Igneous Rocks and Plate Tectonics

1. Igneous Rocks and Plate Tectonics Igneous Petrotectonic Assemblages

2. Igneous Rocks and Tectonics • Igneous rock formation largely tied to tectonic activity • 3 main tectonic settings for making igneous rocks:

3. Igneous Rocks at Divergent Margins: Mid-Ocean Ridges • Magma origin: • Partial melting of ultramafic, fertile mantle rock (lherzolite) • Decompression melting due to mantle convection • Magmatic processes: • Mantle melting produces dominantly “primitive,” mafic magma (basalt); magma may be modified slightly by fractional crystallization • Rocks & structures • “Ophiolite suite” • Cool and crystallize in magma chamber • Erupted onto sea floor as effusive lava flows or pillows

4. Igneous Rocks at Divergent Margins: Mid-Ocean Ridges

5. Igneous Rocks at Divergent Margins: Continental Rifts • Magma origin: • Partial melting of ultramafic mantle rocks • Decompression melting due to rise of material in a hot spot (?) • Magmatic processes: • Initial mantle melting produces “primitive,” mafic magma (basalt) • Magma accumulates in chambers • Fractional crystallization, contamination produce intermediate (andesite) and feslic (rhyolite) magmas • Rocks & structures • Highly varied • Dominated by bimodal volcanic activity • Early: basalt lava flows, shield volcanoes, cinder cones • Later: rhyolite flows and calderas (ignimbrites) • Also mafic through felsic plutons

6. Ocean-Continent Plate convergence Continental arcs Continent-Continent Plate convergence Igneous Rocks at Convergent Margins • Ocean-Ocean Plate convergence • Ocean Island arcs “Recycled” magmas!

7. Igneous Rocks at Convergent Margins: Ocean Island Arcs • Magma origin: • Water released from subducting slab due to T-P increase • Flux melting (partial) of overlying ultramafic mantle rocks • Magmatic processes: • Initial melting produces mafic magma (basalt) • Basalt magma rises into crust; fractional crystallization, melting and assimilation of crust modifies composition (intermediate – andesite) • Rocks & structures • Dominantly andesite stratovolcanoes (“Ring of Fire”) • Mafic to intermediate batholiths (Gabbro, diorite)

8. Igneous Rocks at Convergent Margins: Continental Arcs • Magma origin: • Water released from subducting slab due to T-P increase • Flux melting (partial) of overlying ultramafic mantle rocks • Magmatic processes: • Initial melting produces mafic magma (basalt) • Basalt magma rises into crust; fractional crystallization, melting and assimilation of crust modifies composition (intermediate to felsic) • Rocks & structures • Dominantly andesite to rhyolite stratovolcanoes • Rhyolite calderas and ignimbrites (tuffs) • Vast intermediate to felsic batholiths (granodiorite, granite)

9. Magma origin: Crustal thickening due to thrust faulting during collision Causes partial meting of crustal rocks (anatexis) Magmatic processes: Magma composition depends largely on the source rocks that were melted Dominantly felsic composition Igneous Rocks at Convergent Margins: Continental Collisions • Rocks & structures • Extensive batholiths and plutons of mostly felsic composition (granite) • Compression and thick crust prevent eruption – volcanoes rare

10. Igneous Rocks at Hot Spots • Magma origin: • Partial melting of ultramafic mantle rocks • Decompression melting due to rise of hot material • Magmatic processes: • Initial mantle melting produces “primitive,” mafic magma (basalt) • Heat from mafic magma can melt overlying crustal rocks, producing felsic magmas • Rocks & structures • Initial flood basalts erupted from fissures (associated dikes) • Oceanic: basalt lava flows, shield volcanoes, cinder cones • Continental: bimodal (basalt-rhyolite) volcanic activity; rhyolite calderas and ignimbrites • Also mafic through felsic plutons