IV. Convergent Margin Basalts (IABs and CABs) - subduction zones
A. Association with subduction zones - subduction of oceanic lithosphere
How does magmatism occur? Melting in mantle wedge because of dehydration reactions in subducting slab. Volcanic front overlies slab at 100 km depth. Steeper slab, closer front is to trench. Shallow slab, volcanic front further from trench (or not at all, like central Andes)
Why? Amphibole breakdown at 80-100 km, then phlogopite and serpentine at greater depths, but less release of H2O.
Why not melting in the slab? Slab too cold. Fig 9.5 of book
B. Rock associations - Andes, andesites
1. Basalt - Andesite - Rhyolite association
Diverse suite of rocks with arc basalts being a major component (andesites may be up to 75% of arc assemblage - more later)
Calcalkaline trend most common. Read in text about different series.
2. Volcanism is explosive - Ring of Fire. Pyroclastic material & lava. Plinian eruptions
Why would this be true?
3. Young island arcs: basalt is dominant
- More mature island arcs: andesite
- Continental arcs: andesite to rhyolite
Why? Thicker the crust, more chance for stalling, fractionation, assimilation, melting
Transition - Aleutian to Alaskan peninsula, Kurile to Kamchatka
Cascades versus Aleutians: continental versus island arc.