Evidence for simultaneous contraction and extension at different crustal levels during the Caledonian orogeny in NE Greenland

Abstract

Leucogranites are common in the East Greenland Caledonides. Most granites cut across Caledonian contractional structures and have been interpreted as decompression melts related to late to post-orogenic collapse. To constrain the timing of the collisional event responsible for crustal thickening, zircon, monazite and xenotime were extracted from syn-contractional magmatic rocks from André Land, NE Greenland (73°N) and analysed using the U–Pb isotope dilution thermal ionization mass spectrometry method. The resulting ages range between 429 and 425 Ma. An age of 424 Ma on monazites from a kyanite–garnet–mica gneiss (granite melt source rock) is interpreted as dating late-stage metamorphism and migmatization in the area. A large syn-extensional granite pluton yielded an age of 429 ± 1 Ma. These ages, combined with field observations, demonstrate that subhorizontal upper crustal extension was contemporaneous with mid- to lower crustal subhorizontal contraction, and that the upper crustal strain field is not necessarily representative of the coeval strain fields in the middle and lower crust. The presence of 429–425 Ma migmatites and related intrusive rocks structurally above low-grade metasediments demonstrates that the late and post-tectonic leucogranites are not late orogenic but related to a separate early Caledonian tectonomagmatic phase predating Early Devonian nappe translation.