THE GHOST OF A CRATON'S PAST

Partial melting of the continental crust is an important process in crustal stabilization. Understanding the mechanisms behind partial melting of Archean continental crust is thus paramount to understanding Archean tectonic modes and how stable cratons form. The Beartooth Mountains in MT/WY, USA expose Archean rocks of the Wyoming Craton and are dominantly comprised of a ~2.8 Ga calc-alkaline granitoid batholith known as the Long Lake Magmatic Complex (LLMC). Contained within the LLMC are roof pendants of metasedimentary granulites, which have been interpreted by previous workers to result from contact heating with the LLMC. However, field evidence, bulk Sm-Nd garnet dates, and garnet trace element zoning suggest that high temperature (HT) metamorphism of the roof pendants significantly post-dated LLMC emplacement. Sm-Nd garnet geochronology of microsampled growth zones will place tight constraints on the absolute timing of this metamorphism.

Phase equilibria modeling of metasedimentary granulites suggest that these rocks experienced a clockwise P-T path marked by rapid HT garnet growth during both heating and decompression by ~1 kbar at near-peak T (~780-800 °C). Diffusion modeling of major element zoning in garnet suggests that residence at peak T was brief (<2 Myrs), and that cooling from peak T to ambient mid-crustal temperatures likely lasted <10 Myrs. Phase equilibria modeling of LLMC tonalite suggests that the LLMC experienced minimal recrystallization/melting, if at all during this cryptic HT metamorphic event. We speculate that other large batholiths may experience such ‘ghost’ metamorphic events, only recorded by rocks with kinetically favorable temperature/fluid conditions for metamorphic reactions to progress. The age, duration, and P-T path of HT metamorphism inferred here suggest that metamorphism was likely not due to contact heating with the LLMC, but rather a different mechanism.