Date of Award

6-2014

Document Type

Open Access

Degree Name

Bachelor of Science

Department

Geology

First Advisor

Holli Frey

Language

English

Keywords

ignimbrite, northern, diablotins, dominica, morne

Abstract

Dominica, a 750 km2 island in the Lesser Antilles volcanic arc, has numerous volcanic centers. One of the more active volcanoes is Morne Diablotins, the proposed source of the Grande Savanne ignimbrite. However, reconnaissance geologic mapping suggests that several other pyroclastic deposits in northern Dominica erupted from Morne Diablotins (Smith and Roobol, 2013): Wesley ignimbrite, Bense ignimbrite, and Pointe Ronde ignimbrite. The Grande Savanne deposit is ~70 m thick and includes a block and ash flow overlain by a baked ash layer, and welded and unwelded tuff. The Wesley ignimbrite is a ~27 m thick, layered deposit with a basal unit of 8-10 cm rounded pumice clasts and an overlying layer containing pumice between 2-5 cm. The Bense ignimbrite is ~4 m of unconsolidated lithified crystal rich ash, abundant in andesite lithics (3-5cm). The Pointe Ronde ignimbrite is a ~5-6 m thick pumice rich unconsolidated deposit containing rounded pumice between 8-10 cm. Samples from each unit were characterized petrographically and geochemically to determine any variations within the eruptive sequence. The primary phase assemblage is plagioclase, orthopyroxene, clinopyroxene, iron-titanium oxides, vesicles, and matrix. They contain no hornblende, which is distinctively different from the central Dominican deposits. All units are andesitic ranging from 57-62 wt% SiO2. Although there is some variation within each deposit, the major and trace element chemistry suggests that the samples from Grancde Savanne, Pointe Ronde, and Wesley may reflect differentiation from one source. Bense is the clear outlier from the other ignimbrites. It is unconsolidated lithified ash, distinct in trace elements and major elements, and therefore may be from a different source. For comparison, the data from the Morne Diablotins ignimbrites were examined with respect to a northern volcano (Quill) and southern volcano (Mt. Pelee) of the Lesser Antilles. Morne Diablotins is similar to volcanoes in the southern and northern part of the arc. Like Mt. Pelee and Quill, the volcanic rocks from Morne Diablotins are andesites, averaging from 57-62-wt % SiO2, and have major phenocryst phases of plagioclase, orthopyroxe, clinopyroxene, and magnetite. However, the samples from northern Dominica seem to be tracking more similarly to Mt. Pelee in trace elements. The samples from Mt. Pelee, central Dominica, and northern Dominica show evolution over time. Based on trace elements, the northern ignimbrites appear to have a significant contribution of both fluids from the subducting slab and terrigenous sediment.

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