Hans F. Jepsen

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.

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.

Geochemistry and petrogenesis of S-type granites in the East Greenland Caledonides

In the Kong Oscar Fjord region, East Greenland Caledonides, the formation of peraluminous leucogranite by low-temperature (<800°C) anatexis of metasedimentary rocks, and collection of the resulting melts into granite sheets and plutons can be studied in detail. Field observations suggest that a significant proportion of the biotite in the granites represents remnants of biotite from the metasedimentary source. This complicates geochemical modelling of granite formation, because chemical analyses do not necessarily represent melt compositions. Rb, Sr, Ba, and Eu concentrations in the granites are variable and suggest variations in the proportions of feldspar and mica retained in the residue. Despite uncertainties in modelling, Rb–Sr relationships suggest that granite formation was aided by the presence of externally derived H2O-rich fluids, which could have been introduced along fluid pathways such as shear zones. The distribution of granite plutons indicates that they did not move over great distances, consistent with relatively wet magmas. Some biotite-rich granitoid rocks were formed by partial melting of the same metasediments, but at higher temperatures, during an earlier metamorphic event, while other mafic granites may have a totally different petrogenesis.

Landslide and Tsunami 21 November 2000 in Paatuut, West Greenland

A large landslide occurred November 21, 2000 at Paatuut, facing the Vaigat Strait onthe west coast of Greenland. 90 million m3 (260 million tons) of mainly basalticmaterial slid very rapidly (average velocity 140 km/h) down from 1,000–1,400 maltitude. Approximately 30 million m3 (87 million tons) entered the sea, creatinga tsunami with an run-up height of 50 m close to the landslide and 28 m at Qullissat,an abandoned mining town opposite Paatuut across the 20 km wide Vaigat strait. Theevent was recorded seismically, allowing the duration of the slide to be estimated tocirca 80 s and also allowing an estimate of the surface-wave magnitude of the slideof 2.3. Terrain models based on stereographic photographs before and after the slidemade it possible to determine the amount of material removed, and the manner ofre-deposition. Simple calculations of the tsunami travel times are in good correspondencewith the reports from the closest populated village, Saqqaq, 40 km from Paatuut, whererefracted energy from the tsunami destroyed a number of boats. Landslides are notuncommon in the area, due to the geology with dense basaltic rocks overlying poorlyconsolidated sedimentary rocks, but the size of the Paatuut slide is unusual. Based onthe observations it is likely at least 500 years since an event with a tsunami of similarproportions occurred. The triggering of the Paatuut slide is interpreted to be caused byweather conditions in the days prior to the slide, where re-freezing melt water inpre-existing cracks could have caused failure of the steep mountain side.

Palaeoproterozoic (1740 Ma) rift-related volcanism in the Hekla Sund region, eastern North Greenland: field occurrence, geochemistry and tectonic setting

Abstract Two Palaeoproterozoic volcanic successions, the Hekla Sund (HS) Formation and the Aage Berthelsen Gletscher (AaB) Gletscher Formation, occur within the Caledonian orogen of eastern North Greenland. They consist mainly of mafic pillow lavas, deformed and metamorphosed under greenschist-facies conditions during the Caledonian orogeny. Zircons from a rhyolitic ignimbrite in the HS Formation have yielded an age of 1740±6 Ma. In both formations the volcanic rocks are intercalated with immature sandstones and conglomerates that accumulated in the vicinity of active fault scarps; a shallow marine, rifted basin is implied. Relative concentrations of the more immobile minor and trace elements (Ti, Zr, Nb, Y and rare-earth elements) in both rock suites were unaffected by the metamorphism. Fractional crystallisation of olivine, clinopyroxene and, probably, plagioclase, as well as assimilation of crustal rocks was involved in the petrogenesis of the HS Formation. The AaB basalts have higher Mg, Ni and Cr and lower concentrations of incompatible elements than the rocks of the HS Formation, and they could be regarded as more primitive products of the same magmatic event. However, marked differences in incompatible trace element ratios in the two suites are unlikely to reflect either differences in fractionation histories or variable contamination, and suggest compositional differences in the mantle source rocks. Basalts from the two formations have distinct eNd values (−4.6 and −4.8 for the HS Formation, −5.9–−5.6 for the AaB Gletscher Formation), which is consistent with this interpretation. The volcanic rocks at HS and AaB Gletscher were erupted shortly after a long period of orogenic activity between 2000 and 1750 Ma ago. Following post-orogenic emplacement of granites at ca. 1750–1740 Ma, uplift and erosion took place, and accumulation of extensive immature sediments occurred simultaneously with formation of the volcanic rocks described in this paper. The magmatism that gave rise to the two formations may have been caused by melting during lithospheric extension.