Showing papers on "Granulite published in 1986"

Precambrian Continental Crust of India and Its Evolution

Abstract: The Indian Precambrian continental crust exhibits a variety of geological features fashioned at different times by different geotectonic processes. The bulk of this crust was formed prior to 2600 m.y. ago and remobilized at least twice between 2600-2000 m.y. ago (early Proterozoic Mobile Belt, EPMB) and 2000-1500 m.y. ago (middle Proterozoic Mobile Belt MPMB). Three early Precambrian nucleii: Karnataka (KN), Jeypore-Bastar (JBN), and Singhbhum (SN) appear to have survived in the craton and are characterized by low-grade supracrustals and tonalitic trondhjemite gneisses, formed 3800-2600 m.y. ago. The EPMB event involved sedimentation, amphibolite-granulite facies metamorphism, and $$CO_{2}-K$$ metasomatism and produced amphibolite facies rocks and K-granites in the north, and charnockite and other granulite facies rocks in the south. Gold sporadically distributed in the supracrustal rocks of the craton was remobilized during the EPMB event. K-granites form a garland around the central Dharwar craton, sugg...

Melt-enhanced deformation: A major tectonic process

Abstract: Convergent tectonics between continental crustal blocks result in deep burial and anatexis of supracrustal rocks. Anatectic and/or mantle-derived melts combine to form a melt-weakened zone in the thickened lower crust. The accumulation of melt eventually leads to crustal failure along melt-lubricated shear zones. Rapid (>1 mm/yr vertical component) movements of large crustal blocks result. We refer to these crustal displacements as tectonic surges. The melt-lubricated shears are characterized by the close association of sheared country rocks with foliated or massive igneous sills and plutons. Rocks that formed at different crustal levels are juxtaposed across these shear zones. One result of surges with large lateral component of movement is metamorphic inversion with high-pressure and high-temperature metamorphic rocks structurally over lower pressure and temperature assemblages. Large and rapid vertical surges may displace crust containing abundant melt and may result in high T/P (including granulite facies) metamorphism and preservation of metamorphic textures caused by rapid decompression.

Four zircon ages from one rock: the history of a 3930 Ma-old granulite from Mount Sones, Enderby Land, Antarctica

Abstract: Ion microprobe U-Th-Pb analyses of zircons from a granulite-grade orthogneiss from Mount Sones, Enderby Land, Antarctica, record the ages of four principal events in the history of the gneiss, three of which already have been recognized through previous isotopic dating of other samples. The structure of the zircons indicates at least four different stages of growth. The several zircon ages were obtained by grouping the analyses according to the stage they represented in the observed “stratigraphic succession” of growth and thereby defining separate U-Pb discordance patterns for each stage. The stratigraphically oldest zircon (rare discrete cores) is indistinguishable in age from the most common, euhedrally zoned zircon. Both crystallized when the tonalitic precursor of the orthogneiss was emplaced into the crust 3927±10 Ma ago, making the orthogneiss currently the oldest known terrestrial rock. The outer parts of most grains and some whole grains recrystallized at 2948±31/−17 Ma, during or immediately after possibly ∼100 Ma of high granulite grade metamorphism. The recrystallized zircon was isotopically disturbed by tectonism associated with reactivation of the southern margin of the Napier Complex at ∼1000 Ma. In the intervening time, at 2479±23 Ma, the cores and zoned zircon suffered a major isotopic disturbance involving movement of radiogenic Pb which left most of the crystals with radiogenic Pb deficiencies, but produced local radiogenic Pb excesses in others. A new generation of zircon, characterized by very high Th/U and low U, grew at that time. That event — deformation and possibly a minor rise in temperature — produced widespread perturbations of other isotopic systems throughout the Napier Complex.

Progressive Metamorphism of Mafic Rocks from Greenschist to Granulite Facies in the Dharwar Craton of South India

Abstract: In the Archean Dharwar craton of southern India N-S trending belts of metabasic rocks are exposed which underwent regional metamorphism at about 2.5 Ga ago. The progressive changes in the assemblages and mineral chemistry of metabasites was studied in a N-S traverse covering the Chitradurga and Nagaman-gala belts, the Sargur area, and the Nilgiri Hills. Towards the south with increasing metamorphic grade greenschists ($$chl + act + ab + ep \pm carb$$, qtz) give way to amphibolites ($$hbl + plag \pm qtz$$, gar, cumm) and mafic granulites ($$pyx + plag + gar \pm hbl$$, qtz). The amphibole composition changes from actinolite in the greenschist zone to tschermakitic hornblende in the amphibolite zone and pargasitic-hastingsitic hornblende in the granulite zone. The Ti content of the amphiboles systematically increases in this direction. The plagioclase composition changes from albite in the greenschist zone to oligoclase/andesine in the amphibolite zone and andesine/labradorite in the granulite zone. Almandin...

Western Fiordland orthogneiss: Early Cretaceous arc magmatism and granulite facies metamorphism, New Zealand

Abstract: U-Pb isotopic analyses of zircons from a distinctive suite of previously undated granulite facies metaplutonic rocks, here termed the Western Fiordland Orthogneiss (WFO), in Fiordland, southwest New Zealand, indicate synkinematic magmatic emplacement between ∼120 and 130 Ma ago. These rocks were previously interpreted as possibly being of Precambrian age. Initial Pb and Sr ratios are consistent with arc/subduction related magmagenesis with little or no involvement of ancient continental crust. Subsequent high pressure (>12 kb) metamorphism of the WFO may reflect a major collision event involving crustal thickening by overthrusting of a >15 km thick sequence. Metamorphism ceased ≤116 Ma ago based on206Pb/238U ages of zircon from a retrogressed granulite. U-Pb isotopic analysis of apatite, along with previously published Rb/Sr mineral ages, indicate that final uplift and cooling to <300–400° C was largely completed by ∼90 Ma. The average uplift rate during this period is inferred to have been in excess of 1 mm/yr. Unmetamorphosed gabbronorites of the Darran Complex in eastern Fiordland, inferred by some investigators to be the granulite protolith, yield concordant U/Pb zircon ages of 137±1 Ma. U-Pb ages of apatite, and previously published K/Ar mineral ages indicate that these rocks experienced a rapid and simple cooling history lasting only a few million years. The high-grade WFO and unmetamorphosed Darran Complex are now separated by a profound structural break. However, the ages and similarities in initial Pb and Sr isotopic ratios suggest that both suites are products of the same Early Cretaceous cycle of subduction-related magmatism. The timing of Early Cretaceous magmatism and metamorphism, collision and resultant crustal thickening, and subsequent great uplift and erosion in Fiordland has important implications for terrane accretion and the evolution of relative plate motions along the New Zealand segment of the Gondwana margin.

Deep crustal structure and tectonic history of the Northern Kapuskasing Uplift of Ontario: An integrated petrological-geophysical study

Abstract: The northeast trending Kapuskasing uplift transects the east-west belts of the central Superior Province over a distance of some 500 km. Granulite to upper amphibolite facies rocks of the uplift form three distinct geological-geophysical entities: from south to north, the Chapleau, Groundhog River, and Fraserdale-Moosonee blocks. Uplift of the granulites along a moderately northwest dipping crustal-scale thrust fault is attributed to an early Proterozoic compressional event. Major northeast-striking faults that bound the Kapuskasing zone on the west were examined by modelling of geophysical anomalies to determine dip and by geobarometry of garnet-orthopyroxene-plagioclase-quartz assemblages to determine vertical displacement. Granulites in the Kapuskasing zone have 7- to 9-kbar signatures whereas those in the Quetico belt to the west indicate metamorphic pressure of 4–6 kbar. Individual calibrations of the barometer yield consistent pressure differences of 2–3 kbar, suggesting 7–10 km of west-side-down movement on the faults. Modelling of gravity and aeromagnetic gradients indicates westerly dips of 60°–65°, with west-side-down offset of up to 14 km. These major normal faults probably formed as collapse structures in response to crustal thickening which occurred during the preceding compressional uplift stage. Differences in the configuration of individual blocks of the Kapuskasing zone can be related to variable fault slip and intersection angles between normal and reverse faults. Thus the Groundhog River and southern Fraserdale-Moosonee blocks are perched thrust tips analogous to the Sangre de Cristo Range of the Laramide uplift province, whereas the southern Chapleau block is a tilted slab with similar configuration to the Laramide Wind River Range. Pop-up geometry deduced for the northern Fraserdale-Moosonee block resembles the structure of the Laramide Uinta Mountains. A normal fault crosses the surface trace of the basal thrust fault between the Groundhog River and Fraserdale-Moosonee blocks and causes a 65-km “gap” without granulites. This article contains supplementary material.

Lunar granulites and their precursor anorthositic norites of the early lunar crust

Abstract: Lunar granulities, which are ancient and KREEP-free, represent the best samples of early lunar crust. They can be divided into ferroan and magnesium groups, and each group can be subdivided on the basis of mineral composition and REE concentrations. It is shown that some of the granulites may be derived from distinct anorthitic norite precursors, while some others are clearly polymict, though it is believed that even these granulites had anorthositic norites as their dominant precursors. The granulites have compositions similar to those of the two lunar meteorites, one of which is ferroan, the other magnesian. These meteorites are soil breccias from an unknown location distant from the Apollo landing sites and contain anorthositic norites as abundant clasts. Granulite and lunar-meteorite compositions more closely resemble the average composition of lunar highlands than those of any other returned lunar samples. The predominance of plutonic anorthositic norite precursors in material having the composition typical of highlands suggests that plutonic anorthositic norites were more abundant in the early lunar crust than is implied by their scarcity in Apollo pristine rocks.

The thermotectonic evolution of a Proterozoic, low pressure, granulite dome, West Uusimaa, SW Finland

Abstract: Three fold generations have been recognized in Svecofennian rocks (±1,800 Ma) from West Uusimaa, SW Finland. The first one (F1) might be related to thrusting and imbrication tectonics at plate collision contacts. The main generation (F2) is due to a N-S horizontal crustal shortening, which created at first E-W trending upright folds in the whole region and later tightened these F2 folds in the western part of the belt, whereas conjugate shear zones and tectonic lenses of competent rock bodies developed in the eastern part. Simultaneously the metamorphic conditions rose from amphibolite- to granulite-facies in this eastern part, which is known as the West Uusimaa Complex. The amphibolite- to granulite-facies transition zone along the western boundary of the granulite-facies complex is studied in detail. A number of prograde mineral reactions are telescoped in this transition zone: the breakdown of biotite and amphibole to ortho- ±clino-pyroxene in metaigneous rocks, the appearance of garnet in cordierite-bearing metapelites and the appearance of scapolite in calcareous rocks. Distinct mineralogical changes also occur in this zone which cross cuts all major structures and rock units and are only affected by late-F3 folding (open, disharmonic folds with approximately N-S trending axial planes) and young shear zones, associated with pseudotachylite generation. The absence of any evidence of block faulting and tilting of the crust that could be associated with the granulite complex suggests that the whole region represents one crustal level.

Fluids of Granulite Facies Metamorphism

Abstract: Rocks of the granulite facies of metamorphism occupy a central role in discussions of petrogenesis of the crust. Almost all well-studied examples are Precambrian, which has suggested to many workers that metamorphic temperature regimes operating in the remote geological past were higher than are characteristic of more recent metamorphism. Granulites are widely believed to make up much of the deeper parts of the continents (Fountain and Salisbury, 1981). The depletion of some very high-grade granulites in large-ion lithophile (LIL) elements, such as U, Th, and Rb, relative to normal upper crustal rocks, increases the appeal of the granulite lower crustal model, because of the low heat flow in ancient shield areas (Heier, 1973). The dense minerals pyroxene and garnet, characteristic of quartzofeld- spathic granulites, impart elevated densities and seismic velocities, generally appropriate to the lower crust (Smithson and Brown, 1977). Granulite petrogenesis may therefore be fundamental in the accretion and stabilization of the continents.

Structure and U/Pb geochronology of Central Hoggar (Algeria): A reappraisal of its Pan‐African evolution

Abstract: Discovery of large-scale deep-seated thrusts in Central Hoggar, with a plurifacial evolution ranging from lower amphibolite facies to upper greenschist facies conditions and linked to a regional refoliation, has led us to reconsider the Pan-African tectonic and metamorphic history in that region. Two areas are described, and a review of other thrusts leads to an interpretative cross section of a large portion of reactivated continental crust. The age and kinematics of this structural reworking have been approached using U/Pb zircon dating in the Tamanrasset region. Despite the difficulty of estimating the age of the initiation of the assumed intracontinental A-type subduction, the results provide a time span of 30–40 m.y. between the climax of granitoid emplacement and a late retrogressive offset along the thrust planes. Some key ages were determined: (1) 2075 ± 30 Ma is the age given by granulite facies remnants which escaped from the refoliation, the corresponding lower intercept at 530 ± 70 Ma confirms the Pan-African imprint; (2) 615 ± 5 Ma reflecting the age of syntectonic to late-tectonic granitoids emplaced in reworked gneisses and in preserved granulites; (3) 580 Ma, the concordant age of sphenes and monazites from the same granitoids, which is interpreted as corresponding to the end of medium-grade conditions. No evidence has been found of a ∼1000 Ma age: a Kibaran event does not appear to exist in Central Hoggar. The age similarity between the observed deep intracontinental evolution of Central Hoggar and the collision-related tectonics of Western Hoggar and Iforas suggests a common origin for both phenomena.

P?T conditions for the Arendal granulites, southern Norway: implications for the roles of P, T and CO2in deep crustal LILE-depletion

Abstract: The orthopyroxene-clinopyroxene, garnet-orthopyroxene and garnet-clinopyroxene geothermometers, and the garnet-orthopyroxene-plagioclase, garnet-clinopyroxene-plagioclase and anorthite-ferrosilite-grossular-almandine-quartz geobarometers are applied to metabasites and the garnetplagioclase-sillimanite-quartz geobarometer is applied to a metapelite from the Proterozoic Arendal granulite terrain, Bamble sector, Norway. P–T conditions of metamorphism were 7.3 ± 0.5 kbar and 800 ± 60°C. This terrain shows a regional gradation from the amphibolite facies, into normal LILE content granulite facies rocks and finally strongly LILE deficient granulite facies gneisses. Neither P nor T vary significantly across the entire transition zone. The change in ‘grade’parallels the increasing dominance of CO2 over H2O in the fluid phase. LILE-depletion is not a pre-condition of granulite facies metamorphism: granulites may have either ‘depleted’or ‘normal’chemistries. The results presented herein show that LILE-deficiency in granulite facies orthogneisses is not necessarily related to variations in either P or T. The important mechanisms in the Arendal terrain were (a) direct synmetamorphic crystallization from magma, with primary LILE-poor mineralogies imposed by the prevailing fluid regime, and (b) metamorphic depletion, involving scavenging of LILEs during flushing by mantle-derived CO2-rich fluids. The latter process is constrained by U–Pb and Rb–Sr isotopic work to have occurred no later than 50 Ma after intrusion of the acid-intermediate gneisses, and was probably associated with contemporary basic magmatism in a tectonic environment similar to a present day cordilleran continental margin.

Nd and Sr isotopic systematics of central Australian granulites: chronology of crustal development and constraints on the evolution of lower continental crust

Abstract: Nd and Sr isotopic data are reported for a granulite terrain in the Proterozoic Arunta Block of Central Australia. Sm-Nd data from a wide range of rock types define a crust formation age of 2,070±125 Ma and provide further evidence for voluminous crustal growth in the Proterozoic. An ɛ Nd value of +1.5±0.8 indicates a depleted mantle source for this crustal segment and there is no evidence for a large component of significantly older sialic crust. Field relationships, geochemistry and Rb-Sr data for mafic and felsic granulites indicate that intracrustal differentiation and polyphase deformation were followed by granulite facies metamorphism (Rb depletion) at ∼1,800 Ma. Rb-Sr data for strongly retrogressed granulites define an age of ∼1,700 Ma which is interpreted as the time of retrograde biotite growth. Partial melting at the presently exposed crustal level and anatexis at deeper crustal levels were broadly coeval with the retrograde metamorphism. Sm-Nd and Rb-Sr isotopic systematics of minerals indicate that the terrain cooled slowly, did not experience significant uplift until ∼1,000 Ma and remained at temperatures above 320° C until the late Palaeozoic. The mineral data are consistent with geological relationships and petrological evidence for a prolonged period of isobaric cooling followed by uplift late in the metamorphic evolution of the terrain. The granulite protoliths appear to have formed in a rift which closed within ∼280 Ma of initial separation. Deformation and granulite facies metamorphism at ∼1,800 Ma are interpreted to be a consequence of collision between the continental blocks which defined the rift. Regional retrogression and granitoid magmatism at ∼1,700 Ma are attributed to underthrusting of the granulites by lower grade rocks in the final stages of collision. Subsequent events in the cooling and uplift history appear to have been controlled by the presence of long-lived major faults in the crust and a prolonged history of episodic compression in the continental lithosphere. The results of this study suggest that granulite terrains, in general, cannot be equated with lower continental crust but instead represent assemblages of (mainly) supracrustal rocks which in some instances have been involved in major collision events.

Electrical conductivity of water-undersaturated crustal melting

Abstract: Water-undersaturated melting in the crust can occur at lithostatic pressure in the presence of an H2O-CO2 fluid, of no CO2 or fluid but with all H2O bound structurally in hydrous minerals, or of an insufficient amount of H2O fluid to saturate a melt at liquidus temperatures. The composition of any fluid in equilibrium with possible source rocks depends on the metamorphic grade of the rocks; the fluid at lithostatic pressure in ductile granulite facies rocks would be CO2-rich, while an increased fraction of H2O in a fluid is probable in lithologies of lower metamorphic grade or in rocks of the brittle regime. With limited amounts of pore fluid and hydrous minerals, melting at constant pressure is a highly nonlinear function of temperature and may extend over a broad temperature range. Electrical conductivity of the resultant feldspathic liquids can be estimated to first order from early conductivity measurements on granite under conditions of excess water and recent theories on dissolution of H2O in aluminosilicate magmas. Experimental data suggest especially for the deep crust that conductivity of melt in equilibrium with its source residuum falls as melt fraction increases due to dilution of water in spite of increased temperature. At the lesser pressures of high-level magma chambers, pressure-temperature-water content relations of hydrous melt conductivity should be clarified by further laboratory examination. Melt fraction and temperature estimates derived from field electrical surveys are complicated seriously by melt phase tortuosity in the crystalline matrix, by H2O content of the source rock, and by the possibility of an H2O-CO2 fluid.

A sapphirine-cordierite-garnet-sillimanite granulite from enderby land, antarctica - implications for fmas petrogenetic grids in the granulite facies

Abstract: A quartz-absent magnesian paragneiss layer from Mount Sones, in the Archaean Napier complex of Enderby Land, Antarctica, contains the stable divariant FMAS assemblage sapphirine (XMg=78) — cordierite (XMg=87) — garnet (XMg=51) — sillimanite. Rare green spinel (XMg=53.5, ZnO=2.65wt%) occurs as inclusions mainly within sapphirine, but also within sillimanite and garnet. Late thin coronas of cordierite (XMg=90.5) mantle sapphirine in contact with extensively exsolved anorthoclase. The mineral textures are interpreted to indicate the former stability of a hypersthene-quartz absent assemblage followed by the development of the FMAS equilibrium assemblage sapphirine-cordierite-garnet-sillimanite (sp, hy, qz) and further divariant reaction involving the consumption of sapphirine. The (sp, hy, qz) assemblage uniquely defines the stable P-T reaction topology appropriate to granulites from the Napier Complex, as this paragenesis is allowed in the grids of Hensen (1971, 1986) but is not possible in other grids which assume the stability of a sapphirine-absent ([sa]) FMAS invariant point involving the phases spinel, garnet, hypersthene, cordierite, sillimanite and quartz. The observed mineral assemblages and textures are consistent with peak metamorphism between the [sp] and [hy] invariant points of Hensen (1971), at temperatures of 930–990° C, followed by cooling on a lower dP/dT trajectory towards the (sp, qz) univariant line. The initial spinel-bearing assemblage was stabilized by Zn and to a lesser extent by Ni and Cr, and hence does not require a marked decrease in temperature and increase in pressure to produce the (sp, hy, qz) assemblage. It is inferred that fO2 conditions substantially lower than those used in the experiments of Annersten and Seifert (1981) prevailed in the high-grade metamorphism in the Napier Complex.

Formation of sapphirine during retrogression of a basic high-pressure granulite, Roan, Western Gneiss Region, Norway

Abstract: Sapphirine-bearing rocks occur in the northern part of the Western Gneiss Region, Vestranden, central Norway. The sapphirine-bearing rocks are characterized by a high MgO/(MgO + FeO) ratio, high Al2O3, MgO and CaO, and low SiO2 contents. These rocks form layers within larger complexes which originated as layered magmatic rocks. High PT-metamorphism produced a cpx+ky+gt assemblage. The P and T estimates are P = 14.5±2 kbar and T= 870±50° C. During retrogression, the high-P granulite assemblage broke down to form an intermediate-P granulite mineralogy comprising orthopyroxene, spinel, anorthite, andesine, sapphirine and corundum. Textural relationships suggest that sapphirine formed by the reaction: spinel+kyanite → sapphirine+corundum, and probably also by a reaction between corundum, spinel and orthopyroxene. All reactions took place within the stability field of kyanite. Textural and micro-chemical relationships indicate equilibrium, conditions during the peak metamorphism, whereas pronounced disequilibrium characterizes the mineral associations formed during the early retrogression at low PH2O. The investigation shows that parts of the northern segment of the Western Gneiss Region underwent a metamorphic evolution similar to the Caledonian one recorded from eclogite/granulite terrains further south.

The Southern High-Grade Margin of the Dharwar Craton

Abstract: The amphibolite-facies to granulite-facies transition at the southern margin of the Dharwar craton has been studied in the Krishnagiri, Satnur-Halaguru, and Kushalnagar areas of southern India. In all three areas the transition appears to be a progressive metamorphic overprint on cratonal gneisses and their mafic and metasedimentary enclaves, without major structural interruption. In the Kabbal-Satnur-BR Hills section of Karnataka, a high-grade charnockite massif with pronounced Rb depletion is the culmination of an apparently continuous increase of metamorphic grade southward. In this and the Kushalnagar areas, increase of paleopressure from near 6 to near 8 kbar with increasing grade indicates a depth-zone relationship of the amphibolite and granulite facies. Incipient charnockite replacing Peninsular Gneiss first appears along N-S shears parallel to the regional grain of the craton. Low-P($$H_{2}O$$), high $$CO_{2}$$ vapors were instrumental in the creation of orthopyroxene. Introduction from a deep so...

Mechanisms of Charnockite Formation and Breakdown in Southern Kerala: Implications for the Origin of the Southern Indian Granulite Terrain

Abstract: Field studies in southern Kerala have been successful in locating a Dumber of quarries in which charnockite has partially replaced garnet-biotite gneiss. Four types of charnockite formation have been recognised. Three of these involve the dehydration of gneisses by the influx of CO 2 - rich fluids. Types I and II show charnockite concentrated along shear and foliation planes. In type III, high CO 2 pressures are required by the presence of interlayered, scapolite-bearing calc-silicates. In type IV, charnockite is restricted to the margins of granitic dykes. and formed as a result of the decompression reaction gar + qtz + Na-plag = opx + Ca-plag, rather than a dehydration reaction. The widespread occurrence of arrested charnockitisation in southern Kerala suggests that the process of carbonic metamorphism invoked at Kabbaldurga, Karnataka, was not a local phenomenon, but extended over the entire southern Indian granulite terrain. In contrast to these examples of charnockite in the making, three types of charnockite breakdown have also been recognised. These involve the influx of water-rich solutions by means of granite pegmatites, leucocratic magmas, and pink granites. Present geochronologic data are not sufficient to determine whether the various types of charnockite formation and breakdown were different phases of a single metamorphic event or the result of several distinct metamorphic episodes.

Geochemistry of granulite-facies lower crustal xenoliths: implications for the geological history of the lower continental crust below the Eifel, West Germany

Abstract: Summary Mafic meta-igneous granulite facies xenoliths from Engeln/Eifel display petrographic, chemical and isotopic features which are not easily understood in terms of igneous fractionation or granulite-facies metamorphism. These features are best explained through a metasomatic modification of many of the xenoliths, characterized by the formation of amphibole and increasing Rb/Sr and Nd/Sm ratios as well as shifts in 143Nd/144Nd towards values similar to those of enriched mantle under the Eifel. Granulites which essentially escaped metasomatism have a Sm-Nd depleted mantle model age of ∼1.6 Ga whereas metasomatically altered granulites have younger and highly variable model ages. A Sm-Nd isochron for minerals from one granulite resulted in an unexpectedly young age of 172 ± 5 Ma which can be regarded as the maximum age of metasomatism. This age may be indicative of a thermal event having affected the lower crust under the Eifel during the Mesozoic.

Large-ion lithophile element characteristics of an amphibolite facies to granulite facies transition at Gruinard Bay, North-west Scotland

Abstract: Lewisian grey gneisses from Gruinard Bay, North-west Scotland retain mineralogical and geochemical evidence for Scourian horn-blende-granulite facies metamorphism, and they may be used to assess current models of elemental depletion at granulite grade. Their ‘immobile’major and trace element geochemistry is indistinguishable from that of Lewisian amphibolite and pyroxene-granulite facies counterparts. The K, Rb, Th and U contents of the Gruinard Bay gneisses are depleted relative to amphibolite facies gneisses, but generally the abundances of these elements are above those of comparable pyroxene granulites. U and Th have reached an advanced stage of depletion, but allanite appears to be crucial in maintaining significantly higher U and Th abundances at Gruinard Bay than in pyroxene granulites. K and Rb loss is less extreme, and depends on the stability of the rock-forming minerals: K-feldspar; biotite; and, amphibole. Early removal of K and Rb has resulted in a small rise in K/Rb, but further preferential Rb loss would have been required to generate the characteristically high K/Rb ratios of Lewisian pyroxene granulites. The residence of U and Th in the accessory minerals of granulite facies gneisses, which are often correlated with the residua of intracrustal partial melting, renders unlikely their extreme incompatibility required by such models. Even if such phases are ignored, high mineral-melt partition coefficients for silicic melts argue against partial fusion as an efficient depletion mechanism. On the other hand, the advanced stage of U and Th depletion reached in Gruinard Bay gneisses, which were still partly hydrous, severely restricts the role played by CO2-dominated fluids and a hydrous medium is preferred.

Metamorphic conditions of late Archean high-grade gneisses, Minnesota River valley, U.S.A.

Abstract: A detailed, comparative geothermobarometric analysis for the Minnesota River valley (MRV) Archean gneiss and migmatite terrane yields temperature and pressure ranges of 650–750 °C and 4.5–7.5 kbar (450–750 MPa), based on garnet–biotite, two-feldspar, garnet–clinopyroxene, garnet–cordierite, orthopyroxene–clinopyroxene, and magnetite–ilmenite thermometry and garnet–cordierite–sillimanite–quartz and garnet–orthopyroxene–plagioclase–quartz barometry. The temperature variation observed is interpreted to be primarily a result of varying degrees of re-equilibration of assemblages with falling temperature but may include real temperature variations between areas exhibiting granulite facies versus upper amphibolite facies–migmatitic lithologies. The large pressure range is primarily a result of differences among the pressure calibrations applied but may also record variations in pressure within the terrane. Lower pressures are consistent with the occurrence of cordierite-bearing assemblages at Granite Falls and D...

Nd evidence for extensive Archean basement in the western Churchill Province, Canada

Abstract: To determine the extent of reworked Archean crust in the western Churchill Province, we have examined Sm–Nd crustal residence ages of basement cores in southern Alberta and southwestern Saskatchewan along with crustal residence ages calculated for other Sm–Nd data available from the Churchill Province. The deep drill hole samples from the Interior Platform give Sm–Nd crustal residence ages that average 2.8 Ga. Granulites from northeastern Alberta and composite gneisses from northern Saskatchewan also provide Archean crustal residence ages. These data demonstrate that the presence of reworked Archean crust in the western Churchill Province is more widespread than has been confirmed previously. We suggest that Sm–Nd crustal residence ages are valuable estimates of crust formation times, especially in orogenic areas where other isotope systems have been disturbed.