RESEARCH Age, Structural Setting, and Exhumation of the Liverpool

RESEARCH

Age, structural setting, and exhumation of the Liverpool Land eclogite terrane, East Greenland Caledonides

Lars Eivind Augland, Arild Andresen, and Fernando Corfu DEPARTMENT OF GEOSCIENCES, UNIVERSITY OF OSLO, P.O. BOX 1047, BLINDERN, 0316 OSLO, NORWAY

ABSTRACT

The close spatial relationship between Devonian high-pressure rocks (eclogites) and Ordovician–Silurian calc-alkaline plutonic rocks, as observed in Liverpool Land, NE Greenland, is not easily explained by existing tectonic models for the Caledonide orogen. New ﬁ eld stud- ies and isotope dilution–thermal ionization mass spectrometry U-Pb geochronology demonstrate, however, that the association is just coincidental, because the two rock groups are located within distinct terranes separated by a composite structure. The major element is the Gubbedalen shear zone, a N-dipping shear zone dominated by a penetrative top-up-to-the-S ductile fabric. Superimposed brittle-ductile top- down-to-the-N shear zones are typical of the structurally uppermost part of the shear zone. The contact against the hanging wall is the N-dipping, brittle Gubbedalen extensional detachment fault. A zircon age of 399.5 ± 0.9 Ma for an eclogite body is interpreted to represent the time of high-pressure metamorphism of the footwall. The host gneiss was migmatized between ca. 388 Ma and ca. 385 Ma, as constrained by the ages of a pegmatite predating migmatization and crosscutting granites. Coeval synkinematic granites intrude along amphibolite-grade, top-to-the-S high-strain zones in the Gubbedalen shear zone. Juxtaposition of the Ordovician–Silurian plutonic terrane (hanging wall) against the Early to mid-Devonian eclogite terrane (footwall) is best explained by a tectonic model involving early mid-Devonian buoyancy-driven exhumation followed by late mid-Devonian syncontractional extension related to thrusting on the Gubbedalen shear zone in a dextral strike-slip zone. Subsequent exhumation through the brittle-ductile transition occurred by extension on early semiductile structures and the overprinting Gubbedalen extensional detachment fault, and erosion.

LITHOSPHERE; v. 2; no. 4; p. 267–286. doi:10.1130/L75.1

INTRODUCTION 1935; Hansen and Steiger, 1971), the coexistence are: (1) an autochthonous to parautochthonous of which could not be accommodated by simple Archean to Paleoproterozoic basement with a The timing and kinematics of high-pressure tectonic models. We solved the conundrum by variably preserved cover of Neoproterozoic to (HP) metamorphism, exhumation, and crustal discovering the importance of a major structural Silurian sediments; (2) a far-traveled thrust sheet deformation are critical elements for the devel- boundary, investigated through fi eld work and (Niggli-Hagar thrust sheet) composed, from opment of tectonic models for collisional oro- U-Pb isotope dilution–thermal ionization mass the base upward, of Archean and Proterozoic gens like the Caledonides. Other important spectrometry (ID-TIMS) geochronology. In this gneisses, high-grade Mesoproterozoic supra- factors in such reconstructions include the rela- paper, we describe the major N-dipping compos- crustal rocks (Krummedal Sequence), and a tionships between crustal blocks of different ite shear zone and fault separating the Devonian thick package of Neoproterozoic to Ordovician affi nities and origins brought together by strike- eclogite terrane in the footwall from a terrane of sedimentary rocks (Eleonore Bay Supergroup, slip faulting and/or thrusting or extensional fault- predominately Late Ordovician to Silurian plu- Tillite Group, and Kong Oscar Fjord Group); ing. The Caledonides of Scandinavia and North tons in the hanging wall, report U-Pb ages that and (3) late orogenic Devonian continental East Greenland represent the two fl anks of the date the main steps in the evolution of the eclog- deposits in fault-controlled basins (Haller, 1971; Silurian–Devonian collisional orogen between ite terrane and the shear zone, and describe the Larsen and Bengaard, 1991; Higgins et al., 2004; Baltica and Laurentia, where the remains now role of the shear zone in the exhumation of the Andresen et al., 2007). A major unconformity are exposed in a series of nappes and crustal Liverpool Land eclogite terrane. separates the Middle Devonian deposits from blocks on the two Atlantic margins (Haller, 1971; the underlying folded and faulted Neoprotero- Roberts and Gee, 1985). The study of the rela- GEOLOGICAL SETTING zoic to Ordovician sedimentary rocks (Larsen tionships between the various components of the and Bengaard, 1991; Larsen et al., 2008). Silu- Caledonides is never quite straightforward and East Greenland Caledonides rian leucogranites (ca. 435–425 Ma) intrude the over the years has generated many controversies Krummedal Sequence and the lower part of the and debates. In our study, we address a contro- The Late Silurian to Devonian continent- Neoproterozoic sequence (Watt et al., 2000; versial situation in the southern part of Liverpool continent collision between Baltica and Lau- Hartz et al., 2001; Kalsbeek et al., 2001a, 2001b; Land, in the East Greenland Caledonides. The rentia produced the Caledonian orogen, the White et al., 2002; Leslie and Nutman, 2003; fundamental problem at the outset of the study remnants of which presently straddle both sides Andresen et al., 2007). Kalsbeek et al. (2001a, was an apparent paradox between the spatial of the North Atlantic Ocean (Haller, 1971; Rob- 2001b, 2008) argued that most of these leuco- association of Devonian eclogites (Hartz et al., erts and Gee, 1985). The main tectonic elements granites are S-type granites, derived by anatexis 2005) and Silurian plutonic complexes (Kranck, of the East Greenland Caledonides (Fig. 1) of pelitic units within the Krummedal Sequence.

Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/2/4/267/3038294/267.pdf by guest on 25 September 2021 AUGLAND ET AL.

All these units are crosscut by two major, late to postorogenic, low-angle, E-dipping, top-to- A Nørreland Window POST-CALEDONIAN the-E extensional faults: the Boyd-Bastionen 78°N 78 Upper Paleozoic and Mesozoic N sedimentary/volcanic rocks detachment and the Fjord Region detachment (Fig. 1), subdividing the region into three Devonian sedimentary rocks crustal-scale fault blocks (Hartz and Andresen, North East 1995; Andresen et al., 1998; Hartz et al., 2000, NIGGLI-HAGAR THRUST SHEET Greenland Neoproterozoic (Eleonore Bay 2001). Major sinistral strike-slip shearing also Dronning Louise Land eclogite province Supergroup and Tillite Group) accompanied continent-continent collision and and early Paleozoic rocks postcollisional extension. This led to regions of 76°N 76 (Kong Oscar Fjord Group) bulk transpression and transtension, and associ- Mesoproterozoic (Krummedal ated extrusion and pull-apart structures (i.e., the Sequence) Western fault zone and the Storstrømmen shear Greenland Storstrømmen shear zone zone; Holdsworth and Strachan, 1991; Larsen Reworked Archean and ice cap Fjord region Proterozoic basement and Bengaard, 1991; Seranne, 1992; Torsvik et detachment al., 1996; Krabbendam and Dewey, 1998; Smith LIVERPOOL LAND ECLOGITE TERRANE et al., 2007; Steltenpohl et al., 2009). 74°N 74 In a slightly modiﬁ ed tectonostratigraphic Inferred present-day thrust front scheme proposed by Higgins et al. (2004), the CALEDONIAN FORELAND Niggli-Hagar thrust sheet is subdivided into A A’ Archean and Paleoproterozoic two tectonic units: the Niggli Spids and Hagar gneisses w/cover Bjerg thrust sheets. The nature of major faults Boyd-Bastionen and shear zones separating different units in the detachment ? ? High-angle extensional fault southern East Greenland Caledonides is debat- 72°N 72 able, as evidenced by conﬂ icting interpretations Thrust of maps and cross sections presented in Hig- Liverpool gins et al. (2008). We follow the view of Haller Ice Land Low-angle detachment Western fault zone (1971) and Andresen et al. (2007), who inter- RenlandB B’ preted structural repetitions within the Niggli- Milne LandScoresby Sun Hagar thrust sheet to be a result of large-scale 100 km recumbent folding. 30°W 25°W d

Liverpool Land B PETERMANN BJERG SUESS LAND RØDEBJERG Liverpool Land is an isolated area composed AA’Allochthon of pre-Carboniferous rocks in the southern part of the East Greenland Caledonides, and it is separated from the main outcrop area of Cale- Autochthon Parautochthon donian rocks to the west by a cover of Permian and Mesozoic sediments (Jameson Land Basin; Figs. 1 and 2). The link to the tectonostratig- B B’ raphy established farther west is therefore not Jameson Land Liverpool Land straightforward. Partly migmatized metasedi- basin Hurry Inlet plutonic mentary rocks interpreted as remnants of the terrane Krummedal Sequence (Higgins, 1988; Johnston ? ? Gubbedalen shear zone et al., 2009) have, however, been taken to indi- ? Liverpool Land eclogite terrane cate that the rocks constituting northern Liver- pool Land represent an eastward continuation of Figure 1. (A) Simplifi ed geological map of the main lithotectonic units of the East Greenland Cale- the Niggli-Hagar thrust sheet. donides. (B) Representative profi les. Rectangle indicates the Liverpool Land study area of Figure 2. Previous investigations indicated that most The fi gure is modifi ed from Andresen et al., 2007. of Liverpool Land is dominated by various types of intrusions, most of which were hypoth- esized to be Caledonian in age (Kranck, 1935; Sahlstein, 1935; Cheeney, 1985). Based on alogical information, whereas Buchanan (2008) Hansen and Steiger, 1971). The most prominent quartz exsolution in clinopyroxene from one documented minimum pressures of ~18 kbar intrusion is the multiphased, Late Ordovician to of the eclogite lenses near Kap Hope (Fig. 2), at average temperature of ~870 °C for one of Silurian Hurry Inlet composite pluton (Fig. 2; Smith and Cheeney (1980) argued for high- the eclogite lenses. Muscovite 40Ar/39Ar data Kranck, 1935; Augland et al., 2009; Corfu and pressure metamorphism of rocks in the area. from the host gneiss of the eclogite lenses gave Hartz, 2011). Eclogite-bearing gneisses occur More recently, Hartz et al. (2005) suggested that an age of ca. 379 Ma, which was interpreted to the south of the Hurry Inlet composite plu- metamorphism had reached conditions of >25 to date cooling through the muscovite closure ton and its hosting paragneisses (Kranck, 1935; kbar and 800 °C, but they presented no miner- temperature (Bowman, 2008). The Hurry Inlet

268 www.gsapubs.org | Volume 2 | Number 4 | LITHOSPHERE

Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/2/4/267/3038294/267.pdf by guest on 25 September 2021 Age, structural setting, and exhumation of the Liverpool Land eclogite terrane | RESEARCH

composite pluton was interpreted to be intrusive A Liverpool Land POST CALEDONIAN into the eclogite-bearing gneisses by Coe and Cheeney (1972) and Coe (1975), thus requir- N Upper Paleozoic to ing the high-pressure metamorphism to be older Mesozoic rocks than intrusion of the Hurry Inlet composite pluton. Hansen and Steiger (1971) obtained an HURRY INLET PLUTONIC TERRANE imprecise Silurian Rb/Sr biotite age from the Hurry Inlet composite pluton and considered Krummedal Sequence it to be post-tectonic. These interpretations are difﬁ cult to reconcile with the Devonian age pro- Triaselv granite Carlsberg Fjord posed by Hartz et al. (2005) for eclogitization. To shed more light into these questions, we Hodal Storefjord remapped the transition between the Hurry Inlet pluton composite pluton and Liverpool Land eclogite terrane and show it to be a complex shear 71 ˚10’ N Hurry Inlet composite zone and fault with both subhorizontal contrac- pluton tion and extension. The main element of this Jameson high-strain zone is a N-dipping shear zone, a Land Storefjord fundamental feature with a long-lived displace- basin Mariager Fjord ment history, named the Gubbedalen shear zone undifferentiated gneisses (Fig. 2). Superimposed on the shear zone, there is the brittle Gubbedalen extensional detach- High-angle extensional ment fault (Fig. 2). fault

Hanging-Wall Rocks Gubbedalen extensional In our study area (Fig. 2), the hanging wall 40 detachment fault is dominated by the Hurry Inlet composite plu- 70 ˚50’ N ton and its host paragneisses, but it also includes Gubbedalen shear zone the pyroxene-bearing monzodioritic Hodal- Storefjord Pluton, the Triaselv Leucogranite,

LIVERPOOL LAND ECLOGITE TERRANE and other granitoid plutons (Coe and Cheeney, 1972; Augland et al., 2009). The hanging-wall

Hurry Inlet plutonic rocks have an age range of 445–425 Ma Felsic migmatite (Augland et al., 2009; Corfu and Hartz, 2011). gneisses Except for the Triaselv Leucogranite, all these Mafic (eclogite) and granitoids are amphibole- and partly pyroxene- 34 28 ultramafic lenses bearing and have arc-granitoid geochemical LEA 06-58 40 signatures (Augland et al., 2009). Similarities 40 Granitic sheets and 12 are also noted between the plutonic rocks that LEA 08-47 AA 06-56 small plutons LEA 06-87 30 intrude the high-grade paragneisses of north- d Sample locations ern Liverpool Land and similarly aged plutonic Kap Hope Scoresbysun rocks on Renland farther to the west (Kalsbeek et al., 2008; Augland et al., 2009; Rehnström, 20 km 22 ˚10’ W 22 ˚40’ W 2010). The plutons and paragneisses are cut by a swarm of N-striking Late Permian lamprophyre B Gubbedalen extensional detachment fault Gubbedalen shear zone dikes (Buchanan, 2008). The host paragneisses underwent Cale- NS1640 Ma 445 Ma 383 Ma 400400 MMaa donian high-temperature metamorphism that Hurry Inlet 438 Ma locally transformed them into migmatites plutonic terrane ~425 Ma (Johnston et al., 2009). Impure marbles occur 1645 Ma locally, and primary sedimentary structures are preserved in some quartzites. These supracrustal 385385 MaMa Liverpool Land rocks have been correlated with the Mesopro- eclogite terrane terozoic Krummedal Sequence (Higgins, 1988), and a study of detrital zircons from the paragneisses supports this interpretation (Johnston et Figure 2. (A) Simpliﬁ ed geological map of Liverpool Land (location in Fig. 1), and (B) schematic al., 2009). It is important to note that there is proﬁ le across the Gubbedalen shear zone. Ages in black boxes are reported here. Ages in white boxes are reported in Augland et al. (2009, 2010). Selected representative foliation measurements no evidence of mid-Devonian high-grade meta- are plotted (see also Fig. 4B). The geology of areas north of Storefjord and east of the village of morphism (Johnston et al., 2009). Titanite from Scoresbysund is based on the map by Coe and Cheeney (1972). the Hurry Inlet composite pluton yields U-Pb

LITHOSPHERE | Volume 2 | Number 4 | www.gsapubs.org 269

Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/2/4/267/3038294/267.pdf by guest on 25 September 2021 AUGLAND ET AL.

ages of ca. 420 Ma (Corfu and Hartz, 2011), deformation (see geochronology section). There to undeformed or weakly deformed granitic and fi ssion-track data from titanites from a high is an older generation of pegmatites emplaced sheets appearing structurally lower down. The level in the batholith gave an age of ca. 413 Ma in boudin necks and as dikes cutting the mafi c lower boundary of the Gubbedalen shear zone for cooling below ~285 °C (Gleadow and boudins (Figs. 3A–3C). These pegmatites can- is gradational and marked by a systematic Brooks, 1979; Jacobs and Thomas, 2001). Mus- not be traced from the mafi c boudins into the downward decrease in the frequency and thick- covite 40Ar/39Ar data from a paragneiss yields a surrounding migmatite gneiss, i.e., they do not ness of top-up-to-the-S ductile shear zones younger cooling age of ca. 381 Ma (Bowman, crosscut the migmatite orthogneiss foliation. (Figs. 5A–5D). Widely spaced top-up-to-the-S 2008), but this sample was located some 20 km The pegmatites often contain large amphibole shear zones parallel to the Gubbedalen shear from the sample studied for fi ssion tracks and crystals (~3 cm). A younger, more widespread zone are present throughout the entire eclog- probably refl ects a position deeper in the crust population of crosscutting sheets and some ite terrane. In the transition from the footwall at ca. 400 Ma. small, medium-grained plutons of granites, not toward the Gubbedalen shear zone, the gran- affected by the foliation-forming event, intrude ite sheets are swept into shear planes parallel Footwall Rocks both the migmatite orthogneiss and the mafi c to the mylonite foliation in the Gubbedalen The structurally upper part of the footwall boudins (Figs. 3C and 3G). shear zone (Fig. 2), but away from the shear block consists of eclogite-bearing migmatitic In the footwall, there is no evidence of the zone, the granite is generally undeformed. The granitoid (ortho-) gneisses with small and large Late Ordovician–Silurian magmatism observed N-trending lineations also become less promi- lenses of amphibolite, eclogite, and rare ultra- in the hanging wall. nent with increasing distance downward from mafi tes (garnet-peridotite, serpentinite, and the shear zone. pyroxenite; Augland et al., 2010). The mafi c Gubbedalen Shear Zone In addition, the granite sheets in the Gubbe- and ultramafi c lenses largely appear as strati- dalen shear zone are recrystallized, having form boudins parallel to the foliation, varying in The Gubbedalen shear zone is an ~400-m- K-feldspar with core-mantle textures and myr- size from a few centimeters to several hundred thick N- to NE-dipping, ductile shear zone mekite growth, both indicating that shearing took meters (Figs. 3A and 3B). Based on these fi eld deforming rocks of the footwall (Fig. 2). Based place under amphibolites-facies metamorphic relations, the bimineralic (omphacite + garnet) on descriptions of sheared rocks in the south- conditions (Figs. 5C–5E) (cf. Vidal et al., 1980; nature, and the lack of hydrous phases in the least eastern part of Liverpool Land (Cheeney, 1985), Gapais, 1989; Gates and Glover, 1989; Simpson retrogressed eclogites (see following), they are we interpret the shear zone to continue in a and Wintsch, 1989; Tribe and D’Lemos, 1996; interpreted as original mafi c dikes and sills. The southeastern direction in this part of Liverpool Stipp et al., 1999; Stipp, 2002). The growth ultramafi c lenses are generally serpentinized, but Land (Fig. 2). The Gubbedalen shear zone is of garnets in the same rocks is consistent with they locally contain relics of the original miner- characterized by a foliated and lineated fi ne- recrystallization under amphibolites-facies con- alogy (olivine, orthopyroxene, clinopyroxene, grained quartzo-feldspathic mylonite gneiss, ditions. Continued top-up-to-the-S contractional and rare garnet) (Augland et al., 2010). commonly with small feldspar augen. Less deformation under greenschist-facies conditions The migmatite is homogeneous on a scale of deformed lenses of amphibolite and of the origi- is evident from the presence of highly strained more than 25 km2 and is interpreted as a granit- nal migmatitic gneiss occur locally. Amphibole and locally fractured feldspar porphyroclasts, oid orthogneiss. It has well-defi ned leucosome porphyroblasts in the mylonite are locally com- subgrain rotation recrystallization in quartz, and melanosome domains, indicating extensive mon. Asymmetric feldspar porphyroclasts with development of grain shape fabric constituted partial melting (Fig. 4A). The leucosomes are core-mantle textures, S-C textures, shear bands by quartz subgrains (Figs. 5C and 5D), and dominated by K-feldspar and quartz with only (C′), and rotated relic foliation (of migmatite buckled biotite in the mylonite rocks (Gapais, ~10% plagioclase and minor biotite (Fig. 4F), relics within the shear zone: Fig. 5A) all indicate 1989; Kanaori et al., 1991; Stipp, 2002). whereas the melanosome is dominated by bio- top-up-to-the-S sense of shear. tite and plagioclase in addition to K-feldspar and The Gubbedalen shear zone is locally over- Extensional Structures quartz (Fig. 4E), and in some samples amphi- printed by younger brittle faults that cut and Superimposed on the top-up-to-the-S shear bole. Feldspars in the migmatite are recrystal- partly displace the ductile fabric. We have not fabrics in the upper part of the Gubbedalen shear lized by subgrain rotation, and quartz by grain found evidence, however, for large-scale rigid zone, there is an ~70-m-wide zone of top-down- boundary migration, indicating that the migma- body rotation of the ductile fabric. We therefore to-the-N extensional structures (Fig. 2). Because tites have been deformed at high temperature interpret the fabric geometry to be representative this zone is truncated by the brittle Gubbedalen subsequent to migmatization (Figs. 4E and 4F). of the original orientation and sense of shear. extensional detachment fault, its original thick- The leucosomes are locally oblate (“pancakes”) A pronounced downdip lineation within the ness is not known. The sense of shear is docu- and often folded. Locally, they have a prolate mylonitic foliation, defi ned by parallel-oriented mented by small-scale, listric, (semi-) ductile (cigar-shaped) form (Figs. 4C and 4D). The amphibole, quartz aggregates, and elongated extensional shear zones or faults overprinted by long axes in the prolate leucosomes, the fold feldspar porphyroclasts, trends N-S (Fig. 5F). phyllonites with S-C and C′ structures and nor- axes in the folded oblate leucosomes, and the Granite sheets intruded the Gubbedalen mal crenulation cleavage with associated asym- prominent quartz aggregate lineation all trend shear zone contemporaneously with top-to- metrically folded quartz veins (Figs. 6A–6D). approximately north-south (Figs. 4B–4D). In the-S shearing (Fig. 5A–5B). The granite The phyllonite layers and textures are again the southern part of the Liverpool Land eclogite sheets occur parallel to the mylonitic folia- overprinted by semibrittle top-down-to-the-N terrane, the leucosomes appear to have been fl at- tion and have an internal foliation parallel to extensional faults (Fig. 6D). Less common are tened only (pancakes), and the lineation is only the mylonitic foliation (Figs. 5A–5E). In one asymmetric folds and contractional faults, also weakly developed or absent, except in local case, we also observed part of a sheet sheared indicating top-down-to-the-N movement. The shear zones. into a megaporphyroclast (Fig. 5B). Except for Gubbedalen extensional detachment fault over- Two populations of dikes intruding the local growth of garnet and white mica, these prints all the other structures with a top-down- footwall allow us to bracket the early stages of sheets are mineralogically almost identical to-the-N displacement, bringing the Hurry Inlet

270 www.gsapubs.org | Volume 2 | Number 4 | LITHOSPHERE

A C

4040 cmcm D PPlagag AmphAmph CChlh BBtt RRtt Ilmm GGrtrt B

E Omph-inclOmph nc

GrtGrt DDi-Plag-P ag symplsymp

F G

Ilmlm OOmphmph RtRt GGrtrt

Figure 3. (A) Retro-eclogite lens in felsic migmatitic orthogneiss. Note 10 cm scale bar. (B) Nonretrogressed banded eclogite cut by amphibole-bearing pegmatites. Hammer for scale. (C) Maﬁ c boudin with pegmatites and crosscutting granite dikes. (D) Thin section image of retrogressed eclogite. Degree of retrogression is highest to the left, close to a completely amphibolitized vein. (E) Thin section image (crossed nicols) of retrogressed eclogite, sample LEA 06-59, dated in this study. Note omphacite inclusion in garnet. (F) Thin section image (crossed nicols) of nonretrogressed eclogite, sample LEA 06-61. The width of view in all microphotographs is 5.5 mm. (G) Undeformed tabular, dike-like granites cutting the foliation in the migmatite gneiss (outlined in white). Mineral abbreviations: Amph—amphibole; Bt—biotite; Chl—chlorite; Di—diopside; Grt—garnet; Ilm—ilmenite; Plag—plagioclase; Omph—omphacite; Rt—rutile; sympl—symplectite; incl—inclusion.

LITHOSPHERE | Volume 2 | Number 4 | www.gsapubs.org 271

Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/2/4/267/3038294/267.pdf by guest on 25 September 2021 AUGLAND ET AL.

A B N

Total Data : 99 Equal Area

C D

E Qtz Bt F K-Spar Qtz K-spar Plag Bt Plag

Figure 4. (A) Flattened migmatite leucosomes from the footwall south of the Gubbedalen shear zone. View on an almost 90° corner. Scale bar is 10 cm. (B) Poles to the dominant foliation (crosses) and lineation (contoured open circles) in the migmatite orthogneiss, plotted on a lower-hemisphere stereogram. Lineations are quartz aggregate lineations, mineral lineations, and leucosome stretching axes and fold axes. (C–D) Constrictionally strained leucosomes in the migmatite orthogneiss. C is taken parallel to the stretching direction; D is taken from above, i.e., perpendicular to C. (E–F) Thin section images of the migmatite gneiss: paleosome (E) and leucosome (F). Note the recrystallized quartz and feldspar. Width of both photomicrographs is 2.8 cm. Photomicrographs were taken with crossed nicols. Mineral abbreviations are: Qtz—quartz; Bt—biotite; K-spar—K-feldspar; Plag—plagioclase.

272 www.gsapubs.org | Volume 2 | Number 4 | LITHOSPHERE

A B

C D

QQtztz C′ S K-KK-spar-ssppar MMss QQtQtztz C′ KK-sparspar MsM

E N

Total Data : 33 Equal Area

Figure 5. Geological features from the lower part of the Gubbedalen shear zone. (A) Relict migmatite foliation is rooted into the shear zone concen- trated on a foliated granite vein. (B) “Megaporphyroclast” of granite outlined in white. S-planes (white lines) are defi ned by grain shape fabrics and mica. C planes are defi ned by the large arrows parallel to the upper and lower margin of the “megaporphyroclast.” (C–D) Thin section images of a recrystallized granite dike from the Gubbedalen shear zone. Photomicrographs (5.5 mm wide) were taken with crossed nicols. Note the S-C-C′ textures and the recrystallized K-feldspar and quartz. S planes are defi ned by mica fi sh and quartz grain shape fabric, C′ planes are defi ned by recrystallized mica and fi ne-grained quartz. C planes are parallel to the large arrows. Also note the feldspar porphyroclast with the semi-sigma shape to the left in D. South is to the right in A, B, C, and D, and the shear sense is consistently top-up-to-the-S. (E) Myrmekite growth in K-feldspar of a recrystallized granite dike from the Gubbedalen shear zone (crossed nicols; width is 0.5 mm). (F) Poles to the mylonite foliation (crosses) and lineation (contoured open circles) in the Gubbedalen shear zone plotted on a lower-hemisphere stereogram. Inset shows photo of quartz aggregate lineation. K-spar—K- feldspar; Qtz—quartz; Ms—muscovite.

LITHOSPHERE | Volume 2 | Number 4 | www.gsapubs.org 273

Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/2/4/267/3038294/267.pdf by guest on 25 September 2021 AUGLAND ET AL.

A B

C D

5 m

Figure 6. (A) Top-down-to-the-N (left) ductile shear zone with drag folds overprinting top-up-to-the-S (right) kinematic indicators in the upper part of the Gubbedalen shear zone. Note lens cap for scale. (B) Drag folds associated with top-down-to-the-N shear sense. Semiductile listric extensional structure in the upper part shows the same sense of shear. Pencil for scale. (C) Phyllonite layer with quartz veins superimposed by crenulation cleavage. Vergence of folds and S-C textures show top-down-to-the-N shear sense. Pencil for scale. (D) Semibrittle faults in alternating quartzo-feldspathic and mica-rich layers at the top of the exposed Gubbedalen shear zone show top-down-to-the-N displacement. North is to the left in all photos.

274 www.gsapubs.org | Volume 2 | Number 4 | LITHOSPHERE

composite pluton and its host paragneisses in ) contact with Gubbedalen shear zone and the Liverpool Land eclogite terrane. continued ( COMPOSITION OF PYROXENE IN ECLOGITE AND RETRO-ECLOGITE

Most of the maﬁ c lenses in the footwall are amphibolites (Fig. 3C). They generally show textural and mineralogical evidence, such as diopside-plagioclase or amphibole-plagioclase symplectites (Figs. 3D and 3E), indicative of derivation via retrogression of eclogite-facies minerals (omphacite). Some garnet-pyroxenite lenses have preserved their eclogitic paragenesis (Figs. 3B and 3F) with only limited retrograde overprint. The least retrogressed eclogites are generally bimineralic (omphacite + garnet) and are often very garnet-rich (50%–70%). Microprobe analyses of adjacent garnet and clinopyroxene pairs in eclogite and retro- eclogite are given in Table 1. Analyses from sample LEA 06-61 are from the unzoned interiors of adjacent garnets and pyroxenes. The minerals are generally compositionally homogeneous, but the clinopyroxenes show some enrichment in Ca and Fe near the margins. Sym- plectites of plagioclase-diopside or amphibole, up to ~0.1 mm wide, commonly occur between garnets and omphacites. The eclogite has

omphacite with an average composition of Jd44

and a maximum of Jd46 (Table 1). Clinopyroxene in sample LEA 06-59 is retrogressed, and only the interiors of garnet are unzoned. The analyses on this sample were done on omphacite inclusions in unzoned garnet domains, measuring garnet spots close to the inclusions. There are no visible reaction textures between garnet and omphacite inclusions, so mineral equilibrium is assumed. The data yield jadeite compositions

up to Jd41, similar to those in LEA 06-61, cor- roborating the interpretation that all these rocks experienced high-pressure metamorphism. The variation in chemical composition of the analyzed omphacite inclusions is probably due to garnet growth at different pressures. The compositions of the minerals analyzed here are very similar to those in the eclogites studied by Buchanan (2008), and his sample CP-52A is from the same eclogitic body as TABLE 1. REPRESENTATIVE MICROPROBE ANALYSES OF GARNET AND CLINOPYROXENE IN RETROGRESSED ECLOGITE SAMPLES OF GARNET ANALYSES MICROPROBE 1. REPRESENTATIVE TABLE sample LEA 06-61. Buchanan (2008) provided quantitative pressure-temperature (P-T) estimates of minimum 18 kbar and ~870 °C 0.46 0.04 0.17 0.07 0.50 0.07 0.29 0.08 0.19 0.08 0.23 0.06 0.20 0.08 0.30 0.08 0.25 0.05 0.41 0.07 for eclogite-facies metamorphism. He also 0.04 0.02 0.02 0.04 0.00 0.00 0.01 0.02 0.01 0.04 0.00 0.02 0.00 0.00 0.00 0.01 0.00 0.01 0.02 0.00 12345678910 10.37 22.12 12.11 22.38 10.16 22.40 11.96 22.38 12.21 22.34 12.18 22.16 12.56 22.10 11.18 22.02 11.83 22.38 10.81 22.29 50.87 39.10 54.91 39.98 51.02 39.35 53.11 39.67 54.70 39.65 52.85 39.14 53.84 39.58 52.80 39.50 51.73 39.40 52.17 39.74 showed that the retrogressive P-T path went cpx grt cpx grt cpx grt cpx grt cpx grt cpx grt cpx grt cpx grt cpx grt cpx grt through granulite-facies conditions, as evidenced by secondary orthopyroxene and clino- 3 3 2

pyroxene, the appearance of symplectites with O 3.30 0.03 6.10 0.02 3.04 0.02 4.89 0.00 5.78 0.02 5.15 0.00 5.58 0.02 4.08 0.03 3.51 0.02 3.70 0.00 2 O 2 O 2 O 0.00 0.01 0.01 0.00 0.01 0.01 0.02 0.01 0.01 0.01 0.02 0.01 0.01 0.00 0.01 0.00 0.01 0.00 0.01 0.01 2 Ca-rich pyroxene and plagioclase, and Na-rich 2 Sample LEA 06-59 (70°34.939’N, 22°15.777W’); clinopyroxene inclusions in garnet Sample LEA TotalSiTi 99.39 100.5Al FeTotal 101.04Mg 101.29 1.86 100.19 0.01 100.45 2.95Ca 0.19 100.53Na 0.00 0.45 101.06 1.94 1.28K 101.05 0.00 0.55 1.97 2.99 101.02 0.13Xjd 0.00 0.94 0.50 99.45 0.71 1.85 1.17 100.33 0.23 0.01 0.45 1.97 100.34 0.82 2.97 0.17 0.00 100.71 0.00 0.00 0.89 0.43 23 100.95 0.56 1.90 1.19 0.00 100.20 0.42 0.01 0.57 1.99 0.94 100.68 2.97 0.16 0.00 100.60 0.00 0.00 0.92 0.50 0.75 101.23 1.93 1.19 0.00 0.21 100.90 0.00 0.47 1.98 0.91 2.98 41 0.14 0.00 0.00 0.00 0.90 0.51 0.62 1.91 1.19 0.00 0.34 0.01 0.45 1.98 0.93 2.96 0.15 0.00 0.00 0.00 0.91 0.52 0.56 1.92 1.22 21 0.00 0.40 0.01 0.47 1.97 0.93 2.98 0.14 0.00 0.00 0.00 0.90 0.53 0.59 1.88 1.19 0.00 0.36 0.01 0.45 1.96 0.93 2.99 0.15 0.00 34 0.00 0.00 0.92 0.47 0.57 1.86 1.17 0.00 0.39 0.01 0.53 1.96 0.92 2.97 0.18 0.00 0.00 0.00 0.90 0.50 0.67 1.86 1.22 0.00 0.28 0.01 40 0.51 1.99 0.96 2.98 0.16 0.00 0.00 0.00 0.90 0.46 0.68 1.17 0.00 0.25 0.54 1.97 0.90 0.00 0.00 0.91 36 0.71 0.00 0.26 0.94 0.00 0.00 0.00 39 28 25 26 K MnO 0.00 0.31 0.03 0.33 0.03 0.26 0.01 0.32 0.00 0.30 0.00 0.26 0.01 0.30 0.09 0.28 0.06 0.28 0.01 0.26 NiO TiO SiO Cr FeO 6.14 20.34 4.37 18.75 5.61 18.82 5.25 18.93 4.75 18.95 4.91 19.26 4.66 18.98 4.91 18.47 6.12 19.32 5.52 18.70 Na CaO 18.13 10.15 14.75 11.71 19.30 11.31 16.10 11.57 14.80 11.52 15.38 11.45 15.02 11.47 17.56 11.81 17.58 11.10 18.51 11.65 CrMn 0.00 0.00Total 0.00 0.00 0.00 0.02 0.00 4.00 0.00 0.00 8.00 0.02 0.00 4.00 0.00 0.00 8.00 0.02 0.00 4.00 0.00 0.00 8.00 0.02 0.00 4.00 0.00 0.00 8.00 0.02 0.00 4.00 0.00 0.00 8.00 0.02 0.00 4.00 0.00 0.00 8.00 0.02 0.00 4.00 0.00 0.00 8.00 0.02 0.00 4.00 0.00 0.00 8.00 0.02 4.00 0.00 8.00 0.02 4.00 8.00 Al MgO 10.07 8.36 8.56 8.01 10.52 8.21 8.88 8.08 8.61 8.10 8.72 7.99 8.47 8.18 10.03 8.00 9.59 8.05 10.09 8.18 Analysis no. amphibole. Determining the pressure of these

LITHOSPHERE | Volume 2 | Number 4 | www.gsapubs.org 275

Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/2/4/267/3038294/267.pdf by guest on 25 September 2021 AUGLAND ET AL.

rocks is difﬁ cult because there are no good geo barometers applicable to bimineralic eclogites, and uncertainties in the Fe2+/Fe3+ ratios in the analyzed minerals lead to uncertainties in the Fe-Mg exchange geothermometer of Krogh Ravna (2000) used by Buchanan (2008).

U-Pb GEOCHRONOLOGY 6 4 We analyzed zircon, rutile, and monazite from eclogite and different intrusive rocks (continued) obtained from a traverse across the Gubbedalen shear zone and into the footwall for U-Pb isotopes in an attempt to constrain the age of meta- 3

4 morphism, magmatism, and deformation in the rocks of Liverpool Land eclogite terrane. All

on microprobe at the Department of Geo- U-Pb analyses in this study were conducted by ID-TIMS at the Department of Geosciences, University of Oslo. The methodology is sum- ations: cpx—clinopyroxene; grt—garnet; jd—jadeite. 2 marized in Appendix A. 4

Eclogite

Field Relationship and Sample Description Sample LEA 06-59 is from a retrogressed 4 4 eclogite boudin surrounded by felsic migmatite orthogneiss. The rock consists of ~50% garnet, almost 50% symplectized omphacite (plagioclase, secondary clinopyroxene, and amphibole), and accessory rutile. Garnets con- Sample LEA 06-61 (70°34.633’ N, 22°15.223W’); adjacent clinopyroxene and garnet 06-61 (70°34.633’ Sample LEA 3 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.000.51 0.00 1.98 0.05 0.51 0.01 1.99 0.06 0.51 0.00 1.96 0.00 0.53 0.02 1.96 0.50 1.98 12345 tain omphacite inclusions (up to Jd , Table 1; 4 42 Fig. 3E). Interstitial rutile generally occurs together with ilmenite, which appears to pseudomorphically replace rutile (Fig. 7I). Rutile inclusions in garnet contain thin ilmenite lamellae (Fig. 7H).

Mineral Characteristics 9 0.01 0.01 0.02 0.03 0.00 0.00 0.01 0.00 0.00 0.01 0.00 0.03 Zircons from LEA 06-59 can be divided 3 in three main categories based on morphology, color, size, and internal textures: (1) small (<100 μm in diameter), clear, colorless, inclusion-free, well-rounded zircons (Fig. 7G); (2) medium-sized (100–300 μm in diameter), 8

2 subrounded, relatively clear, colorless zircons, with some inclusions; and (3) large (300 µm to ≥0.5 mm), metamict and inclusion-rich, 0 0 0 0.01 0 0.00 0.01 0.01 0.01 0.00 0.00 0.00 0.00 0.00 0.00 slightly brownish, elongated zircons. Cathodo- luminescence shows that many grains have a (continued) 9 0000 core-rim relationship (Fig. 7) showing dark 11.49 22.11 10.81 22.11 12.28 22.12 12.24 22.32 12.24 22.38 12.81 22.12 12.55 22.01 12.06 22.34 52.42 39.29 52.48 39.29 53.77 39.46 54.35 39.28 54.10 38.96 53.07 39.58 52.97 39.46 54.82 39.41 2 cpx grt cpx grt cpx grt cpx grt cpx grt cpx grt cpxcores grt and cpx bright grt rims (grain A) or the opposite

TABLE 1. REPRESENTATIVE MICROPROBE ANALYSES OF GARNET AND CLINOPYROXENE IN RETROGRESSED ECLOGITE SAMPLES OF GARNET ANALYSES MICROPROBE 1. REPRESENTATIVE TABLE (grains E and F). Grain C has a weakly zoned

Global Positioning System positions of the samples are given in parentheses. Analyses were obtained with a Cameca 19939 electr Global Positioning System positions of the samples are given in parentheses. core with a homogeneous outer rim. Grain B 3 3 0.54 0.08 0.36 0.08 0.26 0.07 0.19 0.06 0.20 0.08 0.31 0.09 0.24 0.07 0.16 0.05 2 O 4.27 0 3.98 0 5.65 0 6.20 0.00 6.43 0.03 6.11 0.01 6.19 0.00 6.74 0.02 2 has more complex zoning, with inner and outer O 2 O 2 O 0.01 d 2 Note: j 2 Analysis no. 11 12 13 Na 0.29 0 0.28 0 0.39 0 0.43 0.00 0.46 0.00 0.43 0.00 0.43 0.00 0.46 0.00 X TiO sciences, University of Oslo, using standard operating conditions of 15 kV acceleration voltage and 15 nA beam current. Abbrevi beam current. sciences, University of Oslo, using standard operating conditions 15 kV acceleration voltage and nA Cr Si 4.008.004.008.004.008.004.008.004.008.00 FeTotal 1.86 2.97Ca 0.16 1.89 1.17Total484848 2.98 0.16 0.67 1.91 1.2 0.95 2.98 0.14 0.68 1.19 0.91 1.93 0.58 2.96 0.94 0.13 1.94 1.08 2.94 0.12 0.55 1.93 1.04 0.97 2.97 0.13 0.53 1.90 1.11 1.00 2.98 0.13 0.55 1.94 1.11 0.95 2.96 0.12 0.57 1.18 0.97 0.54 0.98 MnO 0.02 0.27CaO 0.01 0.27 17.57 0.08 11.77 17.68 0.29 11.77 15.15 11.56 0.01 0.20 0.01 14.48 11.99 0.19 14.15 0.02 12.38 0.28 14.67 11.78 0.00 14.74 0.26 12.04 0.04 14.17 12.15 0.18 NiO TotalTi 101.4Al000000 100.1Cr 100.5Mn 100.13Mg 0.01 100.5 100.3 0 0.48K 1.97 0.52 0.01 0 0.46 0.91 000000 100.06 0 100.43 1.97 0.52 0.02 99.92 0.51 0.91 100.31 0.01 0 99.87 1.97 0.45 0 100.93 0.02 0.89 99.39 100.31 0 100.13 100.95 0.00 0.02 0.01 0.00 0.00 0.00 0.00 0.01 0.00 0.00 0.00 0.44 0.00 0.00 0.01 0.89 0.01 0.00 0.00 0.43 0.01 0.00 0.02 0.88 0.01 0.00 0.00 0.44 0.00 0.00 0.02 0.90 0.00 0.00 0.00 0.44 0.00 0.01 0.89 0.43 0.89 Al K FeOMgONa 5.31 18.53 9.81 5.43 8.08 18.53 9.75 4.83 8.08 18.89 8.49 7.94 4.31 18.59 4.43 8.26 18.45 7.95 4.64 8.36 19.06 7.83 4.50 8.18 18.56 7.99 3.92 8.19 18.76 7.90 8.21 8.00 SiO Sample LEA 06-59 Sample LEA bright rims. Grain D shows an irregular diffuse zoning throughout the grain. In grains B and E, the different domains are almost homogeneous, whereas in the rims of grains A and F, the texture is somewhat more heterogeneous.

276 www.gsapubs.org | Volume 2 | Number 4 | LITHOSPHERE

event represents eclogite metamorphism is sup- ported by: (1) the zircon morphology (Fig. 7G); (2) the clear core-rim and two- or multidomain relationships of the zircons, with structureless or irregularly zoned and patchy texture domains (Figs. 7A–7F); and (3) the low Th/U ratio of the completely recrystallized or newly grown zircons (Table 2), which is common in eclogite- facies zircons (Corfu et al., 2003; Hoskin and Schaltegger, 2003; Bingen et al., 2004). Fur- ther support for this interpretation is given by 10 µm A 20 µm B 10 µm C comparable ages obtained by Corfu and Hartz (2011) on other eclogite bodies from the area.

Analytical Results and Interpretation: Rutile Seven rutile fractions consisting of 20–50 grains or fragments yielded both discordant and concordant data, with two analyses actu- ally being reversely discordant. The latter have 206Pb/238U ages older than those of the eclogite- facies event. There is also some correlation with D 50 µm 10 µm E 20 µm F an increase in 207Pb/206Pb age. This suggests that the unusual discordance could be related to dis- G H I turbances of the system and fractionation of U Ilm and Pb, for example, due to only partial dissolu- Ilm tion of some rutile. The two most precise rutile Grt analyses, numbers 35 and 36, overlap, giving a σ Rut concordia age of 382.4 ± 0.8 Ma (2 ; MSWD = Rut 0.69; Fig. 8B). 40 µm 50 µm It is unclear how the development of ilmen- 100 µm ite inside rutile affected the isotopic behavior of Figure 7. (A–F) Cathodoluminescence images of zircons from dated eclogite sample LEA 06-59 rutile. Root et al. (2004) reported the existence showing complex zoning. (G) Microphotograph of concordant metamorphic zircon (399.5 ± 0.9 Ma), of ilmenite plates in rutile from eclogite, inter- either completely recrystallized or newly grown during eclogite-facies metamorphism. (H) Rutile preting them to refl ect exsolution after peak inclusion in garnet with ilmenite lamellae and a patch of ilmenite. (I) Rutile pseudomorph that is pressures. In our sample, exsolution lamellae partially replaced by ilmenite. are only found in nonretrogressed rutile inclusions in garnets (Fig. 7H), together with nonretrogressed omphacite, where the host represents The U content ranges from ~70 to 1000 ppm, nents. A line calculated through all 30 analyses the preserved relicts of the high-pressure assem- and Th/U ranges from 0.074 to 0.83 (Table 2). yields an upper-intercept age of 1601 ± 25 Ma, blage. By contrast, the interstitial rutile always Rutile grains selected for analysis were clear, but with a high MSWD value of 29. Due to the occurs together with pseudomorphically grow- brown, inclusion-free fragments of variable scatter of the data points, and the fact that there ing ilmenite (Fig. 7I), which may be interpreted size. Their U contents were low, at 27–5 ppm, are no points close to the upper-intercept age, it as a retroreaction product of decompression. with initial common Pb at <0.19 ppm and Th/U is not evident that this age is geologically mean- Thus, rutile may exsolve ilmenite during decom- <0.1 (Table 2). ingful. One option is that the scatter refl ects dif- pression from high-pressure conditions, but ferent zircon populations in the protolith. A sec- ilmenite may also grow from the rutiles where Analytical Results and Interpretation: Zircon ond alternative is that that the scatter is the result there is access to fl uids. Such mechanisms of Thirty different zircon fractions, composed of recent Pb loss, pulling some of the data points exsolution and retroreaction could lead to open- of 1–15 grains, yielded mainly discordant data down from an original mixing line. A third ele- system behavior and reequilibration of U and Pb (Fig. 8A; Table 2). Analysis number 13 is con- ment affecting the interpretation is given by the in rutile. This could explain why the rutile ages cordant; it represents eight very small (~50–60 fi ve least discordant analyses (11, 12, 13, 22, postdate the time of formation of the mineral µm), rounded, clear, inclusion-free zircons giv- and 27), which lie on a line projecting toward (as indicated by zircon). It could also explain ing a concordia age of 399.5 ± 0.9 Ma (mean an intercept age of 829 ± 240 Ma, suggesting why the fi eld-based determination of rutile Pb square of weighted deviates [MSWD] = 0.0084), that zircons in the protolith may have been closure temperature (400–450 °C; Mezger et al., which is interpreted to date eclogite metamor- affected by a Mesoproterozoic-Neoproterozoic 1989; Schmitz and Bowring, 2003) is so much phism (Fig. 8A). One imprecise data point event (dashed, gray error ellipses in Fig. 8). Evi- lower than the experimentally determined clo- (analysis 11) also plots on the concordia curve, dence for a Grenvillian-age signature has also sure temperature (~600 °C; Cherniak, 2000). In and supports the interpretation that 399.5 Ma is been found in zircons from a migmatite gneiss the former case, Schmitz and Bowring (2003) the crystallization age. All the discordant frac- from Liverpool Land dated by Corfu and Hartz noted exsolved and newly formed ilmenite in tions indicate the presence of inherited compo- (2011). The interpretation that the 399.5 Ma and associated with rutile, and hence their rutile

LITHOSPHERE | Volume 2 | Number 4 | www.gsapubs.org 277

Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/2/4/267/3038294/267.pdf by guest on 25 September 2021 AUGLAND ET AL.

TABLE 2. U-Pb ANALYTICAL DATA

206 207 σ No. Mineral characteristics Weight U Th/U Pbc Pbcom Pb Pb 2 (ug) (ppm) (ppm) (pg) 204Pb 235U (abs)

Eclogite, LEA 06-59 (70°34.939′N, 22°13.777′W) 1 13 small, anhedral, clear zircon, colorless 2 696 0.53 0.00 0.9 8343 0.8075 0.0053 2 8 large, anhedral, clear, colorless zircon 29 274 0.83 0.15 6.2 5827 0.6231 0.0019 3 15 rounded, clear, colorless zircon, variable size 13 70 0.35 0.09 3.2 1164 0.5049 0.0047 4 13 anhedral, slightly prismatic, colorless zircon 18 283 0.38 0.04 2.8 9370 0.7719 0.0040 5 8 large, anhedral, clear, colorless zircon 25 298 0.33 0.00 2.1 15,551 0.5895 0.0015 6 6 rounded, clear, colorless zircon 1 693 0.15 0.00 0.7 7337 1.3185 0.0057 7 1 large, anhedral, clear, colorless zircon 23 277 0.20 0.03 2.8 11,579 0.7510 0.0018 8 1 large, anhedral, clear, colorless zircon 14 271 0.51 0.08 3.1 6855 0.8681 0.0022 9 8 small, clear, subrounded, colorless zircon 5 343 Not m. 0.04 2.2 3414 0.5788 0.0122 10 12 large, clear, subrounded, colorless zircon 43 229 0.50 0.00 1.8 30,795 0.8676 0.0020 11 3 large, subrounded, colorless zircon, shady 1 127 Not m. 0.00 1.7 303 0.4729 0.0215 12 6 rounded, clear, colorless zircon, variable size 7 235 0.08 0.00 1.3 4956 0.4923 0.0014 13 8 rounded, clear, colorless zircon 10 151 0.07 0.18 3.8 1594 0.4821 0.0018 14 12 rounded, clear, colorless zircon, variable size 14 151 0.34 0.03 2.5 4283 0.7322 0.0028 15 1 large, anhedral, slightly prismatic, shady zircon 37 236 0.74 0.04 3.6 10,989 0.6260 0.0014 16 7 small, clear, rounded, colorless zircon 5 89 Not m. 0.00 1.6 1199 0.5648 0.0037 17 7 small, clear, rounded, colorless zircon 3 127 Not m. 0.00 1.1 1545 0.5889 0.0077 18 1 large, clear, colorless zircon fragment 19 225 0.41 0.00 1.8 12,040 0.7775 0.0020 19 8 clear, rounded, colorless zircon 14 238 0.62 0.00 1.8 12,067 1.0900 0.0026 20 10 small, clear, rounded, colorless zircon 2 303 Not m. 0.00 1.4 1926 0.5974 0.0078 21 5 small, clear, subrounded, colorless zircon 1 1003 0.44 0.00 1.1 4778 0.7942 0.0075 22 3 anhedral, slightly prismatic, colorless zircon 1 72 Not m. 0.00 0.9 331 0.5007 0.0126 23 8 small, clear, rounded, colorless zircon 1 343 Not m. 0.00 1.1 1415 0.5471 0.0041 24 7 small, clear, rounded, colorless zircon 1 665 Not m. 0.00 1.0 3023 0.6427 0.0049 25 1 small, clear, rounded, colorless zircon 1 101 Not m. 0.00 0.6 1261 1.3443 0.0395 26 6 small, clear, rounded, colorless zircon 3 204 Not m. 0.00 1.1 3765 1.1833 0.0055 27 6 small, clear, rounded, colorless zircon 1 465 Not m. 0.00 1.1 1727 0.5092 0.0034 28 5 large, clear, subrounded, colorless zircon 6 148 0.37 0.12 2.7 1611 0.6782 0.0031 29 5 small, clear, rounded, colorless zircon 1 367 Not m. 0.00 1.8 996 0.6571 0.0052 30 1 large subhedral prism zircon w/incl. 21 193 0.73 3.03 67.6 279 0.5662 0.0107 31 ~ 50 rutile fragm. Brown, clear, incl.-free 410 23 0.02 0.08 34.0 1088 0.4590 0.0226 32 ~ 20 rutile fragm. Brown, clear, incl.-free 12 16 0.01 0.02 2.3 333 0.4371 0.0095 33 ~ 40 rutile fragm. Brown, clear, incl.-free 3 14 Not m. 0.00 1.8 119 0.5352 0.0361 34 ~ 20 rutile fragm. Brown, clear, incl.-free 10 27 0.10 0.19 3.9 376 0.6356 0.0070 35 ~ 25 rutile fragm. Brown, clear, incl.-free 40 21 Not m. 0.05 4.0 822 0.4564 0.0032 36 ~ 35 rutile fragm. Brown, clear, incl.-free 53 22 0.001 0.03 3.8 1181 0.4565 0.0020 37 ~ 40 rutile fragm. Brown, clear, incl.-free 9 5 0.08 0.02 2.2 101 0.4694 0.0333

Pegmatite, LEA 06-18 (70°35.030′N, 22°13.838′W) 1 1 brown, metamict euhedral zircon, high aspect ratio 1 12743 0.12 0.24 2.3 21,737 0.4601 0.0025 2 1 colorless, euhedral zircon, high aspect ratio 1 16633 0.14 11.69 13.7 4588 0.4501 0.0017 3 1 brown, metamict euhedral zircon, high aspect ratio 1 21619 0.13 17.21 19.2 4374 0.4625 0.0132 4 1 brown, metamict euhedral zircon, high aspect ratio 1 21249 0.14 3.62 5.6 14,214 0.4501 0.0021

Granite dike/minor intr., LEA 06-62 (70°35.700′N, 22°17.952′W) 1 2 slightly yellow zircon tips 1 3274 0.32 12.66 14.7 813 0.4260 0.0020 2 1 euhedral zircon, high aspect ratio 1 2226 0.15 2.03 4.0 2111 0.4528 0.0017 3 2 large, euhedral, yellow zircon, high aspect ratio 7 1426 0.19 0.70 6.9 5439 0.4475 0.0015 4 3 small, euhedral, colorless zircon, high aspect ratio 1 9366 0.23 0.08 2.1 17,227 0.4618 0.0022 5 2 yellow, clear monazite fragment 8 1617 16.21 0.47 5.8 8636 0.4579 0.0024 6 2 large, euhedral, colorless zircon, low aspect ratio 1 3298 0.24 7.75 9.8 1291 0.4498 0.0021 7 2 yellow monazite fragments, w/incl. 1 6747 18.76 2.62 4.6 5645 0.4618 0.0029

Sheared granite dike, LEA 06-66 (GSZ; 70°35.779′N, 22°13.479′W) 1 1 large euhedral zircon, high aspect ratio 1 1329 Not m. 0.00 0.9 5652 0.4664 0.0059 2 1 small euhedral zircon, low aspect ratio 1 314 Not m. 0.00 1.5 927 0.5890 0.0169 3 2 small, clear zircon fragment 1 552 Not m. 0.69 2.7 804 0.4591 0.0042 (continued)

278 www.gsapubs.org | Volume 2 | Number 4 | LITHOSPHERE

TABLE 2. U-Pb ANALYTICAL DATA (continued ) No. 206Pb 2σ Rho 207Pb 2σ 206Pb 2σ 207Pb 2σ 207Pb 2σ Disc. 238U (abs) 206Pb (abs) 238U (abs) 235U (abs) 206Pb (abs) (5)

Eclogite, LEA 06-59 (70°34.939′N, 22°13.777′W) 1 0.08408 0.00072 0.70 0.06965 0.00043 520.5 4.3 601.1 3.0 918.1 12.7 45.1 2 0.07279 0.00018 0.89 0.06208 0.00009 453.0 1.1 491.8 1.2 676.8 3.0 34.2 3 0.06477 0.00032 0.59 0.05653 0.00042 404.6 1.9 415.0 3.2 473.2 16.5 14.9 4 0.08224 0.00039 0.94 0.06808 0.00012 509.5 2.3 580.9 2.3 870.9 3.7 43.1 5 0.07070 0.00016 0.92 0.06047 0.00006 440.4 1.0 470.5 0.9 620.5 2.1 30.0 6 0.11820 0.00050 0.95 0.08090 0.00011 720.2 2.9 853.9 2.5 1219.0 2.8 43.2 7 0.08105 0.00017 0.93 0.06720 0.00006 502.4 1.0 568.8 1.0 844.1 1.9 42.1 8 0.08861 0.00020 0.90 0.07105 0.00008 547.3 1.2 634.5 1.2 959.0 2.3 44.8 9 0.07068 0.00151 0.96 0.05939 0.00035 440.2 9.1 463.7 7.8 581.6 12.9 25.1 10 0.08851 0.00018 0.96 0.07110 0.00005 546.7 1.1 634.3 1.1 960.2 1.4 44.9 11 0.06254 0.00240 0.85 0.05484 0.00129 391.1 14.5 393.2 14.7 405.9 51.9 3.8 12 0.06482 0.00016 0.77 0.05509 0.00010 404.9 0.9 406.5 1.0 415.8 4.1 2.7 13 0.06393 0.00015 0.64 0.05470 0.00015 399.5 0.9 399.5 1.2 399.8 6.2 0.1 14 0.07978 0.00029 0.70 0.06657 0.00019 494.8 1.7 557.9 1.6 824.4 5.9 41.5 15 0.07286 0.00014 0.95 0.06231 0.00005 453.4 0.9 493.6 0.9 684.9 1.6 35.0 16 0.06976 0.00019 0.56 0.05872 0.00033 434.7 1.1 454.7 2.4 556.7 12.1 22.7 17 0.07188 0.00050 0.53 0.05942 0.00066 447.5 3.0 470.1 4.9 582.4 23.9 24.0 18 0.08286 0.00020 0.88 0.06805 0.00008 513.2 1.2 584.0 1.1 870.2 2.5 42.7 19 0.10296 0.00022 0.93 0.07678 0.00007 631.8 1.3 748.5 1.3 1115.4 1.8 45.5 20 0.07159 0.00089 0.94 0.06052 0.00027 445.7 5.4 475.6 5.0 622.2 9.7 29.3 21 0.08366 0.00078 0.98 0.06885 0.00014 517.9 4.6 593.5 4.3 894.2 4.2 43.8 22 0.06503 0.00036 0.44 0.05584 0.00130 406.1 2.2 412.2 8.5 446.2 51.1 9.3 23 0.06853 0.00019 0.52 0.05790 0.00037 427.3 1.1 443.1 2.7 525.9 14.0 19.4 24 0.07473 0.00055 0.91 0.06238 0.00019 464.6 3.3 504.0 3.0 687.1 6.6 33.6 25 0.12026 0.00352 0.98 0.08107 0.00046 732.1 20.2 865.1 16.9 1223.2 11.2 42.4 26 0.11057 0.00043 0.88 0.07762 0.00017 676.0 2.5 792.8 2.5 1137.1 4.4 42.7 27 0.06653 0.00021 0.57 0.05551 0.00031 415.2 1.3 417.9 2.3 432.9 12.3 4.2 28 0.07760 0.00024 0.69 0.06338 0.00021 481.8 1.4 525.7 1.9 721.0 7.0 34.4 29 0.07562 0.00036 0.67 0.06303 0.00037 469.9 2.2 512.8 3.2 709.1 12.5 35.0 30 0.06986 0.00060 0.37 0.05878 0.00103 435.3 3.7 455.6 6.9 559.0 37.9 22.9 31 0.06083 0.00296 0.89 0.05473 0.00128 380.7 18.0 383.6 15.6 401.0 51.4 5.2 32 0.05869 0.00062 0.53 0.05402 0.00099 367.7 3.8 368.2 6.7 371.9 40.8 1.2 33 0.07036 0.00043 0.62 0.05517 0.00352 438.3 2.6 435.3 23.6 419.3 136.0 –4.7 34 0.08182 0.00022 0.40 0.05634 0.00058 507.0 1.3 499.6 4.3 465.8 22.5 –9.2 35 0.06105 0.00026 0.64 0.05422 0.00029 382.0 1.6 381.7 2.2 380.0 12.0 –0.5 36 0.06114 0.00015 0.57 0.05415 0.00019 382.6 0.9 381.8 1.4 377.5 7.9 –1.4 37 0.06193 0.00037 0.61 0.05498 0.00371 387.3 2.3 390.8 22.8 411.4 145.0 6.0

Pegmatite, LEA 06-18 (70°35.030′N, 22°13.838′W) 1 0.06136 0.00031 0.96 0.05439 0.00008 383.9 1.9 384.3 1.7 387.1 3.4 0.9 2 0.06000 0.00019 0.91 0.05441 0.00009 375.6 1.2 377.4 1.2 388.2 3.5 3.3 3 0.06172 0.00174 1.00 0.05435 0.00014 386.1 10.6 386.0 9.1 385.4 5.6 –0.2 4 0.05998 0.00027 0.95 0.05442 0.00008 375.5 1.6 377.4 1.5 388.6 3.2 3.5

Granite dike/minor intr., LEA 06-62 (70°35.700′N, 22°1.952′W) 1 0.05680 0.00015 0.60 0.05439 0.00021 356.1 0.9 360.3 1.4 387.2 8.5 8.2 2 0.06066 0.00020 0.78 0.05414 0.00013 379.6 1.2 379.2 1.2 376.7 5.4 –0.8 3 0.05973 0.00018 0.90 0.05434 0.00008 374.0 1.1 375.5 1.0 385.1 3.1 3.0 4 0.06143 0.00027 0.95 0.05452 0.00008 384.3 1.6 385.5 1.5 392.8 3.4 2.2 5 0.06128 0.00031 0.97 0.05419 0.00007 383.4 1.9 382.8 1.7 379.1 3.0 –1.2 6 0.06008 0.00019 0.73 0.05430 0.00017 376.1 1.2 377.1 1.5 383.4 7.1 1.9 7 0.06160 0.00038 0.92 0.05437 0.00013 385.3 2.3 385.5 2.0 386.6 5.5 0.3

Sheared granite dike, LEA 06-66 (GSZ; 70°35.779′N, 22°13.479′W) 1 0.06217 0.00077 0.98 0.05441 0.00015 388.8 4.7 388.7 4.0 388.1 6.2 –0.2 2 0.06918 0.00192 0.97 0.06175 0.00046 431.2 11.5 470.2 10.7 665.6 15.8 36.4 3 0.06137 0.00023 0.53 0.05426 0.00043 384.0 1.4 383.7 2.9 381.9 17.6 –0.6 Note: Global Positioning System positions of the different samples are given in parentheses. Analytical and calculated errors are reported at a 2σ level. All 2σ errors are absolute. Th/U modeled from 6/8-ratio + age. Pbc—initial common Pb in sample; Pbcom—initial common Pb + blank; 6/4—corrrected for spike and fractionation; all ratios—corrected for spike, fractionation, blank, initial common Pb; Abs—absolute; Not m.—not measured; incl—inclusions; fragm—fragment; GSZ— Gubbedalen shear zone.

LITHOSPHERE | Volume 2 | Number 4 | www.gsapubs.org 279

Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/2/4/267/3038294/267.pdf by guest on 25 September 2021 AUGLAND ET AL.

(Fig. 9A) and deﬁ ne a line with an upper-inter- LEA 06-59 Zircons 750 σ A 1628 cept age of 387.7 ± 1.8 Ma (2 , MSWD = 0.40, 0.12 anchored at 0 Ma), which is considered to rep- 1601 829 resent the crystallization age of the pegmatite. 650 420 829 Granite Sheets and Minor Granitic 0.10 410 Intrusive Rocks in Footwall Rocks and in 550 400 the Gubbedalen Shear Zone Field Relationships and Sample Description 0.08 390 Sample LEA 06-62 is from a small, unde- 450 Concordia age 1 zircon formed granite body that clearly cuts across 380 399.5 ± 0.9 Ma the SL fabric in the migmatite. Sample LEA 370 MSWD = 0.0084 06-66 is from a foliated granite sheet parallel U 0.06 to the mylonite foliation in the Gubbedalen 350

238 shear zone (see the section on Gubbedalen 0.4 0.6 0.8 1.0 1.2 1.4 1.6 Shear Zone). Both samples are rich in K-feldspar and quartz, with ~10% plagioclase and

Pb/ 0.09 B LEA 06-59 Rutiles minor biotite that is partly altered to chlorite. 206 Minor muscovite, and accessory zircon, rutile, and opaques also occur. The sample from the deformed granite dike in the shear zone (LEA 0.08 500 06-66) has subgrain microtextures in quartz indicating subgrain rotation recrystallization, 460 and K-feldspar has core-mantle textures.

0.07 Zircon and Monazite Characteristics 420 LEA 06-62 is characterized by a heterogeneous zircon population; many grains contain visible cores and are metamict. These were not analyzed. Zircons in the analyzed fractions are 0.06 weakly yellow to reddish, some are slightly 360 2 rutiles concordia age metamict, and some have a rusty surface indi- 340 382.4-+ 0.8 Ma cating partial alteration. Elongation ratios of MSWD = 0.69 the analyzed grains are generally high, ranging 0.05 from 3.5 to 7. Most of the analyzed grains were 0.35 0.45 0.55 0.65 fragmented, equant prisms with {100} dominat- 207 235 ing morphology. The high elongation ratio is Pb/ U indicative of rapid crystallization, probably from Figure 8. (A) Concordia diagrams for the eclogite data (LEA 06-59). Error ellipses are 2σ. a water-rich melt (Corfu et al., 2003). Analyses (A) Zircon data. Inset shows the concordant and least discordant data. Dashed gray error show high U contents (1425–9366 ppm), and ellipses represent the data used to calculate the Mesoproterozoic-Neoproterozoic upper- Th/U = 0.15–0.32. Sample LEA 06-62 also intercept age. (B) Rutile data. See text for further explanation of the data. MSWD—mean contains monazite, commonly as clear yellow square of weighted deviates. fragments. The U content measured in two fractions ranges from 1616 to 6747 ppm, and Th/U is high at 16.2–18.6 (Table 2). ages may reﬂ ect the exsolution process, whereas Zircon Characteristics and Analytical The zircon population of LEA 06-66 is also the experimentally established closure tempera- Results heterogeneous and very similar to that of LEA ture is related to pure diffusion of Pb. Relatively few zircons occur in this sample. 06-62, but the grains are generally more frag- Those extracted can be divided in two groups: mented, and U content (300−1300 ppm) was not Pegmatite in Retro-Eclogite Boudin Neck (1) partly metamict, brownish to reddish long as high (Table 2). prisms, mostly fragmented and dominated by Field Relationships and Sample Description {110} morphology; and (2) rounded, color- Analytical Results and Interpretation Sample LEA 06-18 was taken from a peg- less, clear grains. The rounded grains are pos- The three most discordant zircon analyses matite within the neck of a retrogressed eclog- sibly xenocrysts from the eclogite and/or retro- of LEA 06-62 are collinear and deﬁ ne an upper- ite boudin (Fig. 3C). The pegmatite does not eclogite and were not analyzed. The U content intercept age of 385.0 ± 2.7 Ma (2σ, MSWD = cut the surrounding migmatite (equivalent to of the brown grains is very high, ranging from 0.24; Fig. 9B). The two most concordant data the thin pegmatite in Fig. 3A). The mineralogy ~12,700 ppm to ~21,700 ppm. Th/U varies points, however, deviate slightly, with one to of the rock is simple, with K-feldspar, quartz, between 0.12 and 0.14 (Table 2). The four anal- the right of the chord suggesting some minor and biotite. yses are concordant or less than 3.5% discordant inheritance, and the other to the left, possibly

280 www.gsapubs.org | Volume 2 | Number 4 | LITHOSPHERE

0.0645 A LEA 06-18 B LEA 06-62

398 0.0630 0.0635

394 Monazite 0.0625 390 U 386 0.0610 380

0.0615 238 382

0.0605 378 Pb/ 370 0.0590

4 zirc. anchored at 0 206 374 0.0595 intercept at 387.7 ± 1.8 Ma MSWD = 0.40 3 zirc. anchored at 0 U 360 intercept at 385.0 ± 2.7 Ma 0.0585

238 0.0570 MSWD = 0.24 0.440 0.450 0.460 0.470 0.480

Pb/ 0.425 0.435 0.445 0.455 0.465 0.0634 206 C LEA 06-66 207Pb/ 235U 0.0630 2 zirc. Concordia age = 384.5 ± 1.3 Ma MSWD (of concordance) = 1.3 392 0.0626 Figure 9. (A) Concordia diagram for zircon from a pegmatite (LEA 0.0622 06-18) cutting a retrogressed eclogite boudin, but not the migmatite. 388 (B) Concordia diagram for data of the undeformed minor granite pluton (LEA 06-62) south of the Gubbedalen shear zone. (C) Concordia 0.0618 diagram for zircon from a sheared granite dike (LEA 06-66) within the Gubbedalen shear zone. Black error ellipses represent zircon analyses, 0.0614 384 and gray ellipses represent monazite analyses. All errors are 2σ absolute errors. MSWD—mean square of weighted deviates. 0.0610 380 0.0606 0.450 0.454 0.458 0.462 0.466 0.470 0.474 207Pb/ 235U

due to superimposed early Pb loss or hydrother- the Gubbedalen shear zone (LEA 06-66) are that underwent high-pressure metamorphism mal effects. One reversely discordant analysis indistinguishable within error. This indicates during the Caledonian orogeny. Zircon ages and of monazite (number 5) with a 207Pb/235U age of emplacement during the same magmatic event fi eld observations suggest that the protoliths of 382.8 ± 1.7 Ma is also slightly younger than the in the Liverpool Land eclogite terrane, prob- the eclogite were mafi c dikes or minor plutons zircon upper intercept, strengthening the pos- ably contemporaneous with top-up-to-the-S that intruded the quartzo-feldspathic rock in the sibility that some Pb loss is responsible for the contractional shearing. These granite sheets late Paleoproterozoic (Corfu and Hartz, 2011). apparent younger ages. Reverse discordance is cut the migmatite gneiss and are synkine- Data reported by Augland et al. (2010) and common in monazite due to excess 230Th, result- matic with top-up-to-the-S displacement under Corfu and Hartz (2011) show that the quartzo- ing in unsupported 206Pb (Harrison et al., 2002; amphibolites-facies conditions. The ages of the feldspathic rock hosting the eclogites formed at Oberli et al., 2004). The second monazite analy- dikes thus provide a minimum age of formation ca. 1640–1645 Ma. Corfu and Hartz (2011) also sis plots, instead, right on top of the zircon chord. of the migmatite, give important constraints on reported a ca. 1600 Ma protolith age from an Two of the zircon analyses of sample LEA 06-66 the timing, and a strong clue to the mode of eclogite lens and a late Mesoproterozoic proto- are concordant, defi ning a concordia age of 384.5 exhumation of the eclogite terrane. lith age from a migmatitic gneiss. ± 1.3 Ma (2σ, MSWD = 1.3), whereas the third The most signifi cant tectonometamorphic analysis indicates the presence of an inherited DISCUSSION event in the Caledonian evolution of Liverpool Proterozoic component (Table 2; Fig. 9C). Land eclogite terrane is high-pressure metamor- The age of 385.0 ± 2.7 Ma obtained on the Chronology of Tectonometamorphic Events phism at 399.5 ± 0.9 Ma, most likely related to small, undeformed pluton far away from the continent-continent collision and crustal thick- Gubbedalen shear zone (LEA 06-62), and the The U-Pb isotope ages document that rocks ening. A subsequent Caledonian tectonic and age of 384.5 ± 1.3 Ma obtained on zircons from constituting the footwall to the Gubbedalen shear metamorphic event was related to the emplace- the deformed and foliated granite sheet within zone represent a fragment of Precambrian crust ment of pegmatites, mostly in fractured eclogite

LITHOSPHERE | Volume 2 | Number 4 | www.gsapubs.org 281

Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/2/4/267/3038294/267.pdf by guest on 25 September 2021 AUGLAND ET AL.

lenses at ca. 388 Ma. These pegmatites do not structures in its upper part, must have played an Burchfi el, 1996; Torsvik et al., 1996; Krabben- cut the migmatites and thus provide a maximum important role in the exhumation of the eclogite dam and Dewey, 1998; Klein et al., 1999; Titus age for the formation of the migmatite foliation. terrane, as is evident from the decreasing meta- et al., 2002; Smith et al., 2007; Steltenpohl et The generation of these pegmatites could have morphic grade of the successively developed al., 2009). In the East Greenland Caledonides, been associated with decompressional melting contractional and extensional structures in the sinistral wrench faults associated with the West- during the initial stage of exhumation through shear zone. Although the observed individual ern fault zone (Fig. 1) that were active until at granulite-facies and subsequent amphibolite- top-down-to-the-N extensional structures in the least the mid-Devonian have been described by facies conditions (Buchanan, 2008; Whitney et upper part of the Gubbedalen shear zone are dif- Larsen and Bengaard (1991). They speculatively al., 2004). Further exhumation, possibly com- fi cult to quantify, it is clear that top-to-the-N dis- linked the Western fault zone to the Storstrøm- bined with introduction of fl uids into the felsic placement must have been signifi cant, because men shear zone farther north (Fig. 1). The Stor- host rock, is likely linked to the formation of the transition from ductile to brittle behavior of estrømmen shear zone is a major ductile shear small granitic plutons and sheeted intrusions at the shear zone rocks is obvious. The amount of zone thought to have accommodated large sinis- ca. 385 Ma (Whitney et al., 2004). These plu- strain in the reactivated extensional semiductile tral displacements (Holdsworth and Strachan, tons and sheeted intrusions cut the foliation in part of the zone is, however, diffi cult to assess 1991; Smith et al., 2007). These workers attrib- the migmatite and, thus, provide a minimum age quantitatively because large parts of the struc- uted the foreland thrusting in Dronning Louise on migmatization in the Liverpool Land eclogite ture could have been excised by the overprint- Land (Fig. 1) to sinistral transpression on this terrane. The age of ca. 385 Ma from the syncon- ing brittle Gubbedalen extensional detachment shear zone, refl ecting the oblique convergence tractional granite in the Gubbedalen shear zone fault, and the true thickness of the zone of semi- of Baltica and Laurentia (Torsvik et al., 1996). indicates that exhumation from amphibolites- ductile extension (prior to the brittle detach- Thrusting in Dronning Louise Land has been facies to greenschist-facies conditions was con- ment) is unknown. dated to ca. 390 Ma (Dallmeyer and Strachan, temporaneous with subhorizontal shortening at Final juxtaposition of the two terranes was 1994), implying that, if the interpretations of midcrustal levels. presumably related to the extensional move- Holdsworth and Strachan (1991) and Smith et ments along the Gubbedalen extensional al. (2007) are correct, large-scale bulk sinistral Gubbedalen Shear Zone as a Terrane detachment fault, which brought crust exhumed transpression in the East Greenland Caledonides Boundary from a depth of more than 50 km in contact with was active at that time. Lower-amphibolite- rocks from a middle- to upper-crustal magmatic facies conditions along the Storestrømmen The hanging wall in Liverpool Land most arc (Fig. 10). Extension in the upper crust must shear zone have been dated to ca. 370 Ma (Dall- probably represents a magmatic arc developed have been important in contributing to the exhu- meyer and Strachan, 1994), showing that the on Proterozoic crust in the latest Ordovician mation of the Liverpool Land eclogite terrane shear zone was still active at ductile conditions and Silurian, with a particularly intense phase synchronously with ductile contractional dis- in the Late Devonian. of magmatism at ca. 445–425 Ma (Augland et placements on the Gubbedalen shear zone (pre– Devonian sinistral transtension and trans- al., 2009; Corfu and Hartz, 2011). There is no 380 Ma; see following). Muscovite 40Ar/39Ar pression have also been reported from the evidence that these rocks were affected in any ages at ca. 380 Ma recorded both in the hanging Western Gneiss Region, and Krabbendam and way by the events that exhumed, deformed, and wall and the footwall (Bowman, 2008) indicate Dewey (1998) suggested this to be the main transformed the footwall mafi c rocks into eclog- that the two terranes were at the same crustal mechanism of exhumation of ultrahigh-pressure ites, and then back to amphibolites, together level, and possibly juxtaposed, at this time. and high-pressure rocks from amphibolites- with the widespread migmatization and gen- facies conditions. It is thus clear that sinistral eration of granitic magmas between 400 and Tectonic Model for Development of the displacement was important across the entire 380 Ma. Conversely, there is no evidence of Gubbedalen Shear Zone Caledonide orogen during the Devonian, and Silurian magmatism or metamorphism in the this sets up a framework for our tectonic model. footwall. The time gap and the radically differ- Two main lines of evidence suggest that dis- In a bulk transpressional setting, shear zones ent types of Caledonian activity and lithologies placement on the Gubbedalen shear zone was that are oblique to the main direction of com- between the hanging wall and the footwall indi- related to oblique motions during the latest col- pression may develop as a response to struc- cate that the Gubbedalen shear zone represents lisional stage of the Caledonian orogeny. First, tural asperities (e.g., inherited structures or an important boundary, possibly separating two the orientation (E-W) of the Gubbedalen shear lithologic boundaries). The oblique orientation terranes of highly different tectonomagmatic zone and several related local shear zones within of the Gubbedalen shear zone compared with and metamorphic histories. the Liverpool Land eclogite terrane (Fig. 2), the main structural grain in the East Greenland The question that is most relevant for the which we interpret as the original orientation Caledonides fi ts with such a setting. An element present paper relates to the timing and mecha- relative to the surrounding units, all are perpen- of transpression in Liverpool Land could also nism of juxtaposition of these two terranes and dicular to the main structural grain of the East explain the constrictional structures observed in the role played by the Gubbedalen shear zone Greenland Caledonides. Second, the kinematics the footwall and the strongly lineated mylonites and the Gubbedalen extensional detachment of the Gubbedalen shear zone, with major top- in the Gubbedalen shear zone itself. fault in this process. The fact that the hanging up-to-the-S contraction, is consistent with N-S The initial phase of the exhumation probably wall was neither metamorphosed at ca. 400 Ma crustal shortening. occurred by nearly isothermal decompression as nor intruded by the 385 Ma granitic dikes that There is regional evidence for a Caledonian indicated from the presence of secondary ortho- are so ubiquitous in the footwall and Gubbedalen (especially mid- to late Devonian) orogenwide pyroxenes and clinopyroxenes and Ca-rich pla- shear zone indicates that the two terranes were left-slip fault system between and within Baltica gioclase in the eclogites (Buchanan, 2008) and separated until after 385 Ma. The Gubbedalen and Laurentia (Steltenpohl and Bartley, 1988; the large degree of melting in the migmatitic shear zone, with its contractional structures in Holdsworth and Strachan, 1991; Larsen and orthogneiss. There are different ways of achiev- the lower part and overprinting extensional Bengaard, 1991; Seranne, 1992; Northrup and ing such nearly isothermal decompression paths

282 www.gsapubs.org | Volume 2 | Number 4 | LITHOSPHERE

A Exhumation from amphibolite to greenschist facies conditions: ~386 – >380 Ma N

Fault Zone Fault Western 10 km

Plane of view GSZ 20 km in fig. to the left 30 km E E LLET

Hanging wall GSZ

Footwall

B Exhumation from lower greenschist facies conditions: < 380 Ma N

NSGEDF

GSZ

Fault Zone Fault Western Hanging Plane of view 10 km wall E in fig. to the left E LLET 20 km Footwall

GEDF

Extensionl faults and shear zones Contractional shear zone

Figure 10. Schematic tectonic model for the exhumation of the Liverpool Land eclogite terrane from amphibolite facies to the surface (cross section to the right and plan view to the left). (A) Formation of the contractional Gubbedalen shear zone was associated with a restraining bend in a local dextral strike-slip zone associated with a bulk sinistral transpressional tectonic regime resulting from the oblique collision of Baltica and Laurentia. Thrusting of the hanging wall above the Gubbedalen shear zone was synchronous with extension and erosion in the hanging wall. Extensional and erosive denudation exceeded the crustal thickening resulting from thrusting, thus leading to exhumation of the Liverpool Land eclogite terrane. (B) The stress regime changed to extensional/transtensional. Displacement on the Gubbedalen extensional detachment fault and related extensional faults was, together with erosion, the main agent of exhumation in the upper crust. Dashed line shows a hypothetical major sinistral fault illustrat- ing the bulk sinistral transpressional setting in parts of the orogenic system in the mid–late Devonian. Abbreviations: GSZ—Gubbedalen shear zone; GEDF—Gubbedalen extensional detachment fault; LLET—Liverpool Land eclogite terrane; HICP—Hurry Inlet composite pluton.

LITHOSPHERE | Volume 2 | Number 4 | www.gsapubs.org 283

Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/2/4/267/3038294/267.pdf by guest on 25 September 2021 AUGLAND ET AL.

(e.g., Whitney et al., 2004). In the case of the ric ﬂ ower structures developing in the brittle mixed 202Pb-205Pb-235U tracer. Zircon and rutile

Liverpool Land eclogite terrane, the initial exhu- crustal regime (Dewey et al., 1998). were dissolved in HF and a drop of HNO3 in mation from eclogite-facies conditions (>50 km Teﬂ on bombs at ~190 °C for 5 d. Monazite was

depth) to granulite-facies, lower-crustal condi- CONCLUSIONS dissolved in 6 N HCl and a drop of HNO3 in tions was probably driven by buoyancy and Savillex vials on a hotplate at ~125 °C for 5 d. back thrusting of subducted crust (Chemenda et The Liverpool Land eclogite terrane is a Dissolved samples weighing more than 0.005 al., 1995; Augland et al., 2010). Partial melting piece of continental crust formed during Eon mg and all monazite were chemically sepa- as a consequence of nearly isothermal decom- 16; it has a moderate latest Mesoproterozoic rated using microcolumns and anion-exchange pression would have further increased the buoy- overprint (Augland et al., 2010; Corfu and resin in order to remove cations that may inhibit ancy and decreased the rheology of the Liver- Hartz, 2011) and was affected by eclogite-facies ionization (Krogh, 1973). U/Pb-solutions were pool Land eclogite terrane. This could have metamorphism at 399.5 ± 0.9 Ma. The terrane dried down and loaded on degassed single Re facilitated exhumation through the lower crust was then exhumed and underwent extensive fi laments with silica gel. by mechanisms of syncontractional fl ow on partial melting around 388 Ma, as high-pressure The samples were measured on a Finnigan low-angle shear zones, buckling, diapirism, or conditions changed to amphibolite-facies via MAT 262 mass spectrometer, using either Fara- combinations of these (Hartz et al., 2001; Whit- granulite-facies conditions. The Gubbedalen day cups in static mode or, for low-intensity ney et al., 2004; Andresen et al., 2007). shear zone, forming the upper boundary of samples, a secondary electron multiplier (SEM) The structural and textural expressions of the Liverpool Land eclogite terrane, is a duc- in peak jumping mode. The 207Pb/204Pb ratios the exhumation from amphibolites-facies condi- tile, N-dipping shear zone with a predomi- were measured on SEM for all samples. SEM tions to greenschist-facies conditions of the Liv- nant top-up-to-the-S sense of motion and was data were corrected for nonlinearity based on erpool Land eclogite terrane at the Gubbedalen active at conditions varying progressively from measurements of the standard NBS 982-Pb + shear zone are linked to contractional top-up- amphibolite-facies conditions to greenschist- U500 (Corfu, 2004). Measurements of the stan- to-the-S displacements. If the exhumation from facies retrogression and, together with several dard are also used to monitor the reproducibility granulite-facies conditions to amphibolites- local shear zones within the eclogite terrane, of the mass spectrometer. facies conditions was accomplished, at least in was responsible for exhumation of Liverpool Measurements were corrected for a 2 pg part, by fl ow on low-angle shear zones, a con- Land eclogite terrane from amphibolites-facies Pb and 0.1 pg U blank, with blank composi- tinuation of such fl ow at lower temperatures conditions. The structurally upper part of the tions: 206Pb/204Pb = 18.3, 207Pb/206Pb = 0.85, and could have led to localizations of shear zones Gubbedalen shear zone is characterized by top- 207Pb/204Pb = 15.555 (Corfu, 2004). Common Pb (i.e., the Gubbedalen shear zone). However, for down-to-the-N brittle-ductile structures, and the corrections were employed using the Pb-evolution continued exhumation to have occurred, thrust- contact to the hanging wall is constituted by model of Stacey and Kramers (1975) at the age in ing must have been accompanied by extensional the brittle Gubbedalen extensional detachment question. U source fractionation was estimated to (and erosive) denudation in the hanging wall fault. Exhumation in the brittle regime was be 0.12%/a.m.u. Pb source fractionation was cor- that exceeded the crustal thickening resulting provided by displacement on the Gubbedalen rected using the measured 205Pb/202Pb tracer ratio from thrusting (Fig. 10A; Hartz et al., 2001; extensional detachment fault and by erosive normalized to the certifi ed value of 0.44050. A Andresen et al., 2007). The formation of con- denudation of the footwall block. The hanging standard fractionation error of 0.06%/a.m.u. was tractional structures observed in the Gubbedalen wall is a distinct magmatic terrane of Ordovi- incorporated in the calculations if 205Pb/202Pb was shear zone was ongoing at ca. 385 Ma, implying cian–Silurian age. The model developed in this determined very precisely and the fractionation that the oblique contraction in this region of the paper links the Gubbedalen shear zone to a corrections became unrealistically precise. If Caledonides lasted until at least the end of the large-scale sinistral transpressional system that 205Pb/202Pb was not determined, or the measured mid-Devonian. brings together different blocks and different 205Pb/202Pb was far of from 0.44050, Pb fraction- Exhumation from greenschist-facies con- crustal levels of the Caledonian orogen. ation was set at 0.1%/a.m.u. Pb fractionation ditions (postclosure of Ar in muscovite at values between 0.02% and 0.16%/a.m.u. were ca. 380 Ma; Bowman, 2008), and juxtaposi- APPENDIX A. ANALYTICAL METHODS: generated by this procedure. tion of the footwall and hanging wall as seen U-Pb GEOCHRONOLOGY The analytical errors and corrections were today refl ect movement along the Gubbedalen then incorporated and propagated using the extensional detachment fault and other brittle After crushing and separating heavy min- ROMAGE 6.3 program, originally developed faults in the area (Fig. 10B). This switch from erals using magnetic and heavy liquid separa- by T.E. Krogh. Graphic presentations and age contraction to extension is interpreted to refl ect tion methods, zircon, monazite, and rutile were calculations were performed using the Isoplot the overall change from convergence to exten- handpicked and discriminated on the basis of program of Ludwig (2003) and the decay con- sion in the East Greenland Caledonides at morphology, transparency, color, and internal stants referred in Steiger and Jäger (1977). All ca. 380 Ma (Fig. 10B). textures. Cathodoluminescence images were errors are reported at the 2σ confi dence interval. An alternative exhumation model for the obtained from a grain mount with a represen- exhumation from amphibolites-facies to green- tative selection of 20 zircons from the eclogite. ACKNOWLEDGMENTS schist-facies conditions could be that an extru- Air abrasion was carried out using the method sion wedge was created above a “master fault” described by Krogh (1982) to remove the mar- We thank G. Bye-Fjeld and M. Erambert for associated with a structural asperity compris- ginal areas of the mineral grains most likely analytical assistance and P.I. Myhre and M. ing a restraining bend, having an extensional to have experienced Pb loss. Mineral samples Steltenpohl for fruitful discussions in the fi eld.

shear zone at the top (Dewey et al., 1998). In were washed in dilute HNO3, ionized water, B. Bingen and M. Steltenpohl read and made such a model, the structures observed in the and acetone using an ultrasonic bath to remove constructive suggestions on late drafts. We also Liverpool Land eclogite terrane would be the any contamination. Each sample was then appreciate critical comments by B. Hacker, W. midcrustal ductile equivalent of asymmet- weighed on a microbalance and spiked with a Hames, and four anonymous reviewers.

284 www.gsapubs.org | Volume 2 | Number 4 | LITHOSPHERE

REFERENCES CITED Gapais, D., 1989, Shear structures within deformed gran- Caledonian orogen: Lithos, v. 57, p. 1–21, doi:10.1016/ ites: Mechanical and thermal indicators: Geology, S0024-4937(00)00071-2. v. 17, p. 1144–1148, doi:10.1130/0091-7613(1989)017< Kalsbeek, F., Higgins, A.K., Jepsen, H.F., Frei, R., and Nut- Andresen, A., Hartz, E.H., and Vold, J., 1998, A late oro- 1144:SSWDGM>2.3.CO;2. man, A.P., 2008, Granites and granites in the East genic extensional origin for the infracrustal gneiss Gates, A.E., and Glover, L., 1989, Alleghanian tectono-thermal Greenland Caledonides, in Higgins, A.K., Gilotti, J.A., domes of the East Greenland Caledonides (72–74°N): evolution of the dextral transcurrent Hylas zone, Vir- and Smith, M.P., eds., The Greenland Caledonides: Tectonophysics, v. 285, p. 353–369, doi:10.1016/S0040 ginia Piedmont, U.S.A.: Journal of Structural Geology, Evolution of the Northeast Margin of Laurentia: Geo- -1951(97)00278-3. v. 11, p. 407–419, doi:10.1016/0191-8141(89)90018-7. logical Society of America Memoir 202, p. 227–249. Andresen, A., Rehnstrom, E.F., and Holte, M., 2007, Evidence Gleadow, A.J.W., and Brooks, C., 1979, Fission-track dating, Kanaori, Y., Kawakami, S., and Yairi, K., 1991, Microstruc- for simultaneous contraction and extension at differ- thermal histories and tectonics of igneous intrusions ture of deformed biotite defi ning foliation in cata- ent crustal levels during the Caledonian orogeny in NE in east Greenland: Contributions to Mineralogy and clasite zones in granite, central Japan: Journal of Greenland: Journal of the Geological Society, v. 164, Petrology, v. 71, p. 45–60, doi:10.1007/BF00371880. Structural Geology, v. 13, p. 777–785, doi:10.1016/0191 p. 869–880, doi:10.1144/0016-76492005-056. Haller, J., 1971, Geology of the East Greenland Caledonides: -8141(91)90003-2. Augland, L.E, Andresen, A., Corfu, F., 2009, From Late New York, Interscience Publishers, 413 p. Klein, A., Steltenpohl, M.G., Hames, W.E., and Andresen, Ordovician-Silurian arc magmatism to Devonian ter- Hansen, B.T., and Steiger, R.H., 1971, The Geochronology of A., 1999, Ductile and brittle extension in the south- rane amalgamation and juxtaposition of Baltican and the Scoresby Sund area: Rapport Grønlands Geolo- ern Lofoten archipelago, north Norway: Implications Laurentian crust: New evidence from Liverpool Land, giske Undersøgelse, v. 37, p. 55–57. for differences in tectonic style along an ancient col- East Greenland Caledonides: American Geophysical Harrison, T.M., Catlos, E.J., and Montel, J.M., 2002, U-Th-Pb lisional margin: American Journal of Science, v. 299, Union Joint Assembly, Toronto, Canada, Program with dating of phosphate mineral, in Kohn, J.M., Rakovan, p. 69–89, doi:10.2475/ajs.299.1.69. Abstracts. J., and Hughes, J.M., eds., Phosphates: Mineralogical Krabbendam, M., and Dewey, J.F., 1998, Exhumation of Augland, L.E., Andresen, A., and Corfu, F., 2010, A Baltican Society of America Reviews in Mineralogy and Geo- UHP rocks by transtension in the Western Gneiss ancestry of the Liverpool Land eclogite terrane, East chemistry, v. 48, p. 523–558. Region, Scandinavian Caledonides, in Holdsworth, Greenland: 2010 EGU General Assembly, Geophysical Hartz, E.H., and Andresen, A., 1995, Caledonian sole thrust R.E., Strachan, R.A., and Dewey, J.F., eds., Conti- Research Abstracts, v. 12, abs. EGU2010-10534. of central East Greenland: A crustal-scale Devonian nental Transpressional and Transtensional Tectonics: Bingen, B., Austrheim, H., Whitehouse, M.J., and Davis, extensional detachment: Geology, v. 23, p. 637–640, Geological Society, London, Special Publication 135, W.J., 2004, Trace element signature and U-Pb geochro- doi:10.1130/0091-7613(1995)023<0637:CSTOCE>2.3.CO;2. p. 159–181. nology of eclogite-facies zircon, Bergen arcs, Cale- Hartz, E.H., Andresen, A., Martin, M.W., and Hodges, K.V., Kranck, E.H., 1935, On the crystalline complex of Liverpool donides of W Norway: Contributions to Mineralogy 2000, U-Pb and 40Ar/39Ar constraints on the Fjord Land: Meddelelser om Grønland, v. 95, p. 1–122. and Petrology, v. 147, p. 671–683, doi:10.1007/s00410 region detachment zone; a long-lived extensional fault Krogh, T.E., 1973, A low-contamination method for hydro- -004-0585-z. in the central East Greenland Caledonides: Journal of thermal decomposition of zircon and extraction of U Bowman, D.R., 2008, Exhumation history of Caledonian the Geological Society, v. 157, p. 795–809. and Pb for isotopic age determination: Geochimica et eclogites in Liverpool Land, East Greenland, and Hartz, E.H., Andresen, A., Hodges, K.V., and Martin, M.W., Cosmochimica Acta, v. 37, p. 485–494, doi:10.1016/0016- comparison with eclogites in Norway [M.Sc. thesis]: 2001, Syncontractional extension and exhumation of 7037(73)90213-5. Auburn, Alabama, University of Auburn, http://etd deep crustal rocks in the East Greenland Caledonides: Krogh, T.E., 1982, Improved accuracy of U-Pb zircon ages .auburn.edu/etd/handle/10415/1109. Tectonics, v. 20, p. 58–77, doi:10.1029/2000TC900020. by the creation of more concordant systems using Buchanan, W., 2008, Tectonic evolution of a Caledonian- Hartz, E.H., Condon, D., Austrheim, H., and Erambert, M., an air abrasion technique: Geochimica et Cosmochi- aged continental basement eclogite terrane in Liv- 2005, Rediscovery of the Liverpool Land eclogites mica Acta, v. 46, p. 637–649, doi:10.1016/0016-7037 erpool Land, East Greenland [M.Sc. thesis]: Auburn, (central East Greenland): A post supra-subduction (82)90165-X. Alabama, University of Auburn, http://etd.auburn.edu/ UHP province: Mitteilungen der Österreichischen Min- Krogh Ravna, E., 2000, The garnet-clinopyroxene Fe2+-Mg etd/handle/10415/1112. eralogischen Gesellschaft, v. 150, 50 p. geothermometer: An updated calibration: Journal of Cheeney, R.F., 1985, The Plutonic Igneous and High-Grade Higgins, A.K., 1988, The Krummedal supracrustal sequence Metamorphic Geology, v. 18, p. 211–219, doi:10.1046/ Metamorphic Rocks of Southern Liverpool Land, Cen- in East Greenland, in Winchester, J.A., eds., Later Pro- j.1525-1314.2000.00247.x. tral East Greenland, Part of a Supposed Caledonian terozoic Stratigraphy of the Northern Atlantic Regions: Larsen, P.H., and Bengaard, H.J., 1991, Devonian basin ini- and Precambrian Complex: Rapport Grønlands Geolo- Glasgow, Blackie, p. 86–96. tiation in East Greenland: A result of sinistral wrench giske Undersøgelse, v. 123, 39 p. Higgins, A.K., Elvevold, S., Escher, J.C., Frederiksen, K.S., faulting and Caledonian extensional collapse: Jour- Chemenda, A.I., Mattauer, M., Malavieille, J., and Bokun, Gilotti, J.A., Henriksen, N., Jepsen, H.F., Jones, K.A., nal of the Geological Society, v. 148, p. 355–368, A.N., 1995, A mechanism for syn-collisional rock Kalsbeek, F., Kinny, P.D., Leslie, A.G., Smith, M.P., doi:10.1144/gsjgs.148.2.0355. exhumation and associated normal faulting: Results Thrane, K., and Watt, G.R., 2004, The foreland-propa- Larsen, P.H., Olsen, H., and Clack, J.A., 2008, The Devonian from physical modeling: Earth and Planetary Science gating thrust architecture of the East Greenland Cale- basin in East Greenland—Review of basin evolution Letters, v. 132, p. 225–232, doi:10.1016/0012-821X(95) donides 72°−75°N: Journal of the Geological Society, and vertebrate assemblages, in Higgins, A.K., Gil- 00042-B. v. 161, p. 1009–1026, doi:10.1144/0016-764903-141. otti, J.A., and Smith, M.P., eds., The Greenland Cale- Cherniak, D.J., 2000, Pb diffusion in rutile: Contributions Higgins, A.K., Gilotti, J.A., and Smith, M.P., eds., 2008, The donides: Evolution of the Northeast Margin of Lau- to Mineralogy and Petrology, v. 139, p. 198–207, Greenland Caledonides: Evolution of the Northeast rentia: Geological Society of America Memoir 202, doi:10.1007/PL00007671. Margin of Laurentia: The Geological Society of Amer- p. 227–249. Coe, K., 1975, The Hurry Inlet Granite and Related Rocks of ica Memoir 202, 369 p. Leslie, A.G., and Nutman, A.P., 2003, Evidence for Neo- Liverpool Land, East Greenland: Grønlands Geolo- Holdsworth, R.E., and Strachan, R.A., 1991, Interlinked sys- proterozoic orogenesis and early high temperature giske Undersøgelse Bulletin 115, 34 p. tem of ductile strike slip and thrusting formed by Cale- Scandian deformation events in the southern East Coe, K., and Cheeney, R.F., 1972, Preliminary results of map- donian sinistral transpression in northeastern Green- Greenland Caledonides: Geological Magazine, v. 140, ping in Liverpool Land: East Greenland: Rapport Grøn- land: Geology, v. 19, p. 510–513, doi:10.1130/0091-7613 p. 309–333, doi:10.1017/S0016756803007593. lands Geologiske Undersøgelse, v. 48, p. 7–20. (1991)019<0510:ISODSS>2.3.CO;2. Ludwig, K.R., 2003, ISOPLOT/Ex; A Geochronological Toolkit Corfu, F., 2004, U-Pb age, setting and tectonic signifi cance Hoskin, P.W.O., and Schaltegger, U., 2003, The composition for Microsoft Excel: Berkeley Geochronology Center of the anorthosite-mangerite-charnockite-granite of zircon and igneous and metamorphic petrogenesis, Special Publication 4, 70 p. suite, Lofoten-Vesteralen, Norway: Journal of Petrol- in Ribbe, P.H., and Rosso, J.J., eds., Zircon: Reviews in Mezger, K., Hanson, G.N., and Bohlen, S.R., 1989, U-Pb ogy, v. 45, p. 1799–1819, doi:10.1093/petrology/egh034. Mineralogy and Geochemistry, v. 53, p. 27–62. ages of metamorphic rutiles: Application to the cool- Corfu, F., and Hartz, E.H., 2011, U-Pb geochronology in Liv- Jacobs, J., and Thomas, R.J., 2001, A titanite fi ssion track ing history of high grade terranes: Earth and Planetary erpool Land and Canning Land, E-Greenland—The profi le across the southeastern Archean Kaapvaal Science Letters, v. 96, p. 106–118, doi:10.1016/0012 complex record of a polyphase Caledonian orogeny: craton and the Mesoproterozoic Natal metamorphic -821X(89)90126-X. Canadian Journal of Earth Science (in press). province, South Africa: Evidence for differential cryptic Northrup, C.J., and Burchfi el, B.C., 1996, Orogen-parallel Corfu, F., Hanchar, J.M., Hoskin, P.W.O., and Kinny, P.D., Meso- to Neoproterozoic tectonism: Journal of African transport and vertical partitioning of strain during 2003, Atlas of zircon textures, in Ribbe, P.H., and Earth Sciences, v. 33, no. 2, p. 323–333, doi:10.1016/ oblique collision, Efjorden, north Norway: Journal Rosso, J.J., eds., Zircon: Reviews in Mineralogy and S0899-5362(01)80066-X. of Structural Geology, v. 18, no. 10, p. 1231–1244, Geochemistry, v. 53, p. 469–500. Johnston, S., Gehrels, G., Valencia, V., and Ruiz, J., 2009, doi:10.1016/S0191-8141(96)00040-5. Dallmeyer, R.D., and Strachan, R.A., 1994, 40Ar/ 39Ar mineral Small-volume U-Pb zircon geochronology by laser Oberli, F., Meier, M., Berger, A., Rosenberg, C.L., and Gierè, age constraints on the timing of deformation and ablation-multicollector-ICP-MS: Chemical Geology, R., 2004, U-Th-Pb and 230Th/238U disequilibrium iso- metamorphism, North-East Greenland Caledonides: v. 259, p. 218–229, doi:10.1016/j.chemgeo.2008.11.004. tope systematics: Precise accessory mineral chronol- Rapport Grønlands Geologiske Undersøgelse, v. 162, Kalsbeek, F., Jepsen, H.F., and Jones, K.A., 2001a, Geo- ogy and melt evolution tracing in the Alpine Bergell p. 153–162. chemistry and petrogenesis of S-type granites in the intrusion: Geochimica et Cosmochimica Acta, v. 68, Dewey, J.F., Holdsworth, R.E., and Strachan, R.A., 1998, East Greenland Caledonides: Lithos, v. 57, p. 91–109, p. 2543–2560, doi:10.1016/j.gca.2003.10.017. Transpression and transtension zones, in Holdsworth, doi:10.1016/S0024-4937(01)00038-X. Rehnström, E.F., 2010, Prolonged Palaeozoic magmatism R.E., Strachan, R.A., and Dewey, J.F., eds., Continental Kalsbeek, F., Jepsen, H.F., and Nutman, A.P., 2001b, From in the East Greenland Caledonides: Some constraints Transpressional and Transtensional Tectonics: Geologi- source migmatites to plutons; tracking the origin from U-Pb Ages and Hf Isotopes: Journal of Geology cal Society, London, Special Publication 135, p. 35–40. of ca. 435 Ma S-type granites in the East Greenland (in press).

LITHOSPHERE | Volume 2 | Number 4 | www.gsapubs.org 285

Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/2/4/267/3038294/267.pdf by guest on 25 September 2021 AUGLAND ET AL.

Roberts, D., and Gee, D.G., 1985, An introduction to the struc- Stacey, J.S., and Kramers, J.D., 1975, Approximation of ter- haug, H.J., 1996, Continental break-up and collision in tures of the Scandinavian Caledonides, in Gee, D.G., restrial lead isotope evolution by a two-stage model: the Neoproterozoic and Palaeozoic; a tale of Baltica and Sturt, B.A., eds., The Caledonide Orogen—Scandi- Earth and Planetary Science Letters, v. 26, p. 207–221, and Laurentia: Earth-Science Reviews, v. 40, p. 229– navia and Related Areas: Chichester, John Wiley and doi:10.1016/0012-821X(75)90088-6. 258, doi:10.1016/0012-8252(96)00008-6. Sons Ltd., p. 55–68. Steiger, R.H., and Jäger, E., 1977, Subcommission on Geo- Tribe, I.R., and D’Lemos, R.S., 1996, Signifi cance of a hiatus Root, D.B., Hacker, B.R., Mattinson, J.M., and Wooden, J.L., chronology: Convention on the use of decay constants in down-temperature fabric development within syn- 2004, Zircon geochronology and ca. 400 Ma exhumation in geo- and cosmochronology: Earth and Planetary Sci- tectonic quartz diorite complexes, Channel Islands, of Norwegian ultrahigh pressure rocks; an ion micro- ence Letters, v. 36, p. 359–362, doi:10.1016/0012-821X UK: Journal of the Geological Society, v. 153, p. 127– probe and chemical abrasion study: Earth and Plan- (77)90060-7. 138, doi:10.1144/gsjgs.153.1.0127. etary Science Letters, v. 228, p. 325–341, doi:10.1016/j Steltenpohl, M.G., and Bartley, J.M., 1988, Cross-folds and Vidal, J.L., Kubin, L., Debat, P., and Soula, J.C., 1980, Deforma- .epsl.2004.10.019. back-folds in the Ofoten-Tysfjord area, north Norway, tion and dynamic recrystallization of K-feldspar augen Sahlstein, T.G., 1935, Petrographie der Eklogiteinschlüsse and their signifi cance for Caledonian tectonics: Geo- in orthogneiss from Montagne Noir, Occitania, south- in den Gneisen des südwestlichen Liverpool-Landes logical Society of America Bulletin, v. 100, p. 140–151, ern France: Lithos, v. 13, p. 247–255, doi:10.1016/0024 in Ost-Grönland: Meddelelser om Grønland, v. 95, doi:10.1130/0016-7606(1988)100<0140:CFABFI>2.3.CO;2. -4937(80)90070-5. p. 1–43. Steltenpohl, M.G., Carter, B.T., Andresen, A., and Zeltner, D.L., Watt, G.R., Kinny, P.D., and Friderichsen, J.D., 2000, U-Pb Schmitz, M.D., and Bowring, S.A., 2003, Constraints on 2009, 40Ar/ 39Ar thermochronology of late- and postoro- geochronology of Neoproterozoic and Caledonian tec- the thermal evolution of continental lithosphere from genic extension in the Caledonides of north-central tonothermal events in the East Greenland Caledonides: U-Pb accessory mineral thermochronometry of lower Norway: The Journal of Geology, v. 117, p. 399–414, Journal of the Geological Society, v. 157, p. 1031–1048. crustal xenoliths, southern Africa: Contributions to doi:10.1086/599217. White, A.P., Hodges, K.V., Martin, M.W., and Andresen, A., Mineralogy and Petrology, v. 144, p. 592–618. Stipp, M., 2002, The eastern Tonale fault zone: A ‘natural 2002, Geologic constraints on middle-crustal behavior Seranne, M., 1992, Late Palaeozoic kinematics of the Møre- laboratory’ for crystal plastic deformation of quartz during broadly synorogenic extension in the central Trøndelag fault zone and adjacent areas, central Nor- over a temperature range from 250 to 700°C: Journal East Greenland Caledonides: International Jour- way: Norsk Geologisk Tidsskrift, v. 72, p. 141–158. of Structural Geology, v. 24, p. 1861–1884, doi:10.1016/ nal of Earth Sciences, v. 91, p. 187–208, doi:10.1007/ Simpson, C., and Wintsch, R.P., 1989, Evidence for deforma- S0191-8141(02)00035-4. s005310100227. tion-induced K-feldspar replacement by myrmekite: Stipp, M., Heilbronner, R., and Stunitz, H., 1999, Dislocation Whitney, D.L., Teyssier, C., and Fayon, A.K., 2004, Isother- Journal of Metamorphic Geology, v. 7, p. 261–275, creep microstructures as indicators of deformation con- mal decompression, partial melting and exhumation doi:10.1111/j.1525-1314.1989.tb00588.x. ditions; microstructural analysis of fi eld examples: Geo- of deep continental crust: Geological Society, London, Smith, D.C., and Cheeney, R.F., 1980, Orientated needles logical Society of America Abstracts with Programs, Special Publication 227, p. 313–326. of quartz in clinopyroxene: Evidence for exsolution v. 31, p. 109.

of SiO2 from a non-stoichiometric supersilicic “clino- Titus, S.J., Fossen, H., Pedersen, R.B., Vigneresse, J.L., and pyroxene,” in 26th International Geological Congress, Tikoff, B., 2002, Pull-apart formation and strike-slip Paris, Program with Abstracts, v. 2, p. 145. partitioning in an obliquely divergent setting, Leka MANUSCRIPT RECEIVED 18 AUGUST 2009 Smith, S.A.F., Strachan, R.A., and Holdsworth, R.E., 2007, ophiolite, Norway: Tectonophysics, v. 354, p. 101–119, REVISED MANUSCRIPT RECEIVED 27 APRIL 2010 Microstructural evolution within a partitioned midcrustal doi:10.1016/S0040-1951(02)00293-7. MANUSCRIPT ACCEPTED 18 MAY 2010 transpression zone, northeast Greenland Caledonides: Torsvik, T.H., Smethurst, M.A., Meert, J.G., Van der Voo, R., Tectonics, v. 26, TC4003, doi:10.1029/2006TC001952. McKerrow, W.S., Brasier, M.D., Sturt, B.A., and Walder- Printed in the USA

286 www.gsapubs.org | Volume 2 | Number 4 | LITHOSPHERE

Downloaded from http://pubs.geoscienceworld.org/gsa/lithosphere/article-pdf/2/4/267/3038294/267.pdf by guest on 25 September 2021