Short-lived reheating events in the Sommartuva shear zone hosted in cold crust: Ar-diffusion modelling constraints for the exhumation of eclogites in Flakstadøy, Lofoten Islands, Norway
文摘
The exhumation of the Norwegian Lofoten eclogites, formed during the continental collision of Laurentia and Baltica, is mainly due to the reactivation of shear zones during the collapse of the Caledonian orogen. The current cooling history [temperature–time (T–t) paths] for the exhumation of these rocks as determined by 40Ar/39Ar dating and geothermometry involves a protracted period of slow cooling (c. − 1.7 °C Ma− 1) from high T. The Sommartuva shear zone, hosted in a Palaeoproterozoic anorthosite, contains retrogressed eclogite and amphibolite-facies lenses and served as an open system for Ar with 40Ar-rich and 40Ar-poor hot fluid infiltration. Ca-rich amphiboles from amphibolites contain excess Ar and are mantled by phlogopites that yield Ar-diffusion loss profiles with plateau ages of 413–400 Ma associated with an extensional event in Lofoten. Locally, the zone was intruded in its late-stage history (c. 333 Ma) by a granitoid rock crystallizing annite and muscovite, previously inferred to be injected synchronously with amphibolite-facies metamorphism. An alternative cooling path related to the exhumation of the eclogites was determined by a combination of i) 40Ar/39Ar ages in minerals that grew during different thermal events, ii) thermal modelling of the Ar systematics in these minerals and iii) exploration of the thermal effects caused by the superposition of each event on pre-existing mineral phases. The best-fit T–t path as constrained by the Ar data reveals that, in order to preserve Ar (radiogenic and excess) within the minerals, the ambient T must have been relatively low (≤ 300 °C) over 80 Ma (413–333 Ma). Likewise, the associated intrusion of the granitoid and the hot fluid that percolated the shear zone and crystallized the phlogopites were likely to have been short-lived (c. 100–25 ka) and at relatively moderate T (500 °C). In contrast to previous thermal models where the thermal events are represented as simple inflexions in a T–t curve, our cold-crust model incorporates short-lived thermal disturbances (including estimates of the magnitude of their maximum temperature and duration) superimposed on protracted periods of slow cooling from high T. A new proposed T–t path obtained for the exhumation of this terrane indicates that the eclogites resided for 80 Ma (413–333 Ma) at relatively cool ambient temperatures (c. 300 °C), highlighting the enormous potential of coupling geochronological data from multiple isotopic systems with thermal modelling to identify and constrain the duration of short-lived events.
