Showing papers on "Metamorphism published in 1982"

Tectonic accretion and the origin of the two major metamorphic and plutonic welts in the Canadian Cordillera

Abstract: The Omineca Crystalline Belt and Coast Plutonic Complex are the two major regional tectonic welts in the Canadian Cordillera in which were concentrated intense deformation, regional metamorphism, granitic magmatism, uplift, and erosion. The welts, which formerly were thought to result from subduction of Pacific Ocean lithosphere beneath the western edge of North America, can now be viewed partly as the result of tectonic overlap and/or compressional thickening of crustal rocks during collisions between North America and two large, composite, allochthonous terranes that were accreted to its ancient western margin. The inner composite terrane, Terrane I, includes four smaller terranes that apparently were together by the end of Triassic time. The outer composite terrane, Terrane II, comprises two terranes, amalgamated by Late Jurassic time. The Omineca Crystalline Belt formed mainly from mid-Jurassic time onward, during and following the collision of Terrane I with North America. This belt straddles the zone of overlap of autochthonous and allochthonous terranes, and its characteristic metamorphism and structure are superimposed on both. The Coast Plutonic Complex formed mainly in Cretaceous to early Tertiary time during and following the attachment of Terrane II to the new, Jurassic, continental margin. It lies along the boundary of Terrane I and Terrane II and involves elements of both terranes. The collisions took place within the overall setting of the North American plate moving relatively westward into various Pacific plates from Jurassic time onward and in conjunction with subduction of Pacific Ocean lithosphere.

Magmatism of the British Tertiary Volcanic Province

Abstract: Synopsis This review concentrates on those aspects of research into the magmatism of the British Tertiary Volcanic Province (BTVP) which have seen rapid accumulation of data and changes of views during the last few years. There seems to be general agreement at present amongst those who have made detailed geochemical studies of the problem that the pervasive hydrothermal metamorphism which affects most BTVP igneous rocks has not caused significant changes in the compositions (other than hydrogen and oxygen isotopes) of samples collected away from the immediate vicinities of the major passageways for hydrothermal fluids. Wide ranges of silica saturation characterize the basic members of both the major traditional BTVP magma types—Plateau and Non-Porphyritic Central (NPC). It is only the evolved members of these suites, the ne-normative Plateau-type hawaiites and the Q-normative NPC tholeiitic andesites, which show clearly the long-emphasized contrast between alkalic and tholeiitic compositions. Prior to their final uprise and eruption or emplacement, the Plateau-type magmas appear to have equilibrated near the base of the continental crust, whereas the NPC magmas evolved at upper-crustal depths. The NPC basalts (as traditionally defined) seem to be polygenetic in terms of mantle-derived magma types. Most of them are related to the MORB(Mid Ocean Ridge Basalt)-like Preshal Mhor basalts of Skye and Mull but some appear to be batches of Plateau-type magma which equilibrated in the upper crust. The Rio Grande Rift, southwestern U.S.A., provides a present-day analogue to the Palaeocene magmatic plumbing envisaged beneath the BTVP. The mantle-derived basic magmas upwelling beneath the BTVP were affected by fractional crystallisation and sialic contamination to varying extents as they rose through the continental crust. The earlier magma batches mostly interacted only with granulite-facies Lewisian gneisses, from which they appear to have melted selectively the most-fusible minor acid members. In addition, there is little doubt that Pb equilibrated selectively, in a vapour or non-silicate liquid phase, between the magmas and Archean sial, whilst Sr- and Nd-isotope ratios in the BTVP igneous rocks may also be in part the products of selective magma-crust interactions. If those BTVP basic rocks which contain negligible components of sialic contaminant are considered, it is apparent that there are consistent differences in various incompatible-element ratios between the Plateau-type basalts of Skye and Mull, the two areas studied in most detail at present. All of the BTVP Plateau-type basalts are exceptionally depleted in the strongly-incompatible elements, relative to worldwide examples of basalts with similar major-element compositions. This feature is consistent with genesis from a volume of upper mantle which had previously lost a very small fraction of strongly-alkalic melt. The Permian lamprophyres of the region are tentatively identified as samples of this postulated pre-Tertiary alkalic magma. A case is also made for considering the Caledonian Iapetus Suture as the tectonic feature along which most of the BTVP igneous centres are distributed. Seen from this point of view, the origins of BTVP magmatism are to be found in upper-mantle convection during and after the subduction which closed Iapetus—subsequently re-activated by the regional tension and Palaeocene mantle convection associated with the opening of the North Atlantic. At the climax of BTVP magmatism, when the bulk of the dyke swarms and the plutons of the central complexes were emplaced, a final increment of the progressive partial fusion of the upper mantle beneath the region gave rise to the MORB-like Preshal Mhor-type magmas. Although it is apparent from their Sr-, Nd- and Pb-isotope ratios that most of the BTVP acid magmas contained appreciable fractions from sialic sources, there are few instances where mantle-derived magma does not appear to have been the dominant ingredient in the hybrid liquids which evolved—by fractional crystallisation—to acid residua. Nevertheless, when the sequence of granite emplacement within individual BTVP intrusive centres is considered, it is apparent that the evolution of each acid magma batch was a complex and variable process, involving combinations of such mechanisms as fractional crystallisation, fusion of both sialic crust and earlier BTVP acid rocks, and mixing of magmas at various states of evolution.

Flow melanges: Numerical modeling and geologic constraints on their origin in the Franciscan subduction complex, California

Abstract: In northern California, the central belt of the Franciscan subduction complex of late Mesozoic to early Tertiary age is largely a zone of chaotically mixed pelitic-matrix melange. The melange contains a diverse assemblage of rocks of various sizes and degrees; of metamorphism. The most abundant rock types are graywacke and greenstone, but the exotics such as blueschist and eclogite are the most distinctive rocks in the belt. The melange belt is traceable for hundreds of kilometres along strike and is up to tens of kilometres wide. Elongate fragments of most rock types show pinch and swell structure that grades into boudinage. This suggests they behaved as relatively rigid bodies in a ductilely deforming matrix. The general planar preferred orientation of elongate blocks, development of foliation in the matrix, wide separation of blocks, and chaotic mixing indicate that large amounts of flow occurred during melange formation. The major mineralogy of the matrix is quartz + albite + chlorite + white mica and rarely kaolinite. Minor amounts of pumpellyite and lawsonite are present. Comparison with burial metamorphic sequences and other blueschist terranes indicates that most of the blocks and matrix are compatible with conditions of metamorphism in the range of 100 A model is proposed in which the pelitic matrix melanges of the Franciscan represent zones where flow, driven by the movement of the descending plate, occurred in the accreted sediment pile. When flow occurs in a low-angle corner, a forced convection drives material back to the surface and transports blocks of blueschist and eclogite upward. A varied assemblage develops because fragments of various lithologies and grades of metamorphism are plucked from many points along the walls of the melange wedge and added to subducted material. Laminar flow causes particles to be dispersed. Most large clasts are broken into smaller, rounded fragments by boudinaging as the melange turns the corner to flow back towards the surface. Mixing is enhanced because differences in block size and density cause differential settling of large blocks within the nonuniform velocity field. Oblique convergence cause;s blocks to take helical paths and may explain the dispersal of blocks such as eclogite along the length of the Franciscan from a few localized sources. The thermal regime in a mature flow melange is such that the downward and upward P-T paths of contained material almost coincide. This accounts for the retrograde blueschist facies metamorphism of the high-grade blocks such as eclogite. The Franciscan melange belt was probably initially produced by the subduction and deformation of a thick continental margin sequence with a high shale/sandstone ratio (the “Knoxville”). Similar chaotic zones that do not contain exotic blocks in other subduction complexes, may have originated as small flow melanges that developed in wide-angle wedges that did not go to great depth.

Igneous and Metamorphic Petrology

Abstract: Preface. 1. Overview of fundamental concepts. 2. Composition and classification of magmatic rocks. 3. Thermodynamics and kinetics: an introduction. 4. Silicate melts and volatile fluids in magma systems. 5. Crystal-melt equilibria in magmatic systems. 6. Chemical dynamics of melts and crystals. 7. Kinetic paths and fabric of magmatic rocks. 8. Physical and thermal dynamics of bodies of magma. 9. Magma ascent and emplacement: field relations of intrusions. 10. Magma extrusion: field relations of volcanic rock bodies. 11. Generation of magma. 12. Differentiation of magmas. 13. Magmatic petrotectonic associations. 14. Metamorphic rocks and metamorphism: an overview. 15. Petrography of metamorphic rocks: fabric, composition, and classification. 16. Metamorphic mineral reactions and equilibria. 17. Evolution of imposed metamorphic fabrics: processes and kinetics. 18. Metamorphism at convergent plate margins: P-T-t paths, facies, and zones. 19. Precambrian rock associations. Appendix A. Appendix B. References Cited. Glossary. Index

Volatile production and transport in regional metamorphism

Abstract: Calculations show that H2O and CO2 produced during devolatilization of an average pelite will occupy ∼12 vol. % of the rock at 500°C and 5 kb. Because the tensional strength of well foliated rock at metamorphic conditions is vanishingly small, such a volume of fluid having any vertical extent will fracture the rock and escape upward owing to its lower density.

The transformation of amphibolite facies gneiss to charnockite in southern Karnataka and northern Tamil Nadu, India

Abstract: Amphibolite facies metamorphic grade gives way southward to the granulite grade in southern Karnataka, as acid gneisses develop charnockite patches and streaks and basic enclaves develop pyroxenes. Petrologic investigations in the transitional zone south of Mysore have established the following points: 1) The transition is prograde. Amphibole-bearing gneisses intimately associated with charnockite at Kabbal and several similar localities are not retrogressive after charnockite, as proved by patchy obliteration of their foliation by transgressive, very coarse-grained charnockite, high fluorine content of biotite and amphibole in gneisses, and high large-ion lithophile element contents in gneisses and charnockites. These features are in contrast to very low fluorine in retrogressive amphiboles and biotites, very low large-ion lithophile element contents, and zonal bleaching of charnockite, in clearly retrogressive areas, as at Bhavani Sagar, Tamil Nadu. 2) Metamorphic temperatures in the transitional areas were 700°–800° C, pressures were 5–7 kbar, and H2O pressures were 0.1–0.3 times total pressures, based on thermodynamic calculations using mineral analyses. Dense CO2-rich fluid inclusions in the Kabbal rocks confirm the low H2O pressures at the first appearance of orthopyroxene. Farther to the south, in the Nilgiri Hills and adjacent granulite massif areas, peak metamorphic temperatures were 800°–900° C, pressures were 7–9 kbar, and water pressures were very low, so that primary biotites and amphiboles (those with high F contents) are rare. 3) The incipient granulite-grade metamorphism of the transitional areas was introduced by a wave of anatexis and K-metasomatism. This process was arrested by drying out under heavy CO2 influx. Charnockites so formed are hybrids of anatectic granite and metabasite, of metabasite and immediately adjacent gneiss, or are virtually isochemical with pre-existing gneiss despite gross recrystallization to granulite mineralogy. These features show that partial melting and metasomatism are attendant, rather than causative, in charnockite development. Copious CO2 from a deep-crustal or mantle source pushed ahead of it a wave of more aqueous solutions which promoted anatexis. Granulite metamorphism of both neosome and paleosome followed. The process is very similar to that deduced for the Madras granulites by Weaver (1980). The massif charnockites, for the most part extremely depleted in lithophile minor elements, show many evidences of having gone through the same process.

Petrology: Igneous, Sedimentary and Metamorphic

Abstract: Part 1 Igneous rocks: introduction to igneous environments igneous minerals and textures chemistry and classification of igneous rocks crystallization of magmas origin of magmas by melting of the mantle and crust evolution of magmas - fractional crystallization and contamination petrology of the mantle igneous rocks of the oceanic lithoshere igneous rocks of convergent margins igneous rocks of continental lithosphere. Part 2 Sedimentary rocks: the occurrence of sedimentary rocks weathering and soils conglomerates and sandstones diagenesis of sandstones mudrocks limestones and dolostones other types of sedimentary rocks. Part 3 Metamorphic rocks: metamorphism and metamorphic rocks isograds, metamorphic facies, and pressure-temperature evolution assemblages, reactions and equilibrium controls of the metamorphic reactions metamorphism of mafic and ultramafic igneous rocks metamorphis of aluminous clastic rocks metamorphism of calcareous rocks.

Suspect terranes and accretionary history of the Appalachian orogen

Abstract: The North American connection, or the Grenvillian continental crust that separated from eastern North America during the initiation of the Appalachian orogenic cycle, is poorly defined within the present orogen. A variety of terranes occur outboard of the North American miogeocline, all structurally uncoupled and therefore suspect. Stratigraphic analysis indicates that Appalachian accretion progressed from the miogeocline outward. The boundaries of the earliest accreted western terranes are marked by melange and ophiolite complexes. Later boundaries between eastern terranes are steep mylonitic zones and brittle faults. Accretionary events, defined by Stratigraphic analysis, correspond to times of major deformation, plutonism, and metamorphism in the history of the orogen.

CORDILLERAN METAMORPHIC CORE COMPLEXES -From Arizona to Southern Canada

Abstract: Cordilleran metamorphic core complexes occur in a sinuous belt that lies west of the Cordilleran fold and thrust belt from Canada to California. It then continues southeastward through the Basin and Range country of Arizona, where it lies athwart the northeast edge of the fold and thrust belt across Arizona, before continuing south into Mexico (Figure 1). This curious geo­ graphic distribution, in addition to the relatively recent recognition of the young age of metamorphic fabrics in many areas , has attracted attention to these complexes and led to a variety of hypotheses for their origin. The Shuswap complex in Canada is the largest and longest recognized metamorphic core complex and is considered the type example (Coney 1980). Only during the past two decades has an awareness of the complexes in the US blossomed. This awareness reached its fullest expression in the recent Geolog­ ical Society of America memoir devoted to discussion of these metamorphic complexes (Crittenden et aI1980). Because of the richness of that source , this review focuses on ideas and interpretations rather than descriptive details, but some examples must be given to demonstrate both variability and common features. The investigation of each complex has tended to follow a similar historical pattern. Prior to 1960 most were regarded as exposures of pre-Phanerozoic crystalline basement or granitic intrusive bodies, and thus not structurally active parts of the Mesozoic orogen. In the 1960s, field work and recon­ naissance K-Ar geochronometry drew attention to the complexes as sites of Mesozoic and Cenozoic deformation and metamorphism affecting Phan­ erozoic supracrustal rocks. During the last two decades numerous detailed

Geochronological studies of the Bohemian massif, Czechoslovakia, and their significance in the evolution of Central Europe

Abstract: U–Pb zircon and Rb–Sr whole-rock analyses from various gneisses and plutonie rocks of the Moldanubian and Moravo-Silesian zones and the stable foreland of the Hercynian (Variscan) orogenic belt indicate that most of the crust in Central Europe was first formed during the Cadomian orogeny which straddles the Precambrian–Cambrian boundary. Zircons, however, have a memory of older ages which correspond with those of events known in Fennoscandia. The new radiometrie data are consistent with the stratigraphie record in that they do not provide any evidence for a major early Palaeozoic tectonothermal event between the Cadomian and Hercynian orogenies.Granulites from two localities in the Moldanubian zone yield U–Pb zircon ages of 345 ± 5 Ma; discordant zircon data points indicate that the granulite facies metamorphism was not of long duration. Tectonic units containing these high grade rocks were emplaced amongst amphibolite facies rocks during an event of widespread shearing which has been dated at 341 ± 4 Ma on the basis of a lower U–Pb zircon intercept age from one of the sheared gneisses and 338 ± 3 Ma U–Pb ages from monazites. Rb–Sr muscovite ages of 331 ± 5 Ma from pegmatites axial planar to asymmetrical folds date the last stage of SE-directed simple shear. A Rb–Sr whole-rock isochron of 331 ± 4 Ma from a principal magmatic type of the Central Bohemian pluton confirms the field evidence that the large NE-trending plutons of the Moldanubian zone were emplaced during a late stage of the deformation. The strong disturbance of the U–Pb zircon isotopic system in the sheared gneisses suggests U loss while a high U/Th ratio in monazite from one of these tectonised rocks suggests the simultaneous passage of hydrothermal fluids. Thus a crustal source is indicated for the uranium deposits of the Moldanubian zone.Critical to any plate tectonic model for the development of the Middle European Hercynides was the existence of an ocean in Early Devonian times which separated a North European continent from a South European continent(s). The northward movement of the South European continent over a shallowly-dipping subduction zone and subsequent continental collision can explain the high T–low P metamorphism and the imbricated tectonic style of the Moldanubian zone and adjacent Moravo-Silesian zone along the southeastern Hercynian foreland. The temporal separation of granulites and granites implies distinct conditions of formation and it has been suggested that the plutonism, following on from the imbrication of the Cadomian crust, was initiated by the subduction of wet oceanic sediments.

Kyanite-eclogite to amphibolite fades evolution of hydrous mafic and pelitic rocks, Adula nappe, Central Alps

Abstract: In the southern Adula nappe (Central Alps), two stages of regional metamorphism have affected mafic and pelitic rocks. Earlier eclogite facies with a regional zonation from glaucophane eclogites to kyanite-hornblende eclogites was followed by a Tertiary overprint which varied from greenschist to high-grade amphibolite facies. Despite a common metamorphic history, contrasting equilibration conditions are often recorded by high-pressure mafic eclogite and adjacent predominantly lower-pressure pelite assemblages. This pressure contrast may be explained by different overprinting rates of the two bulk compositions during unloading. The rates are controlled by a mechanism in which dehydrating metapelites provide the H2O required for simultaneous overprinting of enclosed mafic eclogites by hydration.

A partisan review of Proterozoic anorthosites

Abstract: Most anorthosites of the massif type crystallized in the episode 1.7-1.2 Gyr, with a pronounced peak of the age distribution near 1.4 Gyr. They were emplaced anorogenically at depths as shallow as 7 km, where the ambient temperature of country rocks was probably less than 250'C. Depths of emplacement may have been as great as 25 km or more in rare cases; the greater depths of equilibration estimated from granulite facies metamorphism may be incorrectly interpreted as emplacement depths, but in any case they are demonstrably not required or characteristic of anorthosite emplacement. Penetrative deformation and metamorphism of anorthosites are post-emplacement accidents of the local geologic history, and are not directly caused by the presence of anorthosites. Granitic rocks (mangerite-charnockite suite) associated with anorthosite are in general later or contemporaneous products of crustal anatexis, with chemical and isotopic signatures distinct from the anorthosites and their residua. Such granitic rocks should not, therefore, be summed with the anorthositic rocks to obtain bulk compositions. The magmas that produced most anorthosites were dry, as shown by high-temperature mineralogy and anhydrous mineral assemblages in contact aureoles. Residua from their crystallization are ferrodiorites to ferrosyenites typical of closed-system fractionation. These residua were locally and frequently ejected into contemporaneously molten granite, where they formed pillows and cooled rapidly. The overall chemistry of anorthosites and residua is broadly tholeiitic and consistent with derivation from the mantle. Olivine-bearing magmas locally ranged from leucotroctolite (anorthosite) to later but coexisting picrite or melatroctolite in the same pluton, confirming a wide spectrum of magma types. A signal feature of troctolitic and noritic magmas is their low augite content, implying high content of spinel component. Large anorthosite complexes such as Nain and Harp Lake consist of many plutons representing repeated injections of separate magma batches with varying chemistry. The abundant true anorthosites, richer in plagioclase than magmas cosaturated with a mafic phase, must represent plagioclase enrichment by either mechanical or chemical processes or both. The role of kinetics in nucleation and solidification of such rocks may be centrally important. It is proposed that hyperfeldspathic (plagioclase-supersaturated) liquids were generated by quasi-isothermal extraction of mafic minerals from tholeiitic magma enroute to and at the site of emplacement, and that such a kinetic process was uniquely permitted in an environment of aborted continental rifting. Anorthositic rocks may have much to say about the episodic versus continuous geochemical evolution of the earth's mantle.

Sm–Nd dating and cooling history of Scourian granulites, Sutherland

Abstract: The Scourian gneisses of the Lewisian complex of north-west Scotland have been the subject of many geochronological investigations. Sm–Nd whole-rock measurements suggest that the protoliths of the Lewisian complex differentiated 2.92±0.05 Gyr ago from an approximately chrondritic mantle1. Rb–Sr and Pb–Pb whole-rock ages between ∼2.8 and 2.6 Gyr reflect subsequent depletion in Rb and U (refs 2, 3) which has generally been associated with granulite facies metamorphism. Zircons which are thought to have crystallized during this metamorphism have also given U–Pb ages of 2.66 Gyr (ref. 4). This granulite facies metamorphism, involving pressures in excess of 10 kbar and peak temperatures estimated between 820±50 °C (ref. 5) and 1,250 °C (refs 6,7), affects a terrain including supracrustal rocks7 and thus implies a major tectonic event. We report here the first geochronological data measured on mineral assemblages that have also provided estimates of the conditions of metamorphism. Such measurements are possible because the major minerals of granulite facies assemblages show an unusually favourable spread in Sm/Nd ratios; garnet, especially, is strongly enriched in Sm (ref. 8). Data for three garnetiferous basic gneiss assemblages suggest that closure of the Sm–Nd system occurred significantly later than the peak of the metamorphism.

U-Pb dating of an early paleozoic bimodal magmatism in the french Massif Central and of its further metamorphic evolution

Abstract: In the Marvejols area (Southern french Massif Central), the gneissic Marvejols supergroup is overthrust on the metasedimentary “Serie du Lot”, deposited in part prior to 540 Ma. The allochtonous terranes are characterized by the occurrence of a leptyno-amphibolitic group, a complex association of mafic and felsic rocks of igneous and sedimentary derivation. A 480±10 Ma age has been obtained by U-Pb dating of zircons, for the crystallization of both mafic and felsic meta-igneous rocks. These rocks were emplaced during an important extensional tectonics. Relics of eclogites, pyrigarnites, coronite gabbros and HP-trondhjemites are clear evidence for a further HP-HT event dated at 415±6 Ma on zircons from a HP trondhjemite. Subsequently, the Marvejols supergroup underwent an amphibolite facies metamorphism with incipient mobilization dated at 345±10 Ma.

Geochemistry and geochronology of proterozoic tholeiite dykes of east antarctica: evidence for mantle metasomatism

Abstract: Two episodes of tholeiite dyke emplacement have been identified in Archaean high-grade metamorphics of the Napier Complex in Enderby Land Middle Proterozoic Amundsen dykes are typical continental tholeiites and most of the chemical variation in individual suites can be explained in terms of different degrees of partial melting and low-pressure crystal fractionation Group I Amundsen tholeiites were derived from a relatively homogeneous source region 1,190±200 my ago, whereas that of the group II Amundsen tholeiites was chemically and isotopically heterogeneous Group II dykes have various degrees of enrichment in incompatible elements, and commonly show normalised trace element abundance patterns with negative Nb anomalies These features imply variable metasomatism of the source region by a volatile-rich fluid phase (rather than a melt of any observed igneous composition) enriched in K, Rb, Ba, Th, and possibly La and Ce Early Proterozoic (2,350±48 my) tholeiites were emplaced at considerable depths in the crust during the waning stages of granulite-facies metamorphism and include a high-Mg suite of possible komatiitic affinity, ranging in composition from hypersthene-rich tholeiite (norite) to quartz-rich tholeiite They tend to have higher ratios of highly to moderately incompatible elements (eg, K/Zr, K/Ce), and larger Nb anomalies (ie, higher K/Nb) compared with middle Proterozoic tholeiites, suggesting derivation from more enriched source regions Isotopic data are not compatible with significant crustal contamination, but constrain source metasomatism to a time immediately before emplacement Metasomatism of the source region of the much younger group I tholeiites may have been contemporaneous with that of the high-Mg suite

The early metamorphic history of the Haast Schists and related rocks of New Zealand

Abstract: Rare but widespread relics of sodic amphibole occur in metabasites and metacherts of the Haast Schists and related Caples Terrane rocks. Present main-stage metamorphic assemblages are frequently chemically equivalent to earlier sodic amphibole bearing assemblages, indicating that these rocks underwent an earlier, higher P/Tmetamorphism prior to formation of the present pumpellyite-actinolite and greenschist facies assemblages. The earlier assemblages were stable during and after early isoclinal folding, but were replaced by the present moderate P/T assemblages prior to the last major fabric-forming deformation. The change in conditions was due to thermal relaxation, probably accompanied by uplift and erosion, and peak metamorphic temperatures were about 350–370° C in the pumpellyite-actinolite zone of the Caples Terrane and near 390° C in the greenschist facies chlorite zone near Queenstown. According to Henley (1975) these greenschist facies rocks attained a pressure of at least 6.4±0.4 kb during their history, but a pressure of 4.6±0.6 kb has been estimated for a chlorite zone rock from Middlemarch, and so the 6.4 kb estimate probably refers to the maximum pressure attained during the earlier, higher P/T metamorphism. Similar changes in ‘metamorphic facies series’ with time occur in some older and more complex metamorphic belts such as the Caledonides, and this study suggests that it may be possible to interpret particular elements in the metamorphic development of such belts in terms of specific circum-Pacific analogues.

Essentials of Geology

Abstract: Table of Contents 1. An Introduction to Geology 2. Matter and Minerals 3. Igneous Rocks and Intrusive Activity 4. Volcanoes and Volcanic Hazards 5. Weathering and Soils 6. Sedimentary Rocks 7. Metamorphism and Metamorphic Rocks 8. Mass Wasting: The Work of Gravity 9. Running Water 10. Groundwater 11. Glaciers and Glaciation 12. Deserts and Wind 13. Shorelines 14. Earthquakes and Earth's Interior 15. Plate Tectonics: A Scientific Theory Unfolds 16. Origin and Evolution of the Ocean Floor 17. Crustal Deformation and Mountain Building 18. Geologic Time 19. Earth's Evolution through Geologic Time 20. Global Climate Change Appendix A Metric and English Units Compared Appendix B Topographic Maps Appendix C Landforms of the Conterminous United States Glossary Index

Paleogene metamorphism of an accretionary flysch terrane, eastern Gulf of Alaska

Abstract: Reconnaissance field studies have identified a regional metamorphic complex, at least 200 km long and 25 to 50 km wide, in the eastern Chugach Mountains, southern Alaska. The metamorphic complex was developed within the Cretaceous Valdez Group, a flysch sequence that was accreted to the continental margin during the Late Cretaceous or early Tertiary. The metamorphic complex shows the progressive development from subgreenschist and green-schist facies slate, phyllite, and semischist that is regionally characteristic of the Valdez Group, to biotite-plagioclase-quartz schist (± muscovite, cordierite, andalusite, staurolite, garnet, and sillimanite) in an intermediate zone, and amphibolite facies muscovite-biotite-plagioclase-quartz schist and gneiss (± sillimanite and garnet) in a higher-grade central zone. K-feldspar is characteristically absent in the amphibolite facies schists, but it is present in migmatitic rocks that are common to the central zone. The mineral assemblages and petrographic relations indicate that metamorphism took place under low-pressure intermediate-facies series conditions; partial melting took place at the highest grade of metamorphism. Melting produced granodioritic magmas in migma-titic rocks of the central zone that were emplaced as discrete plutons in both the intermediate zone and peripheral rocks of the complex. The timing of metamorphism as indicated by field relations and K-Ar data is about 50 to 65 m.y. ago and overlaps or is coeval with the emplacement, throughout the Gulf of Alaska region, of early Tertiary plutons that are thought to be anatectic in origin. The distribution of these plutons suggests that a high-temperature low-pressure metamorphic belt, about 1,800 km long, developed along the leading edge of the continental margin in late Paleocene to early Eocene time.

Erratum: Superimposed low-grade metamorphism in the Mount Fisher area, southeastern British Columbia—implications for the East Kootenay orogeny

Abstract: Middle Proterozoic (- 1500-1350 Ma) Belt-Purcell strata exposed in the Purcell and southwestern Rocky Mountains were affected by at least three distinct episodes of deformation and regional metamorphism. The oldest episode (1300-1350 Ma) apparently terminated Belt-Purcell sedimentation and involved foiding, regional metamorphism, and granitic intrusion. The second episode (800-900 Ma) occurred during deposition of the Windermere Supergroup and involved uplift, block faulting, and low-grade regional metamorphism. Mesozoic-Cenozoic metamorphism, deformation, and plutonism overprinted the results of the earlier deformation and metamorphism. Illite crystallinity and muscovite polymorph ratios indicate that Purcell strata in the Mount Fisher area are in the lower greenschist to prehnite-pumpellyite facies of regional metamorphism. In the Steeples and Fisher blocks this metamorphism is related to structures that formed during the Late Cretaceous - Paleocene deformation. However, in the Sand Creek block the regional metamorphism is related to the development of a spaced cleavage that is folded by a Late Cretaceous - Pdeocene nappe. Regional considerations suggest that this cleavage formed during the 1300- 1350 Ma episode of deformation and metamorphism. The "East Kootenay orogeny" as currently defined embraces the two older episodes of tectonism. It is proposed that the term East Kootenay orogeny be restricted to designate the 1300-1350 Ma episode and that the term "Goat River orogeny" designate the 800-900 Ma episode of tectonism. The East Kootenay and Goat Rivet orogenies appearto be correlative with the Racklan and Hayhook orogenies recognized in the northern Canadian Cordillera.

Late Proterozoic rifting in NW Scotland: the genesis of the ‘Torridonian’

Abstract: The two major “Torridonian” successions, viz . the Stoer Group (~968 Ma) and the Sleat–Torridon Groups (~777 Ma) occupy NNE-trending rifts cutting Archaean and early Proterozoic crust. Palaeocurrents, pebble petrography and age, and the initial 87 Sr/ 86 Sr ratios of clay-rich sediments indicate derivation from flanking areas of this old crust, rather than from late Proterozoic Grenville or Morarian metamorphic rocks. The earlier rift seems to be roughly contemporaneous with Grenville metamorphism but genetically unrelated to it. The rifts perhaps mark initiation of an Iapetus-related ocean in the same way as very similar fault-bounded Triassic and Lower Jurassic sediments mark the start of continental break-up and initiation of the present Atlantic. The western margin of both rifts probably coincides with the Outer Hebrides Thrust and the eastern margin with the Moine Thrust Zone. Reactivation of the old rift-margining faults by Caledonian compression may have generated the lower thrusts of the Moine Thrust Zone, while the Moine nappe itself, by analogy with Lower Devonian events in southern Appalachia, represents a slice of the Grenville microcontinent emplaced after closure of the intervening ocean.

Origin and age of the Border Ranges Fault of southern Alaska and its bearing on the Late Mesozoic tectonic evolution of Alaska

Abstract: Local geologic relationships in the western Chugach Mountains, together with regional considerations, suggest that in the Early Cretaceous a subduction zone was formed along the southern edge of the Wrangellia-Peninsular-Alexander composite terrane (Talkeetna superterrane of Csejtey et al., 1982) of southern Alaska. The effects of this event include (1) a shattering of older (pre-Cretaceous) crystalline rocks along a complex fault system, (2) emplacement of a tectonic melange beneath the shattered crystalline rocks, and (3) a subgreenschist facies prograde metamorphism of the melange and retrograde metamorphism of the older crystalline rocks. This event created a regionally mappable structural contact between broken crystalline rocks and the melange; the Border Ranges fault of MacKevett and Plafker (1974). Regional stratigraphy as well as radiometric ages of rocks predating and postdating the Border Ranges fault appear to bracket the age of the Border Ranges fault and its associated deformational effects to the interval between about 135 and 120 Ma. Further regional tectonostratigraphic associations suggest that the deformation along the Border Ranges fault represents the nucleation of a north and (or) east dipping subduction zone beneath the Talkeetna superterrane and that the magmatic arc associated with this juvenile subduction zone is the Gravina-Nutzotin belt of southeast Alaska. Mismatches in the distribution of different elements of this Early Cretaceous arc-trench system are probably a result of Late Cretaceous or early Tertiary strike slip motion, but Early Cretaceous ridge-trench interaction (suggested by the occurrence of Early Cretaceous near-trench plutons) may have played a role as well. The Talkeetna superterrane is generally thought to be an exotic block that collided with the Cordillera in the middle Cretaceous (Coney et al., 1980). The age data commonly cited as evidence for a middle Cretaceous age for the collision are, however, misleading. A model is proposed here in which the initiation of convergence along the trailing edge of the Talkeetna superterrane records the time of initial impingement of two irregular margins whereas intense middle Cretaceous deformation recognized along the leading edge of the Talkeetna superterrane (Csejtey et al., 1982) records destruction of a syncollisional flysch basin during the final phase of the collision.

Polygenetic ophiolite belt of the California Sierra Nevada: Geochronological and tectonostratigraphic development

Abstract: The assumption that ophiolite sequences are generated at essentially one point in geologic time by the process of sea-floor spreading is critical for modern concepts in the tectonics of ophiolites and for topics dealing with their structure and petrology. However, this assumption has only been verified in a few locations by an integrated geochronological and structural-stratigraphic approach. Many ophiolite sections are reconstructed from structurally disrupted sequences with the idealized ocean floor model in mind. Such reconstructions are prone to error without adequate age control on each of the reconstructed fragments. This is a significant problem in structurally complex regions where more than one generation of ophiolite may be present. In this paper new Pb/U zircon ages are presented for key locations along a 375 km segment of the western Sierra Nevada ophiolite belt. These age data are combined with structural-stratigraphic observations and published ages, and significant tectonic implications for the ophiolite belt emerge. Three different ophiolitic assemblages are recognized with igneous ages of about 300, 200 and 160 m.y. B.P. Rocks of the 300 m.y. assemblage are in a completely disrupted array of metamorphic tectonite slabs and serpentinite-matrix melange. Fragments of upper Paleozoic seamounts occur in association with the ophiolitic melange, and together these assemblages constitute the basement framework for the western Sierra. Pb/U and K/Ar isotopic systematics are complex within this framework and indicate a polymetamorphic history. Systematics in the 200 and 160 m.y. assemblages are less complex and give tighter igneous age constraints. Rocks of the 200 m.y. assemblage are in a semi-intact state with only local tectonite and melange zones. Rocks of the 160 m.y. assemblage are intact, but nevertheless deformed. Both the 200 and 160 m.y. assemblages have equivalent age basinal volcanic-sedimentary sequences that lie unconformably above the ophiolitic melange basement. In each case the basinal sequences locally extend conformably into the upper stratigraphic levels of the age-equivalent ophiolite sections. These relations along with vestiges of intrusive contacts between the edges of both younger ophiolites and the melange basement indicate that the younger ophiolites underwent igneous formation in proximity to the melange basement. The Sierran ophiolite belt is considered to have formed by a multistage process initiated by the early Mesozoic tectonic accretion of upper Paleozoic sea-floor in general proximity to the ancient continental margin. Regional metamorphism and ophiolitic melange resulted. This accretionary nucleus became the basement of Jurassic-age primitive volcanic arc terranes which underwent rifting episodes during the production of the 200 and 160 m.y. ophiolites. The rifting episodes resulted in the formation of sedimentary basins which were the depositional sites of volcanic-sedimentary sequences. Non-volcanic sources for the basinal sedimentary rocks include the melange basement and continental margin terranes. Contact zones between pre-existing basement and the juvenile ophiolitic sequences created during the rifting episodes consist of dynamothermal metamorphic aureoles, protoclastic deformation zones and cross-cutting dikes. Such edge-zone assemblages are in most localities obscurred or destroyed by superimposed deformations resulting from convergent and perhaps transform motions along basin edges. Both the 200 and 160 m.y. basins were destroyed by compressional orogenic episodes shortly after their formational episodes. Destruction of young ophiolite floored basins may be a common course of events when small oceanic-type plates are generated along continental margin environments. Such tectonic settings are ideal for the emplacement of young ophiolite sheets.

Ore deposition in a proterozoic incipient rift zone environment: a tentative model for the Filipstad-Grythyttan-Hjulsjö region, Bergslagen, Sweden

Abstract: The 1.9–1.8 Ga Bergslagen Supracrustal Series comprises: an Early Volcanic Stage represented by the Lower Leptite Group, an Initial Rift Stage by the Middle Leptite Group, a Rift Stage by the Upper Leptite-halleflinta and Slate Group, metabasites and the Granite-Granophyre Suite, and a Post-rift Stage by conglomerate beds, remobilized granite-granophyres and the Hyttsjo Gabbro-Tonalite Suite. The formation and subsequent alteration of iron, manganese and sulfide skarn ores in the Supracrustal Series involve: (1) late Initial Rift Stage exhalative-sedimentary processes possibly related to ascending granitic magma, (2) early Rift Stage exhalative-sedimentary and seafloor hydrothermal processes related to basic volcanism and intrusion and subvolcanic granite intrusion, (3) late Rift Stage hydrothermal metasomatic alteration and mineralization around subvolcanic granites, (4) Post-rift Stage deformation and metamorphism, (5) Post-rift Stage post-deformation recrystallization and skarn formation related to Hyttsjo diorites, and (6) post-Supracrustal Series metamorphic modifications.

Oxygen isotope evidence for shallow emplacement of Adirondack anorthosite

Abstract: Oxygen isotopic analysis of wollastonites from the Willsboro Mine, Adirondack Mountains, New York reveals a 400-ft wide zone of 18O depletion at anorthosite contacts. Values of δ18O vary more sharply with distance and are lower (to −1.3) than any yet reported for a granulite fades terrain. Exchange with circulating hot meteoric water best explains these results and implies that the anorthosite was emplaced at relatively shallow depths, <10 km, in marked contrast to the depth of granulite fades metamorphism (23 km). These 18O depletions offer the first strong evidence for shallow emplacement of anorthosite within the Grenville Province and suggest that regional metamorphism was a later and tectonically distinct event.