REACTION DATING AND BAYESIAN P-T-T-D AGE-SEQUENCE MODELING: A NEW TOOL FOR TECTONIC GEOCHRONOLOGY

Constraining the absolute time and duration of geologic processes is one of the great challenges and goals in Earth sciences. Increasingly, the integration of geochronologic constraints with petrologic information is being applied to understanding the timescales of metamorphic, igneous, tectonic, and fluid-related processes. High-resolution compositional mapping and in-situ (high-spatial resolution) dating, combined with simultaneous trace-element analysis can provide age constraints on multiple reactions within a P-T-t-D history. Many geochronometers, including monazite, xenotime, zircon, titanite, and garnet, preserve relative age constraints in the form of compositional zoning, deformational fabric(s), or petrologic context. This information can be integrated using Bayesian statistical analysis (age-sequence modeling), which combines geochronologic data with prior information (compositional zoning, fabric and textural constraints, phase relationships) to generate a probabilistic posterior chronology. The calculations, (carried out in Isoplot 4.0), provide enhanced precision on geochronologic dates and rates, and an integrated model of petrotectonic evolution. For example, Bayesian modeling of complex, concentrically zoned monazite from the Amherst block in the northern Appalachian orogen significantly reduced uncertainties (40-70%). Pairing monazite compositions with coexisting xenotime has yielded a detailed temperature-time history, resolving heating and cooling associated with the Acadian (~405 Ma) and Neoacadian (~380 Ma) orogenies. Application to zoned monazite from an ultra-high temperature granulite sample from the southern Trans-Hudson orogen yielded durations of 0.5⁺⁹/_-0.4 Ma and 20⁺⁵/_-8 Ma for biotite dehydration melting and suprasolidus conditions, respectively. The relatively short intervals of heating and peak conditions suggest that UHT metamorphism occurred in a back-arc tectonic setting. Finally, age-sequence modelling combine with Bayesian step-change analysis have been used to constrain the duration of garnet stability in metamorphic rocks from the New England Appalachians. Age-sequence modeling is a new tool that can be widely applied to refine the absolute time and duration of geologic processes.