Showing papers on "Granulite published in 1969"

Le massif ultrabasique des Beni Bouchera (Rif Interne, Maroc): Etude des péridotites de haute température et de haute pression, et des pyroxénolites, à grenat ou sans grenat, qui leur sont associées

Abstract: The ultrabasic rocks which form the Beni Bouchera massif are schistose and folded isoclinally. They outcrop at the center of a late formed anticline under a series of granulite facies precambrian or lower paleozoic rocks. The ultrabasic rocks appear to have been in situ during the major structural development of the series and were emplaced at the time of metamorphism. The massif itself is essentially a peridotite, apparently layered, which contains pyroxenite bands (about 3% of the total volume of the massif). The peridotites are olivinerich containing two aluminous pyroxenes and spinel. The pyroxenites are however quite varied. They are frequently banded themselves and in several centimeters thickness can vary from enstatite rocks at the exterior of the layers to garnet clinopyroxenites at the center with websterite and garnet websterite as intermediate rock type. They are also present as monofacies bands of enstatite or garnet clinopyroxenite rock. Occasionally the minerals in these bands show exsolution features.

Granulitic and eclogitic inclusions from basic pipes at Delegate, Australia

Abstract: Basic breccia-nephelinite pipes at Delegate (N.S.W., Australia) contain abundant two-pyroxene granulite, garnet granulite and fassaite eclogite inclusions and rare spinel pyroxenite, peridotite and charnockite inclusions.

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

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

K-Rb ratio in high grade metamorphism: A confirmation of the hypothesis of a continual crustal evolution

Abstract: The K:Rb ratio (R) in a metamorphic sequence with increasing metamorphism of the West Alps has been analysed. By a regression analysis a comparison is made with the trends observed for R byShaw (1968) in many magmatic processes. The K:Rb ratio in the metamorphic rocks shows a marked increase with increasing metamorphism in relation to a removal of the K content. The arithmetic averages of R in the rocks of amphibolite and granulite facies are 231 and 505 respectively. In some samples of granulite facies the R values are higher than those of the continental alkalic basalts and approximate to the oceanic tholeiites. The variations in the K:Rb ratio may be imputed, according toShaw, to an anatexis process which took place under high grade metamorphic conditions. The marked increase of the K:Rb ratio in the deep material would be caused by partial separation between K and Rb in the granulite assemblages and by an upward migration of Rb, relative to K, from the deep crustal levels of granulite facies.

Parageneses of garnet in granulite-facies rocks, Lofoten-Vesteraalen, Norway

Abstract: Garnet occurs widely as a secondary mineral in the granulite-facies rocks of the Lofoten-Vesteraalen islands of North Norway. The garnet-forming reactions may be inferred from the resulting textures and are interpreted as being retrograde reactions. Microprobe analyses show that garnets with high proportions of CaO and MgO occur in coronas around olivine grains in anorthosites, whereas the most iron-rich garnets formed by reaction between plagioclase and the very iron-rich pyroxenes of some monzonitic (mangeritic) rocks. Garnets ranging in composition between these extremes formed by reactions involving biotite, plagioclase and magnetite. Textural features indicate that these reactions have been induced by oxidation of the biotite; the liberated water has converted the pyroxenes to amphibole. The net effect of the reactions is to transform the granulite-facies assemblages to amphibolite-facies assemblages.

Charnockites as metamorphic rocks

Abstract: In many areas, charnockites are intimately associated with metasediments belonging to granulite facies. Both have similar textures, common mineralogical characteristics and, in several instances, the same structural pattern. From these occurrences, it is proposed that charnockites as they now occur are themselves metamorphic. Some charnockites also contain relic orthopyroxene and are found with migmatized and granitized rocks, together with some scattered metasedimentary bands. These may be altered from an original granulite facies rock. Acidic charnockites, however, may occur as diffuse pegmatites, cross-cutting veins and dikes, and intrusive bodies, and may be the product of anatexis and palingenesis of original charnockites.

The structural evolution of the Tollie antiform—a geometrically complex fold in the Lewisian north-east of Gairloch, Ross-shire

Abstract: Five phases of deformation D1–5 are recognized in the Tollie gneisses. D1–2 are probably Scourian (s.l.) and ante-date a set of basic dykes. The pre-dyke metamorphism was highgrade, in part granulite facies. D3 was accompanied by almandine–amphibolite facies metamorphism and caused minor folding F3 on nnw–sse axes with an associated penetrative foliation of variable dip. The basic dykes were intruded either before or during D3. D4 produced widely-developed steep penetrative foliation S4 with associated folds F4 and lineation L4 having a nw–se trend slightly oblique to that of F3 and causing widespread transposition of S3. The associated metamorphism was retrogressive over part of the area to lower almandine–amphibolite facies. D5 comprises late- or post-metamorphic structures without penetrative foliation. The Tollie antiform hinge is an F4 fold superimposed on a pre-existing major structure the nature of which is investigated by analysing the variation in L4 attitude using lineation-pitch isogons, and by studying the geometry of the dykes. It is deduced that the F4 folds were superimposed on an upright box-shaped anti-form described by the S3 foliation in the gneisses and by the dykes. Alternative hypotheses are proposed to account for the pre-F4 variation in attitude of the dykes.

Distribution of elements in coexisting minerals and the problem of chemical disequilibrium in metamorphosed basic rocks

Abstract: Distribution of elements in coexisting minerals—biotite, hornblende, augite, hypersthene and plagioclase in charnockitic rocks of West Uusimaa Complex, Finland, is mostly orderly indicating a close approach to chemical equilibrium. The distribution of iron and magnesium in coexisting hornblende and pyroxenes of basic charnockites and other rocks of granulite facies from several different areas is also orderly but the variation in the fugacities of H2O and H2 may cause a disorderly distribution locally in some rocks. The probable oxidation or reduction reactions are discussed on the basis of thermochemical and mineralchemical data.

The Nature and Origin of the Granulites of Mont Tremblant Park, Quebec

Abstract: The Mont Tremblant Park granulites can be divided into two main groups: pink, quartzofeld-spathic rocks and green-gray hypersthenic rocks. The quartzofeldspathic granulites are interlayered with garnet-sillimanite gneisses and locally occur as cores of folds or large massifs. More commonly, they are regionally interbanded with the hypersthene granulites. All the granulites are intruded by the quartz mangerites of the Morin anorthosite suite. Geochemical considerations based on ternary variation diagrams and relationships in the albite-orthoclase-anorthite system show that the two groups of granulites have different compositional trends, converging at the extreme granitic end. Mineralogical and geochemical data are consistent with granulite facies conditions of metamorphism. The composition of the granulites and their field relationships are consistent with their postulated derivation from a graywacke-shale geosynclinal sequence. Under granulite facies conditions, partial anatexis formed granitic melts. This anatectic differentiation caused the graywackes to attain a composition appropriate to form hypersthenic rocks and the shales to produce quartzofeldspathic rocks with residual sillimanite-garnet layers. These rocks were deformed during the intrusion of the anorthosite suite, and the characteristic fabrics of the granulites were produced. Volatiles given off by late-stage granite magmas caused retrograde effects at contacts.

Structural Evolution of the Southwestern Tobacco Root Mountains, Montana

Abstract: In the Sheridan district, southwestern Tobacco Root Mountains, Montana, Precambrian metasediments and metavolcanics are complexly folded and refolded. The exposed metamorphic bedrock is divided into the following major mappable units: quartzo-feldspathic gneiss, amphibolite-hornblende gneiss, anthophyllite-garnet gneiss, intermediate gneiss, and marble. Mapping delineates a series of slightly overturned, isoclinal antiforms and synforms which plunge N. 10° W., 24° NW. The nose of an earlier recumbent fold is also preserved. Primarily, all folding occurred during two major deformational episodes; an early episode of recumbent folding around northeast axes and a second, intense period of refolding around nearly north-trending axes that produced the dominant isoclinal folds. During the second folding phase, the nose of a recumbent fold became detached, flowed to the east, and was refolded. The degree of metamorphism produced during recumbent folding is unclear. Progressive regional metamorphism of Barrovian type occurred during the second folding episode and produced mineral assemblages indicative of the amphibolite and granulite facies. Petrofabric evidence substantiates inferences based on regional analysis. Calcite, dolomite, and quartz petrofabrics also reveal a third, less intense phase of folding which was related to the second phase, but which occurred around a distinct axis. Joint systems within the Sheridan district are a result of both Precambrian and Laramide deformations. Dominant, nearly vertical, joint planes with average trends of N. 75° W., N. 45° W., N. 2° E., and N. 65° E. are thought to be Precambrian in origin on the basis of close agreement with theoretical trends. Dominant fracture systems probably controlled the intrusion of the Tobacco Root batholith and the orientations of the Noble and other post metamorphic faults, all of which are associated with the Laramide orogeny.

Partitioning of Sodium between Coexisting K-Feldspar and Plagioclase from Some Metamorphic Rocks

Abstract: The partitioning of sodium between pairs of coexisting potassium and plagioclase feldspars extracted from metamorphic rocks is examined to test its suitability for use as a geothermometer. Rocks from eight different areas were studied in detail, most areas including two zones of metamorphism; in this way a range of metamorphic grade from middle amphibolite facies to various grades of granulite facies was investigated. Data from the metamorphic areas indicate that the distribution of sodium between coexisting feldspar phases cannot give any indications of relative metamorphic grades. Some factors are suggested to account for the different behavior of sodium partitioning between natural and experimental systems. The results for sodium distribution contrast with the temperature-dependent distribution of trace quantities of strontium between the same feldspar pairs.

Garnet-amphibolites at Yokokawa in the Abukuma metamorphic belt, Japan

Abstract: The mineralogy and petrochemistry of the garnet-amphibolites from the highgrade part of the Abukuma metamorphic belt have been studied, using five analyses of rocks, five of hornblendes, three of garnets and one analysis of cummingtonite, Garnetiferous amphibolites are rich in Fe, whereas non-garnetiferous ones are rich in Mg, especially in cummingtonite-amphibolite. The chemical composition of hornblendes associated with garnet is pargasitic and rich in FeO and poor in CaO, but that of non-garnetiferous rocks is rich in MgO. The garnets are rich in almandine molecule. Mg/Mg + Fe2+ ratios of both hornblendes and garnets correspond with those of the host rocks. The development of garnet in the Adirondack metabasites belonging to the upper almandine-amphibolite and granulite facies is observed in Mg-rich rocks as well as in Fe-rich rocks, in which both garnet and hornblende are rich in Mg respectively. However, under the conditions of the andalusite-sillimanite type metamorphism as shown in the Abukuma Plateau, Fe-rich garnet occurs in Fe-rich basic rocks, but cummingtonite occurs in Mg-rich ones instead of Mg-rich garnet. Finally, the problem of polymetamorphism is discussed. The cummingtonite-amphibolite may be the product of polymetamorphism, and Mg-rich garnet which had been present previously was decomposed to cummingtonite and plagioclase by the subsequent regional metamorphism of andalusite-sillimanite type.

Apercu sur le metamorphisme et la migmatisation dans le Precambrien de l'Amsaga (Mauritanie sud-occidentale)

Abstract: Kata-Mesozonal metamorphism, migmatization in zones of almandine amphibolite metamorphic facies, granulite facies, Amsaguian orogeny (new name 2400-2600 m.y.)

Analysis of equilibria involving garnet in rocks of granulite facies; discussion and reply

Abstract: Charnokitic and granulitic rocks of the Varsberg region in Sweden contain mineral assemblages that are characteristic of the(hornblende)-clinopyroxene-almandine zone of the granulite facies. The three-fold division of the granulite facies: the (biotite)-cordierite-almandine zone, and the (hornblende)-clinopyroxene-almandine zone observed in the Adirondacks, represent metamorphic zoning of the Adirondack type, a sequence of progressive metamorphic changes characterizing the granulite facies of the Adirondack highlands. [See deWaard, Jour. Petrology, v. 6, p. 165-191, 1965; Am. Jour. Sci., v. 263, p. 455-461, 1965.]

Moinian Assemblage of Scotland: Chapter 13: Central Orogenic Belt

Abstract: The Moinian assemblage is composed mainly of metamorphosed arenaceous and argillaceous sedimentary rocks but includes a minor proportion of calcareous or dolomitic beds. Psammitic granulite and pelitic schist occur typically in thick successions. Correlation of at least part of the Moine rocks with the Torridonian generally is accepted as a working hypothesis, suggesting that Moine deposition began 800-850 m.y. ago. The upper part of the Moine succession and the lower part of the Dalradian together might represent the Scandinavian Sparagmite Formation. Thus the Moinian assemblage may comprise both pre-Eocambrian and Eocambrian rocks. The Moine rocks of Scotland are separated into two large geographic divisions--the Northern Highlands and the Grampian Highlands--by the Great Glen fault. The Northern Highlands can be subdivided into three areas: western Inverness-shire, Ross-shire, and Sutherland. In western Inverness-shire, the mapping of minor rock types has proved useful in detailed lithologic work. On a broad scale, the oldest Moine rocks (Morar division) are confined to the west, adjacent to the Lewisian rocks of the foreland, and are succeeded eastward by the younger Glenfinnan and Loch Eil divisions. The Central Highland Granulite is probably the youngest unit of the Moine rocks. A major difficulty in interpreting the regional structure of the Moinian assemblages is the contrast between radiometric dates and the maximum dates of folding episodes established by stratigraphic considerations. Three major tectonic events are known to have affected the Moine rocks. They apparently were deformed and metamorphosed during a long period beginning in the late Precambrian and continuing possibly until Early Devonian time, at least in the central part of the orogenic belt.

Porphyritic Plagioclase-Hornblende-Pyroxene Granulite from Charnockitic Rocks of Chipurupalli, Visakhapatnam District, Andhra Pradesh, South India

Abstract: An unusual occurrence of porphyritic plagioclase-hornblende-pyroxene granulite (porphyritic basic charnockite) has been observed within the 'Charnockite region' of the Eastern Ghats of the Precambrian formations of India. From the X-ray and chemical data the phenocrysts are considered to be augite phenocrysts. Based on petrographic and chemical evidence it is inferred that the basic charnockites of Chipurupalli area in Visakhapatnam district are formed from tholeiitic magma as a gabbroic mass under deep seated plutonic conditions. The gabbroic mass has been recrystallized as plagioclase-hornblende-pyroxene granulite due to later metamorphism, leaving most of the phenocrysts unaffected.

Sr87/Sr86 ratios, K/Rb ratios, and rare-earth element abundances in pyroxene granulites.

Abstract: Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Earth and Planetary Sciences, 1969.

Anorthosites of Southern West Greenland: Chapter 62: Arctic Regions

Abstract: The described anorthosites are confined to the Greenland equivalent of the Kenoran structural province between 61°N and 65°N lat. in southern West Greenland. They occur in gneisses as stratigraphic units up to 2 km wide and can be traced for about 400 km in West Greenland. The anorthosite is found in both the deep levels (granulite and retrogressed amphibolite facies) and higher levels (migmatitic amphibolite facies in linear belts) of the basement complex. In the deep-level Fiskenoesset region, 125 km of chromite-layered anorthosites are associated with layers of hypersthene amphibolite, spinel-layered bronzitite, peridotite, chromite-layered bronzitite, magnetite-layered anorthosite, sapphirine- and corundum-bearing ultramafic rocks, and garnetite. The anorthosite and associated rocks are unmigmatized and contain inch-scale and phase layering and grading; the plagioclase has the composition of An80-90. In the higher level areas is homogeneous hornblende-bearing gabbro anorthosite (mostly with labradorite to bytownite) which is migmatized. > The chromite layering indicates that these anorthosites formed by gravitational accumulation of a basic magma. Thus they contrast with the volatile-rich ilmenite- and magnetite-layered anorthosites emplaced during the middle Precambrian in the Canadian shield. It is suggested that there is an anorthosite series beginning with early Precambrian, early magmatic bottom accumulates of anorthosite and ending with middle Precambrian, late magmatic plutons. Because about three fourths of the West Greenland anorthosite in the higher level areas is not chromite layered, a "metasedimentary" origin should be ascribed with caution to stratigraphic anorthosites.

Aluminous augite in the pyroxenite of the charnockite rocks of Visakhapatnam district, andhra pradesh

Abstract: Aluminous augite from an augite-pyroxenite occurring as a lens in the pyroxene granulites of the charnockite rocks, interbedded with garnet-sillimanite gneisses, quartzites and calc-granulites, is described. The presence of high aluminium in the augite indicates high temperature and pressure conditions of crystallisation and suggests that the ultrabasic and basic members of the charnockite rocks of Visakhapatnam have crystallised from an olivine tholeiitic magma.

Elastic properties of rocks at high pressures; ; granulite massif of Kola peninsula

Abstract: (1969). Elastic properties of rocks at high pressures; ; granulite massif of Kola peninsula. International Geology Review: Vol. 11, No. 9, pp. 1035-1046.