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Title: Magmatic evolution at volcan Sollipulli, southern Andes of Chile.
Author: Murphy, Michael D.
Awarding Body: University of Bristol
Current Institution: University of Bristol
Date of Award: 1996
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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.
Supervisor: Not available Sponsor: Not available
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Subduction; Andes; Magma mixing Volcanoes Plate tectonics Geochemistry