The geochemical variations of the upper cenozoic volcanism along the Calama–Olacapato–El Toro transversal fault system in central Andes (~24°S): petrogenetic and geodynamic implications
Introduction
Since the Jurassic, the evolution of the Central Andes has been dominated by the subduction of the Pacific Plate under the South American Plate. The related arc magmatism, located in the Coastal Cordillera during the Jurassic, has migrated eastward to reach the Western Cordillera in the Miocene–Quaternary (Fig. 1). Upper Cenozoic magmatism in Central Andes shows a very complex spatial and temporal evolution, developing essentially in the main volcanic arc and in a very extensive “back-arc” setting Coira et al., 1993, Roger and Hawkesworth, 1989. This “back-arc” volcanism developed on the Puna plateau, which represents the southern extension of the Bolivian Atiplano extending from 18°S to 28°S (Fig. 1).
Volcanism on the Puna plateau started in the Upper Oligocene time but became widespread only during the Miocene at about 10 Ma de Silva, 1989, Coira et al., 1993. This correlates with the Quechua deformational phase which reflects the cessation of spreading along the Farallon–Nazca ridge and the beginning of spreading at the East Pacific ridge Mayes et al., 1990, Coira et al., 1993. At that time, the Central Andes were affected by a compressional tectonic regime and underwent crustal thickening associated with extensive eastward thrusting developed mainly in the eastern sector of the belt Isacks, 1988, Marrett et al., 1994.
In the northern Puna plateau (~23°S), the Miocene volcanics are extensively exposed and consist mainly of voluminous ignimbrites such as those recognised in Coranzuli, Panizos, Vilama–Coruto Atana centres which show characteristics of crustal-derived magmas Seggiaro, 1994, Ort et al., 1996, Coira et al., 1996, de Silva, 1989. In this sector of the Puna plateau, during the Upper Miocene, ignimbritic volcanism migrated westward, from 6.7 Ma (Coranzuli) to 4.5–3.8 Ma (Atana), (Coira et al., 1993). In the central Puna plateau (between 24°S and 25°S), the Upper Cenozoic volcanism produced several important stratovolcanoes, which are aligned along the NW–SE Calama–Olacapato–El Toro (COT) fault Fig. 1, Fig. 2. The COT, one of the extensive lineaments in the Central Andes, is a left-lateral transcurrent fault-system running from near the Pacific coast of Chile to the Eastern Cordillera in NW Argentina, across the Western Cordillera and the Puna plateau Allmendinger et al., 1983, Salfity, 1985, Marrett et al., 1994. The stratovolcanoes along the COT from NW to SE are: Cordon Puntas Negras, Rincon, Tul Tul-Del Medio-Pocitos (TUMEPO), Quevar, and Tastil. To the southeast of Quevar lies the Miocene Aguas Calientes caldera and related ignimbrite deposits (Fig. 2). During the Quaternary, COT-related volcanic activity on the central Puna produced small monogenetic centres of shoshonitic affinity such as the basaltic–andesitic scoria cones and lava flows of San Geronimo and Negro de Chorillos (Fig. 2). The only COT-related Quaternary stratovolcano in the central Puna plateau is the Tuzgle volcano, located just north of the lineament (Coira and Mahlburg Kay, 1993).
The prevailing model for the evolution of Cenozoic volcanism in the Central Andes is that of Coira et al. (1993) who used geophysical and geochemical data to argue that dip variation of the subducting Nazca Plate beneath the Central Andes was the dominant process for the genesis and evolution of magmas. These authors propose that subduction beneath the northern-central Puna plateau (about 23°S) was shallow in the Early–Middle Miocene and became steeper in the Late Miocene. As a consequence of the widening mantle wedge above the slab, hot astenospheric mantle would have been emplaced under thin lithospheric mantle triggering extensive partial melting in the upper continental crust Isacks, 1988, Coira et al., 1993. Slab steepening could also explain the westward migration of ignimbrite activity on the Puna plateau during the Miocene (Kay et al., 1999). We would argue that, while subduction dip variations as proposed by Coira et al. (1993) may be the overall control for the volcanic evolution in the northern Puna plateau, this model cannot explain the distribution and the evolution of Upper Miocene volcanism in the Central Puna along the COT fault system. In this paper, we present new geochemical and isotopic data from the volcanic belt along the COT and propose a new petrogenetic and geodynamic model for the late Cenozoic volcanism in the sector of Central Andes between 24°S and 25°S.
Section snippets
Geological and tectonic background
A simplified sketch map of the Central Andes between 22°S and 26° S is shown in Fig. 1. The area is characterised by calcalkaline volcanic products of different ages resting on a basement of mainly Palaeozoic metamorphic and igneous rocks. One of the most noteworthy features of this area is the eastward migration of orogenic volcanism related to long-lived subduction of the Pacific Plate under the South American Plate from the Jurassic to the present (Scheuber et al., 1994). In the western
Volcanology
Radiometric ages for volcanics along the COT on the Puna plateau record a short-lived but widespread volcanic episode in the Upper Miocene, around 10 Ma Ramirez and Gardeweg, 1982, Koukharsky and Munizaga, 1990, Olson and Gilzean, 1987, Petrinovic et al., 1999. The available geochronological data indicate that the volcanic activity resumed in the Quaternary time. This activity was characterised by Tuzgle volcano (Coira and Mahlburg Kay, 1993) and some monogenetic cones with shoshonite
Analytical methods
Major elements were analysed by X-ray fluorescence (XRF) spectroscopy at the University of Pisa and corrected for matrix effects using method described by Franzini et al. (1975). Mg and Na were analysed by atomic absorption spectrometry and Fe2+ was determined by titration. Trace element analyses were carried out at the Department of Earth Sciences (Monash University, Melbourne) while Sr–Nd isotopic analyses were done at the Department of Earth Sciences at La Trobe University (also Melbourne).
Discussion
COT-related volcanism occurs in a belt extending 250 km east from the main CVZ arc of the Western Cordillera. The composition of Upper Miocene calc-alkaline volcanics along this transversal structure can be used to make inferences about the magma source regions underlying the Central Andes at approximately 24°S.
Kay et al. (1999) proposed for the northern Puna plateau a slab steepening and a westward migration of arc magmatism from 12 to <3 Ma. The ages distribution and geochemical
Geodynamic implications and conclusions
The variation of the geochemical features of the volcanic products along the COT belt allowed us to propose a new model on the tectonic evolution of the sector of Central Andes at about 24°S. The presented data suggest the following.
- 1.
Trace elements and isotopic composition of TUMEPO volcanics could reflect larger contribution of mafic lower crust Fig. 6, Fig. 8 with respect to Puntas Negras and Rincon products Fig. 5, Fig. 8, set in correspondence of the CVZ volcanic front.
- 2.
Quevar and Aguas
Acknowledgments
This work has been carried out in the framework of a scientific convention between Pisa and Salta Universities and supported by the FOMEC (Salta University) and MURST (fondi di Ateneo, Pisa University) grants. We are grateful to Gerhard Wörner for his suggestions that improved the paper and to an anonymous referee. We thank also Suzanne Mahlburg Kay for the constructive discussion in Rio de Janeiro during the 31st International Geological Congress. M.M. thanks Dr. Louise Frick for her help
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