On Day 5 (Thursday, 18 October), we left the foreland area of the Variscan Orogeny and drove into the hinterland, closer to the suture zone where Gondwana and Laurentia collided.

Deformed Proterozoic Granitic Rocks

We started the day by examining some foliated granites near the village of Pola de Allande.  These granites were dated at 600 million years, making them the oldest known Iberian rocks.  They are calc-alkaline granites, indicating they formed in a subduction environment.  An ocean slab must have been subducting off the north coast of Gondwana, under the Iapetus Ocean.  The granites were later foliated during the Variscan orogeny.

Suki Lopez (BMC '15) shows off a Proterozoic granite that was deformed in the collision between modern North America and modern Africa

Following a Giant Thrust Sheet

We stopped at two outcrops today that had been affected by the giant Mondonedo Thrust Fault:  a 60-km-long thrust fault that happened during the Variscan Orogeny.

At the first stop, at Porcia Beach, we saw a stack of Ordovician-age phyllites, rich in graphite and mica and showing great examples of crenulation cleavage, isoclinals folding, and ptigmatic folding of quartz-chlorite veins.  This sequence was deposited on the passive margin of Gondwana, after Avalonia rifted off but before Laurentia and Gondwana collided.  It was then folded and faulted during the Variscan Orogeny, along the Mondonedo Thrust.

Emily Garcia (BMC '15) points to some crenulation cleavage.

At the second stop, at Binquerencia Beach, we observed Cambrian sandstone with cross-bedding indicating that stratigraphic “up” now pointed down into the ground – the beds had been overturned.  This makes this sequence the bottom lib of a giant recumbent fold: a fold that has been pushed over onto its side.

Gabi Gutierrez-Alonso shows the students cleavage planes

Deformed Ordovician Granitic Rocks

Professor Arlo Weil discusses gneiss deformation

We began the day looking at deformed Proterozoic granites.  We ended by looking at deformed and metamorphosed Ordovician-age granitic gneisses.  The gneisses exposed along Xilloy Beach contain large alkali feldspar grains (“eyes”) that have been deformed into lens shapes.  The original granite protolith formed about 490 million years ago, during the Ordovician, and was later deformed during the Variscan.

Fern Beetle-Moorcroft (HC '14) and Suki Lopez (BMC '15) point to some feldspar lenses in Ordovician gneisses