Zircon growth in very low grade metasedimentary rocks : evidence for zirconium mobility at ~250°C

Zircon outgrowths are present on detrital zircon grains in many very low to low-grade metasedimentary rocks worldwide, ranging in age from mid-Archaean to Palaeozoic. The outgrowths comprise minute (typically < 3 mu m) crystals that form an irregular fringe on detrital zircon grains, and in a few cases, on diagenetic xenotime outgrowths. Textural relationships indicate that while zircon growth postdates diagenetic xenotime precipitation, it precedes or is synchronous with metamorphic xenotime formation. Unlike xenotime, zircon outgrowths are absent in unmetamorphosed sedimentary rocks, and only appear in prehnite-pumpellyite facies rocks, suggesting that zircon growth commences at temperatures of similar to 250 degrees C. The greater abundance of zircon outgrowths in shales than in to other sedimentary rocks may relate to higher halogen concentrations, which have been linked to enhanced zirconium mobility in hydrothermal systems. The growth of zircon in metasedimentary rocks indicates that zirconium was transported in aqueous fluids, possibly as fluorine complexes, during very low-grade metamorphism.