Contrasting metamorphic gradients: Barrovian-type vs. high-pressure metamorphism. An example on the northern margin of Gondwana (NW Iberia)

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Contrasting metamorphism in adjacent terranes is distinctive of large-scale tectonic events that include both collisional and rifting scenarios. When one of those terranes is characterized by the presence of high-pressure rocks, it is more likely to be related to collisional settings, and commonly in locations close to the suture. This contribution shows an example of the aforementioned situation in the Variscan orogenic belt of NW Iberia, where a tectonic slice with high-pressure metamorphism is above rocks that underwent Barrovian metamorphism. The two involved terranes are known as lower allochthon and parautochthon, respectively. The lower allochthon recorded the continental subduction (blueschist- and eclogite-facies conditions; [1,2]) of the most external part of the north Gondwana passive margin during the late Devonian (ca. 370-365 Ma; [1, 3]) at the beginning of the Variscan collision, followed by a buoyancy-driven exhumation triggered by the extensional collapse of the orogenic pile. Contrarily, the underlying parautochthon underwent crustal thickening, resulting in a medium-pressure Barrovian-type metamorphism that possibly was followed by a higher temperature/lower pressure Buchan-type metamorphism that may be related to tectonic exhumation and/or erosion [cf. 4].