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Showing papers on "Granulite published in 1980"


Journal ArticleDOI
06 Nov 1980-Nature
TL;DR: The hot-spot and plate-tectonic models of Precambrian crustal evolution lead to different schemes for CO2 delivery to continental roots as mentioned in this paper, and new tectonic concepts may be needed to explain carbonic metamorphism, minor-element depletions, and local phenomena of arrested development of charnockite in terrains.
Abstract: Stabilization of early crust against melting by high radioactivity and against resorption into the mantle by fast convective overturn requires that water and heat producers were flushed upwards within 50 Myr of accretion. Creation of a refractory base of granulite by metamorphism associated with CO2 vapour explains CO2-rich fluid inclusions in ancient high-grade rocks, minor-element depletions and local phenomena of arrested development of charnockite in Precambrian terrains. The hot-spot and plate-tectonic models of Precambrian crustal evolution lead to different schemes for CO2 delivery to continental roots. New tectonic concepts may be needed to explain carbonic metamorphism and other features of early crustal evolution.

512 citations


Journal ArticleDOI
S. A. Drury1, R.W. Holt1
TL;DR: In this article, a 1:1,000,000 scale LANDSAT imagery and reconnaissance structural analysis has revealed a pattern of tectonic evolution in the South Indian craton extending from Archaean times to the late Proterozoic.

273 citations


Journal ArticleDOI
TL;DR: Agarwal et al. as mentioned in this paper investigated the role of anatexis during uplift in obliterating such high temperature assemblages elsewhere in the world, and found that the unusual isobaric cooling to low temperatures followed by uplift to the surface which these granulites are inferred to have undergone is considered of importance, reflected in the disctinctive type of mineral reaction coronas found in these rocks.
Abstract: Osumilite-sapphirine-quartz granulites from Enderby Land, Antarctica (Ellis et al. 1980) were metamorphosed at 8–10 kb pressure, 900°-980° C under very low $$P_{H_2 O} $$ conditions. Retrograde mineral coronas in these rocks record subsequent cooling from the peak of metamorphism at approximately constant pressure. The inferredP-T cooling-uplift path differs markedly from that evident in many other granulite terrains. Present garnet-cordierite geothermometers imply equilibration at temperatures of 500°–600° C, well within the kyanite stability field. These temperatures are inconsistent with the presence of sillimanite and the high temperature stability fields of the actual mineral assemblages. Examination of available garnetcordierite experimental data suggests a possible large increase in the Gt-Cd Fe-MgK D with increasingX Mg of the cordierite (and pressure). New experiments designed to test this possibility were inconclusive because of the failure to attain satisfactory equilibrium, even at 1,000° C. Possible reasons for the exposure of these unusual granulites in Enderby Land are considered. Although they formed at much higher temperatures than other granulites exposed on a regional scale, suchP-T conditions are not exceptional for the base of the crust. Instead, the unusual isobaric cooling to low temperatures followed by uplift to the surface which these granulites are inferred to have undergone is considered of importance. The unusual tectonic conditions are reflected in the disctinctive type of mineral reaction coronas found in these rocks. The common occurrence elsewhere of mineral reaction during uplift, and the role of anatexis during uplift in obliterating such high temperature assemblages elsewhere in the world are considered.

210 citations


Journal ArticleDOI
TL;DR: In this article, the average abundances in the continental crust are: 98 ppb Cd, 14.8 ppm Pb, 77 ppm Zn and 98 ppm Rb (K/Rb: 223), which are close to the mean concentrations of the 6 elements in sedimentary and low to medium grade metamorphic rocks.

208 citations


Journal ArticleDOI
TL;DR: In this article, the mineral assemblages, reactions and compositional zoning in minerals are discussed in terms of continuous and discontinuous reactions in response to changing conditions of metamorphism, due to the effects of Mg-Fe and (Mg,Fe)-2Al exchange equilibria with decreasing temperature.
Abstract: The pre-Cambrian granulites of Enderby Land Antarctica, contain coexisting spinel-quartz, sapphirine-quartz, hypersthene-sillimanite-quartz and osumilite on a regional extent. Osumilite is present in a variety of mineral assemblages, most of which are documented in granulites for the first time. The mineral assemblages, reactions and compositional zoning in minerals are discussed in terms of continuous and discontinuous reactions in response to changing conditions of metamorphism. The development of many of the mineral coronas can be explained by continuous rather than discontinuous reactions, due to the effects of Mg-Fe and (Mg,Fe)-2Al exchange equilibria with decreasing temperature. The highest P-T conditions of metamorphism (8–10 kb, 900 °–980 ° C, Ellis, in preparation) were beyond the stability limit of coexisting garnet-cordierite. Secondary cordierite has developed through a large number of mineral reactions in response to cooling of these granulites.

193 citations


Journal ArticleDOI
TL;DR: In this paper, rare earth element (REE) data for major elements and 14 other trace elements are presented for ultramafic, mafic and intermediate, intermediate, tonalitic, trondhjemitic, anorthositic and microline gneisses, representative of the range of rock types making up the 2.9b.y.

159 citations


Journal ArticleDOI
TL;DR: In this paper, the authors used radiometric dates and the P-T path of the lower crustal section of the Calabrian massif to determine the minimum age of granulite-facies metamorphism.
Abstract: An approximately 7 km thick, continuous sequence of granulite-facies rocks from the lower crust, which contains a lower granulite-pyriclasite unit and an upper metapelite unit, occurs in the NW Serre of the Calabrian massif. The lower crustal section is overlain by a succession of plutonic rocks consisting of blastomylonitic quartz diorite, tonalite, and granite, and is underlain by phyllonitic schists and gneisses. Discordant apparent zircon ages, obtained from granulites and aluminous paragneisses, indicate a minimum age of about 1,900 m.y. for the oldest zircon populations. The lower intersection point of the discordia with the concordia at 296±2 m.y. is also marked by concordant monazites. Therefore, the age of 296±2 m.y. is interpreted as the minimum age of granulite-facies metamorphism. Concordant zircon ages were obtained from a metamorphic quartz monzogabbronorite sill (298±5 m.y.) and an unmetamorphosed tonalite (295±2 m.y.); they are interpreted as the intrusion ages. Discordant zircon ages from a blastomylonitic quartz diorite gneiss, situated between the lower crustal unit and the non-metamorphosed tonalite, reveal recent or geologically young lead loss by diffusion. The 207Pb/206Pb ages of the two analysed size-fractions point to an intrusion age similar to that of the overlying tonalite. Rb-Sr mineral ages are younger in the granulite-pyriclasite unit than in the overlying metapelite unit. Feldspars from the granulite-pyriclasite unit yield ages of about 145 m.y. and those from the metapelite unit 176±5 m.y. In the same way, the biotite cooling ages range between 108 and 114 m.y. in the granulitepyriclasite and between 132 and 135 m.y. in the metapelite unit and the tonalite. Some still younger biotite ages are explained by the influence of tectonic shearing on the Rb-Sr systems. A muscovite from a postmetamorphic aplite in the metapelite unit yields a cooling age of 203±4 m.y. The Rb-Sr isotopic analyses from migmatite bands do not lie on an isochron, perhaps due to limited isotopic exchange between the small scale layers during the long cooling period after the peak of metamorphism. In the phyllonitic gneisses and schists a Hercynian metamorphism is indicated by a muscovite age of 268±4 m.y., whereas the biotite age of 43±1 m.y. from the same sample can be correlated with an Alpine greenschist-facies metamorphism. On the basis of the radiometric dates and of the P-T path of the lower crustal section deduced petrologically, the following model is presented: the end of the Hercynian granulite-facies metamorphism was accompanied by an uplift of the lower crustal rocks into intermediate crustal levels and by synchronous plutonic intrusions into the lower crust and higher crustal levels, but essentially into the latter. Substantial further uplift did not occur until after cooling from the temperature of the granulite-facies metamorphism to the biotite closing temperature. This cooling lasted for about 185 m.y. in the lower part and for about 160 m.y. in the upper part of the lower crust section. A comparison between the geologic evolutions of the NW Serre of Calabria and the Ivrea Zone of the Alps demonstrates striking similarities. The activity of deep seated faults in both areas at least since late Hercynian time raises the possibility that a fault precursor of the boundary of the Adriatic microplate already existed at this time.

150 citations


Journal ArticleDOI
01 Sep 1980
TL;DR: The high-grade metamorphic rocks of Enderby Land, which form part of the East Antarctic Precambrian Shield, have been subdivided into two major metamorphics complexes, the Archaean Napier Complex and Proterozoic Rayner Complex.
Abstract: The high‐grade metamorphic rocks of Enderby Land, which form part of the East Antarctic Precambrian Shield, have been subdivided into two major metamorphic complexes—the Archaean Napier Complex and the Proterozoic Rayner Complex. The Napier Complex consists predominantly of pyroxene‐quartz‐feldspar gneiss and garnet‐quartz‐feldspar gneiss, with subordinate mafic granulite, pyroxenite, and a variety of siliceous, aluminous, and ferruginous metasediments. The gneisses are intruded by several types of mafic dyke, including an abundant suite of dolerites, the Amundsen Dykes, and locally by granitic rocks and pegmatite. Much of the Rayner Complex probably represents re‐metamorphosed Napier Complex rocks, and mafic dykes occur only as metamorphosed relics. The Rayner Complex is correlated with the Proterozoic metamorphics of the MacRobertson Land coast and northern Prince Charles Mountains, for which Rb‐Sr dates of 800–1100 m.y. have been obtained. Temperatures of at least 950°C, at pressures of 8–10 k...

137 citations


Journal ArticleDOI
TL;DR: The Amiˆtsoq gneisses are depleted in Rb and U relative to those of Ameralik and Isua as discussed by the authors, indicating that this depletion, correlated with the granulite-facies metamorphism occurred ca. 3600 Ma ago.

135 citations


Journal ArticleDOI
TL;DR: In this article, the authors interpreted the low abundances of large ion lithophile elements (LIL), K, Rb, U, Th, Cs, and high K/Rb ratio in rocks varying in composition from gabbro to granite in the Scourian complex, NW Scotland, are interpreted as due to depletion during granulite facies metamorphism.
Abstract: The low abundances of large ion lithophile elements (LIL), K, Rb, U, Th, Cs, and high K/Rb ratio in rocks varying in composition from gabbro to granite in the Scourian complex, NW Scotland, are interpreted as due to depletion during granulite facies metamorphism. Depletion was controlled by the mineralogy of the rock, the composition of the associated fluid phase and its volume relative to the volume of the rock. K-feldspar granites and granodiorites were not depleted in K and only moderately in Rb, but tonalites and trondhjemites were strongly depleted in both K and Rb. Published mineral-fluid partition coefficients for LIL in aqueous systems indicate that between 0.075 and 2.0 rock volumes of fluid phase passed through the host rock in order to achieve the observed selective elemental depletion.

110 citations


Journal ArticleDOI
TL;DR: In this article, a feasibility study was conducted on garnets and coexisting mineral and whole-rock phases from a selection of amphibolite facies rocks, granulite facia rocks and an eclogite.
Abstract: In this feasibility study, Sm/Nd and 143Nd/144Nd ratios have been determined on garnets and coexisting mineral and whole-rock phases from a selection of amphibolite facies rocks, granulite facies rocks and an eclogite. In two micaceous schists and an eclogite the distribution coefficient of the Sm/Nd ratio in garnet relative to other phases is in the range 2-3, i.e. comparable to values for phenocrysts in igneous rocks. In two granulite facies rocks the garnets have Sm/Nd distribution coefficients relative to other phases in the range 13-20 and have yielded precise ages.Garnet from a granulite facies shear zone from the Limpopo belt in southern Africa, of Archaean age, has yielded concordant duplicate ages of 1974±14 Ma and 1988±14 Ma. These results have been interpreted as relating to the last tectonic event in the region, long after the main gneiss-forming event. Garnet from a granulite facies xenolith from a Carboniferous vent in the Midland Valley of Scotland has yielded an age of 356±10 Ma; this is related to the onset of basic magmatism in the region.

Journal ArticleDOI
TL;DR: The geology of 700 km2 of high-grade metamorphic rocks from Doubtful Sound, New Zealand, is described in this article, where Malaspina, Turn Point, and Waipero Cove Gneiss Zones, constituting the granulite, transition, and amphibolite zones of the basement, are separated by doubful sound thrust from the amphibolitic facies cover, consisting of Kellard Point Gneisse, Deep Cove GNEisses, and Straight River Granite The last was intruded into the cover during the Tuh
Abstract: The geology of 700 km2 of high-grade metamorphic rocks from Doubtful Sound, New Zealand, is described Malaspina, Turn Point, and Waipero Cove Gneiss Zones, constituting the granulite, transition, and amphibolite zones of the basement, are separated by Doubtful Sound Thrust from the amphibolite facies cover, consisting of Kellard Point Gneisses, Deep Cove Gneisses, and Straight River Granite The last was intruded into the cover during the Tuhuan metamorphism and has a whole rock Rb-Sr isochron age of 372 ± 12 my BP Regionally important syn-metamorphic faults cut the cover and may link up to form another thrust It is suggested that Tuhuan retrogressive metamorphism of granulite facies basement, which gave rise to the transition and amphibolite zones, occurred during progressive amphibolite facies metamorphism of the cover sequence Synmetamorphic migmatites from the cover give a whole rock Rb-Sr isochron age of 388 ± 23 my bp The unaffected granulite basement is thought to be Precambrian

Journal ArticleDOI
TL;DR: In this article, the authors considered low pressure fractional crystallisation of pyroxene-plagioclase assemblages under conditions of lowfO 2, and the presence of a large number of pregranulite facies potassic pegmatites in the area suggests metasomatism of an originally less potassic suite of rocks.
Abstract: Archaean granulites from the type charnockite locality of Pallavaram, Madras City, Tamil Nadu, southern India consist of a bimodal suite of basic and silicic orthogneisses, associated with metasedimentary gneisses (khondalites). Charnockite is the dominant rock type. Basic granulites display a tholeiitic trend of strong iron enrichment accompanied by an increase in the concentration of the rare earth elements (REE), and the development of appreciable negative europium anomalies. These trends are considered to reflect low pressure fractional crystallisation of pyroxene-plagioclase assemblages under conditions of lowf O 2. Ultramafic granulites may represent the cumulus material of such a process. The silicic granulites (charnockites) are generally K2O rich and in marked contrast to the low K2O (tonalitic) silicic gneisses which dominate most granulite facies terrains. Their REE abundances, however, embrace the complete range of patterns observed in both K-rich and K-poor Archaean gneisses. The presence of a large number of pre-granulite facies potassic pegmatites in the area suggests metasomatism of an originally less potassic suite of rocks. Trace element considerations lead to a model whereby metasomatism and partial fusion of silicic gneisses in the terrain preceded the granulite facies metamorphic event. This sequence of events is best related to fluctuations in the composition of metamorphic fluids in the lower crust.

Journal ArticleDOI
TL;DR: In this article, rare earth element data for the Scourian granulite facies terrain of the Precambrian Lewisian complex of N.W. Scotland indicate that:
Abstract: Medium-to high-pressure granulite facies complexes represent samples of lower crustal material and are, therefore, important in the study of crustal processes. New rare earth element data for the Scourian granulite facies terrain of the Precambrian Lewisian complex of N.W. Scotland indicate that: These new data, along with previously reported major and trace element data, isotopic abundances, and trace element modelling support the hypothesis that the Scourian terrain is the residuum left after genesis and removal of granitic melts.

Journal ArticleDOI
TL;DR: The Lewisian complex of the Scourie-Badcall area is composed predominantly of banded tonalitic gneiss which intrudes layered gabbro-ultramafic complexes.
Abstract: The Lewisian complex of the Scourie-Badcall area is composed predominantly of banded tonalitic gneiss which intrudes layered gabbro-ultramafic complexes. Intrusive into both gabbro and tonalitic gneiss are homogeneous acid sheets which are trondhjemitic to granitic in composition. All rocks were subjected to granulite facies metamorphism.

Journal ArticleDOI
TL;DR: The greenstone belt of Suomussaimi is composed of a lower volcanic sequence of komatiitic and tholeiitic composition, a middle sedimentary unit and a upper volcanic sequence that is mainly andesitic-rhyodacitic in composition as discussed by the authors.


Journal ArticleDOI
TL;DR: In this article, a meta-igneous suite was probably derived from at least two different parental magmas and it may be a part of a gabbroic complex which resembles mafic bodies associated with anorthosites.

Journal ArticleDOI
01 Jul 1980-Nature
TL;DR: In this article, it was shown that if the widespread upper mantle depletion in lithophile elements, as represented by the negative correlation between Nd and Sr isotopes in basalts, has resulted from continental crustal extraction, then extraction of observed Archaean crustal compositions best explains the Nd-Sr isotope relationship.
Abstract: Calculations suggest that if the widespread upper mantle depletion in lithophile elements, as represented by the negative correlation between Nd and Sr isotopes in basalts, has resulted from continental crustal extraction, then extraction of observed Archaean crustal compositions best explains the Nd–Sr isotope relationship. Much of the lower continental crust must, therefore, have the high Nd/Sm and low Rb/Sr ratios typical of Archaean granulites.

Journal ArticleDOI
TL;DR: In this paper, materials balance calculations on amphibolite and granulite facies from the Adirondack region and from some areas of New Quebec demonstrate that the rocks of the two facies differ by near granite-minimum melt compositions.
Abstract: Materials balance calculations on amphibolite facies and granulite facies rocks from the Adirondack region and from some areas of New Quebec demonstrate that the rocks of the two facies differ by near granite-minimum melt compositions. The implication of these results are; granulites of both regions result from partial melting of greywacke-type metasedimentary rocks during upper amphibolite facies metamorphism.

Journal ArticleDOI
TL;DR: In this paper, a high pressure clinopyroxene +garnet±plagioclase assemblage at an estimatedP-T of 12-15 kb and 1,000° C was reported.
Abstract: Ultramafic and mafic granulites from Archaean gneisses in N.W. Scotland (the Scourian) show evidence of two periods of granulite facies mineral growth. The first produced a high pressure clinopyroxene +garnet±plagioclase assemblage at an estimatedP-T of 12–15 kb and 1,000° C. Uplift of the complex caused partial breakdown of the garnet by reaction with clinopyroxene to produce orthopyroxene +plagioclase ±spinel±amphibole symplectites, at an estimatedP-T of 10–14 kb and 800°–900° C. Garnet stability is shown to depend on both whole-rock Fe/Mg ratios and onP-T conditions. The pressures imply crustal thicknesses in the Archaean of least 35–45 km.

Journal ArticleDOI
TL;DR: The Nikubuchi peridotite body occurs as a tectonic body in the Sanbagawa metamorphic belt and is characterized by the presence of aluminous pyroxenes and green spinel as discussed by the authors.
Abstract: The Nikubuchi peridotite body occurs as a tectonic body in the Sanbagawa metamorphic belt. The peridotites consist of dunite, spinel wehrlite, spinel websterite and spinel-bearing metagabbro, and are characterized by the presence of aluminous pyroxenes and green spinel. Textural relationships and mineral chemistry indicate that the peridotites were equilibrated at the lowpressure granulite facies before they were emplaced into the Sanbagawa metamorphic belt and metamorphosed at about 600° C and 8–13 kbar (epidote amphibolite facies). The temperature and pressure of the granulite facies stage were estimated by two-pyroxene geothermometers and experimental data on the high-pressure solidus phase. They were about 750° C and 5–10 kbar. Banded structures and rock types of the peridotites indicate origin as a cumulate from a fractionating magma. Reconstructed cumulus assemblages and available data on the high-pressure liquidus or near liquidus phase indicate that the cumulates were precipitated at a pressure of the order of 10–18 kbar. Comparative study of the peridotites with the ‘ Alpyroxene-rich suite’ ultramafic nodules indicates that they were situated at the lower crust-upper mantle boundary similar to that of the inner zone of southwestern Japan.

Journal ArticleDOI
TL;DR: In this paper, a temperature of 750°±30°C and a pressure less than 7 kb are inferred for Cascade Slide in the Adirondack Mountains, New York.
Abstract: Akermanite (Ak90) coexists with monticellite (Mo92) and wollastonite (Wo99) in an unusual calc-silicate xenolith in anorthosite at Cascade Slide in the Adirondack Mountains, New York. Experimental results bearing on the stability of akermanite have been evaluated through calculations based on thermochemical data and by chemical analysis of experimental products (Yoder 1975). A temperature of 750°±30° C and a pressure less than 7 kb are inferred. These estimates are in agreement with the conditions of regional metamorphism previously inferred from other nearby rock types. When errors are considered, all existing data show that the Cascade Slide mineral assemblages last equilibrated at a slightly higher pressure of 7.4±1 kb and at a temperature of 750°±30° C during regional granulite facies metamorphism. The exotic mineralogy at Cascade Slide (akermanite, monticellite, cuspidine and wilkeite) was stabilized by low carbon dioxide fugacity. Posttectonic anorthosite intrusion is ruled out by the absence of a preserved contact aureole.

Journal ArticleDOI
TL;DR: In this article, a COCORP reflection profile across the Wind River uplift crosses exposed Archean rocks and resolves an unusual complex deep crustal structure at a depth of 24-31 km in an area where depth relations in Precambrian rocks can be inferred.

Journal ArticleDOI
TL;DR: In this paper, wet chemical analyses of forty-one pyroxenes from pyroxene granulites and a few neighbouring rocks from Swat Kohistan and the adjoining Indus Valley are presented.
Abstract: This paper presents wet chemical analyses of forty-one pyroxenes (thirty-nine of them new) from pyroxene granulites and a few neighbouring rocks from Swat Kohistan and the adjoining Indus Valley. The granulites, considered to be derived from plutonic norites of an island arc tholeiitic nature, constitute one of the most extensive belts of its kind in the world. The pyroxenes are not unusual in any respect but they bear a closer resemblance to metamorphic than to igneous pyroxenes crystallized from deep-seated tholeiitic magmas. The distribution of Mg, Fe, and Mn (average KD Mg−Fe=0.57) and the tie-line intersections on the Wo-En join (generally from Wo80.5 to Wo76.5) between the coexisting eighteen pyroxene pairs from the granulites are discussed. Based on eight different methods of geothermometry and other considerations, it is concluded that the pyroxene granulites were metamorphosed at around 800 °C and 7 to 8 kbar.

Journal ArticleDOI
TL;DR: In this paper, two forms of chemical variation can be distinguished: (1) an igneous-type differentiation, involving tholeiitic iron enrichment accompanied by increases in some incompatible elements, and decreases in the transition trace elements; (2) metamorphic fractionation effects resulting in deficiencies in K, Rb, Sr, Ba, Zr, and enhancement in Na in the highest grade rocks.
Abstract: The metabasites were originally minor intrusions which are now characterised by wholly metamorphic textures and mineral assemblages diagnostic of an amphibolite-granulite facies transition. Two forms of chemical variation can be distinguished: (1) an igneous-type differentiation, involving tholeiitic iron enrichment accompanied by increases in some incompatible elements, and decreases in the transition trace elements; (2) metamorphic fractionation effects resulting in deficiencies in K, Rb, Sr, Ba, Zr, and enhancement in Na in the highest grade rocks. These distributions closely parallel those in the host acid-intermediate charnockitic gneisses. The data preclude the possibility that the deficiencies were caused by removal of melts from the granulites. The metamorphism closely followed intrusion, and it is unlikely that the fractionations were caused by secondary dehydration of once hydrous assemblages. The preferred model involves intrusion and crystallisation directly under high-grade conditions, possibly with CO2-rich fluids playing an important role in suppressing the formation of hydrous minerals.

Journal ArticleDOI
TL;DR: A detailed electron probe study of irontitanium oxide intergrowths from slowly cooled granitic rocks from the granulite grade, Archaean Scourian complex of north-west Scotland has yielded a wealth of information about magmatic and metamorphic temperatures, subsolidus cooling, and the behaviour of the fluid phase during cooling.
Abstract: A detailed electron probe study of irontitanium oxide intergrowths from slowly cooled granitic rocks from the granulite grade, Archaean Scourian complex of north-west Scotland has yielded a wealth of information about magmatic and metamorphic temperatures, subsolidus cooling, and the behaviour of the fluid phase during cooling. Five stages are documented in the cooling history of granites and trondhjemites which include: (i) magmatic-subsolidus cooling (io35 ~ 890 ~ (ii) granulite facies metamorphism and the accompanied expulsion of a hydrous fluid phase (890 ~ 83o ~ (iii) subsolidus cooling following the peak of the granulite facies metamorphism (83o ~ ~ (iv) the localized reintroduction of water into the rocks during retrogression (660 ~ ~ and (v) subsolidus cooling and re-equilibration in the presence of a finite amount of H20 (530 ~ ~

01 Jan 1980
TL;DR: The Mtito Andei-Taita area in southern Kenya is situated within the Mozambique bique Belt, a major Proterozoic structural/metamorphic unit extending more than 5000 kms along the eastern coast of Africa.
Abstract: The Mtito Andei-Taita area in southern Kenya is situated within the Mozam­ bique Belt, a major Proterozoic structural/metamorphic unit extending more than 5000 kms along the eastern coast of Africa. The metamorphic lithologies of the Mozambiquian in the area include paragneisses, schists, marbles and amphibolites, considered to be originally mio- and eugeosynclinal volcanic and sedimentary rocks. Ultramafic rocks and poly-metamorphic charnockites and granulites are thought to represent respectively dismembered ophiolites and ancient sialic crust. They were tectonically emplaced during an early deformation phase. Subsequently a major orogenic episode affected all the rocks mentioned at about 800 m.y., pro­ ducing the Barrow-type metamorphism noted which reached the highest amphibolite facies. The structural evolution involved at least three phases of plastic deformation: final cratonization was achieved during the Pan-African orogenesis. Mio-Pliocene ruptural deformation associated with the extrusion of phonolites and basalts is related to the development of the East African rift system. Economic mineral

Journal ArticleDOI
TL;DR: In this paper, the age of the Picherais orthogneiss is estimated to be 1,900 m.y.y., which is more likely Upper Proterozoic than Lower Palaeozoic.
Abstract: Situated within the crystalline metamorphic complex of Champtoceaux NE of Nantes, the orthogneiss of La Picherais (near St Mars-du-Desert, Loire Atlantique, France) show relicts of a granulite facies paragenesis. Comparison with other granulitic rocks in the Hercynian fold-belt suggest possible ages ranging from Lower Proterozoic to Phanerozoic. The Rb-Sr whole rock method yields an errorchron of 570±110 m.y. for the Picherais orthogneiss, whereas the U-Pb zircon method indicates an upper intersection on Concordia at 1,880±120 m.y. and a lower intersection at 423±10 m.y. Several interpretations are possible for these data: the granite emplacement age was (1) 1,900 m.y. ago. (2) more likely Upper Proterozoic — Lower Palaeozoic. The zircons concordant at 1,900 m.y. were either present in the granitic magma at its time of origin or were introduced into the magma during emplacement. These zircons could be derived from sedimentary horizons such as found in the Lower Ordovician sandstones of the Armorican massif whose zircon age data are presented here.