Slow cooling versus episodic fluid injections: Deciphering the Caledonian orogeny in Vestvågøy, Lofoten islands, Norway

Abstract

The determination of the thermal (temperature-time) histories of highpressure metamorphic terranes has been commonly based on the concepts of slow cooling and closure temperatures. In this paper, we find that this approach cannot reconcile a geochronological data set obtained from the amphibolite-facies allochthonous Leknes Group of the Lofoten islands, Norway, which reveals an extremely complex thermal history. Using detailed results from several different geochronometers such as 40 Ar/ 39 Ar, Rb-Sr, and U-Pb, we show that a model invoking multiple, short-lived thermal pulses related to hot-fluid Accepted ArticleThis article is protected by copyright. All rights reserved. infiltration channelized by shear zones can reconcile this complicated data set. This model suggests that hot fluids infiltrated throughout basement shear zones and affected the overlying cold allochthon, partially resetting U/Pb titanite and rutile ages, crystallizing new zircon, and produced identical 40 Ar/ 39 Ar and Rb/Sr ages in muscovite, biotite, and amphibole in various rocks throughout the region. This paper shows the enormous potential of coupling laser Ar-spot data with thermal modelling to identify and constrain the duration of short-lived events.An optimal P-T-t history has been derived by modelling the age data from a previously dated large muscovite crystal (Hames & Andresen, 1996) and using Zr-in-rutile thermometry which is consistent with all geochronological data and geological constraints from the basement zones and allochthon cover. This tectonothermal model history suggests that there has been three episodic hot-fluid and 40 Ar-free infiltration events, resulting in the total resetting of Ar ages during the Scandian (425 Ma) for 1 Ma at 650°C, and two reheating events at 415 Ma for 400 ka at 650°C and at 365 Ma for 50 ka at 600°C, which are modelled as thermal spikes above an ambient temperature of 300°C. Independent confirmation of these parameters was provided by Pb-diffusion modelling in rutile and titanite. The model suggest that the amphibolite-facies rocks of the Leknes Group probably remained cold before being exhumed for at least 60 Ma (425-365 Ma) and successfully explains the presence of different minerals that crystallised or were totally/partially reset in the allochthon and in the basement. The migration of hot fluids for short period of times within conduits extending through the basement and allochthon rock units is likely associated with episodic seismic activity.

Widening of Hydrous Shear Zones During Incipient Eclogitization of Metastable Dry and Rigid Lower Crust—Holsnøy, Western Norway

“…This substantial volume of fluid infiltration can be achieved either by a continuous supply or by repetitive pulses of fluid infiltration. Such pulses have been proposed for shear zones exposed in Lofoten (northern Norway), where similar crustal rocks were locally transformed into eclogite (Fournier et al., 2019). There, however, viscously deforming shear zones did not widen and are restricted to the width of their precursor (Menegon et al., 2017).…”