DM

Arc Magmatism

  • Types of Magma in Arcs:

    • Basalt
    • Andesite
    • Dacite
    • Rhyolite

  • Types of Arcs:

    • Island Arcs:
    • Found along subduction zones
    • Characterized by a diverse and silicic composition
    • Generally more explosive volcanic activity
    • Strato-volcanoes are the most common landform
    • Continental Arcs:
    • Examples include Alaska, Andes, Cascades, and Mexican Volcanics

Structure of an Island Arc

  • Schematic Diagram:
    • Shows typical cross-section of an island arc
  • Volcanic Rock Composition:
    • High proportion of basaltic andesite and andesite
    • Most andesites occur in subduction zones

Example Localities and Composition Proportions

  • Talasea, Papua:
    • B: 9%, B-A: 23%, A: 55%, D: 9%, R: 4%
  • Little Sitkin, Aleutians:
    • B: 0%, B-A: 78%, A: 4%, D: 18%, R: 0%
  • Mt. Misery, Antilles:
    • B: 17%, B-A: 22%, A: 49%, D: 12%, R: 0%

Subduction and Volcanism

  • Two types of arcs related to subduction:
    • Ocean-Ocean → Island Arc (IA)
    • Ocean-Continent → Continental Arc (CA)

Magma Series

  • Key Types:
    • Tholeiitic (related to mid-ocean ridge and subduction)
    • Alkaline (common in ocean-island arcs)
    • Calc-Alkaline (typical in subduction zones)

Origin of Subduction Volcanics

  • Melting Mechanism:
    • Melting is largely confined to the mantle above the subducted slab
    • Results in the generation of abundant basalts, and andesites, dacites, and rhyolites

Factors Influencing Melting

  1. Crystal Fractionation
  2. Assimilation

Magma Source Components

  1. Crustal Element:
    • Altered oceanic crust and forearc sediments
  2. Mantle Wedge:
    • Interaction of fluids and subducted materials influences arc magmatism

Interaction of Slab and Mantle Wedge

  • Dehydration:
    • H2O released from the subducting slab plays a critical role in potentials for melting in the mantle wedge

Geological Indicators

  • Comparison of incompatible elements in mantle-derived basalts (MORBs) and subduction-derived volcanics indicates significant water enrichment in arc magmas

P-T-t Paths in Subduction Zones

  • Temperature and Pressure Paths:
    • Hotter conditions are above the slab, influencing mineralogical changes from blueschist to eclogite

Example Reactions

  • Transition from blueschist minerals (glaucophane + epidote) to eclogite minerals (omphacite + garnet)

Importance of Water in Mantle Wedge

  • Water released from the subducting slab hydrates the mantle, creating mafic magmas

Summary Points

  • Volcanic Activity Related to Subduction:
    • Calc-Alkaline volcanism dominated by high water content and interaction between fluids
    • Melting at depths of around 100 to 120 km significantly contributes to basaltic magma generation
  • Evolution of Magma:
    • Basalt evolves through fractional crystallization to produce andesites, dacites, and rhyolites
  • Cross-Contribution:
    • Subduction zones are a unique melting environment, differentiating them from plume melting typically observed at mid-ocean ridges