DM

Arc Magmatism and Subduction Zone Volcanism Notes

Arc Magmatism Overview

  • Types of Igneous Rocks:

    • Basalt

    • Andesite

    • Dacite

    • Rhyolite

Island Arc Magmatism

  • Typical characteristics:

    • Arcuate volcanic island chains occur along subduction zones.

    • Composition is more diverse and silicic than other volcanic provinces.

    • Basalt is generally subordinate.

    • More explosive eruptions.

    • Strato-volcanoes are the most common volcanic landform.

Examples of Island Arcs

  • Aleutians

  • Marianas

  • Tongan

  • Japan

Structure of an Island Arc

  • Typical cross-section shows complex tectonic settings.

  • High proportion of basaltic andesite and andesite found in subduction zones.

  • Common volcanic rock types classified by their relative proportions, e.g.:

    • Talasea, Papua: 9% B (Basalt), 23% B-A (Basaltic Andesite), 55% A (Andesite), 9% D (Dacite), 4% R (Rhyolite).

Subduction Zones

  • Types of Subduction Zones:

    • Ocean-ocean → Island Arc (IA)

    • Ocean-continent → Continental Arc (Continental Margin, ACM)

Examples of Continental Arcs

  • Alaska

  • Andes

  • Cascades

  • Mexican Volcanics

Subduction Products

  • Characteristic igneous associations include:

    • Basalt, Andesite, Dacite, Rhyolite.

  • These products also display distinctive patterns of metamorphism and orogeny contributing to mountain belts.

Major Elements and Magma Series

  • Three primary magma series:

    • Tholeiitic (MORB, OIT, Subduction)

    • Alkaline (OIA, Subduction)

    • Calc-Alkaline

Origin of Subduction Volcanics

  • Melting related to subduction is confined to the mantle above the slab.

  • Involves andesite, dacite, and rhyolite through processes of:

    • Differentiation

    • Crystal Fractionation

    • Assimilation

Key Components in Magma Source

  1. Altered oceanic crust of the subducted slab

  2. Mantle wedge between the slab and arc crust

    • Presence of water is significant to arc magmatism.

Mantle Wedge Dynamics

  • Temperature and depth influence melting and mineralogy in the mantle wedge.

  • Dehydration leads to the production of hydrous minerals.

Changes in Mineralogy

  • Transition from Blueschist to Eclogite involves dehydration and changes in rock composition.

    • Blueschist: Glaucophane + Epidote.

    • Eclogite: Omphacite + Garnet.

Mantle Wedge Melting

  • Melting begins at ~100-120 km depth.

    • Produces basalts, which evolve to andesite or more silicic compositions through fractional crystallization and assimilation.

Summary

  • Arc magmatism involves complex interactions between slab fluids and mantle dynamics.

  • Subduction-induced melting produces hydrous basalts, followed by evolution to more silicic magma types (andesite, dacite, rhyolite).

  • Comparison of arc volcanics with mid-ocean ridge basalts (MORBs) shows significant differences in trace elements, influenced by slab-derived fluids.