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 isn’t 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 Poincarè