Showing papers on "Granulite published in 1976"

Late Proterozoic Cratonization in Southwestern Saudi Arabia

Abstract: Early cratonal development of the Arabian Shield of southwestern Saudi Arabia began with the deposition of calcic to calc-alkalic, basaltic to dacitic volcanic rocks, and immature sedimentary rocks that subsequently were moderately deformed, metamorphosed, and intruded about 960 Ma ago by dioritic batholiths of mantle derivation ($^{87}$Sr/$^{86}$Sr = 0.7029). A thick sequence of calc-alkalic andesitic to rhyodacitic volcanic rocks and volcanoclastic wackes was deposited unconformably on this neocraton. Regional greenschistfacies metamorphism, intensive deformation along north-trending structures, and intrusion of mantle-derived ($^{87}$Sr/$^{86}$Sr = 0.7028) dioritic to granodioritic batholiths occurred about 800 Ma. Granodiorite was emplaced as injection gneiss about 785 Ma ($^{87}$Sr/$^{86}$Sr = 0.7028-0.7035) in localized areas of gneiss doming and amphibolite to granulite facies metamorphism. Deposition of clastic and volcanic rocks overlapped in time and followed orogeny at 785 Ma. These deposits, together with the older rocks, were deformed, metamorphosed to greenschist facies, and intruded by calc-alkalic plutons ($^{87}$Sr/$^{86}$Sr = 0.7035) between 600 and 650 Ma. Late cratonal development between 570 and 550 Ma involved moderate pulses of volcanism, deformation, metamorphism to greenschist facies, and intrusion of quartz monzonite and granite. Cratonization appears to have evolved in an intraoceanic, island-arc environment of comagmatic volcanism and intrusion.

The interrelations of fluid transport, deformation, geochemistry and heat flow in early Proterozoic shear zones in the Lewisian complex

Abstract: The Lewisian complex of northwest Scotland shows a pattern of evolution typical of a number of early Proterozoic provinces. During the period 2500-1600 Ma, deformation occurred along steeply dipping shear zones, resulting in both vertical and lateral movements. The largest of these shear zones, forming the northern boundary to the Scourian granulites (Archaean), must have penetrated to considerable depth, possibly to the mantle. Modal and chemical analysis of rocks from shear zones are presented and discussed in relation to rocks sampled outside shear zones. The mineralogy and composition of all rocks deformed in the shear zones have been considerably altered by synkinematic metasomatism. In the early stages, immediately prior to and during the intrusion of the regional doleritic dyke swarm, this metasomatic activity involved addition of H$\_{2}$O and Na to the rocks. Subsequently, more significant changes in rock chemistry occurred (addition of H$\_{2}$O, K, Na, loss of Fe, Ca, Mg). These changes resulted from the interaction between large volumes of water and the rocks in the shear zones along which the fluid travelled. A combination of modal and chemical data allow general chemical reactions to be written which describe the evolution of the gneisses during reworking and retrogression from pyroxene bearing granulite facies rocks to hornblende and biotite bearing amphibolite facies rocks in shear zones. The reactions are written as ionic equilibria and suggest that the fluid phase in the shear zones had a low pH. Adiabatic transport of water upwards through the crust will result in moderate warming of the fluid, and can cause large temperature increases above the preexisting geothermal gradient in rocks through which the fluid travels. It is suggested that both deformation and metamorphism in these shear zones are related to transport of fluid by hydraulic fracturing. Grain size reduction by hydraulic fracturing increases the strain rate in the shear zones. Deformation may cease in a shear zone when the fluid pressure drops and hydraulic fracturing no longer occurs. Thus fluid transport, mineral reactions, chemical changes, grain size reduction and convective heat flow will cease. A close relation should exist between the intensity of deformation, the extent of metasomatism and the thermal history in these important shear belts.

Phase relationships in granulitic metapelites from the Ivrea-Verbano zone (Northern Italy)

Abstract: Paragneisses of the Ivrea-Verbano zone exhibit over a horizontal distance of 5 km mineralogical changes indicative of the transition from amphibolite to granulite facies metamorphism. The most obvious change is the progressive replacement of biotite by garnet via the reaction: a $${\text{Biotite + sillimanite + quartz }} \to {\text{ Garnet + K - feldspar + H}}_{\text{2}} {\text{O}}$$ which results in a systematic increase in the modal ratio g = (garnet)/(garnet + biotite) with increasing grade. The systematic variations in garnet and biotite contents of metapelites are also reflected by the compositions of these phases, both of which become more magnesian with increasing metamorphic grade. The pressure of metamorphism has been estimated from the Ca3Al2Si3O12 contents of garnets coexisting with plagioclase, sillimanite and quartz. These phases are related by the equilibrium: b $$\begin{gathered} 3 CaAl_2 {\text{Si}}_{\text{2}} {\text{O}}_{\text{8}} \rightleftharpoons Ca_3 Al_2 {\text{Si}}_{\text{3}} {\text{O}}_{{\text{12}}} + 2 Al_2 {\text{SiO}}_{\text{5}} + {\text{SiO}}_{\text{2}} \hfill \\ plagioclase garnet sillimanite quartz \hfill \\ \end{gathered} $$ which has been applied to these rocks using the available data on the mixing properties of plagioclase and garnet solid solutions. Temperature and f H 2O estimates have been made in a similar way using thermodynamic data on the biotite-garnet reaction (a) and the approximate solidus temperatures of paragneisses. Amphibolite to granulite facies metamorphism in the Ivrea-Verbano zone took place in the P-T ranges 9–11 kb and 700–820 °C. The differences in temperature and pressure of metamorphism between g= 0 and g = 1 (5 kms horizontal distance) were less than 50° C and approximately 1 kb. Retrogression and re-equilibration of garnets and biotites in the metapelites extended to temperatures more than 50° C below and pressures more than 1.5 kb below the peak of metamorphism, the degree of retrogression increasing with decreasing grade of the metamorphic “peak”. The pressure and temperature of the peak of metamorphism are not inconsistent with the hypothesis that the Ivrea-Verbano zone is a slice of upthrusted lower crust from the crust-mantle transition region, although it appears that the thermal gradient was too low for the zone to represent a near-vertical section through the crust. The most reasonable explanation of the granulite facies metamorphism is that it arose through intrusion of mafic rocks into a region already undergoing recrystallisation under amphibolite facies conditions.

Microfossils from a newly discovered Precambrian stromatolitic iron formation in Western Australia

Abstract: THE Nabberu Basin is a newly discovered sedimentary basin of probable early Proterozoic age in Western Australia1–3 (Fig. 1). It covers an area of ∼60,000 km2, and contains ∼6,000 m of shallow-water clastic and chemical sediments. It is essentially unmetamorphosed in the south-east but is metamorphosed to granulite facies in the west, where folding and deformation are more pronounced.

Late Archean metamorphism in the Buksefjorden region, Southwest Greenland

Abstract: Standard state thermodynamic data extracted from experimental studies and applied to mineral assemblages in orthogneisses, metasedimentary gneisses and metabasites show that conditions of late Archean (2,850 m.y.) upper amphibolite facies were P solid≈7.0 kb, T≈630° C, and rose to P solid≈10.5 kb, T≈810° C in adjacent granulite facies. The estimates of solid pressure for the granulite facies suggest a late Archean crustal thickness of ca. 35 km, comparable to present day continental crust. Upper amphibolite facies assemblages were in equilibrium with $$P_{H_2 O}$$ about one half P solid, while granulite assemblages equilibrated at much lower $$P_{H_2 O}$$ , varying from about one tenth P solid in quartzofeldspathic gneisses to one third P solid in more basic layers.

The Grenville Province in Helikian Times: A Possible Model of Evolution

Abstract: It is suggested that the Helikian (1650-1000 million years (Ma) ago) evolution of the Grenville Province in the Canadian Shield was marked by three events: emplacement of anorthosites around 1450-1500 Ma ago, rifting associated with opening of a proto-Atlantic ocean between 1200 and 1300 Ma ago, and continental collision responsible for the Grenvillian 'orogeny' about 1100-1000 Ma ago. Emplacement of rocks of the anorthosite suite (anorthosites and adamellites or mangerites) into continental crust was accompanied by formation of aureoles in the granulite facies. The Grenville Group was deposited in the southern part of the Province between 1300 and 1200 Ma ago and comprises marbles, clastic metasedimentary rocks and volcanics. It occupies a roughly triangular area limited on the northwest by the Bancroft-Renfrew lineament and on the southeast by the Chibougamau-Gatineau lineament. It is thought to have been accumulated in an aulacogen that would have developed along a fracture zone separating two basement blocks. The Grenvillian thermotectonic event may represent a Tibetan continental collision in the sense of Burke & Dewey. The suture zone would now be hidden under the Appalachians. Collision would cause reactivation of continental crust and renewed movement on pre-existing lineaments. The east-central part of the Grenville Province appears to have been more intensively reactivated than the western part.

Replacement of hornblende by garnet in granulite facies assemblages near Milford Sound, New Zealand

Abstract: In hypersthene bearing hornblende metadiorites near Milford Sound garnet replaces hornblende in ∼5 cm wide subplanar zones, patterned almost certainly on joints. The alteration has been accompanied by a change in bulk rock composition (increase of Al, decrease of Na) and the occurrence constitutes good evidence for external chemical control of the overprinting of “hornblende granulites” with garnet.

K/Rb ratios and metasomatism in metabasites from a Precambrian amphibolite–granulite transition zone

Abstract: Potassium and rubidium values are reported for a suite of 181 metabasites from the amphibolite–granulite transition zone in the Precambrian Bamble sector of south Norway. Both alkalis are more abundant, and the K/Rb ratios lower, in the rocks from the amphibolite facies. Rubidium has been strongly fractionated with respect to potassium. The K/Rb ratios exhibit a regular and systematic variation and this defines a linear trend which is markedly different from established igneous fractionation trends. The chemical fractionation occurred as a result of the metamorphism, and the chemical changes are in keeping with the concept of a vertical chemical fractionation within the continental crust. Similar linear trends established in other high grade metamorphic terrains may have the same (metasomatic ?) origin.

The Chinamora Batholith: diapiric intrusion or interference fold?

Abstract: Mapping of the Chinamora Batholith of Rhodesia has revealed a complex suite of rock types which include old gneisses with infolded green-stone belt relics (metamorphosed to upper am-phibolite and granulite facies), intruded by a series of granitic (s.l.) bodies. The youngest of these granites is a distinctive porphyritic adamellite which is shown to have been deformed by two phases of tight folding. It is argued that the intensity of these deformations would effectively obscure any earlier intrusive relationship between the porphyritic adamellite (or older intrusive granites) and the greenstone belt rocks. The granite-greenstone distribution of the Chinamora area is explained as a large scale interference fold and it is suggested that small folds measured in part of the Chinamora Batholith as well as folds apparent on maps of the surrounding greenstone belt rocks can be related to this large scale folding.

The Geochemistry of `Charnockites' and Their Constituent Ferromagnesian Minerals from the Precambrian of South-East Sri Lanka (Ceylon)

Abstract: A chemical and petrographic study of charnockites and pyroxene/hornblende-bearing granulites has been undertaken, and the data used in an interpretation of the petrogenesis of these rocks. Major- and trace-element analyses of fourteen rocks are presented, together with majorelement analyses of seven orthopyroxenes, seven clinopyroxenes, two garnets, and three hornblendes. The compositions of the coexisting ferromagnesian minerals are used to obtain estimates of the likely pressure/temperature conditions of equilibration (5-7 kbar and 700±50°C of the various mineral assemblages.

A palaeomagnetic study of part of the Lewisian complex, north-west Scotland

Abstract: Directions of magnetization have been measured in dykes and gneisses of the Central Zone of the mainland Lewisian and in the Laxford granite sheets. Nine Scourie dykes from an area around Lochinver have yielded closely grouped directions of magnetization giving a mean palaeomagnetic pole at 37.3°N, 274.8°E (dp = 6.9°, dm = 9.6°). The direction for a Scourian granulite falls very close to the mean for the dykes, A regional difference in magnetic stability has been discovered and this, together with evidence concerning variations in the opaque minerals, has formed the basis of an interpretation in terms of metamorphic history. Stable magnetization was probably imposed in the late Laxfordian metamorphism at about 1800 Ma. Upon closure of the Atlantic Ocean there is good agreement with North American apparent polar wander paths.

Seismic velocities in the Lewisian metamorphic complex, northwest Britain—‘in situ’ measurements

Abstract: Synopsis Sixteen reversed seismic refraction profiles, each 1 km long, in Sutherland between Kylesku and Durness have demonstrated a difference in P-wave velocity between the northern and central belts of the mainland outcrop of the early Precambrian Lewisian rocks. The variably-retrogressed pyroxene granulites of the central belt (of andesitic-dioritic composition) yielded a velocity of 6.34 ± 0.17 km/s (6 observations); the quartzo-feldspathic gneisses of the northern belt yielded a velocity of 5.64 ± 0.18 km/s (6 observations); the rocks of the boundary region, near the Ben Stack line, yielded a velocity of 5.28 ± 0.23 km/s (4 observations). Correlations are made with the velocity layering of crustal models derived for the area, confirming the wide occurrence in the upper crust of granulite-facies rocks. Such high values of velocity in surface rocks place strict limits on velocity gradients in such rock types in the deeper continental crust.

The copper deposits of the O'okiep District, South Africa: New data and concepts

Abstract: The copper deposits of the O'okiep District rank amongst the top three S. African producers, with an annual output of 3 million metric tons grading 1.65% Cu on average. They occur in what is now considered the S. extension of the 1200 m. y. Kibaran orogenic belt. Copper mineralization is linked to cross-cutting bodies of "noritoid" RO=0.7126−0.7250) which have been emplaced into the granulite facies metamorphic country rocks 1100 m. y. ago. The results of 150 electron probe analyses show that orthopyroxene in the non-ore-bearing noritoids is En51–52, in the ore-bearing noritoids En60–65. There is a concomitant variation in the composition of phlogopitic mica which carries higher Fe in non-ore-bearing noritoids. The Ticontents of magnetite are below 0.1%; there are significant chromium values. Spinel exsolution lamellae contain up to 14% ZnO. The copper sulphides occur as granular aggregates with silicates, on grain boundaries and on cleavage planes of hypersthene and mica; they also replace altered Fe-Ti-oxides. Pyrite (up to 1.4% Co) and pentlandite (up to 5% Co) are not widespread. The possible derivation of the noritoids from a reservoir of basic magma at depth is considered. A source-bed model, involving generation of noritoid "magma" from Cu-bearing members of the stratigraphic sequence, during the peak of metamorphism (800–1000 °C, 6–8 kb) is tentatively proposed. Stratabound base metal concentrations (Aggeneys, Gamsberg) in lower grade (amphibolite facies) metamorphic terrains to the East have not been mobilized.

Aluminium in coexisting pyroxenes as a sensitive indicator of changes in metamorphic grade within the Mafic Granulite Terrane of the Fraser Range, Western Australia

Abstract: Three pairs of coexisting pyroxenes of mafic granulites from each of two locations 100 km apart show large chemical differences, especially in Al, Fe, Mn, Ti and Na. Al content of the pyroxenes at the higher pressure locality is more nearly independent of Al of the host rock than are the pyroxenes from the lower pressure locality. All the data confirm that although no significant difference in temperature has emerged, there was a large difference in pressure between the two localities. Al is found to be a more effective discriminant of metamorphic conditions than {Mathematical expression}. As the three pairs of pyroxenes cover a wide range of Fe at each locality, the close relationship of Al and Na (and of Ca-tschermak and jadeite) to Fe becomes evident. This shows that a normalizing procedure should be adopted before comparing localities with different Al, Mn, Ti, Na and other elements or derived components such as jadeite and Ca-tschermak.