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EMSC2017 - ROCKS AND MINERALS – Semester 1, 2012 – Lecture 11

EMSC2017 - ROCKS AND MINERALS – Semester 1, 2012 – Lecture 11. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2013 – Lecture 17. Metamorphic Petrology. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2012 – Lecture 11. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2013 – Lecture 17. Metamorphism

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EMSC2017 - ROCKS AND MINERALS – Semester 1, 2012 – Lecture 11

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  1. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2012 – Lecture 11 EMSC2017 - ROCKS AND MINERALS – Semester 1, 2013 – Lecture 17 Metamorphic Petrology

  2. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2012 – Lecture 11 EMSC2017 - ROCKS AND MINERALS – Semester 1, 2013 – Lecture 17 • Metamorphism • Changes in texture and mineralogy in rocks in response to changes in pressure and • temperature (and fluids). • Normally melt free processes, but can be facilitated by fluids (CO2, H2O-rich etc). • P-T conditions vary from diagenesis (compaction and lithification of sediment) up to • conditions approaching the solidus of different rock-types. • Metamorphic rocks often preserve evidence of mineralogical disequilibrium and • non-hydrostatic (directed) stress (shearing, foliation, lineation etc).

  3. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2012 – Lecture 11 EMSC2017 - ROCKS AND MINERALS – Semester 1, 2013 – Lecture 17 Pressure (P) – Temperature (T) conditions of metamorphism c Subduction zones – high P and low T Orogenic belts – intermediate P-T Oceanic ridges or contact aureoles – low P and high T P (kbar)

  4. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2012 – Lecture 11 EMSC2017 - ROCKS AND MINERALS – Semester 1, 2013 – Lecture 17 Protoliths of metamorphic rocks • A major task of experimental petrology is to see through the effects of metamorphism • and determine the precursor rock, i.e. the protolith. • Particular protoliths may metamorphose to distinctive types of metamorphic rocks (although • relationships are not always clear!!) • Ultramafic rocks – peridotites are actually high temperature metamorphic rocks. Low T • versions include rocks containing minerals such as serpentine, talc, brucite, chlorite, • tremolite and magnetite. • Mafic rocks (basalts, gabbroic rocks) – crystalliseactinolite, hornblende, pyroxene, garnet, • epidote, plagioclase, chlorite, pumpellyite • Felsic rocks (granitoids) – quartz, feldspar, biotite, chlorite, amphibole, pumpellyite • Pelitic rocks (clay rich shales) – kyanite, andalusite, sillimanite, codierite, garnet, staurolite, • quartz, mica

  5. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2012 – Lecture 11 EMSC2017 - ROCKS AND MINERALS – Semester 1, 2013 – Lecture 17 Types of metamorphism Ocean ridge metamorphism – rifting environment and tensile cracking of lavas and dykes in newly formed ocean crust allow deep access by seawater, which circulates deeply through hot rock, leading to pervasive hydration of mafic lavas and intrusive (metasomatism). Regional metamorphism – orogenic metamorphism, wide-spread, in roots of orogenic zones. Deformation due to converging lithospheric plates, recrystallisation and mineral reactions due to increases in P-T during crustal thickenning. European Alps, Appalachians of the eastern USA, Caledonides in Scotland and Norway etc. Temperatures in lower crust may lead to calc-alkaline magmatism (granitoid emplacement). Burial metamorphism – in deep sedimentary basins, temperatures up to 200-300°C may be reached. Contact metamorphism – country rock surrounding magmatic intrusions are subjected to heat and circulating fluids, leading to formation of a contact metamorphic aureole. Temperatures decrease with distance from the contact.

  6. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2012 – Lecture 11 EMSC2017 - ROCKS AND MINERALS – Semester 1, 2013 – Lecture 17 Metamorphic facies A suite of mineral assemblages found repeatedly in metamorphic terranes of all ages around the world, with a regular relationship between mineral composition and bulk composition. P-T ranges of major metamorphic facies

  7. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2012 – Lecture 11 EMSC2017 - ROCKS AND MINERALS – Semester 1, 2013 – Lecture 17 Mineral assemblages in metamorphic facies for different rock-types + many, many additional minerals

  8. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2012 – Lecture 11 EMSC2017 - ROCKS AND MINERALS – Semester 1, 2013 – Lecture 17 Pressure-Temperature-time (P-T-t) paths for metamorphic rocks • Burial by crustal thickenning • (i.e. deposition, thrust sheets etc) P (kbar) T (°C) • Decompression (isostacy?), uplift • and exhumation • Temperature at peak metamorphic • pressure • Peak metamorphic temperature • Prograde and retrograde P-T-t paths

  9. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2012 – Lecture 11 EMSC2017 - ROCKS AND MINERALS – Semester 1, 2013 – Lecture 17 Why metamorphic petrology? Metamorphic rocks are a record of orogenesis and mountain building, subduction and exhumation and continental crust formation and evolution. Field relations, fabrics and mineralogy record episodes of deformation and recrystallisation driven by burial, tectonic convergence, crustal thickening during orogenesis, and uplift. Ultra-high pressure metamorphism, recorded in coesite or diamond inclusions in some rocks, indicates burial depths of 100-120 km, even though the maximum crustal thickness is 60-80 km (Central Andes, Tibetan Plateau and Himalayas). How is relatively low density continental material forced so deep through denser ultramafic mantle? What processes cause its rapid exhumation? Our approach – look at major protoliths and their response to metamorphism at particular (relevant) facies.

  10. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2012 – Lecture 11 EMSC2017 - ROCKS AND MINERALS – Semester 1, 2013 – Lecture 17 Metamorphism of ultramafic rocks Peridotites in ophiolites, or other tectonically emplaced mantle fragments, or xenoliths are commonly mildly to completely serpentinised. This is a metasomatic reaction involving H2O-rich (hydrous. Aqueous) fluids reacting with Primary ultramafic mineralogy, particularly olivine 2Mg2SiO4 + 3H2O = Mg3Si2O5(OH)4 + Mg(OH)2 Forsterite serpentine brucite 6.15Mg2SiO4 + 6H2O = 2Mg3Si2O5(OH)4 = 2Mg(OH)4 + 4.3 MgO + 2.15SiO2 Forsterite serpentine brucite removed in solution Serpentine can also contain minor Fe2+ and Fe3+ and these reactions are commonly accompanied by magnetite (Fe3O4) formation.

  11. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2012 – Lecture 11 EMSC2017 - ROCKS AND MINERALS – Semester 1, 2013 – Lecture 17

  12. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2012 – Lecture 11 EMSC2017 - ROCKS AND MINERALS – Semester 1, 2013 – Lecture 17 Serpentinised harzburgite Mesh texture (ol-replacement) Incipient serpentisation in olivine phenocryst + olivine phenocryst pic!! Bastite texture (opx-replacement)

  13. EMSC2017 - ROCKS AND MINERALS – Semester 1, 2012 – Lecture 11 EMSC2017 - ROCKS AND MINERALS – Semester 1, 2013 – Lecture 17 Subduction of serpentinised peridotite Mg3Si2O5(OH)4 = Mg2SiO4 + MgSiO3 + 2H2O antigorite forsterite enstatite serpentine olivine opx Ulmer and Trommsdorf, Science, (1995)

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