Sub-greenschist Facies Research Articles

HRTEM and SAED investigations of polytypism, stacking disorder, crystal growth, and vacancies in chlorites from subgreenschist facies outcrops

Selected-area electron diffraction (SAED) combined with experimental and computed high-resolution transmission electron microscopy (HRTEM) images were used to investigate polytypism, stacking disorder, mixed layering, and vacancies in chlorites from subgreenschist facies outcrops of the Taveyanne Sandstone from the Helvetic nappes, Switzerland. SAED patterns reveal increased ordering of the stacking sequences in chlorite with increasing metamorphic grade. However, semi-randomness and rotational faults occur even if the SAED photographs imply a regular ordered stacking sequence. In diagenetic (T = 210-250 °C, P = 2.1-2.2 kbar) and anchizonal-grade outcrops (T = 270-300 °C, P < 5 kbar) the polytypes Ibb and IIbb of chlorite were found, whereas in epizonal-grade samples (T = 300-360 °C, P < 5 kbar) the exclusive polytype is IIbb. Based on SAED and HRTEM images, a polytypic evaluation in one epizonal sample indicate that the monoclinic polytype IIbb-2 (55%) occurs as frequently as the triclinic polytype IIbb-4 (45%). Our samples suggest that by far the most important influence on polytypism and stacking disorder is temperature. Vacancy clusters occur in the octahedral cation positions within the talc-like layers and brucite-like sheets. The M1 and M2 positions in the talc-like layers are affected more by the cation deficiencies than the M3 and M4 positions in the brucite-like sheets. We suggest that octahedral vacancies are a substantial feature in natural chlorites in rocks of the Taveyanne Sandstone.

Sub-greenschist facies assemblages of metabasites from south-eastern Peloritani range (NE-Sicily)

Pre-Hercynian magmatic rocks are widespread in the Palaeozoic basement of the Peloritani range. The metabasites of the Mongiuffi-Melia and Gallodoro areas represent the largest and the most important lower-Ordovician magmatic products of this range. These rocks were metamorphosed during the Hercynian event and preserve relict igneous minerals and textures. A detailed study of the metamorphic assemblages has allowed the identification of two stages of metamorphic crystallisation; they are mainly distinguished by the differentXCO2 of the fluid phase. The first metamorphic event produced three calcite-free sub-greenschist facies assemblages that contain ubiquitous quartz+albite+titanite+chlorite+epidote along with pumpellyite or prehnite or actinolite. The second metamorphic episode produced a “Calcite+Chlorite” assemblage, non-diagnostic to evaluate P-T conditions of metamorphism. The first stage assemblages are only preserved in small domains of the rock. As even very low amounts of CO2 in the fluid phase drastically inhibit the formation of diagnostic sub-greenschist facies calc-silicate assemblages, it appears that a more CO2-rich fluid must have been introduced during the second event. We suggest that this introduction of more CO2-rich fluid occurred during the development of S2 crenulation cleavage.

Upper crustal fluid migration: an example from the Variscides of SW Ireland

The density and composition of single generation fluids in quartz veins from two similar host lithologies located either side of the Irish Variscan front were investigated using fluid inclusion microthermometry and crush leach analysis. The front marks the northern boundary of a distinctive cleavage developed under sub-greenschist facies conditions and coincides with the margin of a fault-controlled inverted half graben, the Late Devonian Munster Basin. Trapped fluids in Variscan veins north of this basin have medium salinities (4–14 wt% NaCl eq.) and densities generally compatible with the expected P—T conditions of deformation in the area. Fluids in veins south of the half graben margin can be grouped into those in (a) syn-compressional veins with moderate salinities (8–16 wt%) and (b) late stage veins associated with post-orogenic extension which have trapped high salinity fluids (22–27 wt%). Fluid densities for both vein types are broadly compatible with estimated P–T conditions of both trapping events. Crush-leach analyses reveal that all the fluids analysed have Br/Cl ratios close to SMOW. Variations in I/Cl ratios and cation chemistry indicate significant water/rock interaction for medium salinity fluids on either side of the front. The marked internal consistency in Br/Cl ratios from northern and southern fluids coupled with the broad ranges of cation to chloride ratios suggests that an early homogenous marine brine was subsequently modified by differing migration histories during the Late Palaeozoic tectono-sedimentary evolution of the area. It is contended that on a megascopic scale, gravity driven fluid flow controlled by megascopic fold geometries was the dominant mechanism of fluid migration associated with the buckling phase of the orogen, while expulsion of supra-hydrostatically pressurised fluids is associated with late stage orogenic extension.

Low-pressure metamorphism of the mafic volcanic rocks of the Rossland Group, southeastern British Columbia

Mafic volcanic rocks of the Rossland Group have been metamorphosed in the subgreenschist to lower amphibolite faciès. Subgreenschist-facies regional metamorphic rocks are subdivided into prehnite–pumpeilyite zone and prehnite–epidote zone. Fluid inclusions in two subgreenschist-facies veins yielded mean homogenization temperatures of 139 and 151 °C. Assuming a reasonable maximum temperature limit of 275 °C for the subgreenschist fades, the fluid-inclusion isochores indicate a pressure <250 MPa for regional metamorphism in the subgreenschist facies. This is consistent with the widespread occurrence of prehnite–chlorite-bearing assemblages. Metamorphic grade increases sharply northward approaching the large plutons of the Nelson suite. The contact aureoles of the Nelson batholith and the related Bonnington pluton encompass most of the region, producing an extensive region underlain by rocks within the hornblende–oligoclasc zone. Intrusion of the Nelson plutonic suite overlapped with the development of the Hall Creek syncline and Silver King shear zone. The pattern of isograds across the Rossland Group indicates superimposed contact and regional metamorphism rather than progressively deeper structural levels northward.

Sub-greenschist facies metamorphism from the Variscides of SW Ireland an early syn-extensional peak thermal event

Illite crystallinity studies on metaclastics from the Variscides of SW Ireland range in value from 0.18 to 0.25°Δ2 θ indicating a metamorphic grade in the upper anchizone-lower epizone ( c. 275–325°C). Chlorite geothermometry yields a similar metamorphic temperature range (280–315°C). Combining these data with known overburden estimates for the area ( c. 5km) implies late Palaeozoic geothermal gradients in excess of 60°Ckm −1 . Fluid inclusion studies on quartz veins in these metaclastics reveal two vein types characterized by fluid densities of 0.7–0.93 g cm −3 (Group 1) and 0.9–1.0gcm −3 (Group 2) respectively. This variation in fluid density is thought to be dominantly controlled by temperature, with Group 1 veins linked to late Palaeozoic extension and high geothermal gradients and Group 2 veins associated with subsequent Variscan deformation and lower temperature conditions.

Late Jurassic‐Early Cretaceous metamorphic age of Fraser Complex migmatite, Westland, New Zealand

Abstract Discordant U‐Pb zircon data from a migmatitic leucosome in amphibolite facies gneiss of the Fraser Complex define a 157 ± 21 Ma concordia lower intercept date interpreted as the age of peak metamorphism. An upper intercept date of 735 ± 100 Ma indicates inheritance of Proterozoic zircon. The mid‐Mesozoic age for high‐grade metamorphism does not support the existence of a regional Precambrian gneiss basement, incorporating the Fraser Complex and other gneissic rocks in Westland, as suggested by earlier workers. Early Paleozoic Greenland Group, of subgreenschist facies regional metamorphic grade, is separated from Fraser Complex by the Fraser Fault. This structural relationship is similar to that described from Cretaceous metamorphic core complexes in the New Zealand Western Province where lower plate amphibolite facies metamorphic rocks are separated from upper plate Greenland Group strata by detachment faults. Although the mid‐Mesozoic age of metamorphism reported here suggests an affinity with t...

Regional retrograde alteration of sub-greenschist facies chlorite to smectite

Dioctahedral smectite is present as a retrograde alteration product of chlorite in Permian-Triassic red slates of the Malaguide Complex in Sierra de Espuna (Betic Cordillera). Mineral assemblages and textures, illite “crystallinity” indices, and fluid inclusion data indicate sub-greenschist facies conditions that reached at least 180°C in the higher-grade tectonic unit of the Malaguide Complex, preceding formation of smectite. Smectite, having K as the dominant interlayer cation, occurs ubiquitously intercalated with trioctahedral chlorite as thin packets of layers and as individual layers that commonly change to chlorite along layers. Although some chlorite is typically homogeneous and trioctahedral, much chlorite shows signs of alteration and has compositions corresponding to different degrees of smectite contaimination. The incompatibility of metamorphic grade with the occurrence of smectite, the general association of chlorite and smectite, and the textural relations collectively show that dioctahedral smectite is derived through replacement of trioctahedral chlorite. Such replacement occurs on a regional basis and demonstrates that caution must be used in interpreting the occurrence of smectite in pelites as being due to prograde processes. Alteration of trioctahedral chlorite under oxidizing conditions due to introduction of phreatic water after uplift of the Betic Cordillera is proposed as the cause of formation of smectite.

Paleontology of proterozoic shales and mudstones: examples from the Belt supergroup, Chuar group and Pahrump group, western USA

Proterozoic black shales and other fine-grained siliciclastic rocks contain a variety of fossils. These fossils are illustrated by examples from three Proterozoic units in the western USA—the Belt Supergroup (Middle Proterozoic), the Chuar Group (Late Proterozoic), and the upper Pahrump Group (Late Proterozoic). A variety of microfossils and megafossils occur in the Belt Supergroup, western Montana. Acritarchs and filamentous microfossils are moderately well preserved in sub-greenschist facies black shale in the Little Belt Mountains. The biologic affinity of the acritarchs is uncertain; they could represent the vegetative or encystment stage of eukaryotic algae or the outer envelope of colonial coccoid cyanobacteria. Most of the filaments in these shales are of a size and morphology that is consistent with prokaryotic affinities; however, rare 20- and 40-μm-wide filaments could be either prokaryotic or eukaryotic. Higher-grade sub-greenschist facies dark-gray to black mudstones in Glacier National Park contain poorly preserved acritarchs and spheroidal and filamentous pseudomicrofossils. They are of interest because they provide information useful for distinguishing authentic microfossils from pseudofossils in mudstones and shales that have undergone substantial burial metamorphism. Several authentic megafossils occur in lower Belt shales in the Little Belt and Big Belt Mountains, Montana. One of these is the carbonaceous compression Beltina danai , which could represent either fragments of a microbial mat or fragments of a sheet-like megascopic alga. The megafossil Grypania spiralis has the form of a coiled ribbon and tentatively is interpreted as a megascopic alga. Green to purple mudstones and muddy sandstones in the lower Belt Supergroup in Glacier National Park contain markings that resemble a string of beads; they are interpreted as megafossils that may have eukaryotic algal affinities. Black shales in the Late Proterozoic Kwagunt Formation, Chuar Group, Grand Canyon, Arizona, contain a variety of acritarchs and filamentous microfossils. Of special interest is the vase-shaped microfossil Melanocyrillium , which is interpreted as the sporangium of a eukaryotic and probably megascopic alga. Melanocyrillium also occurs in black mudstone in the Late Proterozoic Pahrump Group, southeastern California.

The Ny Friesland Orogen, Spitsbergen

AbstractThe Caledonides of Ny Friesland comprise the type Hecla Hoek sequence of Svalbard, a succession of late Proterozoic to Ordovician strata greater than 18 km thick. Three supergroups constitute the sequence: the Stubendorffbreen Supergroup (Riphean), the Lomfjorden Supergroup (late Riphean-Sturtian) and the Hinlopenstretet Supergroup (Varanger-mid-Ordovician). Basement elements have recently been identified within the Stubendorffbreen Supergroup, but their extent and significance is yet to be established. The Stubendorffbreen Supergroup records the deposition of sediments and volcanics (both acid and basic) in an unstable marine environment. In contrast, the Lomfjorden and Hinlopenstretet supergroups record sedimentation in a shallow-marine, periodically emergent, stable environment without volcanism. The Ny Friesland Orogen is divided into two subterranes by the Veteranen Line, a zone of attenuation along which sinistral strike-slip displacement has occurred. This line separates the strongly deformed Stubendorffbreen Supergroup rocks in the west from the less-intensely deformed Lomfjorden and Hinlopenstretet supergroup rocks in the east. Despite these contrasts and the obvious displacement, there is no evidence that a significant stratigraphie break occurs across it.All the supergroups were deformed and metamorphosed during the late Ordovician-Silurian Ny Friesland Orogeny. Early compressional deformation produced isoclinal folding and nappes in the Stubendorffbreen Supergroup rocks, accompanied by amphibolite faciès metamorphism; deformation in the Lomfjorden and Hinlopenstretet supergroups was less intense with open, upright folds and greenschist or subgreenschist facies metamorphism. Early compression was followed by a Silurian transpressive deformation that generated a pervasive lineation in the Stubendorffbreen Supergroup rocks. Transpressive deformation and the associated sinistral strike-slip was focused where strata were in a near-vertical attitude conducive to displacement. At a late stage in the orogeny, and probably still under a strike-slip regime, batholiths were emplaced into rocks east of the Veteranen Line.As a result of continued sinistral displacement (transpression, transcurrence and transtension) along the Billefjorden Fault Zone, Ny Friesland (part of the Eastern Province of Svalbard) finally docked against the Central Province during the late Devonian Svalbardian movements. At the same time, the Central Province docked against the Western Province. In total, hundreds of kilometres of Caledonian displacement along the Billefjorden Fault Zone brought the Eastern and Central provinces into their present positions. Pre-Carboniferous Svalbard is thus a composite terrane of at least three provinces, each comprising more than one minor terrane.

Tertiary volcanic rocks from the Halmahera arc, Eastern Indonesia

Halmahera is a K-shaped volcanic island arc situated near the junction of the Australian, Eurasian and Philippine Sea Plates. Recent work on Halmahera has identified two important pre-Quaternary intervals of volcanism in western Halmahera. Neogene andesites were produced in the Halmahera Arc during subduction of the Molucca Sea Plate at the western boundary of the Philippine Sea Plate. Pre-Neogene basalts of the Oha Formation are probably the equivalent of volcanic basement rocks found elsewhere in the Philippine Sea region and are interpreted to represent the products of Late Mesozoic or Early Tertiary subduction within the Pacific. Neogene andesites and subordinate basalts contain abundant phenocrysts; plaioclase feldspars, orthopyroxene, clinopyroxene, hornblende and titanomagnetic are common. Like the products of Quaternary volcanism andesitic bulk rock compositions reflect high proportions of acid glass. The Neogene volcanic rocks have evolved by plagioclase, pyroxene, hornblende and magnetite fractionation. They are medium-K to high-K rocks of the calcalkaline series, REE patterns are sloping, typical of arc volcanic rocks, and MORB-normalized element plots show strong depletion of Nb, similar to other West Pacific arc volcanic rocks. Most samples are very fresh. A single zeolite (mordenite) is rarely present and chlorites, smectites and chalcedony occur in a few samples. The local, very low-grade, alteration is typical of geothermal environments. Volcanic rocks of the Oha Formation, which forms the basement of the western arms, are aphyric and phyric basalts, typically with textures which reflect rapid cooling. Plagioclase feldspar, olivine and clinopyroxene phenocrysts are common, orthopyroxene is rare and phenocrysts of hornblende and magnetite are absent. The Oha Formation basalts evolved by olivine, plagioclase and clinopyroxene fractionation. They are depleted in HFS elements, and enrichment in LIL elements is partly due to extensive sub-greenschist facies alteration reflecting deep burial and/or high heat flows. This alteration produced zeolites, chlorites, smectites, and locally, pumpellyite and sphene. Dating of overlying sedimentary rocks shows that the Oha Formation volcanic rocks are older than Late Miocene; a Late Cretaceous-Eocene age is suspected since petrographically and chemically similar arc volcanic rocks of this age are present in the NE and SE arms.

Melange- and sediment-hosted gold-bearing quartz veins, Hodgkinson gold field, Queensland, Australia

Gold-bearing quartz veins in the Hodgkinson gold field are hosted in subgreenschist facies Siluro-Devonian sandstone, conglomerate, carbonaceous shale, clast-in-matrix rock, chert, and spilite. Mineralization occurs in brittle to brittle-ductile shear zones which crosscut two previous deformations and melange. Vein systems are isolated in distinct structural and lithologic domains by postmineralization faulting. Scheelite-quartz veins may locally overprint the gold quartz mineralization, but stibnite-gold-quartz mineralization is restricted to separate domains. The strongest mineralized areas define elongate, elliptical ore shoots which plunge steeply down or across the dip of the host shear zones. Melange-hosted lodes are parallel in strike but have opposite dips to the melange cleavage. Sandstone-hosted shear zones lie within reactivated F 2 axial planes or are refracted through bedding planes. Splaying, fissure intersections, changes in attitude, and lithologic contacts are the main ore shoot controls.The gold quartz vein textural types are ribbon, buck, breccia, assimilation textured, and comb quartz. Multiple injection of quartz and multiple shear zone movement have produced complex zones of quartz separated by seams of gouge. Arsenopyrite, galena, sphalerite, pyrite, pyrrhotite, and gold (600-840 fine) constitute 5 percent of the ore shoots and occur as millimeter-scale clusters in quartz. Fluid inclusions in quartz are liquid and liquid-vapor types and give salinity ranges of 3 to 11 equiv wt percent NaCl, with a slight salinity decrease through time. CO 2 and other gases are not abundant. Homogenization temperatures of 170 degrees to 305 degrees C have been pressure corrected to 270 degrees to 355 degrees C, based on estimated formation pressure of about 1 kbar.Measured and calculated stable isotope compositions of the fluid depositing gold quartz are delta 18 O = 10 + or - 2 per mil, delta D = -100 + or - 20 per mil. Although meteoric water contributions cannot be discounted, geologic characteristics, isotopic data, quartz types, sulfide mineralogy, and ore controls favor gold deposition from an upwardly migrating, homogeneous, metamorphic, or distal magmatic fluid. Depletion of D is attributed to exchange reactions involving organic matter in the metasediments and reduced gas species rather than introduction of meteoric water. Gold mineralization accompanied brittle tectonism which was contemporaneous with regional plutonism and took place late in the evolution of the melange.

Blueschists in major suture zones of China

Blueschist facies rocks and C‐type eclogites occur as allochthonous terranes or as small tectonic blocks in almost all the accretionary fold belts and major suture zones within the interior of China and along the Circum‐Pacific and Tethys‐Himalayan belts. At least forty blueschist localities have been described. Blueschists around the Pacific margins and the Himalaya‐Alpine belts are mostly Mesozoic in age; only those in Taiwan and New Caledonia are Cenozoic. Intracratonal blueschists of China are older than Late Permian, when the collision‐amalgamation presumably occurred. Several alleged Precambrian blueschist localities [Ye, 1987] are shown in the recently published Metamorphic Map of China. A Precambrian(?) blueschist belt in Anhui‐Hubei (central China) extends for about 2300 km. However, except for the pre‐Sinian blueschist terrane in Aksu, the age determination for Precambrian blueschist metamorphism elsewhere has not been confirmed. Characteristic features of Chinese blueschists include the following: (1) blueschist facies metamorphism predates the continental collision‐amalgamation, (2) most blueschists have undergone multistage metamorphism, commonly with increasing temperature and/or decreasing pressure for later events, (3) assemblages include sodic amphibole + epidote + albite + quartz + phengite + chlorite + sphene; lawsonite, jadeitic pyroxene, and aragonite are not common, and (4) protoliths are mainly mafic, pelagic, and clastic rocks. Blueschist facies metamorphism in China may be divided into two types on the basis of the imposed geothermal gradient, hence the progressive change in mineral assemblage. The rare lawsonite‐bearing blueschists produced at lower temperatures and higher pressures are confined to the Inner Mongolia and Yarlung Zangbo suture zones where aragonite and possibly jadeitic pyroxene + quartz occur, and the blueschists are associated with subgreenschist facies rocks. Most other blueschists of China formed at higher temperatures along the transitional blueschist‐greenschist facies boundary where epidote, sodic, and calcic amphiboles are ubiquitous, and the blueschists are interlayered with greenschist facies rocks. The common occurrence of intracratonal blueschists in major suture zones of China and elsewhere in Eurasia indicates pre‐Mesozoic subduction‐accretion of oceanic crust and flysch sediments. These zones of accretion represent growth around the periphery of major Precambrian cratons. This fact, together with the distribution of ophiolite belts and available paleomagnetic data, provides evidence for an extremely mobile history of plate movement in Eurasia. However, the Paleozoic intracontinental blueschists and ophiolites of China have formed before, rather than during, final closure between cratons. They do not usually mark the location of terminal sutures but are the result of earlier accretion and continental growth by subduction, underplating, and imbrication of oceanic materials similar to the accretion history of westernmost North America.

Interaction between Strike-Slip and Thrust-Shear: Deformation of the Bullbreen Group, Central-Western Spitsbergen

Field mapping and strain measurement data are used to infer a deformation history for synorogenic, subgreenschist facies sedimentary rocks of the Ordovician Bullbreen Group in central-western Spitsbergen. Evidence for true extension parallel to fold axes, and comparison of incremental strains preserved by fiber-filled extension veins with numerical deformation models, suggest that strike-slip shear was a significant component in what was generally a thrust-shear deformation regime. Initial dextral strike-slip shear parallel to the fault-controlled Bullbreen sedimentary basin subsequently decreased in importance relative to thrusting perpendicular to the basin margin. Although the presumed Caledonian-aged dextral strike-slip motion described here may have been a local phenomenon, it is difficult to reconcile with existing models of N-S orientated, large-scale, sinistral strike-slip in western Spitsbergen.

Paleomagnetism and rock magnetism of Early Silurian Dunn Point volcanics, Avalon Zone, Nova Scotia

Paleomagnetic results have been obtained from the subaerial volcanics (andesites, rhyolites, ignimbrites) from the Dunn Point Formation exposed in the Antigonish Highlands of the Avalon Zone, Nova Scotia. An Early Silurian age of 430 ± 15 Ma is assigned to these volcanic rocks. The age is in agreement with the fossil evidence from conformably overlying sedimentary rocks. The volcanic units were tightly folded and metamorphosed to sub-greenschist facies during one major and one secondary phase of deformation probably associated with the Mid-Late Devonian Acadian orogeny. This paleomagnetic study is based on 45 samples collected from nine sites. The site-mean directions of the natural remanent magnetization (NRM) make a large deviation from the present Earth's field with NRM intensities ranging from 10 −2 to 10 Am −1. The most commonly observed remanence direction, A( D=153°, I=−28°), obtained from both alternating fields (30–75 mT) and thermal (500–560°C) treatments yields a corresponding direction A′( D=0°, I=−63°) after correction for geological tilt. A large scatter in direction (up to = 60°) is observed in A′ and the fold test is not significantly positive. A second and less common remanence component, B ( D=146°, I=+54°), which presumably postdates folding, is also observed; this component is mainly isolated by thermal treatment in the 540–630°C range. Room-temperature isothermal experiments and optical examination of polished sections indicate the presence of magnetite and hematite as the carriers of both A and B directions. The corresponding reversed paleomagnetic poles fall at A (163°E, 52°N) and B (146°E, 4°N) in close agreement with cratonic North American Upper Jurassic and Late Devonian poles, respectively. In the Mid-Late Devonian time interval, the Avalon Zone was situated slightly to the south but close to the North American eastern margin.

Tectono-magmatic evolution of the 1.9-Ga great bear magmatic zone, Wopmay orogen, northwestern Canada

The 1875-1840-Ma Great Bear magmatic zone is a 100-km wide by at least 900-km-long belt of predominantly subgreenschist facies volcanic and plutonic rocks that unconformably overlie and intrude an older sialic basement complex. The basement complex comprises older arc and back-arc rocks metamorphosed and deformed during the Calderian orogeny, 5–15 Ma before the onset of Great Bear magmatism. The Great Bear magmatic zone contains the products of two magmatic episodes, separated temporally by an oblique folding event caused by dextral transpression of the zone: (1) a 1875-1860-Ma pre-folding suite of mainly calc-alkaline rocks ranging continuously in composition from basalt to rhyolite, cut by allied biotite-hornblende-bearing epizonal plutons; and (2) a 1.85-1.84-Ga post-folding suite of discordant, epizonal, biotite syenogranitic plutons, associated dikes, and hornblende-diorites, quartz diorites, and monzodiorites. The pre-folding suite of volcanic and plutonic rocks is interpreted as a continental magmatic arc generated by eastward subduction of oceanic lithosphere. Cessation of arc magmatism and subsequent dextral transpression may have resulted from ridge subduction and resultant change in relative plate motion. Increased heat flux due to ridge subduction coupled with crustal thickening during transpression may have caused crustal melting as evidenced by the late syenogranite suite. Final closure of the western ocean by collision with a substantial continental fragment, now forming the neoautochthonous basement of the northern Canadian Cordillera, is manifested by a major swarm of transcurrent faults found throughout the Great Bear zone and the Wopmay orogen. Although there is probably no single evolutionary template for magmatism at convergent plate margins, the main Andean phase of magmatism, exemplified by the pre-folding Great Bear magmatic suite, evolves as larger quantities of subduction-related mafic magma rise into and heat the crust. This results in magmas that are more homogeneous, siliceous, and explosive with time, ultimately leading to overturn and fractionation of the continental crust.

Mineralized fracture systems of the Skaergaard intrusion, East Greenland

Extensive fracture systems in the Skaergaard intrusion, including veins, sills and dikes, constitute channels for the flow of magmas and hydrothermal solutions during the subsolidus cooling and deformation of the intrusion. The abundance, orientation, transgressive relations and mineralogy of these fracture systems provides a record of fluid transport and chemical reactions during the Skaergaard's subsolidus cooling history.
 The earliest mineralized fractures in the Layered Series are gabbroic pegmatites that occur as veins, sills and irregular patches. These structures are crosscut by leucocratic granophyres. Veins filled with hydrothermal minerals occur throughout the Skaergaard intrusion and its host rocks. Hydrothermal minerals in veins are similar to mineral assemblages found in metabasalts ranging in grade from upper amphibolite to zeolite facies. The earliest hydrothermal veins contain upper amphibolite facies assemblages. They are <1-2 mm wide, continuous for tens of metres and occur with frequencies of 0.2 to >1.5 and 1.4 to >20 fractures/metre in the Layered Series, and Marginal Border Group and contact metamorphosed basalts, respectively. These early veins are inferred to have formed at temperatures of >500° to 750°C and <925°to 960°C. Later vein types contain mineral assemblages characteristic of lower amphibolite, greenschist and sub-greenschist facies and occur as more localized fracture systems than the early, high temperature vein types. Compositions of secondary minerals from all of these hydrothermal vein types show systematic changes relative to both the overall fractionation trend and local modal variations in the igneous mineralogy of the Skaergaard intrusion.
 ­­Observed phase relations and fracture abundances are, in general, consistent with isotopic and transport characteristics of the Skaergaard magma-hydrothermal system reported by Taylor & Forester (1979) and Norton & Taylor (1979).

Ammonium micas in metamorphic rocks as exemplified by Dome de l'Agout (France)

Sub-greenschist facies metamorphism from the Variscides of SW Ireland an early syn-extensional peak thermal event

The Gaissa Nappe, Finnmark, North Norway: an example of a deeply eroded external imbricate zone within the Scandinavian Caledonides

The Lower Allochthon of the Caledonides of Finnmark, northern Norway, is represented solely by the Gaissa Nappe, which is composed of sub-greenschist facies sedimentary rocks of late Riphean to Tremadoc age. The lithostratigraphic sequence has been shortened by thrusting and folding in an ESE direction. Based on mapping and structural profiling east of Porsangerfjord, the Gaissa Nappe can be divided into four structural segments: the Børsely duplex, developed beneath the Kalak Nappe of the Middle Allochthon, is oblique to an imbricate fan, the Munkavarri imbricate zone, east of which is the Guiverassa duplex zone that is partly covered by the Vuonjalrassa thrust sheet. The sole thrust to the Gaissa Nappe is a flat planar surface which truncates the common N-S folds and associated cleavage in the rocks of the Gaissa Nappe. The Vuonjalrassa-Gaissa thrust cuts down section in the transport direction, possibly as a result of early tectonic downwarping. A balanced cross-section and a hanging-wall diagram have been partially restored, indicating that the metasediments of the trailing edge of the Munkavarri imbricate zone have been displaced by 104 km in their ESE translation direction. Taking the sequence west of Porsangerfjord into consideration, an overall contraction of more than 150 km is possible. In the east, it is argued that the basal Gaissa décollement, formerly thought to die out and pass laterally into an unconformity, extends to the northeast beyond the head of Tanafjord. Folds that occur in front of the sole thrust on the Varanger Peninsula imply the presence of a blind thrust. In an orogenic context, the Gaissa Nappe forms a series of imbricated thrust sheets in the external part to the collision belt, produced during the Finnmarkian orogenic event in late Cambrian to early Ordovician time.

Geological setting and age of the Flemish Cap granodiorite, east of the Grand Banks of Newfoundland

Flemish Cap, a large isolated submarine knoll 600 km east of Newfoundland, consists of a central core of Hadrynian rocks and an onlapping sequence of undisturbed to disturbed Mesozoic–Cenozoic sediments. The central core of the cap, sampled with an electric rock-core drill, comprises pink, fine- to medium-grained granodiorite, dacite, and volcanic siltstone. The granodiorite samples, collected in six out of eight cores, are remarkably similar lithologically, despite a separation of up to 65 km, and probably represent a single pluton. The aphanitic dacite and laminated cherty volcanic siltstone have been metamorphosed to subgreenschist (prehnite) facies but show no evidence of contact metamorphism.U–Pb analyses of coarse and fine zircon fractions from a granodiorite core yielded upper intercepts of 751 and 833 Ma, respectively. Although not precise, these ages probably represent the age of intrusion sometime in the 750–830 Ma range and are older than those reported for similar Hadrynian granitic rocks of eastern Newfoundland. AK–Ar age of 657 ± 29 Ma on hornblende from the same core and a K–Ar age of 615 ± 20 Ma on biotite from a granodiorite core elsewhere in the body probably represent incomplete degassing during the superimposed subgreenschist metamorphism of uncertain age.Thus the Flemish Cap granodiorite and associated rocks are part of the Avalon Zone and may represent a much older part of the Avalon Terrane than parts toward the west on Avalon Peninsula and Burin Peninsula.

Phase equilibria and mixed parageneses of metabasites in lowgrade metamorphism

AbstractThe system Na2O-CaO-MgO Al2O3-SiO2-H2O 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 Fe2O3 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 quantitative effects on P-T displacement and phase compositions are discussed. The XFe3+ isopleths for epidote were plotted to exemplify the transition from the zeolite through prehnite-pumpellyite to prehnite-actinolite facies. T-XFe3+ relations were established for continuous and discontinuous reactions relating such facies transitions. Because of the common occurrence of two or three Ca-Al hydrosilicates in low-grade metabasites, an isobaric Al-Ca-Fe3+ projection from chlorite may be used to illustrate mineral assemblages and compositions of the coexisting Ca-Al silicates in the presence of quartz, albite, 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 pumpellyite and epidote with increasing metamorphic grade are oversimplified.