Abstract
The stratigraphy, chemistry and age of rocks assigned to the eastern portion of the Abanico Formation exposed along the El Volcán river valley, Principal Cordillera east of Santiago (30º50'S/70º12'-70º5'W), are reported and discussed. This ca. 3,300 m thick succession is mainly composed of basalts, basaltic andesites and volcaniclastic rocks. 40Ar/39Ar radiometric dates on plagioclase from the lava flows yield Oligocene-lower Miocene ages with a maximum age of 34.3 ±0.4 Ma for the lower part and a plateau age of 21.4±1.0 Ma for the upper part of the succession. The lava flows show calc-alkaline affinities and have chemical characteristics that are typical of arc volcanic rocks erupted in an active continental margin. A temporal chemical evolution in the sequence is indicated by upward increases in concentrations of LILE and LREE elements and LaN/YbN ratios. This pattern can be attributed to increasing contributions of fluids derived from the subducted lithosphere with time. A chemical comparison of these rocks with Oligocene-lower Miocene volcanic rocks from the Cerro Abanico and Chacabuco areas on the western border of the Principal Cordillera, east of Santiago, and at the northern end of the Central Depression reveals west to east compositional variations. From west to east these variations include: (1) increasing LILE and LREE concentrations, LaN/YbN ratios and Sr and Nd initial isotopic ratios, and (2) decreasing LILE/HFSE and LREE/HFSE ratios. These pattern can be attributed to a west to east decrease in the contribution of slab derived fluids and increase in the influence of crustal contamination processes.
Highlights
The Andean Principal Cordillera in central Chile (33°-34°S) is composed mainly of Cenozoic volcanic and continental volcaniclastic rocks forming a northsouth-trending continuous belt
Petrographic and chemical study of Abanico East formation volcanic rocks exposed in the Andean Principal Cordillera, east of Santiago, suggest that a magmatic evolution dominated by plagioclase, clinopyroxene, olivine and magnetite was the most important control in the geochemical variations from basalt to dacite
Lavas from this series have a calcalkaline character and show chemical and isotopic characteristics typical of a subduction related volcanic arc developed in an active continental margin
Summary
The Andean Principal Cordillera in central Chile (33°-34°S) is composed mainly of Cenozoic volcanic and continental volcaniclastic rocks forming a northsouth-trending continuous belt. These Cenozoic deposits overlie Mesozoic marine and continental units, which are distributed in a tight belt near the Chilean-Argentinian border. Between 33 and 34°S, the Cenozoic deposits are known as Abanico Formation (Aguirre, 1960; Klohn, 1960) of Late Eocene-early Miocene age (Vergara et al, 1999; Charrier et al, 2002) and Farellones Formation (Klohn, 1960) of Miocene age (Munizaga and Vicente, 1982; Vergara et al, 1988). The Abanico Formation forms two parallel north-south-oriented bands, separated by the overlying Farellones Formation (Fig. 1). According to Charrier et al (2002) an unconformity and a hiatus of ca. 80 Ma separate Abanico East formation from the underlying Mesozoic units
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