On Day 6 (Friday, October 19), we moved from the hinterland of the Variscan Orogeny into the orogeny itself. Throughout the day, we saw evidence of the 390-370 million-year-old collision between Gondwana and Laurentia, from tectonic mélange to a piece of ocean crust caught between the two colliding land masses. Together, these stops make up an ophiolite: a cross-section of ocean crust that was scraped off onto the continent during collision.
The Stuff That Got Scraped Off the Top: A Tectonic Melange
We started the day at the beach near the town of Espasante, examining a tectonic mélange: a mishmash of many types of rocks. Melanges form on the edges of subduction zones, when the subducting plate scrapes off its top rocks onto the overriding continent.
At this mélange, we saw many rock types, all metamorphosed and smashed together:
- An amphibolite that had once been an eclogite. Eclogites are the highest temperature and highest pressure rocks on Earth – they only form in subducting ocean crust. This ocean slab had metamorphosed to an eclogite, then been exhumed and thrust up into the subduction zone, where it underwent retrograde metamorphism. Its pyroxenes turned into amphiboles and it became a garnet-bearing amphibolites.
- A serpentinite – a serpentine- and talc-bearing rock that forms from metamorphism of olivine-rich rocks from Earth’s mantle.
- Migmatites – rocks that were heated so hot they began to melt. This migmatite appeared to be granitic, indicating initial melting of sedimentary rocks.
- Greenschists from mafic volcanic
- White marble entrained with serpentinite
These rocks together are indicative of an ocean environment: mantle rocks, ocean crust, sedimentary materials, mafic volcanic, and limestone were all mashed together and metamorphosed, before being scraped up off onto the edge of Gondwana.
The migmatites at this site had been dated at 490 million years, meaning this mélange likely formed at a subduction zone on Laurentia. It was then mashed up onto the edge of Laurentia by the Avalonia collision.
A Piece of the Overriding Plate: Avalonia
This granulite was formed from Cambro-Ordivician sediments that formed on the edge of Gondwana. These sediments then must have rifted off Gondwana with Avalonia, ~480 million years ago, travelled across the Iapetus Ocean, collided with Laurentia ~420 million years ago, and stayed with Iberia when Pangaea finally rifted apart. They have crossed the ocean that is now the Atlantic at least twice.
The Subducting Plate: Eclogites
At Cabo de Ortega, we visited the largest outcrop of eclogites in the world. Eclogites are extremely high-temperature, high-pressure rocks made from the metamorphism of basaltic rocks. They only form in subduction zones, when oceanic plates dive below continental crust and experience extreme pressures and temperatures.
The eclogites at Cabo de Ortega contain the classic omphacite-garnet mineral assemblage that typify the rock.
These eclogites were likely still part of Avalonia. This was the piece that was subducted under Laurentia during Avalonia’s docking around 420 million years ago.
Atop a mountain now lined with giant wind turbines, we found an outcrop of the black rock harzburgite: an olivine- and pyroxene-rich rock that forms when the mantle partially melts below oceanic crust. This mantle rock must have been lifted up onto the edge of Laurentia along with the eclogites from Cabo de Ortega during the docking of Avalonia.
The Ocean Crust: Deformed Gabbros
We saw the mantle below the ocean crust and the sediments deposited on top of the ocean crust. The final piece of the ophiolite is the ocean crust itself. This deformed gabbro is typical of normal mid-ocean ridge basalts (N-MORBs), and likely formed just as the Rheic was opening to rift Avalonia off of Gondwana, ~490 million years ago. Later, most of that ocean slab would be subducted below Gondwana in the Variscan Orogeny – this particular outcrop was caught between the continents and lifted up on top of Gondwana instead.
Taken together, today’s outcrops represent an ophiolite: a cross-section through ocean crust that is lifted up onto a continent during an orogeny. The top of the sequence is the mélange: the material scraped off the top of the subducting slab. Then comes the subducting slab itself, the ocean crust before subduction, and the mantle. Here, the overriding continental crust is also preserved in the form of granulites.