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Geological Society of America Special Paper 265 1991
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Spatial and temporal variations in volcanics of the Andean Central Volcanic Zone (26 to 2aoS)
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James A. Walker and Tari N. Moulds*
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Department of Geology, Northern Illinois University, DeKalb, Illinois 60115
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Department of Geology, Dalhousie University, Halifax, Nova Scotia B3H 3J5, Canada
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Mark D. Feigenson
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Department of Geological Sciences, Rutgers University, New Brunswick, New Jersey 08903
Marcos Zentilli
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ABSTRACT
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Late Cenozoic volcanic rocks from the southernmost extremity of the Central Volcanic Zone (CVZ) of the Andes, near the first-order tectonic discontinuity at 27°S, display geochemical attributes typical of CVZ volcanic rocks: an exclusion of basic compositions, high concentrations of large ion lithophile (LIL) elements and elevated 87Sr/86Sr. Some ofthese geochemical attributes show intriguing and sometimes systematic spatial and temporal variation. Along the Miocene volcanic front, normalized concentrations of K20 and Rb in volcanic rocks erupted from central vents systematically increase as the tectonic discontinuity is approached from the north. K/Rb systematically decreases in the same direction. These along-arc systematics are equivalent to the geochemical differences between the Miocene volcanic front, as a whole, and the Pliocene to Recent volcanic front, which lies some 50 km farther east. Pliocene to Recent volcanic rocks, however, generally also exhibit enrichments in light rare earth elements and La/Yb. In addition, local variations-in L~L element contents rival those along and across the arc. All of these spatial-temporal geochemical variations are attributed to crustal differentiation processes, specifically crustal contamination. The spatial-temporal implications are that crustal contamination is increasingly effective: (1) toward the modern-day tectonic discontinuity at 27°S; and (2) underneath Pliocene to Recent, as opposed to Miocene, volcanoes. Garnet control and hornblende fractionation are important variables in crustal differentiation. Increasing crustal contamination along and across the arc correlates with flattening of the subducted Nazca plate and possibly with increasing crustal thicknesses. If so, the spatial-temporal geochemical variations in the southernmost CVZ strongly support the models of Kay and others (1987, 1988) for the volcanic and tectonic evolution of the present-day nonvolcanic region.
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*Present address: Hawaiian Volcano Observatory, U.S. Geological Survey, P.O. Box 51, Hawaii National Park, HI 96718. Walker, J. A., Moulds, T. N., Zentilli, M., and Feigenson, M. D., 1991 , Spatial and temporal variations in volcanics of the Andean Central Volcanic Zone (26-28°S), in Harmon, R. S., and Rapela, C. W., eds., Andean magmatism and its tectonic setting: Boulder, Colorado, Geological Society of America Special Paper 265.
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J. A. Walker and Others
INTRODUCTION
GEOLOGIC AND TECTONIC BACKGROUND
It is well established that magmatic differentiation in the crust is responsible for the distinctive volcanic-rock compositions of the Central Volcanic Zone (CVZ) of the Andean convergent plate margin (e.g., James, 1982; Thorpe and others, 1984; Harmon and Hoefs, 1984; Harmon and others, 1984). This is not surprising because crustal thicknesses reach about 70 km beneath at least portions of the CVZ (James, 1971). The question arises, however, about the relative importance of crustal differentiation toward the southernmost extremity of the CVZ, where smaller crustal thicknesses have been presumed. The following is a brief summary of the geochemistry of Miocene to Recent volcanic rocks erupted in the southernmost CVZ, augmenting the prior results of Zentilli (1974), McNutt and others (1975), Dostal and others (1977b), Zentilli and Dostal (1977), McNutt and others (1979), Longstaffe and others (1983), and Baker and others (1987). We shall demonstrate not only that magmatic differentiation in the crust is still profound in the southernmost CVZ, but also that the petrogenetic influence of crustal differentiation is temporally and spatially variable.
The CVZ abruptly terminates around 27°S near the imposing Ojos del Salado volcanic complex, the highest volcano on earth (Figs. 1 and 2). This volcanic discontinuity corresponds with a significant tectonic discontinuity, in that the dip of the underthrust Nazca plate is quite shallow south of 27°S, but gradually steepens to 30°, north of 27°S (Barazangi and Isacks, 1976; Bevis and Isacks, 1984; Isacks, 1988). A number of other geologic and tectonic features are also discontinuous near 27°S (Bonatti and others, 1977; Allmendinger and others, 1983; Jordan and others, 1983; Gonzalez-Ferran and others, 1985; Thornburg and Kulm, 1987). Tectonic segmentation at 27°S is thought to have been initiated at about 18 Ma, perhaps as a result of subduction of the buoyant Juan Fernandez Ridge (Pilger, 1981, 1984; Sacks, 1983; Jordan and others, 1983; Jordan and Gardeweg, 1989), with an acceleration around 10 Ma as andesitic volcanism in the main Cordillera died in the present flat-slab region (Kay and others, 1987). Between 26° and 28°S, there is a clearly defined Miocene volcanic front stretching from Volcan Dona Ines in the north to Volcan Jotabeche in the south (Fig. 2). Figure 2 shows that the locus of volcanic activity between 26° and 28°S shifted about 40 to 75 km eastward sometime after initiation of tectonic segmentation. Available age determinations also indicate that activity persisted at Volcan Copiapo, the largest of the Miocene volcanic centers, for almost 5 m.y., a lifespan on a par with that of the prodigious Cerro Galan complex of northwestern Argentina (Sparks and others, 1985). Moreover, there is an unconfirmed report of a 5-m.y.-old pyroclastic flow on the northern flanks of the Copiapo complex (R. Sillitoe, personal communication, 1990). If this age is correct and if the flow originated from Copiapo, then the lifespan of Copiapo was more on the order of 8 m.y. Regardless, volcanic activity at Copiapo overlapped with that of Wheelwright Caldera, a late Miocene to Pliocene volcanic center located to the northeast (Fig. 2; Gonzalez-Ferran and others, 1985). Therefore, migration of the volcanic axis was apparently not a discontinuous abrupt affair. Figure 2 also shows that the most recent volcanism around 27°S has centered around the Chilean-Argentine border. As a result, Gonzalez-Ferran and others (1985) suggest that the present volcanic axis runs almost parallel to the direction of plate convergence, and the southern termination of the CVZ is coincident with the so-called Easter Hot Line on the subducting Nazca plate (i.e., Bonatti and others, 1977). This interpretation seems premature given the lack of information on Pliocene to Recent volcanoes away from 27°S (Fig. 2). In the following geochemical summary, samples from Dona Ines, Cerros Bravos, and Copiapo are used to characterize magmatic compositions along the Miocene volcanic front. Cerros Bravos samples were kindly provided by Jorge Munoz of the Chilean Geological Survey (SERNAGEOMIN). Detailed sample locations are available from the first author. All of the Miocene volcanic rocks are moderately to highly porphyritic and are all
SOUTH
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Figure 1. Tectonic setting of western South America. Regions enclosed by lighter lines are areas of Quaternary volcanism: NVZ, CVZ, SVZ, and A VZ are Northern, Central, Southern, and Austral volcanic zones, respectively. Shaded box approximates study area.
Volcanics of the Andean Central Volcanic Zone
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