Age and petrogenesis of the Tinn granite, Telemark, South Norway, and its geochemical relationship to metarhyolite of the Rjukan Group.

Abstract

The Tinn granite is a Mid Proterozoic, foliated pluton, situated in the central part of the Telemark Sector, South Norway. It is spatially associated with metarhyolite belonging to the Tuddal Formation of the Rjukan Group of the Telemark supracrustal sequence. SIMS U-Pb dating indicates an age of 1476+-13 Ma, which is slightly younger than the 1500-1514 Ma eruption interval for the Tuddal Formation rhyolite. Rare xenocrystic zircon cores give an age of 1506+-10 Ma, which is indistinguishable from the age of the Tuddal Formation. The absence of older inherited zircons and evidence from whole-rock Nd isotopes suggest that no source component older than the Rjukan Group is needed in the source region of the Tinn granite magma. The preferred petrogenetic model for the Tinn granite is a partial melting-mixing process at moderate depth in the crust, within the Rjukan Group volcanic pile. A mafic magma acted as a source of heat and contributed to the bulk chemistry of the granitic magma. Resetting of the lead isotope system of the granite at mineral scale took place in Sveconorwegian time, at 1031+- Ma.