Zircon: The Metamorphic Mineral

A mineral that forms under conditions as variable as diagenesis to deep subduction, melt crystallization to low temperature alteration, and that retains information on time, temperature, trace element and isotopic signatures is bound to be a useful petrogenetic tool. The variety of conditions under which zircon forms and reacts during metamorphism is a great asset, but also a challenge as interpretation of any geochemical data obtained from zircon must be placed in pressure–temperature–deformation–fluid context. Under which condition and by which process zircon forms in metamorphic rocks remains a crucial question to answer for the correct interpretation of its precious geochemical information. In the last 20 years there has been a dramatic evolution in the use of zircon in metamorphic petrology. With the advent of in situ dating techniques zircon became relevant as a mineral for age determinations in high-grade metamorphic rocks. Since then, there has been incredible progress in our understanding of metamorphic zircon with the documentation of growth and alteration textures, its capacity to protect mineral inclusions, zircon thermometry, trace element patterns and their relation to main mineral assemblages, solubility of zircon in melt and fluids, and isotopic systematics in single domains that go beyond U–Pb age determinations. Metamorphic zircon is no longer an impediment to precise geochronology of protolith rocks, but has become a truly indispensable mineral in reconstructing pressure–temperature–time–fluid-paths over a wide range of settings. An obvious consequence of its wide use, is the rapid increase of literature on metamorphic zircon and any attempt to summarize it can only be partial: in this chapter, reference to published works are intended as examples and not as a compilation. This chapter approaches zircon as a metamorphic mineral reporting on its petrography and texture, deformation structure and mineral chemistry, including trace element and isotopic systematics. Linking this information together highlights the potential of zircon as a key mineral in petrochronology.