|Finally, an ultramafic
and blue-shist citadel!
Braganca, Portugal (general location)
This walled city with itís 12th-century medieval citadel and a domus municipalis (town hall) sitting upon an ultramafic dome was a pleasant surprise. Many of the stones used in the citadel appear to be ultramafic rocks likely derived from the Braganca Massif. The rocks in this area have been interpreted as fragments of the mantle below an island arc that existed in NW Spain and Northern Portugal during the mountain building period called Variscan (sometimes called the Herycnian) orogeny. This mountain building period is interpreted as a result of a late Paleozoic period of collision between the attached North America and European continents with Gondwana to form the major portion of the supercontinent Pangaea. Rocks north of this area have been interpreted as parts of the Gondwana continental margin that was subducted or carried deep into the Earth's interior as part of this collision. Economic minerals including podiform chromitite and related platinum-group element mineralization has been of interest to both geologists and miners.
The blueschist wall of the city and view of the countryside.
A close up of the rocks
Garnet(?) bearing rocks
Serpentine and asbestos bearing rocks
The surrounding area has much to discover as well.
This graceful Roman bridge is elaborately constructed of rounded blocks of highly metamorphosed rocks which are much disturbed (note the tight foliation and brecciated rock below).
It also has wonderful looks back in time when oxen were regularly used to bring in the hay!