Neil F. C. Hudson

Pelite facies series and the temperatures and pressures of Dalradian metamorphism in E Scotland

Summary An attempt is made to synthesise data on the regional metamorphism of pelites in the eastern Scottish Dalradian. Zonal sequences (facies series) of mineral assemblages are presented and variants of the traditionally recognised Barrovian and Buchan metamorphism are separated. A Schreinemakers net and petrogenetic grid for the system KFMASH is presented and fixed in P-T space using experimental data. The P-T data for the various facies series are used in conjunction with other constraints to develop a model of isotherm and isobar distribution in the region, and the effects of post metamorphic folding of the isopleths are noted. P-T gradients normal to isobars suggest overall ‘geothermal’ gradients for most of the area to be convex to the T-axis, and it is suggested that they result from magma intrusion at depth. The region adjacent to the Highland Boundary fault shows strong horizontal and vertical temperature gradients which are tentatively interpreted to indicate the presence of a synmetamorphic tectonic boundary to the Dalradian metamorphic belt.

Evidence for a genetic granite–migmatite link in the Dalradian of NE Scotland

The Inzie Head gneisses of the NE Dalradian are syntectonic metapelitic migmatites containing numerous sill-like bodies of granite. Leucosomes preserve evidence for efficient deformation-controlled segregation and interconnection of melt, from intergranular pockets to sheets of leucogranite hundreds of metres thick. Although the physico-chemical system is complex, the geochemistry of in situ decimetre-scale leucosomes and discrete metre- to decametre-scale leucogranite sheets within the migmatites suggests derivation from a common metapelitic source. A comparison with spatially and temporally related Grampian Granite plutons and garnet-bearing aplites that occur at shallower crustal levels supports a genetic granite–migmatite link. The compositions of the granitic rocks are interpreted to be linked via fractional crystallization of a parental magma and escape of the fugitive melt from a cumulate residue. Leucogranite sheets within the migmatites have compositions that are similar to experimentally determined first-formed melts at the appropriate P–T conditions, and are interpreted to approximate parental liquids. Smaller-scale leucosomes contain the accumulated products of fractional crystallization and variable quantities of entrained solids, principally biotite. The Grampian Granites and aplites represent evolved liquids that segregated from source. The evolution of Grampian Granite melts involved varying degrees of K-feldspar-dominated fractional crystallization that is consistent with magma ascent; removal of a maximum of 15–20% cumulate material is implied. The composition of the aplites suggests a multistage evolution that involved plagioclase-dominated fractionation of a fugitive melt batch.

Melt segregation structures within the Inzie Head gneisses of the northeastern Dalradian

Synopsis The Inzie Head gneisses of the NE Dalradian are syntectonic metapelitic schollen migmatites containing numerous sill-like bodies of granite and diorite. Leucosomes with clear evidence of melt-present crystallization exhibit a wide variety of segregation structures. Initial in situ melting occurred within, or at the margins of, fertile metasedimentary horizons and was strongly controlled by variations in bulk-composition. Intergranular melt-pockets coalesced into thin (<10 mm), discontinuous stromatic horizons which subsequently joined laterally and/or became interconnected in extensional shear-zones. A diktyonitic structure formed, enabling buoyancy aided upward migration of melt into low-strain sites such as boudin-necks and shear-zones. Metre-scale leucosome channels containing abundant schollen and having diffuse, unchilled contacts against the migmatites were supplied from this network and may represent the foci of H2O-rich fluid influx. Bodies of nebulitic granite contain abundant ‘ghost’ schollen in the final stages of assimilation. Diorite sills show extensive magma mingling and hybridization with granitic leucosome and were clearly contemporaneous with the crustal melting. A variably developed flattening fabric within the migmatites was in part due to inflation following intrusion of the diorites. Immediately beneath diorite intrusions, melt loss from the migmatites was pronounced indicating that pure-shear dominated deformation is a strong driving force for melt segregation. Garnetiferous aplites at higher crustal levels and contemporaneous with the peak of regional metamorphism represent melt frozen in channelways that potentially contributed to nearby, large-scale Grampian granitic bodies.

Mineral Associations in Diamonds from the Lowermost Upper Mantle and Uppermost Lower Mantle

The chapter reviews the worldwide occurrence of inclusions in diamonds which involve Si-rich minerals and which appear to come mainly from depths in the range ca 550 to 800 km. They are referred to collectively as the Perovskite and Periclase Suite. Attention is focussed upon distinguishing retrograde and primary minerals, and upon identifying associations of different primary minerals within single diamonds; this provides potential equilibrium inclusion assemblages whose depths of formation may be estimated with reference to experimental studies. Associations in the same diamond of separate inclusions of (Mg,Fe)SiO3, (Mg,Fe)O and (Mg,Fe)2SiO4—potentially representing the original phases MgSi-perovskite(mPv), ferropericlase (fPer) and ringwoodite (rw)—indicate formation at the Upper/Lower Mantle boundary. Associations involving MgSi-perovskite and ferropericlase, without ringwoodite, are taken to indicate Lower Mantle assemblages of ultrabasic bulk composition, and these are divided into two: those with low-Al MgSi-perovskite, mPv, from the shallowest Lower Mantle, and those with high-Al MgSi-perovskite, mPv(Al), from greater depths. In assemblages of basic bulk composition, the primary phases of mPv(Al), sodic majoritic garnet (maj-grt), new Al–silicate phase (NAL) and the calcium ferrite structured phase (CF) are all represented by composite inclusions, which include a variety of retrograde products such as olivine, spinel, tetragonal almandine-pyrope phase (TAPP), NaAl-rich pyroxene phase (NaAl-pyrox) and nepheline. In both ultrabasic and basic bulk compositions, the principal Ca-bearing phase appears to be CaSi-perovskite (cPv). The chemical compositions of the primary phases show considerable coherency, including relatively constant Fe–Mg partition coefficients. The postulated mineral assemblages conform well with experimental investigations, and a series of different depths of formation are indicated. In the inclusion assemblages of basic bulk composition, a transition from assemblages with sodic majoritic garnet (maj-grt) to ones with mPv(Al), NAL and CF is believed to approximately coincide with the change from mPv to mPv(Al) in ultrabasic assemblages. The eight minerals mPv–mPv(Al), fPer-mW, maj-grt, NAL, CF, cPv, corundum (crn) and stishovite (stv) [where mPv–mPv(Al) and fPer-mW indicate solid solutions] potentially define an invariant point in the 6-component composition space MgO–FeO–Al2O3–SiO2–CaO–Na2O, giving rise to a series of univariant and divariant mineral assemblages in pressure–temperature space. Consideration of the available experimental data indicates that this invariant point is probably in the pressure range 24–28 GPa under the range of temperatures expected in the uppermost Lower Mantle.

Geometry and population systematics of a quartz vein set, Holy Island, Anglesey, North Wales

Abstract A steeply dipping set of quartz-chlorite-muscovite-biotite veins was emplaced, at depths of at least 14 km and temperatures >400°C, into Môna Complex metasedimentary rocks of Holy Island, after the D4 event. Vein trends range from northeast-southwest to north-northwest-south-southeast. Vein offshoots, consistently oriented c. 10–35° anticlockwise relative to the main vein, are common. Quartz fibres within different veins show a range of orientations, from northwest-southeast to east-west, and are interpreted as tracking vein opening directions. Vein and fibre orientations are integrated into a four-stage model for vein emplacement. At each stage, new main veins either open extensionally, with fibres subnormal to their boundaries, or by hybrid extension-shear. Hybrid main veins commonly have extensional offsoots. Extensional main veins are orientated progressively further anticlockwise with time from a dominantly northeast-southwest trend to a dominantly north-south trend, but hybrid main veins have out-of-sequence orientations and probably utilized pre-existing fractures. The dominance of extensional and hybrid extension-shear vein opening indicates that the differential stress was small and pore fluid pressure was high. A study of length-thickness relationships has revealed that this vein set is self-affine with vein widening progressing more quickly than elongation during growth. Linear transect analyses show that the vein spacings have fractal characteristics.

Constraints on the timing of deformation, magmatism and metamorphism in the Dalradian of NE Scotland

Synopsis The Portsoy Gabbro was emplaced into Argyll Group Dalradian metasediments as part of a suite of mafic intrusives that outcrop around the low-P/high-T Buchan metamorphic domain in NE Scotland. This paper presents new field data together with geobarometry and calculated pseudosections for pelites and metabasites, and geochronological data from the Portsoy Shear Zone, on the western boundary of the Buchan domain, which constrain the relative and absolute timing of deformation, metamorphism and magmatism. Textural and cross-cutting relationships indicate the D2 event, magmatism and Buchan metamorphism overlapped. Pelitic schists containing all three Al2SiO5 polymorphs occur structurally beneath the Portsoy Gabbro. The sequence of aluminosilicate growth in these rocks is andalusite–sillimanite–kyanite, implying an anti-clockwise P–T–t path. This study indicates that the andalusite to sillimanite transformation took place during D2, synchronous with the intrusion of the Portsoy Gabbro, and was followed by kyanite growth after D2. The late growth of kyanite indicates that, at least initially, cooling was not associated with uplift. Magmatic zircons from the Portsoy Gabbro yield a U–Pb age of 471.3 ± 0.6 Ma. A U–Pb zircon age from an undeformed pegmatite that cuts the S2 foliation in the gabbro indicates D2 shearing had largely terminated by ~ 471 Ma. We demonstrate, using a metabasite pseudosection, that the titanite formed during deformation as the gabbros cooled. The cooling rate for the gabbro from 1000°C to 620°C was rapid – at least 49°C Ma−1 – and, allowing that the Portsoy Shear Zone was not an uplift structure, was caused by a change in the thermal regime.

On the deformation sequence in the New Harbour Group of Holy Island, Anglesey, North Wales

Four phases of deformation are recorded by minor structures in the New Harbour Group (NHG) of southern Holy Island. The regional schistosity in these rocks is a differentiated crenulation cleavage of D2 age. An earlier preferred orientation (S1) is commonly preserved as crenulations within the Q-domain microlithons of the S2 schistosity and is demonstrably non-parallel to bedding. F3 folds are widely developed in S2 and, to a lesser extent, in bedding. S3 crenulation cleavage is sporadically developed but can be intense locally. A major antiformal fold exists in the NHG near Rhoscolyn. This fold is of D3 age since it clearly deforms S2 schistosity and is consistent with the vergence of F3 minor structures. All planar structures are deformed by folds of D4 age.

On the origins of layered calc-silicate rocks from the Buchan Dalradian and their bearing on the diagenetic history of the Southern Highland Group

Synopsis Calc-silicate rocks from the Southern Highland Group of the Dalradian at Banff are metamorphosed carbonate concretions. Many original sedimentary features are preserved even though the calc-silicates now have amphibolite facies mineral assemblages. Those occurring in the andalusite and staurolite zones contain layered mineral assemblages that are characterized by the presence of calcite, clinopyroxene, hornblende, cummingtonite, and biotite from centre to margin. It is unlikely that the layering formed principally by synmetamorphic diffusional processes since diffusion modelling demonstrates that the relative values of phenomenological diffusion coefficients required to reproduce this set of layers are inconsistent with known values. The layer order, with pyroxene adjacent to carbonate and amphibole adjacent to pelite, is also inconsistent with their formation by infiltration of an H2O-rich fluid into a pure carbonate body. The layers can be best explained as resulting from metamorphism of compositionally zoned, diagenetic carbonate concretions. If the metamorphism was essentially isochemical, then composition profiles present in the calc-silicates may be compared with those in unmetamorphosed concretions and interpreted in terms of known diagenetic processes. A decrease in CaO from centre to margin results from the progressive elimination of the primary porosity during compaction of the sediment, a conclusion supported by the presence of relict sedimentary structures. Such calc-silicates contain important information about the early burial history of the sedimentary basin where their host rocks were deposited prior to the onset of metamorphism. There is some evidence that the alkali profiles have been modified during metamorphism. However it is not appropriate to use such calc-silicates for studies of synmetamorphic diffusion or other synmetamorphic processes without taking pre-metamorphic composition gradients into account.