Magmatic evolution at volcan Sollipulli, southern Andes of Chile.
Volcan Sollipulli is a Quaternary stratovolcano situated at 38'50'S in the Southern
Volcanic Zone of the Andes of Chile, about 25 km east of the volcanic front. The volcano is
capped by a large (25 km2 approx. ) ice-filled caldera. Sollipulli is unusual in this region of
predominantly basic to intermediate magmatism in that it has erupted a wide range of magmas
from high-MgO (9%) basalt to rhyolite (74% Si02). The last major eruption, the Alpehue
eruption, occurred at about 2,900 B. P, ejecting about 4.7 km3 (D. R. E) of homogeneous
high-Si dacite pumice, forming an extensive plinian airfall deposit and ignimbrite. The
caldera predates the Alpehue eruption and is believed to have formed by passive subsidence
combined with erosion rather than by catastrophic collapse. Withdrawal of magma from
beneath the centre of the structure and effusive eruption induced subsidence.
Magma mixing, fractional crystallisation and crustal assimilation are important
evolutionary mechanisms. Some mixed dacite lavas contain primitive basaltic magmatic
inclusions with diktytaxitic textures indicative of rapid quenching. Strongly resorbed,
reverse zoned sodic plagioclase of dacitic origin occurs in basic inclusions and high-Mg
olivine occurs in dacites. Other sequences appeart o have evolved predominantly by
fractional crystallisation with some crustal assimilation. The amount of crustal assimilation
increasesw ith decreasinga gei n somec ases. Older Sollipulli basic magmash ave evolved as
small batchesp redominantlya t moderatet o high pressurein the mid-lower crust whereas
younger basic magmas have experienced protracted upper crustal histories in a large magma
chamber, fractionating and assimilating crust to produce abundant high-Si dacite.
Sollipulli magmas have an anhydrous mineralogy except for the occurrence of very
minor amphibolei n somem ixed rocks. Magma temperaturesra ngef rom about 118 0'C in
basalts to about 900'C in dacites. The most primitive samples represent hot and relatively
water-poor (<1-2% H20) high-Al basaltic magmas. Crystal-rich andesitesa nd dacites
record lower temperaturesth an crystal-poore quivalents. The cooler porphyritic magmas
appear to have assimilated more crust than the hotter crystal-poor magmas. Most magmas
have evolved at oxygen fugacities close to the NNO buffer curve.
Large volcanic front centres in the region erupt magmas with lower incompatible
elementa bundancea nd higher Ba/Nb than magmase rupteda t minor monogeneticc entresa, nd
at stratovolcanoesto the easto f the front, which have incompatiblee lements ignatures
transitional towards back-arc alkaline magmas (e. g. high Nb, Ce/Y). Older Sollipulli
magmas also have high Ce/Y and Nb similar to magmas at some monogenetic centres but
have lower Ti and Y. Younger Sollipulli magmas have even lower Ti and Y. Sollipulli basic
magmasa re also characterisedb y higher Mg/Ni than all regional magmas. The simplest
explanation is that the high Nb, Ce/Y magmas have assimilated enriched mantle lithosphere.
The Ti, Y, Mg/Ni systematics suggest that the Sollipulli magmasa re generated by higher
degrees of melting of a similar MORB-source-type mantle than the regional magmas. In the
case of the younger Sollipulli magmas, generation from mantle which was slightly depleted
during the earlier phase is also possible but the older magmas show no evidence in their
spinel compositions for derivation from refractory mantle.