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

Caledonian orogenesis in NE Greenland resulted from the collision of Lauren tia and Baltica during the Ordovician-Silurian. Anatectic pelites within th e metasedimentary Smallefjord Sequence record a clockwise P-T path, the res ult of early crustal thickening at c. 445-440 Ma and subsequent exhumation of the high-grade metamorphic core by a combination of ductile extension an d tectonic denudation. The early prograde segment of the path followed a sh allow, near-isothermal trajectory and attained a metamorphic peak of c. 9.0 -10.0 kbar at > 790 and < 850<degrees>C. Prograde metamorphism initiated an atexis of pelites in the kyanite stability field and continued with sillima nite stable. Inclusion trails in the garnet cores are textural remnants of early deformation, which occurred either before or during prograde metamorp hism. The peak metamorphic conditions are anomalously high in the context o f thermal models and P-T paths for continental collision zones. The additio nal heat input required to promote migmatization may have been provided by advection as lower crustal high-pressure rocks and the uppermost mantle wer e uplifted following lithospheric thinning at an early stage in the orogeny . The prograde path was interrupted by the development of retrograde extens ional shear fabrics defined by biotite+sillimanite and associated with garn et breakdown. Field observations indicate that ductile extension was accomp anied by melt extraction, transport and emplacement of intracrustal granite s dated at c. 430 Ma. Regional ductile extension and exhumation probably re sulted from the development of gravitational instabilities within the overt hickened crust during continental collision.