The Late Miocene Guacimal Pluton in the Cordillera de Tilarán, Costa Rica: its nature, age and petrogenesis
Abstract
The Guacimal Pluton situated in the Cordillera de Tilarán in the NW of Costa Rica forms an oval-shaped body with an exposed surface of 60–70 km2. The pluton intruded into basic volcanic rocks of the Aguacate Group (Miocene–Pliocene) and is surrounded by a aureole of calc-silicate metasomatic rocks.
The pluton is mainly formed by monzogranites to granodiorites, which strongly prevail over more basic types. The pluton was emplaced at depth of c. 3 km, crystallized at temperature of c. 760–800 °C under a relatively high oxygen fugacity (1.6–2.1 log units above the NNO buffer) and increased activities of volatiles (H2O, F) upon cooling.
The prevailing, felsic rocks of the Guacimal Pluton are high-K calc-alkaline. Laser ablation ICP-MS dating of zircons from two granite samples yielded statistically identical U–Pb ages of 6.3 ± 0.5 and 6.0 ± 0.4 Ma, respectively.
The Sr–Nd isotopic compositions are rather primitive (87Sr/86Sr6 = 0.70380–0.70413, e6Nd ~ +7.3 to +7.9).