Geochemical and Petrographic analysis of rhyolite tuff from the Mattapan Volcanic Complex, Braintree Massachusetts

Presenter(s)

sophia Oliver

Abstract or Description

This study was part of a year-long investigation in our Petrology course that studied the geochemical and mineralogy data of a rock sample from the Late Proterozoic Mattapan Volcanic Complex in eastern Massachusetts. To determine the mineralogy and texture, the rock was prepared for petrographic analysis by being cut and ground to create a thin section. A separate portion of the rock sample was crushed and pulverized into a fine powder, some to be melted and molded into a fused glass bead. The remainder of the powder was combined with a binding agent and compacted into a pressed pellet using a 25-ton press. A Rigaku ZX-3 X-ray fluorescence instrument was used to analyze the major oxides and selected trace elements.

Petrographic analysis indicates the sample is a volcanic tuff consisting primarily of quartz and feldspar phenocrysts set in a fine-grained matrix. Geochemical results show that this sample plots within the rhyolite field on a Le Bas classification diagram of SiO2 vs Total Alkalis. Tectonic discrimination diagrams suggest the rhyolite tuff was produced in a within-plate tectonic setting likely associated with a continental rifting. The geochemical signature indicates the sample is peraluminous and likely derived from melting of a pre-existing sedimentary rock.

Mentor

Dr Krol