Abstract

The presence of Lower Miocene volcanic rocks in the Coastal Range of central Chile is unique in that the westernmost outcrops of the Oligocene-Miocene volcanic arc occur 80 km to the east. The Las Pataguas Lavas (LLP; 33,8°S) consists of a total exposed area of 1.15 km2 without evidence of feeder conduits nor the construction of a volcanic edifice. The presence of east-west aligned and elongated flow-lobe tumuli suggests a flow from the west, over a gently sloping surface. This suggests a fissure-type eruptive system. The proposed age of the LLP, of ca. 18 Ma is intermediate between Abanico Formation (34-20 Ma) and Farellones Formation (20-10 Ma), and is coeval with littoral facies of the Navidad Formation, exposed 20 km to the west. LLP consists of basaltic trachiandesites, with 52.8-54.5 weight % silica, however their high total alkali contents (6.21-6.26 weight % K2O + Na2O) place these rocks in the alkaline series of differentiation. This is a clear difference with the tholeiitic products of the Abanico Formation, and those typical calc-alkaline of the Farellones Formation. The rare earth element patterns are significantly enriched in light rare earth elements, with a La/Sm ratio of 3.4 and the heavy rare earth elements, present a flat pattern. The silica and MgO contents indicate an advanced differentiation state dominated by fractional crystallization of olivine, clinopiroxene and magnetite. The content of Ba and Nb indicate a significant contribution from the lithospheric mantle. Th and the Ta-Nb trough show a crustal signal as well. The particular petrogenesis and isolated westward forearc setting of the LLP can be explained as a response to the slowing of convergence acceleration at the end of the extensional regime in place until the late Early Miocene, related to the Abanico Formation volcanism. Therefore, the alkaline basaltic andesites of the LLP represent a single, intermediate, volcanic episode between two characteristic tectonomagmatic regimes. © 2005 Servicio Nacional de Geología y Minería.