Arc Magmatism

by

Andy Sutton, Arlëne Hunter, Dee Edwards and Dave Williams
The Open University

This module, aimed at second and third year undergraduates, is designed to cover the main aspects of arc magmatism at mantle wedge and intracrustal levels. Thus it aims to give a complete picture of the processes involved in magma petrogenesis in island and continental arc settings. The aims of the module are:

to provide a strong general background on arc magmatism in relation to destructive plate margins - their evolution and their importance in the generation of crust;
to introduce the concept of evolving magmatic systems - how the petrology of basalts through to rhyolites are related and can be used to produce a petrogenetic model for a system using mineralogical and geochemical criteria;
to introduce current theories on arc magmatism at mantle wedge and intracrustal levels regarding for example magma generation, element mobility and fluxing.

The module comprises seven sections accessed via this main menu

Animated sequences are used extensively to explain concepts

Description of the module

The module occupies about 20Mb and comprises seven main sections. In addition there is a 'How to use this module' section and a final assessment test. The main sections are as follows:

1. Introduction

This gives a general background to the tectonic settings of arc magmatism and explains the importance of arcs at subduction zones. The different types of plate margin are described, discussing sites of preservable crust formation and sites of recycling into the mantle. Further pages discuss island arcs v active continental margins - their global distribution and similarities and differences.

The introduction covers many features of arc magmatism

2. Subduction processes

This section looks at the tectonic settings and geophysical structure of arcs, the components of the crust and mantle present beneath subduction zones and the nature of partial melting processes - for example thermal problems and the need for water. Peridotite melting phase diagrams are used as examples.

3. Petrography

This examines the petrology and mineralogy of common arc rocks in relation to their crystallization histories. The section includes thin section images of rocks.

Examples include detailed petrographic studies

4. Classification

This describes the common methods of classification - geochemical and mineralogical - by which rocks are divided into various series.

5. Intracrustal processes

This looks at some of the factors that act on mantle derived magmas to produce the spectrum of rocks observed in arcs; fractional crystallization, contamination and magma mixing are discussed. The section raises the question of how to recognise these processes from the geochemical composition/mineralogy of rocks.

6. Subcrustal processes

This section discusses partial melting and the varying fluxes from different components that help produce magmas of different composition within the mantle.

7. Volcanism

This gives a brief look at some of the styles of activity, volcanic landforms and deposits associated with arcs.

The last section includes information on styles of volcanic activity associated with arcs

Assessment questions

A final test consisting of ten questions examines how much the user has understood and absorbed from the module.

Running the software

The different sections of the module are accessed via a main menu and navigation around the module is by means of buttons along the base of the screen. A glossary can be accessed by clicking on words in the text or from a pull-down menu at the top of the screen. A listing of further references is available for each section.

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