Showing papers on "Metamorphism published in 1977"

The influence of erosion upon the mineral fades of rocks from different metamorphic environments

Abstract: Metamorphism of tectonically thickened continental crust or subducted sediment wedges is likely to take place in a thermal regime where temperature increases by conductive relaxation whilst concurrently pressure decreases by erosion of the pile. The mineral facies of rocks reaching the surface do not reflect any one geotherm through the pile but lie on a locus of P–T conditions, the metamorphic geotherm, which will generally be concave towards the temperature axis. Maximum pressures on the metamorphic geotherm are significantly less than maximum pressures experienced by rocks during the early stages of recrystallization. The metamorphic geotherm is polychrome, points at lower temperatures reflecting conditions earlier in the development than those at higher temperature; crustal melts are developed after low-medium temperature metamorphism and the amount of such melts could be significant. Blueschists develop on the low temperature end of the metamorphic geotherm and are succeeded in exposure at the surface by greenschist- or amphibolite-facies rocks; the time-scale for this process is consistent with the virtual absence of Precambrian blueschists. Crust thickened by addition of hot magma is likely to yield a metamorphic geotherm convex towards the temperature axis. Recognition of differently curving metamorphic geotherms can be used to assess the part played by magmatic activity in older metamorphic terrains.

Ophiolites: Ancient Oceanic Lithosphere?

Abstract: I. What is an Ophiolite?.- II. Plate Tectonics and Ophiolites.- III. Igneous Petrology.- 1. General Discussion of Ophiolite Assemblages.- 2. Peridotites with Tectonic Fabric.- 2.1 Introduction.- 2.2 Structure.- 2.3 Mineralogy and Petrography.- 2.4 Chemistry.- 3. Cumulate Complexes.- 3.1 Introduction.- 3.2 Structure.- 3.3 Mineralogy and Petrography.- 3.4 Chemistry.- 4. Leucocratic Associates.- 4.1 Introduction.- 4.2 Mineralogy and Petrography.- 4.3 Chemistry.- 5. Dike Swarms.- 5.1 Introduction.- 5.2 Structure.- 5.3 Mineralogy and Petrography.- 5.4 Chemistry.- 6. Extrusives.- 6.1 Introduction.- 6.2 Structure.- 6.3 Mineralogy and Petrography.- 6.4 Chemistry.- 7. Geochemistry and Petrogenesis.- 7.1 Introduction.- 7.2 Trace Elements.- 7.3 Strontium Isotopes.- 7.4 Petrogenesis.- IV. Metamorphic Petrology.- 1. Introduction.- 2. Internal Metamorphism.- 2.1 Serpentinization.- 2.2 Rodingites.- 2.3 Hydrothermal Metamorphism.- 3. External Metamorphism.- 3.1 Metamorphic Aureoles.- 3.2 Continental Margin Metamorphism.- V. Ore Deposits in Ophiolites.- 1. Introduction.- 2. Massive Sulfides.- 3. Chromite.- 4. Deposits Formed by Secondary Processes.- 4.1 Laterites.- 4.2 Asbestos.- VI. Geologic Character.- 1. Ancient Convergent Plate Margins (Sutures).- 2. Associated Sediments.- 3. Melanges.- VII. Emplacement Tectonics.- 1. Introduction.- 2. Obduction - Subduction.- 3. Diapirs.- 4. Gravity Slides, Protrusions, Deep Faults.- VIII. Geologic, Tectonic, and Petrologic Nature of Four Ophiolites.- 1. Introduction.- 2. Bay of Islands Ophiolite, Newfoundland.- 2.1 Geologic Situation.- 2.2 Internal Character.- 2.3 Petrologic, Tectonic, and Geophysical Considerations.- 3. Troodos Ophiolite Complex, Cyprus.- 3.1 Geologic Situation.- 3.2 Internal Character.- 3.3 Petrologic, Tectonic, and Geophysical Considerations.- 4. Semail Ophiolite, Oman.- 4.1 Geologic Situation.- 4.2 Internal Character.- 4.3 Petrologic, Tectonic, and Geophysical Considerations.- 5. Eastern Papua Ophiolite, New Guinea.- 5.1 Geologic Situation.- 5.2 Internal Character.- 5.3 Petrologic, Tectonic, and Geophysical Considerations.- Epilogue.- References.

Metamorphism of alpine peridotite and serpentinite

Abstract: The emplacement of peridotite into continental crust results in changes in its mineralogical, structural, textural, and bulk chemical properties. Accompanying deformation may partially or completely remove from it the evidence of its original stratigraphy and any envelope of contact alteration it might once have possessed. Such rocks have long been known as alpine-type peridotites (Benson 1926) and their field relationships have been well characterized (Wyllie 1967). With advancing metamorphism, many clues as to the original nature of alpine-type peridotites are therefore effaced, but the loss need not be total if the processes involved are understood. To gain this understanding, we need to examine the inter­ relationships between temperature, pressure, stress, mineral transformations and reactions, textures, and diffusion, etc, in a relatively simple chemical system in the earth's crust. It is not the purpose of this review to study the origin of peridotite; melting and fractionation in the upper mantle; upwelling of peridotite at mid-ocean ridges, at fracture zones, or behind island arcs ; the anatomy or significance of ophiolites; or the crystallization of mafic and ultrabasic magma in the crust. The ample literature on these subjects has been well reviewed elsewhere (Wyllie 1967, 1969, 1970, O'Hara 1968, Miyashiro 1975). Further, the emplacement, tectonic environment, and different basic types of alpine peridotite have been extensively discussed in a number of recent papers (e.g. Coleman 1971a, Dewey & Bird 1971, Jackson & Thayer 1972, Moores & MacGregor 1972, Moores 1973). In his textbook on metamorphism, Miyashiro was obliged to comment ( 1973, p. 30): "In contrast to.other classes of metamorphic rocks, however, no detailed petro­ graphic data are available on the progressive changes to take place [ sic] in this (ultrabasic rock) class." It is hoped that, by gathering and reviewing here some of the recent pertinent literature, this data vacuum may at least partially be filled. This lack of systematic information is in curious contrast to the volume of literature on peridotites exotic to their surroundings-xenoliths in kimberlite, alkali basalt, and nephelinite, alpine "cold-slab" peridotites, serpentinites in melange zones, and samples dredged from the ocean floor. However, there do exist studies of ultra basic

Rb-Sr and K-Ar geochronometry of Mesozoic granitic rocks and their Sr isotopic composition, Oregon, Washington, and Idaho

Abstract: Mesozoic orogeny and magmatism began in the northwestern United States soon after deposition of Permian strata, but no rocks have yet been dated from the Permian-Triassic orogenic period. Middle Triassic to Late Jurassic sediment sequences include major unconformities and evidence of several episodes of igneous activity. An early culmination of magmatism occurred in Late Triassic and Early Jurassic time (200–217 m.y. ago) in eugeosynclinal parts of far western Idaho. A widespread and intense culmination in Late Jurassic time was the final major orogenic event in the Oregon eugeosyncline. The Bald Mountain (147 to 158 m.y. old), Wallowa (probably 143 to 160 m.y. old but affected by Cretaceous metamorphism), Deep Creek (at least 137 m.y. old), and many other plutons in the Blue and Klamath Mountain regions in Oregon and in western Idaho were emplaced shortly before the end of Jurassic. The bulk of the Idaho batholith was emplaced during a Cretaceous culmination of igneous activity — the southern (Atlanta) lobe about 75 to 100 m.y. ago and the northern (Bitterroot) lobe about 70 to 80 m.y. ago. Much of the batholith was affected by Eocene magmatism which resulted in widespread resetting of isotopic dates for older rocks to values of 50 m.y. or less. Between 55 and 70 m.y. ago, there was a lull in igneous activity in the northwestern United States. Sr isotope initial ratios change abruptly across a boundary in western Idaho from ∼0.7040 or less, to the west, to ∼0.7060 or greater, to the east. This change marks the boundary between Precambrian crust and Phanerozoic eugeosyncline. The geologic setting of the observed transition and its time independence suggest that it is due to contamination and assimilation processes involving magmas from the mantle and enclosing crustal rocks. Contamination of magmas with radiogenic Sr renders the Sr whole-rock isochron technique useless in dating the Idaho batholith and other intrusive rocks in central and eastern Idaho, areas underlain by Precambrian basement.

Chemistry, thermal gradients and evolution of the lower continental crust

Abstract: Precambrian granulites are representative of the lower continental crust. They are, relative to upper crustal gneisses, refractory and low in heat-producing elements, being 9depleted9 in K, Rb, Cs, U and Th; they have normal or even higher than normal abundances of Zr, Sr, and Ba, highly fractionated REE patterns and high Ba/Rb, Ba/Sr, Ce/Yb, K/Cs and K/Rb ratios. Having low Rb/Sr, U/Pb and Sm/Nd ratios they are likely to be unradiogenic with respect to Sr, Pb and Nd isotopes. They do not seem to have marked positive Eu anomalies which might compensate for the significant negative Eu anomalies observed in upper crustal granitic rocks and sediments. The granulites formed largely under an intermediate goethermal gradient with P of 7–12 kbar and T of 700–1000°C. The continental crust completed a substantial part of its growth by 2500 Ma ago. We suggest that growth took place at Cordillerantype continental margins with underthrusting of oceanic crust, generation and underplating of extensive calc-alkaline tonalitic-granodioritic material containing early remnants of hornblende gabbro/calcic anorthosite complexes, and that it was associated with widespread nappe stacking and imbricate interthrusting. This crustal generation process culminated in the granulite metamorphism deep in the tectonically and magmatically thickened continents.

Ocean floor metamorphism in the Betts Cove ophiolite, Newfoundland

Abstract: The mineralogical and geochemical features of the lower Ordovician Betts Cove ophiolite of northeastern Newfoundland indicate that hydrothermal circulation of seawater near a mid-ocean ridge has been involved in the metamorphism of the complex. The degree of greenschist facies metamorphism increases with stratigraphie depth in the ophioli te. Calcite, hematite and epidote distributions show that the metamorphosing fluid penetrated downward and was reduced with depth. The mobilities of major and trace elements support the hypothesis of the interaction of seawater and basalt: Fe2O3, MgO, Na2O and H2O increase whereas CaO and Cu decrease in the rock after alteration; SiO2, total iron, K2O, Ba and Rb can either be depleted or enhanced in the altered material; TiO2, P2O5, Zr, Y, Cr and Ni remain stable during the metamorphic episode. Finally, the occurrence of massive sulphides and incipient rodingitic gabbro is explicable in a circulatory seawater system.

Oligocene and Miocene metamorphism, folding, and low-angle faulting in northwestern Utah

Abstract: An area of 3,000 km 2 in and around the Grouse Creek Mountains and the Raft River Mountains exposes Precambrian, Paleozoic, and Triassic sedimentary rocks that were folded several times and displaced tens of kilometres on low-angle faults. Overturned folds and local imbrications indicate transport westward and northward during two episodes of metamorphic deformation and transport eastward after metamorphism. Metamorphic grade increases downward in the allochthonous sheets and autochthon and increases westward in the autochthon. Mineral grains are flattened into the horizontal plane, and shear strains increase upward, suggesting that the deformations were caused by gravity acting on a broadly heated dome. Rb-Sr dating of granitic plutons affected by the deformations indicates that (1) the area is underlain by adamellite, about 2.5 b.y. old, in which deformation decreased progressively downward; (2) the first metamorphic deformation probably ended before 38.2 ± 2.0 m.y. ago; and (3) the second metamorphic deformation was still underway 24.9 ± 0.6 m.y. ago. High-grade allochthonous rocks that lie on low-grade parts of the autochthon indicate as much as 30 km of eastward transport after metamorphism. Parts of the dome sagged to form broad basins 12 m.y. ago, and the coarse sediments and tuffs that accumulated in them were overrun by allochthonous sheets measuring at least 11 by 19 km. Two Rb-Sr mineral isochrons and several fission-track ages indicate that some parts of the area cooled below 400 °C only 10 m.y. ago.

The Metamorphism of the Dalradian rocks of Scotland.

Abstract: Synopsis The metamorphism of the Dalradian rocks of Scotland is reviewed and commented on, attention being paid to the nature and spatial distribution of the zones and their significance in PT modelling. Reactions defining Barrovian and Buchan isograds are listed and figured and the importance of textural and chemical disequilibrium discussed. The effect of rock composition on the isograds is examined and a T-X diagram constructed to indicate the probable metamorphic mineral sequence for the average pelitic Dalradian composition. The history of ideas on the genesis of the migmatites is outlined and it is concluded that they may well be polygenetic, and those in the NE may even be of Pre-Caledonian age. The role of oxidation and reduction reactions is briefly mentioned and the late cooling reactions are described, from which it can be seen that the climatic assemblages have often been markedly modified. The pressures and temperatures of the metamorphism, determined from phase considerations are plotted on a PT diagram and compared to the isotope results. Finally, evidence is used to suggest that although the fundamental cause of metamorphism might be a normal heatflow across the mantle/crust boundary and within the tectonic pile, the evidence at the present level indicates some of the metamorphism is due to high level magma and perhaps heat focussing along major structures.

The Sudbury dikes of the Grenville Front region: paleomagnetism, petrochemistry, and K–Ar age studies

Abstract: Northwest trending diabase dikes having alkali olivine basalt chemistry occur abundantly in the Southern Structural Province of the Canadian Shield of Ontario. In several subregions these dikes can be traced across the Grenville Front for a short distance into the Grenville Province. Within the Southern Province the dikes possess a westerly directed primary magnetization yielding a paleomagnelie pole of high precision and accuracy (168° W. 21/2°S, A95 = 21/2°). The geological age of these dikes is probably about 1250 Ma. At distances ranging from 2–8 km (northwest) of the Grenville Front these dikes are re magnetized and are characterized by anomalously high apparent K–Ar ages. The acquisition of this later magnetization having a direction of 111°, +261/2° with a corresponding pole at 161/2° W, 4° S. A95 = 61/2° takes place without any general mineralogical expression of metamorphism within the dikes. A few dikes sharing the above direction of magnetization were intruded during this time of reheating at a...

Formation of peridotites by deserpentinization in the Darrington and Sultan areas, Cascade Mountains, Washington

Abstract: In the Darrington and Sultan basin areas of the Cascade Mountains, mafic and ultramafic rocks occur as sheets and lenses that were tectonically emplaced along high-angle faults. The ultramafic rocks comprise pseudomorphic serpentinite (lizardite-chrysotile), antigorite serpentinite, and metamorphic peridotite (olivine-talc-tremolite). The peridotites are of two types: (1) subordinate black-weathering peridotite characterized by iron-poor olivine (Fo 98 −Fo 94 ) and essential magnetite, and (2) dominant orange-weathering peridotite with more iron-rich olivine (Fo 94 −Fo 85 ) and little magnetite. In contrast to upper-mantle peridotites, olivine composition in the orange-weathering peridotites is variable on all scales down to a few micrometres. Field relations, textural and mineralogical relics, mineral composition, and oxygen isotope values all reflect formation of the Darrington peridotites by deserpentinization. Field relations demonstrate the prograde metamorphic sequence lizardite-chrysotile serpentinite → antigorite serpentinite → metamorphic peridotite. Incipiently dehydrated serpentinites exhibit veinlets and scattered porphyroblasts of new-formed high-Mn olivine. Altered chromite, Fe and Ni-Fe sulfides, magnetite, and Mg carbonates in the peridotites have been inherited from the serpentinite parent. In the black-weathering peridotites, relict textures such as bastite pseudomorphs after pyroxenes are inherited from the parent serpentinites and survive as ghosts marked by the distribution of finegrained magnetite. In the black-weathering peridotites, iron is bound largely in magnetite inherited from the parent serpentinites; thus the olivine is iron poor. The variability of olivine composition in the orange-weathering rocks is thought to be related to the variable content of iron in antigorite of the parent serpentinites. Progressive metamorphism of these rocks reached middle amphibolite facies (500 to 600 °C). Deserpentinization occurred prior to Tertiary tectonic transport and emplacement into fault contact with the unmetamorphosed rocks of the present structural setting.

Origins of some ophiolite-related metamorphic rocks of the “Tethyan” belt

Abstract: A review of the “Tethyan” belt between Yugoslavia and Oman shows that metamorphic rocks, up to the amphibolite facies, occur at the base of many of the allochthonous ophiolite sheets. Although these rocks have generally been interpreted as slivers of older metamorphic basement, we argue for their formation by metamorphism of the tectonically overridden Mesozoic sedimentary and volcanic sequences during ophiolite emplacement. Metamorphism at a high structural level is indicated by the absence of high pressure assemblages. Both frictional heating and the residual heat of thermally immature oceanic crust are theoretically capable of producing amphibolite facies temperatures. Where “old” oceanic crust has been emplaced, frictional heating is likely to dominate, but residual heat may be more important during emplacement of “young,” thermally immature crust. Implications for the geotectonic settings of specific Tethyan ophiolites are mentioned.

Thermodynamic Control of Metamorphic Processes

Abstract: The distribution of cations in coexisting minerals of various metamorphic rocks has been studied, using the principles of Sobolev and Ramberg to explain the relationship between crystal energetic parameters such as polarization of oxygen ions in silicates of differing structures and the distribution of cations. Statistical studies of distribution coefficients in rocks of low to high metamorphic grade have resulted in the formulation of several geothermometers and geobarometers. This formulation should be regarded as an attempt to systematize the theory, data and application of the crystal-chemical and thermodynamic principles to understand metamorphic and igneous processes. The results from the different thermo- and baro-meters are internally consistent and have been used to evaluate partial pressures of O2, H2O, and CO2 in the fluid phase in metamorphic processes. The metamorphic processes are dependent on the geotectonic processes and, therefore, the study of the P-T regimes of metamorphism cannot be separated from the study of the geodynamic evolution of the crust.

Grenville events in Moine rocks of the Northern Highlands, Scotland

Abstract: A 14 point Rb/Sr whole rock isochron obtained from the Morar Pelite in the Morar area of the Northern Highlands suggests an age of 1024± 96 Ma for the main metamorphism of the Morar Moines. Within error this age overlaps that recently given for the Ardgour Gneiss and thus confirms the existence of Grenvillian elements within the Northern Highland Block. It suggests that the tectono-metamorphic history of the southwestern Moines is more complex than hitherto thought.

Tectonic relationships between the Proterozoic Gawler and Willyama orogenic domains, Australia

Abstract: Stratotectonic and morphotectonic data from the two principal exposed domains (pre-Adelaidean rocks) of the Gawler sub-province are used to characterize the Proterozoic Olarian orogeny and to distinguish its effects from those of the later Phanerozoic Delamerian orogeny. The principal metasedimentary sequences in the Gawler domain and in the Willama domain are inferred to have been deposited in a single broad zone of early Proterozoic shallow-water sedimentation on older (presumed Archaean) continental crust. The sequence becomes more pelitic upwards and may be interpreted as a transgressive sequence with more distal facies to the east. Three main phases of deformation are recognized, and each phase has similar characteristics and age in both domains. D1 2nd D2 can be dated between 1850 and 1650 Ma, while D3 appears to be about 1650-1540 Ma. In high grade rocks, D1 gave rise to a layer-parallel schistosity, while D2 is characterized by tight folds with a high-grade axial-plane schistosity. The whole subprovince was characterized by high geothermal gradients so that medium- to highgrade metamorphism affected the lower parts of the succession before and during the D1 and D2 deformation episodes. No distinct tectonic zones can be recognized but large-scale stratigraphic inversions (i.e. nappe tectonics) during D1 have been recognized only in the east of the Willyama domain. The higher parts of the stratigraphic succession are generally less deformed and exhibit only low-grade metamorphism. D3 produced relatively open, upright macroscopic folds and was characteristically associated with retrogression, but was demonstrably of pre-Adelaidea n age. The Gawler domain exhibits D3 structures although it lies in the platform west of the Adelaide Geosyncline and was not affected by deformation during Adelaidean sedimentation or by the subsequent Delamerian orogeny. A network of retrograde shear zones is the principal expression of post-Olarian deformation in the Willyama domain which forms part of the basement to the Adelaide Geosyncline. The trends of D2 and D3 folding in the two domains are similar and it is shown therefore that no large-scale rotations of one domain relative to the other has been produced by the Delamerian orogeny. Large-scale translations on discrete faults or on broad zones of simple shear in the basement are not easily ruled out, but if they exist, are probably largely of pre-Adelaidea n age. However, a significant relationship between Olarian structures and variable Adelaidean fold trends has been deduced. The Olarian orogeny may have occurred in close proximity to a continental margin to the east and may thus be related to subduction processes. It differs from linear gneissic belts in Phanerozoic orogenies since it occurs in a more stable stratotectonic environment and over a wider area.

Structural history and metamorphic modification of Archean volcanic-type nickel deposits, Yilgarn Block, Western Australia

Abstract: The Archean volcanic-type Fe-Ni sulfide ores of Western Australia occur at the base of relatively small, lenslike peridotitic flows or subvolcanic sills that occur beneath thick sequences of ultramafic flows. Massive ores, breccia ores, matrix ores, and disseminated ores occur in varying proportions in the major deposits.The economic volcanic-type ores known to date are confined to amphibolite facies metamorphic domains, commonly of dynamic style, although the important Kambalda deposits occur in a static-style environment. The orebodies have an essentially tabular form elongate subparallel to the penetrative linear fabrics in host rocks (where present) and/or to the trend and plunge of regional and parasitic folds. Locally, orebodies may be confined to footwall embayments or to prominent shear zones in the ore environment. The ores themselves have been metamorphosed and complexly deformed; most massive ores exhibit metamorphic layering and tectonite fabrics, breccia ores are common, and most contacts between massive ores and more disseminated ores appear to be tectonic boundaries.There are gross similarities in the structural-metamorphic histories of a number of deposits (Juan and Lunnon shoots, Kambalda; Windarra, Redross, and Nepean mines). All ores have undergone a complex interplay of largely ductile deformation and heating events during which massive ores were generated from more disseminated ores in some instances and were mobilized relative to less massive ores in all cases. Repeated deformation of ores is indicated by their crosscutting relations with late-metamorphic, largely posttectonic dikes. All ores reverted, at least in large part, to metamorphic monosulfide solid solution (mss) during the metamorphic climax; relict phases within mss included pyrite and spinels. Highly variable Fe/Ni ratios characterize ores that suffered largely premetamorphic deformation as pyrrhotite-pentlandite + or - pyrite aggregates, whereas ores deformed as essentially mss during the metamorphic climax are typified by constant Fe/Ni ratios. Stress-induced diffusion of Cu accompanied the deformation of massive ores under all metamorphic conditions, whereas sulfur diffusion via the vapor phase was common during waning metamorphism in some ores. Mechanical segregation of pyrite and spinels appears to have been important during deformation of mss-rich ores. Considerable modification, and possibly generation, of sulfide ores occurred in metasomatic reaction zones developed at the contact of the host ultramafic unit with footwall rocks in some ores, particularly those from dynamic-style environments.The gross structure of the ores records the major part of the ductile deformation of the sulfides, whereas their small-scale structures and textures record only deformation or annealing events at low metamorphic temperatures following unmixing of the Fe-Ni-Cu sulfides from metamorphic mss.This study demonstrates the significance of metamorphism in modifying preexisting sulfides and emphasises the importance of considering such effects before erection of models for the magmatic stage. Allowing for this metamorphic imprint, there is still considerable evidence favoring the prior existence of magmatic sulfides, although at least in some deposits they may have been less concentrated than the present ores. A remaining, as yet unexplained, anomaly is the variable mean Fe/Ni ratio between individual ore shoots, which contrasts with the relatively constant (magmatic?) mean Ni/Cu ratio of the same shoots.

The geochemistry of central Australian granulites in relation to the chemical and isotopic effects of granulite facies metamorphism

Abstract: Metamorphism to intermediate-pressure granulite grade had a minimal effect on the geochemistry of layered gneisses in central Australia. The overall composition of the terrain is granodioritic and major element compositions have equivalents in igneous and sedimentary supracrustal rocks. K, Rb, Sr and probably Th concentrations, and K/Rb ratios are normal; the initial isotopic composition of Sr shows the usual range of crustal rocks. However, U is strongly depleted and was lost by a pervasive process, probably dehydration, rather than by anatexis. Comparison with other areas in which major chemical depletions and unusually low initial Sr isotopic ratios are postulated leads to alternative interpretations of these areas which do not involve large scale chemical migration. An intermediate composition for the lower crust may result from a high density of basic intrusions rather than chemical processes.

Geochronology of some blueschists from Ile de Groix, France

Abstract: THE Ile de Groix series consists of metavolcanic and metasedimentary rocks, with or without glaucophane. These rocks belong to a South Brittany polymetamorphic complex which is named the ‘domaine de l'Anticlinal de Cornouaille’ by Cogne1. In this complex, anatectic granites occur in the midst of a migmatitic and gneissic terrain1,2. Some meta-sedimentary and metavolcanic rocks which occur in the terrain are cut by Hercynian granites. The Ile de Groix series1,3,4 is characterised by high pressure mineral assemblages with glaucophane, jadeitic pyroxene and lawsonite5—these assemblages are unknown on the adjacent mainland. The series is considered to have formed during two metamorphic periods: the first produced high pressure–low temperature mineral assemblages and the second produced a recrystallisation of mineral assemblages in greenschist facies6. In this note, we shall present new isotopic age data on various rocks and minerals from Ile de Groix, hoping to delineate the time sequence of original magmatism, sedimentation and subsequent metamorphism.

3-Gyr-old stromatolites from South Africa

Abstract: WE report here a new record of early Precambrian stromatolites from the 3.0-Gyr Pongola Supergroup of South Africa (Fig. 1), which is exposed in parts of the southeastern Transvaal, Swaziland, northern Natal and Zululand. These rocks occur in an almost complete section found in the Wit-Mfolozi River valley, about 200km north of Durban. In that part of Natal the volcanic-–sedimentary Pongola Supergroup attains a maximum thickness of about 2,500 m. It is subdivided into two distinct north-east-dipping stratigraphic units; the lower 1,800-m thick Insuzi Group and the upper 700-m Mozaan Group1. Bedding and primary sedimentary structures are ubiquitously well preserved. Deformation is restricted to moderate cleaving of the argillaceous rocks. Evidence of regional metamorphism is restricted to incipient recrystallisation of the arenaceous and carbonate rocks; the lavas show evidence of deuteric alteration. Contact metamorphism is found where isolated diabase sills intrude the strata.

Ancient borderland terranes of the North American Cordillera: Correlation and microplate tectonics

Abstract: Enigmatic Paleozoic and Precambrian sequences rich in volcanic and plutonic rocks form discrete terranes along the outer border of the North American Cordillera. These borderland terranes occur in California in the northern Sierra Nevada and Klamath Mountains, in central Oregon and eastern Oregon–westernmost Idaho, in Washington in the Northern Cascade Mountains and San Juan Islands, in British Columbia in Vancouver Island, and in Alaska in the Alexander Archipelago. The difficulty in relating the geology of the borderland terranes to that of the North American continent, the recognition of ophiolites and suture zones separating the terranes from the continent, plus the Asiatic affinity of certain of the borderland faunas indicate that the terranes are allochthonous relative to the North American continent. Furthermore, major differences in stratigraphy, magmatic activity, tectonic activity, metamorphism, and particularly the ages and types of basement between the terranes — when considered together with discordant paleomagnetic data — suggest that at least six lithospheric plates are represented. The terranes in the Klamath Mountains have an Ordovician ultramafic rock (ophiolite) basement. The Oregon terranes have ultramafic complexes (ophiolites) in close association with volcanic rocks (volcanic arcs) that form the basement. In the Northern Cascade Mountains and San Juan Islands, the terranes have, respectively, Precambrian and Ordovician crystalline metaplutonic (magmatic arc) basement. The terrane in the southern part of the Alexander Archipelago has a Precambrian crystalline meta-volcanic-metasedimentary (remnant arc) basement, but an Ordovician basaltic-andesitic basement (initial deposits of an upward-shoaling island arc) appears farther north. Transcurrent faults segment and truncate parts of the Cordillera, but since the borderlands are in themselves composed of several plates, models of a single allochthonous plate are difficult to apply. More likely, during Precambrian and Paleozoic time, multiple microcontinental plates and volcanic arcs moved outboard and inboard (away from and toward North America) to accommodate a succession of marginal ocean basins opening and closing behind migrating arcs. This was followed in Mesozoic and Cenozoic time by large-scale northwestward drift.