EXTENDING THE BARROVIAN AND BUCHAN CONCEPTS INTO THE 21ST CENTURY

A classic concept in metamorphic petrology is that of progressive metamorphic sequences. These occur in orogens worldwide and are typically classified according to the lithostatic pressure (P) over which much of the temperature changes occurred: moderate P (Barrovian) vs. low P (Buchan). In both cases the highest-grade part of the sequence may be related to migmatite domes or magmatic bodies that provided heat in the later stages of orogeny, including during orogenic collapse by extension or transtension. Geochronology studies of Barrovian and Buchan sequences have determined that the peak of metamorphism in high- and low-grade zones may have been coeval with each other and with associated migmatite/granite, and that the duration of metamorphism was short (<10 m.y.). Although these sequences have classically been framed in the context of collisional processes (crustal thickening/nappe tectonics), these results are consistent with formation and/or exhumation during extension or transtension. Numerical modeling of lithospheric extension shows that Barrovian and Buchan sequences develop at different crustal levels of the same orogenic system, with the latter forming when hot, deep crustal rocks such as migmatite ascend rapidly to shallow crustal levels during extension/transtension. In the case of the Montagne Noire (France) migmatite dome, an andalusite-sillimanite sequence developed in the schist carapace of the dome, coeval with crystallization of migmatite and granite that flowed rapidly from deep to shallow crustal levels. The P-T history of high-P relics in the dome (including kyanite in paragneiss and eclogite assemblages in mafic rocks) testifies to the dynamic history of the dome rocks, and geochronology results document the thermal connection of dome and carapace rocks. The close spacing of isograds in the schist combined with transtensional structures around the dome support a major role of crustal thinning in the metamorphic evolution of the migmatite dome-cored Buchan sequence. The formation of progressive metamorphic sequences, with implications for first-order metamorphic processes, is best understood in a dynamic context, with appreciation for the role of syn-metamorphic extension/transtension.