Showing papers on "Metamorphism published in 1990"

Monazite U-Pb dating of staurolite grade metamorphism in pelitic schists

Abstract: A study of the occurrence of and relations between rare-earth element (REE) minerals in pelitic schists indicates that monazite forms at or near the P and T of the staurolite isograd. Samples at staurolite grade from the Silurian Perry Mountain Formation in the Rumford quadrangle of Maine yield monazite in sufficient quantities to permit accurate dating of the metamorphic events forming the monazites. The bulk chemistry of the metapelites, as seen in the major element abundances and REE patterns, does not vary significantly across the study area. Thus the appearance and disappearance of REE phases is assumed to reflect changes in metamorphic grade. In a sample from the biotite zone, scanning electron microscope and microprobe studies show allanite and monazite intimately associated on a 10 μm scale. The texture suggest that metastable detrital monazite breaks down, distributing its REE components to allanite. From samples below staurolite grade in which monazite is not present, our observations suggest that REEs are partitioned into allanite. At or near the staurolite isograd monazite forms as a metamorphic mineral, initiating its role as a geochronometer. Garnet-biotite geothermometry on samples at this grade from this and other studies places constraints on the minimum temperature necessary to form monazite: 525° C±25°C at 3.1±0.25 kbar. A total of 15 separates from nine schist samples ranging up to sillimanite grade have been dated. Each date is remarkably concordant, even though petrologic and textural studies by previous workers have shown that the rocks in the area have been affected by at least three metamorphic episodes. Calculations indicate insignificant Th disequilibrium in these monazites. The conditions associated with the metamorphic events suggest that monazite remains closed to lead loss provided that subsequent metamorphisms are at or below sillimanite grade. Two distinct metamorphic events are resolved, one at around 400 Ma and one at about 370 Ma. The latter was due to thermal effects of a nearby pluton that yields concordant monazite ages of 363 Ma. This work suggests that in addition to dating plutonism and high-grade metamorphism, monazite should be viewed as a reliable geochronometer for moderate metamorphism of pelitic schists.

Granulites and crustal evolution

Abstract: The NATO ARW granulite conference: a report.- Crustal Evolution.- Genesis of continental crust: evidence from island arcs, granulites, and exospheric processes.- The granulite - granite connexion.- Heat, fluids, and melting in the granulite facies.- Granites, granulites, and crustal differentiation.- Regional Syntheses.- Europe.- Evolution of the lower crust in the Ivrea zone: a model based on isotopic and geochemical data.- The granulite belt of Lapland.- North and South America.- A model for the granulite-migmatite association in the Archean basement of southwestern Montana.- Multi stage late Archaean granulite facies metamorphism in Northern Labrador, Canada.- Archean tectonic setting of granulite terranes of the Superior Province, Canada, a view from the bottom.- The granulites of the Jequie complex and Atlantic Coast mobile belt, Southern Bahia, Brazil - An expression of Archean/Early Proterozoic plate convergence.- From Africa to India.- Pressure-Temperature-Time paths of granulite metamorphism and uplift, Zambesi belt, N.E. Zimbabwe.- Thermal history and tectonic setting of the Namaqualand granulites, Southern Africa: clues to Proterozoic crustal development.- The granulite-facies rocks of the Limpopo belt, Southern Africa.- Crustal evolution of the granulites of Madagascar.- Nature and scale of fluid-rock exchange in granulite grade rocks of Sri Lanka: a stable isotope study.- The granulite terrane of the Nilgiri Hills (Southern India): characterization of high-grade metamorphism.- Fluids and Petrological Equilibria.- Granulites of Satnuru and Madras: A study in different behaviour of fluids.- Scapolite phase equilibria: additional constraints on the role of CO2 in granulite genesis.- Synmetamorphic fluid inclusions in granulites.- Fluid inclusions in granulites: peak vs. retrograde formation.- Thermometry and barometry of mafic granulites based on garnet - clinopyroxene - plagioclase - quartz assemblages.- Geochemistry and Geophysics.- Geochronology in Granulites.- Origin of granulites: geochemical constraints from Archean granulite facies rocks of the Sino-Korean craton, China.- The oxygen isotope composition of lower crustal granulite xenoliths.- Ionprobe investigation of rare earth element distributions and partial melting of metasedimentary granulites.- Geochemistry of intermediate/- to high-pressure granulites.- Thermal data and crustal structure. Role of granites and the depleted lower crust.- Some thermal aspects of granulite history.- Key word/subject index.

Uplift of deep crust during orogenic extensional collapse: A model based on field studies in the Sogn-Sunnfjord Region of western Norway

Abstract: The Western Gneiss Region (WGR) in Norway experienced high-pressure metamorphism during Silurian-Devonian continent-continent collision. The eclogite-bearing lower crust is separated from the middle and upper crust by major detachment zones formed during extensional collapse of the orogen; formation of the Devonian basins is related to the extension. The footwall of the detachment zones comprises three structural and metamorphic zones. The upper zone, zone 1, is characterized by penetrative homogeneous down-to-the-west simple shear developed under retrograde greenschist-facies metamorphism. Zone 2 suffered inhomogeneous simple shear of the same polarity. Petrography and mineral chemistry data from the lower zone, zone 3, show a record of initial eclogite facies metamorphism at 600°C and >16 kbar, which was decompressed almost isothermally to amphibolite-facies conditions at 550°C and 10–12 kbar. Both the eclogite- and amphibolite-facies metamorphism developed in a regime of pure shear with vertical shortening. The rapid decompression records an approach of approximately 20 km to the surface, related to uplift that was probably the result of the removal of a thickened thermal boundary layer in the mantle lithosphere. The pure shear regime, which developed initially in the lower crust, was truncated by zones of simple shear as the lower crust was uplifted to middle and upper crustal levels. Extension by simple shear in the upper crust was rooted in the lower crust where extension occurred by pure shear. The shear zones in zones 1 and 2 did not penetrate the pure shear regime of the lower crust. A considerable amount of tectonic stripping of the orogenic welt predates deposition of the Devonian sedimentary basins.

Evaporitic sediments of Early Archaean age from the Warrawoona Group, North Pole, Western Australia

Abstract: Chemical sediments are common and diverse in the c. 3500 Myr old North Pole chert-barite unit in the Warrawoona Group, Western Australia. Although almost all original minerals were replaced during hydrothermal alteration, metamorphism and deformation, pseudomorphic relics of sedimentary and diagenetic textures and structures show that at least six lithofacies were partly or wholly chemical in origin. These contained five main chemical sedimentary components: primary carbonate mud, diagenetic carbonate crystals, primary sulphate crystals, diagenetic sulphate crystals and diagenetic sulphate nodules. All show a wide range of characteristics consistent only with a marine evaporative origin. Diagenetic carbonate and sulphate crystals, once ferroan dolomite and gypsum, were precipitated within volcanogenic lutites high on littoral mudflats. The other evaporative phases were apparently deposited behind a barrier bar composed of stranded pumice rafts. Primary sulphate crystals, once gypsum and now barite, were precipitated in semi-permanent pools immediately behind the bar. Primary carbonate mud, originally calcitic or aragonitic but now silicified, was deposited in nearby channels and on surrounding mudflats. Within these sediments, diagenetic carbonate crystals (formerly ferroan dolomite) and diagenetic sulphate nodules and crystals (once gypsum) grew during later desiccation. The existence of these evaporites, and more like them in the sediments of other Early Archaean cratons, suggests that shallow marine and terrestrial conditions prevailed over a small but significant portion of the early Earth, contrary to some models of global tectonic evolution. Their overall similarity with more recent evaporitic deposits indicates that there was greater conformity between conditions in modern and primeval sea-shore environments than might be expected, given the great age difference. The attitude implicit in many accounts of Earth's early history, that evaporites were either not deposited or not preserved in Archaean sediments, thus seems to be incorrect.

Granites, Granulites, and Crustal Differentiation

Abstract: Partial melting and the ascent of granitoid magma are among the main processes leading to differentiation of the continental crust. Recent experimental studies and modelling indicate that fertile crustal rocks will typically produce 30 to 40 vol% melt at temperatures of 850°–900°C, even under fluid-absent conditions. Quartz-saturated pelitic rocks will yield 25 to 50 vol% melt at ~ 850°C, while metabasic and intermediate rocks will form 15 to 50 vol% melt at T ≤ 900°C. Production of a large quantity of melt in this temperature range will have the effect of buffering metamorphic temperature in the melting zone. The temperature will not exceed 850°–900°C until the partial fusion process is complete. Fusion of ~ 40% of the lower crust would consume a large amount of energy: the buffering capacity (?H) of a fluid-absent partial melting reaction (at 850°C) being about 50 cal per gram of melting rock. This means that metamorphic T will rarely exceed 850°–900°C during a first, major, thermal event. At this stage, crustal magma production will depend on the fertility of the source rock and the intensity of the thermal anomaly responsible for the metamorphism. With a fertile lower crust, such a thermal event will generate S- and I-type granitoid liquids and leave behind granulitic residues composed of Qtz + Kfs (or Pl) + Grt + Sil + Ru, Qtz + Pl + Opx + Grt, or Opx + Cpx + Pl (± Grt), depending on the composition of the protolith. During a second thermal event (affecting recycled, melt-depleted crust), high-temperature A-type magmas could be produced. During any subsequent thermal event (without introduction of aqueous fluids) there would be no buffering effects linked to either subsolidus or melting reactions. The buffer capacity of the crust would be exhausted, and temperatures of around 1000°C could readily be reached, given a sufficient heat supply. Depending on rock composition, unusual high-T assemblages such as spinel + quartz, sapphirine + quartz, orthopyroxene + sillimanite, and osumilite may develop.

Silurian Orogeny in the Newfoundland Appalachians

Abstract: Mapping and U-Pb age studies in the Central Mobile Belt in the southern Newfoundland Appalachians provide new insights into the tectonic evolution of this region. An older limit for the main Paleozoic deformation in the northern Hermitage Flexure is provided by the $466 \pm 3 Ma$ Bay du Nord Group. The regional D1 deformation of Ordovician strata is bracketed by ages of 429 +5/-3 Ma and $415 \pm 2 Ma$ from the synkinematic Burgeo intrusive suite. Two major metamorphic complexes (Port aux Basques complex, Little Passage gneiss), yield $412 \pm 2 Ma$ and 423 +5/-3 Ma ages, respectively; the latter is intruded by the $421 \pm 2 Ma$ synkinematic Gaultois Granite. A 396 +6/-3 Ma granite of the North Bay Granite Suite provides a younger limit for the main regional deformation and peak metamorphism. The La Poile Group, (420 +8/-2 Ma, 424 +7/-3 Ma and $428 \pm 6 Ma$), the Bear Pond rhyolite (429 +7/-3 Ma), and the Stony Lake volcanics (423 + 3/- 2 Ma), exposed within and north of the Hermitage Flexure, are now do...

The Granulite — Granite Connexion

Abstract: Granulites may be produced in either fluid-present or fluid-absent metamorphism. Fluid-present granulites can be formed by relatively low-T dehydration reactions, in the presence of a fluid dominated by a species other than H2O (e.g., CO2). Small quantities of H2O-rich fluid may be present at the onset of granulite facies conditions (≥ 650°C). This will promote limited degrees of partial fusion and the formation of granulitic migmatites, but will not produce mobile granitoid magma. Much of the lowermost crust is composed of non-restitic metagabbros and cumulates. However, the middle and lower crust also contain a substantial component of granulitic restite. This is derived through fluid-absent partial melting of common crustal rock-types that had been through earlier hydration cycles. Only fluid-absent granulites, produced at T ≥ 850°C, can have intimate, cogenetic connexions with voluminous granitoids. Non-restitic, mafic granulites represent basaltic magma that provided the heat source for metamorphism and melting of the overlying rocks. Restitic granulites are the refractory, residual complements of the granitoid magmas emplaced at higher levels. Silicic magmatism is most commonly a manifestation of crustal growth through under- and intra-plating of mantle-derived magma.

Geodynamic setting of mesothermal gold deposits; an association with accretionary tectonic regimes

Abstract: Mesothermal gold provinces of Phanerozoic age are characteristically associated with regional structures along which allochthonous terranes have been accreted onto continental margins or arcs. A recurring sequence of transpressive deformation, uplift, late kinematic mineralization, and shoshonitic magmatism is consistent with thermal reequilibration of tectonically thickened crust. Mesothermal gold camps in the Superior province are spatially associated with large-scale structures that have been interpreted as zones of transpressive accretion of individual subprovinces or allochthonous terranes: these boundary structures are characterized by the sequence of significant horizontal shortening, uplift, late-kinematic mineralization, and shoshonitic lamprophyres and therefore may have the same geodynamic significance as Phanerozoic counterparts. In this model, thermal re-equilibration of underplated and subducted oceanic lithosphere and sediments in a transpressive regime, over time scales of 10 to 40 m.y., is a necessary precursor to gold mineralization. Hydrothermal fluids are released along boundary faults and their splays during uplift: the uniform temperature, low salinity and mole% CO2 signify uniform source conditions, whereas the variable O, C, Sr, and Pb isotopic compositions of fluids reflect lithological complexity of the source regions and conduits. Ou the basis of this model it is suggested that mesothermal lode gold deposits are the product of subduction-related crustal underplating and deep, late metamorphism, rather than magmatic or metamorphic events in the supracrustal rocks. Secular variations in the generation of Archean, Proterozoic, and Phanerozoic mesothermal Au provinces reflect the timing of collisional orogenies within terranes of these eras.

Chronologic and isotopic framework for Early Proterozoic crustal evolution in the eastern Mojave Desert region, SE California

Abstract: The Early Proterozoic geologic evolution of the eastern Mojave Desert region, as defined by characteristics of its supracrustal rocks, granitoids, metamorphism, structural history, and Pb and Nd isotopic signature, contrasts sharply with other Proterozoic provinces of the southwestern United States. The oldest supracrustal rocks of the Mojave Desert region contain zircons over 2.0 Ga, corroborating Nd isotopic evidence for a much older crust here than elsewhere in the southwestern United States. Granitoids widely emplaced within these supracrustal rocks range from 1.76 to 1.64 Ga. The earlier plutons and surrounding supracrustal rocks were metamorphosed to granulite and high amphibolite facies throughout the province at about 1705 Ma in a migmatite-producing event that we term (informally) the Ivanpah orogeny. Subsequent granitoids, emplaced from 1.69 to 1.67 Ga, were voluminous along a north trending belt in the middle of the Mojave province. Younger plutons were emplaced at about 1.66 Ga in several places and at about 1.64 Ga along the extreme southern part of the province. Commonalities between the Proterozoic evolutions of the Mojave and Arizona crustal provinces do not conclusively establish the time that the provinces were juxtaposed; the data only suggest that the juxtaposition occurred between about 1.76 and 1.64 Ga.

Archaean and Proterozoic high-grade terranes of East Antarctica (40–80°E): a case study of diversity in granulite facies metamorphism

Abstract: In recent years there has been a tendency for workers in high-grade terranes to seek and identify unified and somewhat generalized models for the origin of granulites, usually based on observations in classic terranes such as the Adirondacks (Bohlen 1987) and southern India (e.g. Newton et al. 1980, Hansen et al. 1984). In particular, some studies have emphasized a perceived uniformity in granulite P-T conditions and P-T-t (pressure-temperature-time) paths (Newton & Perkins 1982, Bohlen 1987), and hence have modelled granulite terranes in terms of one particular tectonic setting such as a mag-matic arc. In contrast, Harley (1989a) has emphasized diversity in the important features of granulite terranes, most notably in their P-T conditions of formation and P-T-t paths but also in their lithological constitution and age structures, and has considered a spectrum of possible tectonic settings and mechanisms for granulite formation.

Sm−Nd and Pb isotopic study of mafic rocks associated with early Proterozoic continental rifting: the Peröpohja schist belt in northern Finland

Abstract: The Perapohja schist belt in northern Finland rests unconformably on Archaean granitoids, and marks the early stages of Proterozoic crustal evolution in the Fennoscandian (Baltic) shield. 2440 Ma old layered mafic intrusions predate the supracrustal , and ca. 2200 Ma old sills of the gabbro-wehrlite association intrude the lowest quartzites and volcanics (Runkaus) of the sequence. The Sm-Nd mineral isochron of the Penikat layered intrusion gives an age of 2410±64 Ma. The initial ɛNd-values of the Penikat intrusion (ɛNd(2440) = −1.6) and the Runkausvaara sill (ɛNd(2200) ≈ 0) suggest that these mafic magmas were contaminated by older crustal material. The Sm-Nd and Pb isotopic results on the 2.44–2.2 Ga old Runkaus volcanics indicate mobility of Pb, fractionation of Sm/Nd during “late” greenschist facies metamorphism, and crustal contamination. The Pb-Pb data provide an age of 1972±80 Ma with a high initial 207Pb/204Pb ratio (μ1 = 8.49), while scattered Sm-Nd data result in an imprecise age of 2330±180 Ma, with an initial ɛNd-value of about zero. Secondary titanite gives an U-Pb age of ca. 2250 Ma. The Jouttiaapa basalts, in contrast, ascended from the mantle without interaction with older crust. These LREE depleted tholeiites mark a break in continental sedimentation, and yield a Sm-Nd age of 2090±70 Ma. Their initial ɛNd = + 4.2 ±0.5 implies that the subcontinental early Proterozoic mantle had been depleted in LREE for a long period of time. The first lava flows are strongly depleted in LREE, suggesting that their source was significantly more depleted than the source of mid-ocean ridge basalts today.

A model for low‐pressure facies metamorphism during crustal thickening

Abstract: Low-pressure metamorphic facies (i.e. high T/P ratios) are widespread in a wide range of tectonic settings. Explanations offered for the occurrence of these facies include extensional and/or magmatic models. However, these fail to explain that the low-P facies metamorphism is commonly coeval with a phase of pervasive crustal thickening, with T/P ratios increasing during, or slightly lagging behind, the thickening. We propose an alternative explanation based on the approximate synchroneity of crustal thickening and erosion (thinning) of the mantle lithosphere.

Age constraints on the geological evolution of the Narryer Gneiss Complex, Western Australia

Abstract: Zircon U‐Th‐Pb and mineral K‐Ar and 40Ar/39Ar isotopic studies indicate that the maximum deposition age of the Mt Narryer quartzite (which contains detrital zircons up to 4200 Ma old) is 3280 Ma, or by association with other sequences possibly 3100 Ma. This postdates a major episode of high‐grade metamorphism, granite emplacement and deformation at 3300 Ma, which affected adjacent gneiss terranes and which previously had been considered to have affected metasediments and basement gneisses alike. Prograde metamorphism of the Narryer metasediments to amphibolite facies evidently took place during a younger event culminating ca 2700 Ma, prior to injection of granite sheets ca 2650 Ma in age, by which time the present tectonic framework had been assembled.

Isotopic structure and tectonics of the central Transantarctic mountains

Abstract: Regional patterns of Nd, Sr, and O isotopic ratios of ∼500 Ma granitic rocks are used to identify the ages and areal extents of three crustal provinces in the central Transantarctic Mountains. One of the provinces is the edge of the East Antarctic Craton, which isotopic analyses show is composed of Archean rocks thrust over Proterozoic rocks. The other two provinces compose the Beardmore microcontinent, which we deduce was allochthonous to East Antarctica and was emplaced in late Precambrian or early Paleozoic time. Evidence for a former ocean basin between the Beardmore microcontinent and East Antarctica is provided by basalt and gabbro of mid-ocean ridge character, dated by Sm-Nd at ∼760 Ma, associated with marine sediments now located at the suture. The granitic rocks formed over a westward-dipping subduction zone that was active at ∼500 Ma. The East Antarctic Craton is exposed in the Miller Range, which is a tectonic composite of reworked Archean and early Proterozoic material containing ∼500 Ma peraluminous granites with model ages (TDM) of 2.0 Ga, high δ18O (+11 to +12‰) and high initial 87Sr/86Sr (0.7324 to 0.7417). East of the Marsh Glacier the granitic rocks are metaluminous to weakly peraluminous with model ages of 1.3 to 1.8 Ga, high δ18O (+9 to +13‰) and lower 87Sr/86Sr (0.7068 to 0.7191). East of the Shackleton Glacier, gabbro, tonalite, diorite, and granodiorite have low δ18O (+6 to +7‰), low initial 87Sr/86Sr (0.7045 to 0.7059) and high ϵNd (+0.4 to +1.7). These isotopic provinces correspond to differences in age and composition of the middle and lower crust at the time of formation of the granitic magmas. The boundaries of the isotopic provinces also correspond to discontinuities in provenance, lithology, structural style, and grade of metamorphism of prebatholithic metasedimentary rocks. The isotopic data indicate that the granitic magmas were formed mostly by crustal anatexis in the areas west of the Shackleton Glacier. This is typical of early Paleozoic granitic batholiths elsewhere in the world and has led to speculation that subduction was not involved in granitic magmatism at that time of earth history. However, the granitic rocks located west of the Shackleton Glacier, by virtue of their mantle-like isotopic compositions and their association with metavolcanic rocks, appear to be subduction-related. The tectonic history deduced for the Gondwana margin, as represented in the central Transantarctic Mountains, began with deposition of sediments on an Atlantic-type rifted margin at ∼760 Ma. The Beardmore microcontinent was most likely accreted in association with folding of the clastic sedimentary rocks before middle Early Cambrian time (550 Ma). Carbonate sedimentation and volcanism along the eastern margin of the Beardmore microcontinent commenced in Cambrian time. Folding and metamorphism of all older units occurred in late Cambrian time followed by emplacement of granitic rocks at ∼500 Ma.

Field geology of high-grade gneiss terrains

Abstract: 1 Introduction- 2 Mapping in Gneiss Terrains- 21 Introduction- 22 General Problems- 23 Working Method- 24 Outcrop Analysis- 25 What to Map- 26 Types of Maps 7- 27 Scheme of Events- 28 Sampling- 3 Fabric Development in Gneiss Terrains- 31 Introduction- 32 The Geometry of Ductile Flow in Rocks- 321 Coaxial or Non-Coaxial Flow- 322 Effects of Progressive Deformation- 33 Fabric Elements- 331 Granoblastic Fabrics- 332 Shape and Mineral Fabrics- 333 Layering- 334 Augen Structures in Gneiss- 34 Shear Zones in High-Grade Conditions- 341 Introduction- 342 Development of Fabrics in Shear Zones- 343 Isoclinal Folds in Shear Zones- 35 Fabric Distribution in Shear Zones- 351 Introduction- 352 The Semi-Brittle Deformation Regime- 353 Cataclasite and Pseudotachylyte- 354 The Transition Zone- 355 Relation of Brittle and Ductile Fault Rocks in Gneiss Terrains- 4 Interpretation of Structures and Fabrics- 41 The Inadequate Memory of Rocks- 42 Igneous or Sedimentary Origin of Gneisses- 421 General Guidelines- 422 Apparent Sedimentary Structures- 43 Assessment of Strain Intensity- 44 Shear Zones- 441 Recognition of Shear Zones- 442 Deformation in Shear Zones- 443 Determining Movement Direction Using Lineations- 444 Determining Movement Direction in Absence of Lineations- 445 Shear Sense Criteria- 45 Folds and Boudins- 46 Overprinting Relations Involving Folds and Boudins- 461 Introduction- 462 Fold Interference- 463 Deformed Vein Sets- 47 Overprinting Relations Involving Intrusions- 471 General Guidelines- 472 Intersection Geometry of Planar Intrusions- 473 Interaction of Dykes and Shear Zones- 474 Reactivated Shear Zones- 475 Partial Melt Veins- 48 Outcrop Surface and Fabric Patterns- 481 General Problems- 482 Lineations and Foliation Traces- 5 Metamorphic History of Gneiss Terrains- 51 Introduction- 52 Metamorphic History- 53 Fabric Evidence for Metamorphic History- 54 Metamorphic Conditions- 541 Introduction- 542 Mineral Assemblages in High-Grade Metamorphism- 543 Mineral Assemblages in High to Medium Grade Retrogression- 544 Mineral Assemblages in Low-Grade Retrogression- 55 Sites to Study Retrogression Fabrics- 56 Relative Dating of Metamorphic Events- 57 Isograd Patterns- 58 Geothermobarometry- 6 Geochemistry, Isotope Geochemistry and Geoehronology: Application to Field Studies- 61 Introduction- 62 Geochemistry- 63 Isotope Geochemistry- 64 Geochronology- 641 Introduction- 642 Sm-Nd Method of Dating- 643 U-Pb Dating of Zircons- 7 Origin and Evolution of High-Grade Gneiss Terrains- 71 Introduction- 72 Two Kinds of Gneiss Terrain- 73 Crustal Structure of Gneiss Terrains- 74 Origin and Evolution of High-Grade Gneiss Terrains- 8 Problem Section- 81 Introduction- 82 Informative Misinterpretations- 83 Problems- 9 References- 10 Index

Chronology of Sanbagawa metamorphism

Abstract: By collating age data based on the fossil age of the protoliths, radiometric dating of the metamorphic minerals, and sedimentary records of erosion at the earth's surface, the history of the Sanbagawa metamorphism can be summarized as follows. (1) The pre-metamorphic sedimentary rocks (Carboniferous-Jurassic + Early Cretaceous?) became mixed and formed a thickened packet in the vicinity of an ancient trench through a variety of subduction-related tectono-sedimentary processes, probably in Early Cretaceous time (c., 130-120 Ma). (2) The subducted protoliths underwent progressive metamorphism reaching a maximum depth of c. 30 km in late Early Cretaceous time (c. 116 ± 10 Ma). (3) The high-P/T metamorphic rocks began to rise toward the surface (during the interval 110-50 Ma) with minimum estimates for the average cooling rate around 9-12°C/Ma and an average uplift rate around 0.4-0.5 mm/year. (4) Finally, at some stage after reaching the erosional surface, the high-P/T metamorphic rocks were covered unconformably by the middle Eocene (c. 50-42 Ma) Kuma Group. On the basis of the present chronological summary of the Sanbagawa metamorphism, the areal extent of the Sanbagawa metamorphism is also discussed with respect to the weakly metamorphosed subduction-accretion complex of the next tectonic belt to the south, the Northern Chichibu belt.

Growth, stabilization, and reactivation of Proterozoic lithosphere in the southwestern United States

Abstract: Growth of Proterozoic continental lithosphere in the southwestern United States involved assembly of tectonostratigraphic terranes during several pulses of convergent tectonism ca. 1.74, 1.70, and 1.65-1.60 Ga. Prograde metamorphism accompanied orogenic assembly, and peak metamorphic conditions outlasted deformation. Regions now characterized by the highest metamorphic grades underwent slow isobaric cooling and were not uplifted until more than 200 m.y. after assembly. Regions of low metamorphic grade were not uplifted substantially after assembly. We suggest that (1) relatively thin lithospheric fragments were assembled into isostatically stable, "normal" thickness continental lithosphere; (2) assembly did not erase lithospheric-scale heterogeneities; (3) the present juxtaposition of different crustal levels reflects differential uplift related to 1.4-1.1 Ga tectonomagmatic activity; and (4) the boundaries between different lithospheric blocks were repeatedly reactivated from Precambrian through Tertiary time.

High temperature-low pressure Cretaceous metamorphism related to crustal thinning (Eastern North Pyrenean Zone, France)

Abstract: The Mesozoic sediments in the Eastern North Pyrenean Zone have suffered a high temperature-low pressure metamorphism which reached its climax before the major deformation event. The mineral associations in pelitic rocks are consistent with temperatures of 500°–600°C and a maximum pressure of 3–4 kb. Post-metamorphic brittle deformation has disturbed the initial thermal pattern. The Albo-Cenomanian (98–87 Ma) metamorphism is related to thermal anomalies contemporaneous with the crustal thinning in the North Pyrenean Zone. The distribution of paleotemperatures suggests that the intensity of metamorphism may have been related to the magnitude of crustal attenuation. High-grade rocks are associated with lherzolites and granulites, whereas low-grade rocks are associated with higher-level crustal material (gneisses and micaschists). Crustal thinning and metamorphism developed during sinistral transcurrent displacement of Iberia with respect to Europe.

Proterozoic collisional tectonism in the Trans-Hudson orogen, Saskatchewan

Abstract: Isotopic and structural data from the juvenile Reindeer zone of the Trans-Hudson orogen, northern Saskatchewan, indicate a pre-1.85 Ga thermotectonic event, possibly reflecting arc-continent collision, followed by a more extensive, nappe-forming, ca. 1.83-1.80 Ga thermotectonism during terminal continent-continent collision. Preliminary data from the adjacent, ensialic Cree Lake zone suggest high-grade reworking of Archean crust by the pre-1.85 Ga event. In the Rae province to the west, high-grade metamorphism and reworking of Archean crust occurred about 2.0 Ga and may be related to the formation of the coeval Taltson magmatic zone.