Separating metamorphic events in the Fosdick migmatite–granite complex, West Antarctica F. J. KORHONEN, 1,2 M. BROWN, 2 M. GROVE, 3 C. S. SIDDOWAY, 4 E. F. BAXTER 5 AND J. D. INGLIS 5 * 1 Department of Applied Geology, Curtin University, GPO Box U1987, Perth WA 6845, Australia (f.korhonen@curtin.edu.au) 2 Laboratory for Crustal Petrology, Department of Geology, University of Maryland, College Park, MD 20742, USA 3 Department of Geological and Environmental Sciences, Stanford University, Stanford, CA 94305, USA 4 Department of Geology, Colorado College, Colorado Springs, CO 80703, USA 5 Department of Earth Sciences, Boston University, Boston, MA 02215, USA ABSTRACT The Fosdick migmatite–granite complex in West Antarctica records evidence for two high-temperature metamorphic events, the first during the Devonian–Carboniferous and the second during the Cretaceous. The conditions of each high-temperature metamorphic event, both of which involved melting and multiple melt-loss events, are investigated using phase equilibria modelling during successive melt-loss events, microstructural observations and mineral chemistry. In situ SHRIMP monazite and TIMS Sm–Nd garnet ages are integrated with these results to constrain the timing of the two events. In areas that preferentially preserve the Devonian–Carboniferous (M 1 ) event, monazite grains in leucosomes and core domains of monazite inclusions in Cretaceous cordierite yield an age of c. 346 Ma, which is interpreted to record the timing of monazite growth during peak M 1 metamorphism (820–870 °C, 7.5–11.5 kbar) and the formation of garnet–sillimanite–biotite–melt-bearing assem- blages. Slightly younger monazite spot ages between c. 331 and 314 Ma are identified from grains located in fractured garnet porphyroblasts, and from inclusions in plagioclase that surround relict garnet and in matrix biotite. These ages record the growth of monazite during garnet breakdown associated with cooling from peak M 1 conditions. The Cretaceous (M 2 ) overprint is recorded in compositionally homogeneous monazite grains and rim domains in zoned monazite grains. This monazite yields a protracted range of spot ages with a dominant population between c. 111 and 96 Ma. Rim domains of monazite inclusions in cordierite surrounding garnet and in coarse-grained poikiloblasts of cordierite yield a weighted mean age of c. 102 Ma, interpreted to constrain the age of cordierite growth. TIMS Sm–Nd ages for garnet are similar at 102–99 Ma. Mineral equilibria modelling of the residual protolith composition after Carboniferous melt loss and removal of inert M 1 garnet constrains M 2 conditions to 830–870 °C and 6–7.5 kbar. The modelling results suggest that there was growth and resorption of garnet during the M 2 event, which would facilitate overprinting of M 1 compositions during the M 2 prograde metamorphism. Measured garnet compositions and Sm–Nd diffusion modelling of garnet in the migmatitic gneisses suggest resetting of major elements and the Sm–Nd system during the Cretaceous M 1 overprint. The c. 102–99 Ma garnet Sm–Nd ÔclosureÕ ages correspond to cooling below 700 °C during the rapid exhumation of the Fosdick migmatite–granite complex. Key words: garnet; melt loss; monazite; partial melting; phase equilibria modelling. INTRODUCTION Long-lived active convergent plate margins commonly experience several cycles of high-temperature meta- morphism and crustal growth (e.g. Gower & Krogh, 2002; Glen, 2005). However, constraining the timing of these events is difficult because the geological signifi- cance of ages retrieved from various geochronometers may be ambiguous in terms of the event to which they refer. Studies of metamorphic rocks that integrate petro- graphic, thermobarometric and geochronological data have the potential to provide direct estimates of the P–T–t conditions during burial and exhumation. The dating of rock-forming minerals such as garnet may yield the timing of mineral growth in slowly cooled metamorphic rocks if the mineral remains below the closure temperature of the relevant chemical system, otherwise the timing relates to P–T conditions during cooling (e.g. Christensen et al., 1989; Ducea et al., 2003; Lancaster et al., 2008; Peterman et al., 2009; Dutch & Hand, 2010; Pollington & Baxter, 2010, 2011). Therefore, the Sm–Nd age of garnet formed early in the evolution of a polymetamorphic terrane can be reset (fully or partially) if subsequent *Present address: University of North Carolina, Chapel Hill, NC 27599, USA J. metamorphic Geol., 2012, 30, 165–191 doi:10.1111/j.1525-1314.2011.00961.x Ó 2011 Blackwell Publishing Ltd 165