Rb–Sr and Sm–Nd isotopic study of the Puquı́ complex, Colombian Andes

The Puquı́ Complex is exposed in the northwestern part of the Colombian Central Cordillera and consists of a complex association of metamorphic rocks (amphibolite facies gneisses, migmatites, granulites, and amphibolites) intruded by the Puquı́ Tonalite, a large and mostly undeformed intrusion. The Puquı́ Tonalite is calc-alkaline and displays a mild peraluminous character, whereas the gneissic country rocks are strongly peraluminous (and interpreted as derived from a sedimentary protolith). A peculiar feature of this tonalitic body is the common presence of garnet (up to 7% modal proportion in some localities). The Rb–Sr data for the Puquı́ Gneiss yielded an isochron age of 306±11 Ma with a high initial 87Sr/86Sr ratio (ca. 0.70980). This is interpreted as the result of Late Paleozoic metamorphic resetting of the Rb–Sr isotopic system of the sedimentary/metasedimentary protolith. This metamorphic event is also recognized in other parts of the Colombian Andes. Sm–Nd garnet–whole rock ages, however, indicate that these rocks have also been involved in a much younger metamorphic event, between ca. 140 and 120 Ma. Sm–Nd model ages reveal a Precambrian age for the source of the original sediments (TDM model ages are in the range 1.33–1.53 Ga). The Puquı́ Tonalite was emplaced at ca. 248±17 Ma (whole-rock Rb–Sr isochron) with an initial 87Sr/86Sr ratio of ca. 0.70748, which suggests some contribution from older sialic material in the original magma. This is confirmed by the Sm–Nd isotopic data, which show εNd(248) between −5 and −6.15 and TDM model ages between 1.17 and 2.13 Ga. These results suggest that the original tonalitic magma is strongly contaminated by the Puquı́ Gneiss. These preliminary Nd–Sr isotopic data suggest the presence of Precambrian crustal material in this part of the Colombian Central Cordillera. We also postulate that the Puquı́ Gneisses derived from Precambrian peraluminous crustal sources served as the contaminant of the tonalitic magma.