Showing papers on "Metamorphism published in 1989"

An introduction to metamorphic petrology

Abstract: This second edition is fully updated to include new developments in the study of metamorphism as well as enhanced features to facilitate course teaching. It integrates a systematic account of the mineralogical changes accompanying metamorphism of the major rock types with discussion of the conditions and settings in which they formed. The use of textures to understand metamorphic history and links to rock deformation are also explored. Specific chapters are devoted to rates and timescales of metamorphism and to the tectonic settings in which metamorphic belts develop. These provide a strong connection to other parts of the geology curriculum. Key thermodynamic and chemical concepts are introduced through examples which demonstrate their application and relevance. Richly illustrated in colour and featuring end-of-chapter and online exercises, this textbook is a comprehensive introduction to metamorphic rocks and processes for undergraduate students of petrology, and provides a solid basis for advanced study and research.

Bedrock geology and tectonic evolution of the Wrangellia, Peninsular, and Chugach Terranes along the Trans‐Alaska Crustal Transect in the Chugach Mountains and Southern Copper River Basin, Alaska

Abstract: The Trans-Alaskan Crustal Transect in the southern Copper River Basin and Chugach Mountains traverses the margins of the Peninsular and Wrangellia terranes, and the adjacent accretionary oceanic units of the Chugach terrane to the south The southern Wrangellia terrane margin consists of a polymetamorphosed magmatic arc complex at least in part of Pennsylvanian age (Strelna Metamorphics and metagranodiorite) and tonalitic metaplutonic rocks of the Late Jurassic Chitina magmatic arc The southern Peninsular terrane margin is underlain by rocks of the Late Triassic (?) and Early Jurassic Talkeetna magmatic arc (Talkeetna Formation and Border Ranges ultra-mafic-mafic assemblage) on Permian or older basement rocks The Peninsular and Wrangellia terranes are parts of a dominantly oceanic superterrane (composite Terrane II) that was amalgamated by Late Triassic time and was accreted to terranes of continental affinity north of the Denali fault system in the mid- to Late Cretaceous The Chugach terrane in the transect area consists of three successively accreted units: (1) minor greenschist and intercalated blueschist, the schist of Liberty Creek, of unknown protolith age that was metamorphosed and probably accreted during the Early Jurassic, (2) the McHugh Complex (Late Triassic to mid-Cretaceous protolith age), a melange of mixed oceanic, volcaniclastic, and olistostromal rocks that is metamorphosed to prehnite-pumpellyite and lower greenschist facies that was accreted by middle Cretaceous time, and (3) the Upper Cretaceous Valdez Group, mainly magmatic arc-derived flysch and lesser oceanic volcanic rocks of greenschist facies that was accreted by early Paleocene time A regional thermal event that culminated in early middle Eocene time (48–52 Ma) resulted in widespread greenschist facies metamorphism and plutonism

The metamorphism in the Central Himalaya

Abstract: All along the Himalayan chain an axis of crystalline rocks has been preserved, made of the Higher Himalaya crystalline and the crystalline nappes of the Lesser Himalaya. The salient points of the metamorphism, as deduced from data collected in central Himalaya (central Nepal and Kumaun), are: 1 The Higher Himalaya crystalline, also called the Tibetan Slab, displays a polymetamorphic history with a first stage of Barrovian type overprinted by a lower pressure and/or higher temperature type metamorphism. The metamorphism is due to quick and quasi-adiabatic uplift of the Tibetan Slab by transport along an MCT ramp, accompanied by thermal refraction effects in the contact zone between the gneisses and their sedimentary cover. The resulting metamorphic pattern is an apparent (diachronic) inverse zonation, with the sillimanite zone above the kyanite zone. 2 Conversely, the famous inverted zonation of the Lesser Himalaya is basically a primary pattern, acquired during a one-stage prograde metamorphism. Its origin must be related to the thrusting along the MCT, with heat supplied from the overlying hot Tibetan Slab, as shown by synmetamorphic microstructures and the close geometrical relationships between the metamorphic isograds and the thrust. 3 Thermal equilibrium is reached between units above and below the MCT. Far behind the thrust tip there is good agreement between the maximum temperature attained in the hanging wall and the temperature of the Tibetan Slab during the second metamorphic stage; but closer to the MCT front, the thermal accordance between both sides of the thrust is due to a retrogressive metamorphic episode in the basal part of the Tibetan Slab.

40Ar/39Ar age constraints on deformation and metamorphism in the main central thrust zone and Tibetan slab, eastern Nepal Himalaya

Abstract: In eastern Nepal, structural and petrologic observations suggest that movement on the Main Central Thrust (MCT), the development of an inverted metamorphic sequence, and leucogranite plutonism were genetically related. Samples from across the MCT zone and its hanging wall, the Tibetan Slab, were collected from four locations in the Everest region for 40Ar/39Ar studies: (1) the MCT zone, (2) the lower Tibetan Slab, (3) the upper Tibetan Slab away from intrusive rocks, and (4) the upper Tibetan Slab adjacent to and including leucogranitic material. Within the MCT zone, muscovite and hornblende yield cooling ages of 12.0±0.2 Ma and 20.9±0.2 Ma, respectively. K-feldspar gives a minimum age of 8.0±0.2 Ma (closure temperature (TC) =220±15°C). In the lower Tibetan Slab a sample from a sheared pegmatite yields muscovite and biotite isochron ages of 7.7±0.4 Ma and 7.5±0.6 Ma and a K-feldspar minimum age of 6.4±0.8 Ma (TC = 225±20°C). An adjacent gneiss yields a 9.1±0.2 Ma biotite isochron and a K-feldspar minimum age of 3.6±0.2 Ma (TC = 210±50°C). In the upper Tibetan Slab, samples collected >200 m from visible intrusives yield a biotite isochron age of 20.2±0.2 Ma and a complex hornblende age spectrum suggestive of excess argon. At the fourth location an augen gneiss, a pegmatite, and a tourmaline-bearing leucogranite, all in mutual contact, yield indistinguishable mineral ages. The average biotite and muscovite isochron ages for these samples are 17.0±1.4 and 16.6±0.2 Ma, respectively. The minimum age for K-feldspar from the leucogranite is 15.5±1.8 Ma. As thermobarometric data from the MCT zone in this area suggest synmetamorphic deformation at temperatures of ∼500°–550°C, the MCT hornblende (TC = ∼500°C) dates this event at ∼21 Ma. Diffusion experiments on hornblendes from the MCT zone provide data which support a maximum duration of peak metamorphic temperatures of ≤2 Ma. Biotite ages as old as ∼20 Ma from the upper Tibetan Slab near leucogranites indicate that leucogranite intrusion was essentially coeval with deformation and metamorphism in the MCT zone. Very young ages in a ductile shear zone in the lower Tibetan Slab suggest that there has been deformation within the Tibetan Slab that postdates major movement on the MCT.

Grain-boundary graphite in rocks and implications for high electrical conductivity in the lower crust

Abstract: THE origin of zones of high electrical conductivity in the lower continental crust is a long-standing mystery; possible explanations include the presence of brines1,2, partial melt3, serpentine4 and graphite5. When discussing the occurrence of graphite in the crust many petrologists have considered phase relations as they would have existed at the peak of metamorphism or during igneous emplacement6-10. Here we show that a fine film of graphite is present on the grain boundaries in three rocks from the Laramie Anorthosite Complex. In two of these rocks graphite was stable during igneous crystallization but in the other it was not. We maintain that in all of the rocks the grain-boundary graphite precipitated from a CO2-rich fluid during cooling. The chemical processes that produced the grain-boundary graphite in these rocks are likely to operate in many lower-crustal rocks. We therefore contend that, because the films we observe are capable of producing the high conductivity that is seen in the lower crust, grain-boundary graphite should be considered as a possible cause for some conductivity anomalies.

Thermal model for the Zanskar Himalaya

Abstract: Crustal thickening along the northern margin of the Indian plate, following the 50 Ma collision along the Indus Suture Zone in Ladakh, caused widespread high-temperature, medium-pressure Barrovian facies series metamorphism and anatexis. In the Zanskar Himalaya metamorphic isograds are inverted and structurally telescoped along the Main Central Thrust (MCT) Zone at the base of the High Himalayan slab. Along the Zanskar valley at the top of the slab, isograds are the right way-up and are also telescoped along northeast-dipping normal faults of the Zanskar Shear Zone (ZSZ), which are related to culmination collapse behind the Miocene Himalayan thrust front. Between the MCT and the ZSZ a metamorphic-anatectic core within sillimanite grade rocks contains abundant leucogranite-granite crustal melts of probable Himalayan age. A thermal model based on a crustal-scale cross-section across the Zanskar Himalaya suggests that M1 isograds, developed during early Himalayan Barrovian metamorphism, were overprinted during high-grade MCT-related anatexis and folded around a large-scale recumbent fold developed in the hanging wall of the MCT.

U-Pb systematics of garnet: dating the growth of garnet in the late Archean Pikwitonei granulite domain at Cauchon and Natawahunan Lakes, Manitoba, Canada

Abstract: This study considers the potential of using the U-Pb dating of garnet for determining quantitative P-T-t paths for the late Archean metamorphism in the Pikwitonei granulite domain. Garnets for U-Pb dating were selected mainly from samples that also provide information on pressure and temperature. The garnets used for dating were clear and free of any visible inclusions. Pb concentrations range from 63 ppb to 966 ppb and U from 136 ppb to 1143 ppb. The measured 206Pb/204Pb ratios range from 52.8 to 529.4. The ages are generally discordant with U/Pb ages that may lie above or below concordia. The discordance is caused by a recent disturbance of the U/Pb ratio in the garnets as indicated by replicate analyses on the same garnet separates that reproduce 207Pb/206Pb ages well within analytical uncertainty and in most cases within ±1.5 Ma at 2600–2750 Ma. High grade metamorphism continued over a period of at least one hundred million years, but the garnet-K-feldspar Pb-Pb ages suggest that, during this time, garnet growth has been favored during three distinct periods in the Cauchon Lake area: 2700–2687 Ma 2660–2637 Ma 2605–2591 Ma The ca. 2695 Ma garnet ages from Cauchon Lake date the time of melting and staurolite breakdown during prograde metamorphism, the ca. 2640 Ma ages date the time of extensive migmatization and the last period of metamorphic garnet growth, the ca. 2600 Ma ages date the time of crystallization of igneous garnet in late granitic intrusions. Peak metamorphism occurred around 2640 Ma followed by the intrusions of pegmatites starting at 2629 Ma. The Pb-Pb ages for garnet are similar to the U-Pb ages for zircon that date a leucocratic mobilizate (2695 Ma), a plagioclaseamphibole mobilizate (2637 Ma) and pegmatite (2598 Ma) (Heaman et al. 1986 a; Krogh et al. 1986; this study). Xenocrysts of garnet from 2600 Ma old graphic granites give minimum ages of 2984 Ma and 2741 Ma which are minima for the times of garnet growth in the source of the granites. The agreement of the zircon and garnet ages suggests that the metamorphism may have been punctuated by events that led to the development of melts or encouraged mineral growth at specific times. If so, the prograde and retrograde paths of metamorphism in the area may have contained minor excursions in pressure, temperature or fluid fugacities. In the Natawahunan Lake area some 50 km northwest of Cauchon Lake, garnet growth associated with the prograde breakdown of staurolite occurred at ca. 2744–2734 Ma. This suggests that a similar style of metamorphism may have occurred earlier in the Natawahunan Lake area than at Cauchon Lake area, or higher grades of metamorphism were reached earlier and were of longer duration associated with the somewhat greater depths in the Natawahunan Lake area. These results indicate the these garnets, which are 0.1–1 cm in diameter, have maintained closed system behavior for U and Pb at peak metamorphic conditions, i.e. temperatures up to 800° C and pressures of 7.5 kb.

Fluid flow and metasomatism in a subduction zone hydrothermal system: Catalina Schist terrane, California

Abstract: On Santa Catalina Island, southern California, bluechist to amphibolite facies metasedimentary, metamafic, and meta-ultramafic rocks show veining and alteration that reflect fluid flow and mass transfer at 25-45 km depths in an Early Cretaceous subduction zone. Synkinematic and postkinematic veins record fluid transport and metasomatism during prograde metamorphism and uplift. Vein and host-rock mineralogy and whole-rock compositions demonstrate large-scale chemical redistribution, especially of Si and alkali elements. Veins and host rocks trend toward isotopic equilibration with aqueous fluids with {delta}{sup 18}O{sub SMOW}=+13{per thousand} {plus minus} 1{per thousand}. The likely source for these fluids is in lower temperature, sediment-rich parts of the subduction zone. Carbon isotope systematics support this conclusion and indicate the influence of an organic C source. Quartz solubility relations indicate the importance of fluid-flow paths in chemical redistribution during subduction. These results document large-scale fluid flow and the complexity of possible metasomatic and mechanical mixing processes at intermediate levels of subduction zones. The record of subduction-zone mass transfer in the Catalina Schist is compatible with the record inferred for greater depths from geochemical and petrologic studies of arc magmatism.

Magmatism and the development of low-pressure metamorphic belts: Implications from the western United States and thermal modeling

Abstract: Two-dimensional numerical modeling and geological and geophysical constraints from ancient and modern magmatic arcs demonstrate that magmatic heat advection is sufficient to produce low-pressure metamorphic belts in many areas, and that it is apparently necessary in some areas. In the western United States and other areas, regionally extensive low-pressure facies-series metamorphism (LPM) occurs where intrusions form >∼50% of the upper crust. Numerical models indicate that coalescence of thermal aureoles from multiple felsic intrusions can produce regionally extensive LPM where the abundance of intrusions exceeds ∼50%. This effect does not depend strongly on the rate of emplacement; LPM results even with complete cooling between intrusions. Models with geologically reasonable emplacement rates show that in an active magmatic arc, temperatures are near metamorphic maxima for only a small fraction of the time. Arc magmatism cannot sustain widespread thermal gradients of the magnitude indicated by the final distribution of LPM, a result consistent with heat-flow data in active arcs. Low-pressure metamorphic belts can thus develop through numerous local, short-lived metamorphic events while most of the crust remains considerably cooler. Metamorphic maxima largely depend on the biggest nearby intrusion; emplacement rates and other heat sources affect mainly the magnitude, not the distribution of metamorphism. A simulation based on the distribution and U-Pb geochronometry of the composite Sierra Nevada batholith compares favorably with the observed distribution of metamorphic grades and temperature histories; coeval medium-pressure facies-series metamorphism to the east of the batholith requires a different mechanism. Predicted surface heat flow is qualitatively similar in magnitude and distribution to that measured in modern arcs. Sequential intrusions can produce brief ( 10 km) uplift, and evidence for moderate over-all paleothermal gradients require high-level advection of heat. This model of relatively cool crust punctuated by local, short-lived thermal events differs significantly from the image of a broad, uniform thermal maximum that might be drawn from the spatial distribution of LPM; it has implications for crustal deformation igneous petrogenesis, and the interpretation of high metamorphic gradients in Archean terranes.

Thermobarometric constraints on the thermal history of the Main Central Thrust Zone and Tibetan Slab, eastern Nepal Himalaya

Abstract: The Main Central Thrust (MCT) south of Mt Everest in eastern Nepal is a 3 to 5km thick shear zone separating chlorite-bearing schist in the lower plate from sillimanite-bearing migmatitic gneiss in the overlying Tibetan Slab. The metamorphic grade increases through the MCT zone toward structurally higher levels. Previous workers have suggested that either post- or synmetamorphic thrust movement has caused this inversion of metamorphic isograds. In an effort to quantify the increase in grade and to constrain proposed structural relations between metamorphism and slip on the fault, four well-calibrated thermobarometers were applied to pelitic samples collected along two cross-strike transects through the MCT zone and Tibetan Slab. Results show an increase in apparent temperature up-section in the MCT zone from 778 K to 990 K and a decrease in temperature to ∼850 K in the lower Tibetan Slab, which is consistent with synmetamorphic thrust movement. A trend in calculated pressures across this section is less well-defined but, on average, decreases up-section with a gradient of ∼28MPa/km, resembling a lithostatic gradient. Pressure-temperature paths for zoned garnets from samples within the MCT zone, modelled using the Gibbs' Method, show a significant decrease in temperature and a slight decrease in pressure from core to rim, which might be expected for upper plate rocks during synmetamorphic thrust movement. Samples from the uppermost Tibetan Slab yield higher temperatures and pressures than those from the lower Tibetan Slab, which may be evidence for later‘resetting’ of thermobarometers by intrusion of the large amounts of leucogranite at that structural level.

Are systematic variations in thrust belt style related to plate boundary processes? (The western Alps versus the Carpathians)

Abstract: Within the Mediterranean region, Cenozoic deformation of the Western Alps and the West to East Carpathians has resulted in two different styles of foreland fold and thrust belt. The most prominent difference between the two belts is the presence (Carpathians) or absence (Western Alps) of contemporaneous back-arc extension, but other important differences in structure, topography and metamorphism also exist. These differences in thrust belt style developed mainly during the final stages of thrust belt evolution and appear to reflect fundamental differences in the tectonic settings of the Western Alps and the Carpathians in middle and late Cenozoic time. In particular, they appear to be the result of convergence that is in the first case driven primarily by major plate motions and in the second case only by local motions of small lithospheric flakes or fragments. We suggest that the structural styles developed in these two mountain belts may be useful in identifying mountain belts that have evolved in similar tectonic settings elsewhere in the world. In this respect, the Western Alps and the Carpathians can be regarded as typical examples of two different styles of foreland fold and thrust belt (or more properly as end-member examples within a broad spectrum of foreland fold and thrust belt styles). We propose that continental subduction zones and orogenic belts can be loosely divided into segments that show no major back-arc extensional deformation adjacent to the belt (the Western Alps) and segments that exhibit back-arc extension contemporaneously with thrusting (the Carpathians). The former are found in areas where the rate of overall plate convergence exceeds the rate of subduction, and are commonly typified by extensive involvement of crystalline basement in thrusting, exposure of high grade metamorphic rocks at the surface, high topographic elevation, and large amounts of erosion (tens of kilometers). The latter are found in areas where the rate of subduction exceeds the rate of overall plate convergence and are commonly typified by thrust belts with little to no involvement of crystalline basement in thrusting, low grade to no metamorphism, low topographic elevation, little erosion and, in some instances, an anomalously deep foredeep basin system.

Low‐angle faults above and below a blueschist belt—Tinos Island, Cyclades, Greece

Abstract: New observations from the Island of Tinos, Greece, allow a better definition of the structural position of the Alpine (Eocene) blueschist belt exposed in the islands of the Aegean Sea. These blueschists, over a significant part of the Aegean sea, are delimited from below by a low-angle thrust fault, while from above they are delimited by a low-angle, normal-type fault which omits a substantial crustal interval. Both underlying and overlying rocks were not affected by the high? metamorphism. The rapid uplift and exhumation of the high? rocks was therefore mainly the result of fault movements rather than erosion and whole-crust uplifting. The low-angle normal fault apparently had a major role in the uplift of the blueschists.

Early Paleozoic terranes of New Zealand

Abstract: Lower Paleozoic rocks of New Zealand comprise two major assemblages each with their own distinct sedimentary tectonic, metamorphic and igneous history; they thus represent two distinct tectono-stratigraphic terranes. In Nelson-Westland, the western, or Buller, terrane consists of the Western Sedimentary Belt, together with Ordovician paragneiss at Charleston and in Victoria Range. The sedimentary sequence, ranging in age from basal to Upper Ordovician, comprises continentderived quartz-rich turbidites with black shales inferred to have been deposited in submarine fans and slope basins. The eastern, Takaka terrane (Central and Eastern Sedimentary Belts) is much more varied in lithofacies, composition and age (Cambrian to Silurian) and itself comprises several tectonic, probably thrust, slices. Volcanics, volcaniclastics, siliceous and calcareous siltstone, conglomerate and turbidites dominate the Cambrian part of the sequence and indicate proximity to a Cambrian island arc. The oldest sediments ar...

First evidence of high-pressure, low-temperature metamorphism in the Alpujarride nappes, Betic Cordilleras (S. E. Spain)

Abstract: HP-LT assemblages and minerals are described for the first time in the Alpujarride nappes of the Central and Eastern Betic Cordilleras. These assemblages occur in Permo-Triassic metapelites and include ferroand magnesio-carpholite, aragonite, kyanite and Mg-rich chloritoid. The estimated P-T conditions range from 4-5 kbar at 280-300 °C to 7-9 kbar at 450-500 °C. Hence, these nappes underwent HP-LT metamorphism (thermal gradient of 12-16 °C/km), just as did the underlying Nevado-Filabride units, prior to their lowto intermediate pressure evolution. Key-words : Betic Cordilleras, high-pressure metamorphism, carpholite, aragonite, Mg-rich chloritoid. High-pressure, low-temperature (HP-LT) terrains are widespread in the Alpine Belt. More particularly, the Western Alps and Cor­ sica are characterized by the superposition of low-grade HP-LT units (blueschists and car­ pholite-bearing schists) upon high-grade eciogi­ tic units (Saliot et al., 1980 ; Goffe and Chopin, 1986 ; Caron and Pequignot, 1986 ; Gibbons et al., 1986). In the Betic Cordilleras, HP-LT metamorphism has until now only been reco­ gnized in the deepest zones (Nevado-Filabride Complex) through the occurrence of high-grade assemblages in the eciogite facies (Nijhuis, 1964 ; Kampschuur, 1975 ; Puga, 1977 ; Marti­ nez-Martinez, 1986 ; Gomez-Pugnaire and Fer­ nandez-Soler, 1987). Low-grade HP-LT rocks, on the other hand, were unknown in the belt. DOI:10.1127/ejm/01/1/0139 The higher Betic units (Alpujarride nappes) were thought to have only suffered lowto intermediate-pressure metamorphism of various grades from lower greenschist to granulite facies (Westra, 1969; Torres-Roldan, 1979; Aldaya et al., 1979; Akkerman et al., 1980; Platt, 1986), while the still higher nappes (Malaguides, Sub-Betic) are virtually unmeta­ morphosed. We report he re new petrographical observations from the Alpujarride nappes which decisively change this previous concep­ tion. Geological setting of HP-LT facies rocks HP-LT assemblages and associated relicts can be identified in the Alpujarride nappes of 0935-1221/89/0001-139 $ 1.00 o 1989 E. Schweizerbart'sche Verlagsbuchhandlung, D-7000 Stuttgart 1

Short Paper: The depositional age of the Dalradian Supergroup: U-Pb and Sm-Nd isotopic studies of the Tayvallich Volcanics, Scotland

Abstract: Zircons from a keratophyre associated with the Tayvallich Volcanics in the Dalradian rocks of the SW Scottish Highlands have been dated by U-Pb methods, yielding an age of 595 ± 4 Ma. This age indicates that most or all of the Dalradian is Precambrian in age, and that Dalradian sedimentation may have lasted for about 200 million years. The age also constrains the time interval between cessation of Dalradian sedimentation and subsequent crustal thickening and regional metamorphism during the Grampian Orogeny. Sm-Nd isotopic data for the Tayvallich Volcanins and related metadolerite sills yield initial oNd values of +2 to +4, which are thought to reffect the effects of melting of lithospheric mantle.

Petrologic and age constraints on the origin of a low‐pressure/high‐temperature metamorphic complex, southern Alaska

Abstract: The interrelationships between metamorphism, deformation, magma intrusion, and 40Ar/39Ar geochronology were determined for a low-pressure/high-temperature metamorphic complex which formed from an accretionary prism in the Chugach Mountains, southern Alaska. Compressional deformation, which first produced south verging folds and associated thrusts, was followed by magma intrusion and development of north verging folds. Synmetamorphic southward directed thrusting of metamorphosed flysch over flysch produced increased load in the footwall, as documented by the distribution of mineral assemblages and by pressure-temperature modeling of garnet growth. The initial heating to the greenschist facies may have been accomplished by a combination of advective heating from aqueous fluids and of conductive heating from subducted young oceanic crust. Regionally developed amphibolite facies metamorphism followed intrusion of felsic sills. The peak metamorphic conditions derived from geothermobarometry, mineral assemblages, and fluid inclusions ranged from 400° to 600°C at a depth of ∼10 km. The increased heat from associated synmetamorphic concordant felsic sills raised the ambient temperatures to produce a regional distribution of andalusite and cordierite with a core zone of sillimanite-bearing migmatites. Subsequent cooling was initially rapid (≈55°C/Ma) to ∼350°C based on 40Ar/39Ar dates of 53 Ma for hornblende and 50 Ma for biotite and may have slowed to ∼11°C/Ma until 200°C based on an 40Ar/39Ar date of 35 Ma for plagioclase. Intrusions of felsic sills and at least one pluton were along the initially north dipping foliations associated with south verging folds. This suggests the source region for the melts may have been downdip in the subduction zone rather than from directly below within the accretionary prism.

Late Archean Quetico accretionary complex, Superior province, Canada

Abstract: The 1200-km-long Quetico subprovince consists of monotonous metagraywacke, with derived migmatite and granite, in thrust and/or transcurrent fault contact with the adjacent Wabigoon and Wawa metavolcanic subprovinces. Imbrication of sedimentary wedges derived from volcanic arcs to the north and south produced a 10-100-km-wide, stratigraphically north-facing pile. Thermal relaxation in the 20 m.y. following accretion of the southern arc resulted in melting of the accretionary pile to produce peraluminous granite and associated low-pressure metamorphism.

Evolution and assembly of Archean Gneiss Terranes in the Godthåbsfjord Region, southern west Greenland: Structural, metamorphic, and isotopic evidence

Abstract: The Godthabsfjord region of southern West Greenland comprises several terranes that were assembled between 2750 and 2550 Ma and folded during amphibolite facies metamorphism. The terranes, which are dominated by gneisses of different ages and show different preassembly metamorphic and structural histories, are (1) the Faeringehavn terrane containing the 3820–3600 Ma Amitsoq gneisses, with granulite facies metamorphism at circa 3600 Ma, (2) the Akia terrane containing the 3070–2940 Ma Nuk gneisses, with granulite-amphibolite facies metamorphism at circa 2980 Ma, (3) the Tasiusarsuaq terrane containing circa 2900 Ma gneisses, with granulite facies metamorphism at circa 2800 Ma, and (4) the Tre Brodre terrane containing the 2800–2750 Ma Ikkattoq gneisses, with amphibolite facies metamorphism at 2800–2750 Ma. Metamorphic assemblages and structures formed prior to terrane assembly, were variably overprinted during amphibolite facies metamorphism and heterogeneous strain associated with assembly. Recognition of the region as consisting of several terranes indicates that the anatomy of some Archean high-grade gneiss complexes may resemble that of orogens of Proterozoic and Phanerozoic age that formed as a consequence of plate tectonic processes.

Fluid and enthalpy production during regional metamorphism

Abstract: Models for regional metamorphism have been constructed to determine the thermal effects of reaction enthalpy and the amount of fluid generated by dehydration metamorphism. The model continental crust contains an average of 2.9 wt % water and dehydrates by a series of reactions between temperatures of 300 and 750° C. Large scale metamorphism is induced by instantaneous collision belt thickening events which double the crustal thickness to 70 km. After a 20 Ma time lag, erosion due to isostatic rebound restores the crust to its original thickness in 100 Ma. At crustal depths greater than 10 km, where most metamorphism takes place, fluid pressure is unlikely to deviate significantly from lithostatic pressure. This implies that lower crustal porosity can only be maintained if rock pores are filled by fluid. Therefore, porosities are primarily dependent on the rate of metamorphic fluid production or consumption and the crustal permeability. In the models, permeability is taken as a function of porosity; this permits estimation of both fluid fluxes and porosities during metamorphism. Metamorphic activity, as measured by net reaction enthalpy, can be categorized as endothermic or exothermic depending on whether prograde dehydration or retrograde hydration reactions predominate. The endothermic stage begins almost immediately after thickening, peaks at about 20 Ma, and ends after 40 to 55 Ma. During this period the maximum and average heat consumption by reactions are on the order 11.2·10−14 W/cm3 and 5.9·10−14 W/ cm3, respectively. The maximum rates of prograde isograd advance decrease from 2.4·10−8 cm/s, for low grade reactions at 7 Ma, to 7·10−10 cm/s, for the highest grade reaction between 45 and 58 Ma. Endothermic cooling reduces the temperature variation in the metamorphic models by less than 7% (40 K); in comparison, the retrograde exothermic heating effect is negligible. Dehydration reactions are generally poor thermal buffers, but under certain conditions reactions may control temperature over depth and time intervals on the order of 1 km and 3 Ma. The model metamorphic events reduce the hydrate water content of the crust to values between 1.0 and 0.4 wt % and produce anhydrous lower crustal granulites up to 15 km in thickness. In the first 60 Ma of metamorphism, steady state fluid fluxes in the rocks overlying prograde reaction fronts are on the order of 5·10−11 g/cm2-s. These fluid fluxes can be accommodated by low porosities (<0.6%) and are thus essentially determined by the rate of devolitalization. The quantity of fluid which passes through the metamorphic column varies from 25000 g/cm2, within 10 km of the base of the crust, to amounts as large as 240000 g/cm2, in rocks initially at a depth of 30 km. Measured petrologic volumetric fluid-rock ratios generated by this fluid could be as high as 500 in a 1 m thick horizontal layer, but would decrease in inverse proportion of the thickness of the rock layer. Fluid advection causes local heating at rates of about 5.9·10−14 W/cm3 during prograde metamorphism and does not result in significant heating. The amount of silica which can be transported by the fluids is very sensitive to both the absolute temperature and the change in the geothermal gradient with depth. However, even under optimal conditions, the amount of silica precipitated by metamorphic fluids is small (<0.1 vol %) and inadequate to explain the quartz veining observed in nature. These results are based on equilibrium models for fluid and heat transport that exclude the possibility of convective fluid recirculation. Such a model is likely to apply at depths greater than 10 km; therefore, it is concluded that large scale heat and silica transport by fluids is not extensive in the lower crust, despite large time-integrated fluid fluxes.

Thermal development of low‐pressure metamorphic belts: Results from two‐dimensional numerical models

Abstract: We have used two-dimensional numerical models and analytical solutions to heat flow equations to investigate the mechanisms and the spatial and temporal development of regionally extensive low-pressure facies-series metamorphism (LPM). Two-dimensional models are necessary for describing the evolution of isotherms in the crust where lateral heat flow and local time-dependent heat sources are important. The models demonstrate that felsic intrusions can produce regionally extensive LPM where their abundance through time in the upper crust exceeds approximately 50%. Evenly spaced intrusions elevate vertical metamorphic gradients in regions between intrusions by ∼5° to 10°C km−1 over background values at ∼40% abundance and by ∼15° to >30°C km−1 at ∼70% abundance. Other mechanisms, such as intrusion of mafic magmas in the lower crust, crustal extension, or aqueous fluid advection, cannot independently produce maximum temperatures in such regions. If coeval with intrusion, these other processes will contribute to maximum temperatures; however, the distribution of isotherms and the timing of metamorphism in the upper crust are governed by the shallow intrusions. Intrusion width, abundance, and composition, and timing of emplacement of nearby intrusions have the dominant influences on maximum temperatures between intrusions. For 10-km-wide intrusions, abundances necessary for production of regionally extensive LPM are ∼15% less for gabbros than for granites and ∼20% less when intrusions are emplaced simultaneously than when complete cooling occurs between intrusive events. Intrusion shape, height, convection (magmatic and hydrothermal), and recharge (open system magma chambers) have a significantly less effect on thermal evolution. The total area affected by intrusions depends on their final distribution, not the magmatic flux. In the development of low-pressure metamorphic terranes with abundant intrusions, temperatures in the upper crust are significantly elevated only near recently emplaced intrusions. Consequently, the final distribution of metamorphic grades has little resemblance to the distribution of isotherms at any time—a fundamental difference with LPM in regions lacking abundant shallow intrusions, where metamorphism likely occurs concurrently over large regions. This model of the development of widespread LPM by the juxtaposition through time of local, short-lived thermal maxima should be considered when interpreting temperature-dependent phenomena (for example, deformation) in terranes with abundant intrusions.

Origin and metamorphic history of an Early Cretaceous polybaric granulite terrain, Fiordland, southwest New Zealand

Abstract: Regionally extensive two-pyroxene granulites in Fiordland, southwest New Zealand, are products of metamorphism of a suite of anhydrous magmas which crystallized two pyroxenes. The granulite protolith (igneous charnockitic rock) synkinematically intruded metasediment and other orthogneiss in an Early Cretaceous subduction-related magmatic arc, and during cooling experienced deformation-induced recrystallization to form granoblastic gneiss. The granulites occur side by side with coeval rocks of amphibolite facies. Mineral zoning and textural relationships in both granulites and amphibolite facies rocks provide evidence of two distinct periods of crystallization: 1) an early high temperature, comparatively low pressure event accompanying magmatic intrusion (andalusite-sillimanite facies series recorded locally in the country rock), followed by 2) high pressure metamorphism under conditions of ∼650°–700° C at ∼12–13 kbar. Garnet granulite locally overprinted earlier formed two-pyroxene granulite during the latter event. The pressure increase (∼6 kbar) between the two events is attributed to crustal thickening by overthrusting, and is equivalent to unloading of a ∼20 km thick slab over rocks already buried at mid-crustal depths. Both events occurred over a < 20 m.y. interval, between the time of magmatic emplacement of the granulite protolith and uplift-controlled final cooling of the terrain. The Phanerozoic granulites in Fiordland share some petrologic similarities with Precambrian granulite terrains, suggesting that at least some aspects of the former may serve as a useful model for development of the latter.