TIMING OF THE HP METAMORPHISM RECORDED BY ZIRCON AND MONAZITE IN THE TJELIKEN AND HELAGS GNEISSES, SEVE NAPPE COMPLEX, SCANDINAVIAN CALEDONIDES

The closure of the Iapetus Ocean in Ordovician resulted in the formation of the Scandinavian Caledonides. This process has most likely involved several subducion-eduction events before the final collision between Laurentia and Baltica in Silurian and Early Devonian. Nonetheless, timing and evolution of the orogenic processes forming the Caledonian orogen still needs to be constrained in more detail. The key unit that can provide more information about it is the Seve Nappe Complex (SNC) of the Middle Allochthon. Within this unit, HP-UHP lithologies, including eclogites, garnet peridotites and gneisses are found.

Geochronological data provide relatively wide range of the HP-UHP metamorphic ages of the SNC: ca. 505 Ma in Norrbotten, ca. 460-445 Ma in Jämtland. To resolve this age dispersion, we have examined gneisses from two localities, the Tjeliken and Helags mountains in northern and southern Jämtland, respectively.

The Tjeliken gneiss is composed mostly of Grt-Qz-Ph-Czo-Epi±Fs±Cb with accessory Zrn-Ttn-Ap. Euhedral zircon crystals show oscillatory zoning in the cores and homogeneous rims. U-Pb SIMS analyses were targeted on the zircon metamorphic rims and they yield a concordia age of 459 ± 2 Ma (MSWD=0.57). This age is interpreted to reflect the timing of the near peak metamorphic conditions.

The main mineral assemblage of the Helags gneiss is Grt-Qz-Pl-Bt-Ph with accessory Rt-Zrn-Mnz. One undeformed and one mylonitic gneiss samples have been selected for monazite dating using U-Th-total Pb method. Preliminary chemical ages range from 440 to 425 Ma.

The zircon age from Tjeliken corroborates previous Sm-Nd dating of the UHP event in northern Jämtland. Monazite ages from Helags are in agreement with previous monazite dating in central Jämtland and most probably reflect post HP decompression melting and subsequent thrusting.