The roots of an ancient mountain chain
The smooth, wave-worn boulders of the Frade Plutonic Complex are the geological equivalent of looking under the hood of a mountain range. These rocks are part of a massive batholith—a large mass of intrusive igneous rock—that extends along the coast. They formed deep within the crust during the Brasiliano Orogeny, a series of mountain-building events that occurred between 660 and 500 million years ago. This was a period of intense tectonic activity, as continents collided to form the supercontinent of Gondwana.
The rocks here are primarily charnockites, a type of orthopyroxene-bearing granite that forms under immense temperature and pressure. Petrological studies indicate these magmas crystallized at temperatures between 830-925°C and pressures of 6-9 kilobars. This corresponds to a depth of nearly 50 kilometers, placing their origin in the lower continental crust. The specific mineral assemblages, including orthopyroxene, garnet, and biotite, are stable only under these extreme conditions, showing processes that occur deep within the Earth. The complex is a small part of the larger Serra do Mar Coastal Batholith, the Precambrian foundation of the Serra do Mar mountain range that runs for 1,500 kilometers along Brazil's coast.