Granito Orbicular of Chile

January 24, 2017 SouthAmerica

A rather photogenic and intriguing set of beach boulders in northern Chile are located 10.4 km north of the small port town of Caldera, at a decimal degree location of -26.97274, -70.8950, in fact, the rocks are so unusual that they are in a protected park called “El Santuario de la Naturaleza Granito Orbicular” that was established in 1981. A short drive down a graded dirt road from highway 5 brings one to the shoreline and the spotted rocks. Orbicular igneous textures are made by a variety of magma crystallization processes, and in the cases here at the edge of the Pacific Ocean, the monzogabbro rock, not granite, had variable crystal growth rates that are found simultaneously surrounding each one of the spherical rock orbs. The nuclei are all subrounded and made of coarse-grained monzogabbro, and this center is mantled by radiating even more coarse-grained crystals of black hornblende. Then outboard of this all of the spheres appear to have a period of quenching or rapid crystallization, which may result from sudden temperature drop, depressurization, or even a major event that saw the loss of the magma’s volatile content. Then the crystallizing magma went back to conditions that formed once again a coarse-grained texture in the matrix or groundmass.

The amount of the orbicular igneous rock is limited along the beach exposure, and slowly becoming less with the steady erosive forces on the crashing waves. One particular contact shows that the orbicular forming magma intruded into earlier form coarse-grained monzodiorite, making a dike like body. Rounded clasts of this wall rock can be found floating within the orbicular textured dike. If one walks around the sandy plains up on the terrace above the beach shore a few mafic dikes can be found. These dikes are part of a major swarm that generally runs in a northwest direction, and are related to Jurassic arc magmatism. A study by Farrar and others in 1970 reported a K-Ar isotopic date on biotite separated from the orbicular rock, giving a formation age of 159 (plus or minus 5) million years ago. Several blocks of this orbicular material can be found as decorative material in the University of Atacama in the city of Copiapo. Given how rare such bold textured intrusive rocks are, it is fortunate that outcrops along the Chilean coast were made into a park.