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Showing papers on "Granulite published in 1973"


Journal ArticleDOI
TL;DR: In this article, the authors consider that continental collision is followed by crustal thickening, to accommodate further plate convergence, with ensuing partial melting of the lower crust, resulting in a dry refractory lower crust consisting of pyroxene granulites and anor-thosites.
Abstract: Extensive terranes of basement reactivation are interpreted as resulting from crustal thickening following continental collision. It is suggested that terranes, such as the Grenville Province and much of the Variscan orogenic belt in Europe, have their modern analog in the Tibetan Plateau. The Tibetan Plateau is underlain by a continental crust between 60 and 80 km thick and is characterized by extensive high-potash Neogene vulcanism. Following T. H. Green's arguments that partial melting of a dioritic lower crust may yield potassic granitic liquids and refractory anorthositic residues, we consider that continental collision is followed by crustal thickening, to accommodate further plate convergence, with ensuing partial melting of the lower crust. At high structural levels, silicic-potassic ignimbrites are extruded in intermontane basin-horst terranes, with subjacent granite plutons. At deeper levels, a dry refractory lower crust consisting of pyroxene granulites and anor-thosites is generated.

848 citations


Journal ArticleDOI
TL;DR: During the discussions certain important conclusions have appeared and it seems possible that the mantle processes controlling tectonics in the Archaean, while having some features in common with the present regime, were unique.
Abstract: As chairman of a session of the meeting I would like to add a few comments concerning some of the problems discussed. During the discussions certain important conclusions have appeared. The detailed descriptions presented of the oldest known samples of the Earth indicate that at that time the crust had already a remarkably similar structure and chemistry to that of more modern times. It has also been made clear that rocks of the granulite facies make a massive contribution to the base of the ancient crust and logically could make a large contribution to modern regions under batholith zones of mobile belts, where the crust has been thickened by tectonic events. It also seems possible that the mantle processes controlling tectonics in the Archaean, while having some features in common with the present regime, were unique. Most models of the Earth's thermal history (Birch i965) suggest a very hot initial stage and it is difficult to see how the general geochemical equilibrium state of the Earth could be achieved without a very large degree of melting at some period of Earth history. If a large molten fraction is assumed, it would be difficult to preclude very complex convective structures. Crust will appear when surface temperatures approach 800 ?C whichl could be at a very early stage indeed. When liquid water appears on the surface at 100 ?C or less, a crust a few kilometres thick would rapidly form. Some present models (Finale I97I) indicate that formation of a hydrosphere might be an early event and this process itself would accelerate initial cooling. It might well be expected that the earliest semi-stable crustal fragments would be granitic. Such materials would be capable of floating on basalt liquids while solid basalt would be rapidly engulfed. This crust would also concentrate radioactive species (see Heier, this volume). When an aqueous phase is present, weathering would be intense partly on account of the supply of acids from intense volcanism ancd also from the supply of glassy volcanic debris. Early geochemical separation processes associated with a hydrosphere would thus be intense and synchronous with thicker crust formation. The initially radioactive crust could well have been associated with thermal gradients of the order of 100 ?C km-' and could hardly be thicker than 10 km before belting would occur at the base with the occurrence of igneous overturn of the material. It is now also abundantly clear that the appearance of simple life forms and the hydrosphere 'were separated by a very brief period so that bio-geochemical precipitates and bio-geochemical separations might also operate on the primitive surface at a very early stage.

250 citations


Journal ArticleDOI
K. S. Heier1
TL;DR: The author’s opinion the important problem associated with granulite facies rocks is not that of their origin, but rather of their importance as constituents of the continental crust, and how they attained their present chemistry.
Abstract: Occurrences of granulite facies rocks are widespread in continental regions where they mostly are parts of stable shield areas. Granulite facies terrains are classified as low-, medium- or high-pressure terrains on the basis of critical mineral associations. Special interest is attached to the medium- and highpressure terrains, as they are representative of the deepest crustal levels available for study in any areal extent on the surface, and may give information about the composition of the lower continental crust. Granulite facies terrains are mainly composed of metamorphic and metasomatic rocks, but magmatic rocks with primary igneous textures interpreted as formed by crystallization of magmas under granulite facies conditions are frequent in some areas. Examples of such rocks are anorthosites, gabbros and mangerites. The low-pressure—high-temperature granulite facies rocks are chemically indistinguishable from the amphibolite facies gneisses with which they characteristically occur. It is therefore important to make a distinction between these and the higher pressure types. The medium- to high-pressure granulite facies terrains are characterized by a less ‘acidic’ average major element compositions, and significant depletions in Rb, Cs, Th and U compared with average surface shield compositions. Available data also indicate low initial Sr isotope ratios, even in the gneissic types. In the author’s opinion the important problem associated with granulite facies rocks is not that of their origin, but rather of their importance as constituents of the continental crust, and how they attained their present chemistry.

218 citations


Journal ArticleDOI
TL;DR: 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.
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.

118 citations


Journal ArticleDOI
01 Jan 1973-Lithos
TL;DR: This article showed that eclogite is a stable assemblage within the deeper parts of the continental crust; this in turn requires that dP/dT > 0 for the equilibrium curves for the relevant reactions.

116 citations


Journal ArticleDOI
TL;DR: The Limpopo belt is an extensive ENE-trending linear zone of high-grade metamorphic tectonites which separates the Archaean nucleii of the Rhodesian craton to the north from the Kaapvaal craton in the south as mentioned in this paper.
Abstract: The Limpopo belt is an extensive ENE-trending linear zone of high-grade metamorphic tectonites which separates the Archaean nucleii of the Rhodesian craton to the north from the Kaapvaal craton to the south. The belt consists of reworked Archaean granite-greenstone terrain with an early Proterozoic cover sequence, the Messina Formation, infolded and metamorphosed with the basement. Two major zones of shearing and transcurrent dislocation separate marginal granulite zones from a central zone which consists of complexly infolded cover rocks and reworked basement. The northern granulite zone appears to grade transitionally into the Rhodesian craton to the north, whereas there is some evidence that the southern granulite zone is faulted against the Kaapvaal craton to the south. The whole belt has behaved as a zone of crustal weakness throughout geological time, and is characterized by repeated shear deformation, igneous intrusion and extrusion, despite the cessation of major regional tectono-thermal reactivation about 1900 Ma ago.

112 citations


Journal ArticleDOI
TL;DR: In this article, the formation of the melts which produce intrusions of the granite family are considered to result from the partial fusion of high-grade metamorphic rocks, and the melting behaviour of such materials is considered.

99 citations



Journal ArticleDOI
23 Jul 1973-Nature
TL;DR: In this paper, the granulite facies metamorphism is 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 area.
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.

87 citations


Journal ArticleDOI
TL;DR: The oldest greenstone belts are largely granulites and gneisses, and in west Greenland there is evidence of 1000 Ma of crustal history before the final high-grade metamorphism as discussed by the authors.
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.

78 citations


Journal ArticleDOI
TL;DR: The metamorphism and geochemistry of the major components of a small area of granulite facies rock are described and discussed, and a chemical model for the evolution of anomalous trace element distributions in such materials is suggested as mentioned in this paper.

Journal ArticleDOI
TL;DR: In the early stages of Proterozoic mobility the Archaean crust was traversed by incipient dislocations which developed into ''straight belts' characterized by high strain this paper.
Abstract: Polycyclic gneiss complexes in which early rock-assemblages of granulite or high amphibolite facies have been tectonically and metamorphically reworked occur on a regional scale in the crust. The distinctive properties of the dry, structurally isotropic parent-assemblages have played an important part in determining the style of regeneration which was characteristically inhomogeneous. The reintroduction of water and consequent amphibolization of granulites appears to have been associated with increases of ductility and consequent changes in tectonic behaviour. During the early stages of Proterozoic mobility the Archaean crust was traversed by incipient dislocations which developed into `straight belts' characterized by high strain. Deformation in the mobile provinces as a whole was associated with relative movements of the blocks defined by the straight belts. In these intervening blocks, regeneration was most effective at intermediate depths where pore fluids were available. Many Archaean granulites lying beneath the domain of regeneration remained as closed systems and suffered little tectonic or metamorphic modification.

Journal ArticleDOI
TL;DR: In this article, the authors reported that 80 per cent of the variation in Rb is associated with variation in K. This low value argues against repeated extraction of Rb and K through time on a regional scale, since the major depletion in these lithophile elements occurred in the early stages of crustal evolution.


Journal ArticleDOI
20 Nov 1973
TL;DR: The Fiskenaesset complex has a well-preserved igneous stratigraphy as mentioned in this paper, which occurs as conformable layers up to 1.5 km thick in high grade Early Archaean gneisses.
Abstract: The Fiskenaesset complex, which occurs as conformable layers up to 1.5 km thick in high grade Early Archaean gneisses, has a well-preserved igneous stratigraphy. An upward sequence of lithological zones is observed in the complex as follows: 1. Pyroxene Amphibolite; 2. Ultramafic Group (hornblende-bearing spinel- and magnetite-layered dunites, peridotites, pyroxenites); 3. Layered leuco-gabbro; 4. Dark gabbro; 5. Homogeneous leuco-gabbro; 6. Anorthosite; 7. Chromite horizon; 8. Garnet Anorthosite; 9. Pyroxene Amphibolite. Eighteen whole rock XRF analyses (for major and trace elements) indicate that this sequence is mainly the differentiation product of a basaltic magma. Cryptic layering is present within zones 2 to 8 inclusive and shows that this part of the sequence belongs to a stratiform igneous intrusion, zone 8 being the topmost layer of the original igneous stratigraphy. There is especially a marked upward increase in the Fe/Mg ratio. The amphibolites of zones 1 and 9 are much richer in iron than any rocks belonging to the intrusion and are considered to have been volcanics into which it was emplaced. Metasomatic sapphirine-rich rocks are localised along the contact of zones 8 and 9. The complex has internal sub-complexes within which its upper zones are locally confined. The stratigraphic sequence is repeated symmetrically in reverse order and the differentiation trends of the two halves mirror each other. The original body was flattened so that its layering was deformed into an isoclinal syncline. This deformation accounts for the repeated stratigraphy. The complex was subsequently double folded and metamorphosed under both granulite and amphibolite facies conditions. Mineral assemblages indicate that there may have been appreciable water in the original magma, and this viewpoint is strengthened by the observed differentiation trend.

Journal ArticleDOI
TL;DR: The granulite facies meta-igneous and metasedimentary rocks that surround the Lac St Jean anorthosite yield an Rb-Sr age of 1480 ǫmy Intrusive monzonite, possibly genetically associated with the
Abstract: The granulite facies meta-igneous and metasedimentary rocks that surround the Lac St Jean anorthosite yield an Rb–Sr age of 1480 my Intrusive monzonite, possibly genetically associated with the

Journal ArticleDOI
TL;DR: In this article, the significance of the ubiquitous sub-structures within galena is considered in terms of retrograde effects upon high grade metamorphic textures, including gangue and sphalerite.
Abstract: Two main periods of metamorphism have effected the Broken Hill base metal deposit. The first, at granulite grade, occurred at 1,700 m.y., the second, at lower amphibilite grade, occurred at 500 m.y. The earlier metamorphism correlates with two stages of intense regional folding; the latter occurs as narrow “shears” across the orebody. The prograde metamorphism caused intense brecciation, development of an ore “mush” with ore movement, formation of ore-bearing parapegmatites and boudins and much recrystallization of ore and gangue minerals. The orebody parallels an axial plane schistosity in the wall rocks with numerous ore piercement structures causing local discordancies. Ore in retrograde zones is again brecciated with galena further recrystallizing after destruction of prograde recrystallization. Gangue minerals remain essentially as brecciated fragments without further recrystallization. Secondary hydrothermal veins with rare silver minerals derived from the orebody transect the retrograde zones. Galena is plastically injected into fractures in the retrograde wall rock schists. Prograde ore shows co-recrystallization of various sulphides and gangue minerals yielding characteristic annealed textures. Quartz, garnet, hedenbergite, roepperite and apatite co-recrystallize with galena, sphalerite and chalcopyrite with balanced surface tensions. Retrograde ore shows mainly fragments of gangue and sphalerite set in a matrix of further recrystallized galena or schistose galena with a superimposed sub-grain structure. The significance of the ubiquitous sub-structures within galena is considered in terms of retrograde effects upon high grade metamorphic textures.


Journal ArticleDOI
TL;DR: The Lewisian rocks of Barra as mentioned in this paper are a Laxfordian supra-structure of amphibolite facies gneisses and a Scourian infra structure characterised by orthopyroxene bearing gneses.
Abstract: The Lewisian rocks of Barra are a Laxfordian supra-structure of amphibolite facies gneisses and a Scourian infra-structure characterised by orthopyroxene bearing gneisses. Both units contain representatives of the Scourie Dyke suite; in the supra-structure they are highly deformed and folded; in the infra-structure they are relatively undeformed, unfolded, and retain original discordant relationships. The infra-structure also has several suites of intrusive igneous rocks earlier than the Scourie dyke suite, the most widespread being dykes of dioritic composition. The same sequence of Laxfordian deformation may be recognised in both units. Folding occurred in the supra-structure under amphibolite facies conditions, which continued after deformation ceased. In the infra-structure, evidence for a pre-Scourie dyke pyroxene granulite facies metamorphism is preserved. The Scourie dykes of the infra-structure have pyroxene granulite facies mineral assemblages resulting from intrusion into hot or dry country rock gneisses. Probably a very old cover to basement relationship is preserved in the area, which was first deformed in the Scourian orogeny, and then subsequent Laxfordian deformation of the interface between the two major units produced overall structure of the area.

Journal ArticleDOI
TL;DR: In the granulites of the Brazilian shield, beryllium decreases in the order: intermediate granulite, basic granule, granitic granule and granitic-granulite as mentioned in this paper.

Journal ArticleDOI
TL;DR: In this paper, three new sets of mineral and rock chemical data on basic granulites confirm the trends in compositional relationships, with respect to magnesium and iron, among hornblende, orthopyroxene and calcic pyroxene.
Abstract: Three new sets of mineral and rock chemical data on basic granulites confirm the trends in compositional relationships, with respect to magnesium and iron, among hornblende, orthopyroxene and calcic pyroxene advanced by Sen (1970) and Ray and Sen (1970). Analysing the interrelationships among magnesium-iron distribution coefficients between hornblende-orthopyroxene and hornblende-calcic pyroxene, tetrahedral aluminium contents of hornblendes, and temperature of equilibration, it can be shown that equilibrium temperatures are higher with the Mg/(Mg+Fe) ratios of hornblendes increasing. An application of these relations to ferromagnesian phases of some garnet-bearing basic granulites points to formation of garnets in response to decreasing temperature.


01 Jan 1973
TL;DR: The petrology of the Mellid area, the SE portion of the outer zone of the Ordenes Complex which is one of the upthrusted Precambrian complexes in the axial zone in the Hercynian orogen in Galicia, NW Spain, was studied in this paper.
Abstract: This study concerns the petrology of the Mellid area, the SE portion of the outer zone of the Ordenes Complex which is one of the upthrusted Precambrian complexes in the axial zone of the Hercynian orogen in Galicia, NW Spain. An eugeosynclinal rock sequence is found containing units with different metamorphic evolutions. All units were affected by Precambrian tectonization and retrogressive metamorphism. This orogeny may be subdivided into three metamorphic and four deformation phases which caused definite changes in the mineralogical composition and the texture of the rock. The sequence of metamorphic phases, established in a granulite facies unit is as follows: the first phase of Precambrian metamorphism is characterized by the (hornblende-)granulite facies, more precisely the (hornblende-)clinopyroxene-garnet-sodic plagioclase subfacies of the kyanite-bearing granulite facies. At that time, PH2O must have been very low locally. The second and third phases were marked by the hornblende-clinopyroxene-garnet-sodic plagioclase subfacies and the amphibolite facies, respectively. The other units bear witness to lower grade metamorphic activities. The granulite facies unit comprises metamorphosed basic lavas, metapelitic rocks, garnet-bearing metagabbros and garnet-bearing peridotites. The metapelitic rocks (kyanite-garnet-orthoclase-sodic plagioclase-biotite) and the metamorphosed basic lavas (clinopyroxenegarnet-sodic plagioclase-amphibole) are described in detail. The latter rocks contain Ca-rich inclusions, displaying scapolite-bearing mineral assemblages. The inclusions can be ascribed to deuteric alteration or incipient metamorphism in the basic lavas prior to the granulite facies metamorphism. The other units contain metasedimentary rocks and granitic and granodioritic orthogneisses. Metamorphic conditions during the Hercynian orogeny did not go further than the lower amphibolite facies. Therefore, retrogradation of the Precambrian units continued but a clear conversion of the rock texture cannot be discerned. The most important Hercynian event in the Mellid area was the emplacement of an ophiolitic rock suite.

Journal ArticleDOI
TL;DR: Garnets from the upper mantle peridotites are rich in pyrope and exhibit a homogeneous distribution of the elements and are therefore unzoned to a high degree.


Journal Article
TL;DR: The plagioclase - pyroxene - garnet rocks, occurring in the Manalur area, have been designated as meta-anorthosite by Murthy (1959) as mentioned in this paper.
Abstract: The plagioclase - pyroxene - garnet rocks, occurring in the Manalur area, have been designated as 'meta-anorthosite' by Murthy (1959). The authors however, consider them as a member of meta-sedimentary group of Archaeans which have resulted from the metamorphism of calcareous sediments of varying compositions. Since there is a general misconception among some of the Indian petrologists, who consider these metasedimentary calcareous members as 'anorthosites', it is proposed to deal with these rock types, in order to show that they have no association with anorthosites, which are one of the members of the layered complex.

01 Jan 1973
TL;DR: In this paper, the structural setting of a refoliated belt of sapphirine granulites in northern Uganda and petrography of two selected rocks are described.
Abstract: SUMMARY. The structural setting of a refoliated belt of sapphirine granulites in northern Uganda and petrography of two selected rocks are described. Electron-probe analyses of the following minerals are given: ilmenite, titanian hematite, rutile, magnetite, sapphirine, hyperstheue, brown and green biotite, garnet, and cordierite. Field and experimental data suggest the following paragenesis: deposition of ferruginous shales with siliceous bands, followed by burial and regional metamorphism under granulite facies conditions, and finally rapid unloading associated with refoliation and shearing and crystallization of sapphirine and cordierite. TIlE Labwor Hills of northern Uganda consist of Precambrian metamorphic rocks of amphibolite and granulite facies. In I963- 4 the Uganda Geological Survey recognized the presence of sapphirine-bearing granulites that contain a wide mineralogical range of iron magnesium alumino-silicates. Of particular additional interest is the structural setting of these intriguing assemblages. The sapphirine-bearing rocks are restricted to a zone of NW-trending refoliation, 2"5 miles wide and 8 miles long, that forms a conspicuous photogeological feature truncating a zone of easterly-trending foliation (fig. I). The relationship of these two trends of foliation is particularly well displayed in the SE where banded pyroxene granulites become increasingly tightly folded along NW-trending axes in a westward direction; the end product of this refolding is the development of NW-trending isoclinal folds that have steeply dipping axial planes, and in which the original E-W folds only occur as crescentic-shaped attenuated closures with sheared-out limbs. Petrography Sapphirine is most conspicuous in ferruginous rocks containing ilmenite, garnet, hypersthene, and cordierite, and representative specimens, AR39 and AR5 I, were selected for detailed petrographic study and a third, PHN Io69, for electron probe mineral analysis. In hand specimen, AR39 and AR5I show a lenticular gneissose interbanding of orange garnet-bearing aggregates, with coarse leucocratic minerals. These are silica-deficient and silica-rich bands respectively; the mineral composition, in wt %, determined from heavy liquid and magnetic separations, is: AR39 AR51*