Alain Demant

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.