Improving the resolution of single-grain U/Pb dating by use of zircon extracted from feldspar: Application to the Variscan magmatic cycle in the central Alps

Abstract

The usual discordant U/Pb zircon age patterns for intrusives in orogenic belts impede and exact determination of the time of emplacement. They are the product of multiple overprinting and of the presence of inherited zircon. Dating of monophase zircons crystallized during a single event may yield higher age resolution and results that are easier to interpret. During magma crystallization, zircon is generated over a range of PTX conditions, whereas rock-forming minerals grow under more limited conditions and their relative ages can be determined petrographically. Zircons formed during earlier stages of magma crystallization and included in such mineral constituents are shielded from subsequent reactions outside the carrier mineral. Following partial melting of the protolith or xenolith assimilation, newly crystallizing rock-forming minerals may include zircon xenocrysts as individual grains. Using this insight and to facilitate the analysis of monophase zircons included in petrogenetically defined constituent minerals, we have introduced additional steps in the mineral separation procedure. We found the effort worthwhile in that the magmatic zircons extracted from plagioclase yielded the most concordant intrusive ages, in contrast to zircons from microcline or from the interstices. Also, inherited zircons occurring as individual grains in magmatic feldspar appear to have preserved some rudimentary morphology and a less disturbed radiogenic Pb composition, which allows us to distinguish provenance from discrete protoliths. We were thus able to resolve small but structurally supported age differences between two groups of Variscan intrusives in the central Swiss Alps. We obtained 299.4 ± 1.2 Ma for the crystallization age of the granite-gneiss in the Gotthard massif with inherited zircons that suggest generation from Late Proterozoic (∼ 735 Ma) and Mid-Proterozoic (∼ 1760 Ma) protoliths. The massive leucocratic granites on the margin of the granite-gneiss yielded an intrusive age of 294.3 ± 1.1 Ma with an inherited grain implying a source of Caledonian vintage.