Feiko Kalsbeek

Late Mesoproterozoic to early Neoproterozoic history of the East Greenland Caledonides: evidence for Grenvillian orogenesis?

Detrital zircons from high-grade metasedimentary rocks (Krummedal supracrustal sequence) in the East Greenland Caledonian orogen yield ion-microprobe U–Pb ages mainly in the range 1100–1800 Ma but with a few grains of c. 1000 Ma, different from zircon ages (mainly 1800–2800 Ma) obtained from the crystalline basement that underlies the metasedimentary rocks. These results indicate that original deposition of the Krummedal sequence took place after 1000–1100 Ma ago, and that the sediment was not derived from the underlying crystalline basement, but from younger, at present unknown sources. High-grade metamorphism of the Krummedal sequence and formation of anatectic granites took place around 930 Ma ago. Caledonian granites are also present in the region, but cannot be distinguished on visual criteria in the field from the older granites, unless emplaced into a younger (900–600 Ma) sequence of sedimentary rocks, the Eleonore Bay Supergroup. It is not yet certain whether the high-grade metamorphism and granite formation at c. 930 Ma are related to a ‘Grenvillian’ or slightly younger collisional event, or to an episode of rifting and crustal thinning. If present at all, a ‘Grenvillian’ orogen in East Greenland would be of very different character than that in North America and southern Scandinavia.

From source migmatites to plutons: tracking the origin of ca. 435 Ma S-type granites in the East Greenland Caledonian orogen

Abstract In the Caledonian orogen of East Greenland, superb exposure along steep fjord walls allows direct observation of the origin of ca. 435 Ma S-type granites by anatexis of paragneisses, and collection of the resulting melts into wider sheets and plutons. Field observations suggest that ‘fertile’ metasediments of the late Mesoproterozoic Krummedal supracrustal sequence were the source of the granites, and there is no evidence of participation of the ‘less fertile’ Archaean and Palaeoproterozoic orthogneisses in the region. Comparison of inherited zircons in the granites with detrital zircons in the metasediments by SHRIMP, as well as chemical analyses and Rb–Sr isotope data, support this contention. In addition to the ca. 435 Ma granites, ca. 930 Ma S-type granites are also present; the two age groups cannot always be differentiated on field criteria. The formation of late Caledonian plutons may have been triggered by decompression during gravitational collapse following crustal thickening by Caledonian collision, but structural relationships are too complex to allow a precise interpretation of the tectonic setting of most granites.

Palaeoproterozoic basement province in the Caledonian fold belt of North-East Greenland

Abstract RbSr and SmNd model ages of basement gneisses in the Caledonian fold belt of North-East Greenland show that most of these rocks are related to a Palaeoproterozoic event of crust formation around 2000 Ma ago. UPb within-zircon (SHRIMP) analyses on three samples yield ages of 1974±17, 1764±20, and 1739±11 Ma (2σ values). Archaean rocks (∼ 3000 Ma) are also present, but do not appear to be common. However, SmNd analyses demonstrate that the Proterozoic gneisses may contain substantial proportions of older (probably Archaean) crustal material. The discovery of a major province of mainly juvenile Palaeoproterozoic crust, which extends some 900 km along the coastal outcrops of North-East Greenland, implies that as much as one third of the whole of the Greenland shield may have been generated in the Palaeoproterozoic.

Isotopic and chemical variation in granites across a Proterozoic continental margin—the Ketilidian mobile belt of South Greenland

Abstract Pb-Pb and Rb-Sr isotope systems are compared for Proterozoic granites in the central part of the Ketilidian mobile belt and in the Archaean craton bordering the mobile belt. Very significant differences are found and it is concluded that the granites from the border zone contain large proportions of Archaean crustal material, whereas this is not the case for the granites in the central part of the mobile belt. This shows that most of the Ketilidian mobile belt is not underlain by Archaean crust. The genetic difference between the granites in these two geological settings is also reflected in their chemical compositions. The granites from the central part of the mobile belt have a wider compositional range with, on average, less SiO 2 and more CaO and ferromagnesian constituents than those from the border zone. The latter are leucocratic high-SiO 2 granites, relatively enriched in most incompatible trace elements.

Anatomy of the Early Proterozoic Nagssugtoqidian orogen, West Greenland, explored by reconnaissance SHRIMP U-Pb zircon dating

Reconnaissance SHRIMP U-Pb zircon dating of 90 samples from the Early Proterozoic Nagssugtoqidian orogen of West Greenland, together with previous age and isotope data as well as new field observations, yields information on some large-scale structures of the orogen: 1900–1940 Ma dioritic to tonalitic juvenile arc rocks (positive initial ϵ Nd values) are thrust more than 50 km over Archean gneisses in the central and northern portions of the orogen. Archean and Proterozoic rocks are tectonically interleaved near the southern margin of the orogen. Regional reworking (high-grade metamorphism, shearing, and crustal melting) occurred at 1800–1840 Ma and ca. 1780 Ma. Tectonic emplacement of juvenile Proterozoic arc rocks over Archean gneisses suggests that the Nagssugtoqidian orogen contains slices of unrelated rocks swept together by closure of an ocean of unknown extent.

Geochronology of Archaean and Proterozoic events in the Ammassalik area, South-East Greenland, and comparisons with the Lewisian of Scotland and the Nagssugtoqidian of West Greenland

Abstract The Ammassalik area in South-East Greenland forms a link between the Precambrian terrains of West Greenland and northwest Scotland. It consists mainly of Archaean orthogneisses reworked during a Proterozoic tectonothermal event. Proterozoic ortho- and paragneisses, major leuconorite-charnockite complexes, suites of metadolerite dykes and posttectonic granites also occur. SmNd, RbSr, PbPb and zircon UPb isotope data yield ages of ∼ 3.0–2.8 Ga for Archaean basement gneisses, 2.2-2.1 (?) Ga for Proterozoic gneisses, 1.9-1.8 Ga for the Proterozoic reworking, 1.89 Ga for the leuconorite-charnockite complexes, and 1.7-1.6 Ga for the late granites. Proterozoic reworking has disturbed most Archaean isotope systems, and contamination with Archaean material makes dating of Proterozoic rocks difficult. In several cases discordant dates are obtained from different isotope systems. The chronologies of the Lewisian complex of Scotland and the West Greenland Nagssugtoqidian mobile belt compare closely with that of the Ammassalik area, and support the correlation between these areas suggested by earlier studies. There are, however, also major differences between the Nagssugtoqidian and the mobile belt in the Ammassalik area, and precise correlation across the Inland Ice of Greenland is still ambiguous.

Dating the metamorphism of Precambrian marbles: Examples from Proterozoic mobile belts in Greenland

Abstract Pb-isotope data yield whole-rock isochrons of 1881 ± 20 Ma (mean square of weighted deviates, MSWD, 1.73), 1845 ± 23 Ma (MSWD 1.12) and 1773 ± 22 Ma (MSWD 3.96) on marbles from the Rinkian and Nagssugtoqidian mobile belts of West Greenland and the Ammassalik mobile belt in Southeast Greenland, respectively. The isochron dates are considered to represent the age of metamorphic recrystallisation of the marbles. For the Rinkian and Nagssugtoqidian belts the dates are in good agreement with published ages obtained from RbSr and Pb/Pb data on the other rock types. For the Ammassalik belt there is disagreement with some but not all of the published information; it is possible that the data record two separate phases of metamorphism. The high precision of the dates is due to large variations in Pb-isotopic composition, which appear to be a common feature in carbonate rocks.

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

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

The Nagssugtoqidian orogen in South-East Greenland: Evidence for Paleoproterozoic collision and plate assembly

The 200 km wide, east-west trending Paleoproterozoic mobile belt of the Ammassalik region of South-East Greenland contains a diverse assemblage of Paleoproterozoic and Archean rocks, variably affected by Paleoproterozoic deformations and high-grade low or high pressure metamorphism. By using previous field and geochemical data combined with new zircon dating and zircon trace element geochemistry, this mobile belt is confirmed as a 1870 to 1840 Ma collisional orogen, which contains one or more 1900 to 1880 Ma magmatic suites and northern and southern Archean basement terranes. The most studied 1900 to 1880 Ma magmatic suite is the Ammassalik Intrusive Complex, which is dominated by diorites (with arc-like geochemical signatures and with Paleoproterozoic Nd depleted mantle model ages), which was intruded into sedimentary rocks, (with predominantly Paleoproterozoic detrital zircons). Both these associated rock types show 1900 to 1880 Ma moderate pressure granulite facies metamorphism (7 kbar, ≥ 850°C). Paleoproterozoic mylonites separate the Ammassalik Intrusive Complex from Archean orthogneisses containing 1870 Ma eclogite (≥ 11 kbar, 650 –700°C) and high-pressure granulite facies assemblages in Palaeoproterozoic diabase dike remnants. Associated with these Archean gneisses are pelitic metasediments, marbles and orthoquartzites (with Archean detrital zircon with complex 1870 to 1740 Ma metamorphic rims) that contain kyanite, thereby also showing high-pressure metamorphism. In the Ammassalik area we propose that one or more Paleoproterozoic magmatic arcs with syn-magmatic moderate pressure, high temperature metamorphism were emplaced over the edge of southern Archean continental crust. This resulted in at least doubling of crustal thickness, causing the transient 1870 Ma eclogite to high-pressure granulite facies metamorphic conditions in the buried southern Archean terrane. Archean orthogneisses in the north of the mobile belt preserve low-pressure Archean granulite facies assemblages, and are interpreted as a different terrane at a higher structural level, which was juxtaposed with the southern eclogite-bearing Archean terrane and the lower pressure metamorphism Paleoproterozoic arc rocks during the collisional event(s). This new information from the Ammassalik region is used in a synthesis of Paleoproterozoic crustal accretion and collisional orogeny in Greenland.

The northward extent of the Archaean basement of Greenland — a review of RbSr whole-rock ages

Abstract An outline is given of recent Rb-Sr whole-rock ages from both the Archaean gneiss block of southern Greenland and the areas north of the Archaean block. It is probable that most of Greenland, perhaps with the Ketilidian mobile belt of South Greenland as the only exception, is underlain by Archaean rocks. However, north of the Archaean block sensu stricto, later metamorphism has generally upset the RbSr isotope systems to such an extent that precise age determination by the RbSr whole-rock method is no longer possible.