Petrologia y geoquimica de los Ortoneises aflorantes en los sectores central y este en el Macizo de Santander, Andes Colombianos

Orthogneisses outcropping in the central and eastern regions in the Santander Massif consist in interlayered of hornblende gneiss, biotite gneiss and quartzo-feldspathic gneiss. Biotite and hornblende gneisses are the predominant lithologies. In these orthogneisses, sporadic amphi-bolite lenses appear. The orthogneisses are the result of the synthectonic emplacement of granitoids in the rocks of the Silgara Schist Unit during the metamorphic peak of this meta-sedimentar y unit. Some structural and compositional differences are observed in the orthogneisses of the central and eastern sectors; the most developed foliation is observed in the central sector and the presence of muscovite, apparently it is restricted only to the eastern sector. The geochemical characteristics show that the orthogneisses of the central sector are granitic in composition, while, for those located in the eastern sector, their composition varies between granite and granodiorite. These orthogneisses were formed on an active continental margin. According to rare earth elements - REE behavior the rocks displayed three geochemical trends: 1) The felsic orthogneisses are moderating depleted in Light Rare elements-LREE (La-Sm) and depleted in Heavy Rare Earth elements-HREE (Eu-Lu); 2) negative europium anomaly is typical for orthogneisses; 3) Amphibolite is depleted in LREE and slow increment of HREE without europium anomaly. P-T metamor-phic conditions were estimated for the central region as T = 690-770 degrees C and P = 0.53-0.85 GPa whereas amphibolite from eastern region reached metamorphic thermal peak conditions at T = 680-693 degrees C. Occurrence, field relationship of studied orthogneisses respect to metasedimentary country rocks are very similar that those orthogneisses outcropping along the Andes and southeastern terranes of Mexico as result of magmatic arc developed along the western Gondwana protomargin.

Automated summary

The orthogneisses in the central and eastern regions of the Santander Massif consist of interlayered hornblende gneiss, biotite gneiss, and quartzo-feldspathic gneiss. These orthogneisses were formed during the metamorphic peak of the Silgara Schist Unit. The geochemical characteristics show differences between the central and eastern sectors, with the presence of muscovite restricted to the eastern sector. The occurrence and field relationship of these orthogneisses are similar to those found in the Andes and southeastern terranes of Mexico.