Robert H. Hunter

The differentiation of the Skaergaard intrusion

ConclusionsMcBirney and Naslund have missed the two central points that we reiterate for clarity. First the composition of the cotectic cumulate extract for the LZc, best estimated from the average composition of adcumulate rocks, had a substantially lower silica content than any conceivable basaltic liquid, so silica enrichment is unavoidable. This is true of all the dyke compositions reported by Brooks and Nielsen, even the anomalous rock E with SiO2=46%. Second it is the normal behaviour of tholeiite basaltic magma to produce increasingly iron-rich cumulate extracts with little change in silica until FeTi oxide precipitation occurs when silica-enrichment and compositional divergence begins. Such divergence between equivalent cumulate plutonic and fractionated volcanic rocks hads been widely documented. Skaergaard magmas do have some uncommon features. However, similar assemblages exist in volcanic rocks as in the andesitic to rhyolitic glasses of the Loch Ba ring dyke, Mull (Sparks 1988). It is also a basic principle that elements enriched in cumulate rocks often become depleted in the corresponding liquids. Iron enrichment in the Skaergaard cumulates is complementary to iron depletion in liquids.

Texture Development in Cumulate Rocks

Abstract In the past three decades, the cumulus terminology developed by Wager and co-workers has provided the framework for understanding texture development in crystal mushes. Much of the debate has concerned the conditions necessary for development of adcumulate rocks and has involved discussion of mechanisms of heat and mass transfer within mushes. In this article the historical development of ideas is reviewed and aspects of the nomenclature are discussed. The development of primary and secondary textures in mushes are then discussed, principally with respect to the relative roles of crystal overgrowth, compaction, and cementation. Most crystal accumulation in moderate- to large-sized layered intrusions occurs on the floor, where crystal mushes develop by either in situ crystallization or crystal sedimentation. Except where a preferred crystal shape orientation occurs as a result of directional growth from a substrate, there are no definitive textural criteria for distinguishing in situ crystallization from crystal sedimentation in the accumulation of mushes. Mushes inherit primary textural characteristics that influence the subsequent texture development within the crystal pile. Primary porosity and permeability are influenced by initial packing and clustering characteristics of crystals which are a function of the way in which crystals accumulate and any subsequent mechanical reorganization. Crystal growth, solution/replacement, cementation, compaction, and recrystallization are competing processes involved in the secondary texture development of the crystal pile. The densification of a crystal mush involves the reduction of primary porosity of the cumulus grains. This may be by overgrowth on the grains or compaction. Either process will be restricted by the nucleation and growth of poikilitic grains which cement the granular crystal framework. These processes are analogous to syntaxial overgrowth, compaction and cementation involved in sediment diagenesis. Whether crystals grow under near-isothermal conditions or during cooling depends upon whether the mush is open or closed to melt percolation but is independent of the mechanism of heat and mass transfer within the mush. Compaction, necessarily an open-system process, involves deformation (dislocation creep) or solution/reprecipitation of grains (diffusion creep) and usually results in an increase in the degree of local textural equilibration. However, recrystallization ( e.g. by thermal annealing) also results in textural equilibration. Growth, compaction, and recrystallization are all competing processes and it is commonly not possible to isolate their contribution to any given texture; all produce rocks with the textural characteristics of adcumulates. The extent of densification of a mush of cumulus grains depends critically on the timing of nucleation and growth of poikilitic cementing phases. In any given magma composition this is a function of the local phase relationships. A cyclicity will develop in the texture in a crystal mush that is a function of the balance of densification and poikilitic cementation. Repeated replenishment of a magma chamber may result in suppression of the cementation cycle and allow mushes to become highly densified. On the scale of an intrusion the texture which develops depends upon the interaction of fronts of densification and cementation and hence is dependent on intrusion geometry.

The Rum Layered Suite

Abstract Palaeocene igneous activity in the Rum Central Complex culminated in the formation of the ultrabasic and gabbroic rocks of the Layered Suite. Its three components, the Eastern Layered Series, the Western Layered Series and the Central Series, represent a continuum in time during which replenishments of picritic (MgO 15-20 wt.%) and basaltic magmas ponded in thin sill-like bodies at the Lewisian gneiss - Torridonian sandstone unconformity, each contributing incrementally to a layered cumulate sequence. The magmas were probably guided during ascent by the long-lived Long Loch Fault. Peridotite (olivine - chrome-spinel) cumulates formed from picritic magma. The residual (basaltic) magma mixed with resident residual magma from earlier batches, and with small amounts of siliceous rheomorphic melts from country rocks, forming (isotopically contaminated) allivalitic (= troctolitic), plagioclase - olivine cumulates or, less commonly, gabbroic (plagioclase - olivine - clinopyroxene) cumulates. Residual basaltic magma was probably also intruded as gabbroic sheets and plugs, and extruded as lavas. Widespread slump and shear structures indicate mechanical instability of unconsolidated cumulate mushes, especially in the allivalites. Ultrabasic breccias are common in the Central Series, and are attributed to (i) disruption of earlier cumulates as new batches of magma rose along an elongate, north-south feeder zone and (ii) collapse of cumulates into this zone during episodic magma withdrawal. Equilibrated textures, lack of compositional zoning in olivine and pyroxene, offsets between compositional and modal variation at unit and other lithological boundaries, the occurrence of finger structures and other replacement features, and the compositional modification of ultrabasic rocks adjoining late-stage gabbroic veins, all attest to the pervasive influence of migrating intercumulus liquids during crystallization and consolidation of the cumulates.

Isotopic and chemical constraints on the building of the deep Scottish lithosphere

Synopsis The Scottish late Caledonian granites define a chemical province characterised by high Na, Ba and Sr abundances, particularly towards the north and west where they also have higher Zn, REEs and La/Y and lower Rb and Th. A comparison with data for inclusions found in younger volcanics and hypabyssal intrusions and thought to have been excavated from the mantle and lower crust indicates that the source of at least some of the chemical variation in the granitic magmas lies in the lithospheric mantle. The asthenosphere or postulated subducted slab material from the Iapetus ocean floor cannot have directly contributed significant amounts of these elements to most of the granitoids. Further, isotopic data clearly indicate that lower crust-derived materials are essential components of the more evolved granites, implying that considerable melting took place near the Moho. The age of the lower crust parallels the changes in chemistry in the lithospheric mantle which suggests that the Moho is not a major tectonic discontinuity. The Mid-Grampian line probably represents the rifted continental edge during Dalradian deposition. There is no isotopic evidence for primary crust of Grenville age under Scotland; rather the lower crust appears to be a complicated mixture including some components which are very young (probably Lower Palaeozoic) under the Midland Valley. The only location where clearly enriched mantle has been identified is under the Lewisian of the Outer Hebrides. The lower crust in this region is also Lewisian, apparently metasomatised. We tentatively suggest that the chemical enrichments and depletions in the lithosphere north of the Mid Grampian line are most simply explained by a volatile flushing mechanism.

Xenoliths and their implications for the deep geology of the Midland Valley of Scotland and adjacent regions

Late Palaeozoic alkalic basalts in and around the Midland Valley of Scotland contain a wide variety of ‘plutonic’ xenoliths. Pyroxene-rich ultramark xenoliths (wehrlites, clinopyroxenites and garnet pyroxenites) may be representative of younger components within a dominantly peridotitic upper mantle represented by ubiquitous magnesian peridotite xenoliths. Glimmerites and other biotite-rich ultramafic xenoliths are probable samples of metasomatised upper mantle facies. Xenoliths composed mainly of plagioclase, clinopyroxene ± orthopyroxene ± magnetite are widespread. These pyroxene granulites may typify the lower crustal layers. Garnet granulites are rare; such rocks may formerly have been important with loss of garnet occurring through retrograde metamorphism. Anorthositic xenoliths are relatively common. The lower crust may consist largely of anhydrous rocks, of gabbroic to anorthositic composition, ccurring as stratiform bodies of metacumulates. Other xenoliths of igneous origin include tonalitic and trondhjemitic gneisses. Although these may play some role in the lower crust, they may be more abundant in the mid-crustal domains underlying the deformed upper Precambrian and lower Palaeozoic supracrustal strata. Xenoliths of quartzofeldspathic, granulitic gneisses containing garnet ± sillimanite ± rutile are also of widespread occurrence; many of these are of metasedimentary provenance and are regarded as being derived from the mid-crustal layers beneath the Southern Highlands, Midland Valley and Southern Uplands and their Irish counterparts.

The description of the primary textures of “Cordilleran” granitic rocks

Variation in the primary textures of “Cordilleran” granitic rocks is described relative to three identifiable stages of the crystallisation interval; namely: (1) crystallisation in suspension; (2) growth of a touching crystal framework; (3) interstitial crystallisation. Crystals that initially grow in isolation will start to impinge and form small clusters as crystallisation proceeds and the volume of solid material increases, eventually forming a continuous interconnected crystal framework. Subsequent crystallisation involves solidification of the melt occupying the interstices of the framework, and therefore shows similarities to the way in which the porosity occludes in sedimentary systems. A case study of textural development in Cordilleran granitic rocks from the zoned Linga superunit of the Peruvian Coastal Batholith, reveals that compositional zonation from granodiorite through to syenogranite is accompanied by a systematic variation in the textures, specifically those of the three felsic phases (plagioclase, quartz and alkali feldspar). Plagioclase was the first phase to appear on the liquidus, and was joined by the other two phases as crystallisation proceeded and the melt evolved. The melt fraction at which quartz and alkali feldspar started to crystallise influenced the early growth of plagioclase, and the way in which the texture developed through each stage of the crystallisation interval. The geometry of plagioclase progressively changes from a touching framework of crystals in the granodiorite, to small aggregates or isolated crystals suspended in an equant mosaic of the other felsic phases in the syenogranite. This variation can be explained by an earlier evolution of the melt to the cotectic (i.e. at higher melt fractions) as the rocks become more acidic, and hence a greater contribution of alkali feldspar and quartz to the growth of the framework at the expense of plagioclase and the mafic phases. Textural observations are comparable to the crystallisation pathways of the felsic phases modelled in the quaternary An-Ab-Or-Qz system from the bulk compositions. All compositions lie in the plagioclase volume, and evolved to three-phase saturation on the cotectic via either the quartz/plagioclase divariant surface (granodiorites) or the alkali feldspar/plagioclase divariant surface (monzogranite and syenogranite).

RB-SR AND SM-ND ISOTOPIC VARIATIONS IN DISSECTED CRUSTAL XENOLITHS

Abstract Four crustal xenoliths from Scotland have been dissected in order to evaluate the effects of magma-xenolith interaction on Rb-Sr and Sm-Nd isotopic systematics, and hence the use of the xenoliths for accurate determination of source compositions. Each xenolith was sampled from the edge to the center. The Nd isotopic compositions are virtually uniform across each xenolith; however, there are significant variations in Rb, Sr, and REE concentration, as well as Rb Sr and Sm Nd ratios and Sr isotopic composition. Most of this variation appears to be inherited from the protolith. However, one sample shows systematic variations in Rb and Sr concentration and Sr isotopic composition with distance from the xenolith edges due to interaction with the host magma. This sample displays petrographic evidence for grain boundary melt infiltration. These data are consistent with theoretical considerations of transport times and diffusivities that place limits on the amount of modification that is possible via diffusion alone. The calculated effects of transport of even large amounts of crustal xenoliths on the compositions of xenoliths and host magmas will be small, provided transport times are short. Overall, the isotopic variations within the xenolith suite mirror the source rocks from which they were extracted and are inferred to reflect variations in the deep crust of Scotland. However, small samples, such as most crustal xenoliths, may be of little use in defining meaningful crustal residence ages because of modal variations in metamorphic minerals and isotopic re-equilibration. The one xenolith studied has extremely variable Sm Nd ratios due to the presence of garnet and yields a broad range of geologically meaningless Nd model ages, but yields an Sm-Nd isochron age of 396 ± 49 Ma. The Loch Roag xenolith has a large range of Rb Sr ratio, and the Sr isotopic data define an isochron age of 416±28 Ma. Both ages are taken to represent the timing of metamorphism and/or cooling from 600°C at the end of the Caledonian Orogeny in Scotland.

A Large and Brilliant Fire-ball Meteor

ON Sunday, the 14th inst., about 9h. 45m. p.m., I was entering my house by the back door, when the whole place was so brilliantly illuminated that I momentarily supposed there had been a flash of lightning. That erroneous impression was at once removed by the continuance of the light. Wheeling round, I saw a splendid meteor of the fire-ball type, descending obliquely through the sky. Though the Monday newspapers reported serious fog in London on the previous day, yet the night sky at Loughton was perfectly clear; and it was easy to see that the meteor in its descent was passing a little to the right of the constellation Gemini, in a direction nearly, but not quite, parallel to a line joining Castor and Pollux. The head, which was downwards, was a large oval mass of light. The tail was not a mere thread of silvery radiance, like those of November 1866; it seemed broad, irregular on the edges, and sending out sparks. The fire-ball had not descended far when it vanished among a shower of sparks, which also very speedily disappeared. I heard no rushing sound during its course, and no noise of an explosion when it came to its end.