformation mechanisms. However, our understanding of fluid movement through the deep crust is limited because rocks readily recrystallize, and fluids are invariably lost from grain boundaries. Little is known about how textures evolve during metamorphism since minerals rarely preserve much textural history that can be directly related to grain-boundary processes. Here we report on a study of the surfaces of apatite within gneisses. A variety of textural features is preserved on these surfaces, including imprinted texture of fossil grain boundaries. This texture is linked to deformation that caused a temporary excursion to fine grain size in the surrounding silicates. The geometry of paleoporosity is also recorded by microhollows on the apatite surface and gives a unique insight into both fluid behavior within the crust and the textural evolution of rocks.