Showing papers on "Metamorphism published in 1973"

Metamorphism and Metamorphic Belts

Abstract: My book Metamorphic Rocks and Metamorphic Belts (in Japanese) was published by Iwanami Shoten, Publishers, in Tokyo in 1965. A few years later, Mr D. Lynch-Blosse of George Allen and Unwin Ltd contacted me to explore the possibility of translating it into English. Thus, translation accompanied by rewriting of substantial parts of the book was made in subsequent years, resulting in the present book Metamorphism and Metamorphic Belts. This title was chosen to emphasize the tectonic significance of metamorphic belts. Metamorphic geology has a long history. The microscopic description and classification of metamorphic rocks began in the late nineteenth century. The theory of equilibrium mineral assemblages began in the first half of the twentieth century. Detailed mineralogical studies and the experimental determination of the pressure-temperature conditions of metamorphism began in the 1950s. The importance of metamorphic peirology in our understanding of the tectonic processes has been realized only in the past decade. This book is intended to synthesize the mineralogic, petrologic and tectonic aspects of metamorphism. Advanced treatment of the thermodynamic and structural aspects is not intended. Part I of this book treats the basic concepts of metamorphic petrology and geology. Part II deals mainly with the progressive mineral changes and their diversity in regional metamorphism. Part III deals with the tectonic aspects of regional, ocean-floor and transform-fault metamorphism in relation to the evolution of the crust and lithosphere. A short history of metamorphic geology is given in the Appendix to help readers understand the historical background of concepts and terms and to, emphasize the great contribution of the to Finno-Scandinavian school of petrology in the first half of the twentieth century.nFrom the tectonic viewpoint, regional and ocean-floor metamorphism are the most important of all categories of metamorphism. Abundant data are available for regional metamorphism. Hence, the mineralogy, metamorphic facies, facies series, and pressure-temperature and tectonic conditions of regional metamorphism are discussed in detail. Available data, though they are not abundant, of ocean-floor, transform-fault and contact metamorphism are reviewed.

Sub-sea-floor metamorphism, heat and mass transfer

Abstract: The ophiolitic rocks of E. Liguria, Italy contain a „spilitic” metamorphic assemblage sequence, cross-cut by hydrothermal veins, which developed in the oceanic environment. Metamorphic parageneses indicate that temperatures as high as ∼400°C were realised at depths as shallow as 300 m below the original rock/water interface. The inferred temperature interval was equivalent to a geothermal gradient of ∼1300°C/km. It is suggested that metamorphism took place in a sub-sea-floor geothermal system, and that such systems are an integral part of the sea-floor spreading process. Modern evidence is provided to support this hypothesis, and to suggest that heavy metal rich solutions discharged from such systems are responsible for the formation of a metal enriched sedimentary component. A unified model of sub-sea-floor metamorphism and mass transfer is proposed, and possible differences between sub-sea-floor and terrestial geothermal systems are discussed. In the light of the model, the origins of certain aspects of bedded cherts found associated with ophiolitic rocks, of ophiolitic massive sulphide deposits and of certain trace element patterns are considered.

Evolution of an Early Proterozoic Continental Margin: The Coronation Geosyncline and Associated Aulacogens of the Northwestern Canadian Shield

Abstract: The Coronation geosyncline developed in the early Proterozoic along the western margin of a continental platform (the Slave Province) of Archaean rocks older than 2300 Ma, and culminated between 1725 and 1855 Ma ago with the emplacement of a pair of batholiths (the Bear Province). The evolution of the geosyncline has a strong family resemblance to Phanerozoic geosynclines believed to delineate ancient continental margins and have been controlled by global plate interactions. Such geosynclines are unknown in Archaean orogenic belts, from which it is inferred that creation of the first large rigid continental platforms marked the end of the Archaean and the beginnings of actualistic plate tectonics. The geosyncline began with deposition of a westward-facing continental shelf, consisting of a lower formation dominated by orthoquartzite, derived from the platform, and an upper cyclic stromatolitic dolomite formation. West of the shelf edge, the dolomite passes abruptly into a much thinner mudstone sequence with dolomite debris-flows, and the orthoquartzite into a thick laminated silt and mudstone sequence with quartzite turbidites. The oldest rocks west of the shelf edge, an area interpreted to have been a continental rise, are pillow basalts and volcanic breccias, extruded above a basement of unknown character. The principal turning point in the evolution of the geosyncline came with the foundering of the continental shelf. It is draped by a thin laminated pyritic black mudstone sequence, overlain by a westward-thickening clastic wedge resulting from intrusion and erosion of the batholiths to the west. The clastic wedge begins with a thick sequence of coarse greywacke turbidites that passes eastward into concretionary mudstone on the platform. The mudstone grades upward into laminated shaly limestone with minor greywacke turbidites, overlain in turn by cross-bedded red lithic sandstone. The supracrustal rocks of the geosyncline have been compressed and tectonically transported toward the platform. Adjacent to the batholithic belt, the continental rise and clastic wedge sequences are penetratively deformed and recrystallized by regional low-pressure metamorphism. To the east, the unmetamorphosed continental shelf and clastic wedge sequences have been flexurally folded and overthrust above a basal detachment surface. East of the thrust zone, relatively thin rocks on the platform are nearly flat-lying except around large anticlinal basement uplifts. Unusual features of the platform are its two aulacogens - long-lived deeply subsiding fault troughs that extend at high angles from the geosyncline far into the interior of the platform. During every phase in the evolution of the geosyncline, the aulacogens received much thicker sedimentary sequences, commonly with the addition of basaltic volcanics, than adjacent parts of the platform. Although equal in thickness to the geosyncline, the aulacogens were never subjected to the batholithic intrusions, regional metamorphism or low-angle overthrusting characteristic of the geosyncline. The Athapuscow aulacogen, in the region of Great Slave Lake, is interpreted as having been an incipient rift, located over a crustal arch, during the continental shelf stage of the geosyncline, but sagged to become a crustal downwarp during the clastic wedge stage, ultimately with sufficient transverse compression to produce broad folds. Finally, the aulacogen became part of a regional transcurrent fault system, along which thick fanglomerates accumulated in local troughs. The batholithic belt consists of two batholiths, eroded to different depths, separated by the north-trending 350 km long Wopmay River fault. The Hepburn batholith, east of the fault, is a composite intrusion of mesozonal granodiorite plutons. The foliated and migmatitic borders of the plutons are normally concordant with wall rock sheaths of sillimanitic paragneiss. Along the eastern margin of the batholith, metamorphosed rocks of the continental rise sequence dip gently to the west beneath the batholithic rocks. Belts of intensely deformed and metamorphosed supracrustal rocks within the batholithic terrain include sequences of pillow basalt, pelites and granite-pebble conglomerate, perhaps the lower part of the continental rise deposited during the initial rifting of the continental margin. The Great Bear batholith, west of the fault, consists of discordant epizonal plutons, mostly adamellite, that intrude broadly folded but regionally unmetamorphosed sequences of welded rhyodacitic ash-flow tuff, trachybasalt and derived sedimentary rocks. The volcanic rocks, intruded by dense dyke swarms radiating from the plutons and by felsite plugs, are interpreted to be comagmatic with the plutons. Mapping is as yet insufficient to establish, speculations aside, the possible relations of the two batholiths to arc-trench systems. Furthermore, the western margin of the batholithic belt, a region of critical importance, is covered by a veneer of younger Proterozoic and Paleozoic sedimentary rocks. Until fossil arc-trench systems are outlined, the contention that the Coronation Geosyncline involved global plate interactions is based on indirect evidence - the analogous evolution of the geosyncline east of the batholithic belt with Phanerozoic geosynclines in which fossil arc-trench systems have been found.

Interpretative Synthesis of Metamorphism in the Alps

Abstract: Compilation maps are presented showing major tectonic features, selected lithologies, and zones of progressive metamorphism in the eastern half of the western Alps (scale 1:400,000) and the eastern Alps (scale 1:800,000). Two principal complexes are distinguished on them: (1) the Caledonian and Hercynian metamorphosed terrane in the Southern Alps + Austroalpine sheets, overlain by deformed but largely unrecrystallized uppermost Paleozoic and younger platform-type sedimentary rocks; and (2), the tectonically lower Sesia-Lanzo + Lepontine-Pennine + Helvetic realms, a sequence of Hercynian and pre-Hercynian plutonic igneous + metamorphic rocks and a younger, chiefly Mesozoic cover sequence consisting of shelf, slope, and deep-sea sediments + ophiolites, incompletely to pervasively recrystallized during Alpine metamorphism. 1. The pre-Mesozoic metamorphism involved several cycles in both complexes, but it has not been possible to distinguish these on the maps. Judging from the mineral para-geneses, recrystallization events seem to have taken place under moderate to very high temperatures at low to moderately high pressures. 2. Three contrasting but intergradational, temporally overlapping episodes of Alpine metamorphism are recognized in the Sesia-Lanzo + Lepontine-Pennine + Helvetic terrane: (a) an early, high-pressure, low-temperature event syntectonic with nappe formation, which produced eclogites + albite amphibolites, glaucophane schists, and allied greenschists, with lower grade, more recently metamorphosed sections lying externally (that is, toward the European foreland) relative to the older, progressively higher grade, more internal, imbricated sections lying to the south and east; (b) a middle syntectonic to post-tectonic stage characterized by more “normal” physical conditions, resulting in the partial or complete conversion of the products of event (a) to greenschist (prasinite) and low-rank amphibolite facies metamorphic rocks; and (c), a late, and in most cases syntectonic to post-tectonic recrystallization involving moderately high temperatures and pressures, which locally obliterated the products of both (a) and (b). Of the recrystallization continuum, event (a) is best preserved in the Franco-Italian Alps, and in Switzerland in the cantons of Wallis and Graubunden, (b) is nearly ubiquitous in the Sesia-Lanzo + Pennine + Helvetic complex, and (c) is confined to the Lepontine gneiss area of the Italian and Swiss Alps, and to the central gneiss domes of the Tauern Fenster, central Austria. The timing of Alpine metamorphism evidently varied laterally along and across strike of the belt. For instance, in Austria, event (a) may have begun in Late Cretaceous (?) time, whereas it probably commenced during Paleocene-Eocene time in the western Alps. Moreover, “early” Alpine, low-grade zeolitization occurred in the external parts of the Helvetic realm probably during Oligocene time— nearly contemporaneously with the more internal “late” Alpine higher grade Lepontine recrystallization of event (c). The contact between (1) the Southern Alps + Austroalpine nappes on the one hand, and the structurally lower (2) Sesia-Lanzo + Lepontine-Pennine + Helvetic realms on the other, juxtaposes rocks of markedly contrasting petrologic and tectonic histories. This zone, here referred to as the Alpine Suture, is postulated to represent the crustal expression of a Late Mesozoic-early Tertiary convergent lithospheric plate junction. The early Alpine high-pressure paragenesis appears to reflect subduction and shuffling of the more northerly terrane beneath the stable lithospheric dab capped by the Southern Alps + Austroalpine sheet. If so, the observed blueschist-type metamorphic zoning probably was generated by progressively greater depths of underflow and a consequent depression of the isotherms. A variable rate or time at which the complex was exhumed locally could account for the later establishment of a more normal thermal regime, and thus, succeeding higher temperature mineral assemblages as displayed in the Lepontine gneiss area and the Tauern gneiss domes.

The Archaean craton of the North Atlantic region

Abstract: The North Atlantic Archaean craton, exposed in parts of Greenland, Labrador and northwest Scotland, is a high-grade gneiss terrain which contrasts with Archaean granite-greenstone-belt terrains such as those of southern Africa. The tonalitic to granitic banded or agmatitic gneisses which occupy most of the craton are considered to be derived largely from granitic bodies emplaced within the crust. Early granitic gneisses of this type in Godthabsfiord are at least 3800 Ma in age and it is suggested that a granitic basement of similar age extended over much of the craton. Most of this early basement was reworked and interleaved with metamorphosed supracrustal rocks, with layered anorthositic complexes and with abundant tonalitic gneisses derived from younger intrusions. Identifiable metavolcanics and metasediments, forming narrow belts in the gneisses, occupy less than 20% of the craton; they include highly-metamorphosed basic, ultrabasic and intermediate-acid volcanic rocks with associated intrusions and predominantly chemical sedimentary rocks. Clastic sediments are preserved in the lower part of the Isua supracrustal belt where they are overlain by banded ironstones and metavolcanics. All these rocks suffered profound deformation and metamorphism which destroyed their primary relationships and culminated in the development of fold interference patterns without linear grain and in granulite or amphibolite-facies metamorphism ending at about 2800 Ma. Tectonic and metamorphic episodes over the next thousand million years were more localized and served to differentiate the Archaean craton from border-zones of early Proterozoic mobility.

Petrography of the Sokoman Iron Formation in Part of the Central Labrador Trough, Quebec, Canada

Abstract: The sedimentary textures of the Sokoman Iron Formation are similar to those of limestones; therefore the classification of textural elements in limestone (Folk) can be applied to the iron formation. The authors recognized the following textural elements: (a) femicrite (a matrix of iron silicate and carbonate) and matrix chert, both analogous to micrite; (b) cement chert and carbonatic cements; (c) aggregated particles, comparable to Folk9s allochems: pellets, intraclasts, ooliths, and pisolites. Shard textures are derived from ooliths and intraclasts by compaction. Rock types are defined by the combination of textural elements they contain. The iron formation suffered extensive epigenetic alteration. Dessiccation, shrinkage of silica-gel, compaction, and cementation are early diagenetic. Primocrystallization of quartz concludes the early diagenetic stage. It leads through a cryptocrystalline stage to the end phase of micropolygonal quartz. Quartz re-crystallized further during late diagenesis (burial metamorphism?) and again during a synkinematic to postkinematic regional metamorphism. Hematite dust is the oldest iron oxide. Much of the microscopic magnetite and specularite formed during early diagenesis. Migration of iron occurred; iron has been enriched in magnetite- or hematite-rich layers (“metallic” layers) during early diagenesis. Renewed crystallization of iron oxides occurred during the regional metamorphism. Microgranular siderite is perhaps primary. Porphyroblasts and glomero-porphyroblastic concretions of siderite, ankerite, and ferriferous calcite are early diagenetic. Minnesotaite and stilpnomelane are late diagenetic minerals. Riebeckite (and perhaps talc) formed during the regional metamorphism.

207Pb/206Pb whole Rock Age of the Archaean Granulite Facies Metamorphic Event in West Greenland

Abstract: IN the Archaean basement complex of West Greenland there are several widely separated granulite facies areas, up to 100 km across1. Rocks from three such areas, Sukkertoppen, Nord-land and Fiskenaesset (Fig. 1), were originally selected for this study on the assumption that the granulite facies metamorphism represented the oldest metamorphic event observable in West Greenland1. This idea has been shown to be ill founded by subsequent geological and geochronological work2,3. The granulite facies metamorphism is now recognized as a distinctive time marker which probably has the following chronological relationships with other principal events in the Archaean of West Greenland, particularly in the Godthaabsfjord area2,3: (1) Plutonic development ∼ 3,700-3,750 m.y. ago4,5 of a complex of granites, granodiorites and tonalites. Metamorphism, migmatization and deformation, with production of banded gneisses (Amitsoq gneisses). (2) Intrusion of a basic dyke swarm (Ameralik dykes) into the Amitsoq gneisses. (3) Eruption of basic volcanics and deposition of sediments (Malene supracrustals). (4) Emplacement of basic igneous complexes with prominent calcic anorthosites. Possibly broadly similar in age throughout the Archaean of West Greenland. (5) Tectonic interleaving of earlier rock units. (6) Intrusion of major suite of granites, granodiorites, tonalites and diorites at 3,040 ±50 m.y. ago6, the parent rocks of the Nuk gneisses. (7) Deformation and folding, accompanied by migmatization and metamorphism that culminated in amphibolite facies conditions in the Godthaabsfjord area, but reached granulite facies to the north and south (in the areas we discuss in this communication). Earlier felsic rocks converted to gneisses. (8) Emplacement of Qorqut granite and pegmatites, ˜ 2,600 m.y. ago (Oxford unpublished data). Intrusion of Precamb-rian dolerite dykes.

Potassium-Argon Geochronometry of Mesozoic Igneous Rocks in Nevada, Utah, and Southern California

Abstract: K-Ar dates for 123 mineral separates from 91 samples of granitic rocks, and 3 samples of Orocopia and Vitrefax Schist from Nevada, western Utah, the Mojave Desert, the Colorado Desert, and the northern and eastern Peninsular Ranges of southern California, range in age from 218 to 19 m.y.; however, most of the dates lie between 160 and 50 m.y. A majority of the hornblende and muscovite dates are nearly concordant with, but generally a few m.y. older than, coexisting biotite dates; some are highly discordant. The oldest plutons gave Late Triassic to Early Jurassic dates (El Paso and San Bernardino Mountains, 218 to 194 m.y.). Jurassic igneous activity reached a culmination slightly before 150 m.y. ago over a large area in northern Nevada and western Utah and supplied ash to the Morrison Formation of the Colorado Plateau. Several plutons of the northeastern Mojave Desert yielded similar dates. A similar major culmination of Mesozoic pluton emplacement in early and middle Late Cretaceous time is represented by scattered plutons in Nevada and probably by most of the plutons in the Peninsular Ranges and southern deserts of California. Earliest Tertiary dates were obtained from far-southeastern California (50 to 66 m.y.). Most dates in the Cordilleran batholith belt of the Southwest, which parallels the edge of the late Mesozoic continent, are between 160 and 75 m.y. and are essentially coeval with metamorphism in the parallel high-pressure, low-temperature Franciscan belt to the west (approximately 150 to 75 m.y.). About 75 m.y. ago, this pattern abruptly changed to a Laramide one with abundant 75 to 50 m.y. dates in a belt running through southern Arizona into southeastern California at a high angle to the edge of the continent. Only very sparse dates of this age are found in Nevada. Abundant Laramide dates are not encountered again until the Idaho, Boulder, and Coast Range batholiths. These important changes in Cordilleran tectonics may be effects of a change from dominantly dip-slip subduction during Franciscan time to dominantly north-northeast strike-slip subduction during Laramide time. Furthermore, the initiation of rifting of eastern North America in Triassic time, changes in spreading rate, and reorientation of spreading directions in the Atlantic Ocean appear to correlate with the beginning of widespread pluton emplacement in western North America, changes in the rate of magma generation in the Cordillera, and shifts of the Cordilleran tectonic pattern, respectively, during Mesozoic time.

Relations between metamorphism and orogeny in a typical section of the Indian Himalayas

Abstract: A comprehensive geological and petrological investigation has been undertaken in an area of about 10 000 km2 in the Indian Himalaya (S-Lahul, Himachal Pradesh). The development of mineral assemblages in metamorphic rocks of medium grade is considered to be a dominant Alpine event, although almost exclusively Paleozoic and Precambrian rocks have been involved. The Barrowian type of this metamorphism, ranging from the anchi- to the sillimanite zone, took place under the elavated T-gradient of about 4°C/100 m. It is suggested that “normal” geothermal conditions prevailed only in the outermost zone of this orogenic belt. In the Late Tertiary this metamorphic series has been moved as a huge nappe upon the Lower Himalaya. During this process a unique feature of reverse metamorphism has been formed. It can be shown that this feature was caused by a combination of metamorphism and very rapid tectonic movements.

Tectonic Displacements and Thermal Activity in Two Contrasting Proterozoic Mobile Belts from Greenland

Abstract: The Proterozoic Nagssugtoqidian and Ketilidian mobile belts are comparable in scale with those of the Phanerozoic rather than those of the Archaean. These two Proterozoic belts differ from one another both in the tectonic displacements which gave rise to them, and in their thermal activities as expressed by igneous and metamorphic characteristics. Similar differences between modern tectonic belts have been interpreted in terms of plate tectonics. The Nagssugtoqidian is characterized by considerable crustal shortening, very limited igneous activity, and high-pressure regional metamorphism which may be related to crustal thickening resulting from both ductile and brittle overthrusting of the Nagssugtoqidian rocks over the Archaean foreland. Evidence of crustal shortening in the Ketilidian is limited, but vertical and transcurrent movements are important. Widespread igneous activity throughout the active history of the belt resulted in the formation of mainly acid volcanic supracrustal rocks and widespread granite intrusion. The appinite suite is also well represented. Metamorphism is mainly of low-pressure type. A tentative comparison can be made between the Alpine and Nagssugtoqidian belts on the one hand, and Andean and Ketilidian belts on the other.

Crustal Development in the Precambrian

Abstract: The oldest Archaean rocks in most shield regions are largely granulites and gneisses, and in west Greenland there is evidence of 1000 Ma of crustal history before the final high-grade metamorphism. Archaean greenstone belts are mostly younger than the high-grade terrains although in some areas, such as southern Africa, this has not yet been proved reliably. The greenstone belts may have developed as oceanic crust in connexion with plate movements, the earlier continents being represented by the more deeply eroded high-grade regions. Stabilization of the Archaean cratons is signalled by continental-scale intrusion of dolerite dyke swarms. Proterozoic mobile belts are exposed at two structural levels. Some early linear basins have mio- and eu-geosynclinal parts and may have been located along Proterozoic suture lines. More deeply eroded mobile belts are often floored by extensive, partly reworked, crystalline basement and probably developed along linear rifted zones which acted as loci for high heat flow and igneous activity; they lack ophiolites and are difficult to interpret as collision-type mountain belts. Most probably there were intra-continental plate movements in the Proterozoic.

Age and Origin of Detrital Zircons from the Pre-Permian Basements of the Bohemian Massif and the Alps

Abstract: U-Pb isotopic analyses were made on detrital zircon populations from sandstones and quartzites of the pre-Permian basement in an attempt to shed light on the presedimentary history of the zircons and the age of their primary source rocks. Eight rock samples were collected from the Saxothuringian and Moldanubian parts of the Bohemian Massif, the western part of the Upper Austroalpine Nappes, and the Southern Alps. The heterogeneous populations were separated into fractions of different size, magnetic susceptibility, color, and shape. Because of their typically pitted surface all zircon grains from the sandstones and quartzites appear to be detrital. Only in three samples from the Alps—one from a contact metamorphic aureole—the zircons show surface recrystallization and minor new growth. With the exception of some euhedral crystals in the Saxothuringian quartzites all zircon fractions have highly discordant U-Pb ages. On a concordia diagram their data points scatter slightly around best-fit lines with upper intersections between 2000 and 2300 m.y. From this pattern the following conclusions are reached: The lower intersections of the best-fit lines with the concordia curve cannot be clearly correlated with an episodic disturbance of the U-Pb systems during weathering and sedimentation and/or during regional metamorphism. For the zircons of the Bohemian Massif a disturbing event, about 550 to 600 m.y. ago, is likely. Clear, euhedral, but nevertheless detrital zircons found among the zircon populations of two Saxothuringian quartzites (“Plattenquarzit” of the pre-Ordovician “Arzberger Serie” and Lower Ordovician “FrauenbachQuarzit”) crystallized most probably during the Upper Proterozoic and/or the Assyntian petrogenesis. The highly discordant age pattern of the detrital zircons from the Alps is likely to be the result of the Caledonian and/or Hercynian (=Variscan) metamorphism. Differences in concentration levels of common lead in detrital zircons and the problem of red zircons as indicators of Precambrian origin are discussed.

Oxygen Isotope Geothermometry of the Burial Metamorphic Rocks of the Precambrian Belt Supergroup, Glacier National Park, Montana

Abstract: Isotopic evidence supports the conclusions of other studies that the Precambrian Belt rocks have been subjected to high-grade diagenesis and low-grade metamorphism. Isotopic temperatures calculated from O 18 /O 16 ratios of coexisting quartz and illite range from 225°C to 310°C and are interpreted as being temperatures reached during metamorphism. Isotopic temperatures generally increase with maximum depth of burial. A plot of isotopic temperatures as a function of depth of burial can be extrapolated to 20°C (surface temperature) at approximately 5,500 m above our uppermost sample. This is consistent with the probable amount of overburden above this sample as inferred from stratigraphic evidence. The isotopic temperatures are also consistent with a model of equilibration during burial in a normal geothermal gradient. Feldspar underwent isotopic exchange during metamorphism but does not always appear to have attained isotopic equilibrium with quartz and illite. The isotopic data indicate some degree of disequilibrium between carbonate and quartz and suggest that the carbonate may have been more readily subject to retrograde exchange than was the silicate. The isotopic compositions of whole-rock samples vary with depth in the stratigraphic section, apparently reflecting post-depositional isotopic exchange.

Changes in Mineral Assemblages with Metamorphism of Some Banded Precambrian Iron-Formations

Abstract: The unleached sedimentary oxide facies of unmetamorphosed Precambrian iron-formation generally consists of a finely banded assemblage of chert, jasper or quartz, hematite or magnetite, or both, and locally some hydrous iron oxides. Upon regional metamorphism to kyanite or sillimanite grade, recrystallization of the chert and iron oxides takes place, accompanied by a marked increase in grain size of the constituent phases, especially quartz. Generally no reactions take place among these minerals and original sedimentary textures are frequently preserved in the recrystallized assemblage. The resulting, banded, metamorphic quartz-specularite-magnetite horizons, with quartz-specularite and quartz-magnetite bands alternating on a scale of less than 1 mm, indicate that oxygen has behaved as an internally controlled (buffered) component in these occurrences during metamorphism.The banded sedimentary carbonate facies generally consists of one or more carbonates (calcite, members of the ferroan dolomite-ankerite series, and siderite), lesser amounts of magnetite, chert, or quartz, and locally some iron silicates. If the original carbonate facies consists only of carbonate, magnetite, and quartz, and if the chemical potential of CO 2 during subsequent metamorphism remains locally high enough to prevent the breakdown of the carbonates, no reactions occur among the coexisting phases. Only recrystallization and increased grain size are noted. If, on the other hand, the chemical potential of CO 2 is reduced, chert (or quartz) will react with the carbonates to form new silicates. For example :Ca(Fe,Mg) (CO 3 ) 2 + 2SiO 2 = Ca(Fe,Mg)Si 2 O 6 + 2CO 2 ferroan dolomite clinopyroxeneIf the chemical potential of H 2 O is high, while the chemical potential of CO 2 is low during metamorphism, the following type of reaction occurs:5Ca(Fe, Mg) (CO 3 ) 2 + 8SiO 2 + H 2 O = Ca 2 (Fe,Mg) 5 Si 8 O 22 (OH) 2 + 3CaCO 3 + 7CO 2 ferroan dolomite actinoliteAs a result of medium- to high-grade metamorphic conditions (kyanite to sillimanite zones), much of the banded carbonate facies contains metamorphic silicates, indicating a generai loss of CO 2 during metamorphism. Some parts of the same carbonate facies in the same metamorphic area may, however, still consist of the original, although recrystallized, quartz-carbonate-magnetite assemblage. This implies that the chemical potential of CO 2 has been locally variable in the iron-rich carbonate rocks and that CO 2 , therefore, cannot at all times be considered as a perfectly mobile component. Such carbonate-quartz-magnetite recrystallization has taken place in a system essentially closed to CO 2 .The original silicate facies consists of a complex mixture of hydrous Fe-silicates (greenalite, stilpnomelane, and minnesotaite), carbonates (members of the dolomite-ankerite series, siderite, and calcite), chert or quartz, and iron oxides. This part of the iron-formation shows most consistently the results of general decarbonatization and dehydration during progressive metamorphism. Not only do the carbonates and chert react to form abundant silicates, such as members of the cummingtonite-grunerite series, but also the original silicates give way to clino- and orthoamphiboles, garnet, and ortho-and clinopyroxenes. The production of olivine (fayalite) has been described only from contact metamorphic occurrences such as along the contact of the Duluth Gabbro with the Biwabik Iron Formation in Minnesota. Fayalite assemblages represent conditions of low relative chemical potential of O 2 . Manganese-rich horizons are frequently present. Original mixtures of Mn-oxides, hydroxides, and carbonates (rhodochrosite and kutnahorite) give way, upon metamorphism, to assemblages containing rhodonite, Mn-garnet (spessartite or calderite), manganoan cummingtonite, Mn-rich pyroxenes (such as manganoan aegrine-augite and hedenbergite), bustamite, and rhodochrosite. In the metamorphic assemblages of the Labrador Trough, many Mn-rich horizons of the iron-formation are also Na-rich, as shown by assemblages such as Mn-aegirine-rhodonite-specularite-rhodochrosite.

A plate tectonic model for the Archaean crust

Abstract: The characteristics of Archaean greenstone belt terrains are briefly summarized together with some of the models which have been used to account for their genesis. Crystalline sialic crust is interpreted as having increased with time by separation from the mantle. Many of the problems posed by Archaean greenstone belt terrains may be eased if the first sialic crust is assumed to have consisted of small masses concentrated by plate tectonic processes similar to those still in operation. Even if Archaean plates were of similar size, and were formed and lost at rates similar to those since the Mesozoic, there would be differences in the manner in which the sialic crust was concentrated because so little had separated from the mantle during Archaean times. The original formation of the rocks now forming the earliest tonalite gneisses and migmatites is attributed to very early Archaean times when no large sialic concentrations are likely to have existed on subduced lithospheric plates and the only form of orogeny was of the Island Arc type. Even after sialic concentrations did become incorporated in subduced plates they may for a long time have been too small for significant areas to survive extensive remobilization and addition of magmas from below whenever they were associated with plate boundary zones. Sets of greenstone belts are interpreted as vestiges of former oceans. By the end of Archaean times the sialic crustal concentrations, despite possible fragmentation and periods of independent development, became sufficiently extensive for large areas to survive ocean closure without significant remobilization. This model implies that there is no need for orogeny to have been any more extensive in Archaean times than now; it could merely have been more extensive compared to the area of the sialic crust in existence at the time. Plate tectonic models of Archaean tectonics are distinguished from the alternatives by their implication that large relative motions occurred between the oldest parts of the granitoid masses now on either side of the greenstone belts. Palaeomagnetism may be able to distinguish the relative usefulness of the models if any such relative motions can be recognized through the effects of remobilization of most, if not all, the sialic crust in Archaean times. Other tests are possible but the most useful might be the necessity for any model of the formation of the greenstone belts being adaptable enough to account for the relationships emerging from studies of the Archaean crustal remnants characterized by granulite facies metamorphism and anorthosites.

Geología de los cuadrángulos H-12 Bucaramanga y H-13 Pamplona, departamento de Santander

Abstract: A program of geologic mapping and mineral investigation in Colombia was undertaken cooperatively by the Colombian Instituto Nacional de Investigaciones Geologico-Mineras (formely known as the Inventario Minero Nacional), and the U.S. Geological Survey1 sponsored by the Government of Colombia and the Agency for International Development, U.S. Department of State. The purpose was to study and evaluate mineral resources (excluding of petroleum, coal, emeralds, and alluvial gold) of four selected areas, designated Zonesl to IV, that total abo ut 70.00C km2. The work in Zone III, in the Cordillera Oriental, was done from 1965 to 1968. The northeast-trending of Cordillera Oriental of Colombia swings abruptly to north-northwest in the area of this report and divides around the southern end of the Maracaibo Basin. This section of the Cordillera Oriental is referred to as the Santander Massif. Radiometric age determinations indica te that the oldest rocks of the Santander Massif are Precambrian age and include high-grade gneiss, schist, and migmatite of the Bucaramanga Formation. These rocks were probably part of the Precambrian Guayana Shield. Low to medium grade metamorphic rocks of Late Precambrian to Ordovician age include phyllite, schist, metasilstone, metasandstone, and marble of the Silgara Formation, a geosynclinal series of considerable extent in the Cordillera Oriental and possibly the Cordillera de Merida of Venezuela. Orthogneiss ranging from granite to tonalite is widely distributed in the high and medium grade metamorphic rocks oi the central core oi the massif and probably represents rocks of two ages, Precambrian and Ordovician to Early Devonian. Younger orthogneiss and the Silgara are overlain by Middle Devonian beds of the Floresta Formation which show a generally low but varying degree of meta morphism. Phyllites and argilhtes are common, and infrequent marble and other calcareous beds are fossiliferous. Except for recrystallization in limestones oi the Permian Carboniferous Diamante Formation, sedimentary rocks younger than Devonian are unmetamorphosed. The effects of Precambrian regional dynamothermal metamorphism and plutonism on Precambrian geosynclinal deposits reached the upper amphibolite facies in the Bucaramanga Gneiss. Geosynclinal deposits oi the Silgara Formation were subjected similar conditions in Late Ordovician and Early Silurian time but reached only the green schist or lower amphibolite facies. Orthogneisses generally show a concordance of foliation and lineation with those of neighboring bodies of the Silgara Formation and the Bucaramanga Gneiss rocks, as well as similari ties in grade of metamorphism. Regional dynamothermal metamorphism in Late Permian and Triassic time reached low grade in the Floresta Formation and caused recrystallization of limestone of the Diamante Formation. The Bucaramanga and Silgara metamorphic rocks show eviaence of retrogressive metamorphism with high activity of potassium and water, but whether this occurred at the time the Floresta was metamorphosed or la ter is not clear. Batholiths, plutons, and stocks of igneous rocks in the Santander Massif range from diorite to granite. Radioactive age data indicate that most of them belong to a single plutonic interval. These are referred to as the Santander Plutonic Group, and are Jurassic and Jurassic-Triassic. Two suites of this group are pink granite and quartz monzonite, andgray quartz monzonite and granodiorite. Contact relations indicate that the pink and more granitic rocks are younger than the gray and more mafic rocks, but radioactive age data are in conflict with this. Undated plutonic rocks that are not clearly related to the group are assigned to relatively older or younger age positions. Rhyolite occurs west of the Bucaramanga fault as a small body in one Jocality and as an intrusive sheet with granophyre and intrusive breccias in Triassic sedimentary rocks in another locality. The age is unknown but probably is younger than the Santander Plutonic Group. Felsic, mafic, and lamprophyric, dikes are common in the batholiths, plutons, and adjacent rocks and most appear to be genetically related to the larger igneous bodies, whereas rarer dikes of dacite porphyry, basalt and diabase are not related. Basalt and diabase dikes are widely scattered aild have been found nearly as high in the section as the Jurassi Cretaceous boundary. Dacite porphyry is the only igneous rock that intrudes rocks of Gretaceous age. With the uplift that accompanied emplacement of batholiths in Latest Triassic and Jurassic time, erosion of the roof rocks furnished fine-grained redbeds and conglomerates of the Jordan Formation followed by erosion of the batholiths themselves that provided the coarse-grained and conglomeratic arkosic sediments of the Giron Formation in thick accumulations off the flanks of the uplift. This period was followed by marine invasion and sedimentation of the Cretaceous period. In the Magdalena Valley area, Lower Cretaceous sedimentation began with quartz sands of the Tambor Formation and continued with fossiliferous limestone of the Rosa Blanca Formation, black shale of the Paja Formation, fossiliferous limestone, glauconitic sandstone and black shale of the Tablazo Formation, and still more black shale of the Simiti Formation. In Late Cretaceous time, calcareous black shale with chert and phosphatic beds in the upper part of the La Luna Formation were deposited during the time of most widespread marine transgression. Thereafter gray shales with limonitic beds of the Umir Formation accumulated as marine conditions wore gradually succeeded by continental deposition with coal beds in latest Cretaceous. Cretaceous deposition over the area was mostly uniform in character if not in thickness, and remnants of these rocks that have escaped erosion in the massif are similar to the Cretaceous rocks of the Magdalena Valley to the west and the Maracaibo Basin to the east. Continental conditions prevalied in the Magdalena Valley area through the Tertiary with sandstone and shale containing coal beds in the Paleocene Lisama Formation, followed in the Eocene by thick conglomeratic sandstone of the La Paz Formation and sandstone silstone, and shale of the Esmeraldas Formation, in the Oligocene by shale of the Mugrosa Formation, and shale with coarse conglomeratic sandstone of the Colorado Formation, in the Miocene by still coarser and thicker sediments of the Real Group, and continuing into the Pliocene and Pleistocene in the Mesa Group. The section of Tertiary rocks in the Colombian part of the Maracaibo Basin is mostly similar in origin and lithologic character but thinner than that in the Magdalena Valley. These rocks were eroded from, or were never deposited in the area that is now the highest part of the massif. Alpine glaciation occurred on the Santander Massif during the Pleistocene, and widespread terraces in the lower valleys may date from this period. Orogeny is probably at or near its highest leve! at the present time with streams eroding the flanks of the massif at a high rate, aided by deep weathering and landslides. The Bucaramanga fault, a major fault of regional extent., trends north·northwestward across the area and apparently extends on to the north coast as the Santa Marta fault defining the western boundary of the Santa Marta Mountains. The present investigations indicate a long and complex history for the Bucaramanga fault with earlier lateral displacement, followed by later uplift of the Santander Massif to the east that continues to the present time. West of the Bucaramanga fault are three areas of rather distinct structural character: A wedge-shaped, dwon-faulted block between the Bucaramanga and Suarez faults is mostly an area of mesas, tilted slightly westward, capped by basal Cretaceous sandstone. At the thin north end of thewedge, Quatemary gravels and mudflows accumulated in the fault-formed basinand now form the dissected terrace on which Bucaramanga, the main city of the region, is located. A plateau belt bordering the mesas west of the Suarez fauli consists mostly of dissected beds, undulating to steeply dipping, of the thick Giron Formation. West of the plateau area ali sedimentary rocks from Jurassic to Tertiary plunge westward into the deep trough of the Nuevo Mundo syncline. This narrow syncline is on the deeper easter side of the geosynclinal area of the Magdalena Valley basin. lt is mostly separated from the shallower part of the geosyncline to the west by the north trending La Salina fault, which places Upper Cretaceous rocks on the east side in contact with Oligocene and Miocene rocks on the west. In the high country that continues south and east of the metamorphic and igneous rocks of the Santander Massif, two north trending structural basins are separated by the regional Servita fault. The western basin contains sedimentary rocks ranging from Devonian to Upper Cretaceous and is complexly faulted. Rocks of the eastern basin range from Lower Cretaceous to Eocene and have undergone compressional folding that is more intense toward the west. Many faults were mapped to the east and west of the Bucaramanga fault, and many more are indicated by lineaments on aerial photographs. Most have trends withina range of north-northeast to north-northwest, mostly parallel to the trend of structure. Only a few major faults cut across this trend. On the east and west flanks of the Santander Massif, belts of sedimentary rocks that include mostly Cretaceous formations have escaped erosion on the downthrown sides of long faults. On the east flank the down· thrown sides are on the west, and on the west flank the downthrown sides are on the east, which suggests either more active uplift of the flank areas or collapse of the central area relate to the flanks.

The pre-Silurian rocks of Clare Island, Co. Mayo, Ireland, and the age of the metamorphism of the Dalradian Ireland

Abstract: The Highland Boundary Fault is considered to extend from Stonehaven in Scotland to the major Leek fault of Clare Island in western Ireland. This conspicuous fault of the island has however been erroneously correlated with the Leannan fault of Donegal, a probable branch of the Great Glen Fault. South of the Leek fault Silurian sediments rest unconformably upon a metamorphic basement. This consists of amphi-bolite facies metasediments intruded by basic and ultrabasic rocks that have also undergone amphibolite facies metamorphism. These high grade rocks of uncertain age are considered to be part of a horst which was uplifted within the Moine Dalradian basin of western Ireland during Cambrian times. The horst was probably a southern source of detritus for the Upper Dalradian turbidites now on both sides of the Leek fault. The large uplift to the south which formed this horst conforms with and expands previous ideas on the early history of the Highland Boundary Fault. The age of the Dalradian metamorphism in western Ireland is reconsidered, and it is concluded that the case for a pre- Nitidus Zone age is unsatisfactory and that a mid-Ordovician age is possible.

Progressive niedriggradige Metamorphose glaukonitführender Horizonte in den helvetischen Alpen der Ostschweiz

Abstract: Glauconite-bearing formations of Cretaceous and Tertiary age in the Helvetic zone of the Glarus Alps have been investigated by microscopic, X-ray, wet chemical, electron microprobe, Mossbauer spectroscopic, and K-Ar dating methods. 3 different metamorphic zones with increasing grade can be distinguished (Fig. 3). Original, unmetamorphosed sediments containing glauconite-calcite-quartz±chlorite comprise zone I. The glauconite is very rich in potassium (8–9 wt.%) and the chlorite is Fe-rich. In zone II green stilpnomelane forms by the reaction: glauconite±chlorite + quartz = stilpnomelane + k-feldspar + H2O + O2. The green stilpnomelane contains as much as ten times the amount of K found in brown stilpnomelane, which is believed to be a weathering feature. In zone III biotite appears by the reaction: chlorite + k-feldspar = biotite + stilpnomelane + quartz + H2O. Riebeckite is a possible additional phase in all three zones. Generally, zones I–III are arranged nearly parallel to the Alpine border with metamorphic grade increasing to the south. In the Glarnisch Massif, however, the transition from zone I to zone II is clearly controlled by the overburden of the nappe pile (Fig. 6). The beginning of zone II also seems to coincide with the middle of the anchizone, as defined by illite-crystallinity measurements in adjoining marly shales and slates; this corresponds approximately to the transition from the zeolite facies to the prehnitepumpellyite facies. K-Ar-ages on glauconites regularly decrease when approaching the zone I/II-transition. Field evidence and combined K-Ar age determinations on glauconites, stilpnomelanes and riebeckites point to a peak of the metamorphism during Lower to Middle Oligocene, shortly after the main orogenic phase in this part of the Helvetic Alps.

Ordovician and Silurian history of the southeastern part of the Lachlan Geosyncline

Abstract: During the Ordovician and Silurian Periods that part of the Lachlan Geosyncline lying between Yass, N.S.W., and the Victorian border changed from a deep-sea region with basic volcanics, volcanic graywackes, and cherts, to a shallow marine to subaerial platform dominated by acid volcanics, volcaniclastics, mudstones, and limestones. In the course of this development a quartz-graywacke distal flysch body spread throughout the region during the Late Ordovician. This was deformed during the Benambran Orogeny at the end of the Ordovician by gravity tectonics consequent upon the anatexis, metamorphism, folding, and uplift of the Wagga Metamorphic Belt to the west. The region remained deeply submerged after the Benambran Orogeny, and a series of submarine fans comprising a quartz graywacke proximal flysch was developed along its western margin during the late Llandoverian. Distal equivalents may be present, unrecognized, within the Ordovician flysch to the east. Early in the Wenlockian the Quidongan Oro...

The structural setting and geochronology of basal granitic gneisses in the Caledonides of part of Nordland, Norway

Abstract: Rb–Sr whole rock isochron dates of 1780 ± 43 m.y. and 1731 ± 73 m.y. (λ 87 Rb 1.39 × 10 −11 yr −1 ) have been measured on basal granitic gneisses which underlie Caledonian metamorphic rocks in the Glomfjord-Nasafjall tract of Norway. Rb–Sr data from a part of the region can be interpreted in terms of localized Sr isotope homogenization at around 1200 m.y., but severe Caledonian deformation and metamorphism has had little apparent effect on the whole rock systems on the scale of sampling. The dates are discussed in the context of published Rb–Sr and U–Pb dates from the Baltic Shield, of which a distribution map is given. It is concluded that the gneisses investigated attained their pre-Caledonian form during the Svecofennian orogeny, but may have been affected in part by the Sveconorwegian orogeny.

Geochronological investigation of the quartzofeldspathic rocks of the Lewisian of Rona, Inner Hebrides

Abstract: Rona Island consists of Lewisian quartzofeldspathic gneisses and amphibolites that show the effects of polyphase deformation. The age of the gneisses, from a six point Rb-Sr whole rock isochron, with λ 87 Rb 1.39 × 10 −11 yr −1, is 2790 ± 210 m.y. This is not significantly different from the age of 2710 ± 20 m.y. determined from zircon fractions from a single block of the gneiss. These results are interpreted as reflecting amphibolite facies metamorphism and gneiss formation, associated with the first recognized fold phase, during the Scourian orogenic episode A Rb-Sr age for unaltered muscovite books from a pegmatite emplaced in the hinge zone of a fold of the third deformational phase is 1740 ± 10m.y. and a Rb-Sr whole rock isochron for a later granite sheet, emplaced between the fifth and sixth fold phases is 1680 ± 170 m.y. These igneous events, and the fold phases between them, belong to the later part of the Laxfordian orogenic episode. No isotopic disturbance of the Rb-Sr whole rock systems of the analysed gneisses has taken place during the Laxfordian episode and no isotopic events between the c 2700 m.y. and c 1700 m.y. episodes have been found in the quartzo, feldspathic rocks. K-Ar ages on the muscovite samples suggest minor opening of minerals to argon loss sometime after 1740 m.y.