Showing papers on "Granulite published in 1983"

Origin of Mesozoic and Tertiary granite in the western United States and implications for Pre‐Mesozoic crustal structure: 1. Nd and Sr isotopic studies in the geocline of the Northern Great Basin

Abstract: Mesozoic and Tertiary granitic rocks in and adjacent to the northern Great Basin (NGB) in Nevada and Utah display a wide range of initial 143Nd/144Nd (eND) and 87Sr/86Sr (eSr) values which vary regularly with geographic position. From the Klamath Mountains inland 500 km to central Nevada, granite eNd values decrease regularly from +8 to −6 and correlate with eSr values that increase from −20 to +60. In east–central Nevada, near the trace of the Roberts Mountains Thrust (RMT), the eND values decrease from −6 to an average of −18, while eSr becomes highly variable with values generally greater than +100. These isotopic discontinuities correspond to the west-to-east facies transition from pelagic clastic sedimentary rocks to shelf carbonates and to the shift in the dominant granite bulk composition from metaluminous to peraluminous. In the easternmost NGB a second discontinuity in esr occurs with values dropping to ∼+60; average eND remains at −18. Combined with known aspects of NGB geology the isotopic data suggest that west of the RMT, granites formed via interaction of magma derived from a LREE-depleted mantle reservoir with an inland increasing proportion of assimilated continentally derived pelagic sedimentary rock. Variations in 87Rb/86Sr with Sr, and 147Sm/144Nd with eND, indicate that crystal fractionation accompanied assimilation, but that plagioclase was not an important fractionating phase. East of the RMT, granites appear to be primarily derived from Precambrian continental basement with little mantle input. The isotopic discontinuities near the RMT mark the western edge of Precambrian basement and occur 100–200 km east of the 87Sr/86Sr (=0.7060) line of Kistler and Peterman [1973]. The eSr discontinuity in the eastern NGB marks a boundary between Rb-depleted (granulite?) lower continental crust to the east, and basement that has no ‘depleted’ lower crust to the west. The difference may be a result of lateral variations in metamorphic grade in the orogenic episode at ∼1.7 b.y. ago or a result of ductile attenuation of the lower crust during the late Precambrian continental rifting. The basement age appears to be intermediate between that of Wyoming (2.6 b.y.) and Colorado (1.8 b.y.) but cannot be precisely determined from the granite Nd model ages due to changes in Sm/Nd during granite magma generation. In contrast to the eastern NGB granites, Mesozoic batholith granites of the western United States have a lowest eND of only −8 even where thought to have been emplaced in crystalline basement, a difference which could be related to an inland decreasing flux of mantle-derived magma into the continental crust in the Mesozoic. A model for subduction-induced flow in the upper mantle beneath the continent, resulting in perturbations in the thermal structure of the continental lithosphere, is presented to account, in part, for the spatial difference in mantle magma flux and the distribution of silicic magmatism in the western United States. Overall, the NGB granite data support the concept that mantle magmas are added to continents mainly at the margin, with little mantle magma generation, or continental growth, in the interior, and demonstrate the potential of Nd and Sr isotopic data from granites in discerning fundamental features of deep continental crust.

Density constraints on the formation of the continental Moho and crust

Abstract: The densities of mantle magmas such as MORB-like tholeiites, picrites, and komatiites at 10 kilobars are greater than densities for diorites, quartz diorites, granodiorites, and granites which dominate the continental crust. Because of these density relations primary magmas from the mantle will tend to underplate the base of the continental crust. Magmas ranging in composition from tholeiites which are more evolved than MORB to andesite can have densities which are less than rocks of the continental crust at 10 kilobars, particularly if they have high water contents. The continental crust can thus be a density filter through which only evolved magmas containing H2O may pass. This explains why primary magmas from the mantle such as the picrites are so rare. Both the over-accretion (i.e., Moho penetration) and the under-accretion (i.e., Moho underplating) of magmas can readily explain complexities in the lithological characteristics of the continental Moho and lower crust. Underplating of the continental crust by dense magmas may perturb the geotherm to values which are characteristic of those in granulite to greenschist facies metamorphic sequences in orogenic belts. An Archean continental crust floating on top of a magma flood or ocean of tholeiite to komatiite could have undergone a major cleansing process; dense blocks of peridotite, greenstone, and high density sediments such as iron formation could have been returned to the mantle, granites sweated to high crustal levels, and a high grade felsic basement residue established.

Titanium, aluminum and interlayer cation substitutions in biotite from high-grade gneisses, West Greenland

Abstract: Detailed electron microprobe study, pelitic to mafic metamorphic rocks from two localities. Higher Ti, K/(K + Na), and possible (K + Na), similar Al IV , and lower Al VI in granulite vs. amphibolite grade samples.--Modified journal abstract.

Kinetics and equilibrium in mineral reactions

Abstract: 1. Compositional Zoning of Crystals: A Record of Growth and Reaction History.- 2. Exsolution and Fe2+-Mg Order-Disorder in Pyroxenes.- 3. Geospeedometry: An Extension of Geothermometry.- 4. Mg-Fe Fractionation in Metamorphic Environments.- 5. Geobarometry in Granulites.- 6. The Cordierite-Garnet-Sillimanite-Quartz Equilibrium: Experiments and Applications.- 7. Experimental Investigation of Exchange Equilibria in the System Cordierite-Garnet-Biotite.- 8. Thermodynamics of Complex Phases.

Experimental investigation and application of garnet granulite equilibria

Abstract: Two mineralogic geobarometers based on the assemblages olivine-plagioclase-garnet and orthopyroxeneplagioclase-garnet-quartz have been calibrated from the reaction (1) fayalite+anorthite⇋garnet (Gr1Alm2). The reaction boundary has been determined to within 0.2 kbar using piston-cylinder apparatus. It is located at 4.7, 5.1, 5.5, 5.8, 6.2, 6.6, and 7.0 kbar at 750, 800, 850, 900, 950, 1,000, and 1,050° C, respectively. Summation of ΔG for reaction (1) and fayalite +quartz⇋ferrosilite locates to within 0.3 kbar the following model garnet-forming reaction for quartz-saturated granulites: (2) ferrosilite+ anorthite⇋garnet(Gr1 Alm2) + quartz. Geobarometers based on (1) and (2) are widely applicable in granulite terranes and yield precise pressures that are in agreement with other well-calibrated barometers. Pressures of 7–10 kbar are inferred for many granulite terranes requiring the widespread development of 60–70 km thick continental crust by mid-Proterozoic.

Fluid inclusions in high-grade anatectic metamorphites

Abstract: Fluid inclusions have been studied, by microthermometry, in metapelites and metabasites which have undergone partial melting in the transition zone from amphibolite to granulite facies. Mobilizates contain far more abundant and larger fluid inclusions than non-mobilized or restite layers, and these inclusions occur as several generations of well-defined and chemically contrasted fluids: carbonic (CO 2 ± CH 4 and hydrocarbon), aqueous (H 2 O ± NaCl) and N 2 . A garnet-bearing mobilizate in amphibolite contains 4 generations of relatively pure fluids, most of them trapped after peak metamorphic conditions in the following P - T domains: CO 2 and N 2 . P = 2–8 kbar, T = 550–850°C; CH 4 ., P = 1–2 kbar, T = 400–550°C; H 2 O, P T = 120–400° C.

Regional geothermobarometry in the granulite facies terrane of South India

Abstract: In 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 upper mantle and deep crust beneath the British Isles: evidence from inclusions in volcanic rocks

Abstract: Coarse-grained inclusions in Upper Palaeozoic alkali basalts, occurring in over 60 localities in the British Isles, include samples of the underlying mantle and lower crust. The ultramafic assemblages imply an extremely heterogeneous upper mantle involving a tectonized spinel lherzolite and harzburgite matrix transected by younger bodies of wehrlite, websterite, clinopyroxenite, orthopyroxenite, garnet-pyroxenite and assorted kaersutite- and biotite-rich ultramafites. Associated megacrysts include anorthoclase, sanidine, clinopyroxene, kaersutite, Ti-biotite, garnet, Ti-magnetite, Mg-ilmenite, apatite, zircon and corundum. These originated in part from disaggregation of pegmatoidal bodies of alkaline felsic rocks, pyroxenites, lherzites and glimmerites. Plagioclase-pyroxene rocks (basic granulites) are probably samples of the lower crust, with quartzo-feldspathic gneisses possibly derived from shallower levels in a chemically-zoned crust. Other inclusions described include apatite-magnetite rocks, biotite- and hornblende-albite rocks and unfoliated tonalites, diorites, trondhjemites and granites. A high-grade (granulite-facies) feldspathic basement is inferred to underlie all the major N British structural provinces, including the Midland Valley and Southern Uplands. Anorthosite xenoliths imply that anorthositic rocks compose some part of the 9basement9 in S and central Scotland.

Metamorphic and tectonic evolution of granulites, Arunta Block, central Australia

Abstract: Petrographical and chemical evidence presented here indicate a prolonged metamorphic evolution of the granulite terrain in the northern Strangways Range, Arunta Block, central Australia (NSR in Fig. 1), beginning with migmatization, followed by dry granulite metamorphism at high temperatures (T) and moderate pressures (P) (estimated at 850–920 °C, 8±1 kbar1). This occurred at ∼1,800 Myr (refs 2, 3), very shortly after deposition of the protolith4, a bimodal volcanic suite5 and associated sediments. Granulite metamorphism was followed by isobaric cooling, during which there was an episode of pervasive but partial hydration. Later retrogression and hydration, localized along shear zones, occurred at the normal geothermal gradient in the kyanite stability field. A tensional, rift-like regime best describes the geological environment in which the protolith formed, was deeply buried, metamorphosed with high heat flow, and then remained in isostatic equilibrium. Metamorphic parageneses recording uplift may be matched with mid-Palaeozoic tectonic events in central Australia. If this correlation is correct, the rocks of the northern Strangways Range may have remained deeply buried for 1,400 Myr, during which the shear zones were reactivated several times (Fig. 2).

A Sm-Nd isotopic study of the South Harris Igneous Complex, the Outer Hebrides

Abstract: Sm-Nd geochronology may be used to bracket the age of metamorphism in rocks which are difficult to date by other methods. By coupling whole rock Sm-Nd analyses of the principal members of the South Harris Igneous Complex, with Sm-Nd mineral isochrons on two anothositic gabbros, the age of granulite facios metamorphism has been defined. Whole rock analyses of three pairs of closely spaced samples of the anorthosite give consistent ages averaging 2.18±0.06 Gyr, but in general the data from the anorthosite do not define an isochron as a result of variable contamination of the evolving magma chamber. Whole rock data on the tonalite indicate that it is younger than 2.06 Gyr; its mean TCHUR age is 1.86±0.05 Gyr. Garnet-pyroxene-amphibole-plagioclase mineral isochrons on two anorthosite samples give identical 1.87±0.04 Gyr ages which date cooling after the high pressure granulite facies metamorphism. Together with the tonalite whole rock data this defines the age of that metamorphism and confirms Dearnley's original assignment of an early Laxfordian age.

Metamorphism and tectonics of the Himalaya

Abstract: Metamorphism connected with the main growth and deformation stages of the Himalaya ranges from mid-Cretaceous to the Quaternary. Northward subduction gave rise in the early to mid-Cretaceous to an island are complex with greenschists, amphibolites, granulites and blueschists in the W (Kohistan-Ladakh) and an Andean-type margin with greenschist-amphibolite grade metamorphism in the E (India and Tibet). Ophiolitic nappes were thrust southwards over the Indian continental margin in the Palaeogene during the final stages of closure of the southern arm of Tethys. The Indus-Zangbo Suture bifurcates westwards and between the two sutures lies the Kohistan-Ladakh island arc which was converted to an Andean-type arc in the Palaeocene-Eocene. The Karakorum Range underwent northward subduction at least during the late Cretaceous to give rise to a complex and prominent calc-alkaline batholith. Post-collisional southward thrusting of crustal slabs over the Indian continental margin took place in the early Miocene with production of high-grade metamorphism, inverted isograds and crustal melt granites. Later southward thrusting in the Pliocene to Quaternary gave rise to localized low-grade recrystallization against the thrusts.

Metamorphism, Partial Melting, and K-Metasomatism of Garnet-Scapolite-Kyanite Granulite Xenoliths from Lashaine, Tanzania

Abstract: Xenoliths of garnet-plagioclase clinopyroxenite, garnet websterite, olivine websterite, and garnet anorthosite are relics of an igneous suite of olivine-normative alkali gabbros metamorphosed into granulites under Lashaine, Tanzania, at ∼1200 K and 14 kb. Most clinopyroxene megacrysts recrystallized into polygonal clusters of small grains, and plagioclase laths exsolved from the cores. Clinopyroxene (CATs 11 mole %, Jd 15) and plagioclase (An28–41, Or1–2) reacted into atolls of garnet. Meionitic scapolite with widely variable sulfate/carbonate developed from plagioclase, oxidized sulfides and CO2. Needles of bent, multiply-twinned kyanite pervade the plagioclase. Prolonged metamorphism homogenized most minerals, including plagioclase next to scapolite. Brown glasses (SiO2 38–56 wt %, Al2O3 15–23, K2O 0.5–8) and dark alteration products mainly occur as rims to garnet, clinopyroxene, and scapolite. One glass pocket with quench plagioclase and hollow clinopyroxene contains two glass populations attributed to...

Metamorphic fluids in the deep crust: evidence from the Adirondacks

Abstract: Unusual mineral assemblages of tremolitic hornblende + diopside+enstatite+quartz have been examined to estimate the temperature (T) and water fugacity ( ) at the time of the granulite fades metamorphism in the Adirondacks. The pyroxenes are closely represented by the system CaO–MgO–SiO2 allowing application of experimental data1 with only small corrections for reduced values of activities. Tremolite equilibria2 buffered log = 3.0±0.1, equivalent to 0.9 kbar of water pressure ( ) at 7 kbar, 710 °C. Nearby marbles contain wollastonite+calcite+quartz and buffered = 1.1 kbar, consistent with ⩽5.9 kbar. Inferred local gradients in fluid compositions suggest that deep crustal fluid conditions are complex and heterogeneous indicating that fluid movements, whether of mantle3 or deep crustal origin, are channelized rather than pervasive. Thus some granulite facies rocks were wholly or partly closed systems with respect to externally derived fluids permitting localized buffering of fluid compositions. Recognition of restricted fluid-mixing limits theories that call for massive amounts of pervasive fluid flow to facilitate melting, cause metasomatism, or to stabilize granulite facies mineralogy.

Crustal growth in south‐eastern Australia—evidence from lower crustal eclogitic and granulitic xenoliths

Abstract: Mafic and ultramafic xenoliths in a basaltic cone at The Anakies in south-eastern Australia are geochemically equivalent to continental basaltic magmas and cumulates. The xenolith microstructures range from recognizably meta-igneous for intrusive rocks to granoblastic for garnet pyroxenites. Contact relationships between different rock types within some xenoliths suggest a complex petrogenesis of multiple intrusive, metamorphic and metasomatic events at the crust/mantle boundary during the evolution of south-eastern Australia. Unaltered spinel lher-zolite, typical of the uppermost eastern Australian mantle, is interleaved with or veined by the metamorphosed intrusive rocks of basaltic composition. Geothermobarometry calculations by a variety of methods show a concordance of equilibration temperatures ranging from 880°C to 980°C and pressures of 12 to 18 kbar (1200-1800 mPa). These physical conditions span the gabbro to granulite to eclogite transition boundaries. The water-vapour pressure during equilibration is estimated to be about 0.5% of the load pressure, using amphibole breakdown data. Large fluid inclusions of pure CO2 are abundant in the mineral phases in the xenoliths, and it is suggested that flux of CO2 from the mantle has been an important heat source and fluid medium during metamorphism of the mafic and ultramafic protoliths at the lower crust/upper mantle boundary. The calculated pressures and temperatures suggest that the south-eastern Australian crust has sustained a high geothermal gradient. In addition, the nature of the mineral assemblages and the contact relationships of granulitic rock with spinel lherzolite, characteristic of mantle material, suggest that the Moho is not a discrete feature in this region, but is represented by a transition zone approximately 20 km thick. These inferences are in agreement with geophysical data (including seismic, heat-flow and electrical resistivity data) determined for south-eastern Australia. Underplating at the crust/mantle boundary by continental basaltic magmas may be an important alternative or additional mechanism to the conventional andesite model for crustal accretion.

Geobarometry in Granulites

Abstract: It is apparent from the initial attempts at quantitative tectonic modeling of regional metamorphism (England and Richardson, 1977; Wells, 1980) that a detailed knowledge of metamorphic pressure is essential to the accurate formulation of any tectonic model attempting to incorporate metamorphic pressure—temperature—time relationships. There is, therefore, a need for pressure data of greater accuracy and precision than has been available. Of greatest importance is knowledge not only of the so-called peak metamorphic pressures but also regional variations in the inferred maximum pressures, as well as the prograde and retrograde pressure path of rocks, especially of those that were once buried to the deepest levels of the crust. Unfortunately, at least some of the information necessary to infer the prograde metamorphic path is destroyed by the metamorphic process itself, although in a few terranes, limited data on the prograde pressure-temperature path can be inferred (e.g., Tracy et al., 1976; Phillips and Wall, 1981). Ability to infer such data depends in part on availability of numerous well-calibrated geobarometers applicable in a variety of bulk compositions.

Geochronology and geological evolution of metamorphic rocks in the field islands area, east antarctica

Abstract: Detailed geochronological, structural and petrological studies reveal that the geological evolution of the Field Islands area, East Antarctica, was substantially similar to that of the adjacent Archaean Napier Complex, though with notable differences in late and post Archaean times. These differences reflect the area's proximity to the Proterozoic Rayner Complex and consequent vulnerability to tectonic process involved in the formation of the latter. Distinctive structural features of the Field Islands are (1) consistent development of a discordant, pervasive S3 axial-plane foliation; (2) re-orientation of S3 axial planes to approximate to the subsequent E-W tectonic trend of the nearby Rayner Complex; (3) selective retrogression by a post-D3 static thermal overprint; and (4) relatively common development of retrogressive, E-W-trending, mylonitic shear zones. Peak metamorphic conditions in excess of 800°C at 900 ± 100 M Pa (9 kbar) were attained at one locality following, but probably close to the time of D2 folding. D3 took place in late Archaean times when metamorphic temperatures were about 650°C and pressures were about 600 MPa (6 kbar). Later, temperatures of 600 ± 50°C and pressures of 700 MPa (7kbar) were attained in an amphibolite-facies event, presumably associated with the widespread granulite to amphibolite-facies metamorphism and intense deformation involved in the formation of the Rayner Complex at about 1100 Ma. The area was subsequently subjected to near-isothermal uplift. Rb-Sr isotopic data indicate that the pervasive D3 fabric developed at about 2400–2500 Ma, and this age can be further refined to 2456+8-5 Ma by concordant zircon analyses from a syn-D3 pegmatite. All zircons were affected by only minor (<7–10%) Pb loss and/or new zircon growth during the Rayner event at about 1100Ma. Thus the 450–850 μg/gU concentrations of these zircons were too low to cause sufficient lattice damage over the 1350 Ma (from 2450 Ma) for excessive Pb to be lost during the 1100 Ma event. The emplacement of pegmatite at 522 ± 10 Ma substantially changed the Rb-Sr systematics of the only analysed rock that developed a penetrative fabric during the 1100 Ma event. Monazite in this pegmatite contains an inherited Pb component, which probably resides in small opaque inclusions. A good correlation is found between Rb-Sr total-rock ages and rock fabric. U-Pb zircon intercepts with concordia also mostly correspond to known events. However, in one example a near perfect alignment of zircon analyses, probably developed by mixing of unrelated components, produced concordia intercepts that appear to have no direct geochronological significance.

Deep crustal carbonates as CO 2 fluid sources: Evidence from metasomatic reaction zones

Abstract: Metasomatic reaction zones which developed at marble-pelitic schist contacts in a granulite facies terrane in West Greenland contain a consistent sequence of five mineralogical zones. Outward from the carbonates the zones are characterized by the assemblages grossular-diopside-meionite (I), meionite-anorthite-diopside (II), anorthite-diopside-edenitic hornblende (III), anorthite-enstatite (IV), plagioclase-almandine-sillimanite (V). Sphene is superceded by ilmenite between zones (II) and (III); quartz is present in all zones except zone I. Scapolite, plagioclase, clinopyroxene and mica exhibit a small degree of compositional variation which correlates with distance from the carbonate. These small compositional variations are superimposed on a strong CaO chemical potential gradient. Compositional features, zone distributions and CaO activity calculations demonstrate that the zones developed in response to CaO diffusion along a chemical potential gradient of 2 kcal/m. The CaO source appears to be carbonate rocks which release calcium as decarbonation reactions proceed. The maximum volume of CO2 released in this process, and that released during discontinuous reactions in the marbles, will contribute a total volume of CO2 approximately equivalent to the volume of carbonate in the rock. Calculations demonstrate that a terrane consisting of as little as 8% carbonate will release sufficient CO2 to result in complete dehydration of an amphibolite terrane, at deep crustal conditions. Dehydration through CO2 release will be accomplished either through rapid burial, which would prevent both equilibration of mineral assemblages and CO2 release at intermediate crustal levels, or through diffusion-driven metasomatic reactions which would lead to CO2 release primarily at the high temperatures of deep crustal environments. The latter process would be the dominant CO2 source at deep crustal levels if carbonate rocks occur predominately as relatively thin layers.

A corundum and sapphirine paragenesis from the Limpopo Mobile Belt, southern Africa

Abstract: An assemblage consisting of corundum, sapphirine, spinel, cordierite, garnet, biotite and bronzite is described from the Messina area of the Limpopo Mobile Belt, and consideration given to its petrogenesis. Various geothermometers and geobarometers have been applied in an attempt to determine the temperatures and pressures of metamorphism. A former coexistence of garnet and corundum is suggested to have developed during the earliest high pressure phase of the metamorphism, where temperatures exceeded 800°C and pressures as high as 10kbar may have been experienced. Subsequently, continuous retrograding reactions from medium pressure granulite facies at about 800°C and 8kbar towards amphibolite facies generated spinel, cordierite, sapphirine and possibly also bronzite. The most notable reaction was probably of the form: garnet + corundum = cordierite + sapphirine + spinel.

Petrochemistry and recrystallization history of granulite xenoliths from the Pali-Aike volcanic field, Chile

Abstract: Mafic granulite facies xenoliths found in the Pleistocene-Recent alkali basalts of southern Patagonia, South America, have an unfoliated granoblastic matrix with the mineral assemblage clinopyroxene + orthopyroxene + plagioclase + or - olivine + or - titanomagnetite. Wherever olivine and plagioclase are in close proximity to one another they are separated by symplectic intergrowths of pyroxenes and spinel. Two-pyroxene geothermometry indicates that the granoblastic matrix phases equilibrated at approximately 900 degrees C. The pyroxene-spinel symplectites formed at 860 degrees C, at which temperature the reaction boundary of olivine + plagioclase to produce pyroxenes + spinel occurs at 5-7 kbar. The xenoliths are interpreted as fragments of a gabbroic pluton which exists at a depth of approximately 20 km in the crust. This is consistent with suggestions that mafic igneous rocks comprise a large portion of the lower continental crust. Capture of the xenoliths in the host basalt must have resulted in extensive heating, but the petrologic evidence of this event is restricted to heterogeneously distributed high temperature oxidation effects.--Modified journal abstract.

Chemical Distinction Between Igneous and Metamorphic Orthopyroxenes Especially Those Coexisting with Ca-rich Clinopyroxenes: A Re-Evaluation

Abstract: A previously suggested simple relationship in a diagram of(A1203 vs. MgO + FeO + Fe203)op x for the distinction between igneous and metamorphic ortho- pyroxenes is shown to be inadequate. A revised diagram is presented and its limitations are discussed. A diagram showing Fe 2 +/(Fe 2 § + Mg) vs. 100Ca/(Fe 2 § + Mg + Ca) for orthopyroxenes coexisting with Ca-rich clinopyro- xenes proves to be useful for the discrimination of igneous and metamorphic orthopyroxenes and for evaluation of the extent and physical conditions of regional orthopyro- xene re-equilibration. IN charnockite migmatite terranes textures of basic rocks (e.g. norites, pyroxene granulites) are often inconclusive for discriminating between an igneous or high-grade metamorphic origin. Bhatta- charyya (1971) suggested that such a distinction could be made on the basis of the orthopyroxene composition alone. In a diagram of MgO + FeO + Fe203 vs. A1203 (wt. ~) fields for igneous and metamorphic orthopyroxenes are separated by a clearly defined line. Bhattacharyya observed that complications may arise due to host-rock bulk chemistry. He also noted that in thermally and in some regionally metamorphosed rocks, retention of the igneous compositions may be found. In the past decade a great number of pyroxene analyses have become available. Therefore it was felt necessary to update Bhattacharyya's diagram by supplementing his data with recent data for orthopyroxenes of known origin (excluding volcanic occurrences), because it may present a quick method of elucidating the origin of orthopyroxene-bearing rocks. Re-evaluation of the diagram shows its limita- tions and alternatives are explored.

Uranium enrichment in Archaean crustal basement associated with overthrusting

Abstract: Uranium deposits cumulatively in the 100,000 tonne U3O8 range occur within ductile shear zones transecting Archaean basement gneisses at Lagoa Real, north-east of Caetite, State of Bahia, Brazil1–3 (Fig. 1). The gneisses, dated at 2,600–3,000 Myr, are at amphibolite to granulite facies of meta-morphism. There is a general concensus that such rocks form near the base of the crust, and typically exhibit pronounced depletion of U relative to the upper crust5,6. To the east and west of Lagoa Real the Archaean gneisses overlie Proterozoic meta sediments and volcanics of the Espinhaco supergroup along well documented thrust surfaces7,8. Limited U–Pb dating of the deposits indicates that U-mineralization occured at ∼820 Myr (ref. 9), involving removal of Si and K coupled with Na addition in oxidizing conditions. V, Pb and light rare-earth elements (LREE) were introduced along with uranium, from hypersaline CO2-hydrocarbon-rich fluids, with δ18O–4‰ at temperatures of ∼540°C, depths of ∼15 km, and Pfluid > Pload. We show here that the geological constraints are consistent with release, through hydrofracturing, of ∼1,000 km3 of over-pressurized-meteoric water-recharged formation brines in the Proterozoic sediments up through the hot overriding gneisses.

The Seve—Köli Nappe Complex of the Handöl—Storlien—Essandsjøen area, Scandinavian Caledonides

Abstract: The Seve Nappes of the area comprise three tectonic units, the Blahammarfjallet Nappe overlain by the Snasahogarna Nappe and the Taljstensvalen Complex. The Blahammarfjallet Nappe is dominated by schists and amphibolites; they exhibit an inverted metamorphic trend ranging from upper greenschist to middle or upper amphibolite facies. The overlying Snasahogarna Nappe mainly consists of granulite facies K-feldspar-sillimanite gneisses. The Taljstensvalen Complex comprises amphibolites and kyanite (± staurolite) gneisses and schists. It contains imbricated slices from the underlying and possibly also from the overlying units. The Seve Nappes are overlain by Koli units, the lower of these being the Handol Formation, comprising calcareous mica schists, metavolcanites, metagabbro and ultramafic rocks. This unit is overlain by the more extensive Visjon Formation, dominated by garbenschiefer. The grade of both units is generally upper greenschist to lowermost amphibolite facies. In the Essandsjoen area, t...