Interpreting prograde-growth histories of Al 2 SiO 5 triple-point rocks using oxygen-isotope thermometry: an example from the Truchas Mountains, USA T. E. LARSON 1 AND Z. D. SHARP 2 1 Los Alamos National Laboratory, Los Alamos, NM 87545, USA (tlarson@lanl.gov) 2 Department of Earth and Planetary Sciences, University of New Mexico, Albuquerque, NM 87131, USA ABSTRACT Oxygen-isotope compositions of kyanite, andalusite, prismatic sillimanite and fibrolite from the Proterozoic terrane in the Truchas Mountains, New Mexico differ from one another, suggesting that these minerals did not grow in equilibrium at the Al 2 SiO 5 (AS) polymorph-invariant point as previously suggested. Instead, oxygen-isotope temperature estimates indicate that growth of kyanite, andalusite and prismatic sillimanite occurred at c. 575, 615 and 640 °C respectively. Temperature estimates reported in this paper are interpreted as those of growth for the different AS polymorphs, which are not necessarily the same as peak metamorphic temperatures for this terrane. Two distinct temperature estimates of c. 580 °C and c. 700 °C are calculated for most fibrolite samples, with two samples yielding clear evidence of quartz-fibrolite oxygen-isotope disequilibrium. These data indicate that locally, and potentially regionally, oxygen-isotope disequilibrium between quartz and fibrolite may have resulted from rapid fibrolite nucleation. Pressures of mineral growth that were extrapolated from oxygen-isotope thermometry results and calculated using petrological constraints suggest that kyanite and one generation of fibrolite grew during M1 at 5 kbar, and that andalusite, prismatic sillimanite and a second generation of fibrolite grew during M2 at 3.5 kbar. M1 and M2 therefore represent two distinct metamorphic events that occurred at different crustal levels. The ability of the AS polymorphs to retain d 18 O values of crystallization make these minerals ideal to model prograde-growth histories of mineral assemblages in metamorphic terranes and to understand more clearly the pressure–temperature histories of multiple metamorphic events. Key words: Al 2 SiO 5 polymorph; metamorphic reaction history; oxygen-isotope thermometry; Protero- zoic metamorphism. INTRODUCTION The Al 2 SiO 5 (AS) polymorphs kyanite, andalusite and sillimanite are among the most widely studied meta- morphic index minerals that are used to interpret the physical conditions of metamorphism in a geological terrane. Their usefulness lies in the fact that they are stable over a wide range of temperatures and pressures and have an invariant point that is common to mid- crustal metamorphism (e.g. 505 °C and 3.8 kbar, Holdaway, 1971; 550 °C and 4.5 kbar, Pattison, 1992). Coexisting AS polymorphs have been interpreted as representing an equilibrium-univariant or -invariant assemblage and cited as a temperature–pressure (T–P) constraint in metamorphic terranes (i.e. Ghent et al., 1980; Grambling, 1981; Hodges & Spear, 1982). The refractory nature of the AS polymorphs, however, also allows them to persist metastably through long-lived and multiple metamorphic events. The usefulness of coexisting AS polymorphs in a metamorphic assem- blage therefore presents a double-edged sword: how can coexisting AS polymorphs represent equilibrium univariant or invariant metamorphic assemblages and at the same time persist metastably outside their P–T stability fields? Importantly, how can we successfully determine equilibrium v. metastability for these important metamorphic minerals? This paper presents oxygen-isotope data from a suite of rocks that contain three, two and one AS polymorph(s) from the Truchas Mountains, New Mexico, USA. The data are used to quantitatively model their prograde metamorphic- growth history. Results from this study allow us to test the hypothesis of Grambling (1981) that the AS poly- morph triple-point assemblage in the Truchas Moun- tains is in equilibrium, and to understand better the possible reaction mechanisms that are responsible for growth of AS polymorphs in a variety of rock types. Previous studies that have used coexisting AS poly- morphs to constrain the metamorphic history of a geological terrane have relied on textural interpretations from qualitative petrographical observations to deter- mine if an equilibrium or metastable relationship exists. Qualitative assessment of such an important petrologi- cal indicator is worrisome when one considers the broader implications. For example, Grambling (1981) interpreted distributions and textures of coexisting J. metamorphic Geol., 2005, 23, 847–863 doi:10.1111/j.1525-1314.2005.00613.x Ó 2005 Blackwell Publishing Ltd 847