Geol 2312 Igneous and Metamorphic Petrology

1. Geol 2312 Igneous and Metamorphic Petrology Lecture 16 Continental Arc Magmatism March 4, 2009

2. Continental Arcs vs Island Arcs Affects of Thick Differentiated Continental Crust • Thick sialic crust contrasts greatly with mantle-derived partial melts may produce more pronounced effects of contamination • Low density of crust may retard ascent causing stagnation of magmas and more potential for differentiation • Low melting point of crust allows for partial melting and crustally-derived melts • Subcontinental lithosphere may be more compositionally diverse that suboceanic lithosphere, especially if crust is old

3. Types of Continental Arcs Destructive more common where Continental crust is older e.g. Andean Margin Constructive more common where Continental crust is younger e.g. Pacific NW

4. Andean Continental Arc • Gaps in volcanic activity • shallow subduction • overthickened slab

5. Andean Volcanic compositionsDistribution of Rock Types Melting of Lower Crust generates Felsic Magmas Lower Crust traps Mafic Magmas

6. Andean Volcanic compositionsMajor Elements • Northern Volcanic Zone • more andesitic to felsic • K-rich comps to east Alkaline Rocks • Central Volcanic Zone • more andesitic to felsic • basalts rare • more staging beneath • Precambrian crust Island Arcs • Southern Volcanic Zone • broad range of comps • K-rich comps to east • shallower subduction • angle • Young continental • crust especially • to south

7. Andean Volcanic compositionsTrace Elements CVZ – Assimilation of Precambrian crust and SCLM SVZ - Shallower subduction angle  melting of Gt-free mantle

8. Andean Volcanic compositionsTrace Elements CVZ – Assimilation of Precambrian crust and SCLM Nb-Ta depletion similar to island arc dehydration of subducted slab

9. Andean Volcanic compositionsIsotopic Compositions Crustal Contamination

10. Constructive Continental ArcPacific NW Juan de Fuca Plate – Young, hot, bouyant; dehydrates quickly upon subduction Columbia Embayment - area of young crust and arc construction by rollback or trench jumping

11. Cascade Magma Types over Time Bimodal Volcanism

12. General Model for Continental Arc Magmatism