Chemical variations of zircon compared with morphological evolution during magmatic crystallization; an example from the Valle de Cervo Pluton (Western Alps)

This study correlates the main chemical variations of zircon with its morphological evolution within a suite of coeval rocks (monzonite, syenite and granite from the Tertiary Valle del Cervo Pluton, Western Alps), by coupling microprobe analyses and the study of crystal typology. Zircon concentrates from 8 different rock samples were first studied with the typology method; then, crystals for microprobe analysis (SiO (sub 2) , ZrO (sub 2) , HfO (sub 2) , Y (sub 2) O (sub 3) , P (sub 2) O (sub 5) , ThO (sub 2) , UO (sub 2) ) were hand-picked from 4 selected rock samples. The typologic characters of the studied populations are typical of zircons from shoshonitic rocks. Monzonite and granite zircons are characterized by dominant {100} and {101} forms; the development of the {110} prism slightly increases in the granite samples. Microprobe data evidence that the bulk composition of the studied zircons corresponds to the composition of the main magmatic growth stage in zircons from calkalkaline and K-calkalkaline rocks from literature. All the zircons of the Valle del Cervo pluton exhibit large scale chemical zoning from the inner to the outer portion within single crystals, as well as chemical variations within each rock type. However, from the least to the most differentiated rock types the evolution trends are not continuous. The monzonite zircons mainly show HfO (sub 2) enrichments; the syenite zircons do not show any particular trend, whereas the granite crystals show a late enrichment in UO (sub 2) and, to a lesser extent, in Y (sub 2) O (sub 3) and ThO (sub 2) . These different behaviours, together with typology data, support the hypothesis that syenite and granite are genetically linked, but they are not produced by fractionation of the same magma that generated the monzonite. The higher HfO (sub 2) contents in the monzonite crystals with respect to the syenite and granite ones suggest a larger crustal contribution in the genesis of the monzonite, as suggested by the published isotopic data. Furthermore, in the monzonite crystals the wide variation range in HfO (sub 2) contents and the occurrence of oscillatory zoning (observed in BSE images) could be due to changes in Hf concentration in the magma during zircon crystallization (assimilation or mixing processes). The different patterns of chemical variations in the studied crystals may be explained with the influence of many interacting factors: occurrence of other minerals, which compete with zircon for the same trace elements, and time of their crystallization with respect to zircon; chemical evolution of the magma (through FC, AFC or mixing processes) during zircon crystallization; local disequilibrium phenomena at the crystal surface during zircon growth. As a consequence, the influence of zircon fractionation on the geochemical evolution of magmas cannot be simply modelled by using an "ideal" zircon composition.