Carboniferous basin in Holm Land records local exhumation of the North-East Greenland Caledonides: Implications for the detrital zircon signature of a collisional orogen

Abstract

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 Visean (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.