Palaeoproterozoic and Archaean gneiss complexes in northern Greenland: Palaeoproterozoic terrane assembly in the High Arctic

Abstract

The Precambrian shield of northern Greenland has been investigated by SHRIMP U-Pb zircon dating of 14 orthogneisses and granitoids plus 5 metasediments, integrated with mapping by the Geological Survey of Denmark and Greenland and whole-rock Nd isotopic studies. The Inglefield Mobile Belt is a tract of Palaeoproterozoic sedimentation, plutonism, polyphase deformation and high-grade metamorphism that underlies Inglefield Land and northern Prudhoe Land. In the southern part of the belt at 78°30′N, the E-W-trending Sunrise Pynt Straight Belt is a high-grade, but structurally late, shear zone with contrasts in the geology on either side. South of the Sunrise Pynt Straight Belt, ca. 1980 Ma diorites and tonalites were emplaced into older orthogneisses and metasediments. Detrital zircons from two metaquartzites (deposited on Archaean basement?) yielded complex age spectra from ca. 3250 Ma to 2350 Ma, with 2600-2450 Ma grains dominant. In associated mica schist, low Th/U, 1923 ± 8 Ma zircons date high-grade metamorphism. The most southern orthogneiss investigated (77°45′N) is Neoarchaean (ca. 2600 Ma), in agreement with previously published isotopic data. North of the Sunrise Pynt Straight Belt to 79°10′N an amphibolite-granulite-facies complex with extensive pelitic to psammitic paragneisses are the oldest rocks recognised. Two psammitic paragneisses yielded unrounded zircons with a unimodal detrital age population centred on 2000-1980 Ma. Their source could be ca. 1980 Ma orthogneisses from south of the Sunrise Pynt Straight Belt, or from basement inliers in the North-East Greenland Caledonian fold belt. The metasediments were intruded by tonalites and diorites with dates of 1949 ± 13 Ma and 1943 ± 11 Ma, and then by granitoids (free of zircon inherited from older rocks) with ages of 1924 ± 29 Ma to 1915 ± 19 Ma. The metasediments show development of low Th/U zircon overgrowths at ca. 1920 Ma, coeval with the granitoids. Finally, other granites, some locally transformed into gneisses, have ages of 1783 ± 22 Ma to 1741 ± 15 Ma. Inherited zircons in the latter are up to 2650 Ma old. A granite dyke with a zircon age of 1783 Ma has an TDM age of 2981 Ma and a strongly negative εNd at 1.78 Ga, indicating derivation by melting or reworking of Archaean crust. Thus, by 1800 Ma this juvenile Palaeoproterozoic terrane had probably overridden crust with Archaean components. North of Inglefield Land, Precambrian crystalline rocks are obscured until Victoria Fjord (81°30′N). One reconnaissance orthogneiss sample from there contains ca. 3400 Ma oscillatory-zoned zircons, which probably date the rock, rather than being a xenocrystic component in a younger rock. Thus, from north to south there is an assemblage of Archaean, Palaeoproterozoic and Archaean to early Palaeoproterozoic gneiss terranes. The Inglefield Mobile Belt is dominated by juvenile Palaeoproterozoic arc crust trapped between two unrelated blocks of Archaean crust of contrasting age. The collision, and probably thrusting of a Palaeoproterozoic arc over a southern Archaean foreland, occurred at ca. 1920 Ma-dated by metamorphic zircon. The new isotopic results consolidate the regional mapping of Archaean and Palaeoproterozoic complexes across northern Baffin Bay that show continuity from Canada into Greenland, without displacement across the Nares Strait. Show more