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Granites part 2

Granites part 2. Affects of water. Lower melting T. “dry” feldspar phase diagram. Top: solid solution Bottom: exsolution along solvus Cool comp. A First solid at: 1090°C Last liquid at: 1000°C Don’t reach eutectic. “dry” feldspar phase diagram. Solid of d cools At 780°C hit solvus

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Granites part 2

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  1. Granites part 2

  2. Affects of water • Lower melting T

  3. “dry” feldspar phase diagram • Top: solid solution • Bottom: exsolution along solvus • Cool comp. A • First solid at: • 1090°C • Last liquid at: • 1000°C • Don’t reach eutectic

  4. “dry” feldspar phase diagram • Solid of d cools • At 780°C hit solvus • Exsolution into 2 phases (perthite) • Orthoclase host with lamellae of albite

  5. “dry” feldspar phase diagram • Cool comp I • First solid • At j, 1002 °C • Last liquid • At k, 970°C • Don’t reach eutectic • Cool, hit solvus • Exsolution • Albite host with othoclase lamellae • Called hypersolvus granite if perthite present

  6. What happens with more water?Solidus intersects solvus

  7. Hypersolvus subsolvus

  8. Subsolvus: two distinct feldspars form

  9. Hypersolvus with perthite • Subsolvus: no perthite, 2 discrete feldspars

  10. What are hydrous mafic minerals? • What are anhydrous mafic minerals? • Which granite will likely have more hydrous mafic minerals, hypersolvus or subsolvus?

  11. Ascent of hydrous/anhydrous melts • Think about what water does to shape of solidus • Any difference for which reach the surface?

  12. Generating cont. arc plutons • Why do we usually see andesites, not rhyolites? • Plutons are more evolved. Why? • Consider: • Viscosity of more felsic melts • How far can hydrous melts rise in crust?

  13. Crust only Crust w/ minor mantle Both, more crust Both, unknown proportion Both, more mantle Mantle w/ minor crust Mantle only Don’t know 1% 21% 41% 15% 7% 1% 0% 13% Where do granites come from?

  14. How much recycling? • How much new melt? • Debatable

  15. Model for generating many granites • Continental arc model • Collisional model

  16. Generating cont. arc plutons • Melt mantle wedge with fluids from slab • This makes basalt, only erupted where crust is thin • If crust thick, basalt ponds at base of crust • Called underplating

  17. Generating cont. arc plutons • Fractionation of stalled basalt • Leads to calc-alkaline trend • Plutons are remelts of underplate or melt of crust • Rising felsic magmas differentiate and/or assimilate crust

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