Characterization of magma from inclusions in zircon: Apatite and biotite work well, feldspar less so

Abstract

Detrital zircon grains are employed to decipher sediment provenances and crustal evolution, and they provide unique evidence of Hadean crust-mantle differentiation processes. We demonstrate that mineral inclusions in zircon provide valuable information on the conditions under which zircon crystallized. Zircon grains from selected plutonic rocks from Dronning Maud Land, Antarctica, contain inclusions of apatite, biotite, amphibole, and pyroxenes that accurately reflect the chemical compositions of the equivalent phases in the host-rock matrix, and the compositions of the whole rocks. High concentrations of Y and low concentrations of Sr in apatite inclusions in zircon are diagnostic of evolved, felsic granitoid host rocks. In contrast, the relative abundances and compositions of plagioclase and alkali feldspar inclusions in zircon are decoupled from the composition of the whole rock, and they are generally indicative of granitic melts regardless of the bulk rock. This is best explained by the late crystallization of zircon relative to the bulk of the feldspars. We conclude that inclusions of apatite and mafic phases in zircon constrain the potential source rocks of detrital zircon, whereas feldspar inclusions do not.