Alkali basalt volcanism along a subduction-related magmatic arc: The case of Puyuhuapi Quaternary volcanic line, Southern Andes (44deg20minS)

In Southern Chile, plate configuration is characterized by ridge-ridge-trench
collision in correspondence of Taitao peninsula (Chile triple junction). The
different converging rates of Nazca and Antarctic plates favored the formation
of a forearc sliver (Chiloe block) limited to west by a dextral transcurrent
fault system, Known as Liquine-Ofqui fault system (LOFS). During the Quatenary
time, a series of monogenetic volcanic centers, as Puyuhuapi volcanic centers
(PVC), formed along the LOFS. The PVC lavas have a primitive character; two
groups and can be distinguihed. Group-1 rocks show a K-AlKaline affinity and
are nepheline normative with olivine and plagioclase as dominant phases.
Group-2 lavas have Na-affinity with olivine and hyperstene in the norm; olivine
is the most abundant mineral phase. In contrast with overall alkaline affinity
of PVC, the products from the neighboring central composite volcanoes are
generally calcalkaline with the exception of the lavas from Maca Volcano, which
show tholeiitic affinity.