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The Guacimal Pluton is situated in the Cordillera de Tilarán in the northwestern Costa Rica. It forms an oval-shaped body strongly elongated in the NW—SE direction. Its dimensions are ˜15 × 4—6 km with an exposed surface of 60—70 km2. The pluton intruded basic volcanic rocks of the Aguacate Group (Miocene—Pliocene) and is surrounded by a wide thermal aureole of calc-silicate metasomatic rocks. The pluton is mainly formed by monzogranites to granodiorites, which strongly prevail over more basic types forming scarce and relatively thin dykes and enclaves. The dominant magmatic minerals of this felsic suite are quartz, plagioclase, and K-feldspar with subordinate Mg-rich biotite, amphibole I, and magnetite. Orthopyroxene, Mn-rich ilmenite, Al-poor titanite, rutile, apatite, zircon, thorite, and chalcopyrite are accessory. Secondary minerals, which occur as fillings of miarolitic cavities and interstices, are quartz II, K-feldspar II, epidote, chlorite, actinolite, ilmenite II and Al-rich titanite II. The much less frequent mafic suite (mainly quartz diorite to quartz monzodiorite/monzogabbro) is composed of plagioclase, pargasite, actinolite, K-feldspar, quartz and magnetite, with accessory amounts of opaque minerals, epidote, chlorite, and titanite. The pluton was emplaced at a 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). Increased activities of volatiles (H2O, F) upon cooling are indicated by the presence of highly aluminous, F-rich titanite and other hydrous silicates in miarolitic cavities. The prevailing, felsic rocks of the Guacimal Pluton are high-K calc-alkaline, whereas the mafic suite is nearly exclusively medium-K calc-alkaline in nature. 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, ε6Nd˜ +7.3 to +7.9). Narrow range of these values effectively rules out open-system processes such as magma mixing or assimilation of isotopically contrasting upper continental crust. Instead, the felsic suite is interpreted as either having crystallized from a highly fractionated melt extracted from a plagioclase—amphibole-dominated crystal mush in a putative deep crustal reservoir or a product of partial melting of older arc-related rocks, such as intermediate lavas or volcaniclastics, immature psammitic sediments rich in volcanic material etc. The observed variation in the felsic suite was most likely produced by low degrees of closed-system fractional crystallization of an assemblage dominated by feldspars. At least some of the rocks of the volumetrically subordinate mafic suite may represent lithologies rich in the complementary cumulates

The Late Miocene Guacimal Pluton in the Cordillera de Tilarán, Costa Rica: its nature, age and petrogenesis