Petrology and geochemistry of the orthogneisses outcropping in the eastern and central regions in the Santander Massif, Colombian Andes

Abstract

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 amphibolite lenses appear. The orthogneisses are the result of the synthectonic emplacement of granitoids in the rocks of the Silgará Schist Unit during the metamorphic peak of this meta–sedimentary 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 metamorphic conditions were estimated for the central region as T = 690–770 °C and P = 0.53–0.85 GPa whereas amphibolite from eastern region reached metamorphic thermal peak conditions at T = 680–693 °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.