Moine Metasediments Research Articles

Petrogenesis of the Loch B\xe0 ring-dyke and Centre 3 granites, Isle of Mull, Scotland

The Loch Bà ring-dyke and the associated Centre 3 granites represent the main events of the final phase of activity at the Palaeogene Mull igneous complex. The Loch Bà ring-dyke is one of the best exposed ring-intrusions in the world and records intense interaction between rhyolitic and basaltic magma. To reconstruct the evolutionary history of the Centre 3 magmas, we present new major- and trace-element, and new Sr isotope data as well as the first Nd and Pb isotope data for the felsic and mafic components of the Loch Bà intrusion and associated Centre 3 granites. We also report new Sr, Nd and Pb isotope data for the various crustal compositions from the region, including Moine and Dalradian metasedimentary rocks, Lewisian gneiss, and Iona Group metasediments. Isotope data for the Loch Bà rhyolite (87Sr/86Sri = 0.716) imply a considerable contribution of local Moine-type metasedimentary crust (87Sr/86Sr = 0.717–0.736), whereas Loch Bà mafic inclusions (87Sr/86Sri = 0.704–0.707) are closer to established mantle values, implying that felsic melts of dominantly crustal origin mixed with newly arriving basalt. The Centre 3 microgranites (87Sr/86Sri = 0.709–0.716), are less intensely affected by crustal assimilation relative to the Loch Bá rhyolite. Pb-isotope data confirm incorporation of Moine metasediments within the Centre 3 granites. Remarkably, the combined Sr–Nd–Pb data indicate that Centre 3 magmas record no detectable interaction with underlying deep Lewisian gneiss basement, in contrast to Centre 1 and 2 lithologies. This implies that Centre 3 magmas ascended through previously depleted or insulated feeding channels into upper-crustal reservoirs where they resided within and interacted with fertile Moine-type upper crust prior to eruption or final emplacement.

Accessory Mineral Chemistry of High Ba–Sr Granites from Northern Scotland: Constraints on Petrogenesis and Records of Whole-rock Signature

The Rogart and Strontian high Ba–Sr plutons (Northern Highlands, Scotland) comprise a range of lithologies from felsic to ultramafic rocks. The latter are mantle-derived and their differentiation to produce the felsic components of the plutons is the result of fractional crystallization and variable assimilation of the surrounding Moine metasediments. New results presented here demonstrate that accessory mineral chemistry can provide further insight into their petrogenesis and highlight the petrological potential of apatite and titanite. The main accessory minerals titanite, apatite and zircon contain most of the rare earth elements (REE) in the high Ba–Sr plutons. Results for apatite and titanite show that careful imaging and in situ trace element analysis provide constraints on the petrogenetic history of the host-rock. In both plutons, apatite and titanite record in situ crystallization and fractionation. In Strontian, both apatite and titanite from the granitoids record a mixing event with mafic magma in their rim compositions. Apatite and titanite chemistries are sensitive to the nature of their host-rocks (felsic versus ultramafic) and some elements (e.g. Sr, V) closely reflect whole-rock chemistry and the degree of fractionation. In some cases, whole-rock trace element concentrations can be calculated based on accessory mineral chemistry. Thus, trace elements in accessory minerals can give direct access to the nature and crystallization history of plutonic rocks. This petrological tool may be helpful in provenance studies using accessory minerals, and because high Ba–Sr plutons have recently been equated with Archaean sanukitoids, this might also be important in constraining the temporal distribution of this important magma type.

Petrogenesis of the Neoproterozoic West Highland Granitic Gneiss, Scottish Caledonides: Cryptic mantle input to S-type granites?

The Neoproterozoic (c. 870Ma) West Highland Granitic Gneiss, exposed in the Northern Highlands Terrane of Scotland, has elemental characteristics that are strikingly similar to those of the host Moine metasediments, which are thus consistent with an origin involving major Moine melting. Most of the constituent bodies have compositions significantly removed from minimum melts of pelites, and trace element constraints suggest variable but significant restite entrainment leading to less silicic bulk compositions with enhanced REE, Zr and Y. However, initial Nd and Hf isotope ratios are not coincident with contemporary Moine and imply a significant juvenile contribution. Close association with a regional suite of metabasites prompts consideration of mafic magma input, for which binary mixing models offer qualitative support. Quantitative difficulties with typical Moine metasediments are eased with radiogenic pelites or by partial melting of the mafic component. A possible alternative is currently unexposed Grenvillian felsic crust. Subsequent interaction of the granitic gneisses with meteoric water has significantly perturbed the oxygen and Sr isotope systems, the timing of which is equivocal but probably occurred during Caledonian events.The elemental characteristics of the West Highland Granitic Gneiss show many similarities with Scandinavian (rift-related?) granites of the same age, but since their geochemistry is largely inherited from the protolith it would be unwise to pursue palaeotectonic attribution on this basis. However, the probable incorporation of significant mantle-derived mafic magma of MORB-like affinity is consistent with an extensional setting.

The Cluanie granodiorite, NW Highlands of Scotland: a late Caledonian pluton of trondhjemitic affinity

Synopsis The Cluanie Pluton is a late Caledonian granitoid emplaced into the Glenfinnan Division of the Moine Supergroup in the NW Scottish Highlands. A field investigation of the pluton and its internal facies is presented along with new major- and trace-element whole-rock XRF analyses, and geobarometric and geothermometric studies. Cluanie is predominantly composed of hornblende granodiorite characterized by varying concentrations of distinctive alkali feldspar megacrysts, with minor amounts of biotite granodiorite and rare mingled porphyritic microgranodiorite. The alkali feldspar megacrysts appear to be magmatic in origin. Rare spectacular pegmatitic concentrations most likely represent physical accumulation of the megacrysts. The pluton is geochemically a high Na/K trondhjemite, the only such pluton known among the Newer Granites of Scotland. On the basis of geochemical evidence and a comparison with partial melting experiments, we propose that the magmas were derived by fluid-rich melting of an amphibolitic source leading to relatively low temperature magmas which were significantly contaminated by Moine metasediments. The pluton was emplaced in the mid crust at about 4.3 kbar during an episode of dextral shear on the Glen Glass Fault related to regional strike-slip faulting on the Great Glen Fault system at c . 425 Ma.

The Glenelg-Attadale Inlier, NW Scotland, with emphasis on the Precambrian high-pressure metamorphic history and subsequent retrogression: an introduction and review

Synopsis The Glenelg-Attadale Inlier of the NW Highlands of Scotland represents the largest area of exposed basement within the Moine Supergroup and, thus, affords the best opportunity of studying the pre-Moine history and the relationship of the basement to the Moine. The Inlier can be divided into Western and Eastern Units, based on lithological associations, and the boundary is marked by a major ductile shear zone with a dominantly reverse (top-to-the-west) sense of shear. The presence of eclogites and of other eclogite facies rocks within the inlier is the only unequivocal examples of crustal eclogites exposed within the British Isles. Eclogites in the Eastern Unit (P–T c. 20 kbar and 750–780°C) has been radiometrically dated (Sm–Nd garnet–omphacite–whole rock) to c. 1082 Ma and their retrogression to amphibolite facies (P–T conditions: minimum 14–16 kbar and c. 650°C) have been dated (U–Pb zircon) to c. 995 Ma. This demonstrates that the Eastern Unit underwent tectonic thickening to c. 70 km during the Grenvillian orogeny and was decompressed into the mid-crust by the end of the Grenvillian orogenic cycle. Rare eclogites in the Western Unit appear to have been formed earlier in the Palaeoproterozoic ( c . 1700–1750 Ma) and their juxtaposition against the Eastern Unit eclogites may be fortuitous or may reflect a fundamental tectonic manifestation of Wilson Cycle tectonics in orogenic belts. The basal Moine shows a deformed unconformable relationship against the Western Unit. However, the Eastern Unit is entirely enveloped by profound shear zones against the Western Unit and the Moine Supergroup, suggesting that the Eastern Unit is allochthonous. Direct dating of mylonites within the shear zone between the Western and Eastern Units, containing coaxially sheared Moine metasediments, suggests an age of c. 670 Ma and this may reflect the time at which the units were juxtaposed and the Moine infolded. Final deformation and metamorphism of the Glenelg-Attadale Inlier and the Moine occurred during the Caledonian orogeny, with major folding and extensional shearing operating sometime after c. 437 Ma, resulting in distinctive fold interference patterns. Terminal thrusting of the Moine and GAI westward over the Laurentian Caledonian foreland occurred at c. 435–430 Ma.

Petrogenesis of high Ba–Sr plutons from the Northern Highlands Terrane of the British Caledonian Province

A variety of high Ba–Sr granites and syenites is intruded into the Caledonian terrane of the Northern Highlands of Scotland. Volumetrically rather small, the plutons display a considerable range of compositions from ultramafic and mafic to granitic and syenitic. Most intrude Proterozoic Moine metasediments, though some are exposed within the Moine Thrust Zone in the west, beyond which lies the Archaean Lewisian foreland. Although it is generally agreed that they are related in some way to the subduction of Iapetus Ocean crust beneath Laurentia, their petrogenesis is much debated, especially the enigmatic relationship between alkaline syenites and calc-alkaline granites. New isotope (Sr–Nd–O) data suggest that both may be derived from mantle sources that include small fractions of subducted pelagic sediment. Juvenile material is volumetrically dominant, although subsequent fractionation involved contamination with local continental crust. Direct derivation from a mafic underplate is unlikely given the absence of pronounced HREE and Y depletion, the major element dissimilarity with adakitic magmas, and the presence of coeval and geochemically-contiguous mantle-derived rocks. The parent magmas were derived from a Caledonian Parental Magma Array (CPMA) that extended from isotopically depleted to significantly enriched compositions, with high δ 18O in the latter. The oxygen isotope values and the high Ba–Sr signature in particular strongly suggest that mantle contamination with subducted pelagic carbonates generated the CPMA, and Nd–Sr isotopes constrain the sediment contribution to less than 10%. Differentiation of the parent magmas proceeded by fractional crystallisation and variable concurrent assimilation of small magma batches, involving different crystallising assemblages and different continental reservoirs. Caledonian syenites evolved by extensive pyroxene-dominated fractionation sometimes involving contamination with Lewisian granulites. On the other hand, the granites fractionated amphibole and plagioclase, during which most were contaminated with local Moine metasediment. The extended time interval over which the syenites were intruded (30 Ma) accords with continuing Iapetus subduction beneath the Laurentian margin, while the late pulse of isotopically-variable appinites and granites at about 425 Ma sits comfortably with Scandian slab breakoff. However, the high Ba–Sr granites studied here represent small-volume assimilation-fractionation products of mantle-derived appinitic magmas, rather than the voluminous melts of lamprophyre-underplated crust also predicted by slab breakoff, which are absent from the Northern Highland Terrane.

Flow perturbation folding in shear zones

Abstract Variable patterns of displacement in shear zones may result in arcuate fold and fault traces as recognized by Mike Coward and co-workers in the early 1980s. Within such deformation zones, localized perturbations in flow may undergo acceleration (surging flow) or deceleration (slackening flow) with respect to the adjacent regions. Such flow cells may govern the orientation and geometries of folds and fabrics and thereby provide evidence of the scale and nature of deformation associated with heterogeneous flow in the high-strain zones. The length/width ratio of individual flow cells (measured in the direction of flow) may vary from <1 to >1 for situations when flow cells are, respectively, dominated by layer-parallel shear (LPS) or layer-normal shear (LNS). Folds initiating at high angles to transport are associated with LPS, whereas LNS may generate folds with slight clockwise (sinistral LNS) or anticlockwise (dextral LNS) trends relative to flow. Continued progressive deformation may subsequently modify and reduce angular relationships between folds and fabrics, but these geometric obliquities are generally preserved. Using examples from the Moine metasediments of northern Scotland, we show that folding displays predictable geometric patterns that can be related to the development of flow perturbation cells associated with Caledonian ductile thrusting under mid-crustal conditions. The differing relative timing of folds and individual ductile thrusts reflects the complexity of flow cells within ductile imbricates and additionally highlights the progressive foreland-directed propagation of ductile thrusting. These geometric relationships developed during contractional shear are compared with those generated in extensional systems, to provide an overall framework for the study of perturbation patterns. The geometric arrangement of mean fold axial planes about the flow direction results in their intersection forming parallel to the transport direction. This relationship permits transport directions to be calculated via the axial-planar intersection method (AIM), and also allows comparison with other techniques devised primarily for the study of soft-sediment deformation and palaeoslope analysis.

Shear zone folds: records of flow perturbation or structural inheritance?

Abstract Deformation within shear zones can be both temporally and spatially variable, resulting in multiple generations of folds which display a range of scales and overprinting relationships in mylonitic rocks associated with high strain zones. Despite such complexities, two main fold associations are broadly recognized in many shear zone settings: early tight to isoclinal sheath folds, often with mylonitic limbs that are post-dated by one or more local generations of synshearing folds which are preserved within, or root downwards into mylonitic high strain zones. These latter structures locally fold the mylonitic foliation and lineation whilst displaying geometric characteristics that are kinematically compatible with the movement regime of the major shear zone. Using examples related to ductile thrusting in Moine metasediments of north Scotland, we show that both types of fold display predictable geometric patterns on fabric topology plots. Fold axes and axial surfaces display consistent changes in asymmetry and sense of obliquity relative to local, transport-parallel mineral lineations that can be used to map out a series of culminations and depression zones. The sheath folds preserve more acute, but almost identical geometric patterns compared to the later synshearing folds, with culmination and depression zones often coinciding in location and scale. Detailed analysis also demonstrates that the distribution of finite strain is systematically linked to the architecture of all folds and that clear and predictable relationships exist between the fabric topologies of both the sheath folds and synshearing folds. These consistent topological relationships could be explained in terms of a fold evolution model , where sheath folds represent a more highly deformed and evolved variety of synshearing folds originally generated during perturbations in ductile flow. However, an alternative fold inheritance model predicts that the gross structural architecture generated during sheath folding may subsequently control the geometry and govern the orientation of the synshearing folds. Both models may be widely applicable in a broad range of shear zone environments.

The Life of Frank Cole Phillips (1902-1982) and the Structural Geology of the Moine Petrofabric Controversy

The Life of Frank Cole Phillips (1902-1982) and the Structural Geology of the Moine Petrofabric Controversy

Chromian illite-ankerite-quartz parageneses from the Kintail district of southern Ross-shire, Scotland

Abstract The Coire Dhuinnid fault zone contains emerald green chromian illite-ankerite-quartz rocks that are similar in appearance to the fuchsite(or mariposite)-carbonate-quartz parageneses that are commonplace in Archaean greenstone belts but which are rather rare in Phanerozoic rocks. The chromian illite contains 2.3 wt.% Cr2O3, low K2O (7.1–7.6 wt.%) and high H2O+ (5.7 wt.%), and it is a 1M polytype with ≤10% of an illite/smectite interstratification indicative of a formation temperature of c. 175–200°C. The host rocks contain high concentrations of Ni and Cr, and show low concentrations of Ti, Nb, Y and Zr, suggesting a former primitive mafic protolith (boninitic magma?); they are considered to be retrograde remnants of Lewisian rocks. The latter, and the associated rocks of the Moine Series, have been affected by CO2 metasomatism that was accompanied by the addition of Ca(+Sr), Fe and Mg, and by the removal of Na from, and the addition of H2O to the Moine metasediments. Radiogenic isotope studies of mineral separates and whole rock from sample no. 43 yielded ages of 483±2 Ma (Ar-Ar dating on Cr illite), 413±12 Ma (K-Ar dating on Cr illite), and 322±9 Ma (Rb-Sr dating on minerals and whole rock); the significance of this discrepant pattern is discussed.

The petrogenesis of Tertiary cone-sheets in Ardnamurchan, NW Scotland: petrological and geochemical constraints on crustal contamination and partial melting

Abundant cone sheets form one of the last magmatic stages in the Tertiary central complex on the Scottish peninsula of Ardnamurchan and can be grouped into a younger inner and an older outer suite relative to a gabbro intrusion. Most of the cone-sheets consist of tholeiitic to transitional basalt with MgO contents between 7.5% and 4%, although more evolved rocks also occur (to 0.5% MgO). The mafic samples are slightly enriched in the light rare earth elements (Chondrite-normalized La/Sm ∼1.1), the enrichment increases in the more evolved rocks. The compositional variation of the basaltic rocks is mainly due to crystal fractionation of olivine and clinopyroxene at depths of ∼10 km but trace elements show simultaneous assimilation of Archean Lewisian granulite crust. The andesitic to rhyolitic lavas formed by fractional crystallization from the contaminated basaltic magma coupled with assimilation of Proterozoic Moine metasediments at uppermost crustal levels. The occurrence of composite cone-sheets with basaltic and rhyolitic parts and mixtures between these magmas implies that the melts ascended successively but within a short period of time. The parental magmas of the Ardnamurchan cone-sheets must have formed at relatively shallow depths in the mantle and are comparable to the youngest tholeiitic lavas from the neighbouring island of Mull.

The Loch Scridain Xenolithic Sill Complex, Isle of Mull, Scotland: Fractional Crystallization, Assimilation, Magma-Mixing and Crustal Anatexis in Subvolcanic Conduits

The petrogenesis of a suite of high-level minor intrusions which intruded the Palaeogene lava field and older rocks in the vicinity of Loch Scridain, Isle of Mull, NW Scotland, has been investigated using mineral chemistry data together with whole-rock major and trace element and radiogenic Sr–Nd–Pb isotope data. Three distinct magma groups are present. Group I consists of aphyric tholeiitic basalts and basaltic andesites, the most primitive of which have MORB-like chemical affinities. Group I rocks are markedly xenolithic, containing both crustal and cognate gabbroic xenoliths. Group II comprises plagioclase- and pyroxene-phyric andesites and dacites. Group III consists solely of fine-grained rhyolites. Compositional trends displayed by the Group I basic magmas can be explained in terms of relatively low-pressure fractionation of the assemblage olivine + clinopyroxene + plagioclase, followed by plagioclase + clinopyroxene + low-Ca pyroxene. The extreme incompatible-element enrichment, the high initial Sr isotope ratios, and radiogenic Pb isotope ratios of the more evolved members of Group I can, however, only be explained through a process of combined fractional crystallization and crustal assimilation of the underlying Moine pelitic schists. The isotope and trace element data imply that the contaminant was a high-Nd partial melt of Moine metasediments rather than a bulk addition of Moine pelite. The major, trace element and Sr–Nd isotope geochemistry of the Group III rhyolites is consistent with their derivation mainly from the partial melting of a pelitic crustal source. The geochemical characteristics of the Group II intermediate rocks suggest that they represent simple mixtures between basic magma and Group III silicic melts.

Precambrian ammonium biogeochemistry: a study of the Moine metasediments, Scotland

The content and isotopic composition of ammoniacal nitrogen have been determined in a suite of amphibolite facies metasediments of Precambrian age. Whole-rock samples and mica separates from the Moine Succession (Proterozoic) of Scotland were analysed by a combination of capacitance manometry and static vacuum mass-spectrometry. Prior to the analyses, the presence of the ammonium ion was confirmed by obtaining infrared (IR) absorption spectra from the micas, principally biotites. Whole-rock ammonium contents are high for amphibolite facies metasediments (140–422 ppm). The ammonium ion was not detected in the IR absorption spectra of biotites from one schist; however, this sample contains neither garnet nor white mica. For most of the samples, the δ 15N values are very positive, between +12.4 and +16.6‰, with one sample having a lower value of +8.4‰. The major variations between samples can be interpreted in terms of original sedimentary features. For the ammonium-bearing samples, muscovites have concentrations between 365 and 633 ppm, biotites between 845 and 1739 ppm. For individual samples, [NH 4 +] Mu/[NH 4 +] Bi varies between 0.31 and 0.43, a much narrower range than has been reported previously. There are no significant or consistent differences in δ 15N between co-existing biotite and white mica, nor between the δ 15N values of the mineral separates and their respective whole-rock values. The latter suggests minimal selective partitioning of isotopes between micas and plagioclase feldspar, the only other potential host for ammonium in the rocks. Assuming that the ammonium has been derived from organic matter originally present at the time of deposition, and by comparison with previous studies, it can be inferred that the original δ 15N value of the Moine sediments was somewhere between +5 and +10‰, similar to recent sediments. This suggests that the nitrogen cycle in the Proterozoic shelf-sea, from which the Moine sediments were deposited, was broadly similar to the present day. Overall the study underlines how under-developed the N-system is compared to other stable isotope systems, notably C and S, and we use the opportunity to highlight some areas that could be developed in the future. However, one aspect is clear: future ammonium studies of metamorphic rocks will need to be performed at high resolution (visible scale of heterogeneity), with strong mineralogical, petrological and field controls.

U-Pb zircon geochronological evidence for Neoproterozoic events in the Glenfinnan Group (Moine Supergroup): the formation of the Ardgour granite gneiss, north-west Scotland

The age and Precambrian history of the Moine Supergroup within the Caledonide belt of north-west Scotland have long been contentious issues. The Ardgour granite gneiss is essentially an in situ anatectic granite formed during deformation and regional high-grade metamorphism from Moine metasediments. High-precision TIMS and SHRIMP U-Pb zircon dating shows that the age of the anatectic Ardgour granite gneiss and its enclosed segregation pegmatites is 873 ± 7 Ma. This demonstrates the reality of a Neoproterozoic episode of high-grade metamorphism in the Glenfinnan Group Moine and, contrary to previous evidence, the absence of Grenvillian-aged metamorphism. This conclusion places constraints on Neoproterozoic palaeogeographic reconstructions of the North Atlantic region, indicating that the Moine rocks cannot be used as a link between the Grenvillian belt of North America and the Sveconorwegian orogen in Scandinavia. SHRIMP ages of between c. 1100 and 1900 Ma were obtained from detrital, inherited zircons and reflect the provenance of the Glenfinnan Group Moine sediments which must, therefore, have been deposited between c. 1100 and 870 Ma. Potential sources are found as relatively minor, tectonically bounded basement inliers within the British Caledonides, although more widespread source areas occur outside Britain in both Laurentia and Baltica. The most important feature of the provenance is the absence of detrital Archaean grains. This suggests that the Archaean Lewisian gneiss complex, which forms the basement component of the western foreland to the Caledonides in Britain, was not a major contributor to the Glenfinnan Group basin.

Elemental and O-Sr-Nd isotope geochemistry of the Glen Dessarry syenite, NW Scotland

Elemental, radiogenic isotope (Sr, Nd) and stable isotope (O) data from samples of the 456 Ma Caledonian Glen Dessarry syenite are all compatible with a two-stage crystal-fractionation model for the evolution of the magmas, locally modified by contamination with a partial melt of the surrounding Moine metasediments. The early-crystallizing phases which controlled melasyenite-leucosyenite evolution were biotite, calcic clinopyroxene plus minor apatite and titanite, whilst in-situ crystal-liquid separation also involved alkali feldspar and amphibole with minor allanite, zirconolite and zircon. The diverse accessory mineral suite controlled significant variations in many of the ‘incompatible’ elements, including inordinate Zr/Hf fractionation. Crustal contamination led to severe disturbance in abundances and inter-element ratios of U, Th; Zr, Hf; Nb, Ta; heavy REEs, Sr and Ba. The elemental characteristics of the magmas are typical of shoshonitic subduction-related suites, with high LILEs, relatively low HFSEs and pronounced negative Nb-Ta anomalies. Inferred isotopic characteristics of the parent magmas ( 87 Sr/ 86 Sr ≈ 0.7040, ΣNd = + 2.5 and δ l8 O ≈ + 7%o) are probably derived from a multicomponent source region involving a substantial input from contemporary depleted mantle.

Elemental evidence for crustal contamination of mantle-derived Caledonian syenite by metasediment anatexis and magma mixing

This paper presents elemental evidence that the mafic unit of the Caledonian Glen Dessarry syenite (northern Scotland, U.K.) has been variably contaminated with material derived from the surrounding Moine metasediments. Uncontaminated portions are shown to have trace-element abundances and ratios similar to those of the central leucosyenite, and island-arc high-K or shoshonitic magmas from present-day oceanic settings. By comparison, contaminated samples display grossly disturbed trace-element patterns, including higher U, Th; Zr, Hf; Nb, Ta; and heavy rare earths; lower Ba and Sr; and negative Eu anomalies. These features are matched in the elemental systematics of the Moinian envelope, but bulk incorporation of excessive amounts of metasediment would be required to reproduce the observed perturbations in, especially, the high field-strength elements. Xenoliths within the contaminated mela-syenites, and parts of the adjacent country rock show petrographic evidence of melting. Since the source composition (Moine metasediment) and residual mineralogy (xenoliths) are known, simple geochemical models of melting and subsequent mixing with uncontaminated melasyenite melt have been computed. The results provide close approximations to the observed geochemistry of the contaminated melasyenites, particularly for the REE's. Moreover, the residues of this hypothetical process have similar REE patterns to several samples of metasediment from immediately adjacent to the syenite contact. Nevertheless, contemporaneous crystal-liquid separation is required to explain the full range of elemental data. Contamination by this mechanism of melting followed by magma mixing is not immediately apparent in the major-element relationships of the two melasyenite groups. This seems to be the result of the approximate colinearity of the established melasyenite crystal fractionation path and the proposed “granitic” contaminant, and may be a more general problem in the recognition of in situ contamination with metasedimentary melts in plutons whose fractionation itself proceeds towards a broadly granitic terminus.

The stratigraphy, structure and regional significance of the Moine Rocks of Mull, Argyllshire, W. Scotland

AbstractTwo right‐way‐up Moine lithostratigraphic units—the Shiaba (older) and Assapol (younger) Groups—are distinguished in the Ross of Mull. These were intruded in the west by the post‐tectonic, Ross‐of‐Mull granite complex at 414 ± 3 Ma. Apparently undisturbed inclusions of Moine metasediments within the granite permit the boundary between the lithostratigraphic units to be followed westward almost to the supposed trace of the Moine thrust in the Sound of Iona. The Shiaba and Assapol Groups which have a transitional, albeit attenuated, stratigraphic contact are correlated with the Morar and Glenfinnan Divisions of Inverness‐shire, the Sgurr Beag ductile thrust (slide) normally found at the Morar/Glenfinnan Division boundary being absent. This implies that the stratigraphic relationship between these two divisions, which is elsewhere obscured by the thrust, is uniquely preserved on Mull.Within a local, D1‐D4 sequence, D2 and D3 structures are dominant. Originally subhorizontal D2 folds are intensely curvilinear on all scales about an originally flat‐lying, NNW‐SSE trending stretching lineation (L2). In sections parallel to L2, D2 minor folds display a constant sense of vergence throughout the Ross‐of‐Mull Moine Rocks, overturning generally NW‐NNW. The present day structure is dominated by the almost upright, SSW‐plunging D3 Assapol synform which overprints all earlier structures. Tentative correlation of the deformation sequence with that of Inverness‐shire, suggests that the D1‐D2 structures of Mull, with accompanying moderate‐to‐high‐grade metamorphism, may be Precambrian, while the D3 Assapol synform may be Caledonian.The presence of migmatites of kyanite grade means that metamorphic grade, established during MS1‐MP2, is anomalously high for Moine rocks lying close to the supposed Caldonian front. This suggests that they may lie within one of the higher Caledonian thrust nappes of the North Highland Moine—possibly the Knoydart nappe, where metamorphic grade is comparable. The greenschist facies metamorphism and single phase of deformation affecting the ‘Torridonian’ rocks of Iona presents a significant contrast to the Moine rocks of the Ross of Mull. A major fault in the Sound of Iona is implied, but the Moine thrust itself probably does not outcrop.

Field evidence for turbulence during flow of basalt magma through conduits from southwest Mull

AbstractThe swarm of Palaeocene inclined sheets around Loch Scridain in southwest Mull includes basaltic sheets which contain abundant metasedimentary xenoliths that show evidence for having been partially molten. In contrast with the majority of dyke-like conduits worldwide, which have essentially planar margins, the contact of one of these xenolithic sheets with Moine metasediments is irregular along a segment where it crosses interbedded psammites and pelites. Relatively fusible mica schists have been excavated by the magma, on either side of a massive refractory quartzite horizon. Furthermore, the dolerite of the sheet shows only a slight reduction of grain size, through a zone 0.5 m wide, adjacent to its margin, in contrast with the pronounced marginal chilling which characterizes most intrusions of the Loch Scridain swarm. These field relationships are interpreted as evidence that the basic magma which formed this sheet flowed with local turbulence during its emplacement. The estimated liquidus of the magma (1130 °C at 1 atm) is approximately 200 °C above the solidus of pelitic horizons in the country rock, but far below the solidus of the quartzites.

Two contrasting styles of interaction between basic magmas and continental crust in the British Tertiary Volcanic Province

Some of the magmas that were extruded to form the 59 Ma, early, plateau‐forming, basalt‐dominated floods in the British Tertiary Volcanic Province mixed with small amounts of sialic melt during their uprise through the continental crust, while others underwent concomitant fractional crystallization and crustal assimilation. Magma batches of the first group originated in the uppermost asthenosphere and mostly ponded at the Moho, where they crystallized fractionally to form a basalt‐benmoreite suite. The major element compositions of members of this series show that they underwent negligible further fractional crystallization between the Moho and the surface. Nevertheless, they paused long enough during their uprise to dissolve their high‐pressure phenocrysts and, in many cases, to mix with small amounts of acid melt from lower crustal, granulite‐facies, Archaean Lewisian leucogneisses. Basalts and tholeiitic andesites of the second group outcrop locally at the base of the lava piles in Mull and Skye. Their major element compositions show that they equilibrated within the upper third of the crust, under conditions approximating to anhydrous 1‐atm cotectic equilibria. Most of these lavas are in SW Mull, where Proterozoic Moine metasediments form the uppermost crust. The major and trace element and isotopic compositions of the SW Mull basal lavas show that they assimilated substantial amounts of Moine metasediments progressively as they fractionated within the upper crust. Pb isotope data reveal that, before their final upper crust evolution, some of the SW Mull basal lavas had already mixed with small amounts of lower crust acid melts.

The structural setting and tectonic significance of the Glen Dessary Syenite, Inverness-shire

The Glen Dessary Syenite crops out in the steep belt of the Northern Highland Moine. It is enclosed within a psammite envelope which was previously assigned to the Glenfinnan Division, but which is here correlated with the psammites of the Loch Eil Division exposed to the E of the Loch Quoich Line. Three episodes of deformation have been identified in the Moine metasediments of the Glen Dessary area, but only the latest (D 3 ) is thought to have deformed the syenite, imposing on it a strong, steeply plunging mineral lineation. The syenite is thought to occupy the core of a large, intensely curvilinear, D 3 synform. The D 3 structures which deform the syenite can be correlated with upright structures, which reworked previously flat-lying, already crystalline rocks, recognized at Loch Quoich. Upright reworking must be younger than the Glen Dessary Syenite, dated by U-Pb methods at 456 ± 5 Ma. Two, hitherto unrecorded, small bodies of granitic gneiss, similar to the c. 1020 Ma Ardgour Gneiss, have been identified in psammites of the Loch Eil Division near the W end of Loch Arkaig. Their presence suggests that the Loch Eil Division has a Grenville history, and is part of the 'Old Moine'.