Episodic burial metamorphism in the Andes—A viable model?

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Burial metamorphism of regional extent throughout Mesozoic to Cenozoic sequences in the Andean Mountain belt has been attributed previously to a unique model of metamorphic development, involving episodic ∼40 m.y. cycles of extensional basin formation, burial, metamorphism, and then exhumation. A main premise of this model is that breaks in metamorphic grade occur at major stratigraphic unconformities, so marking successive metamorphic cycles. This model is tested in a Mesozoic–Cenozoic sequence east of Santiago, where three metamorphic episodes have been reported on the basis of sharp breaks in metamorphic grade at two main unconformities. In metabasites from this area, reaction progress in mafic phyllosilicates shows a continuum across the sequence without breaks at the unconformities. There are differences in mineral assemblages between the various stratigraphic units, from which contrasting subgreenschist facies can be recognized. However, consideration of the controls on mineral paragenesis at subgreenschist facies conditions demonstrates that these different facies cannot be used as evidence of sharp breaks in metamorphic grade at unconformities, as has been reported in many previous publications. Thus, metamorphic breaks within this Andean section cannot be confirmed. Accordingly, models of Andean burial metamorphism linked to episodic tectonic cycles throughout the Mesozoic and Cenozoic appear unfounded.