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Tectonic petrology - robust tests of paleotectonic environments

Tectonic petrology - robust tests of paleotectonic environments. GEOS408/508- Lec 7. Magmatism at ocean ridges. Magmatism @ spreading ridges MORB. Most voluminous, decompression melting of peridotite 5-7 km oceanic crust; makes the harzburgite-gabbro-basalt “trinity”

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Tectonic petrology - robust tests of paleotectonic environments

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  1. Tectonic petrology - robust tests of paleotectonic environments GEOS408/508- Lec 7

  2. Magmatism at ocean ridges

  3. Magmatism @ spreading ridges MORB • Most voluminous, decompression melting of peridotite • 5-7 km oceanic crust; makes the harzburgite-gabbro-basalt “trinity” • Tholeiites, low pressure fractionation Fe enrichment, no silica enrichment • Depleted source - trace and isotopes • End ~ 10, O and sr isotopes are variable • Slow spreading ridges are different • Most gets subducted, some survives as ophiolites

  4. Mantle plumes, LIPS and flood basalts

  5. Magmas in continental extension

  6. Continental extension • Same as MORB, small degree melts • Alkalic, and tholeiitic • Bimodal magmatism - rhyolites are lower crustal melts - reflect the isotopic character of the host crust • Different viscosity - no mixing, lead to “Daly gap”

  7. Subduction zone magmas + much more…..

  8. HFSE anomalies

  9. Cordilleran arcs • Calc-alkaline tonalites, granodiorites • Water -rich • Higher silica than island arcs • Crustal recycling significant • MASH zones • Systematic geographic distribution of isotopes • Depletion in HFSE

  10. Magmatism in collisional orogens

  11. Collisional magmatism • Occurs during “hard collision” of continents (Himalayan); • Accompanied by high grade metamorphism and migmatization; • Mimimum granitic melts mostly through dehydration melting of muscovite and biotite; • Caused by radiogenic heating (self ignition) or underplating from the mantle; also possibly by decompression during uplift • This process makes true granites

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