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Cenozoic erosion in the Andean forearc in Central Chile (33°–34°S): Sediment provenance inferred by heavy mineral studies

By
María Pía Rodríguez
María Pía Rodríguez
Departamento de Geología, Facultad de Ciencias Físicas y Matemáticas, Universidad de Chile, Casilla 13518, Correo 21, Santiago, Chile
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Luisa Pinto Lincoñir
Luisa Pinto Lincoñir
Departamento de Geología, Facultad de Ciencias Físicas y Matemáticas, Universidad de Chile, Casilla 13518, Correo 21, Santiago, Chile
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Alfonso Encinas
Alfonso Encinas
Departamento de Ciencias de la Tierra, Facultad de Ciencias Químicas, Universidad de Concepción, Edmundo Larenas 129, Casilla 160-C, Concepción, Chile
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Published:
May 01, 2012

The forearc of Central Chile (33°–34°S) is formed by three N-S–trending morphostructural units, including, from west to east, the Coastal Cordillera, the Central Depression, and the Principal Cordillera. The Cenozoic sedimentary rocks that could represent the erosional material generated throughout the morphotectonic evolution of these units accumulated in the marine Navidad Basin. The age of the marine deposits is controversial, as foraminifer biostratigraphy indicates that marine deposition started during the late Miocene, whereas 87Sr/86Sr data indicate that deposition started during the early Miocene. We carried out single heavy mineral microprobe analysis and standard heavy mineral analysis of these deposits in order to qualitatively identify the geological units subjected to erosion in the central Chilean forearc during Cenozoic times. Our analysis focused mainly on unweathered and unaltered detrital garnet, pyroxene, and amphibole. The textural characteristics and geochemical signature of these minerals were used to determine their original rock type; their magmatic affinity, in the case of pyroxenes of volcanic origin; and their metamorphic grade, in the case of amphiboles of metamorphic origin. We have also compared the composition of detrital garnet, pyroxene, and amphibole with preexisting chemical data of these minerals in the possible source rocks, which, along with the analysis of the detrital heavy mineral suite in each sample, allows us to determine the specific geological unit from which they were generated. Three erosional-depositional stages are recorded by our analysis. Whereas the chemistry of pyroxene and amphibole characterized volcanic-subvolcanic sources within the present-day Central Depression for the first stage, the Central Depression and the Principal Cordillera for the second stage, and the Principal Cordillera for the third stage; the composition of garnet is indicative of metamorphic and plutonic sources within the Coastal Cordillera during all three stages. If marine deposition inside the Navidad Basin started during the early Miocene, the provenance results would record erosion and deposition contemporary with volcanic activity. On the other hand, if marine deposition started during the late Miocene, the provenance results show a retrograde erosive response to landscape for a regional uplift event proposed for that period in the study area. Also, assuming that provenance results are directly related to the action of faults, our data indicate that the main relief-generating fault during the early stages of Andean uplift corresponds to the Los Ángeles–Infiernillo Fault, rather than the San Ramón Fault, as stated by the proposed morphotectonic models for the study area. In addition, the ubiquitous provenance from the Coastal Cordillera is more likely to represent the erosion of nearshore basement rocks affected by faulting along the eastern border of the Navidad Basin, rather than uplift and erosion of the Coastal Cordillera, as previously considered. Single-mineral geochemical analysis of detrital pyroxene and amphibole can be used in other sedimentary basins related to arc-magmatic systems with short transport distances, like the ones in the western Andean border, where these minerals tend to be largely unweathered. In particular, our work represents an advance in this field, as the chemistry of detrital amphibole has not been used before to discriminate source rocks presenting different geochemical signatures.

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GSA Special Papers

Mineralogical and Geochemical Approaches to Provenance

E. Troy Rasbury
E. Troy Rasbury
Department of Geosciences, Stony Brook University, Stony Brook, New York 11794-2100, USA
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Sidney R. Hemming
Sidney R. Hemming
Department of Earth and Environmental Sciences and Lamont-Doherty Earth Observatory, Columbia University, Palisades, New York 10964, USA
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Nancy R. Riggs
Nancy R. Riggs
Geology Program, School of Earth Sciences and Environmental Sustainability, Northern Arizona University, Flagstaff, Arizona 86011, USA
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Geological Society of America
Volume
487
ISBN print:
9780813724874
Publication date:
May 01, 2012

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