Showing papers on "Granulite published in 1968"

Melting and phase relations in the plane tholeiite-lherzolite-nepheline basanite to 40 kilobars with geological implications

Abstract: Six crystalline mixtures, picrite, olivine-rich tholeiite, nepheline basanite, alkali picrite, olivine-rich basanite, and olivine-rich alkali basalt were recrystallized at pressures to 40 kb, and the phase equilibria and sequences of phases in natural basaltic and peridotitic rocks were investigated. The picrite was recrystallized along the solidus to the assemblages (1) olivine+orthopyroxene+ clinopyroxene +plagioclase+spinel below 13 kb, (2) olivine+orthopyroxene+clinopyroxene+spinel between 13 kb and 18 kb, (3) olivine+orthopyroxene+clinopyroxene+ garnet+spinel between 18 kb and 26 kb, and (4) olivine+clinopyroxene+garnet above 26 kb. The solidus temperature at 1 atm is slightly below 1,100° and rises to 1,320° at 20 kb and 1,570° at 40 kb. Olivine is the primary phase crystallizing from the melt at all pressures to 40 kb. The olivine-rich tholeiite was recrystallized along the solidus into the assemblages (1) olivine+ clinopyroxene+plagioclase+spinel below 13 kb, (2) clinopyroxene+orthopyroxene+ spinel between 13 kb and 18 kb, (3) clinopyroxene+garnet+spinel above 18 kb. The solidus temperature is slightly below 1,100° at 1 atm, 1,370° at 20 kb, and 1,590° at 40 kb. The primary phase is olivine below 20 kb but is orthopyroxene at 40 kb. In the nepheline basanite, olivine is the primary phase below 14 kb, but clinopyroxene is the first phase to appear above 14 kb. In the alkali-picrite the primary phase is olivine to 40 kb. In the olivine-rich basanite, olivine is the primary phase below 35 kb and garnet is the primary phase above 35 kb. In the olivine-rich alkali basalt the primary phase is olivine below 20 kb and is garnet at 40 kb. Mineral assemblages in a granite-basalt-peridotite join are summarized according to reported experimental data on natural rocks. The solidus of mafic rock is approximately given by T=12.5 P Kb+1,050°. With increasing pressure along the solidus, olivine disappears by reaction with plagioclase at 9 kb in mafic rocks and plagioclase disappears by reaction with olivine at 13 kb in ultramafic rocks. Plagioclase disappears at around 22 kb in mafic rocks, but it persists to higher pressure in acidic rocks. Garnet appears at somewhat above 18 kb in acidic rocks, at 17 kb in mafic rocks, and at 22 kb in ultramafic rocks. The subsolidus equilibrium curves of the reactions are extrapolated according to equilibrium curves of related reactions in simple systems. The pyroxene-hornfels and sanidinite facies is the lowest pressure mineral facies. The pyroxene-granulite facies is an intermediate low pressure mineral facies in which olivine and plagioclase are incompatible and garnet is absent in mafic rocks. The low pressure boundary is at 7.5 kb at 750° C and at 9.5 kb at 1,150° C. The high pressure boundary is 8.0 kb at 750° C and 15.0 kb at 1,150° C. The garnet-granulite facies is an intermediate high pressure facies and is characterized by coexisting garnet and plagioclase in mafic rocks. The upper boundary is at 10.3 kb at 750° C and 18.0 kb at 1,150° C. The eclogite facies is the highest pressure mineral facies, in which jadeite-rich clinopyroxene is stable. Compositions of minerals in natural rocks of the granulite facies and the eclogite facies are considered. Clinopyroxenes in the granulite-facies rocks have smaller jadeite-Tschermak's molecule ratios and higher amounts of Tschermak's molecule than clinopyroxenes in the eclogite-facies rocks. The distribution coefficients of Mg between orthopyroxene and clinopyroxene are normally in the range of 0.5–0.6 in metamorphic rocks in the granulite facies. The distribution coefficients of Mg between garnet and clinopyroxene suggest increasing crystallization temperature of the rocks in the following order: eclogite in glaucophane schist, eclogite and granulite in gneissic terrain, garnet peridotite, and peridotite nodules in kimberlite. Temperatures near the bottom of the crust in orogenic zones characterized by kyanitesillimanite metamorpbism are estimated from the mineral assemblages of metamorphic rocks in Precambrian shields to be about 700° C at 7 kb and 800° C at 9 kb, although heat-flow data suggest that the bottom of Precambrian shield areas is about 400° C and the eclogite facies is stable. The composition of liquid which is in equilibrium with peridotite is estimated to be close to tholeiite basalt at the surface pressure and to be picrite at around 30 kb. The liquid composition becomes poorer in normative olivine with decreasing pressure and temperature. During crystallization at high pressure, olivine and orthopyroxene react with liquid to form clinopyroxene, and a discontinuous reaction series, olivine → orthopyroxene → clinopyroxene is suggested. By fractional crystallization of pyroxenes the liquid will become poorer in SiO2. Therefore, if liquid formed by partial melting of peridotite in the mantle slowly rises maintaining equilibrium with the surrounding peridotite, the liquid will become poorer in MgO by crystallization of olivine, and tholeiite basalt magma will arrive at the surface. On the other hand, if the liquid undergoes fractional crystallization in the mantle, the liquid may change in composition to alkali-basalt magma and alkali-basalt volcanism may be seen at a late stage of volcanic activity.

Co-existing Sapphirine and Quartz in Granulite from Enderby Land, Antarctica

Abstract: SAPPHIRINE and quartz have been found together in nature for the first time in granulite collected by I. R. McLeod, of the Australian Bureau of Mineral Resources, from a small nunatak (66° 57′ S, 51° 31′ E) about 42 miles east of Amundsen Bay, Enderby Land, Antarctica. This rock belongs to a gneiss–granulite–charnockite terrain (personal communication from I. R. McLeod), and was called blue quartzite in the field. In thin section it is seen to consist of rutilated quartz, cordierite-quartz intergrowths, sapphirine as grains up to 2 mm long, and accessory sillimanite, garnet, hypersthene, rutile, chlorite, zircon and biotite; the first three constituents make up about 98 per cent of the rock. Another granulite from the same nunatak contains no sapphirine, though it may formerly have done so; it consists of quartz, garnet, sillimanite, orthoclase microperthite with unusually low 2 V (35° to 40°), and accessory rutile, sericite and zircon.

A geochronologic investigation of the anorthosite complex, Adirondack mountains, New York

Abstract: Cogenetic uranium-lead isotope systems in zircons have been analyzed for five rock samples from the anorthositic domes of the Adirondack highlands and compared with data from a cogenetic suite in a charnockitic gneiss from the Ticonderoga dome in the eastern Adirondacks. Textural and structural arguments applied to the zircons and rocks from the anorthosite suite indicate that some of the zircon are metamorphic and reveal that conditions producing granulite facies metamorphism exited in different parts of the anorthosite complex at discretely different times during the interval from 1020 to 1100 m.y. ago. The Ticonderoga gneiss sample yields an interpreted zircon age of 1130 ± 10 m.y. Zircon characteristics argue this to be the age of magmatic crystallization of the host rock. Extensive studies have confirmed this age as the prevailing age in many large bodies of charnockite (or syenite) gneiss which are peripheral to the anorthosite domes. Petrological arguments of persistent spatial association and compositional gradation between the anort11osite and a suite ranging from norite to mangerite, syenite, quartz syenite and granite, imply that all of these rock are comagmatic; this leads to the conclusion that the age of the anorthosite itself is 1130 m.y. No evidence has been found for any older ages in any of the Adirondack orthogneisses to support the hypothesis of a pre-"Grenville" basement. The mounting evidence for almost precise age contemporaneity between the Adirondack anorthosite-syenite complex and the Duluth anorthositic gabbro-granophyre complex should provoke a close comparative petrologic and structural scrutiny of these two great stratiform masses.

Geochemistry and origin of metasediments in the Haliburton–Madoc area, southeastern Ontario

Abstract: Ortho- and para-amphibolites and related silicate and carbonate rocks in the Grenville Province in Ontario have been studied in the field and laboratory in order to determine their genesis. X-ray fluorescence chemical analyses for 31 elements in each of 100 rocks of similar types and of greenschist to granulite facies are used to show that the complex of metasediments was originally an association of tuffs and carbonates. The ortho-amphibolites probably are Na-metasomatized tholeiites. Para-amphibolites were basic tuffs; biotite-plagioclase gneisses were intermediate to acid tuffs. The tuffs were deposited in a region of carbonate sedimentation.Scapolite and clino-pyroxene are the dominant minerals in many of the granulite facies rocks. They are developed from decarbonated calcium-rich rocks to which chlorine was added. The average CO2 loss was 9 to 10 wt.%, and the average Cl addition was 0.8 wt.%. Zn and Pb probably were also added to the high-grade rocks.It is demonstrated that pelite-carbonate mixture...

The spatial extent of chemical equilibrium in some high-grade metamorphic rocks from the grenville of southeastern Ontario

Abstract: Many of the recent studies of chemical equilibrium in metamorphic rocks may have been hampered by sampling which did not reflect single regions or domains of local equilibration This paper records results of tests performed to determine the specific volume of rock in chemical equilibrium with respect to a certain phase, under a particular set of conditions Two samples of high-grade metamorphic gneisses from the Grenville Series of southeastern Ontario were selected for intensive study The garnets of one specimen, possessing a biotite-garnet-cordierite assemblage, underwent in situ laser microprobe spectrochemical determinations for Fe, Mg, Mn and Ca Conclusions are thus possible as to the volumes over which free diffusion and chemical equilibrium took place These volumes were found to be on the order of only a few cubic centimeters and their shapes are strongly controlled by rock structures such as foliation and lineation A higher-grade pyroxene granulite was also investigated with respect to garnet and biotite equilibrium Somewhat larger volumes of equilibration were noted in this rock

The fabric of the granulites of Mont Tremblant Park, Quebec

Abstract: The fabric of the highly deformed and recrystallized gneisses formed under granulite facies conditions of metamorphism are described from Mont Tremblant Park, Quebec, Canada. These gneisses are termed granulites and are classified into quartzofeldspathic and hypersthene varieties.The term 'cataclastic-gneissose' fabrics is used to imply varying degrees of deformation and recrystallization without micro-brecciation or cataclasis 'sensu stricto' and a classification of these various fabrics is presented. The modifications in these granulites are progressive and are assumed to have the following sequence:During the evolution from mortar to granoblastic types, the minerals become flattened and increasingly elongated, while the individual recrystallized grains increase in size. The same deformation produced flaser and granoblastic gneisses in the quartzofeldspathic granulites and mortar and augen gneisses in the hypersthene granulites.The mineral relicts (porphyroclasts) in the mortar and augen gneisses are hi...

The densities of Precambrian rocks from northern Manitoba

Abstract: To assist in the interpretation of gravity anomalies in northern Manitoba and northeastern Saskatchewan, a part of the western Canadian Shield, the average densities of 2004 Precambrian rock samples, grouped according to rock type, are tabulated. Relatively positive anomalies may be expected over greenstone belts (2.85 g/cm3), sedimentary gneisses (2.79 g/cm3), granulites (2.73 g/cm3), and mafic to ultramafic intrusions—diorite (2.77 g/cm3), gabbro (3.00 g/cm3), and peridotite (3.19 g/cm3), whereas negative anomalies may be expected over granitic rocks (2.64 g/cm3). These results are consistent with results from other areas.The average composition of a slab of surface rock in this part of the western Canadian Shield is shown to be granodioritic with an average density of 2.67 g/cm3.

Geochronology of the Aldan Shield, southeastern Siberia

Abstract: Crystalline Precambrian rocks exposed in the basins of the Olekma and Aldan Rivers represent the southeastern part of the Siberian Platform. To the north and east the basement is covered by Paleozoic and Upper Proterozoic formations. To the south and west lie younger deformed belts.The Aldan Shield can be divided into four regions. The Tcharskaja massif, in the west, is the most ancient block. Pb–U dates on allanite and zircon show a good concordance at 2950 m.y. The prevalent rock type is gneiss and migmatite.In the Aldan River basin three thick series of Lower Proterozoic rocks were deposited during the time interval 2000 to 2700 m.y. Granulite facies metamorphism of these rocks occurred 1800 to 2100 m.y. ago. The time of deposition of the lowest formations (Iengra Series) is given approximately by measurement of the isotopic composition of lead in different syngenetic mineral phases. Using muscovite and magnetite, the 207Pb/204Pb:206Pb/204Pb isochron yields a value of 2550 ± 300 m.y.In the east the cry...

Partition of Strontium between Coexisting K-Feldspar and Plagioclase in Some Metamorphic Rocks

Abstract: Trace quantities of strontium in forty-four pairs of coexisting natural potassium and plagioclase feldspars extracted from metamorphic rocks were analyzed using an X-ray spectrographic technique. The main objective of this work has been to evaluate the usefulness of strontium distribution coefficients as indicators of metamorphic grade. Rocks from six different areas were studied in detail, most areas including two zones of metamorphism; in this way a range of metamorphic grade from middle amphibolite facies to various grades of granulite facies was investigated. Strontium partitioning between coexisting feldspar phases in isofacial rocks can be described by the Nernst distribution law and is of the type (mol per cent Sr-feldspar in K-feldspar)/(mol per cent Sr-feldspar in plagioclase) = constant. Data from most areas show systematic and significant changes in strontium partitioning with gross changes in metamorphic grade. With increasing grade, strontium is preferentially taken into the K-feldspar phase....

Regional Metamorphism, Metasomatism, and Partial Fusion in the Northwestern Part of the Okanogan Range, Washington

Abstract: The northwestern part of the Okanogan Range, Washington, (in the vicinity of 49°00 ′N., 120°00′W.) comprises regionally metamorphosed rocks, largely of supracrustal origin, plus igneous rocks ranging in composition from quartz diorite to quartz monzonite. Metamorphic rocks include quartz dioritic to trondhjemitic orthogneisses, biotitic and hornblendic schists, gneisses and hornfelses, amphibolites, and calc-silicate gneisses. Mineral assemblages indicate that the region was metamorphosed more or less uniformly under conditions equivalent to the sillimanite-cordierite-orthoclase-almandine subfacies (cordierite amphibolite facies) with possible local transition to granulite facies. The presence of andalusite plus sillimanite indicates metamorphism of the Abukuma type (andalusite-sillimanite facies series) in contrast to the kyanite-sillimanite facies series in the Northern Cascade Range to the west. A single cycle of synkinematic metamorphism followed by a phase or separate cycle of static recrystallization in essentially the same P-T field is shown by rock textures and mineral assemblages. The parent material of the metamorphic rocks included mafic flows, sills and dikes, intermediate composition volcanic rocks and graywackes, minor amounts of carbonate-rich and pelitic sedimentary rocks plus an indeterminate amount of leucocratic igneous rock. Prior to metamorphism the average composition of the parent material was approximately tonalitic. The igneous rocks include intrusive granodioritic and quartz monzonitic rocks (average composition is granodiorite) of the Cathedral batholith (Daly, 1912) with an apparent age of 94.0 ± 2.8 m.y. (K/Ar date on biotite) and older trondhjemitic to leucogranodioritic rocks. The older igneous series comprises partly gneissose syntectonic intrusive rocks, and directionless late to post-tectonic rocks which lack intrusive contacts. The latter are interpreted as anatectites formed by (partial) fusion of rocks of quartz dioritic to trondhjemitic composition during regional metamorphism. Field and petrographic data indicate that the anatectites have not been deformed. The average chemical composition of the older igneous series is trondhjemitic. A small layered gabbroic pluton, characterized by iron-rich olivine and pyroxene, has intruded the metamorphic series. Its age relative to the Cathedral batholith is not known. The metamorphism was pre-late Cretaceous and, by analogy with surrounding areas, probably was pre-mid Jurassic but post-Paleozoic. The age of the eugeosynclinal rocks, which were parent materials for the metamorphic rocks, is unknown, but a mid- to late-Paleozoic age is suggested by comparison with rocks of known age in adjacent areas. These eugeosynclinal rocks were definitely sialic, their average composition being tonalitic. A comparison between the average chemical composition of the metamorphic rocks and syn-metamorphic plutons and the average composition of eugeosynclinal rocks of the Pacific rim, such as graywackes, suggests that the rocks of the Okanogan Range may have been formed by nearly isochemical metamorphism of eugeosynclinal sedimentary and volcanic rocks.

Retrograde contact meta-morphism in the granulite facies terrain of Mont Tremblant Park, Quebec, Canada

Abstract: The Pre-Cambrian rocks of the Grenville province of south-west Quebec in the Mont Tremblant Park area consists of granulites and associated gneisses formed under granulite facies conditions which are intruded by members of an anorthosite suite. At the contacts of these intrusives especially the late-stage members, the granulites and gneisses were found to be retrograded into rocks with mineral assemblages typical of the amphibolite facies. The transformation of the granulite facies rocks into rocks of lower amphibolite grade can be attributed to local introduction of water which was supplied during the emplacement and crystallization of this late-stage, volatile-enriched magma of the anorthosite suite.

Contrasted types of metamorphism of basic intrusions in the Precambrian basement of the Tasîussaq area, South Greenland

Abstract: Metamorphosed rocks of three distinct episodes of basic intrusion can be recognised in the Precambrian basement of the Tasiussaq area, South Greenland. The oldest intrusions, represented by sills and dykes, are pyriclasites and biotitepyriclasites; the second episode intrusions, in the form of dykes, are pyroxenemetadolerites and the third episode intrusions, represented by dykes and small bodies, are metagabbros, metadolerites, metanorites and amphibolites. The metamorphic nature of the rocks of the three episodes is a reflection of age. Chemical and modal analyses of rocks from the three episodes are presented. Fresh diorite sills and dolerite dykes represent later episodes of Precambrian basic intrusion. The basic rocks depict the varying types of metamorphic conditions which affected the area in Precambrian time, and these are seen to differ from the established metamorphic history in areas to the north-west in South Greenland. The pyriclasites and biotite-pyriclasites have been derived through granulite facies metamorphism; the pyroxene-metadolerites by dipsenic metamorphism under conditions corresponding to the amphibolite facies and the metagabbros, metadolerites, metanorites and amphibolites through amphibolitisation during amphibolite facies metamorphism. It is suggested that the metamorphism producing the pyroxene-metadolerites (Sanerutian in age) was controlled by dipsenic conditions inherited from earlier granulite facies metamorphism (Ketilidian in age). This implies that the Ketilidian and Sanerutian metamorphisms in the Tasiussaq area are not separated by a long span of time and that the break in plutonism marked by the pyroxene-metadolerites cannot be regarded as a significant cratogenic hiatus between two separate plutonisms. The importance of water in controlling trends in the metamorphism of dolerites is stressed. The 1st episode intrusions have undergone severe changes since intrusion and no palimpsest features indicative of primary texture or mineralogy remain. The majority of the 2nd episode intrusions display a granular texture, but some display sub-ophitic and relic sub-ophitic textures. The 3rd episode intrusions display a range from ophitic, sub-ophitic and microporphyritic textures to relic stages of these textures. The 1st episode intrusions were emplaced into a geosynclinal pile of sediments and were probably connected with the volcanicity which occurred at the end of sedimentation. The 2nd and 3rd episode intrusions were emplaced into granitic and metamorphic rocks at a later stage in the same 'geological cycle'. Both the 2nd and 3rd episode intrusions are considered to indicate trends in the crust towards brittle conditions marking temporary partial withdrawals of the thermal front. Their preserved ophitic and sub-ophitic textures are not indicative of emplacement and crystallisation in cratogenic conditions.

The Nomenclature and Classification of Granulites and Related Rocks of Mont Tremblant Park, Quebec, Canada

Abstract: The nomenclature of the granulites which are the characteristic rocks of many granulite facies terrains is reviewed. The granulites which are defined in the classical sense are found in two distinct geological settings. In the high-grade metamorphic terrains of the Precambrian shields, the granulites occur associated with large anorthosite bodies as in the case of Mont Tremblant Park, Quebec, Canada. Also granulites are found in cores of old orogens here associated with rocks of eclogitic affinities. A simple classification of granulites consists of two prinicipal groups; quartzo-feldspathic and hypersthene granulites.

Prekambr w polskiej części Sudetów

Abstract: PRE-CAMBRIAN IN THE POLISH PART OF THE SUDETES Summary The gneissic block of the Sowie Mts., situated in the central part of the Sudetes and of their foreland, represents the oldest area of the region considered. The block is of triangle outline (Figs. 1 and 2), built up mainly of plagioclase para-gneisses and migmatites, derived apparently from an Archaean supracrustal complex. These rocks are accompanied by microcline-plagioclase augen gneisses, amphibolites, granulites, small granite bodies arranged concordantly, and some other crystalline rocks. The tectonics of the gneissic complex of the Sowie Mts. has recently been investigated by W. Grocholski (1961, 1967), who has distinguished three main “b” lineations found in this highly folded complex. He has classified these lineations as B 0 , B 1 and B 2 , the B 0 lineation being thought to be the oldest one. According to W. Grocholski, all the three sets of linear structures are of Pre-cambrian age. It should also be stressed here that the block of the Sowie Mts. suffered a considerable remodellnig along its three marginal parts, when the Variscan movements began to operate. This can be observed mainly in the eastern part of the block, where large masses of mylonites and cataclasites were formed due to mechanical trantsformation of gneissic material. The fold pattern of the Sudetes and of their foreland is arranged tangentially to the three margins of the Sowie Mts. triangle (Fig. 2). The pattern consists of three main arches the northern, southern and eastern ones. These fold arches are built up mainly of different metamorphic members referred to Pre-Cambrian, old Palaeozoic or Devonian. However, the stratigraphy of the old crystalline sequences is in most cases rather arbitral. In the northern arch the Eocambrian (Algonkian) sequence is represented by Lusatia greywackes and Radzimowice beds, the latter being known from the Kaczawskie Mts. only. The Radzimowice beds are built up principally of various phyllites, mainly dark grey or black in colour. These consist of quartz, sericite and carbonates, as well as of changing amount of albite. Graphitic pigment, more or less abundant, is usually present. Meta-phtanites, quartzites and quartzite schists are subordinate constituents of the rock. The Pre-Cambrian of the southern arch of the Sudetic fold pattern is represented by a supracrustal member that chiefly consists of mica schists, para-gneisses, amphibolites, crystalline limestones, erlans and other rocks. The rock sequence is accompanied by microcline-plagioclase gneisses thought to be related to the Assyntic tectogenic processes. In Poland, the latter rocks are represented by the Izerskie gneisses that constitute the north-western part of the Karkonosze block (Fig. 1). The Izerskie gneisses are, according to M. Kozlowska-Koch (1965) of polygenetic character. In part, they were formed due to granitization of mica schists, in part, as a result of gneissification of some granitoid intrusions. Traces of granitized hornfelses are observed, as well. Some Pre-Cambrian rock assemblages are exposed in the eastern Sudetic arch on vast areas. In the region of Śnieznik - Lądek, the Pre-Cambrian is represented by the Stronie series. This is a supracrustal assemblage for the most part composed of mica schists end paragneisses. The rocks are accompanied by amphibolites, quartzites, erlans, crystalline limestones and some other rocks. The regional granitization that took place within the Stronie series resulted in formation of different types of microcline-plagioclase gneisses (K. Smulikowski, 1957, 1960). Two main groups of gneisses, differing both in structural feature and in texture, may be distinguished in the region under consideration: Gieraltow gneisses and Śnieznik gneisses. The former are fine-grained, laminated, streaky or migmatic rocks. Locally, they are associated with granulites, or contain small amphibolites lenses, or small eclogite bodies. The latter that belong to a group known as Śnieznik gneisses, are represented by coarse-grained flaser and augen gneisses. The rocks of these two groups occur in both thick and thin layers and are arranged alternately where large masses of the Śnieznik gneisses are in contact with the Gieraltow ones. Both gneissic assemblages are thought to be related to the Assyntic tectogenic processes, although direct evidences are lacking here. The NW trending zone of Niemcza exhibits numerous various rocks, most of them referred rather to Pre-Cambrian. In the western part of the zone, considerable masses of mylonites are found, derived from the Sowie Mts. gneisses, and produced during several periods of high deformations. The deformations were a result of the thrusting of the Sowie Mts. block towards the east. Close to the mylonites, there occur blasto-mylonitic mica schists, accompanied by graphite schists, quartzite schists and quartzites. All these rocks are thought by H. Dziedzic to be rather of Pre-Cambrian age (1961, 1965). Nevertheless, slates have been found to occur in the Niemcza zone, revealing spore flora characteristic of the Visean/Namurian time (H. Dziedzic and T. Gorecka, 1965). These are intercalated between older crystalline member rather concordantly, this being an evidence that in the Niemcza zone the younger Variscan movements were also highly intense. Metamorphic rocks that encase the intrusion of the Strzelin granite are composed of both Pre-Cambrian and Devonian rocks. The rocks referred commonly to Pre-Cambrian consist of mica schists and paragneisses, accompanied by quartzites, amphibolites, erlans and crystalline limestones. Another series of quartzites, quartzite schists, meta-conglomerates and phyllites has been described as Lower, perhaps also as Middle Devonian in age (J. Oberc, 1966). Both sequences are folded concordantly with one another. The main deformation of the metamorphic sequence in the Sudetes may be thought as Variscan in age. In many regions it may have been accomplished in a time period between the Caledonian and Variscan epochs of folding. In certain cases, the high folding processes began already at the Caledonian time, and then developed during several Varisean phases. In various regional unit, however, the period of main movements could have been different (compare H. Brause 1965 and H. Jaeger 1963). The gneissic block of the Sowie Mts. seems to be the only region in which the main deformation can be considered as Pre-Cambrian. Two, or rather three distinct lineations, differing in age and in structural development, are found to occur in all the metamorphic assemblages of the Sudetes. According to this author they may be classified as B 1 , B 2 and B 3 sets of minor structures respectively. The set B1 is everywhere the most prominent one, and represents the most differential assemblage of the minor structures. It runs parallel to the major structural unit and may be interpreted as belonging to the main deformation. In the old crystalline series related to Pre-Cambrian, the set may be polygenetic in character. Some minor structures formed during the Pre-Cambrian deformation, or at the time when the Assyntic movements were in operation. They could have been remodelled and supplemented during the subsequent high folding. Where both structures are arranged parallely to each other, the older minor structures can hardly be discerned from the younger ones. The lineations B 2 and B 3 are younger than the B 1 set of the minor structures. They trend transversally or obliquely to the B 1 lineation and the major fold structures; however, they are parallel arranged to the younger dislocations here. The development of the B 2 and B 3 sets of these structures is rather monotonous. They are represented only by folds that range from microscopic ones up to several metres along the cross section line. 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Structure and Petrography of the Quartz-Magnetite and Associated Rocks of Vemparala Area, Nellore District, Andhra Pradesh

Abstract: In the Nellore and Guntur districts of Andhra Pradesh a series of detached hills composed of interbanded quartz-magnetite rocks, garnetiferous quartzites, pyroxene granulites, hypersthene-magnetitites and ferruginous schists extend for 33 miles from Konijedu in the south to Vemparala in the north. This paper mainly deals with the structure and petrography of the northernmost group of these rocks at Vemparala. In this area the rocks have been metamorphosed and folded, whose axial plane trace only (FI) can be recognised. These have been folded again (FU) with eastward dipping axial planes trending NNW-SSE accompanied by gabbroic and granitic intrusions. The third folding (FIll), in the nature of a cross-fold, is along axes plunging NNE. There are many faults trending roughly N-S and E-W. Mapping of the structure of a part of the eastern margin of the Cuddapah Basin between Cumbum and Markapur by the authors has revealed the presence of an earlier fold axis trending NW-SE and plunging due NW and a later cross-fold trending NNE-SSW and plunging due NNE. A study of the region from here eastwards to the Konijedu-Vemparala area, a distance of about 60 miles, has shown that the earlier fold axes (or axial plane traces) vary in trend, but the cross-fold axes show fairly consistent trend and plunge due NNE. It is therefore suggested that this region as a whole must have been affected by one and the same set of tectonic forces during its last stage of deformation.

Tectonic History of the Area East of Khammam, Andhra Pradesh

Abstract: East of Khammam town in Andhra Pradesh, the Precambrian rocks are classified into three distinct lithological groups as (a) Peninsular Granites (b) Pakhals and (c) Gncissic Complex. Detailed structural studies on foliation, lineation, joints, folds, trends of dykes, basic enclaves, pegmatite and quartz veins of these groups show that the Peninsular Granite area with its NNW trend is structurally quite distinct from the remaining area which shows NW as well as NE trends. Mineralogical variation diagrams drawn for acid granulites, as a part of the quantitative petrological studies, show that the emplacement of these granulites has been structurally controlled by NE trend. Petrological studies in the Gneissic Complex show increasing grade of metamorphism roughly from northwest to southeast. It is concluded that the area in general is affected by two phases of disturbance, one, Dharwarian and another, late' Eastern Ghat' orogeny. It is also suggested that the demarcation line between 'charnockitic region' and 'non-charnockitic region' of Fermor should pass through the contact between Peninsular Granites and Pakhals.