Isograd migration with time during orogenesis: multiple episodic growth of same phase porphyroblasts during prograde metamorphism

Afroz Ahmad Shah, and Tim Bell

A progression of FIAs (foliation intersection/inflection axes preserved within porphyroblasts) in the foothills of the Colorado Rocky Mountains reveals four periods of staurolite growth and two growth phases each for cordierite and andalusite; these FIA based isograds migrated ~2.5 kms across the orogen. This progression of mineral development occurred about FIAs trending successively NE-SW, E-W, SE-NW and NNE-SSW. Granitoids were emplaced during orogenesis in the surrounding region but have no direct relationship to the isograds. Isograd migration took place away from a heat source to the WNW and this, combined with the lack of relationship to pluton boundaries to the north and south, suggests that the latter rocks were not the heat source for metamorphism but rather a product of it. A final period of andalusite, cordierite and fibrolitic sillimanite grew over the matrix foliation and consequently, no FIA was determined for it; the isograds for this last period of mineral growth lie sub-parallel to those mapped by previous workers. A strong correlation between the distribution of FIA trends and the axial trace of all folds present in the area suggests that pockets of low strain are preserved from the effects of subsequent deformation throughout the region in spite of 4 changes in the direction of bulk shortening. They suggest that FIA data can be used to determine the succession of fold development from regional maps at which scale many overprinting criteria cannot be applied.