Paleoproterozoic gneisses of the Ellesmere–Devon crystalline terrane on southeastern Ellesmere Island are deformed by metre-scale, east-striking mylonite zones. The shear zones commonly offset pegmatitic dikes and represent the last episode of ductile deformation. Samples were dated by the 40 Ar/ 39 Ar step-heating method to put an upper limit on the time of deformation. Biotite from one tonalitic protolith and five shear zones give geologically meaningful results. Clusters of unoriented biotite grains pseudomorph granulite-facies orthopyroxene in some of the weakly deformed gneisses, whereas the shape-preferred orientation of biotite defines the mylonitic fabric. The intrusive age of the tonalitic protolith is 1958 ± 12 Ma, based on previous U–Pb dating of zircon. 40 Ar/ 39 Ar analysis of biotite from the same sample gave a plateau age of 1929 ± 23 Ma, which is interpreted as cooling from regional granulite facies metamorphism. Three nearby samples of mylonitic tonalite have 40 Ar/ 39 Ar ages in the range of ≈1870–1840 Ma. Biotite from two granitic mylonites over 80 km away return high-resolution Ar spectra in the same range, implying that widespread ductile shearing occurred at ≈1870–1840 Ma, or ≈90 million years after cooling from regional metamorphism. Although the 2.0–1.9 Ga gneisses of southeastern Ellesmere Island correlate with the Inglefield Mobile Belt in North-West Greenland and the Thelon Tectonic Zone, the late shear zones are superimposed on that juvenile arc long after the 1.97 Ga Thelon orogeny.
Abstract Ultrahigh-pressure (UHP) metamorphism in the Laurentian margin of North-East Greenland formed in the overriding plate of the Caledonian collision with Baltica. The Greenland UHP terrane exhibits a number of characteristics that are incompatible with accepted models for UHP metamorphism in the downgoing slab during continental subduction. UHP metamorphism occurred very late in the collision, and was a long-lived event. Peak temperatures in excess of 950°C are well above the norm for subduction-zone UHP terranes. Exhumation rates, based on new U-Pb SHRIMP results for two boudin neck pegmatites and one hornblende-bearing leucosome, are slower (~3-5 mm/year) than for many other UHP terranes. These atypical features can be explained by intracratonic subduction of Laurentia in the waning phase of the Caledonian collision, analogous to deformation that may be occurring in the Tibetan Plateau today.
(1989). Boudin, augen, horse? A lesson from the Svarttjorna-Turtbakktjorna Lens, Trondelag, Norway. Geologiska Foreningen i Stockholm Forhandlingar: Vol. 111, No. 4, pp. 385-390.
Abstract Garnet–kyanite–staurolite assemblages with large, late porphyroblasts of amphibole form garbenschists in Ordovician volcaniclastic rocks lying immediately south of the Pearya terrane on northernmost Ellesmere Island, Canada. The schist, which together with carbonate olistoliths makes up the Petersen Bay Assemblage (PBA), displays a series of parallel isograds that mark an increase in metamorphic grade over a distance of 10 km towards the contact with Pearya; however, a steep, brittle Cenozoic strike-slip fault with an unknown amount displacement disturbs the earlier accretionary relationship. The late amphibole growth, probably due to fluid ingress, is clear evidence of disequilibrium conditions in the garbenschist. In order to recover the P–T history of the schists, we construct isochemical phase equilibrium models for a nearby garnet–mica schist that escaped the fluid event and compare the results to quartz inclusion in garnet (QuiG) barometry for a garbenschist and the metapelitic garnet schist. Quartz inclusions are confined to garnet cores and the QuiG results, combined with Ti-in-biotite and garnet–biotite thermometry, delineate a prograde path from 480 to 600°C and 0.7 to 0.9 GPa. This path agrees with growth zoning in garnet deduced from X-ray maps of the spessartine component in garnet. The peak conditions obtained from pseudosection modelling using effective bulk composition and the intersection of garnet rim with matrix biotite and white mica isopleths in the metapelite are 665°C at ≤0.85 GPa. Three generations of monazite (I, II and III) were identified by textural characterization, geochemical composition (REE and Y concentrations) and U–Pb ages measured by ion microprobe. Monazite I occurs in the matrix and as inclusions in garnet rims and grew at peak P–T conditions at 397 ± 2 Ma (2σ) from the breakdown of allanite. Monazite II forms overgrowths on matrix Monazite I grains that are oriented parallel to the main schistosity and yield ages of 385 ± 2 Ma. Monazite III, found only in the garbenschist, is 374 ± 6 Ma, which is interpreted as the time of amphibole growth during fluid infiltration at lower temperature and pressure on a clockwise P–T path that remained in the kyanite stability field. These results point to a relatively short (≈12 Myr) Barrovian metamorphic event that affected the schists of the PBA. An obvious heat source is lacking in the adjacent Pearya terrane, but we speculate it was large Devonian plutons—similar to the 390 ± 10 Ma Cape Woods granite located 40 km across strike from the fault—that have been excised by strike-slip. Arc fragments that are correlative to the PBA are low grade; they never saw the heat and were not directly involved in Pearya accretion.
Abstract The juxtaposition of the composite Pearya terrane and the northern Laurentian margin at Ellesmere Island, Nunavut, Canada, has significant ramifications for the Paleozoic tectonic history of the circum-Arctic region. Published tectonic models rely upon interpretation of the subduction-related Kulutingwak Formation as an indicator of Ordovician and/or Silurian accretion (Trettin, 1998). New igneous and detrital zircon U-Pb and Lu-Hf isotopic data from 16 samples collected in the Yelverton Inlet–Kulutingwak Fiord region of northern Ellesmere Island suggest that the Kulutingwak Formation of Trettin (1998) contains structural blocks derived from both the Pearya terrane and Silurian strata associated with the ancestral Laurentian margin. Data from this study demonstrate a complex provenance history for rocks within the Petersen Bay, Kulutingwak Fiord, and Emma Fiord fault zones, with age probability peaks of ca. 470 Ma, 650 Ma, and 960–980 Ma that suggest affinity with the Pearya terrane, and age probability peaks of ca. 1800 Ma and 2700 Ma that indicate connections to the Laurentian margin. The combination of these signatures in Kulutingwak Formation rocks suggests that the Pearya terrane was proximal to the northern Laurentian margin by Late Ordovician time. Silurian and younger strike-slip displacement on the major fault zones resulted in the incorporation of blocks derived from the Pearya terrane basement and Silurian clastic rocks into the Kulutingwak Formation. Silurian displacement along these strike-slip faults, which are integral components of the Canadian Arctic transform system, is recorded by syndepositional deformation structures in the Danish River Formation and prevented the transition from soft to hard collision of the Pearya terrane. The two-stage model for the Pearya terrane—accretion followed by significant translation—provides a process for developing complex steep terrane boundaries with contentious displacement histories that are common in accretionary orogens.
Research Article| June 01, 2002 First evidence for ultrahigh-pressure metamorphism in the North- East Greenland Caledonides Jane A. Gilotti; Jane A. Gilotti 1Department of Geoscience, University of Iowa, Iowa City, Iowa 52242, USA Search for other works by this author on: GSW Google Scholar Erling J. Krogh Ravna Erling J. Krogh Ravna 2Department of Geology, University of Tromsø, N-9037 Tromsø, Norway Search for other works by this author on: GSW Google Scholar Author and Article Information Jane A. Gilotti 1Department of Geoscience, University of Iowa, Iowa City, Iowa 52242, USA Erling J. Krogh Ravna 2Department of Geology, University of Tromsø, N-9037 Tromsø, Norway Publisher: Geological Society of America Received: 22 Nov 2001 Revision Received: 21 Feb 2002 Accepted: 04 Mar 2002 First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (2002) 30 (6): 551–554. https://doi.org/10.1130/0091-7613(2002)030<0551:FEFUPM>2.0.CO;2 Article history Received: 22 Nov 2001 Revision Received: 21 Feb 2002 Accepted: 04 Mar 2002 First Online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn Email Permissions Search Site Citation Jane A. Gilotti, Erling J. Krogh Ravna; First evidence for ultrahigh-pressure metamorphism in the North- East Greenland Caledonides. Geology 2002;; 30 (6): 551–554. doi: https://doi.org/10.1130/0091-7613(2002)030<0551:FEFUPM>2.0.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 The first evidence for ultrahigh-pressure (P) metamorphism in the Greenland Caledonides is reported from kyanite eclogites and associated host gneisses on an island in Jøkelbugt. Polycrystalline quartz inclusions in garnet and omphacite exhibit palisade quartz rims that are a diagnostic feature of quartz pseudomorphs after coesite, thus providing textural evidence for ultrahigh-P conditions. Geothermobarometry on the mineral assemblage garnet + omphacite + kyanite + quartz and/or coesite ± phengite confirms the microstructural interpretation of ultrahigh-P metamorphism. Peak pressure and temperature conditions (∼972 °C at 3.6 GPa) are well within the coesite stability field. The host gneisses are more retrograded than the kyanite eclogites and only record high-P conditions of 2.5 GPa at 826 °C; however, garnet contains polycrystalline quartz inclusions with radial fractures, suggesting that the gneisses also were subject to ultrahigh-P conditions. The presence of high- and ultrahigh-P metamorphism along the Laurentian and Baltica margins, the high temperatures recorded by the ultrahigh-P terranes in Greenland and Norway, and the absence of mantle peridotites in Greenland suggest that crustal thickening may have played an important role in the formation of an extensive orogenic root in the Greenland and Scandinavian Caledonides. 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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