Peak Of Regional Metamorphism Research Articles

Fluid Flow During Contact Metamorphism at Mary Kathleen, Queensland, Australia

Corella marbles in the Mary Kathleen Fold Belt were infiltrated by fluids during low-pressure (∼200-MPa) contact metamorphism associated with the intrusion of the Burstall granite at 1730–1740 Ma. Fluids emanating from the granite [whole-rock (WR) δ18O=8.1–8.6%] produced Fe-rich massive and banded garnet—clinopyroxene skarns [δ18O(WR)=9.1–11.9%]. Outside the skarn zones, marble mineralogies define an increase in temperature (500 to > 575°C) and XCO2 (∼0.05 to >0.12) towards the granite, and most marbles contain isobarically univariant or invariant assemblages in the end-member CaO−MgO−Al2O3−SiO2−H2O−CO2 system. Marbles have calcite (Cc) δ18O and δ13C values of 12.3–24.6% and −1.0 to −3.9%, respectively. A lack of down-temperature mineral reactions in the marbles suggests that pervasive fluid infiltration did not continue after the thermal peak of contact metamorphism. The timing of fluid flow probably corresponds to a period of high fluid production and high intrinsic permeabilities during prograde contact metamorphism. The petrology and stable isotope geochemistry of the marbles suggest that these rocks were infiltrated by water-rich fluids. If fluid flow occurred up to the peak of contact metamorphism, the mineralogical and isotopic resetting is best explained by fluids flowing up-temperature toward the Burstall granite. However, if fluid flow ceased befor the peak of regional metamorphism, the fluid flow direction cannot be unambiguously determined. At individual outcrops, marble δ18O(Cc) values vary by several permil over a few square metres, suggesting that fluid fluxes varied by at least an order of magnitude on the metre to tens-of-metre scale. Fluids were focused across lithological layering; however, mesoscopic fractures are not recognized. The focusing of fluids was possibly via microfractures, and the variation in the degree of resetting may reflect variations in microcrack density and fracture permeability. The marble—skarn contacts represent a sharp discontinuity in both major element geochemistry and δ18O values, suggesting that, at least locally, little fluid flow occurred across these contacts.

Rare metal pegmatites of the Cape Cross-Uis pegmatite belt, Namibia: geology, mineralisation, rubidium-strontium characteristics and petrogenesis

Rare metal pegmatites of the Cape Cross-Uis pegmatite belt (Namibia) containing sub-economic to economic concentrations of Sn, Nb, Ta, Li, occur in three main pegmatite swarms. Unzoned rare metal pegmatites are characterised by disseminated cassiterite mineralisation and varying Li, Ta and Nb concentrations, whereas zoned and poorly zoned pegmatites of the LI-rich rare metal type, beside low grade cassiterite mineralisation, contain higher Nb, Ta and Li concentrations (500 ppm Nb; 800 ppm Ta average). Occurrences of both types are confined to areas of extensive shearing in Pan-African greenschist facies metasediments within the tensional environment of a half-graben structure. Rb/Sr isotope studies of various mineral phases in zoned and unzoned pegmatites in the study area indicate a late Pan-African emplacement age of 490 Ma for the pegmatitic melts. Petrogenetically, high 87Sr/ 86Sr initial ratios reflect partial melting of crustal material (basement rocks). Rare metal pegmatites in the study area are believed to have originated at the peak of regional metamorphism. This is indicated by higher metamorphic grades towards the Atlantic Ocean, which is combined with an increase in Li, Nb and Ta concentrations.

Emplacement and thermal metamorphism associated with the post-orogenic Strath Ossian Pluton, Grampian Highlands, Scotland

AbstractThe multiphase Strath Ossian Pluton was intruded into metasedimentary rocks of the Neoproterozoic Grampian and Appin groups (Grampian Highlands, Scotland) during Silurian or early Devonian times. Emplacementfollowed the main ductile tectono-thermal history of the area and took place during post-orogenic regional uplift and cooling. Early emplacement of dioritic magma in the northern part of the pluton resulted in migmatization of its immediate country rocks with the generation of new ductile structures. The main granodiorite was then emplaced with magma migrating towards the southeast where wall-rock stoping took place. Elsewhere the pluton created its own space with little stoping or veining. Thermal metamorphism caused by granodiorite emplacement resulted in the progressive development of the assemblage quartz+ plagioclase + biotite+ cordierite +andalusite ± K-feldspar in the metapelitic country rocks. Six prograde mineral assemblage zones are identified in the aureole. Final emplacement of a marginal porphyritic microgranite was accompanied by the release of alkaline fluids into the thermal aureole. This produced sillimanite (fibrolite) in association with hydrous phases such as chlorite and white mica. The development of andalusite and cordierite-bearing assemblages is estimated to have occurred at temperatures of 650±50 °C at an estimated pressure of 3.2±0.5 kbars. An approximately isobaric temperature change of 300±50 °C across the width of the main aureole is deduced. The migmatization close to the plutons margins took place at temperatures of about 700 °C. An estimated depth of emplacement of about 11 km is obtained for the Strath Ossian Pluton. This implies considerable regional uplift both prior to, andimmediately after its emplacement. Thus it has been estimated that at the peak of regional metamorphism, probably during the Ordovician Period, the country rocks were at a depth of 15 to 18.5 km, whereas the early Devonian dykes of the Etive dyke swarm, which cut the Strath Ossian Pluton, were emplaced at, or near surface.

Cryptic Grain-Scale Heterogeneity of Oxygen Isotope Ratios in Metamorphic Magnetite

Oxygen isotope ratios measured by ion microprobe in magnetite from granulite-facies marble of the Adirondack Mountains, New York, range from +2 to +11 per mil (standard mean ocean water) across a single grain that measures 3 millimeters by 5 millimeters. Low values are concentrated in irregular domains near the grain boundary but also occur in the grain's interior. In contrast, grains 1 millimeter in diameter that are from a second nearby sample show no significant heterogeneity, except within 10 micrometers of the grain boundary. These data, including large gradients of up to 9 per mil per 10 micrometers, provide important new constraints on the nature and origins of intragrain isotopic heterogeneity and on oxygen isotope thermometry. The differences between these magnetite grains result from contrasting mechanisms of isotope exchange with fluids after the peak of regional metamorphism. Volume diffusion of oxygen through the crystal structure of magnetite contributed to isotope exchange in the rims of small grains, but larger grains are crosscut by healed cracks that are not readily detected and that short-circuited diffusion.

Eocene tectonics in the Omineca Belt, northern British Columbia, Canada: Field, 40Ar-39Ar, and fission track data from the Horseranch Range

A field and geochronometric study of the Horseranch Range, north-central British Columbia, was carried out to determine the cooling and uplift history of the area and to assess the possibility that the range may be a metamorphic core complex. 40 Ar- 39 Ar cooling ages from hornblende, muscovite, and biotite in a variety of rock types and structural positions, and apatite fission track data indicate rapid cooling from about 525 to 50 °C or less in the middle to late Eocene epoch. Uplift and cooling took place by oblique-normal dextral strike-slip along a northwest-striking, moderately to steeply southwest-dipping mylonite zone associated with regional mid-Cretaceous to Oligocene, northwest-striking, dextral strike-slip faulting. Uplift by oblique-normal slip differs from dip-slip displacement reported for metamorphic core complexes in the southern Omineca Belt. The peak of regional metamorphism at 650 °C and 5 to 7 kb was late Early Cretaceous or younger in age and predated mylonitization. There is no evidence of resetting of older minerals. This is in contrast to the nearby Swannell Ranges, which implies to us that differential uplift took place in the northern Omineca Belt, probably due to control of bounding faults.

Metamorphism of magnesium-altered felsic volcanic rocks from Bergslagen, central Sweden. A transition from Mg-chlorite- to cordierite-rich rocks

The ca. 1.9 Ga old felsic volcanic rocks of the Bergslagen province have been affected by synvolcanic hydrothermal Mg-metasomatism. A spatial relationship exists between Mg-alteration zones and calc-silicate/carbonate banded-iron mineralizations and a genetic link is suggested. This is supported by the occurrence of the Mg-rich tourmaline dravite, which exists in the Mg-alteration zones as well as in the wall-rocks to iron-oxide mineralizations. Following later regional metamorphism and deformation, these rocks occur as large zones of Mg-rich schists parallel to sub-parallel with the regional foliation (S1). The dravite crystals have been affected by this regional deformation. A second deformational episode folded S1 along steeply dipping fold axes. Locally, this resulted in a crenulation cleavage and the development of a new generation of micas (S2) crosscutting S1. In the westernmost areas the Mg-schists contain a fine-grained network of Mg-chlorite and muscovite with little or no preferred orientation, indicating low pressures and low-grade metamorphic conditions. The grade of recrystallization and deformation increase towards the east and a boundary between low- and medium-grade metamorphic terrains is crossed. The metamorphic boundary is marked by the disappearance of chlorite and the appearance of cordierite in the Mg-schists. Porphyroblasts of Mg-chlorite or cordierite grew across S1 and cordierite porphyroblasts are transected by S2 micas. This indicates that the peak of regional metamorphism occurred between the two deformational episodes. Retrogression of cordierite to chlorite and muscovite is confined to the vicinity of crosscutting S2 micas.

Prograde and retrograde fluid-rock interaction in calc-silicates northwest of the Idaho batholith: stable isotopic evidence

Carbon and oxygen isotopic analyses of silicate and carbonate minerals indicate that isotopic compositions in metasediments of the Wallace Formation (Belt Supergroup) exposed northwest of the Idaho batholith have been affected by both prograde and retrograde fluid-rock interaction. Silicates retain isotopic fractionations that reflect equilibration at peak metamorphic temperatures. In contrast, calcite oxygen isotopic compositions range from δ18O(Calcite)=+2.3 to +18.6‰ SMOW (standard mean oceanic water) and indicate that some calcites have exchanged with low-δ18O meteorichydrothermal fluids. Values of Δ18O (Quartz-Calcite) as large as +15.5 clearly indicate that the isotopic depletion of these calcites postdates the peak of regional metamorphism. Carbon isotopic compositions of 18O-depleted calcites are not significantly shifted relative to δ13C values in undepleted calcites, suggesting that the retrograde fluid was carbon-poor. Petrographically, retrograde fluid-rock interaction is associated with the occurrence of fine-grained, highly-luminescent calcite overgrowths on less-luminescent, metamorphic calcites, slight to moderate argillic alteration, and pseudomorphing of scapolite porphyroblasts by fine-grained albite. Retrograde isotopic depletions may be related to shallow meteoric-hydrothermal fluid flow developed around the Idaho batholith after intrusion and rapid uplift of the terrane. Peak metamorphic isotopic compositions in the Wallace Formation reflect mineralogically heterogeneous protolith compositions and isotopic fractionation due to devolatilization and/or infiltration. Variability in oxygen isotopic compositions on the order of 4–6‰ within the same rock type can be attributed to the combined effects of inherited isotopic compositions and isotopic shifts resulting from prograde devolatilization. Isotopic and compositional heterogeneity on the scale of mm to m precludes generalization of isotopic gradients on a regional scale. The isotopic data presented here, and metamorphic fluid compositions determined in previous studies, are best reconciled with heterogeneous bulk compositions, dominantly channelized prograde and retrograde fluid flow, and locally low fluid-rock ratios.

Granite magmatism and extensional tectonics in Southern Greenland

AbstractThe southernmost part of the early Proterozoic Ketilidian mobile belt (1·9–1·7 Ga) of southern Greenland comprises a thick series of largely flat‐lying migmatitic metasediments and metavolcanics of upper amphibolite to lower granulite facies. Regional tectonics are extensional and there is little structural evidence for earlier crustal thickening. Several episodes of granite magmatism are recorded, culminating in the late‐ to post‐orogenic rapakivi granites, which are associated with with a coeval suite of norites. These were emplaced at ca. 1740 Ma, roughly synchronously with the peak of regional metamorphism (2–3·5 kbar, 650–800°C). The rapakivi granites have been emplaced into major crustal‐scale extensional shear zones and have ramp‐flat geometries. ‘Black’ and ‘white’ rapakivi granites are found. The mineralogy of the black granites is dominated by orthopyroxene–amphibole assemblages with rare fayalite. In the white granites, biotite and amphibole are the only Fe–Mg silicates. All Fe–Mg silicates have high Fe/(Fe+Mg) ratios. In terms of bulk chemistry, the suite is bimodal. Both norites and granites are strongly fractionated and have Fe/(Fe+Mg) ratios of 0·50–0·65 (norites) and 0·65–0·90 (granites). REE modelling shows that the granites can be derived from 40 per cent partial melting of a metasedimentary protolith. The sequence of magmatic events in this region reflects the progressive metamorphism and dehydration melting of juvenile sediments in an extending basin. The heat source for melting may have come from the underplating of olivine tholeiite melts generated during lithospheric extension.

Archaean hydrothermal zircon in the Abitibi greenstone belt: constraints on the timing of gold mineralisation

Hydrothermal zircons have been found in Archaean mesothermal Au-veins and altered wallrock selvages at Val d'Or, in the Abitibi greenstone belt of Canada. The zircons are paragenetically associated with vein quartz, tourmaline, mica, carbonate, scheelite, pyrite, and gold. Zirconium mobility, and the consequent occurrence of hydrothermal zircon, may be associated with the intense tourmalinisation characteristic of Archaean gold deposits in this district. The SHRIMP ion-microprobe has been used to analyse hydrothermal zircons from four separate mines spatially associated with the Bourlamaque batholith, and has yielded ages constraining formation of the Au-bearing vein systems to within 20 Ma of emplacement of the pluton. The ion microprobe data reveal multiple stages of hydrothermal zircon growth in the vein systems, contemporaneous with the regional metamorphic peak and late kinematic activity along regional structures. Younger (including Proterozoic) ages previously obtained for the veins, using other minerals and isotopic schemes, must reflect either alteration or renewed mineral growth during much later reactivation of fluids along the same structures up to 400 Ma after initial formation of the veins.

Synvolcanic and syntectonic gold mineralization at the Bousquet Mine, Abitibi greenstone belt, Quebec

The Bousquet pyrite gold deposit, consisting of disseminated- and vein-type mineralization in sheared and altered greenschist facies Archean volcanic rocks, is hosted by two vein sets: foliation-parallel veins in ore zone 3 and foliation-oblique veins in zones 1, 2, 4, and 5. Foliation-parallel veins are composed of pyrite and lesser amounts of chalcopyrite, sphalerite, pyrrhotite, arsenopyrite, galena, bornite, rutile, tellurides, and native gold. The foliation-oblique veins of zones 1 and 2 are composed of pyrite, chalcopyrite, and sphalerite, with subordinate amounts of pyrrhotite, arsenopyrite, galena, stannite, gudmundite, rutile, ilmenite, and gold. Quartz and muscovite are the principal gangue minerals in both vein types in ore zones 1, 2, and 3. In zone 4, foliation-oblique veins are composed of pyrite, magnetite, pyrrhotite, and minor amounts of chalcopyrite and gold. The gangue minerals are calcite, ankerite, white mica, chlorite, and tourmaline. The veins of zone 5 contain pyrite, chalcopyrite, sphalerite, bornite, magnetite, rutile, and gold in a gangue of quartz, carbonate, white mica, and chlorite. Disseminated auriferous pyrite occurs in zones 3 and 5.Mineralogical and textural features indicate that gold emplacement and associated alteration occurred during two events, one predating peak regional metamorphism and another postdating peak metamorphism. Early, premetamorphic mineralization and related synvolcanic (?) hy-drothermal alteration produced initial auriferous sulfides and an advanced argillic alteration. Subsequent deformation, recrystallization, and metamorphic remobilization of early sulfides resulted in the currently observed, structurally controlled vein-type gold deposits. The vol-canogenic event is suggested by highly aluminous rocks closely associated spatially with the mineralized zones, by textures preserved through the overprint of metamorphic textures, and by the occurrence of stannite in ore zones 1 and 2.The late, postmetamorphic gold mineralization is considered to be contemporaneous with retrograde alteration of metamorphic minerals formed during late deformation. This alteration produced a pervasive kaolinite-pyrophyllite-diaspore gangue mineral assemblage involving the replacement of syntectonic andalusite porphyroblasts in felsic rocks. In mafic host rocks, retrograde alteration is characterized by carbonate, chlorite, muscovite, pyrite, and gold. The formation of a white mica-diaspore-carbonate-chlorite assemblage and the emplacement of associated gold are interpreted as epigenetic metamorphic events because these late retrograde minerals overprint the greenschist facies mineral assemblage.

Geochronology of the Belmont Lake Metavolcanic Complex and implications for crustal development in the Central Metasedimentary Belt, Grenville Province, Ontario

U–Pb analyses of zircon and titanite were carried out on eight rocks from the Belmont Lake Metavolcanic Complex, a volcano-sedimentary sequence in the Central Metasedimentary Belt of the Grenville Province of southeastern Ontario. The ages of concordant supracrustal rocks within the complex do not accord with stratigraphic position.The youngest volcanic age is [Formula: see text] from a rhyolite near the base of the sequence. The oldest age, [Formula: see text], is from a dacite in the middle. This is overlain by a rhyolite [Formula: see text] in age. A rhyolite at the top of the sequence appears to contain zircon inherited from a source about 1870 Ma old. The complex is therefore interpreted as comprising a lithotectonic sequence composed of structurally interleaved segments of contrasting age. Tectonic emplacement of these segments was most likely along previously unrecognized thrust faults.The supracrustal rocks were subjected to at least two major deformational events as well as a late metamorphism. The earliest event was probably associated with thrusting. The age of the youngest volcanic unit, [Formula: see text], is an upper age estimate for this event. A lower estimate is probably given by the age of the Cordova Gabbro, 1242 ± 3 Ma. The later event, including peak regional metamorphism, should be younger than [Formula: see text], the age of a sheared, recrystallized felsic sill intruded into the supracrustal rocks, and older than [Formula: see text], the age of the undeformed and unmetamorphosed Belmont Granite.Titanite fractions in the Belmont Granite and a volcanic andesite both give an age of 1071 ± 5 Ma. The age from titanite in the Belmont Granite may be due to thermal resetting during slow cooling. Titanite in the andesite is secondary and may have grown as a result of late metamorphic reactions.The youngest age measured, 1039 ± 5 Ma, is from a concordant analysis of a single zircon grain found within the oldest rhyolite. This may be an example of zircon growth from low-temperature, late-metamorphic fluids.

Structural control of greenstone-hosted Gold mineralization by transcurrent shearing: A new interpretation of the Kalgoorlie mining district, Western Australia

The Kalgoorlie mining district encloses the Golden Mile and Mt. Charlotte mining areas, and has produced 1200 metric tons of gold. It is classed as one of the largest Archaean gold deposits in the world. The district is located in the Norseman-Wiluna greenstone belt of the Yilgarn craton, Western Australia. This greenstone belt is characterized by NNW-trending transcurrent shear zones, some of which are traceable over more than 500 km strike length. Mineralization in the district is controlled by second-, third- and fourth-order structures located between principal shear zones of regional extent. An early deformation phase of regional folding (D1), associated with prograde regional metamorphism of low grade, preceded two phases of wrenching. The geometry of mineralized shear zones in both mining areas, as well as field and microstructural studies, allow us to distinguish an early phase of transpressional sinistral shearing (D2) from a later phase of transcurrent dextral shearing (D3). The earlier sinistral shear system in the Golden Mile area was in part reactivated, but its gross original geometry was preserved. These observations are consistent with clay model experiments carried out by the authors. Hydrothermal carbonate-sericite alteration and gold mineralization commenced during the sinistral shear event and continued into the dextral event. Mineralization within individual shear zones forms tabular orebodies, whereas pipe-shaped ore shoots occur at the intersection of shear zones. The ore shoots contain high-grade telluride mineralization locally. Relative timing relationships show that mineralization postdates the peak of regional metamorphism. Hydrothermal activity in the district was preceded by the intrusion of calc-alkaline porphyry dykes of granodioritic to tonalitic composition, and accompanied by the intrusion of lamprophyre dykes.

Polyorogenic40Ar/39Ar Mineral Age Record in the Seve and Köli Nappes of the Gäddede Area, Northwestern Jämtland, Central Scandinavian Caledonides

In the Scandinavian Caledonides, eugeoclinal terranes represented in the Köli Nappes were thrust >500 km onto the Baltoscandian platform during the early to middle Paleozoic. Immediately underlying thrust sheets of the Seve Nappe complex are suspected to have been derived from the outer margin of continent Baltica. Contrasting tectonothermal histories are recorded in Seve and Köli Nappes exposed in the Gäd-dede area, northern Jämtland County, Sweden. Eclogite-bearing Seve rocks underwent an initial amphibolite-grade regional metamorphism (SMI) with different structural units experiencing contrasting high-to intermediate-pressure conditions. Subsequent ductile imbrication (SM2) of high-and intermediate-pressure structural units occurred prior to 460-470 Ma cooling through temperatures required for intracrystalline retention of argon in hornblende. A later thermal overprint (SM3) everywhere rejuvenated Seve biotite and muscovite (and locally hornblende) argon systems at plateau ages of hornblende suggest attainment of peak metamorphic conditions (KM1) in the Lower and Middle Köli Nappes at ca. . Plateau dates of muscovite and biotite in both Seve and Köli units range down to 417 Ma, suggesting relatively rapid post-metamorphic cooling. Initial outboard assembly (KD1) of the Lower and Middle Köli Nappes preceded the regional metamorphic peak. Ductile translation (KD2) of the assembled Köli units occurred during maintenance of near peak KM1 metamorphic conditions. Although this thrusting was clearly later than SM2 ductile imbrication within the underlying Seve Nappe Complex, a widespread thermal overprint (SM3) appears to have accompanied structural emplacement of the Köli Nappes. Comparison with mineral ages reported elsewhere in the central Scandinavian Caledonides suggests a polyorogenic evolution consisting of: (1) variable high-pressure metamorphism of distal Baltoscandian miogeoclinal sequences during their entrainment in an accretionary wedge that developed in the Late Cambrian-Early Ordovician over a westdipping subduction zone; related arc volcanic activity occurred in more outboard, western terranes; (2) ductile imbrication of high-and intermediate-pressure sequences within the accretionary wedge (diachronous cooling and/or chronologically distinct tectonic episodes are indicated throughout the Early and Middle Ordovician); (3) ultimate accretion of a variety of eugeoclinal terranes along the Baltic continental margin; and (4) imbrication and metamorphism of the eugeoclinal terranes and previously deformed portions of the miogeocline during eastward translation onto the Baltoscandian platform in the Silurian and Devonian.

Thermobarometric profile through the Caledonian nappe stack of Western Ofoten, North Norway

P-T estimates from pelite thermobarometry along a transect through the Caledonian nappe stack in Ofoten, north Norway (68° 30° N) plot as a trend in P-T space (ranging from 443° C, 6.9 kb, to 688° C, 10.6 kb). The P-T range for each nappe overlaps the others, except for those from the lowest unit (Narvik Group) which has significantly higher values. Within the P-T ranges for each nappe, the position on the trend of a given sample is independent of its tectonic position. Results support our earlier conclusion, based on structural and petrographic grounds, that the regional metamorphic peak was reached during or after stacking of the nappes in the Siluro-Devonian Scandian orogeny.

Petrologic evidence for a terrane boundary in south‐central Circle Quadrangle, Alaska

Greenschist to amphibolite facies pelitic, mafic, and aluminous calc‐silicate schists from the south‐central Circle quadrangle, east‐central Alaska, are in contact with carbonaceous quartzite and calcareous phyllite recording diagenetic temperatures. Their mutual contact is best explained as a post‐metamorphic fault, indicating that the Yukon crystalline terrane was, at least in part, assembled after the peak of regional metamorphism. Mineral equilibria within the schists indicate metamorphism at minimum conditions of about 440° to 380°C at 0.3 GPa (3Kbar, biotite grade), 500°C at 0.5 GPa and XCO2 = 0.3 (garnet grade), and 550°C at 0.5 GPa and XCO2 = 0.2 (staurolite‐kyanite grade). Reflectance measurements on vitrinite and inertinite from the carbonaceous unit give temperatures of 170° to 230° ± 50°C.

Fluid evolution and transport during metamorphism: evidence from the Llano Uplift, Texas

A field, petrologic and stable isotopic investigation of the marbles and calc-silicates of the 1.15 b.y. Valley Spring Gneiss documents the dilution of internally evolved CO2-rich fluids by externally derived aqueous fluids introduced along channelways. Reaction textures within calcsilicates record the evolution through time of initially CO2-rich fluids toward increasingly more aqueous compositions. Assemblage zonations within calc-silicates require equilibration within local gradients of the mole fraction of CO2 in the fluid, and suggest that the infiltration of aqueous fluids was largely channelized along more permeable lithologies. Localized depletions in 13C and 18O corroborate petrologic evidence for channelized infiltration. Isotopic compositions reflect both devolatilization and the introduction of low-δ18O fluids; estimated minimum oxygen-equivalent fluid-to-rock ratios are near unity. Both mineralogical and stable isotopic systematics document the essential role of infiltration in driving decarbonation reactions during calc-silicate formation. The calc-silicate assemblages which equilibrated with fluids of the lowest mole fraction of CO2 record isotopic exchange equilibrium with fluids of δ18O typical of those derived from “normal granites”, as do the granitic aplites and pegmatites which transect most calcsilicate occurrences. Thus the infiltrating fluids are believed to be genetically linked to the intrusion of a suite of granitic plutons emplaced after the peak of regional metamorphism.

MICROESTRUTURAS E RELAÇÕES METAMORFISMO-DEFORMAÇÃO DO GRUPO SÃO ROQUE NA FAIXA ENTRE O PICO DO JARAGUÁ E A SERRA DOS CRISTAIS - SP

Microstructural studies of rocks belonging to the Sâo Roque Group in an area of about 376 km 2 between Pico do Jaragua (Municipality of Sao Paulo) and Serra dos Cristrais (Municipality of Jundiai) have permitted to recognize their patterns of defornation and metamorphism as a part of a work devoted to the structural analysls of the Sao Roque Group. Three phases of folding affected the metamorphic rocks as determined from characteristics of fold styles, nature of axial-plane foliations, and the interference patterns observable in both outcrops and maps. The relationship between deformation and metamorphism could be recognized with the study of microstructures. The F 1 phase was accompanied by the fornation of axial-plane slaty cleavage or schistosity related to tight folds and it was followed by the regional metamorphic peak under post-kinematic conditions. During this peak porphyroblasts of opaque minerals, gamet, staurclite and sillimanite were developed. This main metamorphic event continued, at least for the staurolite, into the beginning of the second folding episode. The F 2 phase generated a persistent crenulaticn cleavage, frequently both microscopic and of the zonal type, in axial-plane position of the folds are ubiquitous and occur in ali scales, Differentiated banding and some recryatallization of biotite around opaque minerals occurred during this phase. The third folding phase, F 3 , created more spaced crenulation cleavage not of the zonal type so common in the F 2 previous phase. The F 3 phase, which was more intense in the southem part of the area, generated large folds and undulations in regional structures.

Kinematic interpretation of mylonitic rocks in part of the Columbia River fault zone, Shuswap terrane, British Columbia

Mylonitic rocks of the Columbia River fault zone have been examined in a 600 m thick section, 20 km north of Revelstoke, British Columbia. At this locality coarse-grained granite with well-preserved igneous texture lies in the hanging wall of the fault zone. Adjacent to the boundary of the mylonite zone the granite is foliated and mineral segregation gives rise to well-defined compositional layering. The foliation (Sf) is defined by oriented mineral grains and flattened porphyroclasts. In areas of intense strain compositional layering (Sc) is parallel to Sf. At the boundary of the mylonite zone and in weakly deformed areas within the mylonite zone Sc and Sf are inclined at angles up to 30°. Sf is taken to be a plane of flattening, and Sc appears to parallel a plane of shear. After correction for effects of late-stage flexural slip folding the angular relationship of Sc to Sf may be used to determine sense of shear in the mylonitc zone; the hanging-wall rocks have been displaced toward the east relative to the footwall. Formation of mylonitic fabrics and associated displacement of the Selkirk allochthon eastward relative to the underlying Monashee Complex occurred after the Middle Jurassic peak of regional metamorphism and before brittle reactivation and uplift in the Tertiary.

Characteristics and mode of emplacement of gneiss domes and plutonic domes in central-eastern Pyrenees

Gneiss domes and plutonic granitoid domes make up almost 50% of the pre-Hercynian terrains in the Central and Eastern Pyrenees. From a structural study of the shape and internal structure of the domes and of their relationships with the enclosing rocks, it can be shown that both types of domes were emplaced diapirically during the major regional deformation phase and the peak of regional metamorphism. The study also shows that the internal structure, the overall shape and general behaviour relative to the host rocks are similar for plutonic domes and for gneiss domes. This appears to be in good agreement with H. Ramberg's (1967, Gravity Deformation and the Earth's Crust. Academic Press, London; 1970, Model studies in relation to intrusion of plutonic bodies. In: Mechanisms of Igneous Intrusion (edited by Newall, G. & Rast, N.) Geol. J. Spec. Issue 2, 261–286.) model studies showing that dome or mushroom-like structures, similar to those observed, develop when there is a small viscosity ratio between the rising body and its enclosing medium. This implies a high crystal content for the granitoid magma. This crystal content has been estimated by (i) calculating the viscosity and density in natural conditions from petrological data for the magma considered as a suspension, using the model and program of J. P. Carron et al. (1978 Bull Soc. géol. Fr. 20, 739–744.); (ii) using the recent results of experimental deformation of partially melted granites of I. van der Molen & M. S. Paterson (1979, Contr. Miner. Petrol. 70, 299–318.) and (ii) comparing the preceding results with the data obtained by deformation experiments on rocks similar to those enclosing the domes. The minimum crystal content for the development of a dome-like structure has been, thus, estimated to about 70%, i.e. a value very close to that estimated by van der Molen & Paterson (1979) to be the critical value separating the granular framework flow from suspension-like behaviour. The effect of small variations in the viscosity of the rising body are then simulated by centrifuge experiments. These small variations appear to exert a strong control on the shape and rate of rise of the domes. They are thought to be sufficient to account for the variations in shape and structure and the level of emplacement of the different types of gneissic and plutonic domes. Finally, more complex experiments, with models built in order to simulate as closely as possible the natural structural evolution of the region as deduced from petrological and structural data, are reported. Their implications for the regional interpretation of the relationships between gneissic and plutonic domes is then discussed.

Metadolerite age relationships in the Dalradian of northwest Donegal, Ireland and their orogenic significance

AbstractThe structural relationships of several intrusive sheets belonging to the extensive metadolerite suite of the Appin (lower) Dalradian rocks of the Creeslough area, northwest Donegal, Ireland have been examined. It is concluded that, relative to the regional chronology of deformation of the enclosing metasediments, sheet emplacement has occurred: pre‐D2; syn‐D2; between D2 and D3; and syn‐D3. The height of activity came in the post‐D2, pre‐D3 interval and coincided with the peak of regional metamorphism in the area. These relationships in Donegal are compared with those of the pretectonic basic rocks of the southwest Highlands and models are discussed.