K yanite-grade metamorphism in the Evenes and Bogen Groups, Ofoten, North Norway
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Abstract:
Directly north of Ofotfjorden in northern Norway, pelitic schists within the Evenes and Bogen Groups contain the mineral assemblage garnet + biotite ± kyanite ± staurolite + white mica + quartz ± plagio clase. This assemblage implies metamorphic P-T minima of-540C and -4.8 kb. The rocks are thus at a higher grade than suggested by previous reports, which placed them in the greenschist facies. This indicates that several metamorphic allochthons in Ofoten, including rocks of the Narvik, Evenes, Bogen, and Niingen Groups, are all at kyanite grade, supporting recent interpretations which on structural grounds concluded that the metamorphic peak outlasted stacking of these allochthons. A proposed correlation of the Evenes Group with the Middle Ordovician-Lower Silurian Balsfjord Supergroup implies that this stacking and associated kyanite-grade metamorphism are post-early Silurian and are related to the Scandian phase of the Caledonian orogeny.Keywords:
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Field mapping and 40Ar/39Ar age spectrum dating of white mica across the classic metamorphic gradient in Dutchess County, New York indicate that post-peak metamorphic and structural events complicate the interpretation of this sequence as an intact Taconic metamorphic field gradient. Oriented samples were collected and fabrics were measured along two ∼7 km transects. These transects extend from west of the biotite zone (west of Clove Mountain) to the sillimanite zone (Swamp River Valley) in eastern Dutchess County. Field and petrographic analysis reveals three pervasive foliations (S1, S2 and S3). S1 is folded in the lower grade rocks of the sequence, but is increasingly overprinted at garnet and higher zones by a penetrative foliation, S2. S2 dips steeply to the SE and is axial planar to folds in S1. S2 is overgrown by porphyroblasts of biotite, biotite + chloritoid, biotite + garnet ± staurolite ± kyanite ± sillimanite, at respective metamorphic grades, that appear to approximate peak temperature conditions. S1, S2, and peak temperature conditions are constrained as late Taconic based on a depositional age of ∼460 Ma (Potter, 1972) for the Walloomsac Formation and a staurolite age of ∼454 Ma (Lanzirotti and Hanson, 1997) from kyanite-sillimanite grade rocks. S3 is a greenschist facies foliation composed of muscovite ± biotite ± chlorite that truncates or overprints S2 in garnet through sillimanite grade rocks in the eastern part of the study area. S3 dips gently to the S-SE and becomes more penetrative to the east, such that in the garnet zone, S2 is overprinted by a weakly developed spaced S3 cleavage, while in many samples in the staurolite and higher metamorphic zones S2 micas are significantly overprinted by S3 micas. S3 is observed wrapping around and truncating peak temperature porphyroblasts. Three samples of lepidoblastic white mica (with varying proportions of S1, S2, and S3) from the biotite through staurolite zone all produce 40Ar/39Ar age spectra suggesting closure ages of ∼385 Ma. These white mica ages are interpreted to represent the time of cooling through white mica closure (∼350 °C). The 40Ar/39Ar ages in conjunction with field mapping and petrographic analysis suggest that S3 developed in the Devonian Acadian orogeny. Furthermore, the 40Ar/39Ar data constrains the timing of tilting and uplift of the Ordovician Barrovian sequence from the Middle Silurian to Early Devonian. This study suggests mid-crustal wedging during the Salinic orogeny as a possible mechanism for tilting the rocks of this region and exposing the Taconic-aged Barrovian field gradient at the surface today. We conclude the low-grade Acadian metamorphic overprint of these rocks involved deformation that produced a new (third) cleavage that was coincident with regional cooling from peak Taconic metamorphic conditions and thus required strain but no reheating during the Acadian.
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Carbon- and oxygen-isotope analyses were made of both carbonates and graphites from several Precambrian metamorphic samples from British Columbia. The carbon-isotope data are unusual in that the δ 13 C values of many marbles are very high, up to 9.9, and the most positive values occur in the sillimanite-zone rocks. The δ 13 C values of graphite are also relatively high, and the 13 C fractionations between calcite and graphite suggest that (1) the rocks attained and retained carbon-isotope equilibrium during metamorphism, and (2) the temperature of graphite equilibration in marbles from the sillimanite zone is 650–690 °C, in agreement with temperatures estimated from mineralogical phase equilibria.Possible precursor carbonates for the 13 C-rich marbles in British Columbia include (1) Precambrian carbonate-secreting organisms such as algae that participate in reactions with large kinetic isotope effects, (2) carbonates that exchanged carbon isotopes with a large reservoir of organic material prior to metamorphism, and (3) travertines.
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Paleozoic rocks in the Catalonian Coastal Ranges are in their largest part affectedby alow- tovery-low grade Hercynian metamorphism. Amphibolite facies conditions are only found in restricted areas such as the southwestern part of the Guilleries massif where upper amphibolite facies conditions are reached. Metamorphic grade increases from top to bottom of the Paleozoic stratigraphic sequence and the metamorphic peak is diachronous, being progressively older in the lower grade metamorphic zones. The isograd pattern, mineral assemblages, mineral chemistry and preserved reaction textures are consistent with a low pressure metamorphism possibly evolving from a previous Barrovian type event. The metamorphic climax in the high grade zone was reached after the seconddeformational phase. Calculatedpeak P-Tconditions are 620-640 OC and around 3.5 Kb . A latter episode of decompression from the maximum conditions to 1-2 Kb, with an associated temperature decrease to 530-550 OC, is recognized. The intrusion of late Hercynian granitoids produced contact metamorphic aureoles where the pyroxene-hornfels facies is locally reached.
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To constrain the age of deformational/metamorphic events in the south-central Brooks Range, we analyzed 16 samples of white mica, amphibole, and biotite using the 40Ar/39Ar incremental heating technique. Metamorphic rocks in the study area (between 151°W and 148°W) occur in three principal east-west–trending faultbounded belts. These are, from south to north, the pumpellyite-actinolite–facies rocks of the Phyllite belt, the high-pressure/low-temperature (HP/LT) metamorphic rocks of the Schist belt, and the predominantly greenschist-facies metamorphic rocks of the Skajit allochthon. All three belts have been affected by two penetrative deformational/metamorphic events. The oldest of these (D1a) resulted in tight to isoclinal folding and was accompanied by pumpellyite-actinolite–facies metamorphism in the Phyllite belt, blueschist-facies metamorphism in the Schist belt, and blueschistto greenschist-facies metamorphism in the Skajit allochthon. Two white micas samples, one from the Schist belt, and one from the sodic-amphibole–bearing schists north of the Minnie Creek thrust (MCT) yielded convex-upward Ar-release spectra with maximum apparent ages of 142 and 129 Ma, respectively; we interpret maximum apparent ages as a minimum age for D1a deformation and HP/LT metamorphism in the south-central Brooks Range. A second synmetamorphic deformational event (D1b) affected all but the northernmost rocks of the Skajit allochthon, resulting in pervasive dynamic recrystallization accompanied by growth of metamorphic minerals; in the Schist belt and in sodicamphibole–bearing schists north of the MCT, D1b occurred under lower amphiboliteto greenschist-facies metamorphic conditions, and in the Phyllite belt, D1b occurred under lower greenschist(?)-facies conditions. Plateau, preferred, and isochron dates on white mica, amphibole, and biotite are Early Cretaceous in age, and range from 135 to 110 Ma. We argue that the predominance of Early Cretaceous dates result from the degassing of HP/LT metamorphic minerals during dynamic recrystallization associated with D1b folding and the pervasive growth of syn-D1b neoblasts. These ages are significantly older than Late Cretaceous to Tertiary(?) extensional structures in the southern Brooks Range, and support the interpretation that D1b structures formed during Brookian contractional deformation. Gottschalk, R. R., and Snee, L. W., 1998, Tectonothermal evolution of metamorphic rocks in the south-central Brooks Range, Alaska: Constraints from 40Ar/39Ar geochronology, in Oldow, J. S., and Ave Lallemant, H. G., eds., Architecture of the Central Brooks Range Fold and Thrust Belt, Arctic Alaska: Boulder, Colorado, Geological Society of America Special Paper 324.
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Part of the Western Gneiss Region (WGR) of Norway has been mapped in western Sognefjord. Five major tectonostratigraphic units underlying low-grade Devonian metasediments have been recognized: the Basement Gneisses (BG), Lower, Middle, and Upper Mafic Units (LMU, MMU, UMU), and Hyllestad Schists (HS). All the units have been affected by polyphase deformation. Five different styles of deformation have been recognized across the field area, representing up to five discrete tectonic episodes. Eclogite facies mineral assemblages have been found in the BG, either forming mafic pods or as relicts in garnet amphibolites. Temperature and pressure conditions of metamorphism in all units have been estimated using various calibrated reactions. Temperatures range from about 400°C to 700°C and pressures from about 4 kbar in post-D 2 assemblages to 15 kbar in eclogites. Estimated conditions of eclogite formation are consistent with regional studies. High-pressure (eclogitic) conditions have also been inferred from mineral textures in the highest unit (Hyllestad Schists). Mineral assemblages in parts of this unit may represent pressures as high as 14 kbar at temperatures of 600°C. It is suggested that the Mafic Units and HS are allochthonous and were emplaced onto the WGR during an early stage of the Caledonian Orogeny. All units, including the Basement Gneisses, have suffered retrogression during a late extensional phase which continued into at least the Middle Devonian.
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Oxygen, carbon and strontium isotopic profiles across the margin of the Waterville limestone member are used to investigate advective and diffusive transport during metamorphism of the Waterville Formation in south–central Maine, USA. Rb–Sr isotopic systematics were homogenized on the ~10 cm hand-specimen scale at ages that are within error of the 366±6 Ma Rb–Sr whole-rock age of the syn-metamorphic Hallowell pluton. This is consistent with a plutonic heat source for this low-pressure andalusite- and sillimanite-grade Acadian metamorphic terrane. Advective displacements of all three isotope profiles at the garnet-grade Blue Rock Quarry indicate fluid flow to the east into the limestone, and the oxygen-isotope profile implies a time-integrated fluid flux of 3.2±1.5 m 3 /m 2 (2σ error). This cross layer flux is insufficient to cause the observed reaction progress of the muscovite+ankerite–Quartz to biotite+anorthite (in plagioclase)+calcite reaction in the ~100 m thick Waterville limestone member and much of the fluid flow responsible may have been layer parallel. The isotope profiles indicate advective–diffusive homogenization over distances of 1.5 m (δ 13 C) to 6 m (δ 18 O) and such homogenization distances are difficult to reconcile with observations of order of magnitude variations in reaction progression the centimetre scale or less. It is possible that infiltration occurred during events short lived compared with diffusion, that the reactions started at different temperatures dependent on bulk composition or that diffusion of water from layers with less reactants to layers with more reactants was important in driving the biotite-producing reaction. However, variations of fluid composition inferred from the mineral assemblages are apparently inconsistent with diffusion driving reactions progress, and models of precursor assemblages do not indicate significant compositional control of the temperature of the first appearance of biotite in the rocks. Irrespective of the details of flow and diffusive exchange on the centimetre scale, the average reaction progress in the Waterville limestone member requires significant layer-parallel fluid fluxes.
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Research Article| February 01, 1993 High δ13C Neoproterozoic carbonate rocks in western North America Stephen M. Wickham; Stephen M. Wickham 1Department of the Geophysical Sciences and Enrico Fermi Institute, University of Chicago, Chicago, Illinois 60637 Search for other works by this author on: GSW Google Scholar Mark T. Peters Mark T. Peters 1Department of the Geophysical Sciences and Enrico Fermi Institute, University of Chicago, Chicago, Illinois 60637 Search for other works by this author on: GSW Google Scholar Author and Article Information Stephen M. Wickham 1Department of the Geophysical Sciences and Enrico Fermi Institute, University of Chicago, Chicago, Illinois 60637 Mark T. Peters 1Department of the Geophysical Sciences and Enrico Fermi Institute, University of Chicago, Chicago, Illinois 60637 Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1993) 21 (2): 165–168. https://doi.org/10.1130/0091-7613(1993)021<0165:HCNCRI>2.3.CO;2 Article history First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn Email Permissions Search Site Citation Stephen M. Wickham, Mark T. Peters; High δ13C Neoproterozoic carbonate rocks in western North America. Geology 1993;; 21 (2): 165–168. doi: https://doi.org/10.1130/0091-7613(1993)021<0165:HCNCRI>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract 13C-rich marbles from the Neoproterozoic McCoy Creek Group having δ13C values up to +12% have been discovered at three separate localities in eastern Nevada and western Utah. Detailed isotopic profiles measured across individual amphibolite facies, metre-thick marble layers have extremely uniform 13C/12C ratios, which probably represent depositional values. Because individual carbonate horizons have distinctly different but abnormally high values of δ13C, the discovery permits the use of 13C/12C ratios for stratigraphic correlation within the generally nonfossiliferous Neoproterozoic sedimentary sequences of western North America, even where these rocks have been strongly metamorphosed. The anomalous values add to a growing array of data from western North America and elsewhere delineating a large and geographically widespread Neoproterozoic carbon isotope anomaly. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
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