Integration of white mica in situ 87Rb/87Sr with in situ and step-heat 40Ar/39Ar dates in orogenic settings

White mica is common in continental rocks, and often (de)forms in response to structural and metamorphic processes in orogenic settings. It has been most typically dated via step heat or in situ 40Ar/39Ar methods. However, with in situ 87Rb/87Sr geochronology now possible via in line mass-chemical-mass filtering of Rb and Sr isotopes in tandem mass spectrometers, the contrasting accumulation of radiogenic Ar vs. Sr in white mica under deformation and metamorphism should allow for more nuanced and improved interpretations of the geological evolution of white mica. Here, we apply the in situ 87Rb/87Sr dating method to white mica previously dated with the in situ and step heat 40Ar/39Ar methods in three case studies with contrasting structural and metamorphic histories. The results of these comparative studies are used to propose a framework for interpreting paired white mica 40Ar/39Ar and 87Rb/87Sr datasets. In general, white mica 87Rb/87Sr ages are expected to be equivalent to 40Ar/39Ar ages under conditions of low temperature metamorphic, fluid-mediated, or dynamic (re-) crystallization. Under high temperature conditions of igneous or metamorphic white mica (re-)crystallization, 87Rb/87Sr ages are more likely to represent the (re-)crystallization event, with 40Ar/39Ar ages representing postcrystallization cooling. Deformation and/or dynamic (re-)crystallization of white mica can produce complex, spatially controlled age patterns for both chronometers, requiring more nuanced interpretations. These results and framework point to the value of combining both chronometers in orogenic studies to reduce uncertainties in interpretation and test assumptions in dating metamorphic and deformation processes in orogens.