Zircon as a proxy for the magmatic evolution of proterozoic ferroan granites; The Wiborg rapakivi granite batholith, SE Finland

Abstract

A micro-drilling technique was used to detect and study the effect of entrainment of antecrystic zircon in rapakivi texture-forming megacrystic alkali feldspar ovoids in the ferroan granites of the Paleoproterozoic (c. 1630 Ma) Wiborg batholith of southeastern Finland. Texturally controlled analysis of major granite types (wiborgite, dark wiborgite, pyterlite, and porphyritic granite) across the batholith area revealed differences in the textural features, mineral inclusion paragenesis, secondary ion mass spectrometry U-Pb ages, and trace element compositions of inclusion and groundmass zircon populations. This suggests that the material that constitutes the Wiborg rapakivi granites crystallized from separate magmas in at least two stages at different P-T-X conditions. The first stage was characterized by crystallization of alkali feldspar megacrysts from compositionally relatively evolved granitic magmas, recorded by zircon inclusions within the megacrysts during a protracted period of time from c. 1635 to 1628 Ma. The second stage involved relatively rapid final emplacement at c. 1628 Ma, triggered by remobilization of the previously crystallized ovoid material and recorded by groundmass zircon. Differences in the Ti abundance patterns of the dark wiborgite granite type inclusion and groundmass zircon populations can be explained by involvement of coeval mafic (massif-type anorthositic) magmas during the remobilization stage, but the main wiborgitic granite type bears no evidence of mafic magma interaction. Zircon trace element compositions imply that at least in the Wiborg batholith the formation of rapakivi texture has not required mafic magma influence, as rock types without direct evidence of mixing also exhibit prominent rapakivi textures. This suggests that the formation process of the texture was dominated by changes in the intensive parameters (P-T) of the magmatic system. However, more precise quantification of the effects of these factors requires further independent evidence.