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Showing papers on "Metamorphism published in 1979"


Journal ArticleDOI
01 Mar 1979-Geology
TL;DR: In this paper, the nonconformity between crystalline basement and layered cover rocks played an important mechanical and physical-chemical role during metamorphic deformation in the western Cordillera.
Abstract: Metamorphic core complexes evolved in the western Cordillera in early to middle Tertiary time as a response to profound regional extension and thermal incursion; the deformation was punctuated by an unusually thorough tectonic denudation. In southern Arizona examples, the nonconformity between crystalline basement and layered cover rocks played an important mechanical and physical-chemical role during the deformation. The basement stretched and necked in a manner akin to megaboudinage, while simultaneously parts of the layered cover flowed passively during metamorphism and were plated tectonically to crystalline rocks along the unconformity and along ductile normal growth faults. Topographic basins created by regional pinch-and-swell were filled by lower to middle Tertiary sediments and by volcanics that poured out of the rifted basement. Physical-chemical conditions were such that the base of the Phanerozoic section became a vast heat sink as well as a zone of remarkably high fluid pore pressure. Thus, decollement zones, located near the base of the Phanerozoic section, are marked by sharp thermal gradients, wholesale denudation, and a great variety of mechanical and chemical chaos.

316 citations


Journal ArticleDOI
TL;DR: In this article, an isotopic dating investigation (66 K-Ar and 34 Rb-Sr analyses) provided the geochronological framework for the Alpine events of metamorphism and granitic magmatism on Naxos.
Abstract: An isotopic dating investigation (66 K-Ar and 34 Rb-Sr analyses) provided the geochronological framework for the Alpine events of metamorphism and granitic magmatism on Naxos. The oldest phase of high-pressure/medium-temperature metamorphism, M1, was dated by Rb-Sr and K-Ar analyses of paragonites, phengitic muscovites and muscovites at 45±5 Ma (Middle Eocene). Most of the record of the M1 phase has been eraded by a second phase of medium-pressure/high-temperature metamorphism, M2, which induced a metamorphic zonation with anatectic melting in the highest-grade part, the migmatite dome. Most K-Ar dates of M2 hornblendes, muscovites, biotites and tourmalines range from about 21 Ma in the lower-grade part (biotite-chloritoid zone) to about 11 Ma in the migmatite dome. From the pattern of K-Ar mineral dates it is concluded that the M2 phase took place 25±5 Ma ago (Late Oligocene/Early Miocene) and was followed by a prolonged cooling history until about 11 Ma ago (Late Miocene), when the ambient temperature in the migmatite dome had decreased to below 400−360 °C. A Rb-Sr whole-rock isochron analysis of a granodioritic mass dated the intrusion (and the associated M3 phase of contact-metamorphism) at 11.1±0.7 Ma (Late Miocene), with an initial 87Sr/86Sr ratio of 0.7112 ±0.0001. A local phase of low-grade retrograde metamorphism, M4, probably related to Late Alpine overthrusting, was dated at about 10 Ma (Late Miocene).

207 citations


Journal ArticleDOI
TL;DR: K-Ar and Rb-Sr dates on phengites (predominantly 3T) of the best preserved high P/itTmetamorphic rocks from northern Sifnos gave concordant ages around 42 m.y..
Abstract: Polymetamorphic rocks of Sifnos (Greece) have been investigated by Rb-Sr, K-Ar, and fission track methods. Critical mineral assemblages from the northern and southernmost parts of Sifnos include jadeite+quartz+3T phengite, and omphacite+garnet +3T phengite, whereas the central part is characterized by the assemblage albite+chlorite+epidote+2M 1 phengite. K-Ar and Rb-Sr dates on phengites (predominantly 3T) of the best preserved high P/itTmetamorphic rocks from northern Sifnos gave concordant ages around 42 m.y., indicating a Late Lutetian age for the high P/T metamorphism. Phengites (2M 1+3T) of less preserved high P/T assemblages yielded K-Ar dates between 48 and 41 m.y. but generally lower Rb-Sr dates. The higher K-Ar dates are interpreted as being elevated by excess argon. K-Ar and Rb-Sr ages on 2M 1 phengites from central Sifnos vary between 24 and 21 m.y. These ages date a second, greenschist-facies metamorphism which overprinted the earlier high-pressure metamorphic rocks.

201 citations


Book ChapterDOI
TL;DR: In this paper, the authors discuss mineralogy and petrology of burial diagenesis and incipent metamorphism in clastic rocks and propose an integrated picture of the phase changes in different types of sedimentary rocks upon burial.
Abstract: Publisher Summary This chapter discusses mineralogy and petrology of burial diagenesis and incipent metamorphism in clastic rocks The phase-modification processes referred to as “late-diagenetic” or “epigenetic” (or catagenetic) are metamorphic in nature Hence, the distinction between burial diagenesis and lowest-grade regional metamorphism is largely artificial and arbitrary The modification of various kinds of sedimentary rocks upon burial has largely been studied by investigators from different sub-disciplines within the earth sciences: (1) changes in mineralogy of clay-rich sedimentary rocks by sedimentary petrographers and clay mineralogists; (2) authigenic mineral assemblages in tuffs and volcanic sediments by metamorphic petrologists; and (3) changes in the constitution of coaly and other organic matter by coal petrologists, and organic geochemists The growing awareness of the overlap in space of the physical conditions and of the chemical influences controlling these phenomena warrants renewed efforts to correlate the changes in various types of sedimentary rocks in order to arrive, ultimately, at an integrated picture of the phase changes in different types of sedimentary rocks upon burial and incipient metamorphism This chapter intends to contribute to such an integrated picture

167 citations


Journal ArticleDOI
TL;DR: In this paper, the authors synthesize data on the regional metamorphism of pelites in the eastern Scottish Dalradian, and a Schreinemakers net and petrogenetic grid is presented and fixed in P-T space using experimental data.
Abstract: Summary An attempt is made to synthesise data on the regional metamorphism of pelites in the eastern Scottish Dalradian. Zonal sequences (facies series) of mineral assemblages are presented and variants of the traditionally recognised Barrovian and Buchan metamorphism are separated. A Schreinemakers net and petrogenetic grid for the system KFMASH is presented and fixed in P-T space using experimental data. The P-T data for the various facies series are used in conjunction with other constraints to develop a model of isotherm and isobar distribution in the region, and the effects of post metamorphic folding of the isopleths are noted. P-T gradients normal to isobars suggest overall ‘geothermal’ gradients for most of the area to be convex to the T-axis, and it is suggested that they result from magma intrusion at depth. The region adjacent to the Highland Boundary fault shows strong horizontal and vertical temperature gradients which are tentatively interpreted to indicate the presence of a synmetamorphic tectonic boundary to the Dalradian metamorphic belt.

143 citations


Journal ArticleDOI
TL;DR: The Western Australian Shield consists of two large Archaean cratons that are partly covered by remnant Proterozoic sedimentary basins and partly surrounded by Protero-climatic mobile belts.

142 citations


Journal ArticleDOI
01 Apr 1979-Nature
TL;DR: Arrested charnockitic conversion of amphibolitic gneiss at Kabbaldurga, Karnataka State, south India, was studied mineralogically.
Abstract: Arrested charnockitic conversion of amphibolitic gneiss at Kabbaldurga, Karnataka State, south India, was studied mineralogically. Iron-rich pyroxenes were generated from amphibole in patches and stringers without melting. The dark colour of charnockite arises from numerous tiny veins of chlorite and manganese-bearing calcite, particularly in feldspars. The metamorphism was effected by very local, mainly grain-boundary, migration of volatiles low in H2O, and probably dominantly CO2. This was followed by vein alteration at lower temperatures from volatiles richer in H2O. The volatiles are ascribed to massive liberation from the mantle in upwelling areas, and this may have been an important process in the evolution of the deep continental crust.

138 citations


Journal ArticleDOI
TL;DR: In this paper, the conditions of metamorphism are analysed in terms of varying initial thermal gradient, erosion rate constant and the presence or absence of the eclogite transition, and the suite of recorded metamorphic P-T conditions, called the piezothermic array has a slope which is insensitive to the variable conditions.

124 citations


Journal ArticleDOI
TL;DR: In this article, K/1bAr and Rb 1bSr ages are reported for high-grade metamorphic rocks from pre-Hercynian elements in the Hercynian orogen of NW Spain and for an early hercynian suite of granites (half-life 37 Rb = 5.0 · 10 10 10 a; errors 2σ).

113 citations


Journal ArticleDOI
TL;DR: The sequence of deformations, metamorphisms and magmatism in the Arequipa Massif has been established by structural and metamorphic methods and was distinguished by Rb-Sr whole-rock isochrons, at about 1918, 440 and 392 Ma respectively.
Abstract: The Arequipa Massif, between the Andes and the Pacific, is an extensive pre-Devonian metamorphic complex. The sequence of deformations, metamorphisms and magmatism in this complex has been established. Mollendo, Atico and Marcona events are distinguished by structural and metamorphic methods and dated by Rb-Sr whole-rock isochrons, at about 1918, 440 and 392 Ma respectively. The Mollendo event led to partial melting, followed by granulite-facies metamorphism, in sediments buried to about 30 km. Further NW, sillimanite-bearing migmatites and staurolite-andalusite schists are thought to represent the same event. The tectonic trend is uncertain but the structures and metamorphism suggest a collision orogeny which probably pre-dated the Pacific Ocean. The early Caledonian Atico and Marcona events are associated with coast-parallel batholiths, amphibolite- to greenschist-facies metamorphism and penetrative deformations. The Atico and Marcona events are separated by the deposition of the Marcona Formation, which is therefore thought to be Lower Palaeozoic (between about 440 and 392 Ma). The early Caledonian deformations are attributed to a subduction zone near the present Pacific margin. There is no penetrative Hercynian or Andean deformation in the Arequipa Massif. Palaeomagnetic study of Jurassic andesites and dykes suggests that there has been no latitudinal motion of the Arequipa Massif relative to the Brazilian shield during the evolution of the Andes.

111 citations


Journal ArticleDOI
TL;DR: In this paper, geochemical features of the ophiolite suite from the Bay of Islands Complex have been determined by major and trace element analyses of 13 rocks, including pillow lavas and diabases.

Journal ArticleDOI
15 Jan 1979-Lithos
TL;DR: The most widespread record that terrigenous sedimentary rocks have deformed by a pressure solution mechanism is seen in the development of spaced cleavages and transposition structures under conditions of low grade metamorphism as discussed by the authors.

Journal ArticleDOI
TL;DR: In this article, a sequence of five major, temporally discrete, regional structural events is established, which are marked by tight folding and prograde metamorphism, occurred at 1570 ± 20 and 1469 + 20 m.y.

Journal Article
Juhn G. Liou1
TL;DR: Pumpellyite probably crystallized directly from palagonitized basaltic glass as discussed by the authors, and it may contain iron dominantly in the ferric state, but the pyroxenes are well preserved.
Abstract: The basaltic rocks of the fragmented East Taiwan Ophiolite have been subjected to \"oceanfloor\" zeolite facies metamorphism. Depending on the bulk composition and mode of occurrence, various mineral assemblages occur: thomsonite + analcime * chabazite; pumpellyite * chlorite * laumontite in veins of the pillow cores; and pumpellyite * chlorite * K-feldspar; pumpellyite * laumontite * thomsonite; and prehnite (about 5 weight percent FeOr) * hematite in veins of the pillow matrices. Plagioclase phenocrysts were replaced by albite * pumpellyite * Ca-zeolites, pumpellyite * K-feldspar, analcime * chabazite * thomsonite, or by K-feldspar alone, the olivine phenocrysts by brown chlorite * serpentine i pumpellyite, but the pyroxenes are well preserved. Except for local palagonitization along fractures and rims, the pillowed glassy rims are perfectly fresh. The variations in mineral association among pillow cores, rims, and matrices are believed to have resulted from local variations in compositions of the circulating fluid, in the extent of recrystallization, and in temperature. The occurrence of potash feldspar suggests that the basaltic rocks have been subjected to local metasomatic exchange with a hydrothermal solution under zeolite facies conditions. Pumpellyites characteristically contain up to 25 weight percent total Fe as FeO, higher than most reported pumpellyite (except julgoldite). Calculation of their structural formulas indicates that they may contain iron dominantly in the ferric state. Substitution of Fe8+ for Al in this phase evidently enlarges the pumpellyite P-T stability field relative to the zeolite facies assemblages under oxidizing conditions. Pumpellyite probably crystallized directly from palagonitized basaltic glass. The basaltic rocks of the East Taiwan Ophiolite were hydrothermally metamorphosed at I = 150-250'C and depths of 0.6 to 1.6 km, probably at some distance away from the ridge.

Journal ArticleDOI
TL;DR: In this paper, the authors show that the metamorphism of the Alpine schists resulted from frictional heating during the 360-km right lateral slip on the Alpine fault during the Mesozoic (Rangitata) period of fault motion (140-80 m.y. ago).
Abstract: The Alpine fault of New Zealand is a major continental transform fault which has been uplifted on its southeast side 4–11 km within the last 5 m.y. This uplift has exposed the Alpine schists, which have been metamorphosed from the adjacent Torlesse graywackes. The Alpine schists increase in metamorphic grade from prehnite-pumpellyite facies 9–12 km from the fault through the chlorite and biotite zones of the greenschist facies to the garnet-oligoclase zone of amphibolite facies within 4 km of the fault. These metamorphic zone boundaries are subparallel to the fault for 350 km along the strike. The K-Ar and Rb-Sr ages of the schists increase with distance from the fault: from 4 m.y. within 3 km of the fault to approximately 120 m.y. 20 km from the fault. Field relations show that the source of heat that produced the argon depletion aureole was the fault itself. Adopting a friction model for the fault and applying the known geological history of motion of the Alpine fault, we show that the metamorphism resulted from frictional heating during the 360-km right lateral slip on the Alpine fault during the Mesozoic (Rangitata) period of fault motion (140–80 m.y. ago). Fault motion began again in the Plio-Pleistocene Kaikoura orogeny, with 120 km of further right lateral slip and 4–11 km of uplift. Frictional heating during this episode produced the argon depletion. Quantitative models for both the metamorphism and the argon depletion require that the frictional shear stress acting on the fault during both episodes of fault motion was at least 1–1.5 kbars. If, alternatively, it is assumed that all fault motion took place in the last 30 m.y., this does not change our conclusion with respect to the stress required for the argon depletion.

Journal ArticleDOI
TL;DR: In this article, igneous zircons from the Glen Dessary syenite complex give concordant U-Pb ages averaging 456 ± 5 Ma which is taken to date crystallization.
Abstract: Synopsis Apparently igneous zircons from the Glen Dessary syenite complex give concordant U-Pb ages averaging 456 ± 5 Ma which is taken to date crystallization. The complex was deformed and metamorphosed to amphibolite facies subsequent to emplacement so that this age is a maximum for these orogenic events in the western Moine Series. Other age data indicate that amphibolite facies conditions associated with pegmatite emplacement continued until ca. 430 Ma contrasting with the north-east Highlands where tectonism and metamorphism was completed by 460 Ma ago. The disparity between hornblende and mica K-Ar ages suggests that cooling from 500 to 300°C took about 20 Ma. Higher blocking temperatures are indicated for U-Pb in monazite and sphene than for U-Pb in apatite.

Book ChapterDOI
01 Jan 1979
TL;DR: Conodonts are apatitic marine microfossils of Cambrian through Triassic age during incipient metamorphism 500 3000 C they change color from pale yellow to brown to black due to carbon fixing within the trace amount of organic matter in their skeletons as discussed by the authors.
Abstract: Conodonts are apatitic marine microfossils of Cambrian through Triassic age During incipient metamorphism 500 3000 C they change color from pale yellow to brown to black due to carbon fixing within the trace amount of organic matter in their skeletons As thermal metamorphism continues 3000 5500 C conodonts change from black to gray to white to crystal clear as a result of carbon loss release of water of crystallization and recrystallization The conodont color alteration technique provides a unique link between mineral and organic indexing of thermal metamorphism and is best suited for carbonate rocks Conodont color alteration index CAI isograd maps for three stratigraphic intervals in the Appalachian basin show I Conodont color alteration is directly related to the depth and duration of burial and the geothermal gradient 2 Tectonics affect color alteration only where folding and faulting act to significantly increase depth of burial 3 Isograds and overburden isopachs are conformable throughout most of the northern half and in the western part of the southern Appalachian basin in these areas isograd values gradually increase eastward except for a major disruption in the area of the Rome trough 4 South of central Virginia isograds are disrupted and irregular because late Paleozoic thrusting has severed and telescoped original burial metamorphism isograd patterns 5 Basin restoration using conodont CAI isograds indicates a maximum shortening in northeast Tennessee of about 115 miles 185 km 6 The CAI 2 isograd brown conodonts for each stratigraphic interval lies near the eastern limit of oil production for that interval this limit shifts eastward for each successively younger stratigraphic interval concomitant with decreasing overburden 7 Gas production is less related to isograds and depends mostly on primary and or secondary porosity and permeability The CAI 4 isograd brownish black conodonts however approximates the eastern limit of gas production because the temperature depth of burial necessary to produce this high level of organic metamorphism concurrently produces mineral metamorphism that reduces porosity and permeability and the likelihood of commercial reservoirs

Journal ArticleDOI
TL;DR: The metamorphic overprint on the pseudostratigraphy of ophiolite complexes in southern Chile has been studied in this paper, where the authors show that the intensity of recrystallization and the extent of retrograde effects are low; epidote is sparse while secondary biotite and magnetite are common accessory minerals.

Journal ArticleDOI
TL;DR: In this article, the effects of the earlier ridge metamorphism are the main concern of this paper, and the presence of assemblages transitional between greenschist and amphibolite facies for basaltic compositions is suggestive of very low-pressure thermal metamorphisms such as would be appropriate to the crustal portions of an oceanic spreading center.
Abstract: Brecciated mafic+ultramafic plutonic rocks of the East Taiwan Ophiolite occur as detritus and slide blocks in the Pliocene Lichi Melange. These plutonic rocks have been subjected to two stages of post-magmatic recrystallization: (I) pre-brecciation ridge-type metamorphism attended by high-grade greenschist and rare amphibolite facies physical conditions; and (II) later off-axis metamorphism under zeolite to lowest greenschist facies conditions that postdated brecciation, submarine talus accumulation and deposition of associated pelagic sediments. The effects of the earlier ridge metamorphism are the main concern of this paper. (I) Dominant antigorite together with chlorite and talc in some ultramafics suggests that these rocks recrystallized at T>350 ° C. The primary compositions of gabbroic calcic plagioclase have been modified from An 45–70 to An 13–38, and the igneous clinopyroxenes and hornblendes partly replaced by actinolite+chlorite. Stable mineral assemblages in the metagabbros are thus ∼oligoclase+actinolite+chlorite±very rare epidote+sphene, and intermediate plagioclase +actinolite+chlorite+sphene. Amphibolites are less common and consist of more calcic plagioclase (An 25–49)+hornblende. The presence of assemblages transitional between greenschist and amphibolite facies for basaltic compositions is suggestive of very low-pressure thermal metamorphism such as would be appropriate to the crustal portions of an oceanic spreading center. (II) The occurrence of vein albite+actinolite+ chlorite near the base of the brecciated plutonic sequence and vein prehnite+laumontite in the upper part suggests that the brecciated plutonic rocks were later feebly retrograded under conditions of the greenschist and zeolite facies respectively-probably some distance removed from the thermal regime of a mid-oceanic ridge. The East Taiwan Ophiolite probably represents the western termination of the Philippine Sea lithospheric plate. Portions of this oceanic crust and underlying mantle were incorporated in the Lichi Melange of the Coastal Range of eastern Taiwan as a consequence of antithetic faulting and erosion. This process evidently accompanied east-directed underflow of the Asiatic (South China Sea) plate.

Journal ArticleDOI
01 Mar 1979
TL;DR: In this article, the Chapada Diamantina Group is proposed as follows: 1- At 1,7 b.y. formed the first sedimentary basins associated with continental acid voleanism.
Abstract: This paper presents and discursses some of the last geochronological determinations carried out on rocks of the Espinhaco Mountain Belt; by different radiometric methods. For the most part, the basement is composed of Archaean rocks, with formation of magmatites and granites during the Transamazonian cycle (2,0 b.y.). The geologic evolution of the Chapada Diamantina Group is proposed as follows: 1- At 1,7 b.y. formed the first sedimentary basins, associated with continental acid voleanism. 2 - The main stage of folding and epizonal metamorphism occured at approximately 1,2 b.y, with post-tectonic events until 1,0 b.y. These last ones include basicfissure volcanism and tectonic uplift. 3 - During the Brazilian cycle (650-400 m.a.) the tectonothermal phenomena have affected in several ways and with different intensity the age determinations of all geological units.

Journal ArticleDOI
01 Jun 1979-Nature
TL;DR: In this paper, a 40Ar-39Ar age spectra for hornblende separates from two unaltered samples of igneous rocks that are interpreted as the initial igneous products of rifting in the Othris region of central Greece.
Abstract: PRECISE radiometric dating of ophiolites and associated rocks is usually very difficult as the rocks are often altered by ocean floor metamorphism or during tectonic emplacement onto the continental margin. Only age determinations on primary mineral phases can be considered to approximate their initial crystallisation and cooling ages. The K–Ar technique is generally used to date ophiolitic rocks, but even if primary mineral phases are analysed, there is always the uncertainty that the resulting age could have been partially reset by later thermal or tectonic events. The 40Ar–39Ar step heating method is able to detect and often to correct for any partial loss of argon, and thus overcome these problems. We have determined and report here 40Ar–39Ar age spectra for hornblende separates from two unaltered samples of igneous rocks that are interpreted as the initial igneous products of rifting in the Othris region of central Greece. A third hornblende has been analysed from amphibolites believed to have been formed during tectonic emplacement of the Pindos ophiolite (Fig. 1). The ages suggest that the initial rifting began in Othris in the Permo–Triassic (∼248 Myr), while tectonic emplacement of the Pindos ophiolite occurred in the middle Jurassic time (∼180 Myr).

Journal ArticleDOI
TL;DR: In this article, the authors present new evidence from amphibolite and granulite facies terrains which bears upon the mobility of rare-earth elements during high-grade metamorphism.
Abstract: A considerable body of data on the distribution and behaviour of rare-earth elements (REE) in igneous and sedimentary rocks has been accumulated over the past decade. Metamorphosed and metasomatized rocks have by comparison received scant attention. Some investigators have reported REE mobility, but most of the available data suggests the REE levels and patterns are essentially unaffected by metamorphism. However, the hypothesis of REE “immobility” during metamorphism has not been rigorously tested. We present new evidence from amphibolite and granulite facies terrains which bears upon the mobility of rare-earth elements during high-grade metamorphism. Metasomatic zones provide an excellent test of element mobility. Examples are described from an amphibolite facies terrain in which these zones have retained their original REE chemistry, as well as their Ti, P, Y, Zr, Hf, Nb and Ta contents. In the associated metavolcanic suite strong linear correlations exist between the REEs and the other “immobile” elements. Such correlations are frequently explained in cogenetic volcanic suites as being produced by fractional crystallization. It seems unlikely that element migration during metamorphism would preserve such coherent behaviour of elements with different chemical properties. We conclude that in these cases metamorphism and metasomatism has affected neither the REE abundances and patterns of this metavolcanic suite, nor its Ti, P, Y, Zr, Hf, Nb or Ta content. Only these elements provide a reliable guide to the parentage of the metasomatized volcanics. By contrast, our work on Archean granulite-facies gneisses from the Lewisian of N.W. Scotland shows that these deep crustal rocks are depleted in REEs (except Eu) by a factor of 2 or 3 relative to upper crustal values. A distinct positive europium anomaly supports the idea of preferential retention of Eu in the deep crust proposed by Jakes and Taylor (1974) and Nance and Taylor (1976). Mineral REE patterns in the granulites and their distribution coefficients closely resemble those reported for phenocrysts in dacites, suggesting that the minerals equilibrated with a granodioritic liquid. The trace and major element chemistry of this terrain is compatible with the model of a granulite facies residuum, left after the generation of a granodioritic upper crust. Rare-earth elements provide a promising new tool in the determination of the parentage of metamorphosed and metasomatized rocks up to the upper amphibolite facies of regional metamorphism and may prove valuable in assessing the importance of anatexis in the evolution of the lower continental crust.

Journal ArticleDOI
01 Nov 1979-Nature
TL;DR: The Lizard complex of south Cornwall consists of an assemblage of peridotites, gabbros, hornblende schists, metasediments, dolerites and metadolerites as mentioned in this paper.
Abstract: The Lizard complex of south Cornwall consists of an assemblage of peridotites, gabbros, hornblende schists, metasediments, dolerites and metadolerites and acid-basic banded gneisses (the Kennack gneisses). It is bordered to the north by a zone of chaotic sediments with phacoidal inclusions of pillow lavas, greywackes and quartzites (the so-called Meneage crush zone). This zone can be traced eastwards into the Roseland district of south Cornwall1. The age of the complex is uncertain. Dates (largely K–Ar) obtained from the Kennack Gneiss and the majority of hornblende schists are mainly between 370 and 390 Myr, although Green2,3considered the peridotite to be older than the oldest date obtained from the hornblende schists, at 492 Myr. Metadolerite dykes have recently4 been given ages of approximately 400 Myr. All the above are thought to represent minimum ages due to the possibility of argon loss during subsequent uplift, cooling and mild metamorphism. Since the work of Green3, the Lizard complex has been regarded by many as the type example of a hot diapiric mantle intrusion into continental crust. Intrusion is thought to have occurred during a period of regional metamorphism and to have resulted in the imposition of a contact dynamothermal aureole on the hornblende schists at the margin of the peridotite body. The gabbro is regarded as a later and unrelated intrusion, and the Kennack gneisses were attributed to intrusion of acid magma along sheared basic dykes in peridotite. Thayer5 suggested that the complex was ophiolitic, basing his views on the field relationships of gabbros and peridotites which indicated that the two lithologies might be temporally and genetically related. General comparisons have since been made between the Lizard complex and the well-documented ophiolote sequences6–10, although little detail has generally been given. I provide here relevant details which suggest that the Lizard complex is ophiolitic in origin.

Journal ArticleDOI
TL;DR: In this paper, the authors describe the evolution of northwest Buksefjorden in terms of four principal events: the first, D1, includes a series of phases of deformation and associated high-grade metamorphism that may be related to the injection of parental magmas of the Amitsoq gneisses and consequent disruption of sediments and basic igneous rocks of the Akilia association.

Journal ArticleDOI
04 Jan 1979-Nature
TL;DR: In this paper, an empirically derived cooling curve for the Haliburton basic intrusions within the Grenville Province of the Canadian Shield was derived from isotopic data. But the cooling curve was not derived for the entire orogen, only for the upper amphibolite to granulite facies.
Abstract: PALAEOPOLES from the exposed Grenville Province of the Canadian Shield are displaced from those of the remaining shield. It has been proposed that these poles record either a Precambrian plate collision ∼1,000 Myr (refs 1–3) or manifest a large rotation and translation of the entire extant North American crust4–6. Geological evidence in support of the collisional hypothesis is largely lacking; for example, an acceptable suture zone has not been located. The critical evidence for a choice between the two-plate and one-plate tectonic hypotheses must, therefore, come from dating these palaeopoles because if the magnetisations were acquired contemporaneously with those of the Logan Loop (≃1,000–1,200 Myr), then the two-plate or collisional hypothesis is probably correct. We present here an empirically derived cooling curve for the Haliburton basic intrusions within the Grenville Province. These intrusions are located in a region of high-grade metamorphism, upper amphibolite to granulite facies. Their emplacement times are not known, but in the Haliburton Highlands cooling from the culmination of regional metamorphism, not contemporaneous throughout the Grenville Province, began ∼1,050 Myr ago7. This is the first cooling curve for an orogen to be derived solely from isotopic data; that is, the first for which both temperature and time have been determined from the same samples. Using this as a calibration curve, we show here that the thermal remanent magnetisation (TRM) components from the Haliburton basic intrusions were acquired between ∼980 and 820 Myr ago during slow cooling and that, consequently, the two-plate hypothesis is improbable.

Journal ArticleDOI
19 Apr 1979-Nature
TL;DR: In this article, a Caledonian blueschist from the Dalradian Supergroup in South Achill Island, County Mayo was found to occur in a regularly bedded and folded sequence and may imply an early high pressure metamorphic event.
Abstract: THE occurrence of the blue amphiboles glaucophane and crossite in metamorphosed basic rocks is characteristic of terrains metamorphosed at high pressures. Most examples are from relatively recent fold belts. We record here a Caledonian blueschist from the Dalradian Supergroup in South Achill Island, County Mayo (Fig. 1). It is the first ‘Old’ blueschist from Britain to occur in a regularly bedded and folded sequence and may imply an early high pressure metamorphic event in this part of the Dalradian, evidence for which was generally destroyed by either erosion or later metamorphism.

Journal ArticleDOI
TL;DR: In this article, a large part of the fluid present in the thermal dome at Naxos, Greece, during the main stage of metamorphism was CO2-rich and of deep-seated origin.

Journal ArticleDOI
TL;DR: In this article, the fine mineral fractions extracted from pelitic sediments of the Eocambrian upper Damaran Mulden Group were subjected to a detailed isotopic study by the Rb-Sr and K-Ar methods in combination with a mineralogical analysis by X-ray diffractometry.

Journal ArticleDOI
01 Aug 1979-Geology
TL;DR: In this article, a geodynamic model is proposed that attempts to synthesize the orogenic mechanisms in Spain and southern Brittany during middle and late Paleozoic time, and it is therefore suggested that the formation of the Iberian-Armorican arc predates the Hercynian orogeny.
Abstract: Recently assembled geological and geophysical evidence from southern Brittany reveals the organization of submerged structures off the southern Brittany coast. Fragments of a high-pressure–low-temperature belt are caught between a northern high-temperature zone of middle Paleozoic age, and a southern zone of intermediate metamorphism. It is not known whether the rocks in the high-pressure–low-temperature belt belong to a middle Paleozoic allochthonous slab or whether they are parautoch-thonous and the result of Carboniferous obduction. Farther to the south a major geophysical discontinuity is interpreted as a suture, the trace of which cuts across the sedimentary facies zonation established between the Armorican Massif and Spain. It is therefore suggested that the formation of the Iberian-Armorican arc predates the Hercynian orogeny. A geodynamic model is proposed that attempts to synthesize the orogenic mechanisms in Spain and southern Brittany during middle and late Paleozoic time.

Journal ArticleDOI
TL;DR: In this article, the critical reaction 1 zoisite + 1 glaucophane ⇆ 1.2 omphacite+0.8 H2O was investigated in the metamorphic assemblages of Val Chiusella.
Abstract: In the metabasites of Val Chiusella, metamorphic assemblages are present, corresponding to the glaucophane schist facies, i.e. garnet glaucophanites to omphacite-garnet glaucophanites, as well as to the eclogite facies, i.e., glaucophane eclogites, eclogites, and omphacite felses. Both groups of assemblages are divided by the critical reaction 1 zoisite +1 glaucophane ⇆ 1.2 omphacite+0.8 garnet+0.7 paragonite +1.4 quartz+0.8 H2O. From textural evidence it is clear that in the investigated area this reaction proceeded to the right according to a prograde metamorphism. Correspondingly, K garn-cpx D(Fe/Mg) values of coexisting garnet-omphacite pairs in the glaucophane schist facies assemblages are higher than in the eclogite facies assemblages and reflect a temperature increase from about 450 ° C to about 550 ° C at minimum water vapour pressures of 12 to 16 kb.