Crustal thickening and ductile extension in the NE Greenland Caledonides: a metamorphic record from anatectic pelites

Abstract

Caledonian orogenesis in NE Greenland resulted from the collision of Laurentia and Baltica during the Ordovician–Silurian. Anatectic pelites within the metasedimentary Smallefjord Sequence record a clockwise P–T path, the result of early crustal thickening at c. 445–440 Ma and subsequent exhumation of the high‐grade metamorphic core by a combination of ductile extension and tectonic denudation. The early prograde segment of the path followed a shallow, near‐isothermal trajectory and attained a metamorphic peak of c. 9.0–10.0 kbar at >790 and <850 °C. Prograde metamorphism initiated anatexis of pelites in the kyanite stability field and continued with sillimanite stable. Inclusion trails in the garnet cores are textural remnants of early deformation, which occurred either before or during prograde metamorphism. The peak metamorphic conditions are anomalously high in the context of thermal models and P–T paths for continental collision zones. The additional heat input required to promote migmatization may have been provided by advection as lower crustal high‐pressure rocks and the uppermost mantle were uplifted following lithospheric thinning at an early stage in the orogeny. The prograde path was interrupted by the development of retrograde extensional shear fabrics defined by biotite+sillimanite and associated with garnet breakdown. Field observations indicate that ductile extension was accompanied by melt extraction, transport and emplacement of intracrustal granites dated at c. 430 Ma. Regional ductile extension and exhumation probably resulted from the development of gravitational instabilities within the overthickened crust during continental collision.