Petrogenesis of the Pleistocene Tronador Volcanic Group, Andean Southern Volcanic Zone

Abstract

The Tronador Volcanic Group (TVG) is located in the transition between the central and southern segment of the Andean Southern Volcanic Zone (SVZ), »50 km to the east of the current volcanic front. The TVG includes, from older to younger, the deeply eroded Early Pleistocene Garganta de Diablo unit (GDU; »1.3 Ma) and Steffen volcanic complex (SVC), the Middle Pleistocene Tronador Volcanic Complex (TVC; <1.0 Ma to »300 ka), as well as the post-glacial Fonck monogenic volcanic cone. Temporal petrochemical variations observed for older compared to younger samples from the TVG are similar to differences occurring across the active southern SVZ arc, from the current volcanic front to centers east of the front, and consistent with decreasing input of slab-derived fluids, and consequently decreasing percent of partial melting of the mantle source of the TVG. Relatively depleted (K₂O<0.96 wt %; Rb<26 ppm; La<14.6 ppm) Group I basalts and basaltic andesites, and medium-K more differentiated rocks, were erupted to form the GDU and SVC during the Early Pleistocene. These have Ba/La>20 similar to the current volcanic front, but higher La/Yb and lower Yb, consistent with their genesis by 10% partial melting of subarc garnet-peridotite mantle, followed by 20% fractional crystallization involving olivine, clinopyroxene and plagioclase. Younger TVC Group I basic rocks have similar Ba/La, but higher Yb, and were generated by slightly lower degrees (7%) of partial melting in shallower garnet-free mantle. Beginning in the Middle Pleistocene (»500-300 ka, Tronador II and III units of the TVC) relatively enriched (K₂O>0.96 wt %; Rb>31 ppm; La>20 ppm) Group II basalts, with geochemical characteristics similar to volcanic centers east of the current volcanic front (Ba/La<20), were erupted together with Group I basalts. The generation of Group II rocks are consistent with 5% partial mantle melting in drier and shallower plagioclase-bearing mantle modified by less fluids derived from the subducted slab than involved in the generation of Group I rocks. Although these petrochemical changes were gradual, the relative volume of magma erupted across this region of the SVZ arc changed in the late Middle Pleistocene, after »300 ka, with the nearly complete termination of volcanic activity in the TVG and growth of Osorno and Calbuco volcanoes along the current volcanic front at this latitude.