Showing papers on "Metamorphism published in 1971"

Progressive metamorphism and classification of shocked and brecciated crystalline rocks at impact craters

Abstract: The principle of progressive shock metamorphism in nonporous silicate rocks is discussed on the basis of petrographic observations and experimental data. The p-T conditions and the nature of the basic types of shock effects observed in rock-forming minerals are considered as far as they are indicative of the degree of shock metamorphism of the source rock. Particular shock effects that are related to the main regimes of the Hugoniot curves of quartz and feldspar, as well as the shock-melting and vaporization behavior of the whole rock, characterize and define six stages or zones of increasing shock metamorphism. Each stage or zone represents a certain range of peak pressure and temperature. Rocks of stage O are shocked to shock states below the Hugoniot elastic limit of quartz and feldspar that are irregularly fractured. Shock stage I is characterized by diaplectic quartz and feldspar that are released from shock states in the ‘two-phase regime’ of the Hugoniot. Diaplectic glass of quartz and/or feldspar composition occurs within stage II, which is related to the lowest part of the ‘high-pressure-phase regime’ of the Hugoniot. Stage III is characterized by selective melting of feldspar minerals at sufficiently high postshock temperature. Postshock temperatures exceeding the liquidus of the whole rock produce rock glasses on pressure and temperature release (stage IV). Stage V is represented by condensation products of shockvaporized rock material. On the basis of experimental data of several authors, the proposed stages of metamorphism are tentatively correlated with a pressure-temperature scale. It is concluded that the proposed classification is generally valid for shock processes in nonporous rocks and can well be applied to the recognition and distinction of the different kinds of impact breccias of terrestrial or extraterrestrial origin.

Garnet-peridotite stability and occurrence in crust and mantle

Abstract: The boundary between the spinel- and garnet-lherzolite mineral facies is strongly curved between 1300° C and 1450° C; below 1200° C it lies almost parallel to the temperature axis of the T-P diagram. Pressure of at least 12 kb is required to stabilise garnet-peridotite on the geothermal gradient and the depth at which the boundary is encountered is not sensitive to variations in the geotherm. Garnet-peridotite is metastable with respect to spinel-peridotite in normal continental crust. Natural occurrences are mantle derived and have either suffered rapid upward transit in diatremes or, in orogenic zones, either they were emplaced after upward tectonic transport through tens of kilometres, or have originated by in situ metamorphism of pre-existing crustal peridotite in an orogenic root downfolded to depths of at least 40 km.

Metamorphism in the Mid-Atlantic Ridge near 24 degrees and 30 degrees N

Abstract: Metabasites (metabasalts and metagabbros) occur abundantly in association with serpentinites in transverse fracture zones and on walls of the median valley. These metabasites were formed by burial metamorphism probably in deeper parts of the crust and the upper mantle beneath the Ridge crest, and were brought up to the surface of the crust probably by serpentinites rising along fracture zones and by normal faulting along the median valley. The metabasalts are in the zeolite and greenschist facies and a transitional state from the greenschist to the amphibolite facies, whereas metagabbros tend to have been recrystallized at higher temperatures, being in the greenschist and amphibolite facies. Compositionally the metabasites are divided into two groups, I and II. Group I comprises those which retain the approximate composition of the original rocks except for water content, whereas group II comprises those which underwent marked chemical migration, as regards sodium in zeolite-facies rocks and calcium and silicon in greenschist-facies rocks. In rocks of group I, calcic igneous plagioclase remains unaltered, and albite and epidote did not form. This fact, along with the absence of epidote-amphibolite facies rocks, would be due to the low rockpressure during metamorphism. In some rocks of group II, albite and epidote occur. Burial metamorphism takes place probably mainly beneath the Ridge crest where the geothermal gradient is great. The resultant metamorphic rocks are probably of the low-pressure type, and move laterally by ocean-floor spreading to form the main part of the oceanic crust. Contact metamorphic gneisses, probably derived from gabbros, have been found. Some metagabbros were subjected to cataclasis by fault movements along fracture zones and the median valley.

Petrologic and Geophysical Nature of Serpentinites

Abstract: Mineralogically, serpentinites consist predominantly of lizardite, clinochrysotile, and antigorite. Recent work has shown that these minerals are not polymorphs. Chrysotile is the only mineral recognized as a synthetic product in experimental studies of the system MgO-SiO2-H2O. Antigorite seems to be stable at higher temperatures than lizardite or chrysotile. The density of individual serpentine species is dependent on their morphology; the low-density serpentinites (<2.55g/cc) consist predominantly of clino-chrysotile. Seismic velocities and magnetic susceptibilities of serpentinites are related to the degree of serpentinization. The transition of massive serpentinites from ductile to brittle behavior in laboratory experiments at high confining pressures and temperatures above 300°C has been related to dehydration which may provide a mechanism for developing deep-focus earthquakes along Benioff zones. Serpentinite is formed by direct hydration of ultramafic protolith in the crust. The most common ultramafic protoliths are harzburgite, dunite, and Iherzolite. The assemblage generally developed from these is lizardite + chrysotile + brucite + magnetite. In areas of high-grade metamorphism, antigorite is the predominant serpentine mineral. The common, large, alpine-type serpentinized ultramafic masses contain brucite and have MgO/SiO2 ratios similar to those of their protolith, resulting in volume increase during serpentinization. Metamorphic serpentinites and some highly sheared alpine-type serpentinites have lower MgO/SiO2 ratios than their protolith, lack brucite, and appear t o have been formed by volume-for-volume replacement with concomitant loss of magnesium or addition of silica. Many large, young masses of peridotite appear to be slabs of oceanic mantle over-thrust onto continental edges. Subsequent sedimentation, serpentinization, and tectonism have greatly modified these original slabs so that their recognition in older orogenic zones is equivocal. The concept of the tectonic evolution of ultramafic rocks from oceanic crust-mantle slabs invading continental margins and being incrementally serpentinized and moved by later tectonic events provides a working hypothesis that allows a better explanation of the many peculiar and varied occurrences of serpentinite. The evidence does not support Hess' suggestion that the third layer of the oceanic crust consists of partly serpentinized mantle peridotite.

Thermal alteration of clastic organic particles as an indicator of contact and burial metamorphism in sedimentary rocks

Abstract: Most sedimentary rocks contain dispersed solid organic grains of fine sand or silt size. These generally amount to a fraction of 1% of the rock mass, but may be concentrated after macerating the rock with acids. Study by transmitted or reflected light microscopy shows that these grains are largely plant fragments deposited like other grains of the rock; therefore they are termed phytoclasts. Phytoclasts are altered chemically and physically when exposed to increased temperature. This alteration is similar for phytoclasts from rocks near volcanic dikes, from laboratory bombs, from deep wells, and from strata formerly deeply buried. The degree of alteration is quantified best by measuring reflectance of selected phytoclasts—which increases with temperature increase and time of exposure. Reflectance‐temperature standards from laboratory bombs apply to dike metamorphosed phytoclasts, and temperature plots based on phytoclast reflectance in contact rocks sampled are like those expected from a simple m...

A model for the Lower Palaeozoic evolution of the southern margin of the early Caledonides of Scotland and Ireland

Abstract: Synopsis Consideration of the stratigraphical and structural development of the relationship between the early and late Caledonides lends some support to the view that the early Caledonides of Scotland and Ireland lay on a leading plate edge above a northward-dipping descending plate. An early Ordovician trench just north of the Longford/Down Massif and Southern Uplands was the probable site of consumption of an oceanic plate which carried Ordovician/Silurian spilite-argillite-chert-turbidite sequences. The Highland Zone of high temperature metamorphism and gabbroic and granitic intrusion probably lay above a zone of calc-alkaline magma ascent, generated by partial fusion of the crust of a descending plate at depths over 100 km. During Ordovician and early Silurian times the Midland Valley was a southward, oceanward, thickening sedimentary wedge built on deformed Dalradian terrain in the north and on upthrust wedges of early Ordovician oceanic crust and mantle (Ballantrae Complex) in the south. At this time flysch wedges were built out southwards into the ocean and, beginning in late Upper Llandovery times, were progressively deformed to produce a growing land area (Cockburnland). By early Devonian times the sediments of the oceanic region were strongly deformed, forming the lands of the late Caledonides, and the Midland Valley was a graben receiving high rank metamorphic detritus from the Highlands and low rank metamorphic detritus from the Southern Uplands. Ophiolite complexes throughout the Appalachian/Caledonian Orogen are dominantly of early Ordovician age. By analogy with Tethyan ophiolite complexes and the processes involved in the genesis of marginal basins behind, and within, island arc complexes, the Appalachian/Caledonian ophiolites are considered to have originated by sea-floor spreading behind and within Ordovician island-arc complexes.

Hudson Geotraverse: Geology of the Mid-Atlantic Ridge at 45 degrees N

Abstract: A strip across the Crest Mountains and High-Fractured Plateau of the Mid-Atlantic Ridge has been surveyed systematically between latitudes 45 and 46° N. Continuous bathymetric, magnetic and gravimetric data have been obtained. Seismic refraction experiments have revealed a complex structure lineated parallel to the axis of the Ridge. Seismic reflexion studies have revealed a picture of the sediment cover, and have shown the possible existence of block faulting of the underlying rocks, with faults alined both parallel and at right angles to the axis of the Ridge. The major rock types found in 46 dredge stations can be grouped as follows: (1) Ubiquitous basalts and tuffs (ranging from theoleiites to alkali basalts, with a few ferro-basalts and high-Al basalts). Basalts rich in resorbed high-calcic plagioclase xenocrysts are common; these occur both on the slopes of shield volcanoes and in the deepest hole of the Median Valley. A nearby fault scarp yielded coarse-grained gabbros. (2) Serpentinized mafic and ultramafic rocks are not restricted to elongated, presumably blockfaulted seamounts, but are also common on the slopes of what had been interpreted on morphological grounds as shield volcanoes; they are absent, however, on the Median alley floor and its immediate scarp slopes. The pre-serpentinization rock types include dunites, harzburgites, gabbros, troctolitic gabbros and amphibolitic peridotites showing crude cumulate textures. (3) The lower parts of the steep inner walls of the Median Valley have yielded metabasalts and metadiabases showing alteration within the greenschist facies of etamorphism, whilst still retaining original igneous characteristics. (4) Restricted to the fault scarps of elongated seamounts further removed from the Median Valley are higher grade metamorphic rocks of the almandine amphibolite facies of metamorphism. These rocks have lost all igneous textures and exhibit a strong gneissic fabric. (5) Three localities yielded dioritic rocks in association with serpentinized ultramafics. The diorites vary in character from hornblende-rich quartz diorites to more siliceous, almost hornblende-free trondhjemites. The latter show considerable albitization. The whole suite of rocks shows great affinities with similar suites found as late stage intrusives in alpine-type ultramafic complexes. About 23% of the specimens collected included gneissic, granitic and sedimentary rock types of erratic origin, ice rafted into the area in the Pleistocene. A study of their distribution indicates that there are no erratics in the Median Valley, that they are scarce on the mountain ranges immediately flanking the Valley, but beyond these areas they are abundant and are randomly distributed over the whole area. Such a distribution may be a result of ocean-floor spreading, indicating that the Median Valley is younger than the last ice age, or that extrusions subsequent to the last ice age have engulfed any erratics present in the Median Valley. The thickness of manganese coating on extrusive rocks and their K/Ar and fission track ages increase systematically with distance on either side of the axis of the M.A.R., strongly supporting the ocean-floor spreading hypothesis. The ages and coatings both show a marked change in their rate of increase beyond a distance of 50 to 60 km on either side of the axis. The position at which this occurs coincides with the thickening in these areas of sediments found in the inter-volcanic valleys, and the morphological changes between the Crest Mountains and the High-Fractured Plateau. The combined data strongly suggest that there was either a quiescent period sometime in the Pliocene during which ocean-floor spreading was inactive, or that the rate of spreading had accelerated during the Pliocene from less than 1 cm a -1 to a computed 2.5 cm a -1 in Pleistocene times.

Franciscan Mélanges as a model for eugeosynclinal sedimentation and underthrusting tectonics

Abstract: The traditional view representing eugeosynclines as elongate troughs having active volcanism is challenged. Eugeosynclinal rocks are commonly tectonic melanges, representing mixtures of rocks derived from more than one realm of deposition. The Franciscan melange includes ophiolites emplaced at a mid-ocean ridge, radiolarites deposited on an abyssal plain, and flysch turbidites poured into a deep-sea trench. The Franciscan melange was pervasively sheared when the North American plate rode over a Cretaceous Neo-Franciscan plate. The plate junction is not a single overthrust surface. The densely spaced shear surfaces within the melange signify a westward migration of the plate junction as the Franciscan was being underthrust. The Franciscan glaucophane schists, metamorphosed under very high pressure 130–150 m.y. ago, record a pre-Cretaceous Franciscan history. Geological evidence suggests that those once deeply buried rocks were squeezed out during a Late Jurassic alpine type of deformation (Nevadan orogeny), when the North American continent collided with a micro-continent (Salinia). The Cretaceous underthrusting along a consuming plate margin resulted in melange deformation and in lawsonite-schist metamorphism of the neo-Franciscan rocks, which are coeval with the Great Valley sequence.

Sedimentary and Gravity-Slide Emplacement of Serpentinite

Abstract: Large deposits of serpentinite in alpine-type orogenic areas have been formed by sedimentary processes ranging from the detrital accumulation of bedded serpentinite sandstone and shale to the emplacement of chaotic breccias (olistostromes) and gigantic slide blocks. Known occurrences of sedimentary serpentinite are listed, and eight deposits from the circum-Pacific, Caribbean, and Mediterranean areas are described in detail. Sedimentary serpentinites range in age from early Paleozoic to Quaternary, although most are Cretaceous or Tertiary. Most were deposited in eugeosynclinal environments, early in the geosynclinal cycle. Individual deposits range in thickness from a few centimeters to nearly 3 km, and several extend laterally for tens of kilometers. Graded bedding is common, and many deposits contain marine fossils. Serpentinite is the dominant rock constituent, and clasts foreign to the alpine ultramafic assemblage are rare. Chemical analyses often detrital serpentinites show that these rocks contain slightly more silica and alumina than do nondetrital serpentinites, due to contamination by aluminosilicate minerals and quartz during deposition. This and nine other criteria are potentially useful in the recognition of sedimentary serpentinites. Several features suggest that most sedimentary serpentinites were deposited very rapidly by submarine landslides, mudflows, or turbidity currents. The sources of this serpentinite debris are postulated to be upward-migrating serpentinite protrusions which penetrate the seafloor or Earth's surface upslope from eventual depositional sites. Sedimentary serpentinites are much more abundant in alpine-type orogenic areas than is commonly thought, and many ultramafic masses presently regarded as igneous intrusions or tectonic protrusions may in fact be coeval with, instead of younger than, their enclosing sedimentary or metasedimentary rocks. In eugeosynclinal sequences such as the Franciscan Formation, some elongate bodies now regarded as serpentinite sills may be beds of ultramafic detritus whose sedimentary features have been masked by post-depositional shearing; isolated masses may be exotic slide blocks. A sedimentary origin can explain some of the most persistent and perplexing characteristics of many alpine serpentinites: their conformity with enclosing sedimentary rocks, their grossly planar shapes, and the absence of metamorphism along their contacts.

Pan-African Orogeny in Northern Nigeria

Abstract: An area of approximately 12,000 sq km of Precambrian to lower Paleozoic rocks around Zaria in northern Nigeria has recently been mapped for the first time. Results from this survey have been combined with limited data from adjacent areas to determine the nature of the Pan-African event in this part of Nigeria. A crystalline basement complex of gneisses, migmatites (Dahomeyan), and relicts of an ancient metasedimentary sequence (Birrimian) underlies most of the region. Infolded into this basement, as north-south-trending synclinoria, is a younger metasedimentary sequence (Ka-tangan) forming the remnants of an earlier, more extensive supracrustal cover. A suite of syntectonic to late tectonic granites and granodiorites (late Precambrian to lower Paleozoic) has intruded both the basement and its cover. The structure of the area is dominated by north-south trends typical over much of Nigeria. There is evidence to suggest that the whole area was affected by two successive phases of intense deformation resulting in tight isoclinal folding about (1) east-northeast-west-southwest, and (2) north-south axes. The crystalline basement was reactivated and the supracrustal cover regionally metamorphosed. Two phases of progressive metamorphism accompanied the deformations, which were separated and followed by static metamorphic phases. Pressure-temperature conditions of metamorphism appear to have remained fairly constant throughout, giving rise to a low pressure facies series similar to the Buchan type. Extensive migmadzation of the crystalline basement is believed to have accompanied the first deformation, followed by further differentiation during the second phase to produce homogeneous gneisses and intrusive granites. Jointing, fracturing, and faulting followed the orogeny, and a major northeast-southwest-trending transcurrent fault system crosses the area. It is obvious from the structural and metamorphic evidence that the Pan-African event reached the status of an orogeny in northwestern Nigeria.

A radiometric study of polymetamorphism in the Bamble region, Norway

Abstract: Metamorphism-induced parent-daughter isotopic rearrangement yields information concerning the nature and duration of metamorphism in the Bamble Area, Southern Norway. The thermal maximum of the Bamble Sveconorwegian metamorphism was reached at 1160–1200 m.y. ago, according to zircon and sphene U-Pb, and Rb-Sr whole rock results. Dating of post-kinematic pegmatites suggests that the major kinematic episodes took place not much more than 100 m.y. and that not until more than 200 m.y. after the thermal maximum had uplift and cooling resulted in closure of the K-Ar system in micas. Petrological considerations together with radiometric data on the Levang Gneiss Dome suggest that Rb-Sr whole rock samples show open and closed system behaviour under similar temperatures but possibly different $$P_{H_2 O} $$ . Variable recrystallization of zircon in the Levang Gneiss Dome (when taken with accompanying radiometric U-Pb data) appears to substantiate the idea that when $$P_{H_2 O} $$ = P solid, resetting of both the U-Pb systems in zircon and Rb-Sr whole rock systems is greatly facilitated.

Phase relations of biotite and stilpnomelane in the greenschist facies

Abstract: Phase relations of biotite and stilpnomelane and associated silicate minerals have been studied in rocks of the greenschist facies, chiefly from Otago, New Zealand and western Vermont, but also from Scotland, Minnesota-Michigan iron range, and northwest Washington. That stilpnomelane in the greenschicht facies crystallizes initially with nearly all iron in the ferrous state is indicated by chemical analyses, high p-T experiments, and phase relationships. Alteration of stilpnomelane after metamorphism not only oxidizes iron but leaches potassium; corrections for both effects must be made in using analyses of brown stilpnomelane in studies of phase relations. Two discontinuous reactions which produce biotite at the biotite isograd have been identified: Stilpnomelane is stable in muscovite-free rocks throughout the biotite zone, and even up to the grade at which hornblende becomes stable. Phengitic muscovite is stable throughout the biotite zone in New Zealand and thus apparently does not contribute to the formation of biotite until a higher grade is reached.

Retrograde Schists of the Amphibolite Facies at broken hill, New South Wales

Abstract: Aluminous, mafic, felsic, calcareous, and sulphide‐rich rocks have been involved in localized deformation and retrograde metamorphism at Broken Hill, western New South Wales, where retrograde schist‐zones intersect high‐grade, regional metamorphic rocks of the lower granulite facies (or the amphibolite‐granulite facies transition). Although technically retrograde, the schists contain mineral assemblages indicative of the lower amphibolite facies. The schist‐zones were formed by local folding, apparently as part of the third stage of deformation in the Broken Hill area.

Age of the Early Precambrian Rocks of the Saganaga Lake – Northern Light Lake Area, Ontario–Minnesota

Abstract: Whole-rock Rb–Sr isochron and mineral ages from the Saganaga Lake – Northern Light Lake area on the Ontario–Minnesota boundary indicate a major orogeny at 2700 m.y. ago. The sequence of events reconstructed in the geologic history started with the eruption of a volcanic pile of flows and pyroclastic material. During metamorphism and folding the basalt flows were converted to amphibolite and an interlayered series to the Northern Light Gneiss, which is now principally trondhjemite with lesser amounts of amphibolite and metarhyolite. The greenstones and the Northern Light Gneiss were intruded by the Saganaga Granite (tonalite), and the tonalite and the Northern Light Gneiss, in turn, were intruded by a syenodioritic phase of the lcarus pluton.The whole-rock isochron ages for the Northern Light Gneiss, the Saganaga tonalite, and the lcarus pluton are 2740, 2710, and 2690 m.y., respectively. Large uncertainties are attached to these values because of the unfavorable Rb/Sr ratios, but the mineral data that inc...

La evolución estructural de los andes más septentrionales de Colombia

Abstract: The primeval Guayana shield of northern South America is older than the metamorphism of the eugeosynclinal Imataca Complex of south-eastern Venezuela which took place 3.000 m.y. B.P. Metamorphism and at least two granitic magma cycles (Guianese) welded the shield area into a stable craton between 2.250 and 1.850 m. y. ago. To the west and probably to the north the shield rocks are younger; in Colombia, the oldest recorded age of rocks is about 1.350 m.y. (Orinocan). The Precambrian crust extends well into the Andean Cordillera and appears to have provided a platform for ensialic miogeosynclinal sedimentation during much of Phanerozoic time. A peri-continental eugeosyncline developed along and around the western and northern margin of the platform during the Paleozoic Era. In late Paleozoic time miogeosynclinal sediments near the edge of the platform and sediments and volcanic material in the adjoining eugeosyncline were regionally metamorphosed along an arcuate fold belt (late Paleozoic orogen) that incluJed the area of the Central Cordillera, the central part of the Sierra Nevada de Santa Marta, and the Guajira Peninsula, and produced the c:entral northeast-trending arcuate crystalline spinal column of the present Colombian Andes. During the Mesozoic Era, a second eugeosyncline formed outside the late Paleozoic orogenic are; at the close of the Cretaceous the eugeosynclinal deposits were strongly folded but only locally metamorphosed, resulting in the development of the Western Cordillera and coeval fold belts along the northeastern Caribbean coast of Colombia. The immense Antioquian batholith and severa satellitic intrusive bodies were emplaced along the inner side of the late Cretaceous orogenic are. In the middle Eocene and again in the Oligocene-early Miocene, small post tectonic granitic plutons were emplaced at intervals along the orogenic axis. During the middle Tertiary Andean orogeny, the intracratonic Eastern Cordillera rose along sharp marginal flexures, high-angle reverse faults that dip toward the cordillera, and locally along wrench faults. The present Andean cordillera of northernmost Colombia, therefore, are distintive geologically and carne into being at different times. High positive Bouguer anomalies in westernmost Colombia and the presence of abyssal to bathyal lower Tertiary sediments overlying mafic volcanic rocks in eastermost Panama suggest that the lsthmus of Panama probably is of oceanic crustal origin and became connected to and associated with the Andes during the Andean orogeny. Little attrition is recognized along the western edge of the continent at the latitude of Colombia (0° -9°N). To the contrary, accretion is apparent during the Late Paleozoic orogeny, and a width of more than one hundred miles was added to the continental margin during the late Cretaceous orogeny. In northernmost Colombia, east-west Caribbean structures intercept the Andean structures and are therefore younger. Late tertiary epeirogenic uplift accompanied by faulting produced much of the present relief.

Evidence of foundered continental crust beneath the central Tyrrhenian Sea.

Abstract: THE dredging of schists, phyllites and marbles from the faulted margin of a tilted crystal block in the central Tyrrhenian Sea shows that the acoustical basement beneath the centre of this sea basin includes a sequence of rocks similar or perhaps identical to the Palaeozoic and Triassic schists and phyllites of the adjacent Apennine, Calabrian and Sicilian chains, the Pontian Islands1 and Sardinia. Even the low to medium grade metamorphism observed must have occurred beneath the Earth's surface and following metamorphism and deformation we infer that these rocks were uplifted, denuded by subaerial erosion and finally foundered more than 3,000 m below sea level. The Neogene subsidence is still continuing. The metamorphic rocks obtained from the Tyrrhenian acoustic basement appear to support the former existence of the Tyrrhenides and indicate that this ancient upland was underlain by continental crust.

The north greenland fold belt and environs

Abstract: A review of our present knowledge of the North Greenland fold belt and environs is presented. Precambrian crystalline basement, which is exposed at places adjacent to the Inland Ice and can be expected to form larger areas now covered by its northern extremity, is overlain with marked angular unconformity by a Proterozoic to Lower Palaeozoic sedimentary pile. These sediments dip gently northwards forming a platform and hinterland to the North Greenland fold belt which occupies the extreme northern part of Greenland as a roughly E-W zone of deformation and metamorphism. In Peary Land, where the widest part of the zone occurs, the effects of deformation and metamorphism increase northwards towards the assumed centre of the erogenic belt. In eastern Peary Land, the folded Lower Palaeozoic sediments are unconformably overlain by strata of. Pennsylvanian, Permian, Triassic and CretaceousTertiary age. This sequence has been affected by Tertiary earth movements. Field evidence in Greenland, together with evidence from the Innuitian orogenic system in Arctic Canada, suggests that the main Palaeozoic diastrophism affected the sediments of 'the Greenland part of the Franklinian geosyncline between Late Silurian and Late Devonian time. Field work since 1965 on both the.folded and platform rocks of the fold belt has led to a reinterpretation of the structure and stratigraphy of North Greenland. Important results include: 1) the recognition in the western part of the platform of a Palaeozoic reef complex of regional extent with major facies changes between carbonates, and arenaceous and argillaceous sediments, leading to a criticism of the described Silurian unconformities and to a revision of the established Silurian stratigraphical nomenclature; 2) the lowering of the base of the Cambrian in the platform so that rocks which have been previously regarded as Precambrian or Eocambrian (and part of the so-called Thule Group) are now known to be Palaeozoic in age; 3) recognition that the thick Inuiteq So Formation composed of sandstones cut by basic intrusives (which is the oldest unmetamorphosed sedimentary formation in North Greenland) is at least 1000 m. y. old; 4) discovery of both shelly and graptolitic faunas in the folded sediments indicating the presence of Cambrian, Ordovician and Silurian strata in the folded zone; 5) indications that the sediments of the folded zone have passed through a long and complex orogenic history, suffering poly-

Basement Rock facies—Northern North Island

Abstract: Geographic names are proposed for three geosynclinal facies of the northern North Island in preference to those in current use Rocks of the Oparau Facies (synonyms—Hokonui; Shelf; Western; and Marginal, in part) are fossiliferous and tuffaceous, are simple structurally, and show increasing low-grade metamorphism with age They form part of the zeolite mineral facies Rocks of the Morrinsville Facies (synonyms—Alpine, in part; Marginal, in part; and Eastern) are thick monotonous sparsely fossiliferous greywackes, chipwackes, and siltwackes that include neither tuff beds nor lavas They lie in the quartz-prehnite zone of the prehnite-pumpellyite metagreywacke facies Rocks of the Hunua Facies (synonyms—Alpine, in part; Axial, in part; and Marginal, in part) are very thick, commonly sheared, siltwackes and greywackes, with intercalated spilitic lavas, cherts, and limestone They lie in the quartz-prehnite zone of the prehnite-pumpellyite metagreywacke facies, probably somewhat higher grade than roc

Chemical Alteration and Spilitization of the Catoctin Greenstones, Shenandoah National Park, Virginia

Abstract: The Catoctin Formation of Precambrian age is composed predominantly of sodium-rich greenstone derived from metamorphism of basalt. Intimately associated with the spilitic greenstone are irregular masses of quartz-epidote rock (epidosite), estimated to make up about one-third of the total volume of the meta-volcanic rocks. The calculated composition of the greenstone-epidosite complex is almost identical with that of dikes in the underlying basement rocks that are believed to be feeders from which the Catoctin lavas were erupted. Evidently the spilitic greenstone and epidosite are the products of chemical alteration of basalt flows with compositions like those of the feeder dikes. The principal changes involved are enrichment of the greenstone (and depletion of epidosite) in sodium, magnesium, and hydrogen, and enrichment of epidosite (and depletion of greenstone) in oxygen and calcium. Chemical alteration apparently resulted from circulation of fluids with high oxidation potential through a fissure system...

Metamorphic rocks of the hitachi district in the southern abukuma plateau

Abstract: The Paleozoic rocks of the Hitachi district in the southern Abukuma Plateau suffered metamorphism ranging from the greenschist facies to the amphibolite facies through the epidote-amphibolite facies. This area can be divided into the following five zones by the stability relations of some metamorphic minerals. Zone I: chlorite zone Zone II: biotite zone Zone III: epidote-hornblende-sodic plagiocalse zone Zone IV, V: sillimanite zone The metamorphism is characterized by the behavior of the anorthite content in plagioclase, the chloritoid-Fe-chlorite-anorthite assemblage, the optical property of calciferous amphibole, the assemblage of cummingtonite-epidote-sodic plagioclase and the association of Al2SiO5 polymorphism. The association of andalusite-kyanite-sillimanite shows the following relation, These minerals probably grew metastably close by the triple point of these polymorphs. The chloritoid-Fe-chlorite-anorthite assemblage can be interpreted by the Po2 condition at the various stages of the metamorphism; chloritoid and Fe-chlorite are sensitive to Po2 and Fe-chlorite-chloritoid-magnetite assemblage occurs at the low Po2 condition. And as a result of dehydration reaction at the low Po2 condition, anorthite did not decompose to epidote but reacted to Fe-chlorite to produce chloritoid, ilmenite, quartz and calcite. The assemblage of cummingtonite-epidote-sodic plagioclasc indicate the higher PH2O condition of metamorphism; under the metamorphic condition such as the epidote-amphibolite facies, anorthite is unstable and the reaction between anorthite mole and vapor may begin easily to produce zoisite, so that under the higher PH2O and Po2 condition the reaction of producing cummingtonite-epidote-oligoclase may happen with progressive increasing of the metamorphic grade. The anorthite content in plagioclase is generally slower incrcasing with the rising of the metamorphic grade on account of the nature of metamorphism itself and peristerite may occur at a part of the lower metamorphic grade in this district. The optical property of calciferous amphibole is characterized by the small optic axial angle and the wide range of refractive index. Some amphiboles are plotted in field M by Miyashiro's classification. Reflecting on these characters, the metamorphism in the Hitachi district can be explained to be single progressive metamorphism without “polymetamorphism”. The Hitachi metamorphic facies series is of rather higher pressure and/or lower temperature than the central Abukuma Plateau and is characterized by heterogeneous PH2O or Po2 condition of the metamorphism.