VOLCANIC RECORD OF BATHOLITH ASSEMBLY: THE AUCANQUILCHA VOLCANIC CLUSTER, NORTHERN CHILEAN ANDES

Aucanquilcha Volcano, in northern Chile (~21°S), is an active volcano that lies at the center of a cluster of more than 15 dacite and andesite volcanoes that we call the Aucanquilcha Volcanic Cluster (AVC). The temporal, spatial, compositional and mineralogical development of the AVC is consistent with the incremental assembly of a batholith from plutons and bears resemblance to the ~10 m.y. magmatic development of the Tuolumne Intrusive Series. The AVC developed from ~11 Ma to present, but individual volcanoes within the AVC had typical lifespans of a few 100,000 y to about 1 m.y. Assembly of the AVC was fitful. Four main eruptive pulses, 11-8 Ma, 6-4 Ma, 3.5-2.5 Ma, and 1-0 Ma, indicate four plutonic stages. In the first two stages, volcanism was peripheral, compositionally diverse, with a largely dry mafic mineral (55 to 73 wt.% SiO2; ol, cpx, opx, rare amph) assemblage. In the third stage, volcanism became more central, more voluminous and compositionally more restricted (59-68 wt% SiO2) with common modal amphibole. At and after 4 Ma, compositions between 57 and 60 wt.% SiO2 commonly occur as magmatic inclusions rather than as lavas (excepting peripheral Miño Volcano) indicating development of a large partially molten zone that acted as a density barrier to rise of more mafic magma. An upper to middle crustal partially molten zone persists today as indicated by a low-velocity seismic zone. The last stage was Aucanquilcha Volcano itself, an east-west ridge in the center of the AVC. Lavas range from 62-68 wt.% SiO2 and have biotite and coexisiting hornblende and pargasite, indicating the tapping of magma from at least two levels in the upper crust. The trend of compositional homogenization in time is accompanied by a general decrease in 143Nd/144Nd and δ18O and increase in 87Sr/86Sr. We interpret these trends to reflect thermal maturation for about 6 m.y., with increased proportion and shallowing of crustal contribution to magmatism. At ~4 Ma, a thick partial melt zone was established and hydrothermal systems became widespread leading to magmatic cannibalism of hydrothermally-altered products from earlier stages.