2.7 Tectonic Settings for Metamorphic Facies Series

There is a "facies series" button in Figure 2.11 that will show with purple bands the generalized variation of temperature with depth for the three types of regional metamorphism first listed together by Aihiko Miyashiro (1961). The high P/T (blueshist) facies series does not match a geotherm for any crustal thickness, but does match calculated temperatures for subduction zones. The medium P/T (Barrovian) facies series corresponds to the temperatures for the thickened crust of continent-continent collisions. The low P/T (Buchan) facies series temperatures are not produced by crustal thickening with normal crustal parameters, but low P/T conditions can be produced by adding magmatic heat to the crust or by tectonic processes such as thickening extended crust that is already hot. Adding significant magmatic heat on a regional scale is possible in arc settings where magma from subduction processes is abundant.

Medium P/T (Barrovian) metamorphic belts tend to be large in scale and occur in many places around the world. One example can be found in Dutchess County, NY, USA, where the metamorphic rocks appear to have ecaped some of the complexities of collisional metamorphic belts and have been described as "...one of the clearest Barrovian sequences in the world (Donnelly, 1998)." Click on the map to the left to see a larger version with links to outcrop photos of rocks from the metamorphic zones defined by isograds in metamorphic shales. When you have learned more about metamorphic rocks, there is a Metamorphic Activity with images of thin sections of Dutchss Country rocks and a set of advanced questions about interpreting the mineral assemblages in the thin sections.
Dutchess Co. Activity

Metamorphic zones in SW Maine, USA, correspond to the low P/T (Buchan) facies series with andalusite and sillimanite. Evidence for the importance of intrusions in the regional metamorphism of SW Maine can be seen on the map of those metamorphic zones in Figure 14), where the zones wrap around large synmetamorphic plutons. The metamorphic map of Vermont, USA shows a similar magmatic heat boost to the low P/T regional metamorphism in NE Vermont (see VT metamorphic zone map). Other means of producing low P/T metamorphism and especially ultra-high temperature (UHT) metamorphism are described in Kelsey and Hand (2015).

High P/T (blueschist) metamorphic belts are typically found near active or recently active subduction zones such in California, USA, and Japan. These rocks have been carried down a subduction zone, metamorphosed, and then returned to the surface by tectonic processes that are still being debated. In many cases, the premetamorphic relationships among the rocks have been disrupted by the tectonics, producing chaotic mélange zones with erosion-resistant, exotic blocks that have high P/T mineral assemblages (e.g. Jd + Qz) floating in a sea of fine-grained, chlorite-zone rocks that show little evidence of a high P/T history. In a number of locations around the world (e.g the western Alps, China, Norway, see Carswell and Compagnoni, 2003) there are ultra-high P/T (UHP) metamorphic rocks that include metamorphosed sedimentary rocks that were carried by subduction to depths as great as 150 km and returned to the surface.

Because regional metamorphic rocks are produced in plate tectonic collision zones, the details of a regional metamorphic event are complex and not a simple heating followed by exhumation. Continent-continent collisions are messy. Collisions may be oblique, rather than direct. Multiple continental crust blocks may be involved. Island arcs may have been present. Metamorphism may lead to partial melting and magma accumulation. Tectonic unroofing may occur after tectonic burial. Petrologists look for evidence in regional metamorphic rocks to reconstruct their temperature-pressure-time history, which in turn constrains the broader geologic history of the region.

Summary