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| Juncal Complex | |
|---|---|
| Name | Juncal Complex |
| Type | Terrane/tectonostratigraphic complex |
| Age | Late Cretaceous–Eocene (commonly) |
| Primary lithology | Metamorphic schist, gneiss, metagraywacke |
| Other lithology | Ultramafic bodies, amphibolite, marble, serpentinite |
| Region | Coastal ranges, western South America |
| Country | Chile, Argentina |
Juncal Complex is a convergent-margin metamorphic and tectonostratigraphic assemblage exposed in the coastal ranges of western South America. The complex comprises a variable package of high-pressure to medium-pressure metamorphic rocks, ultramafic bodies, and associated metavolcanic and metasedimentary units that record subduction, obduction, and accretionary processes during the Late Cretaceous to Paleogene. Its study has informed regional correlations with adjacent terranes, models of Andean orogenesis, and evaluations of metallogenic potential.
The complex crops out along the Pacific margin where it interfaces with units studied by Rodrigo de Bastidas, Andrés Bello, Diego de Almagro, Ernesto Pinto, and researchers from institutions such as the Universidad de Chile, Pontificia Universidad Católica de Chile, Universidad Nacional de La Plata, Universidad de Concepción, and the Smithsonian Institution. Fieldwork has tied exposures to regional features like the Atacama Fault System, the Magallanes Basin, the Chile–Peru Trench, and the Andes Mountains. Comparative work links the complex to terranes discussed in syntheses by the US Geological Survey, the Geological Society of America, and the International Union of Geological Sciences.
Beds within the complex include metasedimentary successions correlated with formations mapped by teams working in the Coquimbo Region, Valparaíso Region, Aysén Region, and the Magallanes Region. Stratigraphic relationships record sequences similar to those in the Loncura Formation, the Franciscan Complex, and the Chilean Coastal Cordillera where field mapping by geologists from the Instituto de Investigaciones Geológicas and the Servicio Nacional de Geología y Minería has delineated thrusts, nappes, and mélanges. Tectonostratigraphic subdivisions align with frameworks used in studies of the Peruvian Andes, the Patagonian Andes, and the Juan Fernández Ridge.
Petrographic descriptions emphasize schists, gneisses, amphibolites, and serpentinites examined by petrographers from the University of California, Berkeley, the University of Oxford, the University of Cambridge, and the University of Buenos Aires. Mineral assemblages include garnet, staurolite, glaucophane, lawsonite, chlorite, epidote, and talc, comparable to assemblages described in studies of the Sierra de la Ventana and the Sierra Maestra. Ultramafic bodies host olivine, pyroxene, and serpentine minerals analogous to those reported from the Baldissero ophiolite and the Semail Ophiolite. Metamorphic textures and mineral chemistry work by teams at the Max Planck Institute for Chemistry and the CNRS have used thin-section microscopy and microprobe analyses to characterize prograde and retrograde histories.
Tectonic interpretations invoke subduction and accretion linked to plate interactions involving the Nazca Plate, the South American Plate, the Falkland Plate, and the Antarctic Plate. Models reference collisional events contemporaneous with the evolution of the Andean orogeny and episodes described in studies of the Cretaceous normal regression, the Paleogene plate reorganization, and the emplacement histories discussed in reports from the Lamont–Doherty Earth Observatory. Kinematic reconstructions utilize structural analyses comparable to those applied to the Cordillera Darwin and the Sierra de Gredos.
Radiometric ages from metamorphic and igneous components have been reported using U–Pb zircon, Ar–Ar mica, and Rb–Sr systems by laboratories at the ANU Research School of Earth Sciences, the ETH Zurich, and the Geological Survey of Canada. Published age spectra link metamorphism and magmatism to Late Cretaceous and Paleogene intervals also recorded in Patagonia, the Chilean North Patagonian Massif, and the Coastal Batholith of central Chile. Isotopic work (Nd–Sr–Pb) by researchers associated with the Institut de Physique du Globe de Paris and the University of Toronto has been used to constrain provenance and mantle versus crustal contributions in mafic and ultramafic units.
The complex hosts mineral occurrences comparable to mineralization described in the El Teniente, Escondida, Cerro Negro, and Los Bronces districts, including structurally controlled sulfide veins, exoskarn-like bodies, and podiform chromitite in ultramafic lenses. Exploration reports from the Compañía de Salitres y Ferrocarril de Antofagasta and modern studies by consultants linked to BHP, Codelco, Anglo American, and Glencore note potential for copper, nickel, chromium, and platinum-group element concentrations. Metallogenic models adapt frameworks applied to the Cordillera Frontal and the Punta del Cobre province.
Key contributors include mapping and synthesis efforts by geologists in collaborations between the Universidad Católica del Norte, the Universidad Austral de Chile, the Instituto de Geología de Chile, and international teams from the University of Texas at Austin and the University of São Paulo. Seminal papers appear in journals such as the Journal of South American Earth Sciences, Tectonophysics, Lithos, Geological Society of America Bulletin, and the Earth and Planetary Science Letters. Landmark conferences where the complex featured prominently include meetings of the Geological Society of London, the American Geophysical Union, the Iberian–American Geological Congress, and workshops organized by the Comisión Chilena del Cerro Tololo.
Category:Geology of Chile Category:Terranes