Showing papers on "Metamorphism published in 1985"

Magmatism and Metamorphism in the Lhasa Block (Tibet): A Geochronological Study

Abstract: U-Pb systematics in small fractions of zircon, monazite, and sphene (5 to 20 grains each) have been analyzed in order to decipher the age and origin of magmatism in the Lhasa block and to date the tectono-metamorphic events. Additionally, we analyzed a few minerals for Rb-Sr. The results show that the granites and an ignimbrite were formed essentially through anatexis of continental crust, the magma source regions of which consisted of Palaeozoic to Precambrian materials up to 2 b.y. in age. The following episodes can be distinguished: (1) an early Tertiary plutonism-volcanism in the southern Lhasa block (Yangbajing region), strongly related to intracontinental tectono-metamorphism, (2) a late Jurassic to early Cretaceous plutonism in the central and northern Lhasa block, and (3) a middle Jurassic low-grade metamorphism, affecting the basement at the northern margin 171 ± 6 m.y. ago. The early Tertiary magmatism and metamorphism in the south are probably a direct result of Tethys subduction (Indian plate)...

Phase equilibria and mineral parageneses of metabasites in low-grade metamorphism

Abstract: The system Na20-CaO MgO A1203 SiO2-H20 is proposed to model phase equilibria and mineral parageneses for low-temperature metamorphism of basaltic rocks. Univariant reactions marking the transitions between various sub-greenschist facies are identified and some have been experimentally determined. The introduction of Fe203 into the model system at fixed FeO/MgO ratio creates continuous reactions for facies boundaries and discontinuous reactions for invariant points of the model system. Both qualitative and quanti- tative effects on P-T displacement and phase composi- tions are discussed. The XFe 3 + isopleths for epidote were plotted to exemplify the transition from the zeolite through prehnite-pumpellyite to prehnite-actinolite facies. T XFe~+ relations were established for continuous and discontinuous reactions relating such facies transi- tions. Because of the common occurrence of two or three Ca-Al hydrosilicates in low-grade metabasites, an isobaric A1-Ca-Fe 3+ projection from chlorite may be used to illustrate mineral assemblages and compositions of the coexisting Ca-A1 silicates in the presence of quartz, aibite, and chlorite. Reported occurrences in several classic burial metamorphic terrains and ocean- floor metabasites in ophiolites are described. Only the composition of a mineral from a buffered assemblage can constrain the intensive properties for metamorphism; previously reported compositional trends for pumpelly- ite and epidote with increasing metamorphic grade are oversimplified.

On the relationship between deformation and metamorphism, with special reference to the behavior of basic rocks

Abstract: It has long been recognized that deformation and metamorphism are closely interlinked, but although many authors have touched upon various aspects of the subject, there has never been a completely satisfactory understanding of the interrelationships at the mechanistic level. The literature abounds with descriptions of mineralogical changes that mirror the intensity of strain in heterogeneously deformed rocks (e.g., Teall, 1885; Beach, 1973; Kerrich et al., 1977) and with assessments of relative timing of deformation and metamorphism (e.g., Zwart, 1962; Spry, 1969; Vernon, 1977). A number of authors have attempted to identify mechanistic effects whereby the rate of metamorphic equilibration is affected by deformation, the resistance to deformation by particular deformation mechanisms is influenced by metamorphic transformations, or metamorphic segregation might arise during deformation (e.g., White and Knipe, 1978; Rubie, 1983; Gresens, 1966; Beach, 1982; Rutter et al., in press; Heard and Rubey, 1966; Raleigh and Paterson, 1965; Paterson, 1973; Robin, 1978; Gray and Durney, 1979; review of various aspects by Vernon, 1976). The foregoing reference list does not do justice to the number of geologists who have contributed to the subject of deformation/metamorphism interrelationships, which is receiving increasing attention from structural and metamorphic geologists alike.

Continental rifts as a setting for regional metamorphism

Abstract: During very high-temperature/low-pressure Hercynian metamorphism in the Pyrenees the crust began to melt at ∼12 km and stable isotopes show that it was flushed by circulating seawater to that depth. There is no evidence for crustal collision and the tectonic setting for this, and maybe all high-temperature/low-pressure metamorphism, is a zone of continental rifting.

Isotopic dating in metamorphic belts

Abstract: Isotopic dating in metamorphic rocks is concerned with the timing of processes whose operation is controlled by continually changing physical conditions. Of these, temperature has a pervasive influence and the use of physical models to elucidate the ways in which temperature is likely to vary is briefly reviewed. The central concept of closure temperature is critically discussed and quantitative estimates for biotite and hornblende are evaluated, in order to illustrate the factors which control closure temperatures and limit their application in practice. The possibility of dating crystallization using minerals which crystallize below their respective closure temperatures is examined, taking examples involving K-Ar, Rb-Sr, Sm-Nd and U-Pb analyses. The indirect dating of metamorphism via structurally dated plutons is briefly considered, and the problems of interpreting both discordant U-Pb zircon ages and Rb-Sr whole rock isochrons in metamorphosed granites are discussed. The value of near-concordant zircon ages is emphasized. The extent of isotopic equilibration during metamorphism, specifically of Sr-isotopes in metasediments, is shown not to be a simple function of metamorphic grade. The flow of fluids produced by metamorphic reactions appears to be crucial and there is a potential for dating metamorphism using whole rock samples where such flow can he demonstrated independently. The concluding discussion briefly examines existing data for the Dalradian and indicates the potential for further progress using both well-established and novel approaches.

Origin and emplacement of Mariana forearc seamounts

Abstract: Large seamounts occur on the outer half of the Mariana forearc. They represent a new class of seamounts consisting of horsts and diapirs of metamorphosed forearc material. The degree of metamorphism in this material depends on the amount of water available and the pressure-temperature regime of the forearc wedge. The major source for the water involved in the metamorphism is most likely the descending slab. Theoretical models for thermal regimes in convergent margins suggest that the lower grade metamorphic facies will be restricted to the outermost part of a forearc. Zeolite and chlorite facies rocks predominate in dredge hauls from horsts on the outer 50 km of the Mariana forearc. Thermal models indicate that higher grade greenschist facies should occur farther from the trench. Seamounts that were probably formed in response to diapiric emplacement of serpentinite predominate from 50 to 120 km from the trench. Uplift of the horsts and emplacement of the serpentinite diapirs were probably facilitated by vertical tectonic movement in response to subduction of plate seamounts and by fracturing of the Mariana forearc.

Migmatites, Melting, and Metamorphism

Abstract: This is a praiseworthy collection of papers that illustrates the great range of interests necessary to understand migmatite genesis The strength of the collection lies in its broad approach to the study of migmatites and its emphasis on the link between “igneous” and “metamorphic” processes From the point of view of an igneous petrologist it is important to have a firm understanding of the melting process before considering the tectonic significance of voluminous felsic magmas as expressions of their sources at depth (see the work W S Pitcher in this volume for reviews on the diversity of granitic rock types), magma chamber stratigraphy, or volatile evolution Many of the papers written by metamorphic petrologists in this volume give significant clues to such an understanding by providing fundamental constraints of pressure-temperature regimes, fluid evolution, elemental buffering, and thermal evolution of continental crust that is undergoing melting

Structure and deformation history of the Bitlis suture near Lake Hazar, southeastern Turkey

Abstract: Detailed mapping of the Bitlis suture zone near Lake Hazar shows that it is composed of three tectonostratigraphic units which crop out as distinct north-dipping thrust slices. From south to north, these are the Puturge Metamorphic Complex, the Maden Melange, and the Elazig Igneous Complex. The Puturge Metamorphic Complex consists of pre-Tertiary, continental-margin sediments metamorphosed to the greenschist facies during the Campanian-Maastrichtian and deformed by four generations of structures. These are, in sequence (1) isoclinal folds and a transposition foliation, (2) open folds, crenulation cleavage, and north-dipping thrust faults, (3) kink bands, and (4) small-displacement faults. The Maden Melange represents middle Eocene, back-arc-basin sediments and volcanics metamorphosed to the greenschist facies and deformed by three generations of structures: (1) a north-dipping cleavage, (2) kink bands, and (3) small-displacement faults. The Elazig Igneous Complex comprises an imbricated Maastrichtian-early Eocene island arc and young marginal-basin terrain. Thrust faults between units are north-dipping, listric, and form a thin-skinned system. These features suggest the following deformational and tectonic history. The Puturge Metamorphic Complex was generated by isoclinal folding and metamorphism of sediments composing the Arabian continental margin during Campanian-Maastrichtian ophiolite obduction to the south. The Elazig arc developed on the deformed margin because of subsequent southward subduction of oceanic lithosphere. The arc migrated to the north, opening a back-arc basin. By the early Eocene, the back-arc basin was filling with volcaniclastics of the Maden Melange, and the Elazig arc collided with a continent to the north. During middle to late Eocene, convergence caused thrust stacking of the Elazig, Maden, and Puturge Complexes; second-generation deformation in the Puturge Complex; and first-generation deformation in the Maden Melange. Continued convergence with the Arabian plate in the late Miocene caused kink banding in the Puturge and Maden Complexes. Since the late Miocene, this convergence has been accommodated by shortening and thickening along numerous internal faults.

Fluid Regime in Southern Norway: The Record of Fluid Inclusions

Abstract: Fluid inclusions have been studied in representative rocks from Southern Norway, notably in the Bamble granulites. On the basis of the earliest fluid inclusions trapped in rock-forming minerals (mainly quartz), five major types of fluid distribution have been recognized: 2-phase aqueous (H20 dominant, without solid), carbonic (mostly pure CO2, possiDle occurrence of N2 and/or CH4), mixed 1 (aqueous and carbonic inclusions in comparable amounts, but in separate cavities), mixed 2 (aqueous and carbonic fluids in the same cavity, trapped in the miscible state of the H20-C02 system), brines (H20 + solids, NaC1 dominant). Only brines show a relation between a dominant inclusion type and a given protolith; these are especially abundant in 3 well-defined environments: Al-rich metasediments (meta-pelites), skarns and acid volcanics. The distribution of other types is more related to metamorphic grade: high- density carbonic inclusions are typical for the granulite- -facies domain, early 2-phase aqueous inclusions occur almost exclusively in the north-western part of the Bamble and in the Telemark gneiss-granites, mixed (1 and 2) inclusions characterize the complicated transition zone between the amphibolite- and granulite-facies domains north of the orthopyroxene-in isograd. P-T estimates from fluid inclusions are apparently very different for Bamble (maximum C02 density during peak metamorphism) and Rogaland (maximum CO2 density after the peak of metamorphism). Most of the C02 originates from the breakdown of carbonate melts (carbonatites) emplaced as immiscible droplets in deep-seated synmetamorphic intrusives. Southern Norway is a classical example of amphibolite — granulite facies transition in a Proterozoic terrain. Although the age of metamorphism remains controversial (see various entries in this volume, notably by R.H. Verschure, D. Demaiffe and J. Michot. D. Field et al.), the pressure and temperature conditions start to be relatively well understood (Jansen et al., this volume). All recent studies emphazise the key importance of fluids, notably H2O, the fugacity of which decreases suddenly at the granulite — facies boundary. Much information has been derived from the analysis, both experimental and theoretical, of characteristic mineral assemblages; however, since the first discovery of specific, high density CO2 inclusions in many granulites (Touret, 1971), it has become evident that the direct observation of fluids trapped in rock-forming minerals (notably quartz, plagioclase, pyroxene etc.) can provide a great deal of information. CO2-rich fluids, sometimes mixed with other species (N2, CH4) have later been observed in virtually all granulites in the world (Hollister and Crawford 1981, Roedder 1984), leading to the notion of “carbonic metamorphism”, which has provided a new insight into the geology of the lower continental crust (e.g Newton et al. 1980, Newton, this volume). Various studies dealing with Southern Norway have been issued in a number of publications (see e.g. review in Roedder 1984), but the rapid development of fluid-inclusion techniques, notably the possibility of in-situ, non-destructive analysis by micro-Raman spectroscopy, necessitates a critical reevaluation of earlier data. In this paper, a review of all studies performed in Soutern Norway during the last 15 years will be attempted. It will refine earlier syntheses (notably Touret 1981, 1984) and address three fundamental aspects of fluid investigations in high-grade metamorphic rocks: i) The distribution of fluid inclusions in amphibolite- and granulite-facies rocks and the relative importance of lithological composition (protolith) and metamorphic grade. ii) The pressure and temperature estimates derived from fluid inclusions and their comparison with data from solid mineral assemblages. iii) The origin of the CO2 fluids.

Uplift patterns and orogenic evolution in the Scottish Dalradian

Abstract: Rb-Sr and K-Ar mineral ages from the Dalradian rocks of Angus and Perthshire indicate considerable diachronism of cooling, following the Caledonian regional metamorphism. The oldest mineral ages (~515 Ma) are similar to the independent estimates of peak metamorphic age, and hence indicate very rapid cooling; they occur in the highest grade samples of the Angus region and are associated with localized uplift. Regional rapid uplift events at 460–440 Ma and 410–390 Ma may also be recognized and correlated with structural and igneous activity and metamorphic age dates within the Scottish Highlands. Thermal models show the importance of localized and discontinuous uplift in determining both the timing of metamorphism and the shape of the P–T paths. They suggest that the major diachronous metamorphism (and granite production) in the Scottish Highlands is depth controlled rather than representing major lateral diachronism of thickening or heat sources. Localized uplift may exercise considerable control on mineral facies variation within orogenic belts. The local uplift in Angus introduces hot, high-pressure Tarfside Nappe rocks into higher level Tay Nappe rocks. Thus, the uniform block uplift models frequently used in the interpretation of metamorphic and structural phenomena in orogenic belts are an oversimplification of the real events.

Petrologic constraints on imbrication, metamorphism, and uplift in the SW Tauern Window, eastern Alps

Abstract: The pressure-temperature-time-deformation (PTtd) history of two tectonic units in the southwest Tauern Window (Austria/Italy) is evaluated from P-T paths of metamorphism and uplift, geochronologic data, and observations on fabric development. Petrologic data indicate burial of the Lower Schieferhulle (LSH) to depths of ≥ 35 km and the adjacent Upper Schieferhulle (USH) to only 24–28 km; the units are now separated by only 2 km of section. Metamorphism was postkinematic in the LSH and synkinematic in the USH. Correlation of P-T path reversals calculated from zoned garnets in the two units, however, indicates that juxtaposition of the LSH and USH occurred prior to metamorphism. Temperature-time and depth-time diagrams for both units show increasing cooling rates and decreasing uplift rates as the rocks approached the surface. Both the petrologic data and the depth-time profiles for the LSH and USH point to a history of differential uplift in the early stages of decompression. Textural features indicate that the LSH behaved as an essentially rigid body during uplift, whereas the USH experienced extensive ductile shearing. The PTtd data for the LSH and USH are incorporated into a model for continental collision in the Eastern Alps. Initial tectonic uplift of the LSH was accompanied by ductile thinning of the USH and overlying Austroalpine units. Final uplift and erosion affected all of the units “en masse”.

Stable isotopic evidence for large-scale seawater infiltration in a regional metamorphic terrane; the Trois Seigneurs Massif, Pyrenees, France

Abstract: Oxygen isotopic analyses of 95 metamorphic and igneous rocks and minerals from a Hercynian metamorphic sequence in the Trois Seigneurs Massif, Pyrenees, France, indicate that all lithologies at higher metamorphic grades than the “andalusite in” isograd have relatively homogeneous δ18O values. The extent of homogenization is shown by the similarity of δ18O values in metacarbonates, metapelites and granitic rocks (+11 to +13), and by the narrow range of oxygen isotopic composition shown by quartz from these lithologies. These values contrast with the δ18O values of metapelites of lower metamorphic grade (δ18O about +15). Homogenization was caused by a pervasive influx of hydrous fluid. Mass-balance calculations imply that the fluid influx was so large that its source was probably high-level groundwaters or connate formation water. Hydrogen isotopic analyses of muscovite from various lithologies are uniform and exceptionally heavy at δD=−25 to −30, suggesting a seawater origin. Many lines of petrological evidence from the area independently suggest that metamorphism and anatexis of pelitic metasediment occurred at depths of 6–12 km in the presence of this water-rich fluid, the composition of which was externally buffered. Deep penetration of surface waters in such environments has been hitherto unrecognized, and may be a key factor in promoting major anatexis of the continental crust at shallow depth. Three types of granitoid are exposed in the area. The leucogranites and the biotite granite-quartz diorite are both mainly derived from fusion of local Paleozoic pelitic metasediment, because all these rocks have similar whole-rock δ18O values (+11 to +13). The post-metamorphic biotite granodiorite has a distinctly different δ18O (+9.5 to +10.0) and was probably derived from a deeper level in the crust. Rare mafic xenoliths within the deeper parts of the biotite granite-quartz diorite also have different δ18O (+8.0 to +8.5) and possibly represent input of mantle derived magma, which may have provided a heat source for the metamorphism.

Geochemistry, geochronology and petrology of Monte Mucrone: An example of EO-alpine eclogitization of Permian granitoids in the Sesia-Lanzo Zone, Western Alps, Italy

Abstract: A Permian granitoid intrusion, exposed on Monte Mucrone, Sesia-Lanzo Zone, Italy, was metamorphosed under high-pressure conditions. To elucidate the metamorphic history of this granitoid complex, investigations have been made into the chemistry of the protolith, possible mass transfer during metamorphism, phase chemistry and implications for P-T conditions, and the age of metamorphic events. The metagranitoids contain Na-pyroxene, quartz, white mica, garnet and zoisite, and vary from rocks showing a well-preserved granitic texture to strongly deformed schists and gneisses. The rocks are of granitic to granodioritic composition and originated as an I-type granite pluton. With the exception of addition of H2O, the rock series behaved isochemicaliy throughout high-pressure metamorphism and later tectonic and retrograde transformations. P,T estimates based on Na-pyroxenes, garnet-clinopyroxene and garnet-biotite pairs give minimum pressures of ∼ 14 kbar at 600°C for the peak of the eclogite metamorphism. Age determinations by RbSr and KAr methods give 129-Ma for the rejuvenation of the total rock and 114-Ma isochrons for garnet and clinopyroxene, the first eclogite phase minerals. White-mica cooling ages scatter from 90 to 60 Ma.

Phase equilibria in partial melting of pelitic rocks

Abstract: Melting of rocks is a logical extension of normal high-grade metamorphism and there is no question that it occurs: magma exists. Many petrologists ascribe the origins of many migmatites to partial melting with segregation of solids from liquid. However, it is by no means axiomatic that all migmatites are formed essentially by this process. As we have been recently reminded by Olsen (1983), partial melting, metamorphic differentiation, injection or metasomatism are all still processes possible in the formation of migmatites, and more than one of these may be involved in the formation of a given migmatite (e.g. Johannes, 1983).

A Survey of White Mica Crystallinity and Polytypes in Pelitic Rocks of Snowdonia and Llŷn, North Wales

Abstract: Pelitic rocks in North Wales, ranging in age from late Precambrian to Silurian were sampled. XRD analysis was used to determine the mineralogy and white mica crystallinity of separated 0.43Δ°2θ; the 1Md polytype dominates < 2 µm fractions, occurring as K-and Na-micas. Stage II metapelites show variable cleavage development and crystallinities in the range 0.26-0.43Δ°2θ; 1M d and 2M 1 polytypes occur and K- and Na-micas are commonly regularly interstratified. Stage III metapelites are strongly cleaved with crystallinities < 0.26Δ°2θ; the 2M 1 polytype is dominant, occurring as K-mica and paragonite. Pelites bearing pyrophyllite, rectorite, and corrensite are found close to plug-like intrusions and were contact altered prior to regional metamorphism. 1M mica is common in deeply buried but relatively undeformed volcaniclastic rocks of the Arfon Group. Contours of equal crystallinity (isocrysts) are plotted with metabasite zones on a metamorphic map. This shows that stage I metapelites are equivalent to the subpumpellyite zone (≡?laumontite zone). Stage II metapelites are equivalent to the prehnite-pumpellyite facies and stage III metapelites to the clinozoisite and biotite zones of the greenschist facies.

Alpine metamorphism in the south‐east Tauern Window, Austria: 2. Rates of heating, cooling and uplift

Abstract: Existing geochronological data are reviewed and new Rb-Sr, K-Ar and 39Ar–40Ar ages are presented, including a suite of 33 mica ages from a 20 km north–south tunnel section. These data are discussed in relation to the thermal history from the overthrusting of the Autroalpine nappes c. 65 Myr ago to the present. The earliest phase of metamorphism, involving lawsonite crystallization, is associated with emplacement of these nappes. Subsequently, temperatures in the rocks beneath rose, at a mean rate of 3–6°C/Myr, until the climax of metamorphism. At high structural levels, published data indicate an age > 35 Myr for the metamorphic climax. In contrast, a new 39Ar–40Ar step-heating age of 23.8 ± 0.8 Myr on amphibole, from near the base of Peripheral Schieferhulle, closely approximates the age of metamorphism and provides the first clear indication that the climax of metamorphism occurred later at deeper structure levels. Following the climax, near-isothermal uplift and erosion reduced pressure to c. 1 kbar before white mica closure at 19 Myr; this implies uplift at >3 mm/yr. Along the tunnel section, white mica K-Ar ages vary systematically from 24 Myr to 16.5 Myr with position relative to a late 4 km amplitude dome whereas biotite Rb-Sr ages are uniform at 16.5 Myr across the whole profile; doming is thus dated at 16.5 Myr with transient uplift rates >5 mm/yr. At other times uplift rates were <1 mm/yr.

Alpine metamorphism in the south‐east Tauern Window, Austria: 1. P–T variations in space and time

Abstract: The Pennine rocks exposed in the south-east Tauern Window, Austria, contain mineral assemblages which crystallized in the mid-Tertiary ‘late Alpine’regional metamorphism. The pressure and temperature conditions at the thermal peak of this event have been estimated for rocks at four different structural levels using a variety of published and thermochemically derived geobarometers and geothermometers. The results are: (a) In the garnet+chlorite zone, 2–5 km structurally above the staurolite+biotite isograd: T= 490.50°C, P= 7° 1 kbar; (b) Within 0.5 km of the staurolite+biotite isograd: T= 560±300C, P=7.1 kbar; (c) In the staurolite+biotite zone, c. 2.5 km structurally below the staurolite+biotite isograd: T= 610±30°C, P=7.6±1.2 kbar; (d) In the staurolite+biotite zone, 3–4 km structurally below the staurolite+biotite isograd: T= 630±40°C, P= 6.6±1.2 kbar. The pressure estimates imply that the total thickness of overburden above the basement-cover interface in the mid-Tertiary was c. 26.4 km. This overburden can only be accounted for by the Austro-Alpine units currently exposed in the vicinity of the Tauern Window, if the Altkristallin (the ‘Middle Austro-Alpine’nappe) was itself buried beneath an ‘Upper Austro-Alpine’nappe or nappe-pile which was 7.4 km thick at that time. The occurrence of epidote + margarite + quartz pseudomorphs after lawsonite in garnet, indicates that part of the Mesozoic Pennine cover sequence in the south-east Tauern experienced blueschist-facies conditions (T<450°C, P<12 kbar) in early Alpine times. Evidence from the central Tauern is used to argue that the blueschist-facies imprint post-dated the main phase of tectonic thickening (D1A) and was thus a direct consequence of continental collision. Combined oxygen-isotope and fluid-inclusion studies on late-stage veins, thought to have been at lithostatic pressure and in thermal equilibrium with their host rocks during formation, suggest that they crystallized from aqueous fluids at 1.1±0.4 kbar and 420.20°C. Early Alpine, late Alpine and vein-formation P–T constraints have been used to construct a P–T path for the base of the Mesozoic cover sequence in the south-east Tauern Window. The prograde part of the P–T path, between early and late Alpine metamorphic imprints, is unlikely to have been a smooth curve and may well have had a low dP/dT overall; the decompression (presumably due to erosion) which occurred immediately before the thermal peak and possibly also earlier in the Tertiary, was probably partly or completely cancelled by the effects of early- to mid-Tertiary (D2A) tectonic thickening. The thermal peak of metamorphism was followed by a phase of almost isothermal decompression, which implies a period of rapid uplift in the middle Tertiary. The peak metamorphic P–T estimates are compared with the solutions of England's (1978) one-dimensional conductive thermal model of the Eastern Alps, and are shown to be consistent with the idea that the late Alpine metamorphism was caused by tectonic burial of the Pennine Zone beneath the Austro-Alpine nappes in the absence of extraneous heat sources, such as large intrusions, at depth.

40Ar/39Ar and K-Ar data bearing on the metamorphic and tectonic history of western New England

Abstract: 40Ar/39Ar ages of coexisting biotite and hornblende from Proterozoic Y gneisses of the Berkshire and Green Mountain massifs, as well as 40Ar/39Ar and K-Ar mineral and whole-rock ages from Paleozoic metamorphic rocks, suggest that the thermal peak for the dominant metamorphic recrystallization in western New England occurred 465 ± 5 m.y. ago (Taconian). Although textural data indicate a complex metamorphic-tectonic history for Paleozoic rocks, no evidence in rocks at least as high as kyanite grade dictates an Acadian age for the Barrovian metamorphism. Available 40Ar/39Ar and K-Ar data suggest that the low-grade metamorphism and cleavage formation in Taconic allochthons and the higher-grade metamorphism and emplacement of the Berkshire massif allochthon are Taconian. 40Ar/39Ar age data from a poorly defined terrane beginning near the east margin of the Green Mountain massif and extending along the eastern one-third of the Berkshire massif as far south as Otis, Massachusetts, suggest that the area has been retrograded during a metamorphism that peaked at least 376 ± 5 m.y. ago (Acadian). Available age and petrologic data from western New England indicate the presence of at least three separate metamorphic-structural domains of Taconian age: (1) a small area of relict high-pressure and low-temperature metamorphism in northern Vermont (T-1 domain), (2) a broad area in Vermont and eastern New York of normal Barrovian metamorphism from chlorite to garnet grade and characterized by a gentle metamorphic gradient (T-2 domain), and (3) a rather narrow belt of steep-gradient, Barrovian series metamorphic rocks extending from near the Cortlandt Complex northeastward through Dutchess County, New York, to the Berkshire massif in western Massachusetts (T-3 domain). Areas of maximum metamorphic intensity within the T-3 domain coincide with areas of maximum crustal thickening resulting from imbricate thrusting (Berkshire massif) or from recumbent folding (Manhattan Prong) of remobilized North American continental crust in the later stages of the Taconic orogeny.

Metamorphic P?T Paths from pelitic schists and greenstones from the south-west Tauern Window, Eastern Alps

Abstract: Petrological data from intercalated pelitic schists and greenstones are used to construct a pressure–temperature path followed by the Upper Schieferhulle (USH) series during progressive metamorphism and uplift in the south-west Tauern Window, Italy. Pseudomorphs of Al–epidote + Fe-epidote + albite + oligoclase + chlorite after lawsonite and data on amphibole crystal chemistry indicate early metamorphism in the lawsonite-albite-chlorite subfacies of the blueschist facies at P± 7–8 kbar. Geothermometry and geobarometry yield conditions of final equilibration of the matrix assemblage of 475±25°C, 5–6 kbar; calculations with plagioclase and phengite inclusions in garnet indicate early garnet growth at pressures of ∼ 7.5 kbar. Garnet zoning patterns are complex and reversals in zoning can be correlated between samples. Thermodynamic modelling of these zoning profiles implies garnet growth in response to four distinct phases of tectonic activity. Fluid inclusion data from coexisting immiscible H2O–CO2–NaCl fluids constrain the uplift path to have passed through temperatures of 380 + 30°C at 1.3 + 0.2 kbar. There is no evidence for metamorphism of USH at pressures greater than ∼ 7.5 kbar in this area of the Tauern Window. This is in contrast to pressures of ± 10 kbar recorded in the Lower Schieferhulle only 2–3 km across strike. A history of differential uplift and thinning of the intervening section during metamorphism is necessary to reconcile the P–T data obtained from these adjacent tectonic units.

The age and origin of some Norwegian eclogites: A UPb zircon and REE study

Abstract: UPb zircon data from five different eclogites and one amphibolite, all occurring as boudinaged lenses within the 1.7-Ga old Basal Gneisses of western Norway, are reported along with eleven REE patterns on eclogites, amphibolite and country rocks. Based on the size, morphology and zoning patterns of zircon as well as the degree of UPb discordance, protoliths of both basaltic or gabbroic origin can be distinguished. Whereas zircons from eclogites formed from tholeftic and talc-alkaline basaltic protoliths allow dating of the Caledonian eclogite-facies event at ∼ 400 Ma, no precise information on their primary magmatic age is possible. On the other hand, eclogites formed from gabbroic precursors allowed the dating of both the primary magmatic age at ∼ 1.5 Ga, and in one case at more than 1.76 Ga, as well as a Caledonian eclogitization. One amphibolite sample, probably of prograde metamorphic origin, yielded a Sveconorwegian primary age (∼ 950 Ma) and a Caledonian age of metamorphism. REE analyses of the dated amphibolite and eclogites along with REE data on further eclogites within the Basal Gneisses, but also within the younger 1.5-Ma old mangeritic gneisses, are not in agreement with a pre-dominantly continental origin of the respective protoliths. The latter must be postulated if a simple in situ hypothesis for the origin of eclogites is correct. Instead, the observed REE patterns, ranging from LREE-depleted via flat to LREE-enriched patterns with both unfractionated and fractionated HREE, best fit an back arc—island are setting. Major- and trace-element data as well as initial ϵNd-values are in agreement with this hypothesis. Consequently, a tectonic interdigitation of 1.5-Ga old rocks, formed at various depths in a suboceanic mantle, into the Svecofennian grey gneiss basement is suggested. Based on the UPb systematics of some of the analysed zircon fractions together with published age results from the country rocks, a Sveconorwegian tectonometamorphic collisional event is most probable. In the course of such an event not only part of the non-metamorphic precursors of the eclogites but also some ultramafic mantle material, mangerites and other 1.5-Ga old island-arc-derived rocks could have been sub- and/or ob-ducted at various P-T conditions, resulting in a tectonic interdigitation of rocks of different metamorphic and magmatic history and origin. The non-thrusted part of the island arc, however, is thought to be welded to the continental margin of the Baltic Shield. Sveconorwegian gabbroic rocks (∼950 Ma) are transformed into amphibolites in the course of the Caledonian continent-continent collision. However, with the present amount of information on these rocks, it is only possible to say that they probably did not originate in a subcontinental mantle. After closure of the Iapetus Ocean in the Caledonian period, crustal thickening occurred due to subduction of the island-arc system and parts of the attached Baltic Shield underneath the North American-Greenland plate. This led to increasingly higher pressure metamorphism in the subducted rocks. Due to the relative eastward movement of the overriding plate, lithospheric plate thrusting, starting from the lowest eclogitefacies part of the subducted island-arc system, brought eclogites, and probably also ultramafic mantle material as well as other island-arc and continental rocks, from various depths into the crustal level where we observe them today. Similarly, also eclogites derived from older protoliths (> 1.76 Ga) which were already part of the Svecofennian continental crust were involved in such a scenario.

The Evolution of the South Norwegian Proterozoic as Revealed by the Major and Mega-Tectonics of the Kongsberg and Bamble Sectors

Abstract: Detailed surveys of the Kongsberg and Bamble Sectors have shown that both underwent early, major folding, with amphibolite-granulite facies metamorphism (probably associated with C02 flushing and possible mantle degassing) during the Kongsbergian Orogeny (~1600–1500 Ma). A series of ductile shearing episodes mylonitised rocks, particularly at the junctions with the Telemark block, until folding of regional extent occurred again during the second phase of the Sveconorwegian Orogeny(~1100–1000 Ma) which reached greenschist to amphibolite facies grade. The sequence of tectonic events is punctuated by intrusive and metamorphic episodes which can be dated relatively and absolutely. The Kongsberg-Bamble belt probably formed by deposition in a marginal basin of the Svecofennian craton and the Proterozoic Supercontinent. Early shelf deposits were followed by deeper water sediments with intermediate and basic volcanics. Some overlap into the Telemark block is preserved, in places. The major and mega structures suggest a more stable, thicker Telemark block surrounded by a Kongsberg-Bamble mobile belt which has an early antiformal axis still recognisable in the oldest metasediments. The different structural styles and ductile shear directions explain the apparently anomalous NE-SW orientation of the Bamble Sector and the preservation of remnant Kongsbergian ages in certain domains. The more intensive Sveconorwegian deformation and metamorphism in the Telemark Sector is also explained.

The distinction between Caledonian burial and regional metamorphism in metapelites from North Wales: an analysis of isocryst patterns

Abstract: In Snowdonia and Llŷn three prograde stages in metapelites (Gwna Group to Wenlock) are recognized: Stage I, with white mica crystallinity values >0.43Δ°2 θ ; Stage II, 0.43–0.26Δ°2 θ ; Stage III, θ . Isocrysts at 0.03Δ°2 θ intervals are drawn separately for pelites above and below the sub-Arenig unconformity. Approximately concordant isocryst patterns characterize the unconformity in the northwestern and southeastern limbs of the Snowdonia Syncline, whereas discordant patterns are developed northwest of the Aber–Dinlle Fault and at the closure of the Ynyscynhaiarn Anticline. The contrasting patterns are the result of a pre-Arenig burial metamorphism. In post-Tremadoc pelites belts of steep metamorphic gradient coincide with belts of high strain, and a broad area of higher grade Caradoc pelites coincides with the core of the Snowdonia Syncline. The later metamorphism is related to Caledonian deformation rather than to burial. Pelite alteration adjacent to small, intermediate to acid plutons suggests emplacement before the Caledonian regional metamorphism. A close correlation exists between metabasite zones and metapelite stages.

The Scottish paratectonic Caledonides

Abstract: Synopsis The southern margin of the Laurasian continent has been destructive or strike-slip from at least 576 Ma to c. 360 Ma. There may have been a pre- or early Arenig obduction of ophiolite onto it, and this ophiolite now provides the base to the Ordovician part of the Highland Border Complex and could have been responsible for regional metamorphism in Dalradian or related rocks. At c. 480 Ma there was certainly final obduction of the Ballantrae Complex, which formed in an oceanic setting some distance south of its present location. This ophiolite was initially obducted over olistostromes and formed the basement to a later fore-arc basin, the proximal part of which is now exposed at Girvan. The Midland Valley, a Palaeozoic plutonic—volcanic arc, founded on a metamorphic basement, supplied sediments to the fore-arc—accretionary prism on its southern margin. This metamorphic basement is unlikely to have been of local Dalradian type. Growth of the paratectonic zone involved thrusting and probably strike-slip accretion of possibly 6 terranes. Ophiolites and the accretionary prism of the Southern Uplands were thrust NW onto Midland Valley basement. The whole of the paratectonic zone was initially juxtaposed against the Moine—Dalradian terrane by either strike-slip or thrust movements, but the latest interaction between the Midland Valley and Dalradian terranes involved thrusting. Future investigations into the timing and extent of thrust and strike-slip activity will greatly increase our understanding of continental growth in this region.