PETROGRAPHY AND THERMOBAROMETRY OF CRETACEOUS PLUTONISM ACROSS A CENTRAL-WESTERN CORDILLERA TRANSECT, NORTHERN ANDES, COLOMBIA

The Northern Andes (NA) of Colombia have an intriguing geological history of global importance. Rocks in this area record unique subduction-accretion process defined by the interaction of tectonic plates (South America, Nazca-Farallon, Caribbean) and major lithospheric blocks (Panama-Chocó and Nor-Andean).

Cretaceous, subduction-related plutonism in the NA is a key in the development of continental crust. It is concentrated across a latitudinal band (5-6°N) covering the northern Central Cordillera, the Cauca Trough Suture Zone, and the Western Cordillera. From E to W, the most important plutons are Antioquia Batholith, Ovejas Batholith, Altavista Stock, Sabanalarga Batholith, and Sucre Monzogranite. Despite some petrographic similarities described in the literature, these granitoids record different magmatic stages and contrasting thermo-tectonic histories that have influenced the geologic and morphotectonic evolution of the region. We present results of detailed petrography and thermobarometry analyses from 10 samples collected for the units above. Microprobe analyses for Al content in Hb and compositional maps were carried out to compare P-T conditions using the calibrations of Hammarstrom & Zen (1986), Hollister et al (1987), Schmidt (1992), Blundy & Holland (1990), and the thermometer of Anderson & Smith (1995).

Results show that emplacement took place at ~1.32 - 3.10 kbar, i.e., crystallization depth of 6 to 14 km, placing most units in the mesozone. Temperature conditions show tight ranges of 600-700°C. Samples west of the Sabanalarga Fault show different petrographic characteristics from those at the east, which ratifies the contrasting nature of magmatism in the Cauca Trough. Some units show similarities in petrography, as well as in P-T values, which means that certain plutons might be part of the same unit. Spatiotemporal distribution of these units is still an object of debate since the magmatic evolution suggests petrogenetically contrasted environments. Our results are being complemented with geochronology, thermochronology and geochemistry data, still in process. We hope to contribute to the knowledge of regional geodynamics of NW South America by comparing magmatic evolution, mechanisms of emplacement, and tectono-thermal history of the K-plutonism in Colombia.