THE HUINCAN ERUPTIVE CYCLE, SOUTHERN MENDOZA, ARGENTINA: AN EXAMPLE OF ANDESITIC FRONT-ARC-LIKE MIOCENE VOLCANISM IN THE BACK-ARC OF THE SOUTHERN VOLCANIC ZONE

A chain of monogenetic domes and cones erupted during the Huincan cycle (HC) in the northern SVZ (34º - 35º Lat. S, 69º 39` Long. W), for more than 100 kilometers. Ar-Ar amphibole ages range from 10 to 6 Ma. All rocks plot in the sub-alkaline field, and have a narrow, range of SiO2 (55 to 62 % wt) implying they are basaltic andesite and andesite. Rock compositions straddle the boundary between calc-alkaline and tholeiitic sub-series in the FeO*/MgO vs SiO2 diagram, and fall in the medium-K field of the SiO2-K2O diagram. None of these volcanic rocks is primitive as they are not equilibrated with any mantle assemblage. Rocks from HC have relative and absolute contents of trace elements resembling intermediate igneous rocks erupted along magmatic arc fronts. They are characterized by depletions in HFSE relative to other incompatible elements. HSFE depletion of the Huincan rocks is both absolute (compared to primitive mantle) and relative (compared to LILE and LREE). Since HC rocks appear in a belt far to the east of the main volcanic arc and were active at the same latitude and time (e.g. Kay et al., 2005), the HC could be considered a case of back-arc magmatism. A present-day distance of about 75 km from the volcanic front sequence of the Teniente Volcanic Complex (TVC) and the chain of HC provides an estimate of the distance between the fore-arc and the back arc. This minimum distance neglects tectonic shortening during the last 5 Ma. Comparing the geochemistry of back-arc centers of HC to rocks of the volcanic front (TVC) having equivalent SiO2 contents, we note that the abundance of most trace elements is coincident for both these groups. The Sr and Ba contents of the Huincan rocks are significantly elevated relative to volcanic arc-front. In HC rocks LREE are slightly above, or overlap with, the highest values of the volcanic-front lavas. Arc-front lavas have significantly more Th than back-arc rocks. Despite these differences, there is a striking resemblance between the HC back-arc and TVC frontal-arc rocks suggesting that similar petrogenetic processes controlled magmatism in the arc-front and back-arc, which are separated by tens of kilometers.