Crestones summit, of the Ventisqueros lava unit, is situated at the intersection of two major fracture sets. Many of the fractures are open (up to tens of centimeters) and re- mineralized (Figure 24). Additional fractures of sub-vertical dip are well distributed throughout the Crestones summit (Figure 25). The fractures serve as hydrothermal fluid pathways and are observed from the meter to micrometer scale (Figure 8).
Figure 25: Crestones summit. Photo taken north of summit facing south
Epigenetic and deuteric mineralization common to hydrothermal alteration such as: sericite, chlorite, epidote, jasper, tremolite, actinolite, talc, serpentine, Fe, Cu, and Mn oxides, are all observed in thin section and/or in the field (Figures 25-27).
Figure 26: Chlorite-rich zone of hydrothermal alteration. Photo taken near Crestones Summit.
Figure 28: Fe oxide-rich zone of hydrothermal alteration. Photo taken near trail west of Ventisqueros summit.
5 CONCLUSIONS
Chirripó National Park is the highest point in Central America. Typical subduction, prior to the arrival of the Cocos Ridge to the Middle American Trench, formed the Central American island arc. The island arc formed from basaltic volcanism of partially melted peridotite in the mantle wedge. Upon reaching sea level elevation, island volcanoes became a fan delta depositional environment for volcaniclastic/shallow marine sedimentary rock. Progressive sedimentation beginning in the Paleogene, with coeval/intermittent eruptions ranging in composition from basalt to andesite, worked to close the Central American Isthmus in the early Pliocene (Molnar, 2008). Magma chambers below the arc intrude overlying volcanic and sedimentary rock as a complex system of gabbroic and dioritic dikes, stock, sills, and plutons in the mid to late Miocene (Alvarado & Gans, 2012). Arrival of the buoyant Cocos Ridge to the Middle American Trench alters subduction geometry to that of a shallower angle. Shallow angle subduction leads to rapid uplift and exhumation by crustal shortening and isostatic reequilibration (Gräfe, 2002).
A 1:12,500 scale geologic map was produced to ascertain rock units, structure, and stratigraphy. A sample suite of 75 hand specimens was claimed to petrographically describe the rock units and determine their composition. Four rock units were
distinguished: the Piramidé sedimentary unit, the Ventisqueros lava unit, the Talamanca gabbroic unit, and the Talamanca dioritic unit. The four units were correlated to previous studies in the region by De Boer, 1995; Drummond, 1995; Alvarado and Gans, 2012.
Future tasks in the study area include:
• Trace element analysis of the igneous rock units to determine cogenetic relation.
• XRD analysis of secondary mineralization owed to hydrothermal alteration.
• Radiogenic dating of samples to affirm stratigraphic age relations between units.
• Determine processes that led to evolution of intrusive magmas from gabbroic to dioritic composition.
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APPENDICES
Appendix A: Geologic map of Chirripó National Park.
Quaternary glacial alluvium Talamanca dioritic unit
Talamanca gabbroic unit/hypabyssal gabbroic unit
Ventisqueros lava unit Pirámide sedimentary unit