Metamorphic zircon: tracking fluid pathways and the implications for the preservation of detrital zircon
Zircon in greenschist-facies Dalradian metasedimentary rocks of Scotland is characterized by two types: large detrital grains and small euhedral metamorphic grains. The former are concentrated in psammites and contain abundant evidence of dissolution and reaction with metamorphic fluids. The smallest metamorphic microzircons (<9 µm2) represent >65% of zircon grains, are clustered within pelites and form in aligned arrays that cut across the bedding. The combination of textural characteristics with the spatial distribution shows that the growth of microzircon occurred in a small-scale crack network that developed after folding, reflecting precipitation from locally derived metamorphic fluids. Zircons within psammites in the hinge of an antiform show an enhanced metamorphic response suggesting focusing of fluids by the fold. Metamorphic zircon may be successfully used to map detailed fluid pathways, providing a unique approach to monitoring permeability structure in the crust. The scarcity of unmodified detrital zircon, which forms <7% of larger grains, indicates that major biases in the preservation potential of zircon are generated during metamorphism. These biases may feed through into the U–Pb ages of detrital zircons that are analysed from metamorphic rocks and hence into models of crustal evolution.