Lewisian Granulites Research Articles

The Rum Igneous Centre, Scotland

AbstractThe publication of the British Geological Survey memoir on Rum and the Small Isles in 1997 was followed by a period of intense petrological and mineralogical research, leading to some 40 papers, books and other publications. The research progress since then is reviewed here and integrated with the information previously available to provide an overview of the current status of understanding of the centre. New data on the acidic and mixed acid/basic magmas of the early Rum caldera demonstrate that frequent mafic replenishments were the main driver for magmatic activity at Rum right from its initial stages. The caldera is bound by the Main Ring Fault, a structure which probably also exercised an influence on the emplacement of the subsequent basic and ultrabasic intrusions. The later emplacement of gabbros and ultrabasic rocks caused only limited thermal metamorphism of the surrounding Torridonian sandstones, contrasting markedly with the crustal isotope signatures of the early intracaldera ignimbrite magmas and the intense alteration of uplifted masses of Lewisian gneiss within the ring fault. Rare picritic dykes provide an indication of the possible parent magma for the mafic and ultrabasic rocks, but these, as with most other magmatic rocks on Rum, have undergone varying degrees of crustal contamination, involving both Lewisian granulite and amphibolite-type crust but, notably, no Moine metasedimentary compositions as is the case at the nearby Ardnamurchan centre. Detailed textural studies on the gabbroic and ultrabasic rocks allow a distinction between intrusive peridotites and peridotite that forms part of the classic layered cumulate units of Rum and, furthermore, this work and that on the chromite seams and veins in these rocks shows that movement of trapped magma and magma derived from later intrusions, may produce textures regarded previously as of primary cumulate origin. Sulfides in the chromitite seams and ultrabasic rocks, in turn, show possible influences from assimilated Mesozoic sediments. Igneous activity on Rum was short-lived, possibly only between 0.5 and 1 m.y. in duration and commenced at ∼60.5 Ma. The Rum Central Complex was extinct by the time the main activity at the nearby Skye Central Complex commenced (∼59 Ma). From recent apatite fission-track studies it seems probable that Rum, in common with other Palaeogene centres, underwent a brief, but significantly later heating event (∼45 Ma).

Geochemical constraints on the composition of Archaean lower continental crust: Partial melting in the Lewisian granulites

New geochemical data for the Lewisian granulites of NW Scotland show that the protolith to the neoArchaean Scourie granulites was a tonalite containing a mafic hydrous phase, most probably hornblende. This observation when combined with recent thermobarometric calculations and new partial melting studies on the Lewisian makes it very probable that the Lewisian granulites experienced fluid-absent melting. Partial melting calculations based upon a proxy for the unmelted granulite protolith demonstrate that Scourie granulite protolith was depleted in both LILE (Rb, Th, U) and HFSE (Ta) relative to unmelted amphibolite facies Lewisian gneisses of the same age. Both granulite facies and amphibolite facies gneisses are members of the tonalite-trondhjemite-granidiorite (TTG) magmatic suite. Nb/Ta ratios in the TTG granulite protolith are suprachondritic and can be attributed to the partial melting of a basaltic source with a rutile eclogite residue, whereas the amphibolite facies TTG gneisses possess Nb/Ta ratios which are subchondritic and can be attributed to the partial melting of a basaltic source with a garnet amphibolite residue. However, these differences do not account for the differences in LILE between the two suites implying that in addition the two basaltic precursors were different. It is proposed that the depleted character of the tonalitic protolith to the Scourie granulites was inherited from ultra-depleted basalt with very low concentrations of Rb, Th, U and Ta. Felsic melts formed during partial melting are no longer present in the granulite terrane and were probably removed to the upper crust, now removed by erosion. Thus partial melting of the Lewisian granulites contributed to the process of crustal fractionation. The process of fluid-absent melting in the Lewisian took place ca 200Ma after crust formation. This long time interval indicates that the stabilisation and differentiation of the crust was probably a separate event from that of crust formation.

Recent research developments on the Isle of Rum, NW Scotland

The appearance in 1997 of the British Geological Survey's memoir on Rum was followed by a period of intense research, leading to upwards of 35 papers, books and other articles. The scope of these publications, and the research progress over the last 15 years since publication of the memoir, is reviewed here. Igneous activity on Rum was short lived, possibly only ca. 500 ka, and, at about 60.5 Ma. The Rum central complex thus pre-dates the nearby Skye central complex. The earliest, acidic and mixed acidic/basic magmatism on Rum involved both shallow intrusions and ignimbrite eruptions into a collapsing caldera bound by the Main Ring Fault, a structure which probably also exercised a structural influence on subsequent mafic and ultrabasic magmatism. Subsequent emplacement of gabbros and ultrabasic rocks caused only limited thermal metamorphism of the surrounding Torridonian sandstones, contrasting markedly with the intense alteration of uplifted masses of Lewisian gneiss within the ring fault. Detailed textural studies on the gabbroic and ultrabasic rocks allow distinction between intrusive peridotites and peridotite that formed as part of the classic layered units of Rum and, furthermore, this work and that on the chromite seams and veins in these rocks shows that movements of trapped magma and magma derived from later intrusions, may produce textures and structures hitherto regarded as primary features of cumulate rocks. Rare picritic dykes provide an indication of likely parent magma for the mafic and ultrabasic rocks, but these and other magmatic rocks on Rum have all undergone varying degrees of crustal contamination, involving both Lewisian granulite and amphibolite crust but, notably, not Moine rocks as at Ardnamurchan. Sulphides in the chromite seams and ultrabasic rocks show possible influences from assimilated Jurassic sediments. From recent apatite fission track studies it seems likely that Rum, in common with other Palaeogene centres, underwent a brief, but significantly younger (Mesozoic) heating event.

The trace element geochemistry of clinopyroxenes from pyroxenites in the Lewisian of NW Scotland: insights into light rare earth element mobility during granulite facies metamorphism

Clinopyroxenes from layered pyroxenites and from pyroxenite pods in felsic gneisses of the Lewisian granulite complex, NW Scotland, have distinctive chemistries suggestive of different origins. Clinopyroxenes in the layered pyroxenites crystallised from mafic melts in a magma chamber located in the middle to shallow crust, whereas clinopyroxenes in pods in the felsic gneisses crystallised from the tonalitic protolith to the felsic gneisses. In detail clinopyroxenes in the layered pyroxenites are variably enriched in the light REE. Inversion modelling shows that this is not a primary feature inherited from their parent magmas. Rather selective light rare earth element enrichment took place through reaction with a felsic melt generated by the localised partial melting of the hornblende pyroxenites during granulite facies metamorphism. Published isotopic evidence suggests that the light REE mobilisation took place at ca 2.7 Ga, about 200 Ma after the time of crust formation. This observation provides an explanation for the scattered pattern of whole-rock isochron ages from the Lewsian granulites.

On the difficulty of assigning crustal residence, magmatic protolith and metamorphic ages to Lewisian granulites: constraints from combined in situ U–Pb and Lu–Hf isotopes

Abstract Zircons from two granulite facies gneisses from the central region of the Lewisian Complex have been investigated by high spatial resolution ion-microprobe U–Pb dating and laser ablation combined Pb–Hf isotope methods. The ion-microprobe data reveal a complex pattern of zircon ages distributed along the concordia curve between the time of granulite facies metamorphism at c . 2.5 Ga and the oldest zircon in each sample (respectively 2.89 Ga and 3.04 Ga). This Pb-loss pattern complicates assignment of an unambiguous magmatic protolith age to the zircon although cathodoluminescence (CL) imaging is used to suggest a preferred age of c . 2.85 Ga for both samples, with older grains being inherited. In situ Hf isotopes show a larger spread in the sample containing older grains which is also consistent with inheritance and further suggests that several crust extraction events are represented in the inherited population. Comparison of Hf isotope compositions with plausible model evolution curves suggests that crustal precursors to the Lewisian granulites were derived from their mantle source at c . 3.05–3.2 Ga. Supplementary material: Analytical methods and data tables are available at: http://www.geolsoc.org.uk/SUP18395

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.

Zonation of monazite in metamorphic rocks and its implications for high temperature thermochronology: a case study from the Lewisian terrain

Monazite is a valuable chronometer for many geological processes, and a proper interpretation for the age derived from monazite requires an adequate understanding of the behaviour of this phase and its chemical and isotopic systems during geological processes. This contribution reports the results of a detailed study on chemical and isotopic zonation of monazite in Lewisian granulites, which is of general significance for monazite geochronology in high-grade metamorphic rocks. Detailed investigations on two Lewisian granulite samples using SEM, EPMA and SIMS have revealed that Lewisian monazites frequently show petrographic, chemical and age zonation. The petrographical zonation, which reflects the chemical compositional variation, can be characterised as follows on the basis of the BSE images: (1) concentric zoning, (2) patchy zoning, and (3) `intergrowth-like' zoning. The chondrite-normalised REE distribution patterns are remarkably similar between different zones of single monazite grains. This rules out the possibility that any portion of a zoned monazite in these two samples is detrital in origin. Some possible mechanisms are proposed for the formation of monazite zonation. These include: (1) intergrowth of monazite crystals with different composition; (2) episodic growth or regrowth of monazite in response to the change of environment conditions during thermal event(s), which may involve recrystallisation or replacement of the original crystals. In short, the diversity of zoning patterns in monazites observed in this study cannot be explained assuming a single mechanism. Accordingly, interpretation of the chemical and age information from any zoned monazite requires an adequate understanding of its formation mechanism. Line scans of Th, U, Pb, La, Ce, Nd, Sm, Ca and Si across monazite zones have been performed on two monazite grains which experienced granulite facies metamorphism. The very sharp boundaries in these line scans indicate that the diffusive transport of these elements in monazite is low even at the temperature of granulite facies metamorphism. The effective closure of monazite grains may also depend upon other factors such as the armouring effects of the host phase, and recrystallisation of the mineral itself. Thus great care needs to be taken for geological application of the closure temperature concept.

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.

Significance of in situ SIMS chronometry of zoned monazite from the Lewisian granulites, northwest Scotland

In situ studies of Lewisian metamorphic monazites show them to be strongly zones in Th content, Th U ratio and 207 Pb 206 Pb age. Traverses across compositional zones of individual monazites by SIMS and EMPA show that there are no resolvable shifts between 232 Th 238 U and 208 Pb 206 Pb ratios and between Th and Pb contents in any of the profiles obtained. Furthermore, the difference in 208 Pb 232 Th ages between two zones of monazites in the same thin section is indistinguishable from the difference in 207 Pb 206 Pb ages determined by SIMS. These observations together with simple models suggest that transport of Pb relative to Th and U did not occur significantly during younger (e.g., the Laxfordian) events in the Lewisian and the ages preserved in these zoned monazites are close to concordant. Moreover, monazites having different compositions yield indistinguishable 207 Pb 206 Pb ages, which suggests that the closure temperature of the UPb system in these monazites is not significantly dependent upon their composition. Consequently, the pattern of age distribution observed in the monazites is taken to reflect the metamorphic history of the Lewisian. The results are consistent with the recent interpretations of the Lewisian thermal history which suggest two high-grade metamorphic events separated by ∼ 200 Ma.

New evidence for protolith ages of Lewisian granulites, northwest Scotland

Research Article| November 01, 1995 New evidence for protolith ages of Lewisian granulites, northwest Scotland C. R. L. Friend; C. R. L. Friend 1Department of Geology, Oxford Brookes University, Oxford OX3 OBP, United Kingdom Search for other works by this author on: GSW Google Scholar P. D. Kinny P. D. Kinny 1Department of Geology, Oxford Brookes University, Oxford OX3 OBP, United Kingdom Search for other works by this author on: GSW Google Scholar Geology (1995) 23 (11): 1027–1030. https://doi.org/10.1130/0091-7613(1995)023<1027:NEFPAO>2.3.CO;2 Article history first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation C. R. L. Friend, P. D. Kinny; New evidence for protolith ages of Lewisian granulites, northwest Scotland. Geology 1995;; 23 (11): 1027–1030. doi: https://doi.org/10.1130/0091-7613(1995)023<1027:NEFPAO>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract The granulite facies Scourian rocks in the central region of the Lewisian complex of northwest Scotland play an important role in the understanding of the development of mid-Archean high-grade gneiss complexes. Whereas the later, largely Proterozoic history of the Scourian has been clarified in a detailed U-Pb study, unraveling the Archean ages of the protoliths using conventional isotope techniques has proved impossible. In order to advance our understanding of the deep crustal processes responsible for these granulites, it is essential to establish the age(s) of accretion of the protoliths and their subsequent tectonothermal history. A combined cathodoluminescence (CL) and sensitive high-mass resolution ion microprobe (SHRIMP) single-zircon study has revealed a hitherto unrecognized morphological complexity in zircons from the type localities of the granulites. Use of CL allowed identification of relict oscillatory igneous zoning, metamorphic overgrowths, and irregular areas of recrystallization. From the SHRIMP data, an age of ca. 2960 Ma is inferred for the gneiss protoliths, which were altered considerably during an important metamorphic event ca. 2490 Ma. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Depletion of gold and LILE in the lower crust: Lewisian Complex, Scotland

Samples of Lewisian granulite ranging from mafic to felsic in composition, average 0.54 ppb Au, well below the crustal abundance of about 4 ppb Au, but higher than granulite from the Bamble shear belt, Norway, with 0.22 ppb Au. Loss of gold during metamorphism requires modification of the primary sulphide phases, mainly pyrrhotite, that host gold in igneous rocks. Pyrite, formed in part by replacement of pyrrhotite, is the principal sulphide mineral of both the Lewisian and Bamble granulites. Conversion of pyrrhotite to pyrite in the Lewisian was by oxidized fluids during Inverian retrogression at temperatures below 625°C. Some grains show replacement of pyrrhotite by pyrite; ion microprobe analyses of such a grain shows an order of magnitude less gold in the pyrite than in the pyrrhotite. Scourie quartz dolerite dykes, which were intruded after Inverian retrogression, but while the Lewisian complex was still deep in the crust, retain pyrrhotite and average 1.47 ppb Au. Transformation of pyrrhotite to pyrite in the Bamble granulites also occurred well after peak metamorphism. Both groups of granulite are depleted in LILE (large ion lithophile elements) and in gold, but the Lewisian samples are more strongly depleted in LILE (median K/Rb of 1909 v. 1079 for Bamble granulite) and Bamble samples are more strongly depleted in gold. For both the Lewisian and Bamble rocks, LILE depletion was associated with prograde metamorphism, whereas loss of gold was a later and essentially unrelated event.

Chronology and mechanism of depletion in Lewisian granulites

The pattern of Th, U and Rb depletion has been investigated in a suite of contrasting lithologies from the classic Lewisian granulite terrain at Scourie, N.W. Scotland. The study is based on lithologies ranging from ultramafic to tonalitic gneisses, together with calcsilicate and pelitic metasediments, sampled on the metre scale. Because of their close proximity, all of these lithologies have experienced identical P-T-t histories since depletion; however, their expected dehydration and melting characteristics are markedly different. Pb isotope analyses of the samples define the latest time of U/Pb fractionation as 2.665±0.026 Ga, and indicate substantial U and Th depletion relative to Pb in tonalitic and mafic lithologies, and in some metasediments. In contrast, ultramafic samples show no U and Th depletion. Sm-Nd isotope analyses of the mafic and ultramafic samples define an age of 2.707±0.052 Ga, which is interpreted as the time of igneous differentiation of the mafic-ultramafic bodies. This time is indistinguishable from the latest time of Th−Pb and U−Pb fractionation. Overall there is a straightforward relationship between the magnitude of depletion, lithology and the stability of mineral phases. Tonalitic and mafic gneisses are depleted in Th and U relative to Pb, whereas ultramafic gneisses which retain amphibole show no evidence for depletion. The observations are consistent with loss of U, Th and Rb in fluids produced by metamorphic dehydration and melting. A role for externally derived CO2 in the depletion process is not excluded by the data but there is no evidence for it having existed. Limits are placed on the length scale of transport for several elements. U, Th and Rb are depleted regionally from mafic and tonalitic lithologies; transport distances must have been on the kilometre scale. In contrast, movements of Sm and Nd have been more restricted.

Melting, depletion and textural equilibration of lewisian granulites

Melting, depletion and textural equilibration of lewisian granulites

La–Ce dating of Lewisian granulites to constrain the 138La β-decay half-life

The isotope 138La makes up 0.089% of natural lanthanum and exhibits a branched decay, by β− to 138Ce and electron capture to 138Ba. The La–Ce isotope scheme is a potentially powerful adjunct to the Sm–Nd method for petrogenetic studies, but the best determination by counting of the β-decay half-life of La (3.02 x 1011 yr)1 is in conflict with geological half-life determinations based on La–Ce and Sm–Nd mineral isochrons2–4. To test these alternatives, I determined a six-point La–Ce isochron on whole-rock samples of Lewisian granulite-facies Archaean gneiss from NW Scotland. Here I report, based on a half-life of 3.02 × 1011yr, a La–Ce age of 2,990 ±400 Myr (2σ), which falls very well within the error of a whole-rock Sm–Nd date on Lewisian granulites of 2,895±100 Myr (2σ). This half-life (equivalent to a La β-decay constant of 2.30 × 10−12 yr-1) is therefore recommended for geological application of the Ce-isotope method.

Archean granulite gneisses from eastern Hebei Province, China: rare earth geochemistry and tectonic implications

The granulite gneisses and their retrograded products of the Qianxi Group from eastern Hebei Province, China, have been investigated for their isotope and trace element geochemistry. A consistent age of about 2.5 AE has been obtained by the Rb-Sr and Sm-Nd whole-rock isochron methods, in agreement with the zircon U-Pb data (Pidgeon 1980; D.Y. Liu, unpubl.). Geochemical arguments from initial isotopic ratios (ISr and INd) and elemental distribution patterns have led us to conclude that this age of about 2.5 AE represents the time of granulite facies metamorphism, which must have followed closely the primary emplacement of their protoliths. Previous claims for early Archean ages (>3.5 AE) of these granulites are not substantiated. The mineral isotope systematics register an important thermal event at about 1.7 AE, roughly corresponding to the time of the widespread Luliang Orogeny (Ma and Wu 1981) or Chungtiao Movement (Huang 1978). The granulites of the Qianxi Group have diverse compositions ranging from ultrabasic through basic-intermediate to acid. Discriminant function calculations suggest that most analyzed samples have igneous parentage. Only a few show characteristics of metasedimentary rocks. The igneous protoliths apparently belong to two series — tholeiitic and calc-alkaline, with the latter dominating in abundance. The majority of the acid granulites have compositions corresponding to tonalite-granodiorite. Except for ultrabasic and metasedimentary rocks, all REE patterns are significantly fractionated with LREE enrichment. The degree of fractionation, as measured by the (La/Yb)N ratios, is most important in the acid granulites. These rocks often show positive Eu anomalies and HREE depletions that are typical of Archean TTG rocks (tonalitetrondhjemite-granodiorite). The existence of komatiites has been previously reported in this region. Although a few rocks have a major element chemistry similar to that for peridotitic komatiites, the lack of associated members in a komatiitic series and the scarcity of REE data have not confirmed the true komatiite occurrence in this region. Many Qianxi granulites are highly depleted in Rb relative to K and Sr. This preferential Rb depletion during granulite facies metamorphism has led to very high K/Rb and very low Rb/Sr ratios. The most comparable case is found in Lewisian granulites. Although the fractionated REE patterns of the basic granulites somewhat resemble those of continental flood basalts, the highly different abundances in other incompatible elements (Ti, Zr, and Ba) readily distinguish them from each other. Nevertheless, the LREE enriched patterns of the basic granulites may suggest an origin of their protoliths by partial melting of LREE-enriched mantle sources. On the other hand, the REE patterns of acid granulites suggest that their protoliths could be derived by partial melting of quartz eclogite, amphibolite or basic granulite. The close time relationship for a series of geologic events, namely, from initial melting of mantle peridotites, through fractional crystallisation of basaltic magmas, to granulite facies metamorphism, seems to occur in many granulite terrains. This relationship, together with the juxtaposition of lithologies of different origins and the exceptionally high pressure conditions (>10 Kb) can be best explained by crustal underplating combined with intracrustal thin-skinned thrusting and stacking of crustal slices. The “andesitic or island arc” model for the formation of the lower continental crust is not in good agreement with the present geochemical data.

Chemical changes during dyke metamorphism in high-grade basement terrains

Trace element geochemistry is an important tool in deciphering the petrogenesis of igneous, and particularly basaltic, igneous rocks, and in assigning a possible tectonic setting to these rocks1,2. Studies of the trace element geochemistry of metamorphosed igneous rocks often implicitly assume element immobility during metamorphism even though recent studies3–7 have demonstrated the sometimes quite extreme mobility of certain trace elements, and especially of the petrogenetically important rare-earth elements (REE), during low-grade (zeolite–greenschist facies) metamorphism. The mobility of trace elements at higher grades of metamorphism is poorly known, although there is some evidence8 for the mobility of large ion lithophile (LIL—Rb, K, Th, Ba, Sr) elements, but not the REE, during amphibolite fades metamorphism. The REE also seem to be stable through the amphibolite to granulite facies transition9,10. We have studied a suite of ultramafic to mafic dykes (the Scourie dykes) cutting regrogressed Lewisian granulites in the Assynt region, north-west Scotland11 and conclude that the mobility of trace elements is not controlled solely by the degree or grade of metamorphism, but more by the availability of a fluid phase for element transfer.

Rare earth geochemistry of Lewisian granulite-facies gneisses, northwest Scotland: Implications for the petrogenesis of the Archaean lower continental crust

Rare earth element (REE) data, together with data for major elements and 14 other trace elements, are presented for ultramafic, mafic, intermediate, tonalitic, trondhjemitic, anorthositic and microline gneisses, representative of the range of rock types making up the 2.9-b.y. Lewisian granulite complex of northwest Scotland. The data are used to constrain petrogenetic models for the Archaean lower crust. Ultramafic gneisses have flat REE patterns with 3–5 times chondrite abundance. The more Fe-rich mafic gneisses show slight light-REE enrichment, range up to 40 times chondritic and some have negative Eu anomalies. Intermediate gneisses have more fractionated REE distributions (Ce N/Yb N= 4−25) but with rather constant heavy REE. Tonalitic gneiss REE patterns are also strongly fractionated, show variable heavy-REE depletion and have positive Eu anomalies. REE patterns of trondhjemitic gneisses are very strongly fractionated (Ce N/Yb N up to ∼ 300), show strong heavy-REE depletion and most have marked positive anomalies. Anorthosites and microcline gneisses have similar REE distributions to the trondhjemites in spite of different major element compositions. Modelling of the REE and other trace element patterns of processes such as fractional crystallisation and partial melting suggests that whereas the mafic gneisses can be related by low-pressure fractional crystallisation, the more silicic gneisses can only be related by high-pressure partial melting of a mafic source. Most of the gneisses represent liquid compositions; few can be regarded as cumulates or the residues of partial melting. The computed average Lewisian granulite does have a positive Eu anomaly, but this is imparted by the more fractionated tonalites and trondhjemites and not by the more mafic components of the gneiss complex. Elements such as K, Rb, Cs, Th and U are removed from the lower crust by a fluid (CO 2-rich) not a melt phase during granulite-facies metamorphism. The Archean lower crust is more silicic than implied by the andesite model for continental growth. The most suitable tectonic environment to account for the generation and intermixing of the different low-pressure and high-pressure magmas or rock types (and the presence of high-grade metasediments) would be one similar to the Cordilleran belt of South America. Underplating by subduction zone or mantle derived liquids may be a significant process in Archaean continental growth.

Gneisses in diatremes, Scottish Midland Valley: petrology and tectonic implications

Gneissic clasts occur in Carboniferous volcanic vents in the Scottish Midland Valley. Samples from two widely separated areas are closely similar in petrography and are interpreted as fragments of the same gneiss complex. They exhibit mineral assemblages and textures typical of granulite-facies metamorphism. Mineral chemistry is used to suggest that pressure and temperature of metamorphism exceeded 11 kbar and 850°C in part of the complex. The gneisses are geochemically similar to Lewisian granulites in their depletion in K, Rb, and Y. Seismic evidence indicates derivation from a basement now lying 7 to 8 km down which is unconformably overlain by Ordovician and later rocks. This basement complex may have been an area of positive relief in late Precambrian and early Palaeozoic times, and its existence provides an important constraint on plate tectonic models of the Lower Palaeozoic.

Structure of the Crust Beneath the Caledonian Foreland and Caledonian Belt of the North Scottish Shelf Region

Summary This paper presents those results from the sea-to-land North Atlantic Seismic Project of 1972 which apply to the North Scottish shelf region. Time-term analysis based on first arrivals yielded P, of 6.1050.15 km s-', P* of 6.4850.06 km s-' and P, of 7.9950.02 km s-'. The presence of a P* refractor yielding first arrivals distinguishes the northern British crust from that south of the Caledonian belt. Beneath the Caledonian foreland, the P refractor varies in estimated depth between 2 and 16 km and its depth correlates with the Bouguer anomalies; it most closely approaches the surface beneath a 90 mgal Bouguer anomaly high west of the Shetland Islands, and is interpreted in terms of widespread occurrence of Lewisian granulites beneath the region. The Moho beneath the foreland occurs at a remarkably uniform estimated depth of 26+2 km. The P* refractor has been identified beneath the western part of the Caledonian fold belt and beneath the Moray Firth, and may occur more extensively. The crust beneath the fold belt is of the order of 20 per cent thicker than beneath the foreland, except beneath Moray Firth where the crustal thinning to the present estimate of about 23 km may have occurred during the post-Caledonian formation of the Moray Firth basin.

Chemical changes during retrogressive metamorphism of Lewisian granulite facies rocks from Coll and Tiree

Synopsis The chemical compositions of granulite and amphibolite facies representatives of four metamorphic rock types with different bulk compositions and origins, from a Precambrian metamorphic complex in North-west Scotland, are assessed in the light of its tectonic and metamorphic history. Several striking chemical differences exist, which are thought to have been imparted by retrogressive metamorphism of an early granulite facies complex in which all four rock types were present. The main changes incurred in the conversion of granulite facies rocks to amphibolite facies, and common to all four rock types, involved influx of K, Rb, Th and some other elements and loss of Ba, Sr, P and rare earth elements in the whole complex. Some redistribution of elements occurred. Continued retrogression of gneissic rocks to greenschist facies involved further geochemical changes.