Showing papers on "Metamorphism published in 1974"

Al and Ti contents of hornblende, indicators of pressure and temperature of regional metamorphism

Abstract: Chemical analyses of hornblendes from different regional metamorphic terrains and from rocks of different metamorphic grade have been compared. Hornblendes of lowpressure type are distinguished from hornblendes of high-pressure type by their AlVI and Si contents. The Ti content of hornblende is shown to increase with metamorphic temperature from the greenschist-amphibolite transition facies to the hornblende-granulite facies.

The geological development of the southern part of the New England Fold Belt

Abstract: Two major divisions of the New England Fold Belt, Zone A and Zone B, are separated by the Peel Fault. Deposition in these two zones was probably contemporaneous (Lower Palaeozoic ‐ Lower Permian). Terminal orogenesis in both zones was also contemporaneous (Middle Permian) but whereas in Zone A deformation was only moderate, metamorphism was of burial type, and granitic emplacement was uncommon, in Zone B many rocks were severely deformed and regionally metamorphosed, and both syn‐tectonic and post‐tectonic granites are widespread. Pre‐orogenic palaeogeography is envisaged in terms of an evolving volcanic chain ‐ fore‐chain basin ‐ trench system, with an outer non‐volcanic arc developed in the Carboniferous. Cessation of movement on a subduction zone dipping westward beneath the volcanic chain is believed to have caused the Middle Permian deformation, but neither metamorphism nor the granitic rocks are directly related to subduction.

The b 0 value of the potassic white micas as a barometric indicator in low-grade metamorphism of pelitic schists

Abstract: The b 0 of the potassic white micas was measured in 410 samples of low-grade pelitic schists from the Central Pyrenees, the Abukuma belt, Northern New Hampshire, the Eastern Alps, Otago, the Sanbagawa belt and the Central Pontic Chain. Temperature and rock bulk composition can be considered substantially constant all over the sample range, so that the differences in the b 0 values depend primarily on differences in pressure. The analytical data demonstrate that the b 0 increases for increasing pressure, and that it is practically identical in the micas from very different regions belonging to the same baric type of metamorphism. With the help of other analytical data (about 600), a b 0 scale was prepared, which makes the distinction of six different facies series possible. The writers, proposing this very rapid new method of barometric analysis, draw attention to its limitations and point out the main fields of its application.

O 18 enriched ophiolitic metabasic rocks from E. Liguria (Italy), Pindos (Greece), and Troodos (Cyprus)

Abstract: Low grade hydrothermally metamorphosed ophiolitic basic rocks from E. Liguria (Italy), Pindos (Greece) and Troodos (Cyprus) are enriched in O18 relative to the oxygen isotope ratio of fresh basalt (6.0±0.5‰). The maximum observed δO18 value of +13.22‰ corresponds to a positive isotope shift of 7‰ Enrichments in Sr87 relative to Sr86 correlate with hydrothermal alteration. The δC13 values of secondary calcite from E. Liguria are positive, and fall in the range from +0.2% to +3.6‰ Since ophiolitic rocks are considered to be fragments of the oceanic crust and upper mantle, and since the secondary metamorphic assemblages were produced before mechanical emplacement, it is considered that the hydrothermal metamorphism which affected these rocks occurred in the sub-sea-floor environment. The isotope data are directly consistent with the hypothesis that the alteration was produced by interaction of the basaltic material with introduced sea water. Water: rock ratios were sufficiently large to produce the observed isotope shifts. In the Troodos ophiolite sequence δO18 values decrease steadily downwards and change to progressively larger depletions in the Sheeted Intrusive Complex. The trend of δO18 decrease correlates with the original direction of increasing temperature. The O18 depletions, which have also been observed for oceanic “greenstones” (Muehlenbachs and Clayton, 1972b), resulted from water/rock interaction at temperatures greater than the particular temperature range above which whole rock-water fractionations became less than the isotopic difference between fresh basalt and sea water. Since this isotope geochemistry indicates that the water responsible for hydrothermal metamorphism was of sea water origin, the data support the more general hypothesis that convection of sea water within the upper 4–5 kms of the oceanic crust is a massive and active process at oceanic ridges. This process may be completely or partially responsible for (a.i.), the local scatter and low mean value of the conductive heat flux measured near ridges, (a.ii), the transfer of considerable quantities of heat from spreading oceanic ridges, (b) hydrothermal metamorphism, metasomatism and mineralization of oceanic crust, (c), the production of metal enriched, relatively reduced brines during sea water/basalt interaction, d), the high degree of scatter and low mean value of the compressional wave velocities of oceanic basement layer 2 and (e), the low natural remanent magnetization (NRM) intensity of the lower part of layer 2 and upper part of layer 3 of oceanic crust.

The Scandinavian Caledonides

Abstract: Almost the whole of the western edge of the landmass of Scandinavia is occupied by a 1700-km belt of deformed and sometimes metamorphosed rocks on which in the west lie a few isolated masses of relatively undeformed and completely unmetamorphosed Old Red Sandstone. The belt, shown in Fig. 1, where it contrasts with the undivided and unornamented autochthon on its east side, is subdivided in two ways. Firstly and fundamentally there is lateral division into elements running great lengths of the chain. These elements, the five zones of Fig. 1, are distinguished from one another by different rock assemblages as well as different levels of deformation and metamorphism. Boundaries between zones are not steep but gently dipping known, or presumed, thrust faults. Secondly, there is a division into Southern, Central, and Northern Caledonides over the late culminations of Grong and Rombak, which although very convenient for description are not as fundamental since the major zones appear to pass over them.

Geochemistry of the Rare Earth elements in spilites from the oceanic and continental crust

Abstract: Analytical data including major elements, the Rare Earths, Ni, Cu, Zn, Rb, and Sr are presented for twenty-three spilites from the Mid-Atlantic Ridge, the Hercynian part of the Variscan geosyncline in Germany, and several localities in Switzerland. Low grade metamorphism (up to approximately 400° C) and spilitization of basaltic rocks apparently do not alter the original Rare Earth element (REE) distributions. This fact permits comparison of the spilites and unaltered tholeiitic basalts from the Mid-Atlantic Ridge. The relative REE distributions thus appear suitable for delineating the original basalt types of spilites formed by metamorphism.

Deformation and Metamorphism of the Franciscan Subduction Zone Complex Northwest of Pacheco Pass, California

Abstract: The Franciscan Complex northwest of Pacheco Pass, California, includes three fault-bounded units, each characterized by a different deformational style and suite of metamorphic mineral assemblages. Structurally highest is jadeitic pyroxene-bearing metagraywacke semischist. The areally extensive Garzas tectonic melange separates the semischist from the structurally lowest pumpellyite-bearing Orestimba metagraywacke. The Garzas melange is representative of Franciscan melanges in general. These mappable bodies have an internal fabric dominated by penetrative, mesoscopic shear fractures and contain tectonic inclusions of all sizes immersed in a pervasively sheared, generally fine-grained matrix. The shear fractures record brittle deformation of consolidated rock bodies. Many melanges contain exotic inclusions, clearly not derived from adjacent units, and metamorphic mineral assemblages and textures indicate that they were metamorphosed before being tectonically mixed with more voluminous, generally lower grade, metagraywacke inclusions. The structural units are grossly sheetlike in external form and are separated from one another by gently to steeply dipping major faults. Unlike low-angle thrusts in imbricated Cordilleran terranes, the faults do not systematically repeat or offset a normal stratigraphic sequence but rather juxtapose rock units that bear no apparent stratigraphic, deformational, or metamorphic relation to one another. The structural units were separately deformed and metamorphosed under a variety of conditions prior to their tectonic juxtaposition during late Mesozoic continental margin subduction. Field and petrographic evidence permit, but do not prove, the hypothesis that both the semischist and exotic, high-grade melange inclusions once were more deeply buried and have been emplaced upward into their present anomalously shallow structural positions.

Stratigraphy and structure of the western Moine and the problem of Moine orogenesis

Abstract: The stratigraphy and major structure of some 650 km2 of Precambrian, Moine metasediments in western Scotland are discussed and it is shown that at least four major deformation phases involving primary isoclinal folding with fold amplitudes of at least 20 km, and two major metamorphic episodes affect both the Lewisian basement and the Moine cover. It is argued that there is sufficient evidence to regard the first two deformation phases together with the earlier metamorphism as Precambrian in age and the subsequent deformations and metamorphism as Caledonian.

Geochronology of a polymetamorphic and anatectic gneiss region: The moldanubicum of the area Lam-Deggendorf, eastern Bavaria, Germany

Abstract: Rb-Sr isotopic analyses of whole-rocks and biotite and U-Th-Pb analyses of zircon and monazite reveal regional metamorphic events for the Ordovician (Caledonian metamorphism) and the Carboniferous (Variscan=Hercynian orogeny), both accompanied by anatexis The extent of the Caledonian and Variscan anatexis, however, cannot be evaluated, so far, because the field petrographic criteria are not sufficient to distinguish clearly between early and late Paleozoic anatexites Evidence for a Precambrian metamorphism has not been found Rb-Sr whole-rock isochrons obtained on leucosomes and melanosomes of partially molten paragneisses are interpreted as a minimum age of the second, early Variscan anatexis The alternative explanation of the isochrons as a result of local Sr isotopic redistribution without a melt involved is considered less likely Concordant and nearly concordant zircon ages (318–335 my) of a coarse-grained granite and of “diatexites” are regarded as evidence for an intensive early Variscan granitization and palingenesis Concordant zircon ages of “diorite” dykes, crosscutting the anatexites, establish a lower time limit of 309–312 my for the Variscan anatexis Rb-Sr ages of biotite (310-290 my) indicate the end of the Variscan metamorphism

A Model for the Development of Thin Overthrust Sheets of Crystalline Rock

Abstract: Overthrust sheets of granitic rock have numerous features in common with overthrust sheets of ultramafic rock. Regardless of rock type, these overthrust masses are 5 to 10 km thick or more, tens of kilometers wide, and up to hundreds of kilometers long. Their upper parts may consist of undeformed and unmetamorphosed igneous or sedimentary rocks, but near their base the crystalline rocks become tectonites, and retrograde metamorphism is widespread. A necessary condition for the formation of these sheets is a region of unusually steep geothermal gradient such as may be found behind volcanic arcs or in any region of upwelling mantle (for example, young ocean floor). At the time of detachment, the relatively brittle and rigid cover moves away from its underlying thermally softened base. At first, displacement between cover and base is penetrative, but as strain softening (and shear heating?) proceeds, displacement becomes localized along a fault zone that will be parallel to isotherms in the rock, and the overthrust crystalline-based sheet is thus freed from its base. During movement, the underlying rocks of the detached sheet may be metamorphosed, and fragments of the overridden rocks may be picked up. When movement ends, the cooled sheet is emplaced on top of cooler rocks. Fluids driven from overridden rocks during movement, and following emplacement, will act to reduce frictional resistance and will result in retrograde metamorphism of the overlying crystalline mass. K-Ar dates in the lower part of the overriding sheet may be set during movement. Such a model explains the observed structural relations, metamorphic history, and thinness of the overthrust sheets. Rigidity of the detached cover and isostasy prevent the cover (suprastructure) from folding; these conditions require that the cover be displaced laterally for great distances. Lateral constraints and lithologic anisotropy will dictate the direction of overthrusting.

The zonal pattern of regional metamorphism in the Scottish Caledonides

Abstract: The petrology of analysed Moine calc-silicate rocks enables a metamorphic zonal map of the Moines to be drawn. Correlation with pelite isograds in the Dalradian has permitted the production of a zonal map of the entire Scottish Caledonide Belt. The complex zonal pattern in the Moines is attributed partly to the interaction of different metamorphic events, which brought about widespread retrogression and apparent metamorphic inversion, and partly to post metamorphic folding of isograde surfaces.

Spilites and spilitic rocks

Abstract: 1. Introductory and General Papers.- Some Notes on the Problem of Spilites.- Spilitic Magma. Characteristics and Mode of Formation.- Essai de Caracterisation Chimique des Associations Spilitiques.- Pyroxenes and the Basalt. Spilite Relation.- 2. Papers Proposing a Primary Origin.- A Reappraisal of the Textures and the Composition of the Spilites in the Permo-Carboniferous Verrucano of Glarus, Switzerland.- A Series of Magmatism Related to the Formation of Spilite.- Environmental Effects in Magmatic Spilite.- A Statistical Study of Specific Petrochemical Features of Some Spilitic Rock Series.- Middle Triassic Spilite-Keratophyre Association of the Dinarides and Its Position in Alpine Magmatic-Tectonic Cycle.- Petrogenesis of Spilites Occurring at Mandi, Himachal Pradesh, India.- General Features of the Spilitic Rocks in Finland.- The Relationships between Spilites and Other Members of the Oman Mountains Ophiolite Suite.- Gradation of Tholeiitic Deccan Basalt into Spilite, Bombay, India.- 3. Papers Proposing an Autohydrothermal or Autometamorphic Origin.- Vers une Meilleure Connaissance du Probleme des Spilites a Partir de Donnees Nouvelles sur le Cortege Spilito-Keratophyrique Hercynotype.- Spilites of the Lucanian Apennine (Southern Italy).- Quelques Observations Nouvelles Relatives a la Genese des Laves Spilitiques.- Comments on Spilitization of the Permian Eruptive Rocks of the Cho? Nappe in the West Carpathians, Slovakia.- 4. Papers Proposing a Secondary, Diagenetic or Metamorphic Origin.- Spilites as Weakly Metamorphosed Tholeiites.- On the Mineral Facies of Spilitic Rocks and Their Genesis.- The Pillow Lavas of Sakhalin and the Kurile Islands and Their Significance for the Solution of the Spilite Problems.- The Production of Spilitic Lithologies by Burial Metamorphism of Flood Basalts from the Canadian Keweenawan, Lake Superior.- A New Appraisal of Alpine Spilites.- A. Alphabetical List of 1. Literature on Spilites and Keratophyres.- 2. Related Literature (obviously only a selection).- B. Geographical Classification of Occurrences.- C. Classification According to the (proposed) Geological Age (as far as reported.- D. Classification According to the Years of Publication.

Cretaceous Stratigraphy and Structure, Western Andes of Peru Between Latitudes 10 degrees -10 degrees 30'

Abstract: Cretaceous tectonics, sedimentation, and volcanism were dominated by oscillatory vertical movements of strips of basement bounded by major shear belts. Two main episodes of movement occurred: a Valanginian to Senonian episode of general subsidence, and a Senonian to Tertiary episode of uplift. In the West Peruvian trough, the products of the first episode comprise a western shale-graywacke-volcanic facies and an eastern sandstone-limestone-shale facies. The facies meet along the Tapacocha axis, a major steep basement shear zone, which allowed the western side to subside faster than the eastern side. Fracturing of the western block in its descent provided channels for volcanic eruptions, whereas, on the eastern intact block, shelf deposits were laid down. During uplift, the sediments on the eastern block were folded as a single unit by decollement on underlying shale; those on the western block were strongly deformed in narrow belts above steep basement fractures. The belt of strongest deformation is along the Tapacocha axis and was accompanied by metamorphism of greenschist facies. The response of the Andes to lateral movement of oceanic crust beneath was to break into ribbonlike strips, parallel with the continental margin, which oscillated in vertical planes.

Pyrrhotite Isograd in Southeastern Tennessee and Southwestern North Carolina

Abstract: The first appearance of pyrrhotite in the Barrovian-type metamorphic succession of the Blue Ridge province of southeastern Tennessee and southwestern North Carolina is interpreted to represent a metamorphic isograd located in the upper chlorite zone slightly west of the biotite isograd. At lower grades of metamorphism, pyrite is the only recognizable iron sulfide, and it is characterized by textures indicating an arrested state of growth. Concentrations of pyrite are commonly aligned parallel to bedding planes. Pyrrhotite appears as streaks along slaty cleavage and is preferentially developed in pyrite-bearing zones. Hexagonal pyrrhotite is dominant, but monoclinic and monoclinic + hexagonal intergrowths also occur. Where pyrrhotite is present, coexisting pyrite is typically replaced by quartz, chlorite, and biotite. The sulfur and some of the iron contained in pyrrhotite is interpreted to have been derived locally from the replaced pyrite simultaneously with the development of metamorphic minerals.

Rb-Sr Whole-Rock Chronology of Caledonian Events in Northeast Scotland

Abstract: Rb-Sr whole-rock isochrons are presented for four late Caledonian granites from northeast Scotland. These isochrons are believed to define a single 460–m.y. B.P. magmatic event, which caused isotopic homogenization of strontium on a whole-rock scale in slate of the Macduff Group, which is part of the Upper Dalradian Series intruded by the granites. The geochronology of the region supports the suggestion that all Caledonian deformation and metamorphism took place during a relatively short episode between the Early Ordovician sedimentation of the Upper Dalradian Series and the Middle to Late Ordovician granitic magmatism. A premetamorphic granite at Portsoy, which intrudes strata of the Middle Dalradian Series, yields a Precambrian isochron age of 669 ± 17 m.y. This is probably a late or postorogenic granite related to a pre-Caledonian metamorphism already recognized within the Moine Series. This Precambrian age cannot be used to support a recent hypothesis in which the Older basic igneous rocks of northeast Scotland are related to a Cambrian(?) subduction zone. All the granitic rocks of northeast Scotland have initial Sr 87 /Sr 86 ratios of 0.714 to 0.717, irrespective of age. This indicates that the granites are derived from a deep, underlying crustal layer, possibly Lewisian (Precambrian) gneiss, and not from the mantle or subducted oceanic lithosphere.

Geochemistry and Geochronology of the Southern Caribbean–Northern Venezuela Plate Boundary

Abstract: The geochemistry and geochronology of the igneous rocks from the Venezuelan islands in the southern Caribbean, the Netherlands Leeward Islands, and the Caribbean Mountains of northern Venezuela were studied in an effort to test plate tectonics models for the evolution of that part of the Caribbean region. The continental platform north of Venezuela is underlain by an igneous and metamorphic complex that crops out on the Venezuelan islands. The older metamorphic rocks (lower to upper greenschist facies) are similar to those found in the Caribbean Mountains. These rocks were intruded by basic and granitic igneous rocks and andesitic to basaltic volcanic rocks during Early Cretaceous to early Tertiary time. Geochemically, the igneous rocks can be divided into two suites, a calc-alkaline suite consisting of quartz diorite, granodiorite, trondhjemite, and granite, and an olivine and ocean-floor tholeiitic suite consisting of gabbro, diabase, lamprophyre, and orthoamphibolite. The calc-alkaline affinity is shown by high Al 2 O 3 and K 2 O, and low MgO and TiO 2 contents. The K 2 O content increases southward from the Venezuelan islands toward the Caribbean Mountains. The tholeiitic affinity is shown by low Fe 2 O 3 /MgO ratios, low K 2 O and Sr contents, and high K/Rb ratios. K-Ar age determinations on these rocks show that the tholeiitic rock suite is the oldest, which confirms field relations, ranging between 114 m.y. and 130 m.y. (Early Cretaceous). The K-Ar ages of the calc-alkaline rock suite range between 30 m.y. and 84 m.y. (Late Cretaceous to Oligocene). These ages are interpreted to represent a Late Cretaceous–early Tertiary time of intrusion and later metamorphism and also support a late Eocene–Oligocene erogenic event that resulted in the present structural configuration of the central Caribbean Mountains. A dacite in the Carupano area (Araya-Paria Peninsula) yielded a K-Ar age of 5 m.y., or late Pliocene, which is the youngest evidence of igneous activity in northern Venezuela. The dacite is possibly related to the younger volcanic province of the Lesser Antilles. Our data support the hypothesis that the Aves Ridge represents a presently inactive volcanic arc prior to the Lesser Antilles arc.

The Lewisian of Lochinver, Sutherland; the type area for the Inverian metamorphism

Abstract: The Lochinver district contains gneisses, pegmatites and dykes generated in five main episodes: (1) pre-2600 m.y, garnet pyroxene granulite facies gneisses with low angle dips, formed in their final state by the Badcallian metamorphism; (2) potash-rich pegmatites of age range 2540 to 2310 m.y.; (3) 2310–2200 m.y. amphibolite facies gneisses occurring in vertical WNW zones, produced by a broadly isochemical metamorphic event; (4) vertical NW-trending dykes emplaced at 2200 m.y. and perhaps down to 1900 m.y. and (5) epidoteamphibolite facies gneisses produced in narrow, discontinuous NW shear zones during the succeeding Laxfordian events. The petrographic and structural features of each sector of the complex are described and new whole rock Rb–Sr ages given for the pegmatite suite, suggesting a maximum age of 2540 m.y. for the suite (plus intrusion of a further suite at 2310 m.y. or recrystallisation of the first suite at 2310 m.y.). Other pegmatite ages confirm the presence of pre-2000 m.y. activity NE of Scourie and at Gruinard. Field and petrographic evidence at Lochinver demonstrates the existence of amphibolite facies metamorphism in post pegmatite pre-dyke times, which produced well-defned rock types and WNW vertical structures, and is defined as the Inverian metamorphic event.

Mineralogy of New Caledonian metamorphic rocks

Abstract: The compositions of metamorphic pyroxenes from blueschists in northern New Caledonia are investigated. Aegerine-augite occurs in siliceous metasediments and aegerine in some low-grade sodic basic schists. Calcic metamorphic pyroxene (omphacite and chloromelanite) appears first in metabasalts in higher grades of the lawsonite zone and is widespread in metamorphosed igneous rocks and quartzofeldspathic gneisses of the epidote zone. Omphacites in basic rocks have higher Mg∶Fe ratios and are less jadeitic than omphacites from adjacent interbedded quartzofeldspathic gneisses. With increasing metamorphic grade pyroxenes become more jadeitic and diopsidic at the expense of their acmite component. Elemental partitioning between coexisting pyroxenes, garnets and amphiboles from in situ regional metamorphic rocks is generally regular, suggesting equilibrium crystallization. Omphacite appears to be a stable phase within blueschist facies over a temperature range of at least 350° to 550° C. The “eclogitic” assemblage almandine-omphacite is stable within the earth's crust in metamorphosed sediments and igneous rocks over a temperature range of 400° to at least 550° C. No estimate of absolute pressures involved in metamorphism in the Ouegoa district can yet be made.