Jane A. Gilotti

Pressure-temperature evolution of retrogressed kyanite eclogites, Weinschenk Island, North-East Greenland Caledonides

Abstract Caledonian eclogites and associated high-pressure rocks are present in a 400×100-km long belt along the coast of North–East Greenland. Kyanite and clinozoisite eclogites on Weinschenk Island (77°54′N, 21°17′W) occur as lenses within amphibolite facies, quartzofeldspathic gneisses. The eclogites have been extensively overprinted by lower pressure assemblages and exhibit a variety of coronitic and symplectitic reaction textures that record the exhumation path of the rocks. Textural relationships and mineral composition data indicate that the initial high-pressure, plagioclase-free assemblage was garnet+omphacite±kyanite±clinozoisite+rutile. Compositional growth zoning in garnet, together with distribution of mineral inclusions within it, record a prograde evolution from epidote–amphibolite facies progressing into the eclogite facies. Post-peak pressure reaction textures include clinopyroxene–plagioclase intergrowths after omphacite; the replacement of kyanite by sapphirine–plagioclase, spinel–plagioclase and corundum–plagioclase symplectites; and resorption of garnet by amphibole–plagioclase kelyphites. The replacement textures indicate that post-eclogite facies decompression took place while the rocks remained at elevated temperatures (upper amphibolite/granulite facies conditions). The metamorphic evolution of the eclogites is in agreement with eclogite formation by thick-skinned crustal imbrication and subhorizontal shortening of the Laurentian margin during Caledonian collision, exhumation to amphibolite facies by emplacement of the eclogite-bearing thrust sheet on the Laurentian margin and accompanying erosion, and final exhumation to the surface by post-Caledonian crustal thinning.

First evidence for ultrahigh-pressure metamorphism in the North- East Greenland Caledonides

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 Jokelbugt. 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.

Discovery of a medium-temperature eclogite province in the Caledonides of North-East Greenland

Basic and acid-intermediate bodies containing eclogite-facies parageneses were discovered during recent mapping in North-East Greenland. Scattered throughout an orthogneiss complex, eclogitic pods extend from approximately lat 76°409N to the northern limit of the area mapped (78°N), a province comparable in size to the eclogite-bearing Western Gneiss region of Norway. The eclogites are the medium-temperature type typically formed in overthickened continental crust during continent-continent collision. An Early Proterozoic event cannot be ruled out, but field relations and regional correlations suggest that the high-pressure metamorphism may be Caledonian. Paleozoic eclogites in Greenland would negate current tectonic models for Caledonian collision that envision simple subduction of Baltica beneath Laurentia, while Early Proterozoic eclogites would be the oldest bona fide eclogites known.

The foreland-propagating thrust architecture of the East Greenland Caledonides 72°–75°N

Systematic geological mapping of the East Greenland Caledonides demonstrates that the orogen is built up of WNW-directed thrust sheets displaced across foreland windows. The foreland windows in the southern half of the orogen are characterized by a thin (220–400 m) Neoproterozoic to Lower Palaeozoic succession, structurally overlain by two major Caledonian thrust sheets (Niggli Spids and Hagar Bjerg Thrust Sheets). The metasediments of the upper-level Hagar Bjerg Thrust Sheet host 940–910 Ma granites and migmatites formed during an early Neoproterozoic thermal or orogenic event, as well as Caledonian 435–425 Ma granites and migmatites. The uppermost unit of the thrust pile, the Franz Joseph Allochthon, comprises a very thick (18.5 km) Neoproterozoic to lower Palaeozoic sedimentary succession (Eleonore Bay Supergroup, Tillite Group, Kong Oscar Fjord Group). Total westward displacement of the thrust sheets was about 200–400 km, with shortening estimated at 40–60%. Major extensional faults post-date thrusting. Restoration of the thrust sheets indicates that the sequence of Caledonian orogenic events now preserved in East Greenland was initiated several hundred kilometres ESE of present-day East Greenland, as Baltica and its marginal assemblage of Early Palaeozoic accretions began to impinge on the Laurentian margin.

Characteristics of, and a Tectonic Model for, Ultrahigh-Pressure Metamorphism in the Overriding Plate of the Caledonian Orogen

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.

Late-stage extensional exhumation of high-pressure granulites in the Greenland Caledonides

New ages from high-pressure (P) granulites in Payer Land, East Greenland, suggest that exhumation of the deepest crustal levels occurred during extensional collapse late in the collisional history of the Caledonian orogen. The ages set limits on the time of movement on the Payer land detachment fault, which places a little-deformed low-grade metasedimentary sequence on the granulite gneisses. U-Pb SHRIMP (sensitive high-resolution ion microprobe) analyses of metamorphic rims on zircon from high-P (1.4-1.5 GPa) metapsammite yield a weighted mean 2 0 6 Pb/2 3 8 U age of 403 ′ 5 Ma. Zircon analyses from a high-P kyanite-bearing melt pod derived from metapelites yield a weighted mean 2 0 6 Pb/ 2 3 8 U age of 404 ′ 4 Ma. The high-P metamorphism is distinctly younger than the extension associated with generation of 430-424 Ma leucogranites and migmatites at mid-crustal levels. Exhumation of deep-level rocks in Payer Land some time after 405 Ma is roughly coeval with exhumation of eclogite terranes in the Scandinavian Caledonides. Orogenic collapse in the Caledonides appears to have taken place in two stages: synorogenic extension dated by leucogranites, followed 24-30 m.y. later by late orogenic exhumation of the deepest levels. The history of this ancient, well-exposed collisional orogen is an analogue for evolution of the Himalayan system and perhaps offers a predictive model for future exhumation of high-P rocks currently forming at the base of the overthickened Himalayan crust.

Strain softening induced ductile flow in the Särv thrust sheet, Scandinavian Caledonides

Abstract The external zone of the Scandinavian Caledonides is characterized by thrust sheets composed of sedimentary rocks derived from the continent Baltica. The Sarv thrust sheet is unique amongst them because the 4.5–6 km thick, flat-lying Tossasfjallet Group clastics were intruded by a sub-vertical tholeiitic dike swarm prior to thrusting. The Sarv thrust sheet is a large horse (100 km long and 80 km wide) with a classical ramp-flat geometry. The original intrusive relationships of the dikes are preserved throughout most of the thrust sheet and deformation is concentrated at the base in a mylonite zone several tens of meters thick. In this zone, meter-thick dikes rotate into parallelism with the basal thrust and concordant mylonites of both lithologies are intercalated on a centimeter to millimeter scale. The overall rigid block geometry of the Sarv thrust sheet, and the intense strain gradient recorded by the rotation of the dike swarm and the concurrent development of mylonites in the thrust zone, suggest that thrust emplacement took place by ductile yield and on-going pseudoplastic flow. Continual strain softening localized ductile flow in this zone by means of grain-size reduction, reaction-enhanced ductility and grain-boundary sliding — processes inferred from the microstructures.

Leucogranites and the Time of Extension in the East Greenland Caledonides

Leucogranites provide timing estimates for deformation, particularly extension, in many orogens. On this basis, the time of extension in the East Greenland Caledonides has been estimated at 420–430 Ma. U‐Pb SHRIMP (sensitive, high‐resolution ion‐microprobe) analysis of zircon and monazite from selected leucogranites was undertaken to establish the age of the Payer Land detachment zone, an extensional structure that places low‐grade metasedimentary rocks on granulite facies gneisses that record high‐pressure (1.4–1.6 GPa) metamorphism at ca. 405 Ma. Deformed equigranular granites associated with upper plate lithologies give ages of \documentclass{aastex} \usepackage{amsbsy} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{bm} \usepackage{mathrsfs} \usepackage{pifont} \usepackage{stmaryrd} \usepackage{textcomp} \usepackage{portland,xspace} \usepackage{amsmath,amsxtra} \usepackage[OT2,OT1]{fontenc} \newcommand\cyr{ \renewcommand\rmdefault{wncyr} \renewcommand\sfdefault{wncyss} \renewcommand\encodingdefault{OT2} \normalfont \selectfont} \DeclareTextFontCommand{\textcyr}{\cyr} \pagestyle{empty} \DeclareMathSizes{10}{9}{7}{6} \begin{document} \landscape

Kinematic stratification in the hinterland of the central Scandinavian Caledonides

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