Thermochronological Evolution of Retrogressed Caledonian Eclogites, Lofoten Islands, Norway

Abstract

The major challenge in understanding the history in high-pressure continental terranes is dating the timing and duration of fluid infiltration and circulation events related to eclogitization and exhumation. Through a combination of high-resolution 40Ar/39Ar and Rb/Sr dating, coupled with detailed petrological and geochemical studies and numerical Ar-diffusion modelling, the thermal history of the Lofoten Islands – a continental terrane in northern Norway transected by Caledonian (c. 480 Ma) eclogite-facies shear zones formed at P ≈ 15 kbar and T ≈ 680°C – was studied to elucidate its associated fluid infiltration and melt injection histories. Six shear zones were studied from two islands – Vestvågøy and Flakstadøy. 40Ar/39Ar results in eclogitic garnet indicate that the fluids involved in the eclogitization at T ≈ 650–600°C (newly estimated temperatures) were 40Ar-rich. 40Ar/39Ar results and numerical models reveal that isothermal retrogression to amphibolite facies occurred at 425 Ma, lasting an integrated time of ≤ 1 Ma, during which the zones served as pathways for 40Ar-rich hot fluids that migrated through conduits reaching the overlying allochthon of Vestvågøy. The models indicate that the retrogressed eclogites and allochthon were exhumed soon after the eclogitization to shallower crustal levels (depth ≈ 8 km) where the ambient T did not exceed 300°C. These rock units remained at these levels for 90 Ma and 60 Ma, respectively, and experienced episodic injection of hot fluids associated with extensional events and local melt intrusions. 40Ar-poor fluids at 415 Ma infiltrated the zones at T ≈ 500°C for c. 100 ka in the Flakstadøy basement and at T ≈ 650°C for c. 400 ka in the allochthon of Vestvågøy. 40Ar-rich fluids at c. 365 Ma infiltrated the zones in Vestvågøy reaching the allochthonous at T ≈ 600°C for c. 150 ka. In a zone of Flakstadøy, a melt intrusion at 335 Ma reheated the rocks at T ≈ 500°C for c. 25 ka. Short-lived thermal events such as melt intrusions (390 Ma and 320 Ma) and fluid infiltration (280 Ma, 200 Ma, 180 Ma and 55 Ma), continued in different shear zones of Flakstadøy associated with Pangaea breakup and seafloor spreading.