Phase Relations, Reaction Sequences and Petrochronology

Abstract

At the core of petrochronology is the relationship between geochronology and the petrological evolution of major mineral assemblages. The focus of this chapter is on outlining some of the available strategies to link inferred reaction sequences and microstructures in metamorphic rocks to the ages obtained from geochronology of accessory minerals and datable major minerals. Reaction sequences and mineral assemblages in metamorphic rocks are primarily a function of pressure ( P ), temperature ( T ) and bulk composition ( X ). Several of the major rock-forming minerals are particularly sensitive to changes in P–T (e.g., garnet, staurolite, biotite, plagioclase), but their direct geochronology is challenging and in many cases not currently possible. One exception is garnet, which can be dated using Sm–Nd and Lu–Hf geochronology (e.g., Baxter et al. 2013). Accessory mineral chronometers such as zircon, monazite, xenotime, titanite and rutile are stable over a relatively wide range of P–T conditions and can incorporate enough U and/or Th to be dated using U–Th–Pb geochronology. Therefore, linking the growth of P–T sensitive major minerals to accessory and/or major mineral chronometers is essential for determining a metamorphic P–T–t history, which is itself critical for understanding metamorphic rocks and the geodynamic processes that produce them (e.g., England and Thompson 1984; McClelland and Lapen 2013; Brown 2014). Linking the ages obtained from accessory and major minerals with the growth and breakdown of the important P–T sensitive minerals requires an understanding of the metamorphic reaction sequences for a particular bulk rock composition along a well-constrained P–T evolution. Fortunately, the phase relations and reaction sequences for the most widely studied metamorphic protoliths (e.g., pelites, greywackes, basalts) can be determined using quantitative phase equilibria forward modelling (e.g., Powell and Holland 2008). Comprehensive activity–composition models of the major metamorphic minerals in large chemical systems (e.g., White et al. 2014a) allow …