Heterogeneity of initial 87Sr/86Sr ratios within a single pluton: evidence from apatite strontium isotopic study

Abstract

Magmatic apatite in samples of the unmetamorphosed undeformed Cretaceous pluton of the Habu Granodiorite, southwestern Japan, was examined to determine its viability as an indicator of initial 87Sr/86Sr heterogeneity in magmatic bodies. The Habu Granodiorite is a zoned pluton in the Ryoke metamorphic belt of southwestern Japan. A published Rb–Sr whole-rock isochron age of 124.2±10.8 Ma is significantly older than Th–U–total Pb monazite ages of ca. 86 Ma. A Rb–Sr mineral isochron of 90.0±2.5 Ma for a sample of granodiorite from the margin of the pluton has an initial 87Sr/86Sr (SrI) of 0.7056±0.0001, similar to a 87Sr/86Sr of 0.70539 for coexisting euhedral apatite. Similarly, a Rb–Sr mineral isochron of 91.4±2.5 Ma for a sample of adamellite from the center of the pluton has an SrI of 0.7068±0.0001, corresponding to a 87Sr/86Sr ratio of 0.70673–0.70684 for three fractions of coexisting euhedral apatite. The uniformity of 87Sr/86Sr values of apatite in individual samples and accordance of 87Sr/86Sr values of apatite and SrI from the mineral isochron suggest that magmatic apatite can be considered as a reliable indicator of SrI in granitic rocks. The Sr isotopic compositions of euhedral apatite, coupled with those determined through the mineral isochrons and whole-rock data, indicate that SrI varies between samples from 0.7054 to 0.7074. This provides direct evidence of SrI heterogeneity within a compositionally zoned pluton. Such heterogeneity can produce spurious whole-rock age estimates in granitic plutons, even where the true age of a pluton is uniform within measurable error. Assimilation cannot account for chemical differences between granodiorite and adamellite as well as Sr-isotopic heterogeneity. It is likely that the heterogeneity of SrI is caused by magma source materials.