Decoupling of Zr-Hf during contact metamorphic anatexis of metabasalts and timing of zircon growth, Sudbury, Canada

Whole-rock Zr-Hf systematics combined with zircon petrography, geochronology, and geochemistry in partially melted mafic rocks offer new insights into Zr-Hf decoupling processes during metamorphism. Zirconium and Hf are frequently used to interpret the petrogenesis of mafic igneous rocks, but their behavior during dehydration and partial melting is still controversial. The contact aureole of the 1850 Ma Sudbury Igneous Complex (SIC, Canada) includes pyroxene hornfels facies metabasalts with systematically strong negative wholerock Zr/Zr* (down to 0.25) and Zr/Hf (down to 29.7) anomalies. This signature only occurs in the highest grade portions of the aureole where partial melting occurred proximal to the SIC, and indicates that Zr-Hf decoupling is linked to partial melting and melt segregation. In this zone, the metabasalts contain intergranular melt with rare interstitial and poikilitic zircon grains yielding an 1850 +/- 24 Ma U-Pb age, connecting zircon growth to partial melting processes. In addition, zircon texturally overgrows the peak contact metamorphic mineral assemblage and locally shows straight crystal faces in microleucosomes. This is consistent with zircon crystallizing from trapped melt and in agreement with models suggesting that metamorphic zircon does not necessarily grow during peak pressure-temperature conditions. The apparent requirement for melt to facilitate zircon growth and Zr-Hf mobility illustrates the importance of melting for understanding zircon and Zr-Hf behavior in mafic rocks. An increased understanding of whole-rock Zr-Hf decoupling combined with zircon analysis provides the opportunity of better constraining high-temperature crustal processes involving silicate melts.