Carboniferous strata of the Wandel Sea Basin unconformably overlie the Laurentian Precambrian crystalline rocks of the Caledonian hinterland at its northernmost exposures in Holm Land (∼80°N). Complex zircon from an intermediate gneiss gives an upper-intercept age of 1878 ± 71 Ma, a protolith age which fits with the regional 1.8–2.0 Ga calc-alkaline arc. A strongly deformed pegmatite was intruded at 435 ± 17 Ma, and it is a rare example of Caledonian magmatism in the northern sector of the orogen. Omphacite confirms the presence of eclogite (sensu stricto) lenses in the basement complex, thus documenting the northern extent of the North-East Greenland eclogite province formed during the Caledonian collision with Baltica. Holm Land lies in the eastern block of the eclogite province, where an ultrahigh-pressure (UHP) metamorphic event took place at 365–350 Ma. Zircon from a Holm Land eclogite lacks a Eu anomaly, has a flat heavy rare earth element pattern, and gives a sensitive high-resolution ion microprobe U-Pb age of 423 ± 7 Ma, and it is thus interpreted as the time of high-pressure (HP) metamorphism. This age overlaps with the established age of widespread HP metamorphism for the eclogite province (i.e., 415–395 Ma), rather than the younger UHP metamorphism. Westward thrusting of the North-East Greenland eclogite province onto the Laurentian margin after 395 Ma and subsequent exhumation of this uppermost thrust sheet provided a substrate for Carboniferous deposition.
Detrital zircon age spectra from arkosic sandstones of the late Viséan (ca. 330–340 Ma) Sortebakker and early Moscovian (ca. 310–315 Ma) Kap Jungersen Formations record the progressive unroofing of the North-East Greenland Caledonides. All seven samples have a major peak at 1.8–2.0 Ga, and five also have a 1.75 Ga peak, matching the Paleoproterozoic arc and later anorogenic granitoids that comprise the crystalline basement. Paleozoic grains are sparse in the Sortebakker sandstones, but they constitute a pronounced 400 Ma peak in the younger Kap Jungersen Formation. The composition of the detritus—including garnet clasts, the high amount of discordant zircon (40%), and the large numbers of grains with metamorphic rims that cluster around 410 Ma—reflects a local provenance sourced in the North-East Greenland eclogite province, with some input from the overlying thrust sheets. Other Devonian and Carboniferous basins within and peripheral to the Caledonides also show distinct signatures, demonstrating that there is not a simple, representative detrital zircon signature for the Caledonian orogen.
Collisional orogens are an important end-member source of detrital zircon in the sedimentary record (Cawood et al., 2012) that may have a significant influence on the detrital zircon age spectra observed in sedimentary deposits located considerable distances (i.e., thousands of kilometers) from the orogen (e.g., Rainbird et al., 2012). In their analysis of a typical collisional orogenic signature, Cawood et al. (2012) examined foreland basin deposits from the Grenvillian, Cordilleran, Appalachian, and Himalayan orogens. The detrital signature of this setting is characterized by minor components that reflect the depositional age, up to 50% attributed to pre- and syncollision magmatism within 150 m.y. of the depositional age, and 50% or greater related to the age of crustal material involved in the collision. In addition to the foreland setting, intramontane or hinterland basins within collisional orogens may contain a significant volume of detritus (e.g., Horton, 2012). In some cases, such as the Paleozoic Caledonides of Scandinavia and Greenland, the hinterland basins provide the best record of the collisional detrital zircon signature (e.g., Slama et al., 2011). The detrital signature from the Devonian–Carboniferous hinterland basins of East Greenland then becomes the best proxy for the characteristic “Caledonian” signature commonly referred to in analysis of Arctic terranes (e.g., Colpron and Nelson, 2009; Beranek et al., 2010; Anfinson et al., 2012). However, little is known of the spatial and temporal variation of the detrital zircon signature from different hinterland basins in the northern Caledonides.
This contribution provides additional characterization of the late orogenic hinterland basin detrital zircon signature from the Greenland Caledonides and evaluates the spatial variation of the detrital zircon signature within the orogen. In addition, this study addresses the temporal changes in detrital zircon signature and explores the relationship between basement tectonics and basin development during the waning stages of the Caledonian orogeny and beginning of rifting in the northern segment of the Greenland Caledonides. The northernmost occurrence of eclogite in the North-East Greenland eclogite province is documented and dated, together with other components of the crystalline basement that floors the Carboniferous basin. Detrital zircon U-Pb geochronology on seven samples from the Viséan and Moscovian sedimentary rocks in the basin relates exhumation of the basement to sedimentation. The sediments are found to be locally sourced, and they have a quite different provenance than Devonian to Carboniferous basins of central East Greenland (Slama et al., 2011). This result indicates that hinterland basins likely differ in detrital zircon signature from foreland basins, and this finding is significant for interpretations of so-called “Caledonian” detrital zircon signatures in Arctic terranes.
The Caledonides are a >1400-km-long orogen that formed during the Paleozoic collision of Laurentia with Baltica (McKerrow et al., 2000). Laurentia is generally thought to have formed the upper plate in this collision (Gee, 1975; Hossack and Cooper, 1986), which makes Greenland analogous to the Tibetan Plateau on the Eurasian plate in the present-day Himalayan collision. After the Caledonian collision, Laurentia and Baltica were part of the supercontinent Pangea, and they did not rift apart until the Cenozoic formation of the Atlantic Ocean (Faliede et al., 2008). The northern margin of Pangea—including Greenland, Spitsbergen, and Norway—evolved into a rifted margin with half grabens and stable platforms tracing the fringe of the continent during the late Paleozoic. Continental material was eroded to fill the basins, and these sediments provide a record of basement exhumation and erosion during the late Paleozoic.
Regional Geology of North-East Greenland Caledonides
The Greenland Caledonides are exposed along the northeast coast of Greenland and can be split into northern and southern segments at Bessel Fjord (76°N) based on structural and metamorphic patterns (Gilotti et al., 2008). The thrust geometry in the southern sector has been significantly modified by extensional fault systems (Gilotti and McClelland, 2008), whereas the onshore geology of the northern sector (Fig. 1) is a relatively straightforward sequence of west-directed thrust sheets (Higgins et al., 2004; Leslie and Higgins, 2008) that carry progressively deeper parts of the Laurentian margin seen in the east. The foreland of the orogen is best displayed in Kronprins Christian Land, where Silurian to Neoproterozoic sedimentary rocks overlie a cover of autochthonous Paleoproterozoic to Mesoproterozoic clastic sedimentary and volcanic successions (Collinson et al., 2008). These units form the footwall of the Caledonian sole thrust, which displaces the entire section. The frontal thrust sheets contain the unmetamorphosed Silurian to Neoproterozoic rocks, while higher thrusts carry the older, deeper clastic and bimodal volcanic section, including the Independence Fjord Group. The structurally highest Nørreland thrust (exposed in Nørreland; Fig. 1) transports the Precambrian crystalline rocks of Laurentia over the Paleoproterozoic to Mesoproterozoic cover. The Inland Ice obscures the Caledonian sole thrust south of 80°N, but relationships between the Nørreland thrust sheet and the underlying Paleoproterozoic to Mesoproterozoic thrust sheets are well exposed in nunataks, particularly in Dronning Louise Land (Strachan et al., 1992).
The Nørreland thrust sheet contains the deepest crustal section, including the ∼40,000 km2 North-East Greenland eclogite province (Fig. 1; Gilotti, 1993), which is mainly composed of quartzofeldspathic orthogneiss with subordinate mafic and ultramafic lenses. The main protolith of the Precambrian gneiss is a 2.0–1.8 Ga juvenile, Paleoproterozoic calc-alkaline arc that was later intruded by 1.75 Ga anorogenic granitoids (Kalsbeek et al., 2008a, and references therein). The mafic rocks were former dikes, sills, and plutons for which protolith ages are largely unknown, but they were emplaced into the continental crust prior to the Caledonian collision. In the North-East Greenland eclogite province, mafic rocks preserve relict eclogite-facies assemblages consisting of garnet + omphacite + rutile ± quartz ± zoisite ± kyanite ± phengite (Gilotti et al., 2008, and references therein). The eclogites reached high-pressure (HP) conditions between 415 and 395 Ma (Gilotti et al., 2004; Hallett et al., 2014), probably due to overthickening of the crust in the upper plate of the collision with Baltica. Ultrahigh-pressure (UHP) conditions were attained ∼50 m.y. later (McClelland et al., 2006; Gilotti et al., 2014) on a small island at 78°N (informally named Rabbit Ears Island) in the easternmost hinterland of the orogen. Strike-slip faults divide the Nørreland thrust sheet into three blocks. The NNE-striking, sinistral Storstrømmen shear zone separates the western block from the central block (Fig. 1; Holdsworth and Strachan, 1991), while the NNW-striking, dextral Germania Land deformation zone (Hull and Gilotti, 1994) juxtaposes the central and eastern blocks. The ductile strike-slip movement is roughly the same age on both structures and took place from ca. 370 to 340 Ma (Sartini-Rideout et al., 2006; Hallett et al., 2014).
North Greenland, the Norwegian Barents Sea, and Svalbard formed part of the northern Pangean shelf during the Carboniferous and Permian (Stemmerik, 2000; Stemmerik and Worsley, 2005). Mississippian extension produced two linked rift arms: an Atlantic rift between Norway and Greenland that reached northeastward to the Barents Sea, and an Arctic rift between Greenland and Spitsbergen that joined the Sverdrup Basin in the west. As a result, rapidly subsiding half grabens and stable platforms were created, and widespread floodplains formed along the margin of Pangea during Tournaisian and Viséan time, possibly with shallow shelf deposits further to the north (Stemmerik and Worsley, 2005). Uplift and erosion during the Serpukhovian resulted in a regional unconformity (Stemmerik et al., 1991). It was followed by Bashkirian rifting and nonmarine deposition in isolated half grabens. A widespread marine transgression reached the Wandel Sea Basin by the early Moscovian and resulted in deposition of shallow marine siliciclastics and carbonates along the margins and deeper-water sediments along the rift axis (Stemmerik, 2000). This overall pattern continued to the end of the Kazemovian or Gzelian, with warm-water carbonate deposition during sea-level highstands, and temporarily exposed vast areas of the shelves during lowstands (Stemmerik and Worsley, 2005).
Late Paleozoic basins that record the unroofing of the Caledonides fringe the present-day coast of North-East Greenland (Fig. 1). The Wandel Sea Basin in the remote north is partially exposed onshore. Carboniferous to Paleocene sedimentary rocks are deposited directly on Precambrian crystalline rocks of the Caledonides. The sediments exposed in Holm Land and Amdrup Land in the southeastern corner of the basin are linked to the evolution of the rift between Greenland and Norway and have experienced less overburden and structural deformation than the sediments further to the north (Håkansson and Stemmerik, 1989). The maximum burial depth of the Moscovian succession in Holm Land and Amdrup Land is less than 2 km (Stemmerik et al., 1998). The outcrops in Holm Land and Amdrup Land form the northern extension of the offshore Koldewey Platform at the western margin of the Danmarkshavn Basin, a deep offshore basin that occupies most of the present-day continental shelf between the Bivrost fracture zone at ∼74°30′N and the Hovgaard fracture zone at 80°N (Hamann et al., 2005; Gautier et al., 2011). The Danmarkshavn Basin is over 13 km deep in the south and shallows northward, where salt domes are seen; it is thought to preserve a more or less continuous sedimentary record from Devonian to Cenozoic time (Hamann et al., 2005). The sedimentary succession in the Danmarkshavn Basin has not been drilled, but it has been deduced by comparing seismic characteristics with the Jameson Land basin in the south, which has a comparable 8 km of Carboniferous to Cretaceous sediments. The Carboniferous synrift succession in East Greenland consists of Tournaisian–Viséan and Bashkirian terrestrial deposits (Vigran et al., 1999), thus limiting the northern margin of Pangea to an embayment at the Danmarkshavn Basin (Stemmerik, 2000).
GEOLOGY OF HOLM LAND
The northernmost Precambrian crystalline basement in the Greenland Caledonides and the southernmost sedimentary rocks of the Wandel Sea Basin are exposed in Holm Land (∼80°15′N, 17°W; Fig. 2). The basement has been mapped as part of the North-East Greenland eclogite province (Jepsen, 2000), and herein, we present documentation that this is the case. A thick sequence of Carboniferous to Permian sedimentary rocks belonging to the Wandel Sea Basin unconformably overlies the basement (Stemmerik et al., 1998). The poorly exposed, north-striking East Greenland fault zone forms the western boundary of the Wandel Sea Basin (Håkansson and Stemmerik, 1989). It may have started as a strike-slip fault that was the northern continuation of the Devonian–Carboniferous, sinistral Storstrømmen shear zone (Holdsworth and Strachan, 1991; Hallett et al., 2014), and been reactivated as a normal fault during subsequent extension between Baltica and Laurentia (Døssing et al., 2010). Allochthonous metasedimentary rocks of the Paleoproterozoic to Mesoproterozoic Independence Fjord Group lie to the west of the East Greenland fault zone (Leslie and Higgins, 2008). They consist of quartz-rich to arkosic sandstones and conglomerates intruded by dolerite dikes, tholeiitic to andesitic basalts, and rhyolites (Collinson et al., 2008).
The crystalline basement complex is dominated by strongly deformed and metamorphosed quartzofeldspathic gneiss that locally contains mafic layers and lenses (Fig. 3A). The gneiss is garnet rich (Fig. 3B), with meter thick zones of protomylonite and augen mylonite (Fig. 3C). Intrafolial isoclinal folds of gneissosity (Fig. 3D) are ubiquitous, whereas various orientations of superimposed folds are local. The gneiss is assumed to have the same Paleoproterozoic protolith as the crystalline basement south of 79°N (e.g., Kalsbeek et al., 1999), as well as a similar overprint of Caledonian metamorphism. In addition, there are strongly deformed leucogabbros and anorthositic gneisses, which are common on the outer islands of Jøkelbugt, the eastern block (Hull et al., 1994), and the Dove Bugt area (Chadwick and Friend, 1994). Some mafic to ultramafic rocks enclosed by the gneiss contain garnet and clinopyroxene that reflect eclogite-facies metamorphism (Figs. 3E and 3F), although retrograde garnet amphibolite is perhaps more common. Garnet websterite, similar to those found in the North-East Greenland eclogite province and interpreted as the ultramafic parts of layered, lower-crustal intrusions (Gilotti, 1993, 1994), also occurs. Late granitic sheets and pegmatites cut the gneisses and mafic rocks, and are themselves deformed (Figs. 3G and 3H).
The oldest unit of the Wandel Sea Basin, the upper Viséan Sortebakker Formation, rests unconformably on the crystalline basement complex along the southern coast of Holm Land (Figs. 2 and 4). The Sortebakker Formation is ∼1000 m thick and consists of nonmarine, fluvial, siliciclastic sedimentary rocks interbedded with coal (Dalhoff and Stemmerik, 2000). A late Viséan age is assigned based on poorly preserved miospores (Dalhoff et al., 2000). Coarse-grained channel sandstones at the base of the section give way to 0.5–6-m-thick, fining-upward cycles of sandstone and black shale, which are dominated by mudstone (Fig. 5A). A prominent disconformity found approximately one third the way up the section separates the shale-dominated rocks below from the sandstone-dominated, 3–20-m-thick cycles above (Dalhoff and Stemmerik, 2000). Coal is more common above the disconformity. The facies association of channel sandstones, overbank fines, crevasse splay sandstones, and levee deposits points to deposition in a floodplain by a meandering river system. Lacustrine and swamp deposits are found near the top of the section.
A pronounced unconformity separates the Viséan Sortebakker Formation from the overlying Upper Carboniferous Kap Jungersen and Foldedal Formations (Figs. 5B, 5C, and 5D). Deformation of the Sortebakker Formation resulted in dips up to 30° toward the west and increased thermal maturity that destroyed most of the organic matter (Dalhoff and Stemmerik, 2000). Basin models suggest that 2000 m of sediment were eroded prior to the deposition of the Kap Jungersen Formation (Stemmerik et al., 1998). A prominent conglomerate marks the base of the Kap Jungersen Formation. The clasts are subangular to subrounded and poorly sorted and vary in size from 1 to 10 cm; they lie in a coarse sandy matrix. The majority of the clasts are quartzofeldspathic gray gneiss, pink granitic gneiss, mylonitic gneiss, and vein quartz derived from the Precambrian basement (Fig. 5E). Conspicuous single crystals of garnet (Fig. 5F) and K-feldspar point to a proximal source. The 300-m-thick Kap Jungersen Formation consists of thick sandstone and minor conglomerate in the lower part and cyclic shallow marine limestones interbedded with inner-shelf siliciclastic deposits toward the top (Fig. 5D; Stemmerik et al., 1998). The formation is of early Moscovian age based on fusulinids (Stemmerik et al., 1998; Davydov et al., 2001). The overlying Upper Carboniferous Foldedal Formation is a carbonate shelf.
Zircon separates of three meta-igneous and seven sandstone samples from Holm Land were obtained by standard pulverizing, magnetic, and heavy liquid methods. Sample locations are given in Supplemental Table 11 and shown on Figure 2. Aliquots for analysis by secondary ion microprobe spectrometry (SIMS) at the U.S. Geological Survey–Stanford University sensitive high resolution ion microprobe–reverse geometry (SHRIMP-RG) facility were handpicked under alcohol and mounted in epoxy resin with natural zircon standards before being polished to expose grain centers. Representative aliquots for detrital zircon analysis by laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) at Washington State University laboratory and the University of Arizona LaserChron facility were randomly selected from the heavy mineral separates, cleaned of nonzircon under alcohol, mounted in epoxy resin with natural zircon standards, and polished to expose an area on most grains large enough for analysis with a 30–40 μm spot.
Zircon grains from three meta-igneous samples of basement units and metamorphic rims on detrital zircon grains were imaged by cathodoluminescence (CL) to expose intragrain zoning or complexity and aid in placing SIMS spots. The U-Pb (Supplemental Table 22) and trace-element analysis (Supplemental Table 33) was performed simultaneously following routines outlined in Barth and Wooden (2006) and Mazdab and Wooden (2006). Fractionation corrections were calibrated by replicate analysis of the zircon standard R33 (421 Ma; Black et al., 2004; Mattinson, 2010). Ages were calculated from 206Pb/238U ratios corrected for common Pb using the 207Pb method and 207Pb/206Pb ratios corrected for common Pb using the 204Pb method (see Williams, 1998). Initial common Pb isotopic composition was approximated from Stacey and Kramers (1975). The U concentration was calibrated with MAD-green (4196 ppm U; Barth and Wooden, 2010). Samples were analyzed during two separate sessions on two different mounts, with 2σ calibration errors for the R33 206Pb/238U ratios of 0.84% and 0.34%. The trace-element routine collected 139La, 140Ce, 146Nd, 147Sm, 153Eu, 157Gd16O, 163Dy16O, 166Er16O, 172Yb16O, and 180Hf16O, with concentrations calibrated against zircon standard MAD (Mazdab and Wooden, 2006). Data reduction and plotting utilized programs Squid 1.13b (Ludwig, 2001) and Isoplot 3.00 (Ludwig, 2003).
Detrital zircons separated from samples 06–12, 06–19, 06–22, and 06–25 were analyzed for U/Pb using a New Wave UP-213 (213 nm, Nd:YAG) laser system coupled to a Thermo Finnigan Element2 ICP-MS instrument at Washington State University (Supplemental Table 44). The U/Pb analytical routine and data reduction followed Chang et al. (2006). Fractionation factors for the 206Pb/238U and 207Pb/206Pb ratios were established by repeat analysis of natural zircon standards Peixe (564 Ma; Dickinson and Gehrels, 2003), FC-1 (1099 Ma; Paces and Miller, 1993), and R33 at the start, end, and after each block of five unknowns. Zircons from samples 06–20, 06–21, and 06–28 were analyzed at the Arizona LaserChron Center on a Nu Instruments high-resolution ICP-MS instrument attached to a New Wave UP193HE Excimer laser (Supplemental Table 4) following protocols discussed in Gehrels et al. (2006, 2008). Common Pb corrections were made using 204Hg-corrected 204Pb measurements for each analysis, and initial Pb compositions of Stacey and Kramers (1975). Pb/U fractionation and U and Th concentrations were determined through repeat analysis of Sri Lanka (SL) zircon standard (563.5 ± 3.2 Ma; ∼518 ppm U and 68 ppm Th; Gehrels et al., 2008) and R33. Analyses that were >10% discordant, >5% reversely discordant, or had large uncertainties (>10% at the 2σ level) are considered separately in the following discussion. Normalized probability distribution functions were plotted for each sample using the Excel 2003 macro provided by G. Gehrels and available from the Arizona LaserChron Center at www.laserchron.org. The 206Pb/238U ages are preferred for apparent ages younger than 1000 Ma, and 207Pb/206Pb ages are used for analyses older than 1000 Ma. In cases where the 206Pb/238U and 207Pb/206Pb ages straddle 1000 Ma, the age with lower uncertainty was used. Data reduction and plotting utilized in-house programs generated at the Arizona LaserChron Center and Isoplot 3.00 (Ludwig, 2003).
U-Pb GEOCHRONOLOGY OF PRECAMBRIAN BASEMENT ROCKS
A representative eclogite (06–02), intermediate gneiss (06–03), and pegmatite (06–08) were chosen for U-Pb zircon dating and trace-element characterization in order to fit the crystalline basement of Holm Land into the larger context of the North-East Greenland eclogite province.
The eclogite (Figs. 3E and 3F) is strongly deformed and partially retrogressed with small amounts of biotite, amphibole, and oxide minerals. Garnet contains inclusions of the peak pressure mineral assemblage (omphacite, quartz, and rutile) and forms subhedral, elliptical porphyroclasts in a fine-grained, foliated matrix of clinopyroxene, quartz, and plagioclase. Omphacite inclusions in garnet have a maximum jadeite content of Jd28, whereas clinopyroxene in the matrix is diopside with Jd7–17 (Supplemental Table 55). Wormy symplectites of diopside and plagioclase replace former matrix omphacite. Quartz forms thin, very fine-grained, polycrystalline, dynamically recrystallized ribbons. Retrogressed eclogites have been mapped as far north as the north coast of Holm Land (Fig. 1; Jepsen, 2000), and sample 06–02 is now the northernmost eclogite with documented omphacite.
The eclogite pods and layers lie within garnet-rich quartzofeldspathic to mafic gneiss containing locally abundant pegmatite and leucosomes. Sample 06–03 was collected from a homogeneous, strongly deformed, fine-grained layer of a garnet-rich leucogabbroic gneiss (Fig. 3G) located ∼100 m on strike from the eclogite sample. The rock contains garnet, plagioclase, quartz, and clinopyroxene with minor biotite, chlorite, amphibole, and accessory rutile, opaque minerals, and zircon. Kyanite inclusions in garnet and the clinopyroxene-rich matrix point to a HP history.
Sample 06–08 is from a penetratively foliated pegmatite sheet (Fig. 3H) consisting of coarse plagioclase porphyroclasts, ribbon quartz and white mica. The foliation within the pegmatite is parallel to the granulite to amphibolite facies foliation in the adjacent host gneiss and both are locally folded into steeply north-plunging, upright folds.
Results and Interpretation
Eclogite sample 06–02 yielded complex zircon that varies from elongate euhedral to rounded grains with broad oscillatory or sector zoned interiors and variably developed, unzoned to faintly oscillatory zoned mantles and rims (Fig. 6A). Some grains display clear core-rim relations, whereas the distinction in most grains is fuzzy. All of the zircon domains have generally low U (18–80 ppm) and Th (10–106 ppm), and relatively high Th/U ratios (0.2–1.8). The trace-element signature, characterized by flat heavy rare earth element (HREE) patterns (Yb/Gd = 1.1–4.8) and no negative Eu anomaly (Eu/Eu* = 0.9–1.0), is similar for all analyzed domains (Fig. 7A). The U-Pb analyses spread from 400 to 1000 Ma, with clusters near the upper and lower ages and considerable scatter from a simple linear array (Fig. 7A). A planar three-dimensional regression through all of the data yields intercepts with uncertainties >100 m.y., indicating that the zircon systematics are complicated. The four youngest analyses, mostly from sector zoned round grains, give a weighted mean 207Pb-corrected 206Pb/238U age of 423 ± 7 Ma (mean square of weighted deviates [MSWD] = 1.0). The oldest nine analyses, mostly from elongate grain interiors with variable zoning, give a weighted mean 207Pb/206Pb age of 947 ± 63 Ma (MSWD = 1.7) and an upper intercept of 980 ± 70 (MSWD = 1.2) when constrained to a 423 ± 7 Ma lower intercept.
The younger age is interpreted as the approximate age of eclogite-facies metamorphism, although it is slightly older than the 395–415 Ma age determined for the widespread HP metamorphism to the south (Gilotti et al., 2004; Hallett et al., 2014). The upper intercept age of ca. 980 Ma is interpreted either as the emplacement age of the mafic protolith or the timing of a previously unrecognized Neoproterozoic HP metamorphic event. The few eclogites in North-East Greenland that have protolith ages are Paleoproterozoic (Brueckner et al., 1998; Gilotti et al., 2004; Hallett et al., 2014), and thus were originally part of the 2.0–1.8 Ga calc-alkaline arc; however, many obvious crosscutting dikes have never been dated. A Neoproterozoic metamorphic age is consistent with the observed flat HREE pattern and lack of a negative Eu anomaly that is characteristic of eclogite-facies metamorphic zircon (Rubatto, 2002). The only comparable data from the eclogite province are three ca. 950 Ma zircon rim ages from a metagranitoid that cuts an eclogite in Lambert Land (Kalsbeek et al., 1999). Metamorphic ages of ca. 950 Ma have been observed south of the eclogite province (i.e., south of 76°N) in the Fjord region (Strachan et al., 1995; Thrane et al., 1999; Kalsbeek et al., 2000; Watt et al., 2000). Evidence in support of either interpretation is not compelling, and additional work is required.
The leucogabbroic gneiss sample 06–03 taken near the eclogite locality provided a population of equant to elongate zircon grains with CL-dark cores, CL-intermediate mantles and irregular zones of recrystallization, and CL-bright rims (Fig. 6). Some grains are CL-bright and appear to be entirely recrystallized with rare faint relict zoning preserved. The CL-dark cores have trace-element patterns with relatively steep HREE patterns (Yb/Gd = 7–17) and a negative Eu anomaly (Eu/Eu* = 0.35–0.43) typical of igneous zircon (Fig. 7B). The CL-bright grains and rims have lower REE abundances and reduced Eu anomalies (Eu/Eu* = 0.7–1.4), consistent with zircon recrystallization or growth at eclogite-facies conditions. The U/Pb data define a linear array with considerable scatter. Excluding three spots that overlapped epoxy during analysis (11.1, 12.1, and 14.1), the remaining analyses define a discordia with upper and lower intercepts of 1878 ± 71 Ma and 448 ± 34 Ma, respectively (MSWD = 4.6). The upper-intercept age is consistent with Paleoproterozoic ages in the North-East Greenland eclogite province (Kalsbeek et al., 2008a; Hallett et al., 2014) and with a Sm-Nd model age of 2146 Ma for a gabbro-anorthosite from the Dove Bugt area (Stecher and Henriksen, 1994). The poorly defined lower intercept is within uncertainty of 415–395 Ma HP metamorphism observed to the south (Gilotti et al., 2004; Hallett et al., 2014). The considerable scatter in the analyses likely reflects a complex history of recrystallization and Pb loss. We cannot demonstrate that any of the analyses represent entirely newly grown, eclogite-facies zircon; therefore, the lower intercept value could be viewed as a maximum age of metamorphism of the Holm Land basement gneisses.
Granitic pegmatite sample 06–08 contains elongate euhedral featureless CL-dark zircon (Fig. 6) with very high U concentrations (3280–8455 ppm) and low Th/U ratios (0.01–0.02). The analyses show high chondrite-normalized REE concentrations with elevated light (L) REE patterns (Fig. 7C). Three-dimensional linear regression of the data, including three analyses with >10% common Pb and using an unconstrained common Pb value, give a lower-intercept age of 440 ± 18 Ma (MSWD = 5.4). Constraining the common Pb value using the values of Stacey and Kramers (1975) gives a lower-intercept age of 435 ± 17 Ma (MSWD = 5.4). The pegmatite age indicates that Caledonian magmatism occurred as far north as Holm Land. The presence of small intrusive sheets in the eclogite-bearing basement is also seen to the south at Sanddal in the western block (Hallett et al., 2014). The age of the granitic pegmatite is consistent with the age of the much more voluminous 445–432 Ma subduction-related granites in southern East Greenland or the 435–420 Ma leucogranites (Kalsbeek et al., 2008b).
DETRITAL ZIRCON GEOCHRONOLOGY OF CARBONIFEROUS SEDIMENTARY ROCKS
Four sandstones from the Sortebakker Formation were analyzed for detrital zircon; from bottom to top of the section, they are 06–12, 06–28, 06–20, and 06–19 (Fig. 4). Sample 06–12 is a coarse-grained sandstone with abundant plant remains (Fig. 8A) collected from an isolated outcrop near the basal contact reported by Dalhoff and Stemmerik (2000). Sample 06–28 is a 1-m-thick, fine- to coarse-grained, muscovite-rich sandstone layer (Figs. 8B and 8C) collected from the upper portion of the lower shale-dominant unit of the Sortebakker Formation (Fig. 4), immediately west of the unconformable contact with the overlying Kap Jungerson Formation (Fig. 2). Sample 06–20, from the sandstone-dominated rocks immediately above the disconformity (Fig. 4), is a very coarse-grained, 5-m-thick channel sandstone with mud-flake clasts and cross-bedding (Figs. 8D and 8E). The stratigraphically highest sample, 06–19, was collected from the western Sortebakker Formation exposures near palynological sample 420910 reported in Dalhoff et al. (2000). The sample is a medium-grained sandstone with trough cross-bedding (Fig. 8F) collected from a 2-m-thick channel in a >20-m-thick sandstone-dominated sequence. All of the Sortebakker Formation samples are siliceous, submature to mature arkoses with grains of quartz, feldspar, white mica, lithic fragments, and degraded organic material in a partially sericitized silica cement. Many of the quartz clasts are polycrystalline with high-temperature, dynamically recrystallized fabrics (Fig. 9A), indicating derivation from basement gneisses and mylonites. Some of the feldspar grains are perthitic, which also indicates a high-temperature, metamorphic source. Kinked white mica grains (Figs. 9B and 9C) and pressure solution features (Fig. 9D) confirm a significant amount of postdiagenetic deformation consistent with tilting of the section.
Zircon was analyzed from three samples of the Kap Jungersen Formation. The samples are medium- to coarse-grained, moderate to poorly sorted arkosic sandstones. Sample 06–22 is a sand wedge within the basal conglomerate (Fig. 10A). Sample 06–21, also from the lower part of the unit, is a channel sand that drapes a karst surface of limestone (Fig. 10B). Sample 06–25 is a cross-bedded sandstone beneath a conglomerate-filled channel near the top of the formation (Figs. 10C and 10D). The composition is similar to that of the Sortebakker Formation samples, with a relatively high abundance of mica, polycrystalline quartz, single feldspar crystals, and lithic fragments of gneiss and mylonite (Fig. 9E). In addition, up to 5% of the grains are single crystals of garnet (Fig. 9F) and epidote. Fragments of sedimentary grains, such as chert and siltstone, are also found. The arkoses are cemented by calcite; in places, grains are completely surrounded by poikilotopic calcite (Fig. 9F).
LA-ICP-MS Results and Interpretation
Sandstones from the Sortebakker and Kap Jungersen Formations provided populations of equant to elongate, round to subround zircon with oscillatory or patchy zoning and areas of recrystallization. Approximately 30%–50% of the grains have <1-μm- to >20-μm-wide, CL-bright, low-U rims. Between 85 and 168 grains were analyzed for each sample, and of these, between 31% and 52% of the analyses were >10% discordant or >5% reversely discordant (Fig. 11). Discordant data loosely define linear arrays trending down toward 400 Ma, consistent with Pb loss or recrystallization during Caledonian metamorphism. The following discussion of the age populations is based on those grains that pass the 10% normal to 5% reverse discordance filter, plotted in probability density diagrams as black curves (Fig. 12). Exclusion of the discordant data does not significantly alter the age of the major peaks, as shown in Figure 12, where the combined population of discordant and concordant data is plotted as gray curves. These peaks are typically broadened and skewed to the younger side of the peak given by the concordant data alone.
Zircon from the basal Sortebakker sample 06–12 defines a prominent peak at 1955 Ma; minor peaks at 2540 Ma and 2710 Ma are defined by 21 grains that are older than 2100 Ma (Fig. 12). Samples 06–20 and 06–28 display minor to major peaks at 1750–1765 Ma, 1900 Ma, and 1965–1970 Ma, and have 5 and 10 grains that are older than 2100 Ma, respectively. Sample 06–19 has a prominent peak at 1892 Ma and minor peaks at 1760 Ma and 1490 Ma. Minor peaks at 2560 Ma and 2665 Ma are defined by 10 grains that are older than 2100 Ma. Sample 06–22 at the base of the Kap Jungersen Formation defines a major peak at 1980 Ma, contains one Archean zircon, and has a minor peak at 390 Ma defined by 10 grains. Sample 06–21 from the middle of the Kap Jungersen Formation displays two dominant peaks at 1740 and 1980 Ma and a minor peak at 390 Ma defined by eight grains. The stratigraphically highest sample, 06–25, has major peaks at 1740 and 1910 Ma and minor peaks at 340 and 360 Ma defined by 12 grains. Both 06–21 and 06–25 yielded four analyses with ages older than 2100 Ma.
A prominent peak at 1900–2000 Ma is present in all of the samples from the Carboniferous section, but the appearance of the major peak at 1750 Ma is variable (Fig. 12). The 1750 Ma peak is best developed in the two samples in the middle of the Sortebakker Formation, whereas the 1750 Ma peak becomes more prominent up section in the Kap Jungersen Formation. The presence or absence of the 1750 Ma peak (which matches the age of the anorogenic metagranites) most likely reflects heterogeneity in the source region, but the significance of variation in the magnitude of specific peaks is difficult to evaluate due to the low number of analyses per sample (n ≤ 168). There is a subtle shift from more grains older than 2100 Ma in the Sortebakker Formation (6%–28%) to fewer in the Kap Jungersen Formation (1%–4%). This shift is accompanied by an increase in grains with Paleozoic ages from the Sortebakker (0%–1%) to the Kap Jungersen (13%–15%) Formation (Fig. 12).
Zircon that yields Paleozoic ages includes euhedral to subhedral grains with well-developed oscillatory zoning (Fig. 13A), grains with distinct, typically CL-bright, low-U rims (Fig. 13B), and grains with thin rims and varying degrees of recrystallization in the cores, which result in a range of concordant to discordant analyses depending on the degree of recrystallization (Fig. 13C). Paleozoic grains in sample 06–21 have low Th/U ratios (0.002–0.05), suggesting a metamorphic origin, but this characteristic is not definitive. The euhedral, oscillatory zoned Paleozoic detrital zircon is similar in CL-appearance, chemistry, and age to pegmatite and leucosome zircon observed from the North-East Greenland eclogite province (e.g., Sartini-Rideout et al., 2006; Gilotti and McClelland, 2007; McClelland et al., 2009; Gilotti et al., 2014; Hallett et al., 2014). Therefore, some of the Paleozoic grains are new zircon derived from pegmatites and leucosomes, rather than metamorphic overgrowths or recrystallized parts of older zircon.
SIMS Age of Metamorphic Rims—Results and Interpretation
Most of the CL-bright, low-U rims on the detrital zircon grains were too thin to analyze by LA-ICP-MS; therefore, separate fractions were mounted from sample 06–19 from the Sortebakker Formation and samples 06–22 and 06–25 from the Kap Jungersen Formation to analyze by SIMS (Supplemental Tables 2 and 3). Eight core and rim analyses with Proterozoic 207Pb/206Pb ages are concordant to discordant and define a linear array toward a Paleozoic lower intercept (Fig. 14A). Fourteen rim analyses with low U (1–68 ppm) and correspondingly high percentages of common Pb (up to 21%) give Paleozoic 207Pb-corrected 206Pb/238U ages. Many of the REE patterns display elevated LREE indicative of alteration or inclusions (Fig. 14B), and the Proterozoic zircon has higher amounts of REE than the Paleozoic rims. Paleozoic ages from the Kap Jungersen samples range from 495 to 370 Ma and cluster around 410 Ma (Fig. 14C), which is broadly consistent with the age of HP metamorphism observed in the North-East Greenland eclogite province (Gilotti et al., 2004). Accordingly, most of the grains with CL-bright, low-U metamorphic rims were derived from the crystalline rocks of the North-East Greenland eclogite province.
The Precambrian crystalline basement on Holm Land is mapped as the northern continuation of the Paleoproterozoic calc-alkaline arc complex and the North-East Greenland eclogite province based on lithology and field relationships (Jepsen, 2000). The 1878 ± 71 Ma protolith age of a garnet-rich leucogabbroic gneiss (sample 06–03) agrees with ages determined by Kalsbeek et al. (1999) for the nearest dated crystalline basement located ∼100 km to the south in Lambert Land, as well as gneisses further south at Sanddal (Hallett et al., 2014) and Rabbit Ears Island (Gilotti and McClelland, 2011), and it validates this correlation. The 947 ± 63 Ma age from eclogite (sample 06–02) can either be interpreted as a protolith age for the mafic rock or the age of HP metamorphism of the crystalline basement. If the latter interpretation is correct, the metamorphism may either be attributed to the Renlandian orogenic event best defined to the south in East Greenland (Cawood et al., 2010) or inferred to represent the northern continuation of collision-related deformation associated with the Grenville-Sveconorwegian orogen into the Arctic region (Lorenz et al., 2102, 2013). Additional work in the Holm Land region may provide important information necessary to distinguish between these two possible scenarios.
The northernmost eclogites in North-East Greenland are present in Holm Land, which lies in the eastern block of the eclogite province, i.e., east of the Germania Land deformation zone (Fig. 1). The eclogite-facies metamorphism thus has the potential to be as young as 365–350 Ma, the age of coesite-bearing zircon from UHP eclogites on Rabbit Ears Island (McClelland et al., 2006; Gilotti et al., 2014). However, zircon from the Holm Land eclogite gives a metamorphic age of 423 ± 7 Ma, similar to the 415–395 Ma age of the widespread HP eclogites (Gilotti et al., 2004; Hallett et al., 2014), demonstrating that the entire eastern block did not experience UHP metamorphism. The Nørreland thrust sheet was emplaced after the HP metamorphism (i.e., after 395 Ma) but continued to deform locally during UHP metamorphism at 365–350 Ma (Gilotti and McClelland, 2007). The tectonic scenario for producing multiple HP/UHP metamorphic events within a collisional orogen is illustrated in figures presented in Gilotti and McClelland (2007, 2011). The basement was partially exhumed to the surface by the beginning of deposition of the Sortebakker Formation at ca. 340 Ma.
Local Derivation of Detrital Zircon from a Metamorphic Terrane
The detrital zircon signature of the Carboniferous clastic rocks in Holm Land is interpreted to largely reflect ages observed in the underlying eclogite-bearing crystalline basement. The dominant peaks at 1750 Ma and 1900–2000 Ma in both the Sortebakker and Kap Jungersen Formations match the protolith ages observed in the North-East Greenland eclogite province (Fig. 15). Detrital zircon older than 2100 Ma in the Sortebakker Formation requires input from a source other than the Paleoproterozoic arc. Detrital zircon populations of appropriate age have been observed in both the Trekant Series, correlated with the Paleoproterozoic to Mesoproterozoic Independence Fjord Group, and the Cambrian Zebra Series (Fig. 15) exposed in the foreland west of Holm Land (Cawood et al., 2007). Alternate sources include rare Archean enclaves in the eclogite province (Nutman and Kalsbeek, 1994) or Archean basement exposures south of 73°N that give ages of 2630–2825 Ma, 2980–3070 Ma, and 3605 Ma (Thrane, 2002; Johnston and Kylander-Clark, 2013). The southern basement exposures are intruded by abundant Silurian leucogranites (Fig. 16), so the absence of Paleozoic grains in the Sortebakker Formation argues for recycling of the Archean detrital zircon from Mesoproterozoic and Cambrian sedimentary rocks in thrust sheets in the foreland rather than basement exposures to the south.
Sitting above the Serpukhovian unconformity, the basal Kap Jungersen Formation is dominated by garnet-rich clastic rocks with a pronounced 1980 Ma peak and a minor Paleozoic peak that persists up through the section (Fig. 12). The oldest population of grains (older than 2100 Ma) in the Sortebakker Formation is less abundant to absent in the Kap Jungersen Formation. Paleozoic grains ranging from 330 to 420 Ma define peaks at 340, 365, and 390 Ma (Fig. 16). A population ranging from 410 to 420 Ma is consistent with the age of granitic magmatism observed in the basement. Although results of this study indicate that intrusions of this age are locally present in the basement in Holm Land, Ordovician–Silurian granites are much more abundant south of 76°N, where they are dominated by ca. 430 Ma leucogranites (Fig. 16). Since relatively few 410–420 Ma grains are observed, they are inferred to have been derived from ca. 410–430 Ma pegmatites and leucosomes in the local basement gneiss rather than from a region dominated by 430 Ma leucogranites. The observed 410–420 Ma ages are consistent with the 206Pb/238U ages of igneous and metamorphic grains inferred to have suffered Pb loss (Fig. 7; Supplemental Table 2). The large 380–400 Ma population is the same age as igneous and metamorphic basement units that record the timing of HP metamorphism and amphibolite-facies retrogression during exhumation of the North-East Greenland eclogite province (Fig. 16). The 350–365 Ma and the 330–340 Ma populations match the age of igneous and metamorphic grains from both the HP and UHP terranes of the underlying eclogite province.
Deposition of the Viséan Sortebakker Formation on eclogite-bearing basement indicates that exhumation of the Holm Land portion of the North-East Greenland eclogite province was complete by ca. 340 Ma. This phase of exhumation was broadly contemporaneous with displacement along the dextral Germania Land deformation zone (Sartini-Rideout et al., 2006) and sinistral Storstrømmen shear zone (Hallett et al., 2014), as well as UHP metamorphism at 365–350 Ma (McClelland et al., 2006; Gilotti et al., 2014) on Rabbit Ears Island (Fig. 1). The youngest (330–340 Ma) grains in the Kap Jungersen Formation were forming during deposition of the Sortebakker Formation and demonstrate continued metamorphism and exhumation of the eclogite province basement from depth during Carboniferous basin development at the surface. The angular unconformity separating the Sortebakker and Kap Jungersen Formations broadly coincides with late metamorphic and igneous ages, suggesting that local deformation within the Carboniferous basin was associated with the final exhumation of UHP rocks within the eclogite province.
Characteristic Signature of Detrital Zircon from Metamorphic Terranes
Detrital zircon studies typically apply discordance filters to data sets prior to compilation and discussion of the data. It has long been recognized that detrital zircon from metaclastic rocks yields a high proportion of discordant zircon due to postdeposition metamorphism (e.g., Gehrels et al., 1991). Other studies have demonstrated that a high number of discordant analyses may be due to postdeposition alteration (e.g., Morris et al., 2015). In contrast, the high degree of discordance in this study (∼40%) reflects the metamorphic character of the source terrane rather than postdepositional processes. The presence of coarse clasts derived from the metamorphic basement, combined with the abundance of garnet and pyroxene detritus, requires a local provenance for the Holm Land Carboniferous section. The same conclusion is reached based on the high degree of discordance coupled with the presence of metamorphic rims and recrystallization observed within the detrital zircon population (Fig. 13). While a local origin is obvious for the Holm Land Carboniferous strata, the link back to an eclogite-bearing basement source will be more difficult to establish after future and eventual recycling of these grains into more broadly dispersed basins. Analysis of modern river sands has demonstrated that thin metamorphic rims on zircon can survive sedimentary transport (Hietpas et al., 2011). The age of the metamorphic rims, therefore, provides an important additional provenance tool for detrital zircon studies. A comparison of the time of metamorphism recorded in the detritus with basement history can provide a link between deeper-level structural events and basin evolution.
Comparisons to Other Circum-Arctic Carboniferous Basins
Terrestrial basins formed in many parts of the Caledonides during the later stages of the collision (Fig. 17), typically starting in the Devonian within large-scale strike-slip systems that evolved into extensional settings. Middle to Late Devonian extensional basins from western Norway to the British Isles formed within a large extensional step-over in a sinistral strike-slip system (Fossen, 2010, and references therein). Middle Devonian basins in East Greenland also formed within a sinistral strike-slip system (Larsen et al., 2008a, and references therein), and Silurian–Devonian sedimentary rocks in Svalbard were deposited in strike-slip basins as well (e.g., Friend et al., 1997). The Danmarkshavn Basin, lying offshore North-East Greenland (Fig. 1), is thought to be floored by Devonian strata and may also have initiated as a strike-slip basin (Hamann et al., 2005). Devonian sedimentation within the Caledonian orogen and adjacent regions is inferred to reflect continued erosion of and far-traveled dispersion of detritus from the Caledonian orogen (e.g., Anfinson et al., 2012; Slama et al., 2011). Silurian–Devonian clastic rocks are also preserved east of the Caledonian orogen on Novaya Zemlya and in the Oslo rift (Fig. 17). Basin development and clastic sedimentation in the Carboniferous continued in the Caledonian orogen and adjacent areas with the onset of widespread rifting (Stemmerik and Worsley, 2005; Larsen et al., 2008b).
Figure 18 compares the Wandel Sea Basin detrital zircon data (this study) to other studies around the region. Devonian and Carboniferous clastic rocks from East Greenland yield detrital zircon ages with prominent peaks at 440, 1100, 1700, and 2700 Ma (Slama et al., 2011). The 1000–1100 Ma and 1400–1700 Ma grains are inferred to be derived from recycling of Proterozoic sedimentary units of the Eleonore Bay Supergroup exposed within the thrust stack, whereas the older grains are from the associated Proterozoic and Archean basement. The young ca. 440 Ma grains are sourced from local exposure of granite. The slight increase in the percentage of Paleoproterozoic and Archean grains in the Carboniferous strata is interpreted to reflect erosion of the sedimentary cover within the thrust stack, resulting in an increased component derived from the crystalline basement. The lack of Archean grains in Devonian rocks in Norway indicates separation of provenance regions across the orogen (Slama et al., 2011). The striking contrast between the Carboniferous signatures in East Greenland and North-East Greenland suggests that local provenance prevailed along the Laurentian margin. While there are small populations of ca. 1750 and 1900–2000 Ma grains, the East Greenland samples show a much broader array of Mesoproterozoic ages. The prominence of ca. 440 Ma grains reflects the abundance of granite in the local source region rather than the 400 Ma and younger metamorphic and leucosome grains that characterize the detrital zircon signature in North-East Greenland.
Detrital zircon and heavy mineral data from clastic units in the central North Sea are inferred to record sediment input from East Greenland in the Devonian, followed by recycling of material from local sedimentary sequences by the late Carboniferous (Lundmark et al., 2014). Greenland-sourced material is characterized by a broad spectrum of Proterozoic grains and abundant Silurian grains. Similarly, clastic units in the Clair Basin, west of the Shetland Islands, are inferred to be sourced from East Greenland in the Devonian, but reverted to local basement sources in the Carboniferous based on the prominence of locally derived Archean grains in the latter (Schmidt et al., 2012). An East Greenland provenance for Devonian basins in the North Sea region is permissible based on the general similarity of detrital zircon spectra observed in each area (Fig. 18), supporting models for interconnected clastic basins within the orogen in the Devonian. Carboniferous clastic units preserved in the Oslo rift are dominated by late Paleoproterozoic to Neoproterozoic Fennoscandia-derived detrital zircon recycled from sedimentary rocks in the Scandinavian Caledonides with minor contributions of Ordovician–Silurian detritus shed from the Norwegian Caledonides and Devonian–Carboniferous detritus shed from the Variscan orogen to the south (Fig. 18; Kristoffersen et al., 2014). In detail, it is difficult to distinguish the Paleoproterozoic to Devonian detrital signature of the Laurentian-derived East Greenland basin units (Slama et al., 2011) and the Baltican-derived Oslo rift sedimentary units (Kristoffersen et al., 2014). Nevertheless, the age spectra from the Clair Basin more closely resemble East Greenland, whereas sedimentary rocks to the southeast deposited on the flank of the North Sea High are more like the Baltican-derived Oslo rift signature (Fig. 18; Kristoffersen et al., 2014). This observation suggests that the North Sea Devonian to Carboniferous basins may not all be sourced by orogen-parallel transport from East Greenland. None of the southern basins received significant input from the North-East Greenland basement because they lack the 1750 Ma and 1900–2000 Ma Paleoproterozoic signature (Fig. 18). The return to locally sourced material in the Carboniferous basins of the North Sea is consistent with the age of continued deformation observed in North-East Greenland.
In the Arctic realm, Carboniferous strata of Svalbard and Novaya Zemlya (Fig. 17) also display detrital zircon age spectra that are distinct and consistent with derivation from local sources. A single sample from the Carboniferous Billefjorden Group in Svalbard defined multiple peaks from 960 to 1850 Ma, several small peaks from 2470 to 2770 Ma, and two minor populations with four grains between 360 and 400 Ma and four grains between 500 and 700 Ma (Fig. 18; Gasser and Andresen, 2013). The Carboniferous signature is somewhat similar to ages observed in Silurian–Devonian units in the region (Pettersson et al., 2010), but both sample sets lack the prominent 1750 Ma and 1900–2000 Ma peaks characteristic of the Holm Land strata (Fig. 18). Siliciclastic Carboniferous rocks of northern Novaya Zemlya show a major peak between 500 and 600 Ma, with fewer grains up to 800 Ma (Fig. 18; Lorenz et al., 2013). The signature is similar to that from Cambrian to Ordovician units lower in the section, but very different from the underlying Silurian–Devonian clastic strata, which are dominated by Mesoproterozoic ages characteristic of Sveconorwegian–Grenvillian basement inferred to be involved in the Caledonian orogeny (Lorenz et al., 2013). Although there is a minor ca. 1750 Ma peak, input from a 1900–2000 Ma source region in Novaya Zemlya is missing. The lack of a consistent Carboniferous detrital signature in North-East Greenland, Svalbard, and Novaya Zemlya implies deposition in isolated basins within and distal to the northern Caledonides by Carboniferous time.
All of the data from Devonian and Carboniferous hinterland basins in the Caledonides, except for Carboniferous strata in Svalbard, fit the discrimination criteria for the collisional setting of Cawood et al. (2012; Fig. 18B herein). Thus, the broad controls on detrital zircon signature in collisional orogens apparently persist at least through the late orogenic phase, and detrital zircon signatures from hinterland basins have the same general characteristics as foreland basins. The data from North-East Greenland demonstrate that young grains close to the depositional age in late-stage hinterland basins can be sourced from local basement exhumation. Hinterland basin signatures can serve as a proxy for the detrital zircon signature of collisional orogens where the foreland signature is not available. However, the large variation in detrital zircon ages observed among hinterland basins, interpreted to record variation in local basement age along the orogen, suggests that foreland signatures more adequately characterize the detrital zircon signature of an orogen (e.g., Park et al., 2010). Data from a single hinterland basin is not sufficient to establish an orogen-scale fingerprint for evaluating provenance of far-traveled sediments.
The North-East Greenland eclogite province was exhumed by the time of late Viséan deposition of the Sortebakker Formation, supporting the idea that the eclogite-bearing gneiss was the structurally highest thrust sheet in the northern sector of the Greenland Caledonides.
Sediment in the Carboniferous basins in North-East Greenland was locally sourced from the crystalline basement and strata preserved in thrust sheets in the foreland. The Wandel Sea Basin in Holm Land provides an example of the provenance to be expected from the northern Greenland Caledonides, and this signal is different than the southern sector. Local differences are useful for fingerprinting circum-Arctic terranes.
Sediment from the North-East Greenland eclogite province dominates the Moscovian succession, while the Mesoproterozoic Independence Fjord Group signal from thrust sheets in the foreland goes away.
The prominent Serpukhovian unconformity between the Sortebakker and Kap Jungersen Formations is probably related to exhumation of the UHP rocks.
This work was funded by a Shell International Exploration and Production, Inc., grant to McClelland, National Science Foundation grants EAR-0208236 to Gilotti and EAR-0208158 to McClelland, and a University of Iowa grant from the Office of the Vice President for Research to Gilotti. Ramarao acknowledges a Geological Society of America student research grant and funds from the Department of Earth and Environmental Sciences at the University of Iowa. We are grateful to Joe Wooden and the staff at the U.S. Geological Survey–Stanford sensitive high-resolution ion microprobe–reverse geometry laboratory and George Gehrels and the staff at the Arizona LaserChron facility for help with the analytical sessions. We thank Meredith Petrie for analyzing the eclogite at the University of California–Davis electron microprobe laboratory. The review by Jarek Majka improved the manuscript.