metamorphic petrology gly 262 - .from turner (1981) metamorphic petrology: mineralogical, field,

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  • Metamorphic Petrology GLY 262 Metamorphism and plate tectonics

  • Regional Metamorphism in a broad sense: metamorphism that affects a large body of rock, and thus covers a great lateral extent

    Three principal types: Orogenic metamorphism Burial metamorphism Ocean-floor metamorphism

  • Orogenic Metamorphism is the type of metamorphism associated with convergent plate margins

    Dynamo-thermal: one or more episodes of orogeny with combined elevated geothermal gradients and deformation (deviatoric stress)

    Foliated rocks are a characteristic product

    Most studies focus on orogenic belts, and the term, regional metamorphism is often used synonymously with orogenic metamorphism

  • Orogenic Metamorphism

  • Orogenic metamorphism (a) = the incipient stages of subduction

    (b) orogenic welt created by compression, crustal

    thickening, thrust stacking of oceanic slices, and addition of magmatic material from below

    Underthrusting in the forearc migrates trenchward, adding successive slabs to the base of the outer welt (tectonic underplating)

    Heat added by rising plutons, magmatically underplated magma, and induced mantle convection

    Temperature increases both downward and toward the axial portion of the welt where plutons concentrated

  • Orogenic Metamorphism

    Uplift and erosion Metamorphism often continues after major

    deformation ceases Metamorphic pattern is simpler than the

    structural one Pattern of increasing metamorphic grade from

    both directions toward the core area

  • The Types of Metamorphism Orogenic Metamorphism

  • Orogenic Metamorphism Most orogenic belts have several episodes of deformation and

    metamorphism, creating a more complex polymetamorphic pattern Continental collision involves interaction of a continental arc with a

    continental mass having a passive margin and an apron of sediments extending from the continental shelf. Such collisions will usually produce even more complex structural, magmatic, and metamorphic patterns

    Although batholiths are usually present in the highest grade areas of regional terranes, the metamorphism isnt considered contact metamorphism because it develops regionally, and the pattern of metamorphic grade does not relate directly to the proximity of the igneous contacts

    Contact metamorphism typically occurs locally within regional terranes

    In many cases intrusive rocks may be plentiful and closely spaced, so that it is difficult or impossible to distinguish regional metamorphism from overlapping contact aureoles. Spear (1993) calls such situations regional contact metamorphism

  • (1) (2)

    (1) Typical clockwise mono-metamorphic P-T path associated with orogenic metamorphism (2) Retrograde portion of clockwise P-T path with the addition of an isobaric heating path Indicative of late-stage, post-orogenic pluton emplacement

  • The stability field of andalusite occurs at pressures less than 0.37 GPa (~ 10 km), while kyanite sillimanite at the sillimanite isograd only above this pressure

    The P-T phase diagram for the system Al2SiO5 showing the stability fields for the three polymorphs andalusite, kyanite, and sillimanite. Also shown is the hydration of Al2SiO5 to pyrophyllite, which limits the occurrence of an Al2SiO5 polymorph at low grades in the presence of excess silica and water. The diagram was calculated using the program TWQ (Berman, 1988, 1990, 1991).

  • Paired Metamorphic Belts of Japan

    The Sanbagawa and Ryoke metamorphic belts of Japan. From Turner (1981) Metamorphic Petrology: Mineralogical, Field, and Tectonic Aspects. McGraw-Hill and Miyashiro (1994) Metamorphic Petrology. Oxford University Press.

    Ryoke or Abukuma style metamorphism is the same as Buchan-style i.e. Low P/T series

  • Paired metamorphic belts These belts are of the same age, suggesting that they developed together The NW belt (inner belt, inward, or away from the trench) is the Ryoke

    (or Abukuma) Belt Buchan-type of regional orogenic metamorphism Dominant meta-pelitic sediments, and isograds up to the sillimanite

    zone have been mapped Although a Low P/T series high-temperatures are reached and granitic

    plutons are common Outer belt, called the Sanbagawa Belt It is of a high-pressure-low-temperature nature

    Only reaches the garnet zone in the pelitic rocks Basic rocks are more common than in the Ryoke belt, however, and in

    these glaucophane is developed (giving way to hornblende at higher grades)

    Rocks are commonly called blueschists

  • To summarize

    Regional metamorphism of the medium P/T series is associated with continent-continent collision

    Regional metamorphism of the Low P/T series coupled with the high P/T series is indicative of island arc settings

    Contact metamorphism is associated with both

  • The scientist does not study nature because it is useful; he studies it because he delights in it, and he delights in it because it is beautiful. If nature were not beautiful, it would not be worth knowing, and if nature were not worth knowing, life would not be worth living

    Henri Poincar

    Slide Number 1Slide Number 2Slide Number 3Slide Number 4Orogenic metamorphismSlide Number 6Slide Number 7Slide Number 8Slide Number 9Slide Number 10Slide Number 11Paired metamorphic beltsSlide Number 13To summarizeSlide Number 15

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