VII.—The Old Granites of the Transvaal and of South and Central Africa
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Abstract:
Mr. Geo. G. Holmes and others have observed the remarkable and highly suggestive fact that not only do the schists (which contain in many places interlaminated beds of quartzites and massive crystalline limestone, e.g. in the Dwarsberg on the Magalakwin River in the Northern Transvaal) appear interbedded in the gneisses, but that the strike foliation and planes of schistosity of these old schists and of the gneisses seem invariably to be parallel.Keywords:
Foliation (geology)
Felsic
Ultramafic rock
Dharwar Craton
Syncline
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In the Sundarijal- Melamchi area, the following lithological units arc identified based on detailed field mapping: the Talamarang Formation (dark grey feldspathic schist and banded gneiss with garnet and kyanite), Gyalthum Formation (thin- to thick banded, light grey to grey, laminated quartzite with mica partings and bands of feldspathic schist. garnetiferous schist, and gneiss), Bolde Quartzite (grey to light grey, very thick-banded, massive quartzite with mica parting and schist bands containing sillimanite). Timbu Formation (light grey to dark grey, intensely deformed and folded quartzite, gneiss, and migmatite with abundant sillimanite), and Golphu Formation (grey to dark grey feldspathic schist and banded gneiss with large pegmatite veins) from bottom to top, respectively.
The Main Central Thrust (MCT) crosses the Indrawati River north of Majhitar and continues towards the Dhand Khola to SE and joins with the Mahabharat Thrust. It is moderately steep and dips essentially due NW. The footwall of the MCT is made up mainly of slate and phyllite and the grade of metamorphism is quite low in comparison with that of the hanging wall. The inverted metamorphism is conspicuous in the hanging wall of the MCT, where at first garnet schist is observed, which passes rapidly upwards to kyanite schist and then to sillimanite-bearing schist, gneiss, and migmatite.
Augen and banded gneisses invade the upper part of the northern limb and most of the southern limb of the Patibhanjyang Anticline. Apparently, the Sheopuri Injection Gneiss Zone is a continuation of the Kutumsang Gneiss Zone in the north, and the rocks were folded after the emplacement of the injection gneisses.
Sillimanite
Phyllite
Migmatite
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Abstract The Sanbagawa metamorphic terrain of the study area is divided into two units, the Shirakura and Sejiri units. The metamorphic thermal structure is interpreted on the basis of the degree of graphitization (GD) of carbonaceous material in pelitic schists. The areal variations of the metamorphic grade are presented by the distribution of GD calculated using the Lc and d 002 of carbonaceous material. As a result, the two units are classified into four metamorphic zones, respectively: A 1 , A 2 , B 1 and B 2 for the Shirakura Unit; and I 1 , I 2 , II 1 and II 2 for the Sejiri Unit. The metamorphic grades of A 1 , A 2 , I 1 and I 2 are included in the chlorite zone, and that of B 1 , B 2 , II 1 and II 2 in the garnet zone of the Sanbagawa metamorphism. The degree of graphitization at the boundary between A 2 and B 1 zones is the same as that between I 2 and II 1 zones. Detailed study on the variation of GD suggests that the present‐day structure of the study area is best interpreted as a model of shuffled‐cards structure. An estimated minimum thickness of a stack of continuous cards is about 25 m. The compositions of garnet in pelitic schists and of amphibole in basic schists are different from those in the identical metamorphic range of the Shirakura and Sejiri units. It is suggested that rocks of the Shirakura Unit were metamorphosed under higher P/T conditions than those of the Sejiri Unit.
Pelite
Isograd
Amphibole
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Microflore of Pre-Cambrian LImestones of the Duszniki Zdroj Region (The Central Sudeten)
Metamorphic rocks of the Bystrzyckie Mountains, which also comprise crystalline limestones of Duszniki Zdroj, have been known to many authors. H. Wolf (1864) included these rocks among the Pre-Cambrian; at the same time he distinguished red and augen gneisses as older ones, while mica-schists with limestone lenticles, hornblende schists and gneisses were considered to be younger ones. L. Lepla (1910) included metamorphic rocks of this region among the Archaean. W. Petrascheck (1910, 1944) distinguished the older, axial part built of gneisses, and the younger one — a cover of mica-schists with limestone lenticles; he included both parts among the Pre-Cambrian. K. Smulikowski (1951) included mica-schists among the Algonkian, with a reservation, however. J. Gierwielaniec (1957, 1965) supposes that crystalline limestones of the Duszniki Zdroj region are of the Cambrian age, although their affiliation to the Algonkian is not unlikely. A synthetic stratigraphical-lithological profile of metamorphic rocks of the Bystrzyckie Mountains was drawn up by M. Dumicz (1964). This author includes the metamorphic rocks of the Bystrzyckie Mountains among the Pre-Cambrian; at the same time he remarks that micaschists with limestone lenticles are older than gramte-gneisses. J. Obere (1966) included mica-schists of this region among the Proterozoic, while H. Teisseyre (1968) claims their Pre-Cambrian age. The author of the present paper has carried out his studies in one of non-working limestone quarries situated in the Duszniki Zdroj region (fig. 1). Shaly limestones, compact and brecciated, dark-pink and lightgray in colour occur in this outcrop (fig. 2). In .microsections directional arrangement of mineral grains have been found (table I, fig. 1, 2), among which calcite prevails, quartz constitutes about 25%, while micas and feldspars are less numerous. Ferric oxides form pigment that gives colour to the rock. Brecciated limestones are outcropped on one of the walls (table I, fig. 3). Samples for micropalaentologic studies were taken from all outcrop ped layers. The samples were used for making cubes of a size 8 X 5 X 6 cm; the latter were then dissolved in hydrochloric acid, in acetic acid and in EDTA. Some were frozen in liquid nitrogen and then unfrozen. 550 preparations and also 35' microsections and 40 surficial sections were made; the former were then preserved in the Canada balsam. The microflore found there is usually poorly preserved. Sporangia and fructifications of Fungi and Sphaeromorphidae are comparatively best preserved, while Cyanophyta are much worse. The Sphaeromorphida group is the most abundant one in genera and species. Species: Prostosphaeridium flexuosum T i m., Protosphaeridium laccatum Tim. according to B. W. Timofiejew (1966, 1969) are widely spread in Vendian and Riphaean (late Precambrian), but there are only few in the Cambrian. Others, such as: Protosphaeridium vermium Tim. , Pterosperomopsimorpha annulare T i m., Leiosphaeridium — type R. Pf lug, Synsphaeridium sorediforme T i m., are quoted by this author from Riphaean. Among Cyanophyta the genera: Cephalophytarion, Paleonacystis, Sphaerophycus parvum S с h., Catinella polymorpha — the form F. Pf lug, Miliaria implexa Pflug were described by J. W. Schopf (1968), J. W. Schopf et J. M. Blacic (1969), H. Pflug (1966) from the Upper Proterozoic of Australia and the Rocky Mountains in North America. The genera: Gunflintia grandis Bargh., Gunflintia minuta and Eosphaera come from the Lower Proterozoic of Canada (E. S. Barghorn et S. A. Tyler 1965). Also the genera Fibularix, Scintilla and Tormentella were described by H. Pflug from the Upper Proterozoic. The genus Belaiaella, described by P. N. Kolosow (1970), as well as Phycomycetes, presented by B. W. Timofiejew (1969), come from the Riphaean of Siberia. Ascomycetes have not been quoted from the Pre-Cambrian, so far. Having analysed the stratigraphie range of the described genera of microflora (vide specification of the described microflora), the Archaean age of limestones, suggested by A. Lepla (1910), should be thus excluded. Assemblages characteristic of the Cambrian are also absent here, which would not confirm J. Gierwielaniec’ s suggestion (1957, 1965) of the possible Cambrian age of limestones, either. The Upper Proterozoic age seems to be the most reasonable one, which is indicated by the prevalence of genera and species of the microflora. At the present stage of investigations it is difficult to decide whether other crystalline limestones in this area are of the same age as the ones described from the Duszniki region, or whether they are younger or older. Further studies will allow a solution of this problem.
Hornblende
Outcrop
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The Nellore schist belt (NSB) is one of the prominent schist belts of southern India. It is thrust over an unmetamorphosed Proterozoic sedimentary sequence (Cuddapah Basin) in the west and in turn is overthrust by the Eastern Ghats Granulite Terrain (EGGT) in the east. Metamorphic grade has been considered to be either greenschist to amphibolite facies or else to show high- and low-grade groups of rocks. Detailed mineralogical and P, T studies on representative rock types across the high- to low-grade groups (metapelite and metadacite from the high- and low-grade groups respectively) near Vinjamuru reveal that there are high (M1) and medium (M2) grade metamorphic events. While metapelite documents both M1 and M2 events, metadacite shows only the M2 event, thus indicating the presence of high- and medium-grade groups of rocks in the NSB. The stable mineral assemblage during the early prograde part of M1 was Qtz+St1+Ms1+Pl1+Bt±Grt, which subsequently suffered high-grade metamorphic conditions (T∽715–765°C and P∽8·6–9·2 kbar) leading to the demise of St1 and melting of Ms1. In contrast, medium-grade metamorphism (M2) is characterized by the stable association of St2+Qz. Quantitative geothermobarometry suggests T=520–570°C and P=6·1-6·8 kbar, and these results show good consistency with the average P, T estimates obtained from the THERMOCALC program. This metamorphic episode is probably coeval with the 900 Ma granulite facies metamorphic event in the EGGT. The superimposition of M2 over M1 probably has led to widespread retrogression of the high-grade rocks to medium grade, thereby giving the general impression of a greenschist to amphibolite facies gradation. © 1998 John Wiley & Sons, Ltd.
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The region discussed in this paper lies in Chester County, Pa., and is included in the eastern half of the Coatesville quadrangle. (See fig. 3.) It is within the belt of crystal-line schists and gneisses of the Piedmont Plateau. The northern half of the area, which will be called the Doe Run region, from the village of that name (see Fig. 4, p. 15), has been surveyed by Eleanora F. Bliss in connection with the problem of the relation of the Wissahickon mica gneiss to the Octoraro schist.
BLISS
Quadrangle
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Mr. Geo. G. Holmes and others have observed the remarkable and highly suggestive fact that not only do the schists (which contain in many places interlaminated beds of quartzites and massive crystalline limestone, e.g. in the Dwarsberg on the Magalakwin River in the Northern Transvaal) appear interbedded in the gneisses, but that the strike foliation and planes of schistosity of these old schists and of the gneisses seem invariably to be parallel.
Foliation (geology)
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