Showing papers on "Metamorphism published in 1975"
TL;DR: In this paper, five minor and trace elements have been variously combined to produce a set of binary diagrams in addition to total alkali-SiO2 diagrams, that discriminate between fresh tholeiitic and alkali basalts.
744 citations
213 citations
TL;DR: In this paper, the authors suggest that metamorphism occurred in a newly started subduction zone, where an inverted thermal gradient developed below the hot hanging-wall peridotite.
Abstract: On Santa Catalina Island, blueschist is structurally overlain by glaucophanic greenschist, which is overlain in turn by a unit of amphibolite and ultramafic rock. These three units are juxtaposed along sub-horizontal postmetamorphic thrusts; tectonic blocks of amphibolite are distributed along the thrust between the greenschist and the blueschist. Physical conditions of metamorphism are estimated to be approximately 300°C and 9 kb for blueschist, 450°C and 8 kb for greenschist, and 600°C and 10 kb for amphibolite. I suggest that metamorphism occurred in a newly started subduction zone, where an inverted thermal gradient developed below the hot hanging-wall peridotite. Postmetamorphic eastward underthrusting along surfaces of varying dip can explain the present structural relationships. Tectonic blocks of glaucophane-epidote schist, amphibolite, and eclogite elsewhere in the Franciscan Complex may be disrupted remnants of similar metamorphic zones. The inverted thermal gradient will only exist in the early stages of subduction, which explains why the blocks are the oldest rocks in the Franciscan Complex. The gross decrease in age and metamorphic grade westward across the Franciscan results from successive underthrusting and accretion of progressively younger slices of supercrustal material, concurrent with uplift and erosion. Pressure-temperature (P-T) conditions of metamorphism in each east-dipping tectonic slice will increase down-dip. At any given time, older, more easterly slices will have been uplifted further, hence metamorphic grade in the exposed edges will increase eastward and structurally upward. If erosion is faster than accretion for a time, younger slices will be metamorphosed at lower pressures than were the older higher ones. Simple reverse faulting can then produce the observed interleaving of rocks of different metamorphic grade.
167 citations
TL;DR: The early Alpine tectonics and relatively high-pressure metamorphic parageneses of the Helvetic, Pennine and Sesia-Lanzo realms are compared with analogous circumpacific terranes in western California, southwestern Japan, west-central Chile, and southern Alaska as mentioned in this paper.
160 citations
TL;DR: In this paper, it was shown that antigorite-bearing serpentinite bodies containing zoned grains of accessory chromite occur beneath Paleozoic sediments southwest of the Manitoba Nickel Belt.
151 citations
TL;DR: The Archaean complex of Greenland consists of a layered sequence of gneisses whose history can be traced back from 2,600 m.y. to before 3,750 m.dy as discussed by the authors.
144 citations
TL;DR: Some 80 stratabound pyrite deposits occur in Precambrian volcanic and sedimentary rocks in the Skellefte district of northern Sweden as discussed by the authors, which are closely associated with keratophyric and dacitic volcanics, and contain lead isotopes which are coincident with the primary growth curve.
Abstract: Some 80 stratabound pyrite deposits occur in Precambrian volcanic and sedimentary rocks in the Skellefte district of northern Sweden. These deposits have been considered previously to have originated by concentration of metals from the supracrustal rocks during the formation of a late-kinematic palingenetic granite.However, the ores show all the characteristics of deposits associated with submarine volcanic rocks from other parts of the world and in other geologic epochs. In particular, the deposits are stratabound, closely associated with keratophyric and dacitic volcanics, and contain lead isotopes which are coincident with the primary "growth curve". The Pb-Cu-Zn and Pb:Zn ratios are also characteristic of these deposits. Sulfur isotope ratios are remarkably homogeneous over the whole area and fall at about the meteoritic standard.Furthermore, the ores have been subjected to regional metamorphism which is related mainly to the formation of the late-kinematic granites. Macroscopic ore metamorphic features include tight folding, brecciation, and boudinage. The structural setting of many of the deposits is similar to that expected from the process of bedding transpositions during regional metamorphism. The pyrite of the ores has been studied microscopically using structure-etching techniques. The ores commonly show pyrite micromosaics and these have been examined statistically. It is concluded that the pyrite has been annealed and subsequently deformed to various degrees in different deposits. Relict textures have been studied in the etched pyrites. Rogenpyrit, framboidal, and colloform textures have been tentatively identified in these relicts.It is concluded that the ores were formed in association with volcanism in a shallow marine environment during the formation of a geosyncline and subsequently subjected to at least two phases of deformation.
139 citations
TL;DR: The Tari-Misaka ultramafic complex, which is emplaced into the Paleozoic sediments and thermally metamorphosed by two younger granitic masses, is divided into four zones on the basis of the mineral assemblage as mentioned in this paper.
Abstract: The Tari-Misaka ultramafic complex, which is emplaced into the Paleozoic sediments and thermally metamorphosed by two younger granitic masses, is divided into four zones on the basis of the mineral assemblage They are, in order of increasing metamorphic grade: Zone I antigorite-olivine-orthopyroxene-clinopyroxene Zone II olivine-talc Zone III olivine-anthophyllite Zone IV olivine-orthopyroxene Strongly serpentinized clinopyroxene-bearing harzburgite in Zone I is similar to ordinary Alpine-type harzburgite In Zonne II, two kinds of olivine are recognized One is Mg-rich olivine (Fo93 to Fo97) with opaque inclusions and is probably a recrystallization product of serpentine with talc The other is Fe-rich olivine (Fo88 to Fo93) free of opaque inclusions and is probably a relic of the primary peridotite Olivine in Zone III and Zone IV is also relatively Mg-rich (Fo91 to Fo95) Chromitite in Zone IV commonly has an assemblage, olivine+cordierite+Mg-Al spinel (Mg/Mg+Fe2+, more than 09) Enstatite is rare and coexists with less magnesian Mg-Al spinel (Mg/Mg+Fe2+, less than 09) Petrological and mineralogical characters of the ultramafic rocks can be well explained by thermal metamorphism of strongly serpentinized peridotite by granitic intrusion Metamorphic zones are consistent with the experimental results in the system MgO-SiO2-H2O The assemblage olivine+cordierite indicates that the metamorphism occurred at relatively low pressures (<3kb)
127 citations
TL;DR: Metagabbros with a transitional greenschist-amphibolite-facies mineralogy are exposed on the lower slopes of the eastern wall of the axial valley of the Mid-Atlantic Ridge at 06°N as mentioned in this paper.
Abstract: Metagabbros with a transitional greenschist-amphibolite-facies mineralogy are exposed on the lower slopes of the eastern wall of the axial valley of the Mid-Atlantic Ridge at 06°N. These metagabbros are associated with metabasalts in the greenschist facies and with only mildly or nonmetamorphic basalts. Among the metagabbros some are finely banded, consisting of alternating amphibole-rich and plagioclase-rich zones, each about 1 mm thick. Specimens showing gabbro-basalt contacts are interpreted as representing the margins of basaltic dikes originally crossing through the gabbroic lower oceanic crust. The chemical composition of the metagabbros appears to be nearly unchanged from that of their parent rocks, which probably were gabbros with intermediate Fe/Mg ratio, and with olivine tholeiite normative composition and low K content characteristic of abyssal tholeiites. One of the metagabbros was dated at between 2 and 3 m.y. by the fission-track method on zircon crystals. The metamorphic processes which gav...
118 citations
TL;DR: In this article, a modified plutonic igneous hypothesis was proposed for the origin and history of the Scottish Mainland Lewisian and their amphibolised equivalents. But this hypothesis was rejected in favour of an underplating process, with crystallisation occurring under granulite-facies conditions accompanied by synchronous deformation.
87 citations
TL;DR: In this paper, the internal metamorphic fabrics of the main nappe sequence of Finnmark, northern Norway, developed during an Early Ordovician phase of the Caledonian orogeny (Grampian?); the cleavage development in the autochthon also belongs to this phase.
Abstract: On the basis of regional, structural, and metamorphic studies combined with age determinations, it is demonstrated that the internal metamorphic fabrics of the main nappe sequence of Finnmark, northern Norway, developed during an Early Ordovician phase of the Caledonian orogeny (Grampian?); the cleavage development in the autochthon also belongs to this phase. The final mise-en-place of the thrust-nappe sequence, however, appears to belong to a later phase of Caledonian orogenic development, probably toward the end of the Silurian Period, as does the deformation and metamorphism of the Silurian sequence of Mageroy. The geochronological results also give information regarding the positioning of the lower boundaries to the Cambrian and Ordovician Systems.
TL;DR: In this article, the Upper Wakatipu district, South Island, New Zealand, has been studied using optical and X-ray diffraction methods, and electron microprobe analyser.
Abstract: Metamorphic minerals from pumpellyite-actinolite and contiguous facies in the Upper Wakatipu district, South Island, New Zealand, have been studied using optical and X-ray diffraction methods, and electron microprobe analyser. Analyses are given for chlorite, garnet, prehnite, stilpnomelane, alkali-amphibole, actinolite, pumpellyite, epidote, and muscovite. The area is divided into four progressive metamorphic zones. Mineralogical change is summarised. Plagioclase feldspars are all low albite. Muscovite is ubiquitous in all zones except for a few mafic rocks. Analysed muscovites are mildly phengitic. The composition is not related to grade, but is related to host rock composition. Refractive indices of muscovites are rather low and indicate a lower celadonite content than those of eastern Otago and the Sambagawa zone. Basal spacing of muscovites do not differ much between zones or rock types. Epidote is common throughout the area, especially in mafic rocks. The compositional range is fairly narro...
TL;DR: In this article, it was shown that chloritoid and kyanite-bearing acid metavolcanic rocks of the Abitibi greenstone belt have acquired an aluminum surplus by weathering prior to metamorphism.
Abstract: Chloritoid- and kyanite-bearing acid metavolcanic rocks of the Abitibi Greenstone belt have acquired an aluminum surplus by weathering prior to metamorphism. The weathering increases from the top of the volcanic unit downwards, as shown by increasing values for both Niggli-t and the Zr/P ratio. The depositional environment of these rocks is postulated to be either shallow marine or terrestrial.
TL;DR: The grey gneiss complex of 0.7014 ± 0.0007 was derived from upper mantle source regions not more than 100-200 m.y. prior to the Scourian metamorphism as mentioned in this paper.
Abstract: Rb-Sr and Pb/Pb whole rock isochrons for the “grey gneiss” complex yield typical Scourian ages of 2690 ± 140 m.y. and 2640 ± 120 m.y. respectively, probably dating Scourian metamorphism. Subsequent events ( 40 Ar from minerals, but left the whole rock samples closed systems to Rb, Sr, U and Pb. The suggestion of an early Laxfordian granulite facies event followed by regional retrogression to amphibolite facies is thereby rendered most unlikely. Consideration of the initial 87 Sr/ 86 Sr ratio for the grey gneiss complex of 0.7014 ± 0.0007, as well as of the Pb isotope ratios, strongly suggests that the precursors of the “grey gneiss’ complex were derived from upper mantle source regions not more than 100–200 m.y. prior to the Scourian metamorphism, and rule out the possibility that the complex represents the reworking of a significantly older gneissic basement complex with anything like normal crustal Rb/Sr ratios.
TL;DR: In this article, a one-dimensional conductive relaxation model for the thermal history of terrains which have undergone tectonic burial is presented for the Tertiary history of the Eastern Alps, since the time interval between overthrusting and the peak of metamorphism is quite well known.
TL;DR: In this paper, the distribution of iron and magnesium between coexisting garnet and clinopyroxene is expressed by the distribution coefficient K D ga-cpx, which is used to determine the temperature of equilibration of natural eclogites.
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01 Jan 1975
TL;DR: In this article, the authors studied the topology of metamorphic phase diagrams and calculated the slopes of reaction curves, as well as the location of the reaction curves. But they did not study the relationship between the number of phases and the stability of one phase on the other.
Abstract: 1 Background Discussion.- Scope of Metamorphism.- Temperatures and Pressures of Metamorphism.- Movement of Material in Metamorphism.- Mineral Assemblages ('Pangeneses').- Microstructures of Metamorphic Rocks.- Preferred Orientation in Metamorphic Rocks.- Metamorphic Grade.- Metamorphic Facies.- Tectonic Setting of Metamorphism.- References.- 2 Equilibrium Aspects of Metamorphic Reactions.- Driving Force for Metamorphic Reactions.- Types of Metamorphic Reactions.- Variance of Metamorphic Reactions.- Metamorphic Reactions and Resulting Assemblages.- Sliding Reactions at Isograds.- Effect of One Phase on the Stability of Another.- Criteria of Stable Coexistence of Metamorphic Minerals.- Domains of Local (Mosaic) Equilibrium.- Calculation of the Topology of Metamorphic Phase Diagrams (Schreinemakers' Analysis).- Calculation of Slopes of Reaction Curves.- Experimental Location of Reaction Curves.- Reversal of Experimental Reactions.- Discrepancies Between Calculated and Experimentally Determined Reaction Curves.- Open Systems in Metamorphism.- Fluid Phase in Metamorphism.- Pressure of Fluid Phase.- Effect of Fluid Pressure on Reaction Curves.- Effects of Composition of Fluid Phase (Fugacities of Mixed Volatile Components) on Reaction Curves.- Effect of Oxygen Fugacity.- Coexisting Magnetite and Ilmenite.- Effect of fO2 on Hydrous Silicates Without Cations of Variable Valency.- Distribution of Elements Between Coexisting Minerals.- References.- 3 Kinetics of Metamorphic Reactions.- Diffusion in Metamorphism.- Nucleation and Growth.- Application of Reaction Kinetics to Metamorphism.- Zoning in Metamorphic Minerals.- Problems of Aragonite in Metamorphic Rocks.- Kinetics of the Reaction: Calcite + Quartz ? Wollastonite + CO2.- References.- 4 Reactions in Metamorphic Rocks.- Mechanisms of Metamorphic Reactions.- Reactions in Metapelitic Systems.- Reactions in Mafic and Ultramafic Systems.- How Relevant Are Available Experimental and Theoretical Determinations of P-T-X Equilibria?.- Retrograde Reactions.- Reactions Involving Zoned Minerals.- Reactions Involving Zoned Garnet.- Compositional Changes During Metamorphism ('Metamorphic Differentiation').- References.- 5 Stable Metamorphic Microstructures.- Unstable Grain Arrangements.- Stable Grain Arrangements in Isotropic Single Phase Aggregates.- Anisotropic Single Phase Aggregates.- Aggregates with More than One Phase.- Spatial and Size Distribution of Grains in Metamorphic Rocks.- Environments of Grain Adjustment in the Solid State.- References.- 6 Deformation, Recovery and Recrystallisation Processes.- Brittle Versus Ductile Deformation.- Crystal Plasticity.- Primary Modes of Intragranular Plastic Deformation.- Secondary Modes of Deformation.- Dislocations.- How Do Dislocations Assist Deformation?.- Experimental Detection of Dislocations.- Mobility of Dislocations.- Production of Dislocations.- Dislocations in Relation to Work-Hardening and -Softening.- Partial Dislocations.- Dislocations in Non-Metals.- Surface Imperfections 160 Effect of Grain Boundaries on Deformation (Intergranular Deformation).- Recovery and Recrystallisation 166 Development of Crystallographic Preferred Orientation by Recrystallisation.- Development of Preferred Orientations by Deformation.- Some Generalities.- Appendix: Methods of Detecting Slip Systems.- References.- 7 Deformation, Recovery and Recrystallisation of Some Common Silicates.- Quartz.- Microstructural Features of Deformed Quartz.- Experimental Deformation, Recovery and Recrystallisation of Quartz.- Natural Deformation, Recovery and Recrystallisation of Quartz.- Preferred Orientation.- Variation in Quartz Microfabric with Metamorphic Conditions.- Stress Indicators in Deformed Quartz.- Olivine.- Experimental Deformation, Recovery and Recrystallisation of Olivine.- Natural Deformation, Recovery and Recrystallisation of Olivine.- Preferred Orientation of Olivine.- Layer Silicates.- Deformation of Mica Single Crystals.- Deformation of Fine-Grained Layer Silicate Aggregates.- Recovery and Recrystallisation of Mica Single Crystals.- Experimental 'Recrystallisation' of Fine-Grained Layer Silicate Aggregates.- Preferred Orientation Mechanisms in Layer Silicates.- Systematic Study of Natural Deformation and Recrystallisation of Mica.- Plagioclase.- Experimental Deformation of Plagioclase.- Natural Deformation of Plagioclase.- Experimental Recrystallisation of Plagioclase.- Natural Recrystallisation of Plagioclase.- Generalisations.- References.- 8 Relationships between Chemical and Physical Processes in Metamorphism.- Effect of Chemical Reactions on Rock Mechanical Properties.- Effect of Differential Deformation on Mineral Assemblage.- Examples of Metamorphic Layering Formed By Preferential Removal of Minerals.- Large-Scale Metasomatism and Deformation.- Pegmatites and Deformation.- Migmatites.- Time Relationships between Deformation and Mineral Growth.- Solution Transfer.- Spinodal Decomposition?.- References.- Author Index.
01 Jan 1975
TL;DR: Amphibolites associated with alpine peridotites in the Central Ophiolite zone in Yugoslavia have K-Ar ages of 160-170 m.y.
TL;DR: The appearance of garnet can be considered in terms of two complex reactions: Olivine + plagioclase 1 → (Ca, Mg, Fe) garnet + PlagiClase 2 + quartz (quartz-normative).
TL;DR: In this article, the setting of the K-Ar ages in the regional context of the Mesozoic Rangitata orogeny allows a reassessment of previous models of metamorphism and uplift of the schist belt, and a new model of two-stage uplift and metamorphisms in Late Jurassic and late Cenozoic times is proposed.
Abstract: Potassium-argon ages from the Alpine schist belt in south-east Nelson, New Zealand, range from 6 to 143 m.y.; the youngest ages are from rocks closest to the Alpine fault. The K-Ar ages cannot be consistently correlated with metamorphic grade, and there is evidence that degassing of minerals has occurred on a small scale, producing a smooth radiogenic argon concentration gradient spatially related to the Alpine fault. Degassing does not appear to have involved net argon loss or gain from the schist belt as a whole. The setting of the K-Ar ages in the regional context of the Mesozoic Rangitata orogeny allows a reassessment of previous models of metamorphism and uplift of the schist belt, and a new model of two-stage uplift and metamorphism in Late Jurassic and late Cenozoic times is proposed. Metamorphism of schist occurred in Jurassic time followed by uplift and cooling during mid-Cretaceous time. In late Cenozoic time, vigorous vertical movements of the Alpine schist belt initiated degassing of radiogenic argon, resulting in a spectrum of disturbed “Rangitata” ages from 4 to 140 m.y. along a narrow belt close to the Alpine fault. It is estimated that 5,000 m of uplift commenced 4 ± 2 m.y. ago.
TL;DR: In this paper, U-Pb data for zircons from post-tectonic monzonite, syenite and norite, and country rock gneiss and migmatite from the Kap Farvel-Prins Christian Sund area of South Greenland indicate two distinct intrusive episodes at ∼1740 m.y.
TL;DR: In this article, the distribution of andalusite, kyanite, and sillimanite in an important part of the NE Dalradian is mapped, showing that the heat source for the Sillimanite-grade metamorphism in this area was the Newer (or Younger) Basic magma.
Abstract: The distribution of andalusite, kyanite and sillimanite in an important part of the NE Dalradian is mapped. Sillimanite represents a late episode overprinted on the regional Andalusite and Kyanite Zones. The spatial distribution of sillimanite confirms that the heat source for the sillimanite-grade metamorphism in this area was the Newer (or Younger) Basic magma. The sillimanitic aureole of the Newer Basic intrusives is divisible into a Sillimanite Zone and a higher-grade Sillimanite—potash feldspar Zone. Relict andalusite is common in the Sillimanite Zone. Coarse muscovite replacing other minerals is primary in both zones, and should not be confused with late retrograde effects. Electron-probe analytical data on the Sillimanite–potash feldspar zone paragenesis of muscovite are presented. In this, its type area, the so-called ‘Boyne Line’ does not exist; the concept of such a tectonic dislocation arose through misinterpretation of the metamorphic effects.
TL;DR: A pre-Upper Ordovician age for the Greenland Group is suggested in this paper, which is not necessarily related to the age of the sedimentary source area, but dates the metamorphism and perhaps formation of slaty cleavage in the sediments during the uppermost Ordovicians.
Abstract: Potassium-argon (K-Ar) whole-rock ages of Greenland and Waiuta Group slates from Buller and Westland yield a wide range of ages from 298 to 438 m.y. The younger ages ranging from 298 to 370 m.y. almost certainly reflect the thermal overprint associated with Tuhuan igneous and/or metamorphic events, especially at about 300 m.y., whilst an older group of ages in the range 395–438 m.y. reflect a pre-Devonian event. It is argued that the older ages are not “detrital”, (i.e., related to the age of the sedimentary source area), or related to diagenetic recrystallisation during sedimentation, but date the metamorphism and perhaps formation of slaty cleavage in the sediments during the uppermost Ordovician. Accordingly a pre-Upper Ordovician age for the Greenland Group is suggested.
TL;DR: In this article, it is suggested that temperature and load pressure, fluid pressure and fluid composition (H2O, CO2, Cl, SO3) were important factors for the transformation of gabbro to eclogite in the environment of the Lufilian Arc.
Abstract: Eclogites formed by the Lufilian (post-Katanga) metamorphism of gabbros intruded into rocks of the Katanga System (Upper Proterozoic) occur in the Central Province of Zambia. Typical constituents of these rocks are omphacite, almandine garnet with significant contents of the pyrope and grossular components, kyanite and rutile. Eclogites from some localities display well preserved, relict ophitic texture inherited from the original gabbro. Hornblende-scapolite metagabbro and coronite metagabbro often accompany the eclogites, and metalherzolite and serpentinite occur at a few localities. The Zambian eclogites are broadly associated with a variety of relatively high-pressure, kyanite-bearing mineral assemblages some of which are possibly in the eclogite facies. It is suggested that, in addition to temperature and load pressure, fluid pressure, fluid composition (H2O, CO2, Cl, SO3) and permeability of the gabbro due to deformation were important factors for the transformation of gabbro to eclogite in the environment of the Lufilian Arc.
01 Jan 1975
TL;DR: In this paper, the role of pre-Caledonian metamorphism and diastrophism using evidence from isotopic age determinations and from Precambnan unconformities is evaluated.
Abstract: The Caledonian fold belt of Svalbard (Spitsbergen) representing the northern extremity Cj of the circum-Atlantic Caledonian mountain system comprises between 15 and 20 km of sediments and metasediments with a variable share of volcanics and metavolcanics (Hecla Hoek succession). The main phase of Caledonian deformation and metamorphism occurred during the late Ordovician Ny Friesland orogeny and was followed by late-orogenic migmatisation and plutonism around the boundary of Silurian and Devonian and by lateorogenic deformations involving plate motions during the Devonian. An attempt is being made to evaluate the role of pre-Caledonian metamorphism and diastrophism using evidence from isotopic age determinations and from Precambnan unconformities so far generally neglected or underestimated in reconstructions of the evolution of the Hecla Hoek geosyncline. Three successive geosynclinal basins are analysed, the Precambrian Torellian (eugeosynclinal) and Jarlsbergian (miogeosynclinal) basins and the Early Palaeozoic Hornsundian (miogeosynclinal) basin. Diastrophic events recognised in the Precambrian column of the Hecla Hoek are tentatively correlated with igneous activity and metamorphic events within Svalbard and elsewhere.
01 Jan 1975
TL;DR: Agarwal et al. as mentioned in this paper found that scapolite is a metamorphic derivative of Archean saliferous sediments and that the scapolitic gneisses and schists of the Aldan shield are derived from the metamorphism of evaporites, either by itself or associated with plagioclase, quartz, diopside or hornblende.
TL;DR: Biotite and hornblende from a portion of the Blue Ridge Precambrian basement terrane that was progressively retrograded during Paleozoic metamorphism have been analyzed by the 40 Ar 39 Ar dating technique to determine if incremental release spectra can distinguish thermally altered samples as discussed by the authors.