Evaluation of Zircon Entrainment in Megacrystic Granites by Micro Drill Sampling and High-Precision in situ U-Pb Geochronology

Abstract:

A detailed petrographic investigation (optical and SEM), micro drill sampling, and U-Pb (SIMS) and trace element (LAQ-ICP-MS) analysis of zircon from three rapakivi-textured granites of the c. 1.62–1.65 Ga Wiborg batholith in Finland was conducted in order to study the potential effect of zircon antecryst entrainment on their geochronological interpretation. Zircon inclusions were extracted from within the rapakivi texture -forming alkali feldspar ovoids and compared to zircon from the groundmasses of respective samples.

U-Pb SIMS-analysis reveals statistical differences in the mean ²⁰⁷Pb/²⁰⁶Pb ages of the inclusion zircon populations in two of the studied granites (1635±3 Ma, 2σ, n=15 and 1634±3 Ma, 2σ, n=22), when compared to the groundmass zircon (1628±3 Ma, 2σ, n=18 and 1628±3 Ma, 2σ, n=19 respectively). The pattern is similar in the third sample, but the populations have overlapping mean ²⁰⁷Pb/²⁰⁶Pb ages (ovoids: 1631±3 Ma, 2σ, n=18; groundmass: 1629±3 Ma, 2σ, n=18). The ovoid inclusion zircon with regular chondrite-normalized REE-patterns and oscillatory zoning textures (in BSE images interpreted as primary magmatic growth zoning) record higher Hf and U abundances than comparable zircon in the groundmass populations.

These observations suggest that earlier bulk sample zircon U-Pb analyses from the Wiborg batholith granites may have been substantially affected by entrained zircon populations and overestimated the actual intrusion ages of the rocks. The ovoid material crystallized at an earlier stage and was remobilized during a later emplacement stage that is registered by the groundmass zircon. The similarity of groundmass ages in all samples (c. 1628 Ma) may point to a relatively short period of intensive magmatism involved with the emplacement of the Wiborg batholith. Based on the zircon trace element data the first stage magmas were either more fractionated or generated by lower degrees of crustal partial melting than the magmas of the second stage.