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Generating magmas in the crust

Generating magmas in the crust. Generalizations of granite. Appear in continental arcs, not oceanic Need preexisting cont. crust to generate Appear where crust thickened by orogeny Need thermal disturbance to make them Can’t make large volumes of felsic magma from differentiating basalt.

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Generating magmas in the crust

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  1. Generating magmas in the crust

  2. Generalizations of granite • Appear in continental arcs, not oceanic • Need preexisting cont. crust to generate • Appear where crust thickened by orogeny • Need thermal disturbance to make them • Can’t make large volumes of felsic magma from differentiating basalt

  3. Where granites appear • Continental arcs above subduction zone • Melting with high T from underplating basalt • Continental rifts • Mantle plume provides heat • Thickened crust • Geothermal gradient raised in collision zone

  4. Partial melting • Called anatexis • Can by anhydrous • Hard, need high T • Won’t generate much melt • So many hydrous minerals around • Mostly hydrous • Water in minerals or in pore spaces

  5. Partial melting of various rocks • Shale • Muscovite-rich at 850°C • Biotite-rich over wide range • Tonalite • 875-900°C with hornblende breakdown

  6. Melting a basalt • Smallest % melt are granite • More melt, more tonalite • Solid line: dry melting • Dashed line: wet melting

  7. How to generate granitic melt • Slow heating drive off water in pore spaces • 680°C muscovite dehydrates to induce melt • Little muscovite, only 10% melting • 760°C biotite dehydrates • Up to 60% melt • Higher, amphibole dehydrates

  8. Biotite + quartz = hydrous felsic melt + garnet

  9. Melt composition • High in Si, K, Na, water • Low in Mg, Fe, Ca relative to source • Can be peraluminous, metaluminous, peralkaline

  10. Melt segregation: how to get it to collect and rise? • Volume of melt: muscovite breakdown is modest. Biotite/hornblende breakdown generates large volumes • Dehydration melting will make more melt • Deformation can mobilize melt

  11. Assimilation • Remember granites have long path through crust, lots of chance for assimilation • Which rock types might noticeably alter chemistry of magma? • Limestone • Granites are carbonate poor • Shale • Clay minerals are Al-rich

  12. Alphabet granites • Lachlan fold belt • Paleozoic granitic rocks • Eastern Australia

  13. ID 2 magma sources • S-type granites from sed source of clays • Very Al-rich • Sillimanite • Garnet • Muscovite

  14. I-type granites • Magma source: igneous rocks • More Na, Ca • Amphibole • Biotite • Not universal

  15. Continental arc batholiths • Batholith: nested plutons • Plumbing of arc volcanoes • Tonalite and granodiorite mainly • From granite to gabbro • Sierra Nevada:

  16. Sierra Nevada • Emplaced Triassic to late Cretaceous (210-85 Ma) • Country rock variably deformed • Lots of biotite, hornblende • I-type granites • Zoned intrusions, how can this happen?

  17. Peru batholith

  18. Peru Batholith • AFM: Calc-alkaline Harker diagrams: fractional crystallization of what? • Ca: • Mg: • Fe: • Ti: • Na:

  19. How to make batholith • Mantle melts pond at base of crust • Heats lower crust, makes tonalite • Tonalite differentiates forming granodiorite and granite

  20. How do you explain this trend?

  21. Granites in continent collision zones • Collision started 55 Ma between India and Tibet • 1000-1500 km crustal shortening • Crust thickened up to 80 km • No volcanics here!

  22. Granites in continent collision zones • 20 Ma magmatism • Granitic plutons along crest of mountains • Min: qtz, K-spar, plag, biotite, muscovite, garnet, tourmaline • Magma source? • S-type magmas

  23. Overthickened crust can disturb geotherm and allow melting of hydrated seds. • Chem of exposed sediments closely matches chem of granites Figure 18-6. A simple modification of Figure 16-17 showing the effect of subducting a slab of continental crust, which causes the dip of the subducted plate to shallow as subduction ceases and the isotherms begin to “relax” (return to a steady-state value). Thickened crust, whether created by underthrusting (as shown) or by folding or flow, leads to sialic crust at depths and temperatures sufficient to cause partial melting. Winter (2001) An Introduction to Igneous and Metamorphic Petrology. Prentice Hall.

  24. How to generate melt in collision zones • Fluids released by heated footwall rocks • Induce water-saturated melting • Or shear heating if not water-saturated • Decompression of hot, thick crust as Himalaya unroofed

  25. Other granites • A-type granite • Created in anorogenic areas • Alkali-rich w/ alkali feldspar • anhydrous • Noncompressional environ. • Extensional ocean islands (Azores, Canary) • Continental rifts & extension (White Mnts., St. Francois Mnts)

  26. Characteristics of A-types • High alkali content • High halogens (F, Cl) • High incompatibles (U, Th) • High magma T • > 900°C (not 800°C) • Anhydrous • Hypersolvus perthite common • Anhydrous mafic minerals common

  27. Source of A-types • No single process • Mixing of mantle and crustal components • Mantle: plume? • Mafic magmas melt crust already melted once? • Melting of crust with low biotite, amphibole?

  28. Many A-types seem to be post-orogenic • Many are Proterozoic

  29. Tectonic association of granites • I and S-types in continental arcs • S-type in cont-cont collision • A-type are anorogenic and Proterozoic • Archean rocks are tonalites • Characterize by trace elements? • Useful but not perfect

  30. Generating all types on granite in one locality through tectonic changes

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