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NASA NTRS1986

Age and origin of gneisses south of Ameralik, between Kangimut-Sangmissoq and Qasigianguit

Gneisses which crop out along the southern coast of Ameralik between Kangimut-sangmissoq and Qasigianguit (K-s-Q) are the subject of long-standing controversy concerning their relationship to the early Archean Amitsoq gneisses of the Godthaab district. On the basis of field observations, it was argued that gneisses at Kangimut-sangmissoq and Qasigianguit are correlatives of the early Archean Amitsoq gneisses. The data were reexamined and it is concluded that the K-s-Q gneisses represent an addition of substantially juvenile mantle-derived material to the Archean craton of West Greenland during the late Archean times. Some of the parent magmas have undergone interaction with older crust, as indicated by Pb isotope evidence for contamination with Amitsoq-derived Pb. However, the positive epsilon Nd(I) value for the K-s-Q gneisses firmly rules out any significant material contribution from the Amitsoq gneisses to the K-s-Q gneisses.

Jones, N. W.↗

NASA NTRS1988

U-Pb ages and Sr, Pb and Nd isotope data for gneisses near the Kolar Schist Belt: Evidence for the juxtaposition of discrete Archean terranes

Uranium-lead ages and Sr, Pb, and Nd isotopic data for gneisses near the Kolar Schist Belt and their interpretation as evidence for the juxtaposition of discrete Archean terranes were presented. The granodioritic Kambha gneiss east of the schist belt has a zircon age of 2532 + or - 3 Ma and mantle-like initial Sr, Pb, and Nd isotopic ratios. Therefore these gneisses are thought to represent new crust added to the craton in the latest Archean. By contrast, more mafic Dod gneisses and leucocratic Dosa gneisses west of the schist belt (2632 + or - 7 and 2610 + or - 10 Ma) show evidence for contamination of their magmatic precursors (LREE-enriched mantle-derived for the Dod gneisses) by older (greater than 3.2 Ga) continental crust. Fragments of this older crust may be present as granitic and tonalitic inclusions in the 2.6-Ga gneisses and in shear zones. The antiquity of these fragments is supported by their Nd, Sr, and Pb isotopic compositions and by 2.8 to greater than 3.2 Ga zircon cores.

Krogstad, E. J.↗

NASA NTRS1992

Thermobarometric studies on the Levack Gneisses: Footwall rocks to the Sudbury Igneous Complex

Granulite and amphibolite facies gneisses and migmatites of the Levack Gneiss Complex occupy a zone up to 8 km wide around the northern part of the Sudbury Igneous Complex (SIC). Orthopyroxene- and garnet-bearing tonalitic and semipelitic assemblages of granulite facies grade occur within 3 km of the SIC together with lenses of mafic and pyroxenitic rock compositions normally represented by an amphibole +/- cpx-rich assemblage; amphibolite facies assemblages dominate elsewhere in this terrain. These 2.711-Ga gneisses were introduced by (1) the Cartier Granite Batholith during late Archaean to early Proterozoic time and (2) the SIC, at 1.85 Ga, which produced a contact aureole 1-1.5 km wide in which pyroxene hornfelses are common within 200-300 m of the contact. A suite of 12 samples including both the opx-gt and amphibole-rich rock compositions have been studied. Garnets in the semipelitic gneisses are variably replaced by a plg-bio assemblage. Thermobarometric calculations using a variety of barometers and thermometers reported in the literature suggest that the granulite facies assemblages formed at depths in the 21-28 km range (6-8 kbar). Textures and mineral chemistry in the garnet-bearing semipelitic rocks indicate that this terrain underwent a second metamorphic event during uplift to depth in the 5-11 km range (2-3 kbar) and at temperatures as low as 500-550 C. This latter event is distinct from thermal recrystallization caused by the emplacement of the SIC; it probably represents metamorphism attributable to intrusion of the Cartier Granite Batholith. These data allow two interpretations for the crustal uplift of the Levack Gneisses: (1) The gneisses were tectonically uplifted prior to the Sudbury Event (due to intrusion of the Cartier Batholith); or (2) the gneisses were raised to epizonal levels as a result of meteorite impact at 1.85 Ga.

James, R. S.↗

NASA NTRS1986

Tectonic evolution of greenstone-Gneiss association in Dharwar Craton, South India: Problems and perspectives for future research

The two fold stratigraphic subdivision of the Archean-Proterozoic greenstone-gneiss association of Dharwar craton into an older Sargur group (older than 2.9 Ga.) and a younger Dharwar Supergroup serves as an a priori stratigraphic model. The concordant greenstone (schist)-gneiss (Peninsular gneiss) relationships, ambiguities in stratigraphic correlations of the schist belts assigned to Sargur group and difficulties in deciphering the older gneiss units can be best appreciated if the Sargur group be regarded as a trimodal association of: (1) ultrabasic-mafic metavolcanics (including komatiites), (2) clastic and nonclastic metasediments and paragneisses and (3) mainly tonalite/trondhemite gneisses and migmatites of diverse ages which could be as old as c. 3.4 ga. or even older. The extensive occurrence of this greenstone-gneiss complex is evident from recent mapping in many areas of central and southern Karnataka State.

Rao, Y. J. B.↗

NASA NTRS1986

Early Archean tonalite gneiss in the upper peninsula of Michigan

Geochronological results on tonalite gneiss of northern Michigan that is 3.56 Ga or slightly older is presented. Tonalitic augen gneiss and structurally overlying biotite gneiss and schist are exposed in a dome near Watersmeet. They are part of an extensive gneiss terrane of southern Minnesota, Wisconsin and Michigan that includes rocks of early to late Archean ages and lies south of the Wawa volcanic subprovince. Two samples of the augen gneiss and one of the biotite gneiss show zircon grains of similar shape, zoning, color, and development of crystal faces. These zircons give Pb/U isotopic ratios that plot on a chord of 3,560 + or - 40 m.y. upper intersect and of 1,250 + or m.y. lower intersect. The 3,560 m.y. number is believed to be a minimum age because analysis of one of the least discordant zircon fractions by ion microprobe that gave a nearly concordant age of 3,650 m.y. The 1,250 m.y. lower intersect is without geological significance: it is interpreted to be a result of multiple lead loss at 2.7, 1.8, and 0.5 Ga by U/Pb in zircon. Archean rocks 10 to 25 km northwest of the Watersmeet dome give a 2.75 Ga age on zircons. Quartz monzonite here is dated at 2.65 Ga.

Peterman, Z. E.↗

NASA NTRS1986

Regional variation in the Amitsoq gneisses related to crustal levels during late Archean granulite facies metamorphism: Southern west Greenland

The dominant lithology at Kangimut sangmissoq is described as nebulitic tonalitic gneiss containing highly distended plagioclase phyric amphibolites. The gneiss amphibolite complex was intruded by Nuk gneiss between 3.05 and 2.90 Ga and later (2.6 to 2.7 Ga) by post granulite facies granitoid sheets. The amphibolites are though to be Ameralik dikes and the older gray gneiss are then Amitsoq by definition. The problem arises when the isotopic data are considered, none of which indicate rocks older that about 3.0 Ga.

Nutman, A. P.↗

NASA NTRS1988

Origin and evolution of Gneiss-Charnockite rocks of Dharmapuri District, Tamil Nadu, India

A low- to high-grade transition area in Dharmapuri district was investigated petrologically and geochemically. The investigation confirmed the presence of a continuous section through a former lower crust, with felsic charnockites predominating the lower part and felsic gneisses the upper part. The structure of original gneisses is preserved in charnockites and the latter show petrographic evidence for prograde metamorphism. The prograde metamorphism is of isochemical nature as revealed by the similarity of compositions of tonalitic gneisses and tonalitic charnockites. However, the depletion of LIL elements particularly Rb, caused variation in K/Rb ratios from low values (345) in the gneisses in upper part to higher values (1775) in the charnockites in the lower crust. This variation in K/Rb ratio in a north to south traverse is related to the progressive break-down of hydrous minerals under decreasing H2O and increasing CO2 fluid conditions. Metasomatism and partial melting has also taken place to a limited extent along shear planes and weak zones. During cooling the H2O circulation affected substantial auto-regression in the transition zone resulting in the formation of second generation biotite.

Rao, D. Rameshwar↗

NASA NTRS1981

Lu-Hf total-rock age for the Amitsoq gneisses, West Greenland

Lu-Hf total-rock data for the Amitsoq gneisses of West Greenland yield an age of 3.55 + or - 0.22 billion years, based on the decay constant for Lu-176 of 1.96 x 10 to the -11th/year, and an initial Hf-176/Hf-177 ratio of 0.280482 + or - 33. The result is in good agreement with Rb-Sr total-rock and U-Pb zircon ages. In spite of severe metamorphism of the area at 2.9 billion years, zircons from two of the samples have remained on the total-rock line, and define points close to the initial Hf ratio. The initial Hf-176/Hf-177 lies close to a chondritic Hf isotopic evolution curve from 4.55 billion years to present. This is consistent with the igneous precursors to the Amitsoq gneisses having been derived from the mantle at or shortly before 3.6 billion years. Anomalous relationships between Hf concentration and the Lu-176/Hf-177 ratio may suggest that trace element abundances in the Amitsoq gneisses are partly controlled by processes related to metamorphism.

Pettingill, H. S.↗

NASA NTRS1986

Spatial greenstone-gneiss relationships: Evidence from mafic-ultramafic xenolith distribution patterns

The distribution patterns of mafic-ultramafic xenoliths within Archaean orthogneiss terrain furnish an essential key for the elucidation of granite-greenstone relations. Most greenstone belts constitute mega-xenoliths rather than primary basin structures. Transition along strike and across strike between stratigraphically low greenstone sequences and xenolith chains demonstrate their contemporaneity. These terrains represent least deformed cratonic islands within an otherwise penetratively foliated deformed gneiss-greenstone crust. Whereas early greenstone sequences are invariably intruded by tonalitic/trondhjemitic/granodioritic gneisses, stratigraphically higher successions may locally overlap older gneiss terrains and their entrained xenoliths unconformably. The contiguity of xenolith patterns suggests their derivation as relics of regional mafic-ultramafic volcanic crustal units and places limits on horizontal movements between individual crustal blocks.

Glikson, A. Y.↗

NASA NTRS1988

Gneiss-charnockite transformation at Kottavattam, Southern Kerala (India)

At Kottavattam, leucocratic granitic garnet-biotite gneisses (age less than 2 Ga) were partially transformed to coarse-grained charnockite along a system of conjugate fractures (N70E and N20W) and the foliation planes (N60 to 80W; dip 80 to 90 SW) about 550 m.y. ago. To examine and quantify changes in fabric, mineralogy, pore fluids and chemical composition associated with this process, large rock specimens showing gneiss-charnockite transition were studied in detail. The results of the present study corroborate the concept that charnockite formation at Kottavattam is an internally-generated phenomenon and was not triggered by the influx of carbonic fluids from a deep-seated source. It is suggested that charnockitization was caused by the following mechanism: (1) near-isothermal decompression during uplift of the gneiss complex led to an increase of the pore fluid pressure (P sub fluid greater than P sub lith) which - in a regime of anisotropic stress - triggered or at least promoted the development of conjugate fractures; (2) the simultaneous release of pore fluids from bursting fluid inclusions and their escape into the developing fracture system resulted in a drop of fluid pressure; and (3) the internal generation and buffering of the fluids and their, probably, limited migration in an entirely granitic rock system explains the absence of any significant metasomatic mass transfer.

Raith, M.↗

NASA NTRS1988

Retrograde, charnockite- gneiss relations in Southern India

The high-pressure charnockites (P = 8 to 9 Kb., T = 700 to 800 C CO2-rich fluid regime) of the Nilgiri hills show evidence of retrogression related to shear deformation within the Moyar and Bhavani shear belts. Two types of retrogression have been noticed: (1) retrogression along shear planes, and (2) retrogression along pegmatitic veins. Initial stages of retrogression results in the formation of irregular, 2 to 3 cm to one meter wide bleached zones with the removal of greasy grey color of charnockites. Minor structures which were earlier obscured in charnockites are clearly seen in bleached areas. In intensely shear areas, formation of highly fissile grey gneiss results often with the development of flaser and mylonitic structures. Fluid inclusion studies and geochemical investigations carried out for serial samples collected from charnockite to gneiss indicate following features: (1) there is a gradual decrease in density of CO2-rich fluids from 1.073 to 0.821 g/cu cm; (2) interestingly, in many sections of the gneisses studied, there is almost complete absence of fluid inclusions suggesting that they would have decrepitated (this may be due to large pressure difference (2 to 3 Kb.) created between the interior and exterior of the fluid inclusions); and (3) presence of mixed CO2-H2O inclusions were noticed.

Srikantappa, C.↗

NASA NTRS1992

A comparison of the chemistry of pseudotachylyte breccias in the Archean Levack Gneisses of the Sudbury structure, Ontario

The Archean Levack Gneisses of the North Range host millimeter-thick veins and centimeter-thick lenses of pseudotachylyte, as well as substantially larger meter-wide, dykelike bodies of pseudotachylytic 'breccia'. The 'breccia' occurs up to several tens of kilometers away from the Sudbury Igneous Complex and is commonly sited within or near joints and other natural weaknesses such as bedding, dyke contacts, and lithological boundaries. The larger 'breccia' dykes comprise a generally dark matrix containing rounded to subrounded and occasionally angular rock fragments derived predominantly from Levack Gneiss. Selected samples of bulk Sudbury Breccia and Sudbury Breccia matrices were chemically analyzed and compared to existing data on the Levack Gneisses and Sudbury Breccia. The matrices are apparently enriched in Fe and, to a lesser extent, Mg, Ti, and Ca compared to the wallrocks and the majority of clasts. This enrichment can be partly explained by the preferential cataclasis and/or frictional melting of hydrous ferromagnesian wallrock minerals, but also appear to require contamination by more basic exotic lithologies. This suggests that certain components of pseudotachylitic Sudbury Breccia have undergone significant transport during their formation.

Thompson, Lucy M.↗

NASA NTRS1983

Ion microprobe zircon geochronology of the Uivak Gneisses: Implications for the evolution of early terrestrial crust in the North Atlantic Craton

Ion microprobe U-Pb results for zircons from three Uivak I gneisses and one specimen of Uivak II gneiss, from the Saglek-Hebron area of Northern Labrador are reported. These results are compared with interpretations based on published conventional U-Pb zircon results and with conclusions about crustal evolution in the NAC derived from Rb-Sr, Sm-Nd and Pb-Pb isotopic studies.

Collerson, K. D.↗

NASA NTRS1988

Growth of early archaean crust in the ancient Gneiss complex of Swaziland and adjacent Barberton Greenstone Belt, Southern Africa

The relationship between early Archean greenstones and high grade gneisses in the Ancient Gneiss Complex of Swaziland and the neighboring Barberton greenstone belt in Southern Africa is discussed. New high precision zircon analyses reveal a complex history in individual zircons from tonalitic orthogneisses, with ages as old as 3644 + 4 Ma. This suggests the presence of continental crust prior to the formation of the supracrustal rocks of the Barberton greenstone belt, which have been previously considered the earliest rocks in the area. The author suggested that these data are incompatible with the intraoceanic settings that have been widely accepted for this terrane, and favors either a marginal basin or rift environment. By using the detailed age information obtained from zircons in combination with Ar-40 and Ar-39 and paleomagnetic measurements, the author estimated that plate velocities for this part of Africa craton were about 10 to 70 mm/yr, during the period 3.4 to 2.5 Ga. This is not incompatible with the idea that Archean plate velocities may have been similiar to those of today.

Kroener, A.↗

NASA NTRS1983

Sm-Nd isotopic systematics of the ancient Gneiss complex, southern Africa

In order to shed some new light on the question of the absolute and relative ages of the Ancient Gneiss Complex and Onverwacht Group, a Sm-Nd whole-rock and mineral isochron study of the AGC was begun. At this point, the whole-rock study of samples from the Bimodal Suite selected from those studied for their geochemical characteristics by Hunter et al., is completed. These results and their implications for the chronologic evolution of the Kaapvaal craton and the sources of these ancient rocks are discussed.

Carlson, R. W.↗

NASA NTRS1984

Micas in experimentally shocked gneiss

Powder-propellant guns are used to shock biotites and muscovites from a gneiss at pressures between 18 and 70 GPa. It is shown that shock in biotite and muscovite can produce homogeneous and devolatilized glasses within microseconds. Shock-deformed micas are found to exhibit fracturing, kinking, and complex extinction patterns over the entire pressure range investigated. Localized melting of micas commences at 33 GPa and reaches completion at 70 GPa. Even though melted biotite and muscovite are opaque optically, they exhibit extensive microvesiculation and flow when observed with the SEM. It is confirmed through electron diffraction that biotite and muscovite have transformed to a glass. The distribution of vesicles in shock-vitrified mica reveals escape of volatiles within the short duration of the shock experiment. It is noted that experimentally shocked biotite and muscovite undergo congruent melting. It is noted that the compositions of the glasses are similar to the unshocked micas except for volatiles (H2O loss and and K loss). These unusual glasses produced from mica can be quenched by rapid cooling conditions during the shock experiment. On the basis of the results, it is pointed out that the extremely low H2O content of tektites can be reconciled with a terrestrial origin by impact.

Lambert, P.↗

NASA NTRS1986

Strontium and neodymium isotopic variations in early Archean gneisses affected by middle to late Archean high-grade metamorphic processes: West Greenland and Labrador

Relicts of continental crust formed more than 3400 Ma ago are preserved fortuitously in most cratons. The cratons provide the most direct information about crust and mantle evolutionary processes during the first billion years of Earth history. In view of their polymetamorphic character, these terrains are commonly affected by subsequent tectonothermal events. Hence, their isotope systematics may be severely disturbed as a result of bulk chemical change or local isotopic homogenization. This leads to equivocal age and source information for different components within these terrains. The Sr and Nd isotopic data are presented for early Archean gneisses from the North Atlantic Craton in west Greenland and northern Labrador which were affected by younger metamorphic events.

Collerson, K. D.↗

NASA NTRS1988

Gneiss-charnockite-granite connection in the archaean crust of Karnataka Craton, India

It is explained that the big picture of the Indian crust contains essentially three components: (1) the upper crust, which is composed of platformal sedimentary sequences (including banded iron formations), (2) the middle crust, which is represented by the Peninsular gneiss, and (3) the deep crust, which is composed predominantly of charnockites. The charnockitization controversy was addressed by stating that CO2 is absolutely not essential for charnockite formation, and it is suggested rather that molecular hydrogen movements play an important role.

Ananthaiyer, G. V.↗