Showing papers on "Metamorphism published in 1970"

The transformation of clay minerals during diagenesis and low-grade metamorphism: a review

Abstract: Summary The results obtained by the author in the study of clay-minerals diagenesis are compared critically with the principal publications in this field, giving a general picture of the transformation of sheet silicates. Kaolinite minerals are related to the surficial zones of the earth's crust where they are formed. They are characterized by the hexacoordination of aluminium. They furnish paleogeographic indications in ancient sediments. During diagenesis they are very sensitive to the geochemical environment, stable in acid conditions, unstable in alkaline conditions. However, the increase in temperature by burial causes their destruction sooner or later. In the transitional zone to metamorphism (anchizone), kaolinite is not present. Only dickite and nacrite can be observed, provided that the environment is acid. Montmorillonites are hydrated minerals. The rise in temperature and above all in pressure during burial expels water from the interlayers. Concentrated interstitial solutions of diagenesis provide cations which replace molecules of water between the layers. It is an irreversible reaction which produces 14-A minerals (chlorites) or 10-A minerals (illites), passing generally through mixed-layer structures. The lack of montmorillonite is normal in formations which have undergone a marked burial. Mixed-layers are intermediate stages which occur during degradation by weathering and during aggradation by deep diagenesis. This aggradation is the result of an incorporation of certain cations taken up from interstitial solutions, and of a rearrangement within the lattice. There are two major pathways: a potassium and sodium pathway, which produces the illites, then the micas, passing possibly by regular mixed-layering of the allevardite-rectorite type; and a magnesium pathway, which produces the chlorites, passing possibly by a regular mixed-layering of the corrensite type. These mixed-layers can remain stable until the border of meta-morphism (anchizone). Micaceous clay minerals or illites form a very heterogenous group in the sediments which have been hardly diagenetized. Particles of diverse origin are found. They become more regular during burial. In deep diagenesis and the anchizone, crystallo-graphic parameters of the illite are sufficiently well defined to serve as a scale of recrystallization, a zoneographic index. The morphology of the particles changes. Polymorphic types 1Md and 1M are replaced by the 2M-type. The sharpness of the 10-A peak, conventionally called “crystallinity”, is an interesting quantitative criterium, together with the intensity ratio of the 5-A and 10-A peaks, which is related to the chemical composition of the octahedral layer. Micas in low-grade metamorphism, called sericites by petrographers, replace the illites discussed above. They are different from the true micas by a weaker layer charge, less than 0.9 by half-cell. They often contain sodium (paragonitic muscovites and paragonites). The octahedral charge (zero for the muscovite) is generally high, due to the replacement of Al by Fe2+ and Mg (phengites). These transformations should not obscure the fact that metamorphism is also accompanied by crystalline growth and massive neoformation. Chlorites are the least well-known clay minerals in diagenesis. Detrital particles can be aggraded to chlorite during early diagenesis by passing through the mixed-layer stage of corrensite. A massive growth of chlorite is observed in late diagenesis and the anchizone. Illite and chlorite slates give place to sericite and chlorite schists. At present, general data are not available on the crystal chemistry of chlorites in the anchizone and the greenschist facies. The stages in the diagenetic evolution of clay minerals are too little understood to be able to give them precise limits. However, the following provisional scheme can be proposed: (1) Early diagenesis (= “diagenesis” of Russian authors; = the “shallow-burial stage” of Muller, 1967a). In this stage all the clay minerals are stable; some undergo aggradation by adsorption of Mg, K and Na (various mixed-layers); some are neoformed (montmorillonites). (2) Middle diagenesis (= “early catagenesis or epigenesis” of Russian authors; the “deep-burial stage” of Muller, 1967a, includes this stage and all the following until metamorphism). In this stage the sediment becomes compact. It has lost at least 50% of its connate water. Porosity is high and circulation still plays an essential part. Some detrital minerals, such as biotite, are unstable. All the clay minerals are still stable, but many types of replacement take place, due to interstitial circulation. Dickitization of kaolinite and illitization of montmorillonite can already be observed. (3) Deep or late diagenesis (= “late catagenesis or epigenesis” of Russian authors). In this stage the temperature is greater than 100 °C, pressure increases and porosity becomes very weak. Montmorillonites and irregular mixed-layers disappear. Kaolinite recrystallizes as dickite in acid environment. These changes are irreversible. (4) Anchizone (= “metagenesis” of Russian authors; = “zone anchimetamorphique” of Kubler, 1964). This is the transitional zone to metamorphism. It agrees with temperatures around 200 °C. Illite and chlorite are almost the only sheet silicates. However, dickite can be observed as well as pyrophyllite generally associated with allevardite. The crystallographic parameters of illite define the limit of the following zone, the metamorphic epizone or greenschist facies. The crystallochemical processes that take place during the diagenetic evolution of clay minerals are schematically the following: (1) Gradual tetracoordination of aluminium. (2) Filling of octahedral sites either by interlayer cations, either by cations derived from outside the lattice, without the distinction dioctahedral-trioctahedral becoming very clear. (3) Interlayer exchange between crystal lattice and interstitial solution. Gradual closing of the layers by alkaline cations or octahedral brucite-like sheets. (4) Massive crystalline growth in the anchizone and the epizone. These processes are roughly symmetrical with those which occur during weathering. This review is a summary of the conclusions drawn in a Docteur-es-Sciences thesis (G. Dunoyer de Segonzac, 1969: Les Mineraux argileux dans la Diagenese. Passage au Metamorphisme, 339 p., 45 tables, 110 illus.) to be published as part of the series Memoires du Service de la Carte Geologique d'Alsace et de Lorraine. Most of the evidence on which these conclusions have been based is not cited directly in this article, but can be found in the thesis mentioned above, to which the reader is referred.

Relations of andesites, granites, and derivative sandstones to arc‐Trench tectonics

Abstract: Andesitic volcanogenic sequences, granitic batholith belts, and derivative graywacke-arkose sedimentary successions are prominent rock assemblages associated with alpinotype peridotite-gabbro belts and other characteristic tectonic features in orogenic regions or mobile belts where repeated crustal deformation and metamorphism have occurred. Field relations in the circum-Pacific region indicate that andesitic eruptive suites and granitic intrusive suites are commonly consanguineous and roughly contemporaneous and that they have shed voluminous detritus into coeval graywacke-arkose belts nearby. Modern systems of oceanic trenches and parallel magmatic arcs are probable analogues of the tectonic settings in which the three related rock assemblages formed. Data on crustal geophysics, trace-element geochemistry, and strontium-isotope ratios preclude participation of sialic crust in the generation of andesitic magmas at shallow levels but permit alternative hypotheses of primary partial melts from the mantle, derivative melts differentiated from primary basaltic melts, or melts from oceanic lithosphere slabs descending along inclined seismic zones beneath the volcanic arcs. In Quaternary andesitic suites, areal petrologic variations, particularly in potash content, are consistent tranverse to active volcanic chains regardless of longitudinal variations in crustal thickness. Levels of potash content in different suites correlate well with depths to the inclined seismic zone beneath, although significant scatter of points is apparent. Petrologic data from older andesitic terranes can be used to plot approximate positions and inclinations of paleoseismic zones. The anatectic hypothesis for the origin of magmatic plutons in intrusive batholiths is challenged by apparent comagmatic associations with andesitic eruptives, common sequences of intrusion from mafic to felsic, doubtful presence of suitable geosynclinal roots in some areas, available strontium-isotope ratios, difficult geothermal inferences, and unexpected episodicity or periodicity of repeated intrusive events that are correlative throughout large longitudinal segments of batholith belts. Consistent positions of batholith belts along the trends of relatively high-temperature and low-pressure members of paired metamorphic belts suggest that the granitic plutons were emplaced in the roots of complex volcanoplutonic arcs, and that granitic intrusive magmas may be derived from the same deep sources as andesitic eruptive magmas. Transverse petrologic asymmetry within Mesozoic batholiths of western North America is reminiscent of the similar petrologic asymmetry within Cenozoic volcanic terranes, and may be used to construct speculative paleoseismic zones for the volcanoplutonic arcs whose roots the batholiths may represent. Graywacke and arkose sequences that lie on the Pacific side of andesitic volcanogenic and granitic batholith belts are composed mainly of first-cycle volcanic and plutonic detritus and commonly form large parts of the relatively low-temperature and high-pressure members of paired metamorphic belts. Detritus eroded during and between successive episodes of volcanism and plutonism in the adjacent volcano-plutonic provenances was deposited in parallel subsiding belts, where it was progressively buried as an inverse record of the successive magmatic increments to the arc regions. The graywacke-arkose belts commonly include two parallel divisions. Distal facies of strongly deformed trench and continental-rise deposits were ground against and beneath the seaward flanks of the volcanoplutonic arcs. Proximal facies of more orderly strata were deposited in sediment traps between trenches and arcs in the tectonic position occupied by shelves, slopes, and troughs of varied bathymetric character in modern arc-trench systems. The interpretations in this paper attempt to bring petrologic inferences about orogenic rock assemblages in line with current mobilist tectonic concepts that are supplanting previous stabilist views. The formation of the three rock assemblages discussed is probably the principal means by which continental crust is formed from the mantle.

Earth: An Introduction to Physical Geology

Abstract: 1. An Introduction to Geology 2. Plate Tectonics: A Scientific Revolution Unfolds 3. Matter and Minerals 4. Magma, Igneous Rocks, and Intrusive Activity 5. Volcanoes and Volcanic Hazards 6. Weathering and Soil 7. Sedimentary Rocks 8. Metamorphism and Metamorphic Rocks 9. Geologic Time 10. Crustal Deformation 11. Earthquakes and Earthquake Hazards 12. Earth's Interior 13. Divergent Boundaries: Origin and Evolution of the Ocean Floor 14. Convergent Boundaries: Origin of Mountains 15. Mass Wasting: The Work of Gravity 16. Running Water 17. Groundwater 18. Glaciers and Glaciation 19. Deserts and Winds 20. Shorelines 21. Global Climate Change 22. Earth's Evolution through Geologic Time 23. Energy and Mineral Resources 24. Planetary Geology

Polytypism of Chlorite in Sedimentary Rocks

Abstract: Four chlorite polytypes of Bailey and Brown (962) have been identified by X-ray diffraction in clay-size chlorites of soils, sediments, and sedimentary rocks: (1) IIb, the polytype of common metamorphic and igneous chlorites; (2) Ib(s = 90°); (3) Ib(s = 97°); (4) Ia. An additional stacking arrangement. Iba, is defined herein as disordered chlorite which lacks an h0l diffraction band in the 2.4–2.5 A region. Most type-I chlorites are authigenic, as demonstrated by thin-section petrography. Type-I chlorites form during diagenesis, or less commonly during halmyrolysis, at temperatures and pressures less than those of low-grade metamorphism. A type-1 crystallization sequence is proposed, from least to most stable: Iba → Ib(s = 97°) → Ib(s = 90°). Conditions of low-grade metamorphism usually are necessary to cause conversion of Iba(s = 90°) to IIb, the most stable and common polytype. Chemical composition has little or no influence upon polytype relative stabilities; temperature is much more important. Sediment source areas with high relief, abundant rainfall, cold climate, and which contain IIb-chlorite-bearing metamorphic rocks, may yield essentially unweathered IIb chlorite to sites of deposition. Thus, clay-size IIb chlorite in unmetamorphosed sedimentary rocks can be interpreted as detrital. Caution is required, however, because IIb may be able to form authigenically at submetamor-phic temperatures, because it is the most stable polytype. Petrographic evidence is useful in such cases. Chlorite polytypism as a geothermometer can be applied to several geologic problems: (1) the authigenic versus detrital origin of clay minerals in sedimentary rocks, particularly in graywacke matrix; (2) the recognition of diagenetic facies or gradients, areally and stratigraphically, within given geologic provinces; (3) the detection of hydrothermal and incipient metamorphic effects. Chlorite polytypism merits general application as an interpretive tool.

XI.—The Evolution of the Scottish Caledonides in Relation to their Isotopic Age Pattern

Abstract: Three main stages are recognized in the evolution of the Scottish Caledonian orogen. Firstly, the development of the depositional framework from late Pre-Cambrian to early Ordovician times is outlined, The thick Moine and Dalradian sediments, accumulating on a continental rise, are shown to be equivalent to the shelf sequence of the foreland, and likely correlations are suggested. Secondly, the events comprising the Caledonian deformation and metamorphism of these sediments are reviewed and related to the development of a Benioff zone and coupled oceanic trench along the southern margin of the orogen through the Ballantrae complex. Comparison of stratigraphic and isotopic evidence for the age of these events leads to the conclusion that all major deformation and metamorphism occurred during a relatively short climactic episode 480–510 m.y. ago, within the Arenigian stage of the Ordovician. The third factor in the evolution of the orogen is post-climactic uplift and erosion continuing throughout Upper Ordovician, Silurian and Lower Devonian times. Contours of K–Ar mica ages are presented and related to the geological evidence for this prolonged period of isostatic recovery and thermal adjustment. The style and timing of granitic plutonism, which is closely associated with this third stage, may be indicative of crustal behaviour during uplift rather than continued metamorphism at depth.

The step from diagenesis to metamorphism in pelitic rocks during alpine orogenesis

Abstract: Summary Pelitic rocks were followed from unmetamorphosed clays and marls of Upper Triassic and Lower Liassic to anchimetamorphosed phyllites of the Alpine border region in Switzerland. Phengite and Al-rich chlorite formed from mixed-layer illite/montmorillonite; pyrophyllite from kaolinite. For the formation of paragonite, the following sequence is proposed: irregular mixed-layer illite/montmorillonite regular mixed-layer mica/montmorillonite mixed-layer paragonite/phengite paragonite. Besides the formation of new minerals in the transition zone (anchizone) between diagenesis and greenschist facies, other changes with increasing metamorphic grade are: the crystallinity of illite (in the sense of Kubler, 1967) increases, the intensity ratio 002/001 of the illite basal reflexions increases, 1Md illite changes to 2M1 phengite; the slates change colour from red to pink; the mean density increases; and textural changes due to reactions between clastic quartz and clay cement occur.

Potassium–argon ages of slates and phyllites from south-west England

Abstract: Potassium-argon dates from slates and phyllites will give the date of dynamic metamorphism provided that all radiogenic argon was expelled from the rocks at the time of metamorphism and none has been lost since. Where whole-rock slate ages can be compared with ages on separated mica the agreement is normally quite good. Some instances of incomplete argon loss are known, but these are mostly associated with other evidence of incomplete recrystallization. Significant losses due to continuous diffusion in a stable geological environment have not been observed, but post-metamorphic losses may occur in a tectonic environment. In general K–Ar dates from slates are thought to give useful younger limits to the age of metamorphism, provided that certain statistical and petrological criteria can be satisfied. Sixty slates from south-west England have been dated by the K–Ar method. Material from the Torquay area and north Devon gave scattered results, but the remainder fall into four quite well defined regional groups. (1) A 365–345 m.y. group from the Dodman phyllites and Gramscatho Beds in south Cornwall, associated with sw–ne structural trends, probably corresponds to a ‘Bretonic’ phase of folding in the Upper Devonian or Lower Carboniferous. (2) A group in the range 340–320 m.y. from the e–w Devonian slate belt on the southern limb of the synclinorium, considered in relation to stratigraphical evidence, suggests a Namurian (‘Sudetic’) phase of tectonism. (3) The Start schists and associated slates give 290-310 m.y., but the character of the event which these figures represent is not known. (4) Dates in the range 270–310 m.y. from Port Isaac, the Boscastle–Tintagel area and the southern boundary of Culm, represent minimum ages for the known late Carboniferous folding, and may in part date uplift and cooling at the end of the orogeny.

The geochronology of the basic igneous complexes

Abstract: Synopsis Rb-Sr whole-rock isochrons for the Upper Zone of the Insch mass and aureole schists of the Haddo House mass indicate crystallisation of the basic sheet of NE. Scotland at 486± 17 m.y. ago (t½ = 4.85 × 10 10 y. for Rb 87 ). It is argued that intrusion occurred into hot country rocks during, or very soon after, the migmatite overprint phase of the climax of regional metamorphism (M 3 ) in the Dalradian Series, so that this is also dated at about 486± 17 m.y. Small bodies of gneissose rocks in the Arnage mass give a whole-rock isochron age of 479± 12 m.y. and are interpreted as intrusions slightly younger than the Newer Gabbros. Post-metamorphic granites at Insch and Haddo House are dated at 459± 8 m.y. K-Ar mica ages from the gabbro complex are extremely uniform at 461 ± 5 m.y., whereas Rb-Sr ages on some of the biotites range from 471 ± 22 m.y. to 428 ± 9 m.y. Together, these ages preclude the hypothesis of overprinting by any discrete metamorphic event younger than the main climax. Instead, they are interpreted as indicating rapid post-orogenic uplift in NE. Scotland at about 460 m.y. ago.

Age of the Morton and Montevideo Gneisses and Related Rocks, Southwestern Minnesota

Abstract: Granitic gneisses in the vicinities of Morton and Montevideo in the Minnesota River Valley are dated at 3550 m.y. ago and are the oldest rocks so far found in North America. The gneisses were altered in varying degree by younger events of which two have been dated at 2650 m.y. and 1850 m.y. old. The event which occurred 2650 m.y. ago was a high-grade metamorphism accompanied by the intrusion of a large volume of granitic magma. Only the U-Pb zircon and the Rb-Sr whole-rock ages survived this event, and both types are discordant. A two-stage model that explains the U-Pb discordant ages combines a primary discordance produced during the metamorphism of 2650 m.y. ago with a secondary discordance developed approximately 100 m.y. ago when uplift and erosion brought the rocks close to the surface. This secondary discordance is also shown by the zircon from granite near Sacred Heart (2650 m.y. old) and from a younger granitic pluton (1850 m.y. old) near Granite Falls. The discordance in the Rb-Sr whole-rock ages is attributed primarily to the loss of radiogenic Sr 87 that probably occurred largely during the metamorphism of 2650 m.y. ago. Some later loss, however, is indicated in the younger ages of biotite and K-feldspar. Granitic material introduced or mobilized during the metamorphism is also a complicating factor. The 1850-m.y.-ago event was a low-grade metamorphism that reset the K-Ar and Rb-Sr ages of biotite in the rocks between Granite Falls and Ortonville. A number of small plutons, ranging in composition from gabbro to granite, and basaltic dikes were emplaced in the gneisses at this time, but only the granitic pluton near Granite Falls has been dated by both U-Pb and Rb-Sr methods. The mineral ages show variations that are difficult to explain, and the low apparent ages of the biotite may be in some way related to epeirogeny and the stabilizing of the K-Ar and Rb-Sr systems. The southeastern part of the valley, underlain by the Morton Gneiss and the granite at Sacred Heart, was stabilized 2400 to 2600 m.y. ago, but the northwestern part, underlain by gneiss in the Granite Falls-Montevideo area and by granite in the Ortonville area, was not stabilized until 1700 to 1850 m.y. ago. The Morton Gneiss was formed by synkine-matic intrusions of trondhjemitic and granitic magmas, and the structure dates back to the time of the intrusions, 3550 m.y. ago. A similar origin as a synkinematic intrusion of granite is favored to explain the gneiss at Montevideo. The country rock appears to have been a layered series of basaltic lavas, sedimentary rocks, and possibly some sill-like masses of diabase or gabbro. The structure of the region probably was considerably modified during the high-grade metamorphism 2650 m.y. ago. The rock types that were involved in the Mortonian event 3550 m.y. ago are similar to more recent crustal rocks and do not represent a protocrust.

Strontium 87/strontium 86 initial ratios in pyroxene granulite terranes

Abstract: Rb-Sr isotopic analyses of granulite facies rocks from East Africa, India, Guyana, the Adirondacks (New York), and southeastern Ontario give regression plots that restrict initial Sr87/Sr86 to a narrow range 0.700–0.707, comparable with anorthosite and continental basalt. The ratios agree with published values for granulite from India, Scotland, and Ivory Coast. Both geophysical and geochemical evidence suggest that these rocks developed in the lower crust and that most of the examples investigated were of igneous origin. Although subsequent regional metamorphism at the granulite facies level may have rearranged the assemblages, initial Sr87/Sr86 has been unaffected.

Geological investigations in West Nepal and their significance for the geology of the Himalayas

Abstract: In the sedimentary sequence of the Lower Himalayas monotonous geosynclinal deposits are overlain by various shallow-water deposits and a carbonate rock complex at the top. The age of this nonfossiliferous sequence is discussed. Transgressive Jurassic-Tertiary beds are evidence for a minimum thrust distance of 90 km of the Chail Nappes. The facies distribution points to the importance of an axis parallel to the Himalayan strike direction since Lower Paleozoic at least. The metamorphism of the overthrust units is described and the relation to the evolution of the orogen is discussed. An outline of the structure is given. The type of deformation and other features characteristic of the different tectonic units, as well as the relationship to the geology of the whole Himalayas are discussed. It is emphasized that structural development during the Himalayan orogenesis was largely determined by the earlier paleogeographic history. The picture of the structural development in successive stages is given.

Metamorphic framework and plutonic styles in the Prince Rupert region of the Central Coast Mountains, British Columbia

Abstract: The metamorphic framework in Prince Rupert – Skeena region of the Coast Mountains of British Columbia comprises schist, gneiss, and migmatite displaying progressive regional metamorphism that overlaps the Barrovian and Idahoan Facies Series. Although part of the circum-Pacific metamorphic zone, the Coast Mountain metamorphic belt is apparently not paired. Plutonic rocks, which were probably an integral part of the early metamorphic framework, have apparently been mobilized during metamorphism and continued to move out of their original environment while metamorphism waned, some even deforming the pre-existing fabric.Within the framework, four main plutonic styles have been recognized:1) Autochthonous, migmatitic, plutonic complexes.2) Para-autochthonous, steep-walled (tadpole) plutons.3) Para-autochthonous, tongue-shaped, recumbent plutons.4) Allochthonous, intrusive plutons.Quartz diorite and granodiorite are the most common plutonic rocks. Diorite and quartz monzonite are less common: gabbro and especia...

Cover and Basement Tectonics in the Cordillera Oriental of Colombia, South America, and a Comparison with Some Other Folded Chains

Abstract: The Cordillera Oriental of Colombia is built of Mesozoic and Tertiary strata, deformed chiefly during Tertiary orogenesis. These materials form a sedimentary cover, overlying a basement of lower Paleozoic (and Precambrian?) metamorphic and igneous rocks, and of little deformed and nonmetamorphic Upper Paleozoic rocks, which often behaved mechanically as part of the cover. No metamorphism took place during the Tertiary orogenesis, and the main structural features of the chain are determined by the relationship between basement and cover. The basement crops out within the chain in two massifs, Santander and Quetame-Garzon, arranged en echelon. The general structure of the chain is that of a raised block between the Magdalena Valley and Los Llanos of eastern Colombia. Tilting, differential lift, and faulting produced differences in height of the order of 10,000 m in the upper surface of the basement. The faults in the basement are reverse faults, often high-angle reverse faults (some of them reactivated normal faults of pre-Cretaceous age), some of which have been active until Quaternary times (Suarez fault). It is possible that strike-slip occurred along some faults, as has been suggested for the Santa Marta (= Bucaramanga) fault and for many faults in the Caribbean area. A rather narrow strip in the central part of the chain, between the two massifs shows a more tightly folded structure. The sedimentary cover shows different reactions to basement tectonics: (1) passive behavior (undeformed or tilted cover on basement blocks); (2) simple draping structures (monoclinal flexures overlying basement faults; asymmetrical gentle folds); (3) more complex, tight structures, where the basement has a less rigid behavior; (4) independent behavior of the cover, due to gravity (collapse structures), to injection (salt of the “Sabana de Bogota”), and to disharmonic relationships. Thus, the Cordillera Oriental of Colombia shows a series of different examples of the relationship between basement and cover structures. The Cordillera Oriental constitutes the external zone of the Andean System in Colombia, in sharp contrast to the internal zones (eugeo-syncline), with their volcanism of Cretaceous age, their plutonism, and their metamorphism. Such mountain chains as the Appalachians, the Cantabrian Chain, the Alps and the Betic System are characterized by tangential tectonics (nappes), structures facing regularly toward the outside of the chain, and a general decollement of the cover. However, the Cordillera Oriental shows a fan structure, more related to the structure of such mountain chains as the Pyrenees.

A Permian Disturbance of K-Ar Radiometric Ages in New England: Its Occurrence and Cause

Abstract: Approximately 200 K-Ar mineral and whole rock ages from New England, half of which are previously unpublished, are used to delineate an area of Permian thermal disturbance. The disturbed area, as outlined by K-Ar mica ages, forms a north-northeast-trending belt 60–80 mi wide that extends from the coast of Long Island Sound in southern Connecticut to southwestern Maine, where it terminates against rocks displaying older radiometric ages. Several possible mechanisms that may have affected the radio-metric systems of pre-existing rocks are examined: (1) contact metamorphism related to contemporaneous igneous activity, (2) alteration associated with major faulting, (3) regional metamorphism in late Paleozoic time, and (4) burial followed by uplift and erosion. Evidence is given that each of these mechanisms was operative locally, especially in the southern portion of the belt. The general lack of late Paleozoic tectonism in New Hampshire and Maine suggests that only burial is a likely cause of the disturbance there.

Metamorphic facies series of the crystalline basement of Chile

Abstract: The main rock types and mineral assemblages of the metamorphic rocks of the territory of Chile are described, and the metamorphic facies are identified. The crystalline basement of Chile consists of predominantly low-grade, semipelitic metamorphic and granitic rocks, exposed mostly along the coastal area of Central and Southern Chile, and in isolated areas of Northern Chile. Radiometric ages of both metamorphic and igneous rocks indicate Late Paleozoic. Three metamorphic series were recognized in Central Chile and used for a tentative classification of the metamorphic rocks from the remaining areas. The series correspond to intermediate-high pressure, intermediate-low pressure, and low pressure conditions of metamorphism, respectively. The formation of these series, their relation toMiyashiro's circumpacific paired belts, and the role of the basement during the Andean orogeny, are briefly discussed.

Composition of hornblendes formed during the hercynian progressive metamorphism of the Aracena metamorphic belt (SW Spain)

Abstract: This paper attempts to illustrate the chemical variations of metamorphic hornblendes regarding host rocks and prograde variations. Changes related to bulk chemistry (orthoamphibolites) mainly concern Si, Al, Mg, Fetot and Ca. The Mg, Fe2+ and Fe3+ contents of hornblendes are, however, not strictly related to host rook compositions and Mg enrichments are correlated with increasing Fe3+ contents in the amphiboles. Thus, variations of oxygen fugacity may control the Mg contents of the Ca amphiboles studied but this does not show clear relations with the prograde metamorphism. The most sensitive but irregular variation related to the metamorphic conditions is the prograde enrichment of the alkalis into the “A” vacant position and an increase of the (Na+K)tot/Na+K+Ca ratios of the amphiboles. Increasing Ti and AlIV contents as well as decreasing AlVI concentrations are also, but much less evidently, related to increasing T and P. A variation trend from tschermakitic to edenitic hornblendes may be drawn using Shido's end members calculation; this tendency and the relative deficiency of AlVI contents in the low-grade members suggests that the amphiboles studied were subjected to conditions of a low-pressure metamorphism type. Such a conclusion is in agreement with the occurrence of andalusite-cordierite/sillimanite-cordierite associations in the metapelitic rocks, and the absence of Fe-rich garnet and epidote from the orthoamphibolites of the amphibolite facies at Aracena. Comparisons with Ca amphiboles from other metamorphic areas show, in agreement with various authors, that “Abukuma” hornblendes are similar to those encountered in high-grade thermal aureoles and tonalitic intrusives but different from the hornblendes of “Barrovian” metamorphism types.

Oxygen-isotope ratios in metamorphic eclogites

Abstract: Rocks having δO18-values less than those of normal basalts (5 to 7 permil) are very rare. However, anomalously low δ-values are common in European B-type eclogites, including those from N. W. Spain (1.5 to 5.6 permil), W. France, S. Germany, and W. Norway. Since isotopic exchange with sediments during metamorphism would have increased their δ-values, we tentatively conclude that the isotopically light eclogites descended from basaltic rocks which interacted with light meteoric waters at high temperatures, as did the Tertiary igneous rocks of W. Scotland. Isotopically heavy eclogites from S. Germany (8.6 to 10.5 permil) are isotopically similar to calc-silicate rocks from N. W. Spain and are possibly derived from dolomitic pelites. Coexisting quartz and rutile effectively fractionate the isotopes of oxygen and thereby constitute a potentially useful geothermometer. The average quartz-rutile fractionation in B-type eclogites is 6.5 permil.

The Precambrian geology of the “Borborema”-Belt (States of Paraíba and Rio Grande do Norte; northeastern Brazil) and the origin of its mineral provinces

Abstract: In the crystalline area of the “Borborema” in NE Brazil the final geologic event was a strong static recrystallization, which is manifest in regional granitization, hybride granite, or pegmatite veins. The manifestation varies within the mapped area, it depends on lithology, thickness and structure of the Precambrian metasediments. The age of this phase is between 550 and 450 M. years. Recrystallization is accompanied by weak folding of pre-existing anticlines as indicated by the structural control of the Borborema pegmatites. The principal phase of deformation and the accompanying regional metamorphism are considered to be older, approximately 850 M. years or more. This older regional metamorphism produced rocks of the almandine-staurolite subfacies of the amphibolite facies; locally higher (cordierite and sillimanite subfacies) or lower (greenschist facies) degree of metamorphism occurs. The pyroxene-hornfels facies, which is found in the contact aureoles of hybride granites, occurs exclusively in the tactites (metamorphic dolomitic marls). In clastic metasediments, high grade metamorphic facies is restricted to the contact aureole of a small hypersthene diorite stock.

The Green Beds of the Scottish Dalradian Series: Geochemistry, Origin, and Metamorphism of Mafic Sediments

Abstract: The Green Beds of the Scottish Dalradian Series are greenschists and amphibolites of metasedimentary origin. Field relations, petrography, and chemical data are used to demonstrate that these rocks, approximately of basaltic composition, were derived by metamorphism of basic tuffs deposited in an area of pelite sedimentation. The Dalradian Epidiprites, a series of orthoamphibolites associated with the Green Beds are less siliceous and less magnesian, in general, than the metasediments. Pelitic material is more abundant in the Green Beds of the Southwest Highlands than to the northeast. Amphibole occurrence and abundance are related to calcium content of the rocks. Fifty-eight metasediments and three Epidiorites were analyzed for thirty elements each, by X-ray fluorescence methods.

Deformational and metamorphic history of Moinian and Lewisian rocks between Strathconon and Glen Affric

Abstract: Synopsis Seven distinct sets of fold structures have been recognised within 450 square kilometres of Moinian and Lewisian rocks. The earliest structures are only locally preserved, and are deformed by major folds which originally had approximately NE.–SW. axial traces and very variable axial directions. The formation of these folds was associated with high-grade Barrovian metamorphism and with the development of major slides. A pair of large N.–S. folds subsequently deformed this major fold pattern, apparently under conditions of fairly low metamorphic grade. High-grade metamorphism persisted or was re-established in part of the region during the formation of later structures oblique to the dominant regional trend. Subsequently a widespread set of folds whose style and orientation vary with varying metamorphic grade was superposed. In the area of higher grade these have approximately NE.–SW. axial traces, but their attitude in rocks of lower grade is more variable. Formation of another locally-developed set intervened between this widespread folding and the development of the last structures, which are mainly small NE.–SW. folds formed under low-grade conditions. Possible correlations with events in other parts of the Northern Highlands are discussed.

Burial metamorphic episodes in the Andean geosyncline, Central Chile

Abstract: The burial metamorphism of the Andean geosynclinal deposits of Central Chile is studied. The stratigraphic units under consideration, Lower Jurassic to Upper Cretaceous and/or Lower Tertiary in age, have an accumulated thickness of 15,000 to 28,000 m, cover an area of nearly 2,500 km2, and consist predominantly of basic and acid lavas, ignimbrites and volcanic-clastic sediments, deposited in marine and continental environments. Unconformities break up the sequence into several stratigraphic-structural units. A specific burial metamorphic pattern, or series, characterizes each of the units. Each series consists of a succession of mineral assemblages, the metamorphic grade increasing downwards. The isograds are essentially parallel to the bedding planes. The grade range in each series covers different intervals between the zeolite and greenschist facies. Each unconformity corresponds to a mineralogie break in the alteration, higher grade assemblages generally overlying lower grade assemblages. The downward increase in metamorphic grade as well as the mineralogie breaks seem to be unrelated to the volcanic versus sedimentary character and to the continental versus marine depositional environment of the rocks. The repeated pattern of burial metamorphism between unconformities suggests a history consisting of several burial metamorphic episodes, each taking place prior to a folding period and leaving unaffected the underlying units, which were apparently sealed up by the previous episode. This mechanism provides an explaination for the presence of such low-grade burial metamorphic facies at the bottom of a stratigraphic column as thick as that of the Andean Geosyncline.

Effects of Incipient Metamorphism on Organic Matter in Mudrock

Abstract: Comparison of organic matter in unmetamorphosed rocks with organic matter in metamorphosed rocks shows three consistent differences: (1) the absolute amounts of hydrocarbon material present are less in metamorphosed rocks than in unmetamorphosed rocks, (2) the relative amount of the total organic matter present as hydrocarbon material is less for metamorphosed than unmetamorphosed rocks, and (3) the extractable material and the isolated hydrocarbons from metamorphosed rocks have a higher proportion of saturated (compared to aromatic) species, and have lighter colors, lower melting points, and lower average molecular weights than analogous material from unmetamorphosed rocks. In addition, anomalously high carbon isotope ratios (^dgrC13) were observed in two samp es believed to have undergone higher degrees of incipient metamorphism. These results suggest that the nature and characteristics of organic matter in sedimentary rocks may provide a useful new basis for recognizing and classifying degrees of incipient metamorphism.

Metamorphic History of the Blue Ridge Province of Tennessee and North Carolina

Abstract: A study of metamorphic index minerals in 354 panned alluvial samples has enabled delineation of metamorphic isograds in the Blue Ridge province of Tennessee and North Carolina. A well-defined axis of maximum metamorphic intensity (sillimanite and kyanite grades) is flanked to the southeast and northwest by lower metamorphic zones. Based on radiometric age determinations, metamorphic isograds are interpreted to have formed during the 320 to 375 m.y. event. In northeastern Tennessee and northwestern North Carolina, the isograds form a retrograde sequence in rocks previously metamorphosed during the Granville orogeny (≅1000 m.y.). In the southeastern Blue Ridge, a classical prograde sequence of the Barrovian type has developed in the Late Precambrian Ocoee Series. Mid-Paleozoic granites and pegmatites are essentially confined to the kyanite and sillimanite zones. The granites are interpreted to have formed by anatexis. Pegmatites may have formed by differentiation of granitic magmas in some areas, and by local anatexis or metamorphic differentiation in others. Ultramafic rocks are also confined mainly to the kyanite and sillimanite zones. These rocks were probably implaced during an early stage of regional metamorphism. The relationships between metamorphic isograds and faults indicate that the major period of movement of the Blue Ridge thrust sheet took place after mid-Paleozoic metamorphism.

Metamorphic Waters from the Pacific Tectonic Belt of the West Coast of the United States

Abstract: Waters unusually rich in ammonia, boron, carbon dioxide, hydrogen sulfide, and hydrocarbons are found in more than 100 localities along the Pacific coast of the United States. The waters are believed to be products of low-grade metamorphism of marine sediments. The marine sedimentary rocks would have to be tectonically emplaced below crystalline rocks in many places. Mercury ore deposits are probably also products of the low-grade metamorphism.

Evolution of the Labrador Geosyncline

Abstract: The Labrador “trough” is the preserved portion of an Early Proterozoic (Aphebian) geosyncline, extending from the Grenville Front at Wabush to Payne Bay, over 600 mi farther north-northwest. Archaean gneisses constitute the foreland to the west of the geosyncline, and remetamorphosed Archaean gneisses compose much of its eastern hinterland. It is the purpose of this paper to outline and correlate the essential geological features of the Labrador trough and to propose a working hypothesis on its origin. The basin filling represents two, or possibly three, tectonic cycles, each initiated by an orthoquartzite-limestone sequence (including iron formation in the second and third cycles), and culminating in deposition of shales and of flysch-type deposits in the external zone of the geosyncline, and in intense volcanic activity in the central and internal zones. The paleogeography of the first cycle is briefly examined. A rapidly uplifted area situated east of the geosyncline is the probable source of much clastic material. The basin center migrated westward during the preophiolitic stage, and a geanticline emerged during the early phase of the ophiolitic stage in the center of the geosyncline. Very large volumes of mafic volcanic and plutonic rocks erupted in the eastern and central parts of the geosyncline during the latter part of the tectonic cycles. The eugeosynclinal mafic rocks are mainly K-deficient tholeiites. Spilites and transitional types of basalts occur in subordinate volumes. Serpentinized ultra-basic sills are present. The Aphebian rocks of the Labrador trough were folded during the Hudsonian Orogeny. A tectonic section across the geosyncline is discussed. The basement below the western part of the geosyncline has not been involved in the Hudsonian deformation, whereas the Archaean underlying the central and eastern portions of the geosyncline was folded jointly with its cover. The relations between structural style, depth of burial and metamorphic grade are briefly examined. A late-kinematic and post-kinematic metamorphism of an intermediate pressure series overprinted the internal zones of the geosyncline. Hudsonian granites and migmatites are absent, and possible causes of their absence are discussed.