Calcareous Metasediments Research Articles

Coupling restites and mobilizates – Geological and litho-chemical investigations of paired belts of calcsilicate fels and quartzite (SE German Basement) – Quo vadis David London’s pegmatology?

In some areas of the Variscan orogen felsic mobilizates (pegmatitic and aplitic rocks) are closely associated with stratiform and stockwork-like bodies enriched in Ca minerals (e.g. wollastonite, diopside-hedenbergite s.s.s., grossular-spessartine s.s.s., siderite..) and bodies aligned to them similar in structure but abundant in quartz, plagioclase and mica. Geological mapping and lithochemical studies are the tools to decipher the nature of these crystalline rocks which are common to the Hagendorf-Pleystein Pegmatite Province, SE Germany, and present in many ensialic orogens elsewhere. Geological and chemical data suggest paired belts of a restite-mobilization system. The Ca and Si metasomatites are different from calcareous metasediments and quartzites elsewhere in the SE German basement devoid of mobilizates (parent rocks: limestones and cherts). Mobilization conducive to this paired belt of metasomatites involved silica mobilized from a deep level of the crust as a result of metamorphic-metasomatic alteration of Precambrian to Early Paleozoic metagreywackes during retrograde metamorphism from HP to LP metamorphism around 680–600 °C. The arrangement of mobilizates and restites in the field has been denominated as metamorphic differentiation sensu lato. The zone of silica mobilization is transitional into a zone of pegmatoids and aploids that overlaps with another one characterized by rocks derived from Ca metasomatism the footwall facies of which developed in the range 750–400 °C while in the hanging wall metamorphic rocks of rare-element pegmatites 570–430 °C occurred. The intensity of Ca metasomatism diminishes from the footwall to the hanging wall rocks and reflects a subcrustal impact. These investigations call attention among exploration geologists and petrologists to an alternative origin of “metasilica” and “metacarbonate” rocks being encountered in a zoned arrangement with felsic mobilizates (pegmatitic and aplitic rocks). The current study also raises the question “Quo vadis” pegmatology? It is an amendment to the mainstream geoscientific handling of pegmatitic rocks as “..texturally complex igneous rocks” genetically linked to granitic plutons (see review of London (2018) in Ore Geology Reviews). Taking a holistic approach can give us a reality check and prevent pegmatology from converting into a one-way street (granites-only) that eventually ends up in a dead-end street. The field evidence is the litmus test for all our models created in the laboratory and on the PC. There is no ore geology without field geology.

High-grade calcareous metasediments from the Sawtooth Metamorphic Complex, Idaho, USA: evidence for passive margin strata and polymetamorphism within the Idaho batholith

ABSTRACTThe Sawtooth Metamorphic Complex (SMC) of central Idaho contains exposures of metasupracrustal rocks, in a crucial location between the Archaean Wyoming craton to the east and the adjacent Mesozoic terranes to the west, that provide constraints on Precambrian crustal evolution in the northwestern United States. Mineral textures, whole-rock geochemistry, and thermobarometry of calc-silicate gneisses record multiple stages of crustal evolution, including protolith deposition, burial and multiple metamorphic and deformational overprints. Whole-rock signatures are consistent with derivation from post-Archaean, continental sources that have undergone sedimentary maturation and recycling typical of clastic sedimentation in passive-margin environments and with a metamorphosed sequence of calcareous sandstones and marls, containing varying proportions of clay and quartzo-feldspathic detritus. The sandstone-to-marl continuum may reflect a shallow-to-deep water transition in the depositional environment of the calc-silicate protoliths. Two metamorphic events and three deformational events are preserved. The assemblage clinopyroxene + quartz + plagioclase + K-feldspar + rutile represents peak metamorphic conditions (M1), estimated at 750–775°C with relatively oxidized metamorphic fluids, during development of D1a foliation. Deformation twinning in clinopyroxene records a widespread, high strain D1b event at high temperature. A second static thermal event (M2), associated with randomly oriented amphiboles overprinting M1 and D1b features, records conditions of ~550–725°C and H2O-rich fluids. Late-stage, brittle–ductile D2 deformation is characterized by mylonitic lenses of quartz, fractures within M1 clinopyroxenes that crosscut D1b deformation twins, and localized fracturing of M2 amphiboles. Geochemical and mineral chemical signatures of SMC calc-silicates preserve fingerprints of the original protolith through burial to mid-crustal conditions and high-grade metamorphism and suggest crustal thickening to about 20 km. The SMC may reflect rocks at depth beneath the Idaho batholith. These data, indicative of newly recognized post-Archaean terrain with passive-margin, continental sediments suggest that a continuous Cordilleran passive margin sequence may have extended along the western edge of Laurentia.

Incipient charnockite: Characterization at the type localities

Fifty years after the discovery of incipient charnockite (orthopyroxene-bearing granitic rocks) partially replacing amphibole–biotite gneiss in southern India, this phenomenon has been found in transitional amphibolite facies to granulite facies terrains on every continent, and may be a characteristic feature of progressive deep-crustal metamorphism. The alteration is essentially a metasomatic transformation. Mass balance calculations for localities in southern India and Sri Lanka for which analytic data are most comprehensive show that the main reaction producing orthopyroxene in orthogneiss is breakdown of hornblende by reaction with biotite, with loss of FeO, MgO, TiO2, and CaO, as well as H2O from the host rock. There is concomitant gain of SiO2. No significant change of alkalis can be demonstrated as essential to the transformation, though advanced charnockitic alteration at the type locality of Kabbal, Karnataka may also show large increase of K2O, with removal of mafic constituents.Very similar orthopyroxene-bearing veins in garnet–biotite gneiss at the Ponmudi type locality in Kerala show definite, but more subtle metasomatism, with loss of FeO and MgO and increase of SiO2 and alkalis. Both the Kabbal and Ponmudi alterations occurred at mid-crustal levels: 5–6kbar and 700–750°C.It is postulated that both types of charnockitic alteration may have been produced as open-system processes by much the same kind of metasomatizing fluids – nearly coeval CO2-rich and ultrasaline fluids, based on fluid inclusion evidence. The paragneiss alteration is mainly biotite breakdown with some involvement of garnet, and leads to a distinct metamorphic facies type. The presence of graphite in some of the garnet–biotite gneisses at Ponmudi would have resulted in lower oxygen fugacity and, probably, lower H2O activity than in the similar Kabbal-type metamorphism. The fluid/rock ratios for both types were small (about 10 mass percent of the rocks), and the fluids, whatever their origin, were largely rock-governed.The source of the metasomatizing fluids has not been definitely identified. Carbon isotopes of fluid inclusions seem to show an igneous or primitive mantle source at both type localities, but ultrasaline fluids may be more plausibly explained by metasedimentary sources, and calcareous metasediments have been identified as a source of CO2 in some occurrences. Much remains to be learned about incipient charnockite and its significance in processes of crustal evolution.

Heterogeneous arsenic enrichment in meta-sedimentary rocks in central Maine, United States

Arsenic is enriched up to 28 times the average crustal abundance of 4.8mgkg−1 for meta-sedimentary rocks of two adjacent formations in central Maine, USA where groundwater in the bedrock aquifer frequently contains elevated As levels. The Waterville Formation contains higher arsenic concentrations (mean As 32.9mgkg−1, median 12.1mgkg−1, n=38) than the neighboring Vassalboro Group (mean As 19.1mgkg−1, median 6.0mgkg−1, n=38). The Waterville Formation is a pelitic meta-sedimentary unit with abundant pyrite either visible or observed by scanning electron microprobe. Concentrations of As and S are strongly correlated (r=0.88, p<0.05) in the low grade phyllite rocks, and arsenic is detected up to 1944mgkg−1 in pyrite measured by electron microprobe. In contrast, statistically significant (p<0.05) correlations between concentrations of As and S are absent in the calcareous meta-sediments of the Vassalboro Group, consistent with the absence of arsenic-rich pyrite in the protolith. Metamorphism converts the arsenic-rich pyrite to arsenic-poor pyrrhotite (mean As 1mgkg−1, n=15) during de-sulfidation reactions: the resulting metamorphic rocks contain arsenic but little or no sulfur indicating that the arsenic is now in new mineral hosts. Secondary weathering products such as iron oxides may host As, yet the geochemical methods employed (oxidative and reductive leaching) do not conclusively indicate that arsenic is associated only with these. Instead, silicate minerals such as biotite and garnet are present in metamorphic zones where arsenic is enriched (up to 130.8mgkg−1 As) where S is 0%. Redistribution of already variable As in the protolith during metamorphism and contemporary water–rock interaction in the aquifers, all combine to contribute to a spatially heterogeneous groundwater arsenic distribution in bedrock aquifers.

Thermobarometry of metasediments and metabasites from the Eclogite zone of the Hohe Tauern, Eastern Alps, Austria

Metamorphic P– T conditions of calcareous metasediments and metabasites from the eclogite zone of the Hohe Tauern region were evaluated based on the application of various geothermobarometers, e.g. programs PTt PATH, TWEEQU and THERMOCALC. Average P– T values in metasediments are c. 9.0–10.2 kbar, 552–565 °C due to the late tertiary alpine metamorphic event. In one sample, only the inclusion of omphacite in garnet testifies the earlier alpine high P event. In metabasites, however, this event dominates with conditions at c. 19.2–22.4 kbar, 594–627 °C. Retrogression at lower pressure is only rarely documented in the formation of albite. Pseudosections calculated with Thermocalc in the simplified NCFMASH system for various bulk compositions related to sample eclogite 2447 are in approximate accordance with a sequence of mineral assemblages: omphacite+paragonite+zoisite+hornblende (prograde inclusions in garnet)>garnet+omphacite+kyanite (matrix phases at P– T maximum)>paragonite+zoisite+hornblende (secondary phases on retrograde path). Formation of these hydrous phases probably took place during decompression at slightly lower P– T conditions in the presence of restricted amounts of H 2O.

Interaction of aqueous fluids with calcareous metasediments during high‐T, low‐P regional metamorphism in the Qadda area, southern Arabian Shield

The S.W. Nabitah Mobile Belt, Saudi Arabia, contains a Proterozoic island‐arc complex. In the Qadda area, the metavolcanic‐dominated supracrustal sequence records amphibolite facies regional metamorphism of high‐T , low‐P type. Calcsilicate rocks and aluminous dolomitic marbles within the supracrustal sequence have been studied in detail to refine estimates of peak metamorphic P–T conditions and assess the role of fluids during prograde and retrograde metamorphism.Fluid‐independent thermobarometers (including the calcite–dolomite thermometer and P‐sensitive equilibria involving grossular, wollastonite, anorthite, meionite, quartz and calcite) yield peak P–T conditions of c. 650–660 °C, 4 kbar, both higher than previous estimates, giving a revised average thermal gradient of c. 45 °C km–1.The close match between the peak temperatures implied by calcite–dolomite thermometry and those recorded by univariant devolatilization equilibria suggests that the calcareous rocks were fluid‐bearing during late‐prograde and peak metamorphic stages. These fluids were essentially binary H2O–CO2 mixtures with low NaCl and HF concentrations. Most were H2O‐rich, with XCO2 between 0.02 and 0.2, but values of c. 0.6 are recorded by two samples. High modal abundances of the solid products of decarbonation reactions (e.g. c. 10–50% wollastonite) in many of the rocks that record low‐XCO2 equilibrium fluids implies infiltration of significant quantities of externally derived aqueous fluid during late‐prograde metamorphism, but not enough to exhaust the buffering capacity of the rocks. Calculated minimum time‐integrated fluid‐to‐rock ratios of five wollastonite‐bearing calcsilicate rocks range from 0.7±0.22 to 1.39±0.46 (1σ); those of six marbles range from c. 0 to 4±1.4. The latter variation occurs on a metre‐scale, implying focusing of fluid flow. Diopside‐rich rocks record fluid‐to‐rock ratios of up to 88±48. Penetrative wollastonite lineations indicate a temporal link between infiltration and distributed ductile deformation. Infiltrating fluids were probably derived both from the prograde dehydration of adjacent metabasalts and metatuffs and from crystallization of voluminous pretectonic granitoid intrusions. In general, fluid‐to‐rock ratios deduced for the metavolcanic‐dominated Qadda area are similar to those recorded by rocks in the metasediment‐dominated terrane of N. New England.The occurrence of post‐tectonic retrograde hydration textures in both carbonate‐bearing and carbonate‐free rocks otherwise lacking hydrous minerals testifies to infiltration of aqueous fluids during retrograde metamorphism in the absence of penetrative deformation. Minimum fluid‐to‐rock ratios calculated for secondary grossular reaction rims in some calcsilicates are c. 0.04. Later patchy hydration of scapolite probably utilized static, pore‐filling fluids remaining after the early retrograde infiltration.

Chemical mineralogy and geothermometry of the Middle Shire granulites, Malawi

The Middle Shire area of Malawi lies roughly midway between the southern tip of Lake Malawi and the Zambesi river. It includes the western side of the Shire Highlands and the central section of the Shire rift valley. It is occupied by gneisses and granulites of the Mozambique Belt, a polymetamorphic fold belt, which extends down the eastern side of Africa from Ethiopia to southern Mozambique. In the Middle Shire section the eastern part of the area is occupied by two-pyroxene granulites together with subordinate skarns and biotide-garnet granulites. The western part consists of biotite-gneisses and hornblende-biotite gneisses together with subordinate pelitic and calcareous metasediments. A well defined transition zone lies between them separating prograde amphibolite facies gneisses from the granulite facies to the east. Later retrograde effects are related to a subsequent episode of folding and produced secondary amphibole and biotite. Mineral compositions reflect a temperature range of 680–1000°C, during the prograde regional metamorphism and pressures of 6–8 kbars in the granulites. Transition zone temperatures were 550–680°C at 4–7 kbars. Amphibolite facies temperatures were 450–600°C at pressures of 4–5 kbars.

Contact metamorphism of pelitic, psammitic and calcareous sediments in the Southern Highlands of New South Wales

Contact metamorphism has been recognized along a 4 km wide belt adjacent to the shallow‐dipping eastern margin of the Arthursleigh Tonalite, an Early Devonian pluton of the Marulan Batholith, eastern New South Wales. In Ordovician psammitic and pelitic rocks three zones of progressive contact metamorphism range from muscovite + biotite + chlorite assemblages in the outer zone to K‐feldspar + cordierite assemblages adjacent to the pluton and in metasedimentary xenoliths. Retrograde phenomena include extensive replacement of metamorphic minerals by ‘sericite’ and chlorite. Calcareous metasediments adjacent to the tonalite typically contain assemblages of quartz + calcic plagioclase + ferrosalite + sphene, or wollastonite + calcite + diopside with minor grossularite and vesuvianite. Thermal effects in volcanic rocks along the western margin of the pluton are confined to recrystallization of the groundmass. The regional geology indicates confining pressures of approximately 1 kbar at the time of emplacement of the tonalite. Contact metamorphic temperatures were estimated from two‐feldspar geothermometry to attain a maximum of approximately 590°C for rocks in the innermost zone of the aureole and 700°C for the xenoliths. Fluid compositions attending progressive contact metamorphism were water‐rich (Xco2<0.2) and, during cooling, these fluids probably account for the extensive retrograde hydration observed in the aureole.

Stratigraphic setting and mineralogy of the Arctic volcanogenic massive sulfide prospect, Ambler District, Alaska

The Arctic prospect is the largest (>36 million metric tons) volcanogenic massive sulfide deposit known within the Ambler district in northwestern Alaska. These deposits are hosted by the low-greenschist-grade, Devonian-Mississippian Ambler sequence (informal name) of bimodal, basaltic, and rhyolitic volcanic and volcaniclastic rocks, pelitic, graphitic, and calcareous metasediments. Detailed field mapping, core logging, petrography, X-ray diffractometry, and electron microprobe and whole-rock analyses of hydrothermally altered rocks were used to determine the emplacement history and setting of sulfide deposition. The Arctic prospect contains mineralization at three separate stratigraphic intervals. The largest and most economically important of these three intervals, termed the Main horizon, contains 90 to 95 percent of the known ore reserves. Main horizon minera]ization consists of one to possibly as many as 14 semimassive sulfide lenses within a well-developed, horizontally stratified sequence of alteration types, including pyritic (pyrite-phengite-calcite) and chloritic (chlorite-quartz) alteration, dolomite, phlogopite, albite, talc, and white mica. These alteration types together form a pervasively altered zone ( 3 which is asymmetric to the vent area for the ore fluids.The deposit is high in Cu and barite, has moderate amounts of Zn and Ag, and is low in pyrite, Fe, Pb, and Au. Ternary plots of Cu, Pb, and Zn abundances are very similar to those of massive sulfides formed at divergent plate boundaries, with total metal content at Arctic in the top 1 percent (Cu), 3 percent (Zn), and 4 percent (Pb and Ag) of volcanogenic massive sulfides worldwide. Strong lateral zonation of metals (Cu overlying the vent area, Zn intermediate, and Pb and Ag distal) parallels, but is offset from, an asymmetric distribution of alteration. Most sulfides were deposited not over the vent but on a relatively fiat area and in topographic channels between an elongate vent area pa]eohigh and a carbonaceous mudbearing basin to the northwest.No single volcanic unit can be genetically tied to mineralization. The onset of hydrothermal activity was apparently related to fault activity in the vent area and continued during emplacement of subsequent (postore) rhyolites. Mineralization at Arctic took place along a synvolcanic fault in a tectonically and volcanically active basin within a rifting continental margin, possibly related to an active oceanic rift. Jurassic(?) to Cretaceous, low-greenschist-grade (chlorite-muscovite subfacies) regional metamorphism was essentially isochemical on a macroscopic scale and preserved many primary features of the deposit and its host rocks.

The problem of anomalous local limestone-pelite successions within the Moine outcrop; I: metamorphic limestones of the Great Glen area, from Ardgour to Nigg

Synopsis Small outcrops of folded calcareous metasediments, surrounded by otherwise lime-poor Moine rocks, occur along both sides of the Great Glen: (from SW to NE) in Ardgour, beside Loch Lochy, in Gleann Liath and Glen Urquhart, and near Inverness, Rosemarkie and Nigg. Further examples lie in Glen Dessarry to the NW. Outcrops cover from a few m 2 to several km 2 , and vary from pure limestones, with much rarer dolostones, to Ca-Mg-Fe-silicate ‘skarns’. The few previously described examples have been contradictorily assigned to the Lewisian, Moine, Dalradian, and even Durness Limestone. The mineralogical range in each outcrop is generally similar, indicating amphibolite-facies metamorphism. Several limestones are intimately associated with graphitic, kyanite or sillimanite-rich pelites (also atypical of the local Moine), and with the margins of synformal (D 2 or D 3 ), foliated intrusions. Chemically, the calcareous rocks are strikingly homogeneous, as revealed by cluster analysis, and by very high correlation coefficients between ‘immobile’ trace elements (Ti, P, Y, Zr, Nb, Ce, Th), between Cr, Co and Ni, and between K and Rb, Ba or Pb. Mg also correlates inversely with Sr, as the dolomite/calcite ratio varies. This homogeneity suggests that most of the calcareous rocks belong to a single, now dismembered lithostratigraphical formation. Over 100 analyses of Lewisian ‘limestones’, and over 400 covering most of the Dalradian limestone formations, show quite different and far less homogeneous geochemistries. In particular, the Great Glen limestones cannot be equated with the geographically nearest Dalradian limestones, in the Appin–Ballachulish–Lismore area, as revealed by discriminant and correlation analysis. Field and regional considerations argue that the limestones are unlikely to be either Cambro-Ordovician, or an integral part of the Moine succession. As a working hypothesis, they should be assigned to a separate, provisional lithostratigraphical assemblage, whose relationship to the Lewisian, Moinian and Dalradian remains to be established.

Alpine metamorphism of calcareous metasediments in the Western Hohe Tauern, tyrol: mineral equilibria in COHS fluids

During Tertiary regional metamorphism in the Western Hohe Tauern, reaching maximum P, T conditions around 6 kb, 550° C in calcareous metasediments, reaction of pyrite to pyrrhotite is suggested by regional distribution and textural relations. In rocks without graphite pyrite is common at all metamorphic grades. In graphite bearing rocks, however, the dominant Fe-sulfide is pyrite at lower grade and pyrrhotite at higher grade. Furthermore, in graphite bearing high grade rocks, pyrite is restricted to assemblages with Mg-rich silicates. Several factors control pyrite-pyrrhotite relations. Increase of temperature is most effective by increase of pyrrhotite vs. pyrite stability field, shift of silicate-sulfide reactions toward the stability field of pyrrhotite, creation of sulfur free fluids from devolatilization reactions, and increase in the proportions of sulfur bearing fluid species. Presence of graphite also favours progress of pyrite to pyrrhotite reaction, as shown by different \(f_{S_2 } \)-stabilities and changes in the amount of minerals and fluid during metamorphic heating of graphite bearing and graphite free assemblages. An opposite effect is shown by assemblages with low Fe-contents in Fe-Mg silicates, due to the enlarged stability field of such minerals with increasing Mg (and F) content. Another inhibition of pyrite to pyrrhotite reaction is suggested to be due to relatively high sulfur contents of H2O rich infiltrating fluids.

Genesis and mobility of the H 2 O-CO 2 fluid phase during regional greenschist and epidote amphibolite facies metamorphism: a petrological and stable isotope study in the Scottish Dalradian

During high-pressure, low-temperature greenschist and epidote-amphibolite facies metamorphism in Dalradian rocks of the SW Scottish Highlands, mineral assemblages in metabasites and calcareous metasediments were dominantly controlled by infiltration of hydrous fluids; consequently, mineral assemblages capable of buffering the fluid phase composition were rare. Equilibrium prograde H 2 O-CO 2 fluids usually contained less or much less than about 1–2 mol% CO 2 . Three fluid infiltration events are recognized. During prograde greenschist facies metamorphism, metabasic sills were infiltrated by large volumes of CO 2 -bearing hydrous fluid; carbon isotope studies indicate that the CO 2 was locally derived by widespread oxidation of graphite or other organic carbon in adjacent metasediments. This may have occurred under approximately lower greenschist facies conditions as a result of mixing of fluids of varying f O 2 , initiated by thermal expansion of water during heating, decompression and consequent hydraulic fracturing. In the epidote-amphibole facies (garnet zone), dehydration reactions in metabasites generated large quantities of water, which removed carbonate from metabasites on a regional scale and infiltrated calcareous metasediments to produce assemblages containing grossular, diopside, K-feldspar, amphibole, clinozoisite and sphene. A late retrograde infiltration of CO 2 ,-bearing hydrous fluid under lower greenschist facies conditions generated assemblages containing K-feldspar + chlorite + rutile ± dolomite in calcareous rocks and albite prophyroblast schists in zones of intense secondary deformation. Large-scale infiltration of fluid into greenschist-facies metadolerite sills was intimately related to, and possibly controlled by, penetrative deformation and, in the absence of a penetrative deformation, grain-boundary diffusion by itself was an ineffectual mechanism of fluid transport.

Gold, base metals, and diamonds in Botswana

The geological framework of Botswana includes cratonic regions comprising parts of ancient stable blocks of Archean metamorphosed rocks, mobile regions which have suffered tectonism at various periods during the Proterozoic, and platform regions on which unmetamorphosed Proterozoic and Phanerozoic sedimentary and volcanic rocks have been preserved. Superficial deposits mantle much of the country, rendering mineral exploration unusually difficult.Gold is confined mainly to the Archean greenstone belts of the Rhodesian Craton. Many abandoned mines in the Tati have recently been reassessed; a few occurrences may have some potential but most are subeconomic.Base metal mineralization is becoming increasingly significant in the economic outlook of the country. Major nickel-copper sulfide orebodies, with reserves of over 30 X 10 6 metric tons, have been proved at Pikwe and Selebi within the Limpopo mobile belt. Two medium-sized nickel-copper deposits are known in the Tati belt. Important copper occurrences in calcareous metasediments within the Matsitama schist belt on the margin of the Rhodesian Craton are being reinvestigated, and extensive stratiform copper mineralization has been delineated in the Damaran mobile of northwestern Botswana. The geological environment of the latter is similar to that of the Zambian Copperbelt, but exploration is not yet sufficiently advanced to have proved a viable orebody.Banded iron-formations occur principally in the belts and as part of the sedimentary succession in the platform regions, but none is currently of economic interest. Minor manganese deposits are also associated with the platform sequence overlying the Kaapvaal Craton and Limpopo belt.Kimberlites appear to be restricted to two distinct regions. The Orapa kimberlite district in central Botswana contains 30 known pipes, including the Orapa diamond mine, and at least two other economic occurrences. A group of four concealed kimberlites and other isolated pipes have been detected in southern Botswana.The metallogenic evolution of the various mineral districts is discussed and it is concluded that the potential for further discoveries in Botswana is good.

Mineralogy of New Caledonian metamorphic rocks

A Cretaceous to low-Tertiary sequence of interbedded pelites, cherts, basic and acidic volcanics and calcareous lenses has been metamorphosed by an Oligocene event. A complete intergradational metamorphic sequence is exposed in the Ouegoa destrict. The following metamorphic zones have been recognised: — (1) lowest-grade rocks consisting of quartz-sericite phyllites and pumpellyite metabasalts (2) lawsonite zone, characterized by the association of lawsonite and albite (3) epidote zone, characterised by epidote-omphacite-sodic hornblendealmandine bearing metabasalts and epidote-albite-almandine-glaucophane bearing metasediments; calcareous metasediments may also carry omphacite. The epidote and lawsonite zones are separated by a narrow belt of transitional rocks.

Remnants of an Early Metasedimentary Assemblage in the Lewisian Complex of the Outer Hebrides

Abstract: Remnants of a varied association of rocks, including psammitic, pelitic, semi-pelitic and calcareous metasediments, have been mapped in all the larger islands of the Outer Hebrides. Most of the remnants are concentrated in relatively narrow zones in which they are associated with, and pass into, quartzo-feldspathic gneisses. These zones are flanked by areas made predominantly of quartzo-feldspathic gneisses which appear to lack metasedimentary remnants. The metasediments were metamorphosed, migmatised and partially transformed into gneisses prior to the emplacement of the Scourie Dyke suite. Their least-modified portions are in many places intimately associated with bodies of basic rock which are thought to have shielded them from the effects of successive phases of metamorphism. Their present mineral assemblages and fabrics date from the Laxfordian episodes of regeneration. Relict mineral assemblages suggest that the earliest recognised Laxfordian metamorphism was of amphibolite facies over much of the Hebrides but was locally of granulite facies. The metasedimentary rocks are regarded as derivatives of a supracrustal group deposited prior to the main Scourian period of gneiss-formation and much of the gneiss is thought to be derived from such a group. A still older basement could, however, be represented by some of the areas of quartzo-feldspathic gneisses which intervene between the metasedimentary zones, though the existence of such a unit remains open to discussion.