U-TH-PB AND 230TH/238U DISEQUILIBRIUM ISOTOPE SYSTEMATICS: PRECISE ACCESSORY MINERAL CHRONOLOGY AND MELT EVOLUTION TRACING IN THE ALPINE BERGELL INTRUSION

Abstract

In order to investigate the potential of combined Th-U-Pb isotope and 230Th/238U disequilibrium systematics for tracing magmatic crystallization and melt evolution, conventional high-resolution single-crystal TIMS techniques have been applied to zircon, titanite and fragments of geochemically characterized growth zones of allanite. These minerals were extracted from a single tonalite specimen collected from the feeder zone of the Tertiary Bergell pluton (Southern Steep Belt, S Switzerland/N Italy). The isotopic results document an extended history of crystallization and melt evolution of at least 5 Ma, with well-resolved zircon ages defining an early interval of 33.0 to 32.0 Ma, followed by crystallization of zoned allanite from 32.0 to 28.0 Ma and formation of magmatic epidote possibly as late as 26 Ma. Trace and major element patterns in zoned allanite closely mirror melt evolution, characterized by increase of U concentration and sharp decrease of Th and LREE, reflecting early crystallization of phases low in U and, in particular, the dominating control by allanite precipitation. Preservation of substantial quantities of excess 206Pb derived from initial excess 230Th in all analyzed allanite grains indicates that their isotopic systems have not been reset by loss of radiogenic Pb during prolonged residence at magmatic conditions and regional-metamorphic cooling, and that the measured sequence of 208Pb/232Th dates translates into a real age sequence. Major loss of radiogenic Pb from compositionally zoned allanite by volume diffusion would have resulted in a data pattern grossly different from the observed one, as demonstrated by numerical modeling of 232Th-208Pb-238U-230Th-206Pb isotopic evolution. The results therefore suggest closure temperatures >=700°C for magmatic allanite. Quantification of 230Th/238U disequilibrium relationships reveals a smooth, initially steep decrease of Th/U in the magma from values of 2.9 at 32.0 Ma to < 0.1 at 28.0 Ma in equilibrium with sequential allanite zoning. Comparison of calculated (Th/U)magma with measured total-rock Th/U = 0.79 requires fractional crystallization of allanite at an early stage. This process of removal of allanite, and thus Th, from the melt was operative at least until ˜31.5 Ma, providing an upper limit for the time of emplacement of the studied magma batch. The fact that bulk zircon and a large part of allanite crystallization predate emplacement cautions against equating ages determined on refractory minerals from deep-seated plutons with intrusion ages. The persistence of magmatic conditions over a period of ˜5 Ma, concentrated in a narrow collision zone between the European and the African plate, has been a major controlling factor for the Oligocene evolution of the adjacent Insubric fault, the main tectonic lineament of the Alps being active from late Cretaceous to Miocene times.