MEGASCOPIC AND PETROGRAPHIC RELATIONSHIPS WITHIN THE MONARCH CANYON METAMORPHIC CORE COMPLEX AND THIER IMPLICATIONS ON TIMING OF PEAK METAMORPHIC CONTITIONS

Traditional interpretations of the age constraints on peak metamorphism within the Monarch Canyon metamorphic core complex suggest a possible mid- to-Late Cretaceous age.  However, certain megascopic relationships within the core suggest a much older age.  Preliminary structural and petrographic studies in Monarch Canyon suggest a relative timing of structural fabric development.  We suggest that four main deformational events are represented by fabrics in Monarch Canyon, as follows: D₁ –D₂ represented by S₁-S₂ foliation, isoclinal F₂ folds, and L₂ mineral lineations, defined by aligned kyanite crystals, D₃ represented by west vergent F₃ folds that deform the S₁-S₂ foliation and L₂ mineral lineations and a crenulation cleavage in the hinges of these west vergent folds; boudinaged pegmatite dikes and sills display D₄ extensional deformation. The west-vergent F₃ folds predate the ductile extension related to boudinage of the pegmatites. If these interpretations are correct, the peak metamorphic events can not be Late Cretaceous in age as suggested by previous authors from age constraints on the boudinaged pegmatites. We also observed that there are discrepancies with the specific locations of shear zones mentioned by previous authors and that some of the proposed ductile shear zones are brittle fault zones.