DOI

10.17077/etd.rx3wdk3y

Document Type

Dissertation

Date of Degree

Spring 2017

Degree Name

PhD (Doctor of Philosophy)

Degree In

Geoscience

First Advisor

Ukstins Peate, Ingrid A

First Committee Member

Peate, David

Second Committee Member

McClelland, Bill

Third Committee Member

Finzel, Emily

Fourth Committee Member

Ramos, Frank

Abstract

Instances of fault-controlled monogenetic volcanism across the subduction arc of the Central Andes at ~ 23°S illuminate the nature of different parental melts being delivered to the crust. Evidence of magmatic history is preserved in bulk rock geochemistry, the content of melt inclusions, and mineral compositions. Volcanism in this region is dominated by felsic and intermediates lavas as the thickened crust (55 – 65 km) and vast volumes (> 500,000 km3) of mid-crustal magma beneath the Altiplano-Puna high plateau region prevent mafic magmas from reaching the surface (Davidson & De Silva, 1991; Beck et al., 1996; Perkins et al., 2016). However, small volumes of relatively undifferentiated lava have been delivered from the lower crust to the surface along zones of crustal weakness without extensive processing by crustal assimilation and/or extended storage in sub-volcanic magma chambers. Monogenetic eruptions of less-differentiated lava provide important constraints on compositions normally obscured by crustal processing in the Central Andes.

Basaltic andesite sampled within the frontal arc (Cerro Overo maar) is a regional mafic end-member and approximates the composition of parental arc magmas derived from partially-molten lower crustal regions where mantle-derived magmas interact with the surrounding lithosphere and undergo density differentiation (MASH zones). Basaltic olivine-hosted melt inclusions from Cerro Overo provide a glimpse of less-evolved melt composition from this region and suggest mobilization of MASH magma by injection of basaltic melt. Basaltic andesite sampled from the eastern (back) margin of the frontal arc (Puntas Negras – El Laco) is another regional mafic endmember, representing a mantle-derived magma composition that is transitional between subduction arc magmatism and intraplate magmatism of the back-arc. The internal crystal architecture revealed by major and trace element zoning of olivine phenocrysts indicates Cerro Overo magma experienced continuous ascent, while Puntas Negras magma experienced a brief period of stalling or storage near the brittle-ductile transition zone (~ 25 km). Aphyric intermediate monogenetic lavas sampled west of (before) the frontal arc display Adakite-like signatures (e.g. high Sr/Y and Sm/Yb) represent small amounts of melt generated with a significant contribution from direct melting of the metabasaltic slab or delaminated lithospheric root at high pressure. These three magmatic regimes sampled at monogenetic centers approximate different end-member compositions being delivered to the lower crust of the Central Andes from which the range of intermediate main arc volcanism in the Altiplano-Puna region is ultimately derived.

Public Abstract

This work presents several small-volume eruptions of lava which provide new constraints on different compositions of mantle-derived magmas delivered to the base of the crust of the Andes. Melting is generated deep in the mantle beneath northern Chile as the tectonic Nazca plate beneath the Pacific Ocean sinks beneath the west coast of the South American continent. As this melt propagates upwards, it forms magmas of a wide range of compositions as separate batches experience variable amounts of crystallization, assimilation of surrounding rock, mixing with other magma(s), density fractionation, and/or prolonged storage in the crust of the Earth. The style and magnitude of volcanic eruptions at the surface is highly dependent on the composition(s) of the magma which are erupted as lava or ash. The thick (> 55 km) and complex lithosphere beneath the Andes of the Altiplano and Puna plateaus of northern Chile exerts a heavy influence on ascending mantle-derived magmas. Volcanism in the region dominantly erupts lava with compositions reflecting lithospheric alteration processes. The monogenetic (single eruption) volcanos presented in this study represent instances where fault systems reaching to the base of the crust have delivered magma to the surface with relatively minimal degrees of alteration since their genesis in the mantle. The origins of these volcanoes include melting of the mantle under the influence of fluids derived from the subducted Nazca plate, decompression melting of the mantle as it upwelled to fill space created at the base of the lithosphere by foundering of dense material, and melts derived directly from the lithospheric material sinking into the mantle.

Keywords

Andes, Cerro Overo, igneous petrology, monogenetic volcanism, olivine, Tilocalar

Pages

xxv, 399 pages

Bibliography

Includes bibliographical references (pages 381-399).

Copyright

Copyright © 2017 Brennan Martin Edelman de Roo van Alderwerelt

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