Showing papers on "Metamorphism published in 1976"

Garnet composition and zoning in the determination of temperature and pressure of metamorphism, central Massachusetts

Abstract: The distribution of Fe, Mg, Ca, and Mn within individual zoned garnets of medium- to high-grade metamorphosed pelitic rocks from west-central Massachusetts has been studied in detail with an automated electron microprobe, and is consistent with variations predictdd for the Fe-Mg-Mn continuous reactions taking place at each metamorphic grade. This regular behavior allows use of compositions of garnet and coexisting phases for geothermometry and geobarometry tf P-T-X relations of appropriate continuous reactions can be calibrated. Preliminary calibrations of garnet-biotite and garnet-cordierite Fe-Mg exchange reactions and several Fe-Mg-Mn continuous reactions have been used to evaluate P-I conditions of metamorphism in central Massachusetts. Results indicate pressure in the range of 5 to 7 kbar and a west to east temperature gradient from about 580-605"C for kyanite-staurolite grade to 650-700.C for sillimanite-orthoclase-cordierite grade. Pressure and temperature estimates are consistent with regional tectonic reconstructions indicating synmetamorphic uplift in the Bronson Hill anticlinorium and downbuckling in the Merrimack synclinorium.

Elemental mobility during zeolite facies metamorphism of the Tertiary basalts of eastern Iceland

Abstract: Major and trace element analyses are reported for 70 lavas from the Tertiary succession of eastern Iceland The low grade regional metamorphism responsible for the zeolite zones appears to have caused significant mobilization of some elements, particularly Si, Mg, K, Rb, Sr and light rare earth elements (LREE) In contrast, values for Ti, P, Zr, Y, Nb, Ta, Hf and some of the rare earth elements show a high degree of correlation and this is taken to imply that these elements have been relatively unaffected by metasomatic transport The demonstrated mobility of Sr and LREE suggest that Sr isotopic and rare earth data obtained from the eastern Iceland lavas must be interpreted with caution

Rb-Sr ages of granitic rocks in southeastern China and their tectonic significance

Abstract: Rb-Sr age data and initial Sr 87 /Sr 86 ratios (I values) are reported for the Mesozoic granitic batholiths along the coast of southeastern China. Two major thermal episodes are recognized: 90 to 120 m.y. B.P., derived from the mineral isochrons, and 165 ± 13 (2σ) m.y. B.P., derived from a whole-rock isochron. These episodes appear to be correlative with periods of rapid spreading of the Mesozoic Pacific Ocean floor and embrace two orogenic phases of the Yenshan orogeny. The I values of granitic gneisses (0.7060 to 0.7159) vary systematically with the Rb/Sr ratios of their whole-rock samples. The high I value (0.7112) of a pegmatite dike cutting a granitic gneiss in Chinmen suggests that the pegmatite is derived from remelting of upper crustal rocks. The narrow range of I values for more mafic intrusive bodies in Matsu (0.7065 to 0.70695) suggests that they are genetically related and are probably derived from an upper mantle source and have some crustal contamination. An un-metamorphosed granitic sample from Changlo has a mineral isochron age of about 120 m.y., which is interpreted as the time of intrusion. Its rather low I value (0.70546) suggests that the magma source is likely in the upper mantle. The Yenshan orogenic belt in southeastern China may belong to the inner belt of the Pacific-type orogeny; it is characterized by polycyclic deformation, magmatism, and regional metamorphism. Unlike a classical geosynclinal regime, the magmatic activities in this region were not immediately preceded by a major marine geosynclinal subsidence. The apparent absence of marine deposits after Triassic time suggests that marine transgression is not a universal result of rapid sea-floor spreading. The extensive development of magmatic belts along the eastern margin of the Asian continent appears to require large-scale consumption of oceanic lithosphere at the edge of the continental plate. The age distribution and structural trend data of the magmatic rocks suggest that the presumed subduction zone may have dipped westward or northwestward in the western Pacific region during late Mesozoic time.

Late Proterozoic Cratonization in Southwestern Saudi Arabia

Abstract: Early cratonal development of the Arabian Shield of southwestern Saudi Arabia began with the deposition of calcic to calc-alkalic, basaltic to dacitic volcanic rocks, and immature sedimentary rocks that subsequently were moderately deformed, metamorphosed, and intruded about 960 Ma ago by dioritic batholiths of mantle derivation ($^{87}$Sr/$^{86}$Sr = 0.7029). A thick sequence of calc-alkalic andesitic to rhyodacitic volcanic rocks and volcanoclastic wackes was deposited unconformably on this neocraton. Regional greenschistfacies metamorphism, intensive deformation along north-trending structures, and intrusion of mantle-derived ($^{87}$Sr/$^{86}$Sr = 0.7028) dioritic to granodioritic batholiths occurred about 800 Ma. Granodiorite was emplaced as injection gneiss about 785 Ma ($^{87}$Sr/$^{86}$Sr = 0.7028-0.7035) in localized areas of gneiss doming and amphibolite to granulite facies metamorphism. Deposition of clastic and volcanic rocks overlapped in time and followed orogeny at 785 Ma. These deposits, together with the older rocks, were deformed, metamorphosed to greenschist facies, and intruded by calc-alkalic plutons ($^{87}$Sr/$^{86}$Sr = 0.7035) between 600 and 650 Ma. Late cratonal development between 570 and 550 Ma involved moderate pulses of volcanism, deformation, metamorphism to greenschist facies, and intrusion of quartz monzonite and granite. Cratonization appears to have evolved in an intraoceanic, island-arc environment of comagmatic volcanism and intrusion.

The interrelations of fluid transport, deformation, geochemistry and heat flow in early Proterozoic shear zones in the Lewisian complex

Abstract: The Lewisian complex of northwest Scotland shows a pattern of evolution typical of a number of early Proterozoic provinces. During the period 2500-1600 Ma, deformation occurred along steeply dipping shear zones, resulting in both vertical and lateral movements. The largest of these shear zones, forming the northern boundary to the Scourian granulites (Archaean), must have penetrated to considerable depth, possibly to the mantle. Modal and chemical analysis of rocks from shear zones are presented and discussed in relation to rocks sampled outside shear zones. The mineralogy and composition of all rocks deformed in the shear zones have been considerably altered by synkinematic metasomatism. In the early stages, immediately prior to and during the intrusion of the regional doleritic dyke swarm, this metasomatic activity involved addition of H$\_{2}$O and Na to the rocks. Subsequently, more significant changes in rock chemistry occurred (addition of H$\_{2}$O, K, Na, loss of Fe, Ca, Mg). These changes resulted from the interaction between large volumes of water and the rocks in the shear zones along which the fluid travelled. A combination of modal and chemical data allow general chemical reactions to be written which describe the evolution of the gneisses during reworking and retrogression from pyroxene bearing granulite facies rocks to hornblende and biotite bearing amphibolite facies rocks in shear zones. The reactions are written as ionic equilibria and suggest that the fluid phase in the shear zones had a low pH. Adiabatic transport of water upwards through the crust will result in moderate warming of the fluid, and can cause large temperature increases above the preexisting geothermal gradient in rocks through which the fluid travels. It is suggested that both deformation and metamorphism in these shear zones are related to transport of fluid by hydraulic fracturing. Grain size reduction by hydraulic fracturing increases the strain rate in the shear zones. Deformation may cease in a shear zone when the fluid pressure drops and hydraulic fracturing no longer occurs. Thus fluid transport, mineral reactions, chemical changes, grain size reduction and convective heat flow will cease. A close relation should exist between the intensity of deformation, the extent of metasomatism and the thermal history in these important shear belts.

The Grampian Orogeny

Abstract: Synopsis Review of existing plate tectonic theories of the Lower Palaeozoic development of the north and northwest British Isles leads to the conclusion that ridge-trench impingement theory can be used to integrate all known geological observations within one unified plate tectonic theory for the area. Consideration of the consequences of the progressive impingement of a ridge-transform system with a northwesterly-dipping subduction zone in Arenig time gives explanations for: (a) termination of Dalradian sedimentation in the Arenig; (b) initiation of Barrovian metamorphism at the same time; (c) development and history of the Girvan-Ballantrae complex; (d) initiation of the Highland Boundary Fault as a dextral transform fault in the Arenig; (e) its absence in western Scotland; (f) the postulated presence of a boundary fault southeast of Connemara and perhaps at Pomeroy; (g) the peak of metamorphism (Barrovian and Buchan types) in the Dalradian and associated basic intrusions in the Arenig in both Scotland and Connemara; (h) the development of the Moine Thrust (and related foreland folds and thrusts) also in the Arenig and (i) the origin of the South Mayo trough on oceanic crust. Related events explained by this model include the location of the Donegal batholith, the generally low-grade metamorphism of Donegal and southwest Scotland, the location of the Great Glen Fault and all known faunal province and Ordovician sedimentological data from the Midland Valley and Southern Uplands. It is proposed that this Lower Ordovician sequence of related events be termed the Grampian Orogeny, distinct from the later Caledonian Orogeny, which is tectonically and geographically separate.

The Upper Precambrian of South America

Abstract: This paper deals with the structural organization and tectonic evolution of South American Continent basement during the Upper Precambrian. The South American Platform is the old platform of South America. It has more than half of this extension covered by sediments and volcanic rocks of Phanerozoic age; the basement is exposed in three vast shields and several little massifs. In the exposed basement some cratonic nuclei have been distinguished, with structures developed in the Middle Precambrian (Trans-Amazonic) and Lower Precambrian (Jequie and Guriense). The Lower Precambrian structures are described in small scattered nuclei, all the rest seeming to have been remobilized by tectonic, magmatic and thermal processes of Trans-Amazonic age. These processes affected large areas but are still insufficiently understood. In the Upper Precambrian, these cratonic nuclei underwent intense process of reactivation, in large areas, with formation of volcano-sedimentary covers, acid, basic and alkaline intrusive rocks, cataclastic zones and thermally affected zones. During the Upper Precambrian, geosynclinal evolution processes developed at the borders and between the cratons, generating fold belts and regions. The firstly developed belt is located in Central Brazil, related to the Uruacuano Cycle (ca. 1300-1000 m.a.). The Espinhaco and Uruacu Belts are attributed to this cycle. The other units are related to the Brasiliano Cycle ( 1000 m.y. to Cambro-Ordovician time). The fold belts are located in marginal position and the fold regions are between cratonic areas: both show different characteristic of organization, sedimentation, structures, tectonism, metamorphism, magmatism and metallogenesis. In the southern border of the South American Platform a fold region developed, which possibly represents the extension of the Southeastern Fold Region, and continues to the Andean Belt. The South American Platform consolidated during Cambro-Ordovician time. Its western and southern adjacent areas were places of geosynclinal evolution up to the Devonian in the Patagonian Platform and up to the Cenozoic in the Andean Chain. The eastern half of the South American Platform had a platformal evolution since the Silurian

Dewatering of a metamorphic pile

Abstract: Hydraulic fracturing, induced by thermal expansion of water, is invoked as a common phenomenon by which metamorphic fluid is progressively lost from a sediment pile undergoing metamorphism and subsequent orogeny On linear thermal gradients, water loss may occur by this mechanism for all gradients greater than 12°C/km at depths greater than 5 to 10 km During burial on lower gradients, water is retained in the pile and may cause widespread metasomatism

Thermal causes of the Dalradian metamorphism in the central Highlands of Scotland

Abstract: Synopsis Analysis of carbonate and pelitic mineral assemblages at the base of the Dalradian Series near Spean Bridge, Inverness-shire, leads to the conclusion that the temperature at the peak of regional metamorphism reached 535°C at a depth of 18.5 Km. In the light of the general parallelism of the garnet isograd (taken as nearly isothermal) and the base of the Dalradian, the thermal event is modelled in one dimension using an average heat production for the Dalradian of 4.0 × 10 −1 3 cal/cm 3 s and a temperature-dependent thermal conductivity with room-temperature average value of 7.8 × 10 −3 cal/cm s K. It is argued that the thermal inertia of the upper crustal rocks during metamorphism is offset by their concurrent erosion and that a steady state model is a reasonable approximation to the complex transient thermal history over the 20–40 m.y. period during which deformation, heating and erosion were taking place. The heat flow through the base of the Dalradian at the metamorphic climax was about 1.3–1.6 × 10 −6 cal/cm 2 s; this is attributed to heat production within an underlying 10–20 Km thick Moinian basement together with a normal, or slightly higher than normal, conductive flux from the mantle beneath. If a Lewisian-like lower continental crust underlies the Moinian it would have reached temperatures high enough to permit partial fusion and could be the source of calc-alkaline largely post-metamorphic-climax igneous rocks found in the area.

Early and Middle Proterozoic history of the Great Lakes area, North America

Abstract: Two major supracrustal sequences, the Huronian Supergroup in Ontario and the Marquette Range Supergroup and Animikie Group of Michigan and Minnesota, overlie an Archean basement. These sequences are about 2200—2300 Ma and 1900-2000 Ma old respectively. The major Early Proterozoic tectonic event is the ‘Penokean Orogeny’, which occurred about 1850-1900 Ma ago and included deformation, high-grade regional metamorphism, and extrusive and intrusive igneous activity. This was followed by formation of rhyolitic, ignimbritic volcanic rocks and emplacement of associated granites about 1790 Ma ago. The entire region was subsequently subjected to low-grade regional metamorphism 1650-1700 Ma ago, followed by emplacement of anorogenic quartz-monzonite, in part rapakivi, plutons 1500 Ma ago. Late Proterozoic Grenville and Keweenawan events represent the youngest major Precambrian activity in the region. The rocks involved in the Penokean Orogeny lie along the southern margin of the Archean craton of the Superior Province and are interpreted as representing Early Proterozoic cratonic-margin orogenic activity. The distribution of rocks types and structures associated with the Penokean Orogeny and with similar orogenic belts along the margin of the Archean craton of North America suggest that these orogenic belts may have formed as a result of processes similar to modern plate tectonics, although the data are far from conclusive at present.

U-Pb zircon and Rb-Sr whole-rock dating of low-grade metasediments example: Montagne Noire (Southern France)

Abstract: Three detrital, Proterozoic zircon suites extracted from siltstones progressively metamorphosed between chlorite- and staurolite-grade independently date the major Caledonian metamorphism within the gneiss dome of the Montagne Noire (Southern France). From this, the following conclusions concerning U-Pb systematics of zircons in low-, medium- and highgrade metamorphic rocks can be drawn: 1) Temperatures of at most 350–400 °C are sufficient to open U-Pb systems of metamict zircons or domains within zircons. 2) The observed open U-Pb system behaviour during metamorphism of the host rocks was found to be due to a low-temperature recrystallisation of highly radiation damaged zircon lattices, probably enhanced by high concentrations of fluid phases in the dehydrating rock volumes. 3) Recrystallisation of metamict zircons under low temperatures causes maximum U-Pb ages for the thermal climax of metamorphism of medium-and high-grade metamorphic rocks, as annealing and accompaning closing of U-Pb systems took place before the maximum temperatures of metamorphism were reached. 4) Low-temperature recrystallisation of old — generally Proterozoic—zircons can readily help to explain the fact that the numerous zircon suites from ancient shield areas yield “lower intercept ages” which are not correlated to any known geological event. Thus, either a weak thermal pulse, not necessarily registered by the mineral assemblage of the host rock, and/or elevated temperatures during burial in the crust might supply enough energy for a structural reordering and simultaneous lead loss of at least the most disordered lattice domains.

Igneous and metamorphic processes associated with the formation of Chilean Ophiolites and their implication for ocean floor metamorphism, seismic layering, and magnetism

Abstract: A metamorphic overprint on the pseudostratigraphy of ophiolite complexes in southern Chile shows an extremely steep vertical metamorphic gradient passing downward from zeolite to amphibolite facies in 2 km, followed by a transition to fresh gabbros. Burial metamorphism does not explain either the steep metamorphic gradient or the abrupt termination of this metamorphic effect. A combination of hydrothermal and contact metamorphism associated with circulation of seawater and igneous and tectonic activity at a spreading center can better explain these observations. Such a model is supported by 18O/16O isotope data. Disequilibrium textures indicate that spreading rapidly removes amphibolized metagabbros from the zone of effective metamorphism, while overlying rocks undergo greenschist metamorphism in a less restricted region in the vicinity of the spreading center. This model is consistent with metamorphic features displayed by other ophiolites and ocean floor metamorphism. The metamorphism produces a zone of metagabbros (amphibolites) underlain by fresh gabbros, which may account for the transition from seismic layer 3A to 3B. The amphibolites in the Chilean ophiolites contain abundant skeletal networks of secondary opaque minerals similar to those observed in ocean floor amphibolites from dredge hauls, which preserve higher-remnant magnetism than either fresh gabbros or the gabbros, dikes, and pillow lavas in the greenschist facies. A zone of amphibolitized metagabbros with high natural remnant magnetism could make a significant contribution to magnetic anomalies, particularly away from the ridge, where the remnant magnetism of the quenched lava of layer 2 is progressively reduced by metamorphism from below and weathering from above.

Paleomagnetism of the Haliburton Intrusions: Superimposed magnetizations, metamorphism, and tectonics in the Late Precambrian

Abstract: Three components of remanent magnetization have been isolated by af and thermal demagnetization of highly metamorphosed dioritic and gabbro-anorthositic rocks from Haliburton County, Ontario, in the Grenville structural province of the Canadian Precambrian Shield. The predominant A component, a normal magnetization, defines a paleoinagnetic pole at 142.5°E, 35.6°S (α95 = 6.3°, k = 37.6), near southeast Australia, in agreement with published poles from widely separated areas in the Grenville Province. Paleopoles for the normal B and reversed C magnetizations fall much farther north, at 172.3°E, 24.5°N (α95 = 15.7°, k = 9.4) and 167.4°E, 3.0°S (α95 = 6.6°, k = 53.5), respectively. The C pole is close to secondary poles of other studies, but the B pole has not been detected elsewhere within the Grenville Province. It lies among poles for late Keweenawan rocks of the Lake Superior area. The A and B components are frequently superimposed in individual samples. The three components have distinctive coercivity and blocking temperature spectra, which we interpret as recording mineralogy and physical conditions (temperature and duration of heating) at different stages of slow cooling following high-grade regional metamorphism of the Grenville Province. They constitute a record, from a single locality, of apparent polar wander between about 1200 and 950 m.y. The A magnetization was acquired at high temperatures, the B at much lower temperatures. Although both B and C magnetizations nay have been set during subsidiary thermal pulses, a single-stage heating-cooling model of Grenville metamorphism is not ruled out. The proposed age sequence is A-C-B, a sequence most naturally explained by convergence of separate Grenville and North American plates. However, one-plate models which would incorporate the A-C-B sequence into a ‘Grenville Loop’ in the North American polar wander path are also possible.

Rare-Earth Abundances in Basalts and Metabasalts from the Troodos Massif, Cyprus

Abstract: Abundances of some rare-earth elements (REE) in twelve pillow lavas from the Upper Pillow Lavas and Axis Sequence of the Troodos Massif are reported. The samples consist of three fresh basalts and nine zeolite facies metabasalts, metamorphosed at temperatures between 0 and 200° C. All give similar light rare-earth element (LREE) depleted patterns, indicating that hydrothermal metamorphism within this temperature range does not appreciably affect these REE patterns. The LREE depletion is consistent with a petrogenetic model in which Troodos formed at a spreading axis. Variations in profile shape indicate that mantle melting beneath the axis may have taken place during a series of discrete episodes.

Petrology of the Sokoman Iron Formation in the Howells River area, at the western edge of the Labrador Trough

Abstract: The Proterozoic Sokoma-n Iron Formation in the Howells River area, at the western edge of the Labrador Trough, has undergone extensive diagenesis but probably only very low grade metamorphism. Comparison with oxygen isotope data for very similar assemblages in the Biwabik Iron Formation suggests that the iron-formation of this study has been subjected to temperatures of about 150 degrees C or somewhat less.The iron-formation assemblages can be divided into four facies: sulfide, silicate, magnetite-carbonate, and hematite-carbonate. In almost all of the assemblages strong recrystallization has taken place of the original, sedimentary precursors. In some assemblages, however, as in the underlying orthoquartzites and the green chert member of the iron-formation, very fine, fibrous and colloidal textures are still preserved in chalcedony and chert. Representative assemblages in the various facies are as follows: Sulfide facies-chamosite-siderite-ankerite-pyrite-chert-carbon; ankerite-siderite-chamosite-"Al greenalite"-chert-pyrite-carbon. Silicate facies-magnetite-greenalite-stilpnomelane-chert - minnesotaite - ankerite - siderite; chert - ankerite-siderite-stilpnomelane-magnetite; greenalite-stilpnomelane-minnesotaite-ankerite-magnetite-siderite-chert. Magnetite-carbonate facies-quartz-magnetite-hematite-siderite-stilpnomelane; chert-ankerite-siderite-magnetite-stilpnomelane; chert-magnetite-siderite-calcite-hematite. Hematite-carbonate facies-quartz - magnetite-hematite-calcite-ankerite-stilpnomelane; quartz-hematite-magnetite-dolomite-stilpnomelane.Minnesotaite, in the above assemblages, is considered to be a late diagenetic or very low grade metamorphic reaction product. All other phases are concluded to be recrystallization products of sedimentary precursors. Electron microprobe analyses of the co-existing phases in the above assemblages in general show internally consistent element fractionation as shown by the general lack of tie-line crossings in graphical representations. This indicates that, even at very low temperatures, equilibrium was approached or in part attained. The textures and assemblages are interpreted in terms of possible sedimentary precursors.

Late Archean metamorphism in the Buksefjorden region, Southwest Greenland

Abstract: Standard state thermodynamic data extracted from experimental studies and applied to mineral assemblages in orthogneisses, metasedimentary gneisses and metabasites show that conditions of late Archean (2,850 m.y.) upper amphibolite facies were P solid≈7.0 kb, T≈630° C, and rose to P solid≈10.5 kb, T≈810° C in adjacent granulite facies. The estimates of solid pressure for the granulite facies suggest a late Archean crustal thickness of ca. 35 km, comparable to present day continental crust. Upper amphibolite facies assemblages were in equilibrium with $$P_{H_2 O}$$ about one half P solid, while granulite assemblages equilibrated at much lower $$P_{H_2 O}$$ , varying from about one tenth P solid in quartzofeldspathic gneisses to one third P solid in more basic layers.

Radiometric dating of time of thrusting in the disturbed belt of Montana

Abstract: Mesozoic sedimentary rocks overridden by thrust plates in the disturbed belt of northwestern Montana have been metamorphosed by burial beneath these plates. Bentonite in the Cretaceous section has been converted to potash bentonite by this metamorphism, allowing radiometric dating of the emplacement of the thrust plates by the K-Ar method. Ages determined thus far range from 72 to 56 m.y. B.P. The oldest ages coincide with estimates for the beginning of thrusting from stratigraphic and structural evidence. Field evidence does not allow a reliable estimate for the end of thrusting. Thus, the younger ages, which indicate completion of thrusting at the end of Paleocene time, represent additional data of interest to the structural history of the disturbed belt.