Showing papers on "Granulite published in 1979"
TL;DR: In this paper, isotope systematics of Lewisian gneisses from North-west Scotland suggest that their igneous calc-alkaline precursors separated from previously undifferentiated mantle 2,920±50 Myr ago.
Abstract: Sm—Nd isotope systematics of Lewisian gneisses from North-west Scotland suggest that their igneous calc-alkaline precursors separated from previously undifferentiated mantle 2,920±50 Myr ago. Approximately 200 Myr elapsed before this material was finally differentiated and stabilised as granulite and amphibolite fades crust.
241 citations
TL;DR: In this article, the authors compare the geochemical characteristics of high-level and deep-level tonalites and trondhjemites, and show that the latter is a more basic field than the former.
157 citations
TL;DR: In this article, K/1bAr and Rb 1bSr ages are reported for high-grade metamorphic rocks from pre-Hercynian elements in the Hercynian orogen of NW Spain and for an early hercynian suite of granites (half-life 37 Rb = 5.0 · 10 10 10 a; errors 2σ).
113 citations
TL;DR: In this paper, a generalised crustal structure of Fiordland is proposed, based on the recognition of a basement granulite facies lower crustal material separated by a regional thrust zone from a cover sequence.
84 citations
TL;DR: In this paper, the authors studied the geochemical properties of the tonalitic and trondhjemitic gneisses of the Archaean high-grade terrain of Scotland and East Greenland.
Abstract: High Na/K gneisses of tonalitic-trondhjemitic composition dominate the Archaean high-grade terrain of Scotland and East Greenland. Gneiss compositions range from ultramafic to trondhjemite, but the compositional distribution is bimodal, particularly in the amphibolite facies gneiss zones. The tonalitic gneisses have moderately high Al 2 O 3 (mean 2 is mostly below 70 percent except in the minor trondhjemitic bodies and veins. The levels of Ba and Sr are high, and Sr concentrations remain high even in siliceous gneisses. There are marked negative correlations between Y and SiO 2 and between TiO 2 and SiO 2 Ce N /Y N ratloS increase with SiO 2 , primarily as a result of strong depletion of heavy rare earth elements in the more siliceous gneisses, and may reach values in excess of 100. However, total rare earth levels and Ce/Yb ratios are lower in some trondhjemitic rocks. Many of the siliceous gneisses have prominent positive Eu anomalies, though the magnitude of the Eu anomaly is related to total REE content, and some trondhjemitic rocks have slight negative Eu anomalies. The geochemical relationships suggest strongly that if the range of gneiss compositions are related by fractional crystallization, then hornblende has been a major controlling mineral phase, and the whole gneiss complex may have evolved under high p H 2 O conditions deep in the crust. However, partial melting models involving amphibolite and particularly garnet amphibolite as the primary source material seem best able to explain the geochemical features and field relationships. The deeper parts of the gneiss complex inverted to dry granulite facies assemblages during or shortly after the phase of crustal generation, probably in response to changing fluid composition (CO 2 -rich). Expulsion of hydrous fluids was accompanied by loss of K, Rb, Th and U, thus enhancing the tonalitic/trondhjemitic characteristics of the gneiss complex.
83 citations
TL;DR: In this paper, the Uivak II gneisses are considered to have formed by partial melting of granulite facies tonalitic and trondhjemitic, possibly with some fractional crystallization of the derived melt.
Abstract: Early Archean (pre-3.6 Ga) continental crust in Northern Labrador is composed predominantly of either migmatitic or porphyroclastic (augen) gneiss, containing inclusions of supracrustal lithologies, leucogabbro and anorthosite. The migmatitic and augen gneiss are subdivided into two suites (designated the Uivak I and Uivak II gneisses), which both fall on the 3622 Ma Rb/Sr isochron. The Uivak I suite, which is regarded as being derived mainly from trondhjemitic to granodioritic parents, experienced a complex structural and metamorphic history prior to the intrusion of the protoliths of the Uivak II gneisses. The early metamorphic history of the terrain involved the formation of both granulite and amphibolite facies assemblages at different structural levels. Microstructural and paragenetic relationships in the Uivak I gray gneiss indicate that retrogression from granulite to amphibolite facies occurred after the establishment of the planar anisotropy in the suite. This retrogression involved the development of biotite and secondary hornblende from hornblende and orthopyroxene in the early granulite facies gneisses. It was also associated with an increase in modal microcline and quartz. We interpret alteration in the suite to be the result of K and Rb metasomatism associated with the emplacement and deformation of the parents of the Uivak II gneisses. The increase in Rb produced lower K/Rb and higher Rb/Sr ratios than are normally observed in trondhjemitic-tonalitic associations. The age indicated by the isochron is considered to date this allochemical event. Highly fractionated REE patterns and variable Ce/Y ratios in the Uivak I gneisses are interpreted to be induced either by metamorphic or igneous processes. These processes involved the transport of heavy REE's and Y (as well as other LIL elements) in the form of soluble complexes with volatiles derived from the lower crust or upper mantle. We present a model whereby the trondhjemitic and tonalitic parents of the Uivak I gneisses are generated by partial melting of mafic granulite under the influence of a high early Archean geothermal gradient and massive volatile outgassing from the mantle and lower crust. The production of early Archean sialic crust is envisaged as a cumulative, cyclic process, mafic and ultramafic volcanism occurring synchronously with the partial melting at depth of earlier formed mafic granulite facies crust. The Uivak II gneisses are considered to have formed by partial melting of granulite facies tonalitic and trondhjemitic gneisses, possibly with some fractional crystallization of the derived melt.
77 citations
TL;DR: In this article, the authors present new evidence from amphibolite and granulite facies terrains which bears upon the mobility of rare-earth elements during high-grade metamorphism.
Abstract: A considerable body of data on the distribution and behaviour of rare-earth elements (REE) in igneous and sedimentary rocks has been accumulated over the past decade. Metamorphosed and metasomatized rocks have by comparison received scant attention. Some investigators have reported REE mobility, but most of the available data suggests the REE levels and patterns are essentially unaffected by metamorphism. However, the hypothesis of REE “immobility” during metamorphism has not been rigorously tested. We present new evidence from amphibolite and granulite facies terrains which bears upon the mobility of rare-earth elements during high-grade metamorphism. Metasomatic zones provide an excellent test of element mobility. Examples are described from an amphibolite facies terrain in which these zones have retained their original REE chemistry, as well as their Ti, P, Y, Zr, Hf, Nb and Ta contents. In the associated metavolcanic suite strong linear correlations exist between the REEs and the other “immobile” elements. Such correlations are frequently explained in cogenetic volcanic suites as being produced by fractional crystallization. It seems unlikely that element migration during metamorphism would preserve such coherent behaviour of elements with different chemical properties. We conclude that in these cases metamorphism and metasomatism has affected neither the REE abundances and patterns of this metavolcanic suite, nor its Ti, P, Y, Zr, Hf, Nb or Ta content. Only these elements provide a reliable guide to the parentage of the metasomatized volcanics. By contrast, our work on Archean granulite-facies gneisses from the Lewisian of N.W. Scotland shows that these deep crustal rocks are depleted in REEs (except Eu) by a factor of 2 or 3 relative to upper crustal values. A distinct positive europium anomaly supports the idea of preferential retention of Eu in the deep crust proposed by Jakes and Taylor (1974) and Nance and Taylor (1976). Mineral REE patterns in the granulites and their distribution coefficients closely resemble those reported for phenocrysts in dacites, suggesting that the minerals equilibrated with a granodioritic liquid. The trace and major element chemistry of this terrain is compatible with the model of a granulite facies residuum, left after the generation of a granodioritic upper crust. Rare-earth elements provide a promising new tool in the determination of the parentage of metamorphosed and metasomatized rocks up to the upper amphibolite facies of regional metamorphism and may prove valuable in assessing the importance of anatexis in the evolution of the lower continental crust.
77 citations
TL;DR: In this paper, the average composition of the lower crust of the Bournac pipe was estimated for major and twenty-four trace elements, which suggests that lower crust is not always depleted in LILE; however, if a residual character may be attributed to some rocks, it seems unlikely to consider all lower crust as a restite.
Abstract: From the analyses of xenoliths scavenged by the Bournac pipe (Massif Central) an estimated average composition of the lower crust is presented for major and twenty-four trace elements. This composition, relatively basic and close to the average of some andesites, suggests that the lower crust is not always depleted in LILE; if a residual character may be attributed to some rocks, it seems unlikely to consider all the lower crust as a restite. The heat-flow production (1.6–1.7 H.P.U.) is relatively high for lower crustal granulite facies rocks.
71 citations
TL;DR: This paper showed that pyroxene thermometers are sufficiently inconsistent, inaccurate and imprecise that they should not be used as quantitative thermometers in metamorphic rocks, and they also pointed out that the pyroxenes are sensitive to ferrous-ferric ratios with variations of up to 50°C when Fe3+ is estimated from pyroxne stoichiometry of pyroxen compositions obtained by microprobe analysis.
62 citations
TL;DR: In this article, a set of rocks from the Archaean Lewisian metamorphic complex, north-west Britain, have been measured in the laboratory at confining pressures to 8 kbar.
Abstract: Summary
P- and S-wave velocities on a set of rocks from the Archaean Lewisian metamorphic complex, north-west Britain, have been measured in the laboratory at confining pressures to 8 kbar. From estimates of velocities at crustal pressures and temperatures, it is concluded that the composition of the three crustal layers below the Scottish Caledonides are most likely to be, in order of increasing depth: (1) mixed metamorphic rocks in amphibolite or lower facies, plus granites, (2) mixed pyroxene-granulite facies rocks, of overall intermediate composition and (3) mafic garnet granulites. Poisson's ratios of isotropic aggregates of mafic amphibolites and serpentinized peridotites in the Lewisian are high (0.28-0.31) because of the presence of hydrous minerals. Only a highly preferred orientation of minerals in such rocks in the lower crust would lead to observed values of Poisson's ratio of about 0.25.
61 citations
TL;DR: In this paper, a retrograde metamorphic event under slightly lower pressures and temperatures led to the breakdown of the assemblage garnet + clinopyroxene thereby forming coronas of plagioclase II (An 75 ) + orthopyroxenes II + Ti-magnetite ± brown hornblende.
TL;DR: In this article, the compressional and shear wave velocities in quarzite, granite, and granulite are determined at a fixed confining pressure of 2 kb as a function of temperature up to 720° C. The role of the α-β quartz transition as a possible cause of low-velocity layers is discussed.
Abstract: The compressional and shear wave velocities in quarzite, granite, and granulite are determined at a fixed confining pressure of 2 kb as a function of temperature up to 720° C. The high-low quartz transition of the constituent quartz minerals is associated with a pronounced decrease in velocity of the compressional waves when approaching the transition and with a significant velocity increase after the transition. In contrast, the effect of the α-β transition on shear wave velocities is small. The drop of V
P is explained by the elastic softening of structure of the constituent quartz minerals near the α-β transition and the opening of grain-boundary cracks, caused by the very high volumetric thermal expansion of the quartz relative to the other component minerals. The velocity increase in the β-field may be attributed to an elastic hardening of the quartz structure. Poisson ratios computed from the velocity data are anomalous for a solid: they become negative within the transition regime. The transition temperature, as indicated by the minimum velocities, is higher in the polycristalline rocks than is expected on grounds of single crystal behavior, and the discrepancy is more marked in granite than in quartzite. The shift of the transition temperature to higher values is explained by internal stresses that arise from the anisotropy of the thermal expansion and compressibility of individual grains and the differences in thermal expansion and compressibility between different component minerals. The role of the α-β quartz transition as a possible cause of low-velocity layers is discussed.
TL;DR: A 1500-2000 m sequence of amphibolites exhibits progressively increasing metamorphic grade from greenschist to the amphibolite/granulite facies transition.
TL;DR: In this article, the authors found abundant garnet-clinopyroxene-plagioclase-hornblende granulite xenoliths in minette and serpentinite diatremes.
Abstract: Abundant garnet-clinopyroxene-plagioclase-hornblende granulite xenoliths in minette and serpentinite diatremes of the Colorado Plateau are mostly of olivine tholeiitic composition and display slight LREE enrichment and moderate Eu anomalies. Although mineral assemblages and equilibration temperatures (∼600 °C) are compatible with a lower-crustal origin, the very low jadeite contents of the clinopyroxenes (0% to 2%) suggest equilibration at relatively shallow depth (possibly 9 to 15 km). Xenoliths of sillimanite-garnet-biotite schist, felsic granulite, and potassic granite may represent subordinate components of the granulite terrane. Extensive alteration under greenschist-facies conditions affected the granulites in the serpentinite diatremes but not those in the minettes. This alteration is suggested to be a result of “stewing” of the xenoliths in the serpentinite host, possibly during slow diapiric emplacement of the serpentinite along deep fault zones prior to the final stage of fluidized eruption that formed the diatremes.
TL;DR: In this article, the rate earth element compositions of a suite of high-grade metamorphic and plutonic rocks which outcrop in the Varberg region of southwest Sweden are presented.
TL;DR: In this article, the relationship between high-grade granulite and low-grade granite-greenstone terrains is assessed in the light of our knowledge of the 2.9-b.y. event.
TL;DR: A sequence of amphibolite to granulite facies metasedimentary and mafic metaigneous rocks from the western Italian Alps has been analyzed for rare earth elements (REE) as mentioned in this paper.
TL;DR: In this paper, the authors carried out an isotopic investigation of the Precambrian granulite terrain of eastern Bahia, Brazil, and obtained Rb-Sr and K-Ar isochron ages of 3160±60 Ma, 2730±50 Ma, 2300−2450 Ma and 1800−2000 Ma, respectively.
TL;DR: In this article, mafic and ultramafic inclusions from Abiquiu, from Cieneguilla, and from Elephant Butte in the northern and central Rio Grande rift in New Mexico have been examined in detail.
TL;DR: The δ18O values of Archean granitoids from northwestern Ontario range from 3.2 to 11.7 °/oo, and reflect a myriad of magmatic, metamorphic, sedimentary, and alteration processes as mentioned in this paper.
Abstract: The δ180 values of Archean granitoids from northwestern Ontario range from 3.2 to 11.7 °/oo, and reflect a myriad of magmatic, metamorphic, sedimentary, and alteration processes. The Pakwash and Footprint gneisses contain some samples with δ180 values of 6 to 7 °/oo, much lower than measured for other portions of these rocks. The low δ180 values probably reflect isotopic exchange with a low 180 external reservoir (basalt?) during partial melting accompanying upper amphibolite facies metamorphism. The metasedimentary Group A Twilight gneiss, which has been metamorphosed to granulite facies, appears to have isotopically exchanged with the surrounding volumetrically more important metaigneous granitoids. Other less metamorphosed parts of these and other gneisses retain oxygen isotope compositions diagnostic of their clastic sedimentary (Pakwash gneiss) or metaigneous (and probably metaplutonic) protoliths (Footprint, Cedar Lake, and Kenora area gneisses). Some Archean granitoids of lower metamorphic grades have also been depleted in 180 but such occurrences are localized. Greenschist facies felsic metavolcanic rocks in the vicinity of the Burditt Lake granodiorite pluton have been chemically altered and depleted in 180, by interaction action with stock-derived magmatic fluids. Portions of the Jackfish Lake Complex granodiorites also have low δ180 values due to exchange with low 18O fluids (probably ground waters). The chemical composition of these rocks, including their LREE abundances also have been affected by this activity. Unaltered portions of the plutonic Jackfish Lake Complex range in δ18O from 6.4 to 9.2 °/oo, higher values accompanying the progressive differentiation of the Complex from dioritic to granodioritic compositons. The unaltered Archean metaplutonic granitoids of tonalitic to granitic composition have δ 18O values of 7.3 to 9.3 °/oo, somewhat lower than Phanerozoic analogues. Such results probably reflect the more juvenile nature of the sialic material added to the continental crust during this period of Archean crustal evolution. Most Archean granitoids while not grossly disturbed, are nevertheless out of oxygen isotopic equilibrium. This disequilibrium is indicated by variable and usually large quartz-microcline fractionations, large quartz-biotite fractionations, and, for many of the gneisses, low δ18O values for biotite (less than 3 °/oo). Quartz-magnetite temperatures calculated for the Kenora area gneisses' provide a reasonably good minimum estimate of the real temperatures achieved near the culmination of metamorphism; the low (400°) quartz-biotite “temperatures” calculated for the gneisses, however, reflect only the preferential isotopic exchange of biotite (relative to quartz and magnetite), most likely with low 18O fluids.
TL;DR: The coexistence of clinopyroxene (Wo~8 En32 Fs44 Rho6) and pigeonite (Wo8 En35 Fs5o Rho7) in a recta-ironstone from Scourie is described in this article.
Abstract: The coexistence of clinopyroxene (Wo~8 En32 Fs44 Rho6) and pigeonite (Wo8 En35 Fs5o Rho7) in a recta-ironstone from Scourie is described. By comparison with experimental data, the temperature at which they were in equilibrium is estimated as at least IOOO ~ The exsolution history of the two pyroxenes is documented. As the rock is an unambiguous metasediment, its presence among granulite facies rocks containing evidence of metamorphism at pressures greater than IO kbar constrains at least part of the Scourie complex to a supracrustal origin and indicates that tectonic processes of burial were operative in the late Archaean.
TL;DR: In the Utralanama Block, an intermediate pressure granulite facies terrain in the Arunta Block, central Australia, has revealed several anomalous features, not consistent with the depletion of granitophile components generally considered to accompany granulitic facies metamorphism as mentioned in this paper.
Abstract: Geochemical investigations in the Utralanama Block, an intermediate pressure granulite facies terrain in the Arunta Block, central Australia, has revealed several anomalous features, not consistent with the depletion of granitophile components generally considered to accompany granulite facies metamorphism. However, other geochemical features are indicative of depletion. The mean K2O for the Utralanama Block is exceptionally low relative to most other granulite facies terrains, but Rb contents are comparatively high. Consequently, the mean K/Rb ratio is relatively low for granulite facies terrains as is the mean Ba/Rb ratio, whilst mean K/Sr and Rb/Sr ratios are much higher than usual for such terrains. Only the K/Ba ratio shows equivalent values to depleted terrains elsewhere. Comparison of these ratios for the three main compositional groups of rocks in the Utralanama Block reveals that for mafic rocks all the above ratios are characteristic of extreme depletion, whereas, for all but the K/Ba ratio, mean ratios for the pelitic rocks, and to a lesser extent for the quartzofeldspathic rocks approach normal crustal values or values for metasomatic rocks. The abnormally high Rb/Sr ratios of these rocks compared to average crustal rocks suggest, however, that metasomatism is the cause of the anomalous geochemical features of the Utralanama Block, and this is supported by field and microstructural evidence. Thus, Rb/Sr ratios appear to be useful indicators of metasomatism where no gross mineralogical or microstructural evidence for metasomatism is obvious, and under such conditions the K/Ba ratio may be more reliable than the K/Rb ratio for indicating prior depletion of the terrain.
TL;DR: The deformation and recrystallization microstructures in biotite from the Woodroffe Thrust mylonites are described and interpreted in this paper, where the degree of strain and the nucleation mechanisms differ across the mylonite zone, associated with the contrast in water content between granulite and amphibolite facies felsic gneisses on either side of the zone.
TL;DR: A review of granulite occurrences in the South American Platform is presented in this article, which is composed of the Caraibas Complex, the Jequie Complex, Salvador Complex and several minor granulites occurrences along the Brazilian coast in the States of Espirito Santo and Rio de Janeiro.
TL;DR: In this paper, a suite of specimens representative of these fabrics has been examined both by optical universal stage, to determine quartz c-axis orientation, and by X-ray diffraction, to obtain orientation data from r, z, m and a. The data are combined to yield inverse pole figures of schistosity and lineation.
05 Mar 1979-The Journal of the Japanese Association of Mineralogists,Petrologists and Economic Geologists
TL;DR: The Oki metamorphic massif consists of pelitic gneiss, migmatitic gnesis, augen gneis, amphibolite, and calcareous gnesiss, and this massif has a characteristic dome structure as mentioned in this paper.
Abstract: The Oki metamorphic massif consists of pelitic gneiss, migmatitic gneiss, augen gneiss, amphibolite and calcareous gneiss, and this massif has a characteristic dome structure. Sillimanite and orthoclase are stable in the pelitic gneiss and this coexistence indicates the higher grade metamorphism. Amphibolites can be divided into two groups:(a) plagioclase-brown hornblende-clinopyroxene-orthopyroxene-quartz-ilmenite-pyrrhotite (-magnetite)(b) plagioclase-brown hornblende-quartz-ilmenite (-clinopyroxene-magnetite). Orthopyroxene bearing amphibolite (Group a) has higher FeO and SiO2 contents, higher FeO/(FeO+MgO) ratio and lower Fe2O3/(Fe2O3+FeO) ratio. Orthopyroxene is considered to be formed by the reaction of hornblende+quartz=orthopyroxene+clinopyroxene+plagioclase+water. In this reaction orthopyroxene can be easily formed under the Fe2+-rich environment. Geological and petrographical evidences indicate that orthopyroxene-bearing amphibolite and orthopyroxene-free amphibolite are formed under same metamorphic condition, which is transitional from the amphibolite facies to the granulite facies.
TL;DR: In this article, it was shown that eclogite-facies metamorphism in the Glenelg inlier, NW Scotland, occurred after a phase of basic dyke intrusion (Scourie dykes), and before a period of?Caledonian mylonitisation.
Abstract: Summary Eclogite-facies metamorphism in the Glenelg inlier, NW Scotland, occurred after a phase of basic dyke intrusion (?Scourie dykes), and before a period of ?Caledonian mylonitisation. It possibly occurred during the Precambrian Morarian event since banded eclogitic rocks share the same ESE-dipping foliation as the overlying Moines. P-T conditions of metamorphism estimated as 750–600°C and 10–15 kb, suggest that the inlier is an upthrust segment of stable continental lower crust. Granulite-facies rocks of the NE Ox Mountains inlier, W Ireland, show evidence of Caledonian retrogression. Inferred P-T conditions (900–750°C, >10 kb) are those of a heated, lower crustal environment. The rocks are very similar to xenolithic material from the Scottish Midland Valley.
TL;DR: In this paper, a suite of crustal xenoliths from the Craters of the Moon lava field (COM) in southern Idaho are characterized by variable and high 87Sr/86Sr ratios (0.708-0.712) interpreted as reflecting variable degrees of contamination.
Abstract: The petrology and geochemistry of deep crustal rocks are poorly understood due to the inaccessibility of such materials. Most inferences of the lower crust are based on studies of ancient high-grade metamorphic terranes now exposed after a history of deep burial. However, direct sampling of deep crustal rocks is feasible when they are carried to the surface as accidental fragments (or xenoliths) in lava flows. Ferro-basalt to ferrolatite lavas from Craters of the Moon lava field (COM) in the Snake River Plain, southern Idaho are characterised by variable and high 87Sr/86Sr ratios (0.708–0.712) interpreted as reflecting variable degrees of crustal contamination1,2. Some of these lavas contain xenoliths of crustal derivation (silicic charnockites and pyroxene granulites) that are found nowhere in the vicinity in surface outcrops. I present here initial isotopic and trace element results for a suite of these crustal xenoliths. As part of a detailed Pb isotopic study of the lavas (in preparation), three granulites were analysed to test the contamination hypothesis. The results indicate an affinity between the xenoliths and many exposed granulite-facies metamorphic terranes, and the unusually primitive lead isotopic ratios recorded establish the presence of early Archean crust beneath south-central Idaho.
TL;DR: This paper showed that trondhjemites are low in Al and depleted in K, Rb, Cs, but they have high content of other lithophile elements compared to oceanic plagiogranites and granites.
Abstract: Layers of trondhjemite occur in eclogites and high pressure amphibolites belonging to the leptyno-amphibolitic complex of Rouergue (Southern Massif Central). These trondhjemites are low in Al and depleted in K, Rb, Cs, but they have high content of other lithophile elements compared to oceanic plagiogranites and granites. Trace element data imply at least two stages in the genesis of these rocks. The textures and initial mineral assemblage (quartz, plagioclase, kyanite, zoisite, garnet) suggest that these rocks crystallized from a melt under high pressure granulite facies conditions (P ≈ 12.5 - 20 kb; T ≈ 750-840°). Solid-solid reactions, such as zoisite-Al-epidote, indicate a subsequent retrogression under conditions of the amphibolite facies.
(Abbreviations for phases in text and figures: qz = quartz, pl = plagioclase, Ab = albite, ol = oligoclase, an = anorthite, ky = kyanite, sill = sillimanite, ga = garnet, stl = staurolite, zo = zoisite, al ep = Al epidote, cpx = clinopyroxene, amph = amphibole, hb = green hornblende, mus = muscovite, bi = biotite, co = corundum, sp = green spinel, rt = prismatic rutile, il = ilmenite, sph = sphene, V = H2O vapor, L = liquid.)
TL;DR: A recent study on the Proterozoic Namaqua mobile belt in the southwestern part of Africa has revealed the widespread presence of granolites (orthopyroxene-bearing metamorphites) and charnockitic intrusions as mentioned in this paper.
Abstract: Recent studies on the Proterozoic Namaqua mobile belt in the south-western part of Africa have revealed the widespread presence of granolites (orthopyroxene-bearing metamorphites) and charnockitic rocks (orthopyroxene-bearing intrusions) throughout the 800-km exposed length of the belt. Intermediate-pressure granolites are preserved in at least nine widely scattered areas, in places partially overprinted by later amphibolite facies metamorphism. Numerous charnockitic intrusions, in places partially amphibolitized, have been reported from fifteen areas, in at least seven of which they are associated with granolites. The abundance and scale of these occurrences greatly amplify the importance of the mid-Proterozoic granulite metamorphism proposed by T. N. Clifford.