Showing papers on "Granulite published in 1977"

Chemistry, thermal gradients and evolution of the lower continental crust

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

Feldspar and Oxide Thermometry of Granulites in the Adirondack Highlands

Abstract: Thermometry of regionally metamorphosed granulites of the Adi- rondack Highlands has been undertaken using feldspar and iron-titanium- oxide equilibria. Electron microprobe analyses of 20 coexisting oligoclase (An18 3o) and microcline perthite (Ors7 87) pairs from charnockites and granitic gneisses give KD(Na/(Na + Ca + K)plag/(Na/(Na + Ca + K))or = 2-3 yielding temperatures of 650 ~ to 750~ in comparison to Seck's (1971) experimental and Stormer's (1975) calculated temperatures for inferred pres- sures of 8 kilobars. Microprobe analyses of 10 coexisting titaniferous magne- tite (ulv6spinels~ 1645) and ilmenite (hematitess 4.7-6.5) pairs from the Marcy massif anorthosite and related gabbros give temperatures of 620 ~ to 800 ~ C in comparison to Buddington and Lindsley's (1964) experimental data. Oxygen fugacities buffered by this assemblage range between 10-2~ and 10 16 and always lie within 10 +1 of thefo2 buffered by fayalite-magne- tite-quartz. Exsolved albite in alkali feldspar and ilmenite (oxidized ulv6spinel lamellae) must be reintegrated to infer metamorphic temperatures. Both thermometers give internally consistent, reproducible and geologically rea- sonable results. The inferred 750 ~ and 700~ isotherms wrap around the anorthosite massif in roughly concentric circles. Maximum metamorphic temperatures (790_+ 50 ~ C) occur between Saranac Lake and Tupper Lake, New York.

Feldspathic hornblende and garnet granulites and associated anorthosite pegmatites from Doubtful Sound, Fiordland, New Zealand

Abstract: Feldspathic hornblende granulites from Doubtful Sound, New Zealand with the assemblage plagioclase+hornblende+clinopyroxene+orthopy-roxene +oxide+apatite are criss-crossed by a network of garnetiferous anorthosite veins and pegmatites The feldspathic gneiss in contact with anorthosite has a reaction zone containing the assemblage plagioclase +garnet+clinopyroxene+quartz+rutile+apatite The garnet forms distinctive coronas around clinopyroxene The origin of these rocks is discussed in the light of mineral and whole rock chemical analyses and published experimental work It is thought that under conditions leading up to ∼ 750 °C, ∼ 8 kb load pressure and ∼ 5 kb H2O pressure, partial melting occured in feldspathic hornblende granulites The melt migrated into extensional fractures and eventually crystallised as anorthosite pegmatites and veins The gneisses adjacent to the pegmatites from which the melt was extracted changed composition slightly, by the loss of H2O and Na2O, so that plagioclase reacted simultaneously with hornblende, orthopyroxene, and oxide to form garnet, clinopyroxene, quartz and rutile

Two-billion-year granulites in the late precambrian metamorphic basement along the southern peruvian coast.

Abstract: Uranium-lead data indicate that the high-pressure granulitic and charnockitic nuclei within the medium-grade metamorphic complex of the Peruvian coastal area must be related to an orogenic event 2 x 109 years ago. As in western Africa and Brazil, this old granulitic basement is reworked by Late Precambrian orogeny. Its presence along the Peru-Chile Trench must be taken into account in interpreting the anomalously high strontium isotopic ratios of recent calc-alkaline volcanism.

Granulite xenoliths from Lesotho kimberlites and the lower continental crust

Abstract: Six clinopyroxene–almandine granulite facies xenoliths have been analysed for major, trace and rare earth elements. Preliminary microprobe analyses suggest an origin near the base of the crust at temperatures of 660–830 °C and pressures > 9 kbar. Major element analyses indicate minimal chemical variation from a basaltic composition while the trace and rare earth element variation may be attributed to either magmatic or metasomatic activity.

Variation in lithology and tectonometamorphic relationships in the Precambrian basement of northern Saskatchewan

Abstract: The southwestern part of the Precambrian Shield of northern Saskatchewan is divisible into five lithostructural domains. The lithological, structural, and metamorphic characters of these domains are described. The Mudjatik and Virgin River domains constitute parts of a more extensive mobile belt extending further east and share a common tectonometamorphic history. The western boundary of this (Hudsonian) mobile belt is marked by the Virgin River shear zone, a zone of repealed movement marked by a narrow belt of mylonite gneisses and sheared marginal granitic intrusions. To the west of the shear zone the Western Granulite and Firebag domains comprise elements of an earlier (Archean?)cratonic foreland, formed by granulite facies gneisses which are only mildly affected by later Hudsonian events. These two cratonic elements are separated by the Clearwater domain, a narrow mobile belt with a history broadly similar to that of the main mobile belt to the east.The Mudjatik and Virgin River domains are formed pre...

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

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

The metamorphic geology of the Windmill Islands, Antarctica: A preliminary account

Abstract: Rocks of the Windmill Islands, Antarctica (Lat. 66°S, Long. 110°E) consist of a layered sequence of schists, gneisses, and migmatites (the Windmill Metamorphics) intruded by a charnockite (the Ardery Charnockite) and a porphyritic granite (the Ford Granite), and cut by two swarms of easterly‐trending dolerite dykes. The rocks have undergone four deformations. The first two produced tight isoclinal folds, the third developed broader less appressed concentric folds, and the last deformation produced gentle warps which plunge steeply southwards. The metamorphic grade of the Windmill Metamorphics ranges from (i) upper amphibolite facies (sillimanite‐biotite‐orthoclase) in the north, through (ii) biotite‐cordierite‐almandine granulite to (iii) hornblende‐orthopyroxene‐granulite in the south. The boundary between (i) and (ii) above is marked by the incoming ortho‐pyroxene and also the outgoing of sphene, and that between (ii) and (iii) by the outgoing of cordierite. Other metamorphic variations, appare...

Metagabbronorite from DSDP hole 334: an example of high-temperature deformation and recrystallization near the Mid-Atlantic Ridge

Abstract: Five gabbros and three peridotites from hole 334 were examined for postmagmatic deformational changes and metamorphic recrystallization. The condition of the gabbros ranges from unaltered, to slightly altered and deformed, to highly deformed and metamorphosed, the latter type being confined to narrow shear zones. Petrofabric studies and electron microprobe analyses of mineral phases of a foliated metagabbronorite from such a shear zone suggest the following post-magmatic history: (1) strong plastic deformation and anhydrous synkinematic recrystallization of igneous plagioclase (An90−85), clinopyroxene, and orthopyroxene to the granulite facies mineral assemblage of strongly zoned plagioclase (An80−65), diopside, and orthopyroxene; relics of the igneous minerals survived as highly strained porphyroclasts; (2) synkinematic growth of amphibole consisting of a patchy intergrowth of tremolite and actinolitic hornblende; the amphibole fabric is compatible with strain estimates by the centre-point method; (3) ma...

The origin of the albite-rich rocks enclosing the cobaltian pyrite deposit at Thackaringa, N.S.W., Australia

Abstract: A stratabound disseminated and massive cobaltian pyrite deposit at Thackaringa, 30 km SW. of Broken Hill occurs in banded albite-quartz-biotite rocks which are conformable with regional structure and stratigraphy. The albite rocks are associated with pelitic and psammitic metasediments, amphibolite and minor quartzgahnite rocks. The deposit has undergone granulite facies metamorphism and there is no evidence that pyrite has reacted to form pyrrhotite. It is suggested that the albite rocks were an analcimized tuffaceous rock in which disseminated pyrite formed by a volcanic exhalative process. A massive cobaltian pyrite body is enclosed by a metamorphosed hydrothermal alteration zone characterised by an increase in quartz, magnesian fluorobiotite and Rb.

Two-feldspar geothermometry in granulite facies metamorphic rocks

Abstract: A geothermometric technique based on equilibria between coexisting plagioclase and alkali feldspar was applied to quartzo-feldspathic granulites from Salvador, BA, Brazil. The conditions of metamorphism were determined to be in the range 750 ° C–800 ° C, 4–8 Kb, by comparison with experimental data on the stabilities of sapphirine, phlogopite and other minerals occurring in the associated rocks. Selected feldspar data gives temperatures near, but slightly below, this range. Several variants of the Wood and Banno model, as well as an empirical two-pyroxene geothermometer, were also tested and found to give temperatures which were apparently 50 °–100 ° high. The solubility of Al2O3 in orthopyroxene indicates temperatures which are about 200 ° to high, suggesting that Fe in the natural assemblages significantly changes relationships observed experimentally in MgO-Al2O3-SiO2 systems.

Rb-Sr geochronology and evolution of the Grenville province in northwestern Quebec, Canada

Abstract: The La Verendrye Park area extends from the Grenville front near Val d'Or, Quebec, to the beginning of the cover formed by rocks of the Grenville Supergroup some 200 km southeast of the front. Most of the rocks in the study area are layered quartzofeldspathic gneisses of uncertain origin, metamorphosed to the granulite facies. For the first 50 km southeast of the front, the gneisses yield fairly precise Rb-Sr isochron estimates of the times of their metamorphism 2,450 to 2,850 m.y. ago. Throughout the central part of the area, lithologically similar rocks have commonly been affected by a later metamorphic event; they yield a variety of results ranging from a random distribution of isotopic data to very poorly defined ages of about 2,000 m.y., which are interpreted as being intermediate between the ages of two metamorphic episodes. Demonstrably intrusive igneous rocks are very rare in the northern and central parts of the area. A metasomatic origin for the fairly abundant granitic (microcline-rich) layers in the gneisses is supported by isotopic and petrographic evidence and by their relationship to the enclosing quartz-oligoclase gneisses. Similar evidence suggests that this granitization is an Archean phenomenon in the northern half of the area. Farther south, the zone of migmatite between 150 and 200 km southeast of the Grenville front consists of granitic and charnockitic rocks that yield ages of about 1,150 m.y. and locally high initial 87Sr/86Sr ratios. The host rocks are interfolded Archean and Aphebian quartzofeldspathic gneisses. The latter are a group of metasedimentary rocks that are distinct in lithology from the Archean gneisses and the sedimentary rocks of the Grenville Supergroup and yield a reliable metamorphic age of 1,650 m.y. and a low initial 87Sr/86Sr ratio. When this information is added to that from other more southern regions, the evolution of this part of the shield is inferred to be dominantly accretionary but includes a very significant overlap of Grenville Supergroup rocks on Aphebian continental crust. The Grenville front in this area probably represents a zone of vertical movement that exhumed a 100-km-wide strip of high-grade Archean rocks, which presumably had lain under and adjacent to the Abitibi foldbelt. South of the La Verendrye Park area, Archean rocks have not been identified in close association with rocks of the Grenville Supergroup, and older rocks have not contributed significantly to the generation of plutonic rocks of Grenville age except in the boundary zone described in this paper.

Sub-cratonic crust and upper mantle models based on xenolith suites in kimberlite and nephelinitic diatremes

Abstract: Kimberlite and nephelinitic magmas have sampled the upper mantle and crust beneath the ancient cratons en route to the surface from their points of origin within the upper mantle. Study of the xenolith suites at most localities favours the hypothesis that the Moho is a chemical break, with lower crustal granulite facies rocks overlying harzburgitic or dunitic upper mantle rocks. The model for Kansas, USA, however, favours the hypothesis that a broad zone of granulite-facies rocks straddles the Moho. Bulk chemical analyses of garnet-pyroxene granulites from Matsoku, Lesotho and Lashaine, Tanzania, show them to be similar to gabbroic rocks, but certain other features (high K/Rb, low Rb/Sr) suggest that they may arise by ‘basification’ of rock of intermediate composition during high-grade metamorphism with associated anatexis. These lower crustal granulites are not chemically similar to upper mantle eclogites.

The stability of osumilite in high grade metamorphic rocks

Abstract: Osumilite, approximate composition K(Mg,Fe)2 Al5Si10O30, has been reported recently from two granulite localities. The mineral has been synthesised in a model pelitic composition at 1000 and 1100 ° C and 3.6–6.3 kb under conditions of low water and oxygen fugacity. Osumilite coexists, apparently stably, with hypersthene, cordierite and quartz (?) thus duplicating the mineral assemblage of one of the natural occurrences. Osumilite is in a divariant reaction relationship with cordierite and hypersthene i.e. osumilite ⇄ cordierite + hypersthene + orthoclase+quartz. This reaction runs to the right with increasing pressure. Experimental data and field observations suggest that the joins osumilite-garnet and osumilite-sillimanite are not stable. It is suggested osumilite is involved in an invariant point in the system K2O-MgO-FeO-A12O3-SiO2 with the phases cordierite, hypersthene, sapphirine, spinel, orthoclase and quartz. The invariant point should occur at 1000 ± 100 °C and 7± 2kb.

Uranium and thorium in Archean granulite facies terrains of Bahia (Brazil)

Abstract: Th and U distributions in Archean granulite-facies rocks from Bahia (Brazil) have been investigated by means of gamma-ray spectrometry. Thorium shows a very large range of values (from < 0.3 to 72.2ppm) with high concentrations in only a few samples. Uranium is more homogeneously distributed at abundance levels below 1 ppm. The Th/U ratios vary irregularly depending on the variable Th contents but are normally much higher than averages for magmatic and metamorphic rocks or crustal averages. Differential behaviour of Th and U during metamorphism is considered as the main cause of the high Th/U values observed. Selective depletion of uranium in metamorphic fluids may have resulted from an early event as documented by other very old crustal materials. Surface heat production data indicate that the Bahia granulitic block is a low-heat production region (average H.P.U. 3.32). Emphasis is placed on the need for a future study of the vertical compositional profile of the crust in shield areas with exposed high-grade metamorphic rocks using combined heat production rate and heat flow data.

Kinetics of a Garnet Granulite Reaction

Abstract: It is necessary to understand the mechanisms of disequilibrium reactions in metamorphic rocks in order to (1) model the rate of reaction in response to changing state variables during tectonic process, and (2) interpret the assemblages of natural disequilibrium samples in terms of tectonic history. A sample was selected from an area of known tectonic history to examine in detail and document the kinetics of reaction. The sample preserves evidence of the garnet granulite to gabbro transition.

Sulfide petrology of the Broken Hill region, New South Wales

Abstract: The Proterozoic Willyama Complex hosts metamorphosed massive lead-zinc-silver sulfide deposits (including the main Broken Hill lode) distributed across three prograde regional metamorphic zones ranging from middle amphibolite to granulite facies. FeS contents of sphalerites coexisting with pyrrhotite are consistent within a single sample but range widely within a deposit and across the complex showing no correlation with metamorphic grade. Primary (metamorphic) pyrrhotite was iron rich and now consists of hexagonal pyrrhotite with exsolved troilite. Although most primary pyrrhotites were altered to monoclinic pyrrhotite, pyrite, marcasite, or mixtures of these minerals during retrograde or postmetamorphic events, sphalerites were not affected significantly. Closely spaced samples from No. 3 Lens at North mine in the main Broken Hill lode indicate that the domains of sulfide equilibria are less than 30 cm and may not be much larger than the dimension of a polished section.The partitioning of FeS between sphalerite and hexagonal pyrrhotite is a function of P, T, and a FeS . The determination of a FeS for the bulk compositions of pyrrhotites together with analyses of coexisting sphalerites permits an estimation of pressure during regional metamorphism provided that the pyrrhotites have not been altered and the temperature of sulfide equilibration is known. Four samples from the main Broken Hill lode in granulite rocks give mean pressures of 8.1 + or - 0.6 kb for an assumed temperature of final sphalerite equilibration of 650 degrees C, as suggested from sulfur isotopic data, or 9.9 + or - 0.7 kb at 800delta C which is the estimated peak metamorphic temperature.

Structural and Metamorphic Evolution of the Bengal Anorthosite and Associated Rocks

Abstract: The anorthosite is emplaced along the core of a doubly plunging (east-west) antiformal structure developed on the lithologic contacts of the associated metasediments and metanorites. In the first phase of deformation (prior to the emplacement of anorthosite) the country rock together with numerous noritic sills was metamorphosed to pyroxene granulite facies. During the second phase of deformation a hydrous gabbroic anorthosite magma was synkinematically emplaced and slowly consolidated expelling considerable amount of water-rich volatiles which permeated the associated rocks. Metanorites ( pyroxene granulites ) and anorthosites were then metamorphosed to hornblende granulite facies followed by local diaphthoresis to greenschist-amphibolite transition facies. At the waning of deformation, palingenesis of the leptynites at places resulted in younger microcline-granites which brought about metasomatic changes in the anorthosite.

African Archaean mobile belts and granite–greenstone terrain

Abstract: ARCHAEAN granite–greenstone terrains such as the west African and Rhodesian Cratons, are frequently bordered by pene-contemporaneous, long-lived, high-grade mobile belts containing granulite facies supracrustals, enderbites, granite gneisses and anorthosites. For example, in Sierra Leone1, the granite–greenstone terrain contains low-grade supracrustal belts along NNE–SSW trends, while the discordant Kasila Group granulite facies belt trends NW–SE, and forms the coastal border of the West African Craton. A similar relationship is found between the Rhodesian and Kaapvaal Cratons, which are separated by the east–west trending Limpopo Belt2,3. The metamorphic, structural and lithological contrasts between mobile belts and granite–greenstone terrain have been ascribed to contrasts in heat-flow regime. I attempt here to correlate field and experimental data to elucidate Archaean mobile belt–craton relationships.

Early basic bodies in the type Laxfordian complex, NW Scotland and their bearing on its origin

Abstract: Two views on the origin of the Laxfordian gneiss complex have been proposed (1) that it was formed by the repeated modification of gneisses originally produced during early Scourian ( = Badcallian) metamorphism at about 2700 Ma and (2) that it was derived from metamorphism of a post-Badcallian supracrustal series. The paper describes, from the type Laxfordian complex, remnants of metamorphosed layered basic-ultrabasic bodies which can be matched in terms of individual rock-types and of the total rock-assemblage with pre-Badcallian layered intrusives in the adjacent Scourian complex. The wide distribution and structural relationships of these remnants show that much of the Laxfordian complex is derived from pre-Badcallian parent rocks. Field evidence supports the view that the distinctive geochemical features of the Laxfordian complex (high K, Rb, U, Th relative to the depleted granulites of the Scourian complex) were established in pre-Laxfordian times and suggests that differences in the history of granitic intrusion during Scourian events may have been a more important factor in establishing these features than differences in the crustal level at which metamorphism took place.

Sediments, Ores, and Metamorphism: New Aspects

Abstract: Recent years have witnessed the discovery of new large stratabound deposits of base metals, tungsten and uranium, as well as intensified research efforts in the investigation of known occurrences. Three problems are of particular interest: (1) The response of stratabound metal concentrations to varying degrees of metamorphism, and the limits of ‘survival’ of their geometrical and compositional parameters, (2) the distribution of manganese in the ore environment of sulphide deposits, and (3) the participation of elements not traditionally associated with the sedimentary cycle, such as tungsten, in the formation of stratabound deposits. The northwestern Cape Province, S. Africa, has been selected for an initial study of these aspects because of its wealth in large and economically significant base metal concentrations. These occur in volcano-sedimentary successions of probable Kheis age (2600 Ma) which have been metamorphosed during the 1200 Ma Kibaran orogeny. Metamorphism was of amphibolite facies grade (600-700 °C, 3- 4.5 kbar) in the east (deposits of Aggeneys and Gamsberg) and of granulite facies grade (800-1000 °C, 6—8 kbar) in the west (deposits of the O’okiep Copper District, including the Wolfram Schist). In amphibolite facies terrain, the stratabound nature of orebodies is generally well preserved, although complicated by several phases of folding. Electron probe analyses reveal high MnO-contents in silicates of the ore environment (garnets 20 %, pyroxene 5 %, stilpnomelane 11 %). These findings correlate with recent data from the Broken Hill, N.S.W. deposits (Stanton 1976) and from the Red Sea (Cronan et al. 1976). Their potential value lies both in the fields of ore genesis and of exploration geochemistry. Major concentrations of copper ore in transgressive ‘ noritoids ’ are limited to the granulite facies terrain of the O’okiep Copper District. Their isotopic, geochemical and petrological parameters favour crustal rather than mantle origin. A source bed model deriving the noritoids by partial melting of Cu-bearing members of the stratigraphic sequence, is thus proposed. Small tungsten deposits have in the past been mined in the granulite facies terrains of Namaqualand: concordant quartz-ferberite veins occur in the 9 Wolfram Schist ’ and have previously been interpreted as hydrothermal and pegmatitic. Their distinct limitation to a specific stratigraphic horizon as well as new quantitative data on the age and grade of metamorphism in the area suggested a review of this genetic concept. The recent discovery of stratabound tungsten mineralization in Austria, Norway and Rhodesia, and the recognition of the sedimentary origin of the Sundong, S. Korea, deposit further stimulated these considerations. The Namaqualand tungsten deposits are thus interpreted as the product of granulite-facies metamorphism of sedimentary scheelite concentrations. These investigations necessitate the combination of many geoscientific disciplines, including field work, structural geology, geophysics, petrology, isotopic studies, geochemistry, ore microscopy and experimental mineralogy, here termed ‘the comprehensive geoscience approach’. This clearly requires team work; the author has had the privilege of closely cooperating with groups led by Professor T. N. Clifford (Witwatersrand University) and Mr J. Marais (O’okiep Copper Company Limited).