Facies Terranes Research Articles

Shear zones of the Limpopo Belt and adjacent granitoid-greenstone terranes: implications for late Archaean collision tectonics in southern Africa

Granulite metamorphism in the > 2.65 Ga Limpopo Belt of southern Africa is related to crustal thickening, due to late Archaean collision tectonics. The Central Zone of the belt is a major thrust sheet of distinctive lithological, structural and isotopic character, separated from adjacent terranes by crustal-scale shear zones, and the Northern and Southern Marginal Zones are up-thrust deep-level granulite facies terranes of the adjacent cratons. Geometrically the marginal zones are mirror images and the Limpopo Belt is essentially symmetrical in N–S section. Conventionally regarded as a N–S collisional orogeny, the gross structural and lithological differences between the Central Zone and the marginal zones, kinematic data from shear zones and geophysical data showing the three-dimensional geometry of the Central Zone are here used to suggest that the direction of emplacement of the Central Zone was also the collision direction. This was from northeast to southwest, with the upthrust, outwardly directed, marginal zones developing in mechanical response to this collision.

Reflectivity and seismic properties of the deep continental crust

The recovery of drill core from a section of upper amphibolite facies terrane located in the Inner Piedmont of South Carolina provides a rare opportunity to study in detail the seismic character of rocks which were once part of the deep continental crust Seismic reflection profiles over the Inner Piedmont in the vicinity of the drill hole reveal subhorizontal high‐amplitude reflections. The origin of these reflections has been investigated by measuring compressional wave velocities as a function of confining pressure and densities in 150 samples obtained from 305 m of drill core. Synthetic seismograms generated from the velocity and density measurements demonstrate that reflections originate from subhorizontal metamorphic layering, probably originating from lower crustal ductile flow. Fine tuning of the metamorphic layering, which is temperature and pressure related, can produce bright spots in the deep crust. Anisotropy, resulting from preferred mineral orientation, is another important property of this section. Velocities are low for vertical wave propagation, whereas horizontal velocities are relatively high. Horizontal velocities show only a slight variation with azimuth, which is unlikely to be detected using standard refraction techniques. Reflectivity decreases with increasing anisotropy. Critical to this study has been the continuous coring of a major section of high‐grade metamorphic terrane, which has provided the necessary stratigraphic and lithologic control for the generation of accurate synthetic seismograms.

Dynamic recrystallization and associated exsolution in perthites: Evidence of deep crustal thrusting

Mylonitic gneisses from Parry Sound, Ontario, are intensely deformed and occur within a major deep crustal shear zone developed during thrusting of granulite facies rocks (800°C, 900–1100 MPa) northward onto an amphibolite facies terrane. We have examined, by optical and electron microscopy, microstructures in perthites from these mylonites that preserve the deformation‐recrystallization cycle which operated during high‐ductility deformation of the crystals. The exsolution and recrystallization microstructures are intimately associated. Exsolution in the perthites occurred in two stages. A calcic plagioclase (oligoclase) exsolved first and subsequently coarsened and deformed during the single‐crystal ductility phase of shear zone deformation. A progressive increase in size and boundary incoherence of oligoclase domains during deformation led to recrystallization of the host perthite crystals as discrete orthoclase and oligoclase grains. The principal mode of recrystallization was subgrain rotation, with a component of grain growth that followed the formation of mobile high‐angle boundaries. Misorientation of subgrains to form grains at the recrystallization front is abrupt and occurred over one or two subgrain diameters. The second exsolution stage postdated perthite deformation and saw formation of fine albite lamellae in both host and recrystallized orthoclase. Temperatures inferred from the exsolution textures for independently determined confining pressures indicate that this deformation occurred under at least granulite facies conditions. The perthites thus record deformation from crustal depths of the order of 34–42 km. Continental megathrusts, such as the Parry Sound shear zone, provide mechanisms for both the generation of large strains observed in otherwise nonductile minerals and the preservation of ensuing microstructures.

Metamorphic studies: research in progress

Report of a meeting of the Metamorphic Studies Group held at the University of Sheffield, 11 March 1987. The meeting was organized by K. H. Brodie and D. A. Carswell. A Metamorphic Studies Group meeting on ‘Metamorphic Studies: Research in Progress’ and the sixth Annual General Meeting of the Group were held on 11 March 1987. For the first time the annual meeting was held outside London, in the Department of Geology at the University of Sheffield. It was very well attended, and 10 papers were presented on a range of metamorphic topics, including an invited contribution from T. Bell (James Cook University, Australia). The morning session concentrated on high grade and regional metamorphic topics. C. R. L. Friend, in a joint contribution with A. P. Nutman and V. R. McGregor, presented the results of a reinvestigation of an area of amphibolite and granulite facies metamorphism in SW Greenland, involving systematic isotopic studies. The area can be divided into zones with significantly different tectono-metamorphic histories that have been tectonically juxtaposed prior to amphibolite acies metamorphism and deformation which affected the whole area. P. J. Treloar continued the topic of P–T histories of granulite facies terranes in a joint paper with J. Carney and M. J. Crow, involving an example from NE Zimbabwe. Attempts have been made to date two distinct metamorphic events that have affected the rocks, the second of which involved decompression associated with uplift of the area. D. A. Carswell discussed the reality of the postulated inflection

The vertical distribution of radiogenic heat production in the Precambrian crust of Norway and Sweden: Geothermal implications

The present geology of southern Scandinavia offers the unique opportunity to sample deep and intermediate levels from the same crustal section for both heat flow and heat production. In the central part of southern Norway, amphibolite facies terranes appear to lie on top of the same deeper crustal formations which crop out on their western and eastern margins. An extensive data set on the geochemistry of all types of rocks in the region culled from the literature is used to derive values for radiogenic heat production for each kind of crustal facies. Using constraints from heat flow data in the same area allows a reliable model of the distribution of crustal heat production. The average heat production of granulite facies terranes is 0.4 µW/m³, similar to values in other parts of the world. For amphibolite facies rocks, the value is 1.6 µW/m³. The present shield also includes heat producing element enriched granites formed in later events and the average heat production of presently exposed crust is 2.7 µ/m³. Using heat flow and radioactivity data from ten stations, the reduced heat flow in the area is 22 ± 2 mW/m². This corresponds to the heat flow at the top of 28 km of deep crustal facies, implying that the mantle heat flow is probably as low as 10 mW/m². Over the whole crustal thickness, the average amount of radiogenic heat is 31 mW/m².

Geochronology of Early to Middle Paleozoic Tectonic Development in the Southwest Newfoundland Gander Zone

Southwest Newfoundland's Gander Zone was once considered part of the collided eastern margin of an early Paleozoic Iapetus ocean, distinguished from the rest of the Gander Zone by the extensive exposure of a polydeformed amphibolite to greenschist facies sequence considered Devonian in age, overlying presumed Precambrian basement. It was later found that truly Devonian strata were confined to the Cape Ray Fault system, the local northwestern boundary of the Zone. U-Pb zircon isotopic ages of and on metavolcanic rocks in the upper part of the sequence southeast of the fault system indicate that Ordovician and possibly older rocks underlie the Gander Zone itself. A temporal framework for the three main stages of regional deformation, corresponding to pre-metamorphic recumbent folding and transpression ( and ), is presented using U-Pb and dates of granitoid and metamorphic rocks. ) is bracketed as late Ordovician or Silurian by means of U-Pb zircon isotopic ages on the metavolcanic rocks and a 412 Ma U-Pb minimum age on a post- granitoid intrusion. Spatial domains of contrasting biotite and hornblende cooling ages reflect the and episodes, which led to post-metamorphic cooling following uplift of different segments of the amphibolite facies metamorphic terrane. Early Devonian differential uplift on the Cape Ray Fault in the northeast was associated with and caused rapid post-metamorphic cooling through Ar retention temperatures to yield nearly concordant biotite and hornblende ages between 374 and 388 Ma. Reverse and oblique-slip faulting during produced oblique northwesterly uplift of the still-buried portion of the amphibolite facies terrane against Devonian strata in the southwest part of the Cape Ray Fault zone. Late Devonian to early Carboniferous post-metamorphic cooling in this segment is confirmed by plateau dates of 345-353 Ma for biotite and 361 Ma for hornblende. The Gander Zone as a whole is imprinted with a strong penetrative structural grain similar in pattern to and structures in the southwest Newfoundland Gander Zone. K-Ar mineral dates are likewise earliest Devonian and younger, in contrast to numerous pre-Devonian K-Ar ages in the Humber and the Dunnage Zones. The Gander Zone, already identified as a wide, Acadian ductile shear belt and noted for widely variable metamorphic grade, might represent a mobile zone where coherent relics of the "Iapetus" or prior stages of Appalachian history were exhumed during Devonian orogeny, many after Silurian or earlier tectonic burial.

Fluids and melting under upper amphibolite facies conditions

Internal buffering paths on temperature-water activity diagrams for the pelitic model system: NaAIO 2 - KAIO 2 - CaAI 2 O 4 - Al 2 SiO 5 - SiO 2 - H 2 O give rise to sequences of assemblages which correspond to those observed in upper amphibolite facies terranes. In particular, the persistence of the assemblage quartz-alkali feldspar-plagioclase-muscovite-sillimanite across a zone and the rapidly changing composition of plagioclase across this zone can be accounted for in a natural way.

The role of CO 2 in the chemical modification of deep continental crust

Compositional differences between granulite facies rocks and equivalent amphibolite facies rocks and the observation of CO 2-rich fluid inclusions in granulites, have led to the suggestion that CO 2 must play a role in modifying the composition of deep continental crust. How CO 2 effects this change has remained unclear. Using the thermodynamic properties of aqueous ions in a fluid of evolving CO 2 H 2O ratio, it is possible to model the incongruent dissolution of feldspars under conditions appropriate for granulite facies metamorphism. The results demonstrate that dissolution will be strongly enhanced at high CO 2 H 2O ratios, with ion solubilities being Na + > K + ⪢ Ca ++. This enhancement is compatible with the reported compositional contrasts between granulite and amphibolite facies rock, but requires large fluid volumes. To test the dissolution model, a detailed field and petrologic study was conducted in a well exposed granulite facies terrane in West Greenland. Strong correlation between fluid composition and bulk rock chemistry can be documented; CO 2-rich regions contain rocks which consistently have low a Na 2O a CaO ratios, while H 2O-rich regions consistently have high a Na 2O a CaO ratios. Magnetite rims on sulfide grains are ubiquitous in high ƒ Co 2 regions and are absent in high ƒ H 2O regions, and they provide evidence that CO 2 was introduced into the region. These correlations and observations are predictable from the properties of the dissolution process. These considerations, along with observations regarding graphite petrogenesis, provide strong arguments that the total fluid volume interacting with the rock during metamorphism was very large, in some cases equaling or exceeding total rock volume. Such large fluid volumes can lead to significant compositional modification of the crust, and will mask the original protolith chemistry. Such processes should lead to Ca- and Al-enriched, Na-, K-, S- and Si-depleted residues in the deep crust.

Regional geothermobarometry in the granulite facies terrane of South India

ABSTRACTIn the southern part of the Archaean craton of South India, an approximately 3.4–2.9 b.y. old migmatite–gneiss terrane (Peninsular gneiss complex) has been subjected to granulite facies metamorphism about 2.6 b.y. ago. During this event, the extensive charnockite-khondalite zone of southern India developed. A younger metamorphism (Proterozoic?) led to retrogression of the charnockites and khondalites, mainly under the conditions of the amphibolite facies.The physical conditions of metamorphism have been evaluated by applying methods of geothermobarometry to the widespread charnockitic assemblages with garnet, orthopyroxene, clinopyroxene, plagioclase, and quartz. The interpretation of the P–T estimates includes a critical discussion of potential error sources, e.g. errors of the analytical data and the calibrations of the models, and takes into account the complex metamorphic history of the rocks and the kinetics of the mineral equilibria.P-T estimates were obtained for seven subareas from the rim compositions of the coexisting minerals: Shevaroy Hills 680±55°C—7·4±1 kb; Kollaimalai area 680±40°C—8·6± 1 kb; Nilgiri Hills 680±90°C—6·6±0.8kb (upland massif) and 705±60°C—9·3±0.8 kb (northern margin); Bhavani Sagar area 650±50°C—7·2± 1 kb; Sargur-Mysore area 690±60°C—7·6 kb; Bangalore-Kunigal-Satnur area 760±50°C—6 kb. Except for the last subarea, the P-T model data reflect the conditions of a late annealing stage probably related to the retrogressive metamorphism. Conditions near the peak of granulite facies metamorphism (730–800°C—6·5–9·5 kb) are recorded by the core compositions of the minerals. Although a rather uniform cooling history of the main part of the charnockite-khondalite terrane is suggested from the temperature data, differential uplift of smaller blocks is indicated by the regional variation of the pressure data.

The significance of metamorphic fluorite in the Adirondacks

Thermodynamic calculations for selected silicate-oxide-fluorite assemblages indicate that several commonly occurring fluorite-bearing assemblages are restricted to relatively narrow ƒ O 2 -ƒ F 2 fields at constant P−T. The presence of fayalite-ferrohedenbergite-fluorite-quartz ± magnetite and ferrosalite-fluorite-quartz-magnetite assemblages in orthogneisses from Au Sable Forks, Wanakena and Lake Pleasant, New York, buffered fluorine and oxygen fugacities during the granulite facies metamorphism in the Adirondack Highlands. These buffering assemblages restrict ƒ F 2 to 10 −29 ± 1 bar and ƒ 0 2 to 10 −16 ± 1 bar at the estimated metamorphic temperature of 1000K and pressure of 7 kbar. The assemblage biotite-magnetite-ilmenite-K-feldspar, found in the same Au Sable Forks outcrop as the fayalite-fluorite-ferrohedenbergite-quartz-magnetitie assemblage, restricts H 2O fugacities to less than 10 3·3 bar. These fugacities limit H 2 and HF fugacities to less than 10 1 bar for the Au Sable outcrop. The data indicate that relative to H 2O, O 2, H 2, F 2 and HF are not major species in the fluid equilibrated with Adirondack orthogneisses. The calculated F 2 fugacilies are similar to the upper limits possible for plagioclase-bearing rocks and probably represent the upper ƒ F 2 limit for metamorphism in the Adirondacks and in other granulite facies terranes.

Cordierite—garnet bearing assemblages as geothermometers and barometers in granulite facies terranes

Pressures and temperatures of metamorphism can be determined for rocks containing either of the divariant assemblages cordierite—garnet—hypersthene—quartz or cordierite—garnet—sillimanite—quartz. Both these assemblages are common in granulite facies terranes. Experimental reversals of the divariant equilibria and complete electron microprobe analyses support the data of Hensen and Green (1971, 1972, 1973). Recent data from both field studies and experiments present compelling evidence that the distribution coefficient of Fe2+ and Mg between cordierite and garnet Kd(Cd-Ga) decreases with increasing temperature contrary to the data of Currie (1971). It is suggested that in the rare occurrences where high values of Kd(Cd-Ga) are found in rocks otherwise indicating high temperature, re-equilibration by cation-exchange between cordierite and garnet or between cordierite and biotite has occurred at lower temperature. In Precambrian granulite facies terranes temperatures of 900°C and above are indicated by the data, suggesting the presence of unusually high geothermal gradients, probably related to high levels of magmatic activity in Precambrian times. Minerals typically associated with cordierite and garnet in such environments include sapphirine—quartz, hypersthene—sillimanite-quartz and osumilite—hypersthene—quartz.

The Precambrian Gneisses and Supracrestal Rocks of the Western Shore of Kaipokok Bay, Labrador, Newfoundland

In the Kaipokok Bay area the amphibolite facies Hopedale Complex is overlain by the English River Greenstones. The Hopedale Complex is formed of gneisses, amphibolites, and associated granites and shows indications of being polycyclic, possibly derived from an older (Aphebian?) granulite facies terrane. The English River Greenstones are a supracrustal sequence of dominantly metavolcanic rocks and are the equivalents of the Aillik series. Hudsonian orogenic effects were superimposed on both rock units and caused locally intense refoliation of the gneisses and the development of a strong foliation in the greenstones. During this orogenic cycle an early period of intense zonal flattening was followed by a more regionally developed folding and weak tectonite fabric development. All of these rocks were cut by later acidic and basic intrusions. The geological history of the area shows a general similarity to that of southwestern Greenland.

Experimental studies on eclogites: The roles of magnetite and acmite in eclogitic assemblages

Reactions between albite, magnetite, garnet and clinopyroxene solid solutions have been studied at pressures up to 36 kbar at 1,100–1,300°C. A low pressure clinopyroxene + magnetite + plagioclase assemblage is replaced by a high pressure clinopyroxene + garnet + magnetite assemblage, or at intermediate pressures and higher temperatures, by a transitional clinopyroxene + magnetite ± quartz assemblage. Examination of polished thin sections of natural eclogites demonstrates that primary magnetite is rare or absent in eclogites from both glaucophane schist and amphibolite facies terranes. It is concluded that while clinopyroxene ss + magnetite + sodic plagioclase may be stable in high pressure granulite facies it is replaced under eclogite facies conditions by the assemblage clinopyroxene ss + garnet ss ± quartz. Haematite or magnetite may occur in the latter assemblage in rocks with high Fe 2O 3/pyroxene contents.

Geochemical investigations of deep-seated rocks in the Australian shield

Medium to high pressure granulite facies rocks are relatively uncommon at the earth's surface and are probably formed at considerable depth in the continental crust. Low pressure granulite facies rocks are rather common in shield regions and frequently grade into upper amphibolite facies rocks. Medium to high pressure granulite terranes have less ‘acidic’ average major element compositions, and have significant depletions of U, Th, and Rb compared with typical low pressure granulite and amphibolite facies terranes and the overall surface shield. It is considered that the medium to high pressure granulite terranes reflect chemical trends with increasing depth in the continental crust. These trends are the results of modifications of the original compositions by metamorphic (including melting) processes. The strong upward concentrations of Rb, U, and Th in the continental crust indicated by these studies will affect estimates of their crustal and terrestrial abundances. The U and Th distribution is in accord with surface heat flow measurements.

Biotite bigh-grede metamorphic rocks

The phase equilibrium relationships governing the stability of biotite in high-grade pelitic schists can be studied in the system K20-MgO-FeO-AI203-Ti02-H2O-O2. Assuming that potassium feldspar and quartz are present in excess and that water and oxygen are mobile components, the phase equilibrium relationships may be interpreted by a tetrahedral diagram having A1203-Ti02-MgO-FeO at its apices. The phase equilibeium relationships between biotite, garnet and orthopyroxene from the northwest Adirondack Mountains, Lutzow-Holm Bay area, and the charnockitic terranes in Madras and Granuligebirge are dicsussed. Based on the biotite composition field, it is concluded that the rocks in the northwest Adirondack Mounains and the Lutzow-Holm Bay ares belong to a similar subfacies of the granulite facies, and those in the Granulitgebirge and Madras terrane belong to another subfacies of the granulite facies that represents higher temperatures than the former. It is also concluded that the biotite composition fieid of rocks in the Emeryville area and other amphibolite facies terranes have larger stability field than the granulite facies biotite.