Showing papers on "Metamorphism published in 1986"

Dynamics of orogenic wedges and the uplift of high-pressure metamorphic rocks

Abstract: Subduction-accretion complexes can be approximated as wedge-shaped continua with a rigid buttress behind and a subducting litho-spheric slab beneath. Thick wedges undergoing prograde metamorphism have a negligible long-term yield strength and are likely to exhibit a complex nonlinear viscous rheology. Such a wedge will tend to deform internally until it reaches a stable configuration, in which the gravitational forces generated by the wedge geometry balance the traction exerted on its underside by the subducting slab. Accretion of material at the wedge front will lengthen the wedge and cause it to shorten internally to regain the stable geometry. This shortening will be expressed as late (out-of-sequence) thrusting, backthrusting, and folding. Conversely, underplating of sediment or crustal slices will thicken the wedge, which may need to extend internally to regain stability. Extension will cause listric normal faults that may merge downward into zones of ductile extension. Continued underplating at depth and compensating extension above provides a mechanism for bringing high-P/low-T metamorphic rocks to upper levels in the rear of the wedge, where they are commonly observed. Many major tectonic boundaries in convergent orogens (such as the Coast Range thrust in the Franciscan Complex, major nappe contacts in the Alps, and the contact between the Nevado-Filabride and Higher Betic nappe complexes in the Betic Cordillera) show abrupt increases in metamorphic grade downward across them. This is consistent with their origin or reactivation as uplift-related, extensional structures.

Fluids expelled tectonically from orogenic belts: Their role in hydrocarbon migration and other geologic phenomena

Abstract: This paper presents and supports a speculative hypothesis, the essence of which follows. When continental margins in zones of convergence are buried beneath thrust sheets, fluids expelled from the margin sediments travel into the foreland basin and the continental interior. These tectonic fluids have key roles in phenomena such as faulting, magma generation, migration of hydrocarbons, transport of minerals, metamorphism, and paleomagnetism. The thrust sheet, crudely speaking, acts like a great squeegee, driving fluids ahead of it and producing widespread geologic consequences.

Paleozoic terranes of the central Argentine-Chilean Andes

Abstract: The recognition of accreted terranes and their importance in orogenesis has spurred the search for allochthonous fragments along the western and southern margins of South America. Here we present stratigraphic and petrologic data from Chile and Argentina between 29° and 33°S latitude that demonstrate the “suspect” nature of several major terranes, which we infer to have been accreted during the Paleozoic. Three lower-middle Paleozoic terranes are described (from east to west): (1) the Pampeanas terrane, a Cambrian-Devonian magmatic and metamorphic province built on late Precambrian basement at the margin of South America, (2) the Precordillera terrane, a Cambrian-Devonian shelf-slope-oceanic basin assemblage bounded by melanges on both sides and bearing many stratigraphic similarities to the lower-middle Paleozoic of the Northern Appalachians, and (3) the “Chilenia” terrane, which has largely been obliterated by late Paleozoic magmatism and metamorphism. The distribution of Carboniferous continental, deltaic, and marine strata demonstrates that these three terranes were sutured together and part of South America by the end of the Devonian. Subsequent Permo-Carboniferous plate interactions more closely resembled the modern Andean margin, with eastward subduction, accretionary prism formation, and minor terrane emplacement exposed along the present coast of Chile and eastward migrating arc magmatism from the present coast of Chile to western Argentina.

Precambrian Continental Crust of India and Its Evolution

Abstract: The Indian Precambrian continental crust exhibits a variety of geological features fashioned at different times by different geotectonic processes. The bulk of this crust was formed prior to 2600 m.y. ago and remobilized at least twice between 2600-2000 m.y. ago (early Proterozoic Mobile Belt, EPMB) and 2000-1500 m.y. ago (middle Proterozoic Mobile Belt MPMB). Three early Precambrian nucleii: Karnataka (KN), Jeypore-Bastar (JBN), and Singhbhum (SN) appear to have survived in the craton and are characterized by low-grade supracrustals and tonalitic trondhjemite gneisses, formed 3800-2600 m.y. ago. The EPMB event involved sedimentation, amphibolite-granulite facies metamorphism, and $$CO_{2}-K$$ metasomatism and produced amphibolite facies rocks and K-granites in the north, and charnockite and other granulite facies rocks in the south. Gold sporadically distributed in the supracrustal rocks of the craton was remobilized during the EPMB event. K-granites form a garland around the central Dharwar craton, sugg...

Melt-enhanced deformation: A major tectonic process

Abstract: Convergent tectonics between continental crustal blocks result in deep burial and anatexis of supracrustal rocks. Anatectic and/or mantle-derived melts combine to form a melt-weakened zone in the thickened lower crust. The accumulation of melt eventually leads to crustal failure along melt-lubricated shear zones. Rapid (>1 mm/yr vertical component) movements of large crustal blocks result. We refer to these crustal displacements as tectonic surges. The melt-lubricated shears are characterized by the close association of sheared country rocks with foliated or massive igneous sills and plutons. Rocks that formed at different crustal levels are juxtaposed across these shear zones. One result of surges with large lateral component of movement is metamorphic inversion with high-pressure and high-temperature metamorphic rocks structurally over lower pressure and temperature assemblages. Large and rapid vertical surges may displace crust containing abundant melt and may result in high T/P (including granulite facies) metamorphism and preservation of metamorphic textures caused by rapid decompression.

40 Ar/ 39 Ar dating of white micas from an Alpine high-pressure metamorphic belt on Naxos (Greece): the resetting of the argon isotopic system

Abstract: Overprinting of white micas from high pressure, low to medium temperature (M 1) metamorphic assemblages in pelitic schists on Naxos during subsequent thermal dome (M 2) metamorphism ranges from minor in the southeast of the island to complete recrystallization in the amphibolite facies rocks near the migmatites in the centre of the dome. The original (M 1) minerals are phengites (Si4+=6.7–7.0) and the overprinting minerals are muscovites (Si4+=6.0–6.45). 40Ar/39Ar step heating analyses of white mica separates from rocks in the area where phengite and muscovite occur together yield complex age spectra, characterized by low apparent ages in the first and the last stages of gas release and high apparent ages in between. These upward-convex age spectra are shown to be caused by mixing of two generations of micas, each of which has a different age spectrum and argon release pattern. Seemingly good plateaus in some age spectra from white micas of the area must be interpreted as providing meaningless intermediate ages. Further, the upward-convex age spectra have been used to trace the isotopic signature of phengites toward increasing M 2 metamorphic grade, and suggest that as long as phengites can be observed in the rocks upward-convex age spectra occur. On Naxos, crystallization of muscovite at the expense of phengite appears to be the main mechanism of resetting argon isotopic ages in white micas. However, there is also good evidence for argon loss by volume diffusion from phengites. Simple diffusion calculations suggest that the M 2 metamorphism was caused by a shortlived heat source.

Roberts victor eclogites: Ancient oceanic crust

Abstract: The two groups of Roberts Victor eclogites may be further distinguished by their 18O/16O ratios; one above (I) and one below (II) the mantle value (5.7). In the group II suite systematic variation of the major elements with oxygen isotopes matches that calculated for oceanic volcanic rocks altered by circulating seawater in ridge crest hydrothermal systems. A negative linear correlation between 18O/16O and 87Sr/86Sr(WR) and an associated positive correlation with K2O (and Rb) of the anhydrous eclogite assemblage with 87Sr/86Sr(WR) are also products of hydrothermal alteration. The chemical and isotopic similarity of the eclogites to hydrothermally altered oceanic crust supports the conclusion that they are subducted fragments that have been metamorphosed from metabasalts to eclogites. During metamorphism the elements are fractionated into two components. One, enriched in the radiogenic isotopes of U, Th, K, and Rb, and light rare earth elements, is concentrated in a hydrous, interstitial, mobile component, and the other, enriched in Nd relative to Sm, is concentrated in the anhydrous, immobile, garnet and clinopyroxene host. The mobile component is comparable in chemistry to rocks of the MARID suite and may well play an important role in mantle metasomatism. Nd/Sm and Pb/Pb ages indicate an Archean age (approximately 2.47 b.y.) for the metamorphism. Depth estimates indicate a skewed distribution with most eclogites coming from the depth range 165–190 km. The concentration of eclogites correlates with the inflection in the Lesotho geotherm and appears to mark a disrupted layer of subducted Archean oceanic crust separating a shallower region of undeformed, refractory, Mg-rich peridotites from deformed, Fe-rich, more “fertile” pyrolite. This boundary may be interpreted as the “petrologic” base to the continental lithosphere.

Fluid infiltration into fault zones: Chemical, isotopic, and mechanical effects

Abstract: Fluid infiltration into fault zones and their deeper-level counterparts, brittle-ductile shear zones, is examined in diverse tectonic environments In the 27 Ga Abitibi greenstone belt, major tectonic discontinuities, with lateral extents of hundreds of kilometres initiated as listric normal faults accommodating rift extension and acted as sites for komatiite extrusion and locally intense metasomatism During reverse motion on the structures, accommodating shortening of the belt, these transcrustal faults were utilised as a conduit for the ascent of trondhjemitic magmas from the base of the crust and of alkaline magmas from the asthenosphere and for the discharge of thousands of cubic kilometres of hydrothermal fluids Such fluids were characterised by δ18O=+6±2, δD=−50±20, δ13C=−4±4, and temperatures of 270 to 450°C, probably derived from devolatilisation of crustal rocks undergoing prograde metamorphism Hydrothermal fluids were more radiogenic (87Sr/86Sr=07010 to 07040) and possessed higher μ than did contemporaneous mantle, komatiites or tholeiites, and thus carried a contribution from older sialic basement A provinciality of87Sr/86Sr and δ13C is evident, signifying that fault plumbing sampled lower crust which was heterogeneous at the scale of tens of kilometres Mineralised faults possess enrichments of large ion lithophile (LIL), LIL elements, including K, Rb, Ba, Cs, B, and CO2, and rare elements, such as Au, Ag, As, Sb, Se, Te, Bi, and W Fluids were characterised by XCO 2≈01, neutral to slightly acidic pH, low salinity ≤3 wt-%, K/Na=01, they carried minor CH4, CO, and N2, and they underwent transient effervescence of CO2 during decompression Clastic sediments occupy graben developed at fault flexures The40Ar/39Ar release spectra indicate that fault rocks experienced episodic disturbance on time scales of hundreds of millions of years At the Grenville front, translation was accommodated along two mylonite zones and an intervening boundary fault The high-temperature (580°C) and low-temperature (430 to 490°C) mylonite zones, formed in the presence of deep-level crust-equilibrated fluids of metamorphic origin Late brittle faults contain quartz veins precipitated from fluids with extemely negative δ18O (−14 per mil) at 200 to 300°C The water may have been derived from downward penetration into fault zones of precipitation of low18O on a mountain range induced by continental collision, with uplift accommodated at deep levels by the mylonite zones coupled with rebound on the boundary faults Archean gneisses overlie Proterozoic sediments along thrust surfaces at Lagoa Real, Brazil; the gneisses are transected by brittle-ductile shear zones locally occupied by uranium deposits Following deformation at 500 to 540°C, in the presence of metamorphic fluids and under conditions of low water-to-rock ratio, shear zones underwent local intense oxidation and desilication All minerals undergo a shift of −10 per mil, indicating discharge of meteoric-water-recharged formation brines in the underlying Proterozoic sediments up through the Archean gneisses, during overthrusting; ≈1000 km3 of solutions passed through these structures The shear zones and Proterozoic sediments are less radiogenic (87Sr/86Sr=0720) than contemporaneous Archean gneisses (0900), corroborating the transport of fluids and solutes through the structure from a large external reservoir Major crustal detachment faults of Tertiary age in the Picacho Cordilleran metamorphic core complex of Arizona show an upward transition from undeformed granitic basement through mylonitic to brecciated and hydrothermally altered counterparts The highest tectonic levels are allochthonous, oxidatively altered Miocene volcanics This transition is accompanied by an increase of 12 per mil in δ18O, from +7 to +19, and a 400°C decrease in temperature Lower tectonic levels acted as aquifers for the expulsion of large volumes of higher-temperature reduced metamorphic fluids and/or evolved formation brines The Miocene allochthon was influenced by a lower-temperature reservoir inducing oxidative potassic alteration; mixing occurred between cool downward-penetrating thermal waters and the hot, deeper aqueous reservoir In general, flow regimes in these fault and shear zones follow a sequence, from conditions of high temperature and pressure with locally derived fluids at low water-to-rock ratios, during initiation of the structures, to high fluxes of reduced formation or metamorphic fluids along conduits as the structures propagate and intersect hydrothermal reservoirs Later in the tectonic evolution and at shallower crustal levels there was incursion of oxidising fluids from near-surface reservoirs into the faults In general, magmatism, tectonics, and fluid motion are intimately related

Geochemistry of bimodal basalt-subalkaline/peralkaline rhyolite provinces within the Southern British Caledonides

Abstract: Bimodal associations of basalt and rhyolite of Upper Ordovician age which were erupted in a submarine environment occur within the Caledonian orogenic belt of South Britain at Parys Mountain (Anglesey), in Snowdonia (North Wales) and at Avoca (SE Ireland). The volcanic rocks have experienced hydrothermal alteration and low-grade metamorphism, and therefore immobile elements (e.g. Ti, Zr, Nb, Y) have been used to identify the original geochemical characteristics. The basalts have characters transitional between volcanic ‘arc’ and ‘within plate’ types consistent with eruption on an extensional part of an active continental margin. Two groups of rhyolites have been identified. A low-Zr group (Zr 500ppm), represented at Snowdonia and Avoca, is interpreted as originally being peralkaline in composition; their high Zr/Nb ratios (>10) are typical of peralkaline rhyolites erupted above subduction zones. The bimodal nature of the associations and the peralkaline character of some rhyolites indicates magma production in a complex tectonic setting, transitional between an active continental margin/island arc and an extensional environment. Associated sulphide mineralization is volcanogenic and probably syn-sedimentary. High-level, rhyolitic magma chambers are thought to have driven convection of the hydrothermal fluids from which the sulphides precipitated.

Archean gold-bearing quartz veins at the Sigma Mine, Abitibi greenstone belt, Quebec; Part I, Geologic relations and formation of the vein system

Abstract: The Sigma deposit consists of an Archean gold-bearing vein system in metavolcanic rocks intruded by an irregular body of porphyritic diorite, both affected by regional deformation and in turn cut by younger undeformed feldspar porphyry dikes. The veins cut all rock types and are composed chiefly of quartz and tourmaline, with minor amounts of carbonates, pyrite, chlorite, scheelite, and free gold. They were emplaced during or after a greenschist facies metamorphism postdating the intrusion of feldspar porphyry dikes. The veins were not significantly affected by any subsequent metamorphism or tectonism.Two principal types of veins occur in the mine; subvertical veins and subhorizontal veins. They are contemporaneous and cogenetic. Subvertical veins occur within steeply dipping, east-west-trending ductile shear zones along which reverse subvertical displacements have taken place. The veins occupy openings created by overriding of undulations and irregularities of foliation and slip planes during progressive shearing movements within the shears. Subhorizontal veins occupy extensional fractures which developed between ductile shear zones, preferentially in the more competent host rocks. These veins were formed by one or several episodes of open-space filling.Geologic relations indicate that the mineralized veins were emplaced progressively in a dynamic tectonic environment during and shortly after the formation of ductile shear zones. A structural analysis has shown that the ductile shear zones and the two vein types can be related to a single strain ellipsoid of the following orientation: the Z-axis trending north-south and horizontal, the Y-axis plunging 10 degrees west, and the X-axis plunging 80 degrees east. The deformation responsible for the generation of ductile shear zones and emplacement of the veins is ascribed to a late north-south-trending compression postdating regional deformation.

Role of plutonism in low-pressure metamorphic belt formation

Abstract: Wickham and Oxburgh1 recently proposed that low-pressure/high-temperature (low-P/high-T) metamorphism in the eastern Pyrenees, and possibly all low-P/high-T metamorphic belts, resulted from anomalously high mantle heat flow brought about by rifting. Their model is largely constrained by the presence of nearby synmetamorphic rift-related sedimentary rocks and the interpretation that the migmatites and granites are the product of in situ melting in the presence of an anomalously steep geotherm. Here we present an alternative model, in which low-P/high-T metamorphism (pro-grade reactions at pressures near or below the Al2SiO5 triple point) results from contact effects near sill-like igneous intrusions at intermediate crustal levels. Low-P/high-T conditions can be achieved through this process in regions of continent–continent collision with normal mantle heat flux as well as in zones of extension. Our model is based on studies of the low-P/high-T metamorphic terrane in the New England Appalachians.

Isotopic and Structural Responses of Granite to Successive Deformation and Metamorphism

Abstract: Within the Sybella Granite west of Mount Isa in northern Australia, differential response of U-Pb and Rb-Sr isotopic systems to deformation/metamorphism enables ages of emplacement and three discrete deformation/metamorphic events to be determined. Based on zircon U-Pb systems, two of the plutons were emplaced at $1671 \pm 8 Ma$ and 1668 $\pm ^{26}_{21}$ Ma, and the third at $1610 \pm 10 Ma$. A separate microgranite body gives aberrantly old U-Pb ages of up to 1780 Ma, due to partial inheritance of older zircon. The three discrete deformation/metamorphic events ($D_{1}, D_{2} and D_{3}$) occurred at $1610 \pm 13 Ma, 1544 \pm 12 Ma, and 1510 \pm 13 Ma$ respectively, based on whole-rock Rb-Sr isotopic systems. The youngest body was emplaced syntectonically and has possible significance to the origin of the Mount Isa Ag-Pb-Zn orebodies. Homogenization of Rb and Sr isotopes via extensive syntectonic fluid migration, associated with deformation partitioning and resultant solution transfer, appears to have been...

Progressive Metamorphism of Mafic Rocks from Greenschist to Granulite Facies in the Dharwar Craton of South India

Abstract: In the Archean Dharwar craton of southern India N-S trending belts of metabasic rocks are exposed which underwent regional metamorphism at about 2.5 Ga ago. The progressive changes in the assemblages and mineral chemistry of metabasites was studied in a N-S traverse covering the Chitradurga and Nagaman-gala belts, the Sargur area, and the Nilgiri Hills. Towards the south with increasing metamorphic grade greenschists ($$chl + act + ab + ep \pm carb$$, qtz) give way to amphibolites ($$hbl + plag \pm qtz$$, gar, cumm) and mafic granulites ($$pyx + plag + gar \pm hbl$$, qtz). The amphibole composition changes from actinolite in the greenschist zone to tschermakitic hornblende in the amphibolite zone and pargasitic-hastingsitic hornblende in the granulite zone. The Ti content of the amphiboles systematically increases in this direction. The plagioclase composition changes from albite in the greenschist zone to oligoclase/andesine in the amphibolite zone and andesine/labradorite in the granulite zone. Almandin...

Metallogenesis of early Paleozoic graptolite shales from the Graefenthal Horst (northern Bavaria-Federal Republic of Germany)

Abstract: The Graefenthal horst, situated in the northern part of Bavaria, forms part of the Saxothuringian geotectonic domain of the mid-European Variscides. Psammo-pelitic sediments of Ordovician through lower Carboniferous age were folded during the Sudetian movement (transition from lower Carboniferous to upper Carboniferous). Within this sequence the Silurian lower graptolite shales (about 30 m thick) and Lower Devonian upper graptolite shales (10-20 m thick) are of widespread occurrence. These bioliths (phosphorites, alum shales, and carbonaceous matter-bearing chert) are of potential economic interest because of their metal content (maximum value, V = 9,500 ppm, U = 4,800 ppm). They formed under reducing conditions in the outer shelf, in the presence of abundant H 2 S. The terrigenous input into that basin may be disregarded, and based on mineralogical as well as chemical studies, any volcanic intercalations within the stratigraphy of that basin may be ruled out. Elements were mainly introduced from underlying strata into the bottom waters enriched in H 2 S and from coastward upwelling along the shelf abundant in phosphorus.Two different stages of diagenesis (early and late) may be distinguished. The late stage is the most significant one by reason of its abundance of sphalerite, chalcopyrite, galena, and tetrahedrite. The black shales were first subjected to very low grade or early stage conditions during Variscan regional metamorphism. Subsequent metamorphism to lower greenschist facies involved shearing processes subparallel to the S planes. During dynamometamorphic mobilization, only rock-forming elements were able to migrate. Minor amounts of Zn were concentrated in ruby sphalerite.Strata-bound, fault-hosted Fe-Cu-Zn-Pb-U mineralization was initiated by subsequent block faulting. Its mineral assemblage (pyrite, marcasite, sphalerite, galena, and sooty pitchblende) points to low-temperature conditions and an element derivation from the enclosing carbonaceous sediments.During the Upper Devonian the Ag content of these black shales was locally upgraded by contact metamorphic processes caused by diabase sills, which were intruded along the boundary of the graptolite shales and tentaculite beds. Impregnations of Ag-enriched galena as well as marmatitic sphalerite are encountered in the diabases near both contacts. Hornblende hornfels facies was achieved during that contact metamorphism. Subsequent supergene alteration during Pliocene to Pleistocene times caused uraniferous Fe-Al phosphates to be concentrated along fractures by decomposition of pyrite of the black shales.These metalliferous black shales may be categorized genetically as syndiagenetic-metamorphic polymetallic low-temperature, fault-hosted deposits formed by mobilization. Though not economically viable at the present time, they may be considered as a future reserve for certain metals (such as vanadium and uranium).

High-pressure metamorphism of the Zermatt-Saas ophiolite zone, Switzerland

Abstract: Within the Zermatt–Saas ophiolite zone of the western Alps a record of polystadial high-pressure metamorphism is well preserved. Early blueschist assemblages, retained as inclusions in garnets, were succeeded by eclogitic assemblages which in some rocks contained lawsonite, kyanite, talc and chloritoid. These eclogitic assemblages formed by prograde reaction from the early blueschists. Subsequently, reaction of these eclogites with a mixed H 2 O-CO 2 , vapour phase led to the replacement of kyanite by paragonite and the partial replacement of omphacite-garnet paragenses by assemblages containing glaucophane, paragonite and ankerite. High-pressure, eo-alpine metamorphism took place under conditions of 550–600°C and 17.5–20 kbar. Values of a H 2 O between 0.55 and 1 are high enough to accommodate equilibration with a water-rich vapour phase under the highest-grade conditions. The presence of such a fluid phase is locally indicated by the presence of quartz-rich veins containing omphacite and kyanite. This vapour phase was absorbed during retrogression. Later, rehydration is limited to areas close to albite veins, tectonic contacts and bodies of metasediment. The metamorphic conditions determined for the Zermatt-Saas zone are compatible with those suggested for over- and under-lying units. These conditions, and the P–T path inferred by comparing the reaction histories with a petrogenetic grid for the system Na 2 O-CaO-MgO-A1 2 O 3 -SiO 2 -H 2 O, suggest that metamorphism occurred during subduction to depths of between 60 and 70 km and subsequent exhumation during the Alpine orogeny.

40Ar/39Ar mineral dates from retrogressed eclogites within the Baltoscandian miogeocline: Implications for a polyphase Caledonian orogenic evolution

Abstract: Late Proterozoic, rift-facies dolerite dikes within Baltoscandian rocks of the Seve Nappe Complex locally underwent eclogite metamorphism during Caledonian orogenesis. Hornblende from retrograde amphibolite selvages developed around two eclogite boudins exposed at Grapesvare, Norrbotten County, Sweden, record identical 40Ar/39Ar plateau dates of 491 ± 8 Ma. Phengitic muscovite from host schists records plateau dates of 447 ± 7 Ma and 436 ± 7 Ma. Coexisting biotite yields plateau dates of 594 ± 10 Ma and 808 ± 13 Ma. The biotite dates are interpreted to reflect the presence of extraneous argon components. The hornblende and phengitic muscovite ages are interpreted to date times of postmetamorphic cooling through argon retention temperatures. Together with previous 40Ar/39Ar mineral ages from Jamtland, Sweden, they confirm that a significant pre-Scandian tectonothermal event is recorded regionally in allochthonous sequences which originated within the Baltoscandian miogeocline. The eclogite assemblages are interpreted to have formed during westerly subduction of distal portions of the miogeocline with attendant development of an accretionary wedge. The latter was subsequently uplifted and eroded, providing a source for Middle Ordovician through Lower Silurian clastic successions which accumulated in both eastern and western basins. These, together with previously metamorphosed older portions of the miogeocline, were imbricated, folded, and variably metamorphosed during Late Silurian to Early Devonian transport onto the Baltoscandian Platform.

Rb-Sr, K-Ar, and Stable Isotope Evidence for the Ages and Sources of Fluid Components of Gold-Bearing Quartz Veins in the Northern Sierra Nevada Foothills Metamorphic Belt, California

Abstract: Gold-bearing quartz veins occur in and near major fault zones in deformed oceanic and island-arc rocks west of the main outcrop of the Sierra Nevada composite batholith. Veins typically occupy minor reverse faults that crosscut blueschist to amphibolite-grade metamorphic rocks whose metamorphic ages range from early Paleozoic to Jurassic. Vein micas and carbonate-quartz-mica assemblages that formed by hydrothermal metasomatism of ultramafic wall rocks in the Alleghany, Grass Valley, Washington, and Mother Lode districts yield concordant K-Ar and Rb-Sr ages. The dated veins are significantly younger than prograde metamorphism, penetrative deformation, and accretion of their host rocks to the continental margin. New and previously published mineralization ages from 13 localities in the Sierra foothills range from about 140 to 110 m.y. ago, with mean and median between 120 and 115 m.y. The age relations suggest that mineralizing fluids were set in motion by deep magmatic activity related to the resumption of east-dipping subduction along the western margin of North America following the Late Jurassic Nevadan collision event.CO 2 -bearing fluids responsible for metasomatism and much of the vein mica, carbonate, albite, and quartz deposition in several northern mines were isotopically heavy (delta 18 O [asymp] 8-14ppm; delta D between about -10 and -50ppm) and do not resemble seawater, magmatic, or meteoric waters. Metasomatic and vein-filling mica, dolomite, magnesite, and quartz in altered ultramafic rocks generally formed from fluids with similar Sr and O isotope ratios at a given locality. Consistent quartz-mica delta 18 O fractionations (delta 18 O (sub Q-M) = 4.5-4.9ppm) from various localities imply uniform equilibration temperatures, probably between 300 degrees and 350 degrees C. On a local (mine) scale, fluids responsible for both carbonate alteration of mafic and ultramafic wall rocks and albitic alteration of felsic and pelitic rocks had similar Sr isotope ratios.Samples from three veins in the central Alleghany district fit a 115.7 + or - 3-m.y. Rb-Sr isochron with a ( 87 Sr/ 86 Sr) i value of approximately 0.7119. Inferred 87 Sr/ 86 Sr ratios of metasomatic fluids from mines in different parts of the foothills region vary considerably (0.704-0.718), suggesting that Sr was derived from sources ranging from "western assemblage" Mesozoic ophiolitic or arc volcanic rocks to early Paleozoic continent-derived clastic rocks of the Shoo Fly Complex. Systematic geographic variations in both Sr and O isotopes can be rationalized by assuming extensive fluid interaction with rocks similar to the ones that are exposed within a few kilometers of the veins, but the ultimate sources of the fluids, and of Au and other constituents, may be independent of these. Isotopically lighter (meteoric?) fluids deposited some late quartz overgrowths and occupied secondary fluid inclusions in earlier vein quartz.

Fluid Flow during Metamorphism and its Implications for Fluid—Rock Ratios

Abstract: The study of metamorphic rocks is traditionally approached as an investigation of mineralogical changes occurring as responses to variations only in pressure and temperature. Despite the losses of large volumes of fluid during prograde metamorphism, rocks are often regarded, on the thin section scale, as systems able to buffer the chemical potentials (µ) of all components and therefore as essentially closed. There are several important reasons, apart from its simplicity, for adherence to this model. First, there is the observation that metasediments maintain their original sharp compositional discontinuities even in the amphibolite facies (e.g., Chinner, 1960). Second, low variance assemblages (implying internally buffered µ’s) are common. Finally, metasomatic changes and externally controlled activities generally lead to simple mono- or bimineralic zones such as those found in skarns or veins (e.g., Tilley, 1951; Burnham, 1959; Thompson, 1959). Such zones occur between interbedded metapelites and carbonates but are not a generally observed phenomenon in mixed pelitic/psammitic units.

Étude pétro-structurale des chevauchements ductiles himalayens sur la transversale de l'Everest–Makalu (Népal oriental)

Abstract: Tectonic and microtectonic data in eastern Nepal indicate that the major observed thrusting (100 km) on the Main Central Thrust (MCT) postdates the Barrovian metamorphism of the High Himalaya gneisses. This result, at variance with the famous "reverse metamorphism model," better explains the abnormal metamorphic superpositions in the Himalayas and accounts for the lack of high-pressure assemblages under the thick, allochtonous High Himalaya Tibetan slab.Pressure and temperature estimates by microprobe analysis on plagioclase, biotite, garnet, kyanite, sillimanite, and cordierite assemblages are presented for samples collected along the MCT shear zone and across the gneiss slab in the Everest–Makalu area. Since there is very little difference in pressure at the front of the slab (Kathmandu Klippe) and its root, these estimates support the existence of important late metamorphic thrusting. The decrease of pressure towards the top of the gneiss pile, combined with a small temperature increase, explains the k...

Fluid Migration and Veining in the Connemara Schists, Ireland

Abstract: A knowledge of how fluids behave during regional metamorphism is central to any understanding of metamorphic processes, but studies of regional fluid migration are still in their infancy, and there is considerable controversy about the possible role of migrating fluids in metamorphism. There can be no doubt that fluids are driven off from many rock types during progressive metamorphism and that fluid can reenter rocks after the peak of metamorphism to give rise to retrograde changes. However, major uncertainties remain concerning the permeabilities and porosities of rocks undergoing metamorphism, and even the widespread assumption by metamorphic penologists that fluid pressure (P f ) is approximately equal to lithostatic pressure (P l ) can seldom be independently verified.

P-T paths from high temperature shear zones beneath ophiolites

Abstract: Mctamorphic rocks of the St Anthony Complex of north-western Newfoundland are best interpreted in terms of a high-temperature shear zone formed between down-going continental margin rocks and overriding oceanic lithosphere in a subduction zone. High-grade rocks, immediately beneath the oceanic lithosphere peridotite, display retrograde meta-morphism in high-strain zones, whereas lower grade rocks, near the base of the metamorphic complex, display prograde metamorphism in high-strain zones. Mylonite zones in meta-basitcs at all levels in the complex contain the assemblage epidote-hornblende-albite-sodic oligoclase. These observations suggest that the ‘inverted metamorphic gradient’within the St Anthony Complex results from the fortuitous preservation of residual metamorphic assemblages from different crustal levels within an epidote amphibolite facies shear zone. The degree of re-equilibration is strongly dependent on the degree of strain, and is best achieved in synmetamorphic mylonite zones. This interpretation of the St Anthony Complex can be extended to other sub-ophiolite metamorphic sheets, which show very similar relationships. It is proposed that most metamorphic sheets beneath ophiolites are high temperature shear zones, the P-T paths of which preserve records of burial and exhumation in subduction zones.