Extensive fractional crystallization in strongly peraluminous granitoids recorded by zircon Zr isotopes

Abstract

Strongly peraluminous granitoids, classically considered «S-type» melts derived from metasedimentary sources, often exhibit large compositional variations, particularly in their alumina saturation and maficity. Such variations are generally attributed to (1) selective entrainment and/or unmixing of restitic and/or peritectic minerals in anatectic melts or (2) fractional crystallization (FC) of a parental magma. Distinguishing between these two models solely based on chemical compositions is challenging. Here, we use zircon Zr stable isotopes (delta Zr-94/90 relative to NIST RM 8299) to investigate the petrogenesis of the archetypal Kinsman granitoids and associated ultramafic-mafic cumulates from the New England Appalachians, USA. Autocrystic zircon from the Kinsman granitoids records a similar to 0.3 parts per thousand increase in delta Zr-94/90(NIST) values (from -0.23 to +0.12 parts per thousand) that correlates with increasing whole-rock SiO2 (58-76 wt%). This systematic isotopic evolution, which is decoupled from zircon epsilon(Hf)(t), negatively correlates with magma temperature (688-812 degrees C). These characteristics argue against open-system processes such as restite entrainment or continuous crustal assimilation during magma ascent in the crust. Instead, they suggest a nearly closed-system evolution dominated by FC. The lower delta Zr-94/90(NIST) values (-0.45 to -0.04 parts per thousand) of zircon from associated olivine-pyroxene hornblendites and orthopyroxenites are consistent with their origin as earlier cumulates genetically linked to the Kinsman granitoids. Our results demonstrate that while sediment assimilation in the lower crust could be responsible for the peraluminous signature, FC is the dominant driver of compositional diversity in these systems.