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Metamorphic facies

Metamorphic facies. Chapter 14 and 18. Metamorphic terranes. Geologic setting where metamorphism occurs Tracts of exposed, distinct rock from adjacent rock. Metamorphic terranes. Ocean-ridge metamorphism Seen in ophiolites Lots of metasomatism Hydration of minerals, adding Na.

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Metamorphic facies

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  1. Metamorphic facies Chapter 14 and 18

  2. Metamorphic terranes • Geologic setting where metamorphism occurs • Tracts of exposed, distinct rock from adjacent rock

  3. Metamorphic terranes • Ocean-ridge metamorphism • Seen in ophiolites • Lots of metasomatism • Hydration of minerals, adding Na

  4. Metamorphic terranes • Regional metamorphism • Most common in cont. crust • Deformation with contractional stress, P and T up • Alps, Appalachians, Caledonides

  5. Metamorphic terranes • Burial metamorphism • Thick sed basins with T up leading to recrystallization

  6. Metamorphic terranes • Contact metamorphism • Intrusions bake surrounding coutnry rock, fluids too • Skarns form here

  7. Metamorphic terranes • Ductile shear zones • Brittle at surface, ductile at depth • San Andreas fault, Brevard fault

  8. Metamorphic grade • Equilibration T during metamorphism, P not restricted • Mineralogy establishes grade • Low grade or high grade with most hydrous minerals? • For met. grade, consider rocks of similar bulk comp. • Typically low, medium, high grade

  9. Metamorphic zones • Zone defined by distinctive fabric or mineral assemblage with similar protolith • Sequence of zones shows gradient of metamorphism • If correspond to a thermal gradient, basically same as met. Grade • Zones based on fabric are different

  10. Metamorphic zones • In skarns: • Forsterite zone, monticellite zone, vesuvianite zone, garnet zone • Each mineral is an “index mineral” marking a new zone

  11. Metamorphic zones • Pelites in Scotland • Zones ID’ed by first appearance of index mineral • Chlorite zone, biotite, garnet, staurolite, kyanite, sillimanite (ascending zones) • Mapped by isograds: a mappable line marking 1st appearancce of mineral

  12. Metamorphic facies • Comparison of suites of mineral assemblages • Looking at changes of met. conditions in diverse rock types • Compare met. conditions of pelites with mafic rocks • Facies: used to contrast P-T conditions of metamorphism

  13. Metamorphic facies • See Table 14.2 • Greenschist, amphibolite, granulite, blueschist facies • Minerals expected in different rock compositions.

  14. Metamorphic facies • High P/T series: subduction zones • Medium P/T series: regional met. • Low P/T series: contact met.

  15. Key terms • Metamorphic zone: linked to minerals • Metamorphic facies: linked to P-T conditions and composition independent

  16. P-T-t paths for common tectonic scenarios

  17. P-T-t paths for common tectonic scenarios • Crustal thickening • Shallow magmatism • Underplating of basalt, magma generation at base of crust

  18. P-T-t paths for common tectonic scenarios • Crustal thickening • Shallow magmatism • Underplating of basalt, magma generation at base of crust

  19. P-T-t paths for common tectonic scenarios • Crustal thickening • Shallow magmatism • Underplating of basalt, magma generation at base of crust

  20. Metamorphic field gradient • Series of P-T-t paths in one area • Link maximum T from each loop, form metamorhpic field gradient

  21. Metamorphic field gradients

  22. P-T-t path exercises

  23. Mafic metamorphic rocks • Protolith: basalts • Start with few minerals, not much variety after met. • Plag. & amphiboles common • Greenscist facies: epidote, chlorite, actinolite, plag, quartz, calcite, titanite common

  24. Mafic metamorphic rocks • Amphibolite facies: • epidote and chlorite down • Amphibole and garnet up

  25. Mafic metamorphic rocks • Granulite facies • Hornblende reacts to form pyroxene • At high P: • Blueschist facies with glaucophane • Eclogite facies: omphacite (px) and garnet, no plag

  26. Expected minerals in mafic rocks for different met. facies

  27. Mineral changes in medium P/T series, mafic rocks

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