Zircon textures and composition

Refractory recorders of magmatic volatile evolution?

S. Erdmann*, N. Wodicka, S. E. Jackson, D. Corrigan

*Corresponding author for this work

    Research output: Contribution to journalArticle

    24 Citations (Scopus)

    Abstract

    Zircon textures and composition have been used to infer magmatic processes including closed-system fractional crystallization, magma mixing or replenishment, and country-rock assimilation. Here, we propose that zircon textures and composition may also be refractory recorders of magmatic volatile evolution. We present field, whole-rock chemical, textural, mineral chemical, and U-Pb age data from evolved, fine-to-coarse-grained granite intrusions on Melville Peninsula, Nunavut, Canada. Zircon forms two main populations in these granites, Type-1 and Type-2 zircon. Type-1 zircon is present in all samples, but predominant in fine-grained granite. Crystals are euhedral and inclusion-rich and show periodic, fine-scale oscillatory zoning, comparatively low concentrations of U (< 2,200 ppm) and Hf (< 1. 6 wt%), high Zr/Hf (~40-62), and pervasive alteration. Type-2 zircon is predominant in coarse-grained granite. Crystals form overgrowths on Type-1 zircon and individual crystals. They are subhedral and inclusion-poor and show weak, irregular, large-scale oscillatory zoning, high U (up to ~7,250 ppm) and Hf (1. 5-2. 0 wt%), low Zr/Hf (~37-44), and only local alteration. Compatible trace-element concentrations and Zr/Hf change sharply across the boundary of Type-1 to Type-2 zircon; 207Pb/206Pb ages preclude a significant hiatus between crystallization of the two types. We argue against magmatic versus hydrothermal crystallization, country-rock assimilation, or magma mixing as causes for the crystallization of Type-1 and Type-2 zircon. We propose instead that Type-1 zircon formed from volatile-undersaturated magmas and that Type-2 zircon formed from volatile-saturated magmas. Magmas fractionated by volatile-driven filter pressing into crystal-rich mush and crystal-poor magma. Crystal-rich mush with abundant Type-1 zircon crystallized to fine-grained granite. Volatile-rich magma crystallized to Type-2 zircon and coarse-grained granite. While Type-1 zircon was pervasively altered by exsolving magmatic volatiles, Type-2 zircon was only locally affected by subsolidus hydrothermal alteration.

    Original languageEnglish
    Pages (from-to)45-71
    Number of pages27
    JournalContributions to Mineralogy and Petrology
    Volume165
    Issue number1
    DOIs
    Publication statusPublished - Jan 2013

    Keywords

    • Alteration
    • Evolved granite
    • Fluid
    • Magma
    • Zircon

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