Showing papers on "Granulite published in 1972"

A Comparison of Recent Experimental Data on the Gabbro-Garnet Granulite-Eclogite Transition

Abstract: Recent experimental data presented by Ito and Kennedy on the mineralogical variations in the gabbro to eclogite transformation are shown to be consistent with earlier experiments on a variety of basaltic compositions. The new results support, for a specific composition, two of the major conclusions of our previous work: (1) the gabbro to eclogite transformation takes place through a rather broad transition interval in which mineralogies appropriate to high-pressure granulite assemblages are formed, and (2) the slope of the gabbro to eclogite transition interval is such that eclogite is the stable mineralogy for dry basaltic rocks along normal geothermal gradients in the continental crust (stable or shield regions).

Zircon U-Pb ages of granulites from the Central Region of the Lewisian, northwestern Scotland

Abstract: The overprinting of the high grade rocks of the Scourian episode, during the Inverian and Laxfordian episodes, has so far prevented a reliable estimate being made of the age of the Scourian granulite–facies metamorphism in the Central Region of the Lewisian of northwestern Scotland. In an attempt to solve this problem the U–Pb isotopic systems of zircons from two samples of hypersthene granulite from the Kylesku group have been examined. These zircons have discordant apparent ages complicating the interpretation of the results. However, by measuring a number of size fractions from each of the two zircon suites it has been possible to demonstrate that a major isotopic event or events took place approximately 2700 m.y. ago in these granulite–facies rocks. It has also been shown that the two discordance trends found for the two zircon populations can be explained in terms of episodic disturbances of the U–Pb isotopic systems at times which correspond to the known ages of ‘Laxfordian’ and ‘Caledonian’ events.

Ages of Zircons from the Northern Cascade Mountains, Washington

Abstract: Pre-Late Cretaceous crystalline rocks in the core of the Northern Cascade Mountains of Washington are predominantly heterogeneous supracrustal gneiss and schist, and dioritic, quartz dioritic, and trondhjemitic plutons and bodies of orthogneiss. Migmatite derived from plutonic and supracrustal rocks is also widespread. Isotopic age determinations of zircons indicate that, of the supracrustal gneiss and schist, the Swakane Gneiss and possibly the Skagit Gneiss may have been deposited > 1,650 m.y. ago. Alternatively, some of these rocks may be younger but were derived at least in part from a >l,650-m.y.-old source terrane. A second group of supracrustal rocks, the Younger gneissic rocks of the Holden area, includes a metavolcanic unit about 265 m.y. old. Plutonic rocks and orthogneisses comprise four main age groups: 1,452 to 2,000 m.y., pyroxene gneiss of the Yellow Aster Complex; about 460 m.y., the Turtleback Complex of the San Juan Islands and younger orthogneiss of the Yellow Aster Complex; 220 m.y., plutonic and gneissic rocks of the Marblemount belt (Marblemount Meta-Quartz Diorite, Dumbell Mountain plutons); and 92 m.y., the Eldorado Orthogneiss. Strongly metamorphosed rocks of the Chelan Complex, chiefly quartz dioritic, have zircon Pb 206 /U 238 ages ranging from 100 to 183 m.y. These ages appear to be discordant, and the Chelan Complex may represent remobilized rocks from the 220-m.y.-old Marblemount belt. Ages of metamorphic minerals, zircons from pegmatitic material associated with migmatite, and zircons from synkinematic intrusive rocks indicate two major episodes of metamorphism in the Northern Cascade terrane, one about 415 m.y. ago and the other, 60 to 90 m.y. ago. Both episodes culminated in metamorphism to amphibolite facies grade—and in the case of the older episode, metamorphism to granulite facies grade—and both resulted in extensive migmatization of pre-existing rocks.

The catazonal poly-metamorphic rocks of Cabo Ortegal (NW Spain), a structural and petrofabric study

Abstract: The petrological study of the southern part of the Cabo Ortegal area is a complement of Vogel’s (1967) investigation of the northern half. The present investigations include a structural as well as a petrofabric study. The rocks belong to an eugeosynclinal sequence which during the Precambrian underwent prograde metamorphism from the staurolite-almandine-muscovite and the kyanite-almandine-muscovite subfacies of the almandine-amphibolite facies through the clinopyroxene-pyralmandine (± hornblende) granulite facies into the eclogite facies (M1). Several of these zones, bounded by isogrades, which are sometimes tectonic in nature, have been mapped. From the fact that the banded gneisses are only metatexitic and from the jadeite content of omphacite, the P/T conditions for the eclogite facies are estimated as: T\u2248700°-750°C, P\u224811-13 Kb, and water vapour pressures: very low. In the gneisses isoclinal folding (F1) accompanied the metamorphism. The axial planes are subhorizontal and the fold axes plunge just west of north. The fabric analyses of eclogite (point-maximum for [010]) and basic granulite (point-maximum for [001]) show that the same stressfield which produced the F1-folds in the paragneisses, influenced the preferred orientation of clinopyroxene. A second Precambrian metamorphic phase retrograded the rocks into the hornblende granulite facies (M2). Before the onset of this phase pronounced cataclasis caused the formation of thick mylonitic horizons, followed by E-W trending drag folding (F2). The fabric diagram for clinopyroxene does not show a preferred orientation. During this deformation phase the M1 metamorphic zoning was turned upside down by a combined process of folding and thrusting. In the paragneisses the hornblende granulite metamorphism is marked by a second generation of kyanite. Gabbros, intruded along thrust planes, were partly metamorphosed into garnet-coronites. During this second metamorphic phase isoclinal folds (F3) with subhorizontal axial planes and N-S axial directions were formed. The fabrics of these folds show a marked orientation of the c-axes of (metastable) diopside and brown-green hornblende parallel to the fold axis direction. A third metamorphic phase caused further retrogradation of the rocks into the amphibolite facies (M3). The characteristic amphibole of this phase is a blue-green hornblende. The former metabasites were metamorphosed into (garnet-)amphibolites. Intruded gabbros were transformed along their margins into ‘flaser’ amphibolites. Folds with vertical axial planes and N-S axial directions reflect the synchronous F4-deformation. The large syn- and antiform structures are products of this phase. The fabric of the hornblende in the amphibolites is determined by the stress field of F4. Older hornblende orientations were destroyed. Whether a Hercynian age should be attributed to the amphibolite facies is not certain; if so, F4 is the first Hercynian deformation phase. After the overthrusting of the complex over its low-grade country rocks, a phase of chevron folding (F5) was active locally. On the thrust plane small folds of the second Hercynian folding phase can be discerned. A third Hercynian folding phase can be seen in the Paleozoic rocks but not in the Cabo Ortegal Complex proper. Local greenschist retrogradation (M4) and the emplacement of dolerite dykes are late Hercynian. The tectonic history ends with a phase of normal block faulting which caused the E-W faults.

An example of structural and metamorphic relationships in the Musgrave orogenic belt, central Australia

Abstract: The western Musgrave Ranges are broadly divided into three groups of metamorphic rocks. A central granulite‐facies core is bounded on the north by rocks of amphibolite grade and on the south by rocks transitional between the granulite and amphibolite facies. Faults trending east‐west separate the three groups of rocks. The detailed structural relationships between the granulites and the lower grade rocks are described and discussed. The granulites are structurally relatively simple and are characterised by the presence of a strong southwesterly‐plunging, mineral‐streaking lineation. In marked contrast, the transitional rocks are more complexly folded on a macroscopic scale and they also have a well‐developed mineral lineation plunging to the southeast. These two lineation orientations are considered to be directions of maximum elongation. The amphibolite‐facies rocks are also complexly folded and at least two lineations related to different phases of deformation have been recognized. A suite of f...

Sapphirine-bearing rocks from Wilson Lake, Labrador

Abstract: Optical properties, associated mineral assemblages, granulite facies conditions, abundant hematite, paragneisses

Comments on: "A Comparison of Recent Experimental Data on the Gabbro-Garnet Granulite-Eclogite Transition"

Abstract: We are delighted that Green and Ringwood are in close agreement with our recently published experimental results on the details of the transition of gabbro to garnet granulite to eclogite. Our apparent disagreement is in the extrapolation of the data and interpretation of the results. In our view, the seismic discontinuity in the mantle between $$V_{p}$$ velocities of 7.5 km/second and 8.2 km/second may well represent the transition from garnet granulite to eclogite. In addition, we believe that where an intermediate velocity zone with $$V_{p}$$ of about 7.5 exists in the mantle, this zone may well be made of garnet granulite and we suggest that its upper limit is that of a rate process boundary. We know of no evidence, either field or laboratory, that confirms their view that "eclogite mineralogy is stable in dry basaltic rocks along normal geothermal gradients throughout the continental crust in stable or shield regions."

Reactions involving hydration of cordierite and hypersthene

Abstract: Petrographic evidence for the hydration reactions: cordierite + water = gedrite + kyanite + quartz, and aluminous hypersthene + water = aluminous anthophyllite, is shown by some granulite facies rocks in the Arunta Complex, near Alice Springs, central Australia. Electron microprobe analyses enable the reactions to be written in some detail. Preliminary thermodynamic and experimental information suggests that the reactions took place at relatively high confining pressures. The reactions may have proceeded more in response to addition of water to the system, rather than a major change in P-T conditions.

Granulites in Ethiopian Basement

Abstract: IT is well known1 that rocks of the granulite facies including charnockites are common in the mobile belts of Africa. But they are less important than metamorphics of amphibolite facies comprising the greater part of the belts. In the Mozambique Belt the granulites are widely reported1,2. Some authors2 believe them to be remnants of older structure incorporated in the younger tectonic framework. It is significant that the oldest absolute age in the belt3 (3,600±100 m.y.) originates from charnockite.

The Precambrian Gneisses and Supracrestal Rocks of the Western Shore of Kaipokok Bay, Labrador, Newfoundland

Abstract: In the Kaipokok Bay area the amphibolite facies Hopedale Complex is overlain by the English River Greenstones. The Hopedale Complex is formed of gneisses, amphibolites, and associated granites and shows indications of being polycyclic, possibly derived from an older (Aphebian?) granulite facies terrane. The English River Greenstones are a supracrustal sequence of dominantly metavolcanic rocks and are the equivalents of the Aillik series. Hudsonian orogenic effects were superimposed on both rock units and caused locally intense refoliation of the gneisses and the development of a strong foliation in the greenstones. During this orogenic cycle an early period of intense zonal flattening was followed by a more regionally developed folding and weak tectonite fabric development. All of these rocks were cut by later acidic and basic intrusions. The geological history of the area shows a general similarity to that of southwestern Greenland.

The principles of petrology : an introduction to the science of rocks

Abstract: I Introduction.- The Science of Petrology.- The Earth Zones.- The Barysphere.- Composition of the Earth Shells.- Chemical Composition of the Crust.- Rocks and their Composition.- The Rock-forming Minerals.- The Classification of Rocks.- I The Igneous Rocks.- II Forms and Structures of Igneous Rocks.- Forms of Igneous Rocks.- Lava Flows.- Intrusions and Their Relations to Geological Structure.- Forms in Unfolded Regions.- Sills.- Laccoliths.- Lopoliths.- Dykes.- Ring-dykes and Cone-sheets.- Volcanic Necks.- Forms in Folded Regions.- Phacoliths.- Chonoliths.- Batholiths.- Multiple Intrusions.- Composite Intrusions.- Differentiated Intrusions.- Structures of Igneous Rocks.- Structure and Texture.- Vesicular and Amygdaloidal Structure.- Block Lava and Ropy Lava.- Pillow Structure.- Flow Structure.- Jointing, Sheet, and Platy Structures.- Columnar and Prismatic Structures.- Rift and Grain.- III Composition and Constitution of Magmas.- The Magma.- Composition of Magmas.- The Pyrogenetic Minerals.- The Physico-chemical Constitution of Magmas.- Primary Magmas.- IV The Formation of Igneous Rocks.- Glass and Crystals.- Crystallisation of a Unicomponent Magma.- Grain of Igneous Rocks.- Formation of Glass.- Crystallisation of Binary Magmas.- Eutectics-Mixed Crystals.- Crystallisation of Ternary Magmas.- Crystallisation of some Ternary systems.- The Reaction Relation.- V Textures and Microstructures.- Definition and Description.- Crystallinity.- The Beginnings of Crystallisation. Crystallites and Microlites.- Devitrification.- Granularity.- Shapes of Crystals.- Mutual Relations of Crystals.- Equigranular Textures.- Inequigranular Textures.- Porphyritic Texture.- Poikilitic Texture.- Directive Textures.- Intergrowth Textures.- Reaction Structures.- Xenolithic Structures.- Orbicular Structure.- Spherulitic Structure.- Fracture Forms.- VI Classification of Igneous Rocks.- Bases of Classification.- The Factor of Chemical Composition.- The Factor of Mineral Composition.- The Factor of Geological Occurrence and Texture.- Tabular Classification.- Nomenclature.- Granite, Granodiorite, and Diorite.- Syenite, Nepheline-syenite, and Related Alkaline Rocks.- Gabbro, Anorthosite, and Peridotite.- Dolerite and Lamprophyre.- Rhyolite and Dacite.- Trachyte and Phonolite.- Andesite and Basalt.- VII The Distribution of Igneous Rocks in Space and Time.- Consanguinity.- The Diagrammatic Representation of Igneous Rock Series.- Kindreds of Igneous Rocks.- Petrographic Provinces and Periods.- Igneous Action and Earth Movements.- Time Sequences in Igneous Rocks.- VIII The Origin of Igneous Rocks.- Variations in Igneous Rocks.- Evidences of Differentiation. Variation within a Single Rock Body.- Theories of Differentiation.- Differentiation by Liquid Immiscibility.- Differentiation by Crystallisation.- Gravitational Differentiation.- Filtration Differentiation.- The Role of Volatile Constituents in Differentiation.- Pegmatite and Aplite.- Assimilation and Hybrid Rocks.- Origin of Alkaline Rocks.- II The Secondary Rocks.- IX Introduction.- General.- The Breaking-down of the Rocks.- Decomposition of Rocks.- Disintegration of Rocks.- Transport.- Deposition.- Classification of the Secondary Rocks.- X The Residual Deposits.- Residual Deposits in General.- Terra Rossa.- Clay-with-Flints.- Laterite and Bauxite.- Soils.- XI Sedimentary Rocks: Mineralogical, Textural, and Structural Characters.- Mineral Composition.- Grain Size.- Heavy Minerals in Sands and Sandstones.- Shape and Rounding of Grains.- Cohesion.- Stratification.- Minor Structures and Markings.- XII Sedimentary Rocks. Descriptive.- Classification.- Rudaceous Rocks: Breccias and Conglomerates.- Arenaceous Rocks: Sands and Sandstones.- Silt and Siltstone.- Argillaceous Rocks: Clays and Shales.- XIII Deposits of Chemical Origin.- Chemical Deposits in General.- Concretions.- Secretions.- Colloids.- Siliceous Deposits.- Carbonate Deposits.- Ferruginous Deposits.- Salts.- XIV Deposits of Organic Origin.- Organic Deposits in General.- Organic Rocks of Calcareous Composition: The Limestones.- Phosphatic Deposits of Organic Origin.- Siliceous Deposits of Organic Origin.- Carbonaceous Deposits: Peat and Coal.- III The Metamorphic Rocks.- XV Metamorphism.- General Character of Metamorphism.- Agents of Metamorphism.- Kinds of Metamorphism.- Facies and Grades of Metamorphism.- XVI Metamorphic Minerals, Processes, and Structures.- Influence of Original Composition.- Influence of Heat and Uniform Pressure.- Influence of Directed Pressure.- Textures and Structures in Metamorphic Rocks.- Shapes of Metamorphic Minerals.- Growth and Mutual Relations of Minerals in Metamorphic Rocks.- Structures of Metamorphic Rocks.- Classification and Nomenclature of Metamorphic Rocks.- XVII Cataclastic Metamorphism and Its Products.- Cataclastic Metamorphism in General.- Slates and Slaty Cleavage.- Crush-breccia and Cataclasite.- Flaser Rocks and Mylonite.- XVIII Thermal Metamorphism and Its Products.- Thermal Metamorphism in General.- Thermal Metamorphism of Clay Rocks.- Contact Metamorphism of Clay Rocks.- Pyrometamorphism of Clay Rocks.- Thermal Metamorphism of Limestones.- Thermal Metamorphism of Arenaceous Rocks.- Thermal Metamorphism of Basic Lavas and Tuffs.- XIX Dynamothermal Metamorphism and Its Products.- Dynamothermal Metamorphism in General.- Dynamothermal Metamorphism of Argillaceous Rocks.- Dynamothermal Metamorphism of Quartzo-Felspathic Rocks.- Dynamothermal Metamorphism of Basic Igneous Rocks and Tuffs.- XX Plutonic Metamorphism and Its Products.- Plutonic Metamorphism in General.- Granulite, Leptynite, Leptite.- Pyroxene-gneiss, Pyroxene-granulite, and the Charnockite Series.- Eclogite and Garnet-amphibolite.- XXI Metasomatism and Additive Processes of Metamorphism.- Metasomatism.- Metasomatic Processes.- Pneumatolytic Metamorphism.- Injection Metamorphism and Autometamorphism.- Lit-par-lit Gneiss, Composite Gneiss, Anatexis and Palingenesis.

K-Ar and Rb-Sr Geochronology of the Canopus Pluton, Hudson Highlands, New York

Abstract: A six-point whole-rock isochron of 1032 ± 45 (Iσ) m.y. has been established for the Canopus diorite-monzonite pluton in the Hudson Highlands of New York (41°20′ N.-73°55′ W.). The Canopus pluton intrudes Precambrian gneisses of hornblende granulite metamorphic grade that formed during a dynamothermal event about 1150 m.y. ago, based on previously reported zircon data (Long and Kulp, 1962). Structural studies indicate intrusion was accompanied by right-lateral strike-slip faulting along the ancestral Ramapo fracture system (Ratcliffe, 1971), which was contemporaneous with a period of F2 folding in the gneisses along a N. 40° E.-trending fold system. The pluton is thought to have been emplaced during a period of strike-slip faulting and the F2 folding episode. K-Ar and Rb-Sr mineral ages range from 700 m.y. to 524 m.y. in the igneous rocks. The lower ages are found near the fracture zone west of Canopus Hollow on the east side of the pluton and may have been produced by reactivation of the fault zone in Phanerozoic time, or may be a consequence of proximity to a region affected by Paleozoic metamorphism. Phlogopite from a phlogopite-talc calcitic marble shear zone in diopside calcite marbles has a K-Ar age of 893 ± 10 m.y. This phlogopite establishes a minimum age for cataclasis and retrogression along the western mylonite zone at Canopus Hollow that is consistent with the Rb-Sr whole-rock age of the pluton, and suggests that this marble is of Precambrian age as stated by Berkey and Rice (1919).

Structure and metamorphism of the Lewisian of East Tiree, Inner Hebrides

Abstract: Synopsis A study of the Lewisian of east Tiree has shown the presence of previously unmapped psammitic, calcareous and semipelitic paragneisses which occur, often with basic gneisses, in the ‘grey gneiss’ of the area. Four main phases of folding have been determined: F 1 —isoclinal intrafolial folds; F 2 —tight similar folds; F 3 —close and open folds with NW.–SE. axial planes; and F 4 —open and gentle folds with NE.–SW. axial planes. Fold phases (F 1 –F 3 ) have been related to successive metamorphic episodes of granulite facies, upper amphibolite facies and lower amphibolite facies which have been assigned to the Scourian, Early and Late Laxfordian respectively on the basis of their structural relationship with intrusive basic bodies which are now amphibolites. The relations of the ‘grey gneiss’ to the paragneisses and basic gneisses suggest the existence of a pre-Scourian basement. The region is cut by late shears associated with the formation of pseudotachylite. The sequence in Tiree is related to the structural and metamorphic sequences given by other authors for the Outer Hebrides.

Tektonika podłoża krystalicznego prekambryjskiej platformy w Polsce

Abstract: TECTONICS OF THE CRYSTALLINE BASEMENT OF THE PRECAMBRIAN PLATFORM IN POLAND Summary As a result of the differentiated mobility of the crystalline basement, expressed in the form of a general deformation, i.e. bending or curvature of some of its parts, an erosional plan of the surface distribution of the Gothian, Sub-Jotnian and Jotnian complexes have developed. Primarily they were more wide-spread. An “undulation” of the crystalline basement, still before the sedimentation of the Wendian cover, led to a complete or almost complete removal of the Gothian, Sub-Jotnian and Jotnian complexes from the elevated portions of the basement. They persisted only in the depression areas of the basement. The Mazowsze, Dobrzyn and Pomerania granitoid massifs are, as far as their area is concerned, the largest tectonic units of the crystalline basement. They are surrounded with the Svecofenno-Karelian systems. The older Precambrian complexes made here the substratum of the granitoid massifs. Within the Svecofenno-Karelian geosyncline these massifs most probably played a part of the central massifs, and in their marginal portions were deformed. In the Gothian cycle they underwent regeneration, expressed as granitization and migmatization of the older substratum, as well as local mobilization of the rheomorphic and anatectic granitoids. On these granitoids the Gothian formations accumulated, later on metamorphosed and then eroded almost completely, except for small isolated patches only. The activation, expressed in the form of disjunctive tectonics, was responsible for the formation of grabens and depressions on the massifs, where the Jotnian molasse accumulated due to the lowering and uplifting movements. The deep rupture zones were used by the anorogenic, alkalic-gabbroidal, alkalic-ultrabasic and alkalic intrusions, as well as by the Jotnian subvulcanites and vulcanites. The differentiated vertical mobility of the basement led also to the wide deformations of the sedimentary cover and to the use of the disjunctive zone by the Wendian and Lower Palaeozoic vulcanites and subvulcanites. Svecofenno-Karelian metamorphic complexes. The Svecofenno-Karelian branches, represented by the Podlasie, Ciechanow and Kaszuby complexes, which link the granitoid massifs, represent the well-directed systems of the crystalline basement. In the synclinorial units the Svecofenno-Karelian complexes are represented by gneisses and migmatites, and in the anticlinal (or anticlinorial) ones - by primorogenic, metamorphic granitoids, locally accompanied with the Gothian palingenetic granitoids. Within the more eroded areas, the gneisses are not too thick, and occur as patches in granitoids, disclosing numerous post-orogenic vein granites, pegmatites and palingenetic granitoids. The middle part of the Podlasie complex is most eroded. It consists of several synclinoriums and anticlinoriums of lower order. The petrological analysis demonstrates that the synclinorial intra structures and the separating anticlinorial ones are built of the formations related to the deeper, almost axial geosynclinal zones. This is proved by a considerable percentage of pyroxene granulites, anderbites, charnockites, gabbro-amphibolites, leucogabbro-anorthosites in the metamorphic series. They represent altered, pre-inversion volcanogenic-sedimentary formations, and are basal formation of a large metamorphic complex. The next, deeply eroded part of the Podlasie complex is the Sejny-Augustow . structure. It is disturbed by a zone of tectonic fractures, thrown, and almost completely hidden under the Gothian macrostructure. Thus, in the substratum of the porphyroblastic, hornblende-biotite granitogneisses pyroxene granulites and enderbites can be expected to occur. The rocks of the Slawatycze elevation are here the third deeply eroded part of the Svecofenno-Karelides of the Podlasie complex. Here are found strongly granitized products of preorogenic volcanism. The Svecofenno-Karelian system is characterized by the intense folding of rocks which dip under an angle of about 70–90°, A high anisotropism of the structures, a complicated folding, and an abrupt inclination of the rocks were favourable for the multiple rejuvenation of the mylonitization zones, cataclasis, and brecciation showing directions concordant with the general structural plan, and those of transverse character. These processes were particularly common within the boundary zones of various objects, e.g. isotropic granitoid massifs and anisotropic units of the Svecofenno-Karelian system, as well as between the gneissic intrastructures and primorogenic granitoids, also within the gneissic complexes. A tectonic animation took place in the Gothian cycle, when the Svecofenno-KareIian basement was being covered with the supracrustal rocks subjected then to metamorphism. Such a specific tectonics in the basement was posthumously favourable also for the Wendian activation and basalt outflows. The Ciechanow branch of the Svecofenno-Karelides has so far insufficiently been penetrated by drillings. The recently recognized Kaszuby branch of the Svecofenno-Karelides occurs in the East Pomerania area. It has so far been established that the rock complexes of this branch come from a deeper part of the geosyncline, and are built up of pyroxene granulites, charnockites and pyroxene gneisses. Gothian metamorphic complexes. The reconstruction of the crystal .. line basement, begun in the Svecofenno-Karelian cycle, continued in the Gothian cycle. The Gothian rocks overlying the Pre-Svecofenno-Karelian substratum, are represented by metamorphic schists and gneisses. In the basement of the Peri-Baltic depression, metamorphic schists and gneisses are found sporadically. Predominant are here porphyroblastic granite gneisses, rapakivi-like granitoids, and anorthosite-norite intrusions. In the granitoid massifs – regenerated during the Gothian cyde – land within the marginal zones of the Svecofenno-Karelian system, are found palyngenetic and anatectic granitoids surrounded with metasomatic granites and migmatites. The supracrustal Gothian formations were developed in one metamorphic cycle only. Their dip changes, ranging for the most part from 40 to 70°. Their deformation structures are less complicated than in the polymetamorphic Svecofenno-Karelian formations. On the Svecofenno-Karelian basement these rocks are more disturbed, their granitization and mylonitization being more strongly expressed. The contact zone of the Mazury complex of the Gothides with the Svecofenno-Karelian system and its central granite massifs was in the Sub-Jotnian and Jotnian times an area of both tectonic and magmatic activation. To this zone are related anorogenic alkalic-gabroidal, alkalic-ultrabasic and alkalic intrusions. The Karelian-Gothian cycle ends with the molasse sedimentation in grabens and depressions, accompanied with the alkalic magmatism and subvolcanism. The Jotnian rocks are found in patches showing a meridional elongation related to the system of the Gothian dislocations. The analysis of thicknesses, ranges, and formational and facial development of the individual structural complexes in the sedimentary cover proves the differentiated vertical movements of the crystalline basement of the Precambrian platform, from the Precambrian time to the present day. The vertical movements of the crystalline basement are, due to the displacement of the masses in its substratum, caused by the equalization of their density or thermal differentiation. After the crystallization period of the crystalline basement these movements were intensified by the erosional processes, which removed huge masses of the folded rock massif and leveled its surface. The displacement of the material and its accumulation in the form of molasse at other sites were also the effect of the differentiated vertical movement of the basement. 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Hornblende–garnet Granulites from Hatton, Ceylon

Abstract: Quartz–microperthite–hornblende–garnet–plagioclase granulites belonging to the Highland Series of Ceylon are described. The invariable presence of accessory graphite suggests their metasedimentary origin and their chemical composition indicates a possible derivation from feldspathic sandstones. Although the mineral assemblage of these granulites is diagnostic for the upper almandine–amphibolite facies, they are interlayered with orthopyroxene-bearing charnockites of the granulite facies. It is concluded that PHO was initially higher in the sandstones than in the surrounding rocks, and that it remained so during metamorphism under granulite facies conditions, thereby retarding the dehydration reactions that took place elsewhere in the vicinity.

Evolution of the granulites and sub-division of the granulite facies in Ceylon

Abstract: Three subfacies of the granulite facies are recognized: garnet–diopside–quartz subfacies; pyroxene–granulite subfacies; and hornblende–granulite subfacies The erection of a garnet–diopside–quartz subfacies follows De Waard's suggestion (1965) of a garnet–clinopyroxene subfacies. Two major divisions of the hornblende–granulite subfacies are now recognized, (i) garnet–biotite division, (ii) cordierite division. Three sub-divisions of the former are suggested on the basis of critical basic assemblages. Since wollastonite has been found to have a limited areal occurrence within the latter, a two-fold sub-division of the cordierite division is suggested on the basis of the assemblages, (a) calcite–quartz, (b) wollastonite. A seven-fold sub-division of the granulite facies is thus proposed for Ceylon rocks. An attempt is made to trace briefly the major events in the metamorphic history of the rocks supported by available geochronological evidence.

The significance of potassium-argon ages in the vicinity of the charnockitic Ernabella Adamellite, Australia

Abstract: Potassium-argon dates from hornblende-bearing pyroxene granulites in a large xenolith near the margin of the charnockitic Ernabella Adamellite, central Australia, are interpreted in the light of oxygen isotype studies of associated mineral phases which give a detailed thermal history of the Adamellite and associated rocks.

The geology and petrology of the Pre-Cambrian basement between Sirdal and Åseral, Vest Agder, Norway

Abstract: The field relations and petrography of the rocks of the PreCambrian basement complex between Sirdal and Aseral, comprising two series of high-grade metamorphic gneisses separated by a structural discontinuity, syntectonic granites, intrusive quartz monzonites with thermal metamorphic aureoles and basic dykes, are described. During orogeny the gneisses were subjected to intense poly-phase deformation, three regional and two localised phases of which have been recognised. Minor fold relics within augen gneiss in the lower gneiss sequence suggest that this rock was involved in earlier deformation. the climax of metamorphic crystallisation occurred at the low-pressure granulite facies-amphibolite facies boundary with mineral parageneses corresponding closely with the sillimanite-cordierite-orthoclase subfacies of the Abukuma-type cordierite amphibolite facies except for the additional occurrence of orthopyroxene. Major and trace elements X.R.F. analyses of gneissic and some igneous rocks are presented. These data reveal significant differences between basic rocks of the two gneiss series, basic gneisses with different mineral assemblages and to a lesser extent different lithostatigraphical units in the upper gneiss series. Electron microprobe analyses of alkali feldspar, plagioclase, biotite, hornblende, clinopyroxenes, orthopyroxene, sphene, magnetite, ilmenite, chlorite, and garnet from several rock types are presented. With the exceptions of alkali feldspar, magnetite and ilmenite all minerals are chemically homogeneous and represent original equilibrium compositions. The chemical inhomogeneity of alkali feldspar resulted from post-crystallisation leaching and redistribution of alkalies, resistance to which is related to grain size. Equilibrium during original feldspar crystallisation is indicated by the restricted composition of plagioclase coexisting with alkali feldspar. The distribution of titanium and magnesium between coexisting silicates indicates equilibrium compositions, influenced by oxygen fugacity, the nature of the coexisting iron oxides and the tetrahedral aluminium content of the hydrous phases in addition to the rock composition. The application of several means of multicomponent paragenesis analysis reveals that the various mineral assemblages can be interpreted in terms of variations in major element rock composition and oxygen fugacity. The widespread molybdenite mineralisation is considered to have been transported from depth in siliceous hydrothermal solutions into the gneisses, especially where the strike of the gneissic layering coincided with deep fractures. Fixing of the metal as sulphide occurred particularly in the vicinity of pre-existing fahlband sulphide due to release of sulphur in the local environment of increased oxygen fugacity.

On the Alkali Syenites Occurring near Kundulur, Khammam District, Andhra Pradesh

Abstract: Introduction: The occurrence of syenites near Kundulur (17°40' : 81 °24'), a small village in Khamman district, Andhra Pradesh, was reported by Nair and Mahadevan (1968). To the north-west of Kundulur, two seperate syenite bands, intervened by biotite gneiss, extend in a NE-SW direction. The northern band is about one mile, and the southern band is a little over one mile in length. The maximum width of the bands is about 3 furlongs. The present paper deals with the petrographical, mineralogical and chemical aspects of these syenites. Geological setting: The country rocks for the syenites comprise garnetiferous biotite gneiss, amphibolite and pyroxene granulite. The alkali rocks stand out conspicuously as positive topography from the surrounding low country. Within the alkali syenite bands, a central core of nepheline syenite, pegmatoid at places, is fringed successively by biotite-nepheline syenite, hornblende-nepheline syenite and perthite-pyroxene syenite. Nepheline forms pods and lenses at places within biotiteand hornblende nepheline syenite. The syenite complex is emplaced within biotite gneiss and granulites. In the syenite bands well marked foliation is developed with dips varying from 50° towards 1300 to 66° towards 145° and is conformable with the attitude of foliation of the country rocks. The syenites are gneissose on account of the parallel disposition of the elongated clusters of dark and light minerals in alternate bands. Mineral lineation is developed occasionally in the syenites, the general attitude of plunge being 18° towards 190°. Petrography of the country rocks: Garnetiferous biotite gneiss is a medium grained rock with well developed foliation having an assemblage of quartz, biotite (Ny = 1.582 to 1.590, pleochroic from straw yellow to deep brown, X

Pyroxene-garnet granulite from Koppal, raichur district, mysore state

Abstract: The results of the investigation carried out on pyroxene-garnet granulite which is closely associated with quartzite and ferruginous quartzite in the granitic rocks of Koppal area, Raichur District, Mysore State, have been described and discussed. Optical and chemical studies of light green pyroxene and pale pink garnet suggest that the clinopyroxene is ferrosalite with Tsch17Jd3Hd50Di30 (Yoder and Tilley, 1963) and the garnet is almandine rich with Alm72 Sp12 Py8 Gro8. On the basis of the distribution coefficient of Fe+2 and Mg+2 in co-existing pyroxene and garnet it has been concluded that the present mineral assemblage of the granulite is possibly due to regional metamorphism of ferruginous shale rich in lime, under conditions intermediate between glaucophane schist and granulite facies. Evidences are given in support of this conclusion.