regional metamorphism

Regional metamorphism involves large volumes of rock subjected to heat and intense compression during tectonic events such as subduction and orogeny.

Four mineral assemblage series are recognized as resulting from temperature-pressure environments in regional metamorphism: Buchan, Barrovian, Sanbagawa, and Franciscan.

(image at left – click image to toggle mineralogical metamorphic facies –
(di = diagenetic alteration)
(h = hydrothermal metamorphism)
(c = contact metamorphism [VLPHT])
regional metamorphism:
a = Abukama or Buchan Facies Series [LPHT];
b = Barrovian Facies Series [MPHT];
S = Sanbagawa Facies Series [HPMT];
F = Franciscan Facies Series [HPLT].

The arrows trace conditions at : 'c' = contact aureoles around plutonic intrusions; a = island arcs, oceanic ridges, contact aureoles; b = continental orogenic belts; s = accretion zones; f = subduction zones.)

In Paired Metamorphic Belts, circumpacific belts of oceanic-side belts of high pressure/low temperature metamorphism are found associated with continent-side belts of high pressure/high temperature metamorphism. Two examples of paired metamorphic belts are seen in the Sanbagawa Belt/Ryoke-Abukuma Belt of Japan and in the Franciscan complex/Klamath Mts.-Sierra Nevada roof pendants of California. Other paired metamorphic belts are seen in New Zealand, Indonesia, Washington State, Chile, Jamaica, the Alps, and the northern coast of South America.

Most areas with paired metamorphic belts show evidence of convergent plate subduction associated with low geothermal gradients that generated the oceanic-side high pressure/low temperature metamorphism. Evidence of many such belts has probably been lost because the hydrous minerals of blueschist facies metamorphism are overprinted by facies resulting from normal geothermal gradients. Overprinting occurs when circulating fluids convert blueschist minerals to greenschist and amphibolite facies mineral assemblages.

Continental-side high pressure/high temperature belts form beneath the island arc or continental margin volcanic arc. These regions are, during emplacement of the arc, subject to higher than normal geothermal gradients, producing Buchan and Barrovian Facies Series metamorphic Rocks. These belts of high T/high P metamorphism can be subequently uplifted and exposed at the surface by emplacement of batholiths and isostatic adjustment.

The Japanese paired belts probably remained adjacent to one another because subduction moved farther off the coast after compressional tectonics accreted the island arc and trench complex to Japan at the end of the Mesozoic. The paired California belts are separated because the oceanic ridge was subducted, and then the margin of the Farallon plate converted from compression-subduction to a transform fault margin dominated by strike slip faulting. Isostatic rebound of the highly deformed Franciscan Complex has resulted in its exposure at the surface.

Regional metamorphic changes are classified according to metamorphic facies, which are recognizable terranes or zones. Metamorphic facies display an equilibrium assemblage of key minerals that were in equilibrium under specific temperature/pressure conditions throughout the orogenic terrane.

Metamorphic Facies:
low T - low P : Zeolite Facies
mod to high T - low P : Prehnite-Pumpellyite
low T - high-P : Blueschist Facies
mod to high T - mod P : Greenschist - Amphibolite - Granulite
mod to high T - mod P : Eclogite

--- ◘ Buchan or Abukama Facies Series develops at moderate pressures due to either regional heating by plutonic intrusion at shallow to moderate depths, plate collisions at convergent margins, or crustal thinning. The moderate pressures of Buchan metamorphism are lower than that of the aluminum silicate triple point, producing a critical sequence of aluminum silicates that differ from higher pressure Barrovian rocks in the Amphibolite Facies, where andalusite and cordierite appear:
kaolinite → pyrophyllite → andalusite → sillimanite

--- ◘ Barrovian Facies Series develops at higher pressures than the Buchan series, and are found in Paleozoic and some Precambrian mountain belts. Barrovian pressures are higher than that of the aluminum silicate triple point, producing the mineral series:
kaolinite → pyrophyllite → kyanite → sillimanite

--- ◘ Sanbagawa Facies Series are named for the Sanbagawa Belt, which is part of a complex that was accreted to Japan at higher temperatures than the Franciscan complex (Mesozoic), but lower temperatures than Buchan/Barrovian. The Facies sequence is:
Zeolite → Prehnite-Pumpellyite → Blueschist → Greenschist → Amphibolite Facies

--- ◘ Franciscan Facies Series develop along low geothermal gradients, and are exhibited in the mélange Cretaceous Franciscan Complex of California. [wp] Franciscan metamorphism results in a series of rocks that pass through the Facies sequence
Zeolite → Prehnite-Pumpellyite → Blueschist → Eclogite Facies.

◙ subduction zone magmas ◙

more: Contact Metamorphism Dynamic Metamorphism Regional Metamorphism and Regional Metamorphism Metamorphic Rock Identification