U-PB SPHENE AND ZIRCON GEOCHRONOLOGY OF THE TARFALA VALLEY, NORTHERN SWEDISH CALEDONIDES: DECIPHERING THE CONSTRUCTION OF A THRUST-STACK

The Tarfala Valley in the northern Swedish Caledonides offers a complete cross-section through a thrust-stack created by at least two orogenic events: the Finnmarkian Orogeny (~500-475 Ma) and the Scandian Orogeny (~440-390 Ma). Structural patterns throughout the valley are quite consistent, with a gently west-northwest dipping foliation and a down-dip mineral lineation, and do not suggest two or more periods of deformation. In order to better understand the timing and significance of the deformation that formed the thrust stack, sphene from amphibolite was dated via U-Pb TIMS. Additionally, zircon from a leucosomal layer and a dike within the amphibolite were dated via U-Pb SHRIMP-RG. The zircon dating has been combined with CL images and REE data to provide greater insight into the origin and history of the zircon.

Results show that the amphibolite cooled through the closure of U-Pb in sphene (650-700 degrees C) at ~452 Ma. Inherited cores are ubiquitous in zircons from both the leucosome and the dike with 207-Pb/206-Pb ages in the range of 1800-950 Ma. Analyses targeting the rims surrounding the inherited cores produce a spectrum of concordant ages mainly falling in the range of ~642-482 Ma. REE data was useful in distinguishing reset inherited crustal zircon from overgrowth zircon with a different geochemical pattern. The zircon geochronology indicates that the leucosome was metamorphosed at ~642 Ma with some of the zircons being partially or completely reset by an event that included intrusion of the younger dike at ~482 Ma.

Intriguing implications are: 1) both the leucosome and the dike had a diverse (sedimentary?) zircon source, 2) the leucosome, of unknown significance, records an event ~35 m.y. older than the established age of the protolith of the amphibolite, 3) The Finmarkian Orogeny constructed the majority of the thrust stack and emplaced the younger dike (~482 Ma) synkinematically(?), 4) the majority of the thrust stack cooled uniformly through the closure of U-Pb in sphene at ~452 Ma, 5) Scandian Orogeny deformation is likely restricted to the variably strained greenschist facies basement rocks found at the bottom of the thrust stack.