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The Greenland Caledonides have a tectonic architecture built of Laurentian-margin Precambrian crystalline complexes and younger sedimentary successions that were metamorphosed during the Paleozoic collision with Baltica. Caledonian metamorphic patterns correspond to the gross structural levels of the orogen. The patterns are superimposed on earlier metamorphic histories in the Precambrian crystalline complexes, but they account for the sole metamorphism of Neoproterozoic to early Paleozoic sedimentary units. We describe the Caledonian metamorphism by dividing the orogen into two parts, a northern and a southern segment separated at 76°N by Bessel Fjord. North of Bessel Fjord, metamorphic grade increases eastward toward the hinterland in progressively higher thrust sheets, where it ultimately reaches ultrahigh-pressure conditions. The metamorphic pattern in the southern segment is complicated by regional extensional detachment faults. Very low-grade sedimentary rocks of the foreland are overlain by the deepest structural level, the Niggli Spids thrust sheet, which contains widespread relicts of high-pressure metamorphism. The overlying Hagar Bjerg thrust sheet is composed of a midcrustal-level migmatite complex that records high temperatures in the amphibolite to granulite facies. The Neoproterozoic to Ordovician sedimentary rocks of the uppermost unit, the Franz Joseph allochthon, reached greenschist- and locally amphibolite-facies conditions (garnet + staurolite) at their base. The Devonian and younger sedimentary basins are not significantly metamorphosed. Each of the main structural levels has a characteristic pressure-temperature path. Three main periods of metamorphism are currently recognized. The oldest, ca. 440–415 Ma, relates to the formation of migmatites and granites at midcrustal levels. This was followed by widespread high-pressure granulite- and eclogite-facies metamorphism from 410 to 390 Ma. A very late pulse of ultrahigh-pressure metamorphism occurred at 360–350 Ma and marked the end of the Caledonian collision.

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