Regional Metamorphism

• Buchan Facies Series forms under pressures, which, in the middle grades of metamorphism, are lower than that of the aluminum silicate triple point. Consequently, the critical sequence of aluminum silicates is kaolinite -->pyrophyllite -->andalusite --> sillimanite.
• Barrovian Facies Series, in contrast, develops where pressures in the middle grades of metamorphism are higher than that of the aluminum silicate triple point. The resulting aluminum silicate mineral sequence is kaolinite -->pyrophyllite -->kyanite --> sillimanite.

The low-grade assemblages are virtually identical to those of the Barrovian Facies Series described below. Similarly, Greenschist Facies rocks are mineralogically similar to their equivalents in Barrovian Facies Series. It is in the Amphibolite Facies, where andalusite and cordierite appear, that the Buchan Facies Series is distinguished from the higher-pressure Barrovian rocks.

The various phase assemblages developed in each metamorphic zone of the Buchan Facies Series indicate various reactions. In pelitic (shale) rocks, at the lowest grade, the Zeolite Facies contains assemblages such as

kaolinite-illite-smectite-chlorite-quartz-analcite-K feldspar

At slightly higher-grade conditions, where assemblages of the Zeolite Facies are replaced by those of the Prehnite-Pumpellyite Facies, some minerals, such as K feldspar, are absent from many rocks, and new phases appear, such as white mica, prehnite, pumpellyite and albite. Smectites and K feldspar are among the first minerals that may disappear from aluminous rocks. Kaolinite also commonly disappears from pelitic assemblages during development of Prehnite-Pumpellyite Facies assemblages.

Typical assemblages in Greenschist Facies pelitic rocks include

white mica-chlorite-quartz-albite-magnetite-epidote-pyrophyllite-biotite-garnet-ilmenite

The Greenschist-Amphibolite Facies boundary is a broad zone. The disappearance of albite marks the maximum upper limit of the Greenschist Facies. Both albite and pyrophyllite are absent from Amphibolite Facies rocks, whereas cordierite and the aluminum silicates andalusite (at lower grades) and sillimanite (at higher grades) characterize aluminous bulk compositions. Additional phases that may occur in pelitic rocks include, but are not restricted to, chloritoid, alkali feldspar, tourmaline, apatite, and sphene. Reactions distinctive of Buchan Facies Series are those defining the appearance of andalusite and cordierite, which combined with the disappearance of albite, mark the transition to the Amphibolite Facies.

Example: Buchan Metamorphism, Northern New England, U.S.A.

Differences and Similarities Between Contact and Buchan Facies Series

The Paleozoic orogenic belts are clearly associated with convergent plate margins. Both Barrovian and Buchan Facies series develop at such margins. In convergent zones, regional heating due to the rise of plutons into the overlying plate (the plate above the subduction zone) is the general cause of metamorphism, but migrating fluids may also transport heat.

The zones of metamorphism in the Scottish Highlands originally described by Barrow (1893) include six distinct mineral assemblages that occur in the rock types listed below:

Chlorite Zone (slates, phyllites, and schists)

quartz-albite-white mica-chlorite-microcline ± calcite

Biotite Zone (phyllites and schists)

quartz-albite-white mica-chlorite-biotite ± microcline ±calcite ± epidote

Almandine (Garnet) Zone (phyllites and schists)

quartz-albite-white mica-biotite-garnet ± chlorite

Staurolite Zone (schists)

quartz-oligoclase-white mica-biotite-garnet-staurolite

Kyanite Zone (schists)

quartz-oligoclase-white mica-biotite-garnet-kyanite ±staurolite

Sillimanite Zone (schists and gneisses)

quartz-oligoclase-biotite-sillimanite ± kyanite ± K-feldspar ± white mica

At the lowest grade, in the Zeolite Facies, which forms under conditions just above those of diagenesis, assemblages are characterized by clay minerals. Assemblages may include

kaolinite-illite-smectite-chlorite-quartz-analcite

At slightly higher-grade conditions, assemblages of the Zeolite Facies are replaced by those of the Prehnite-Pumpellyite Facies. New phases appear, including albite, white mica and stilpnomelane. As was the case in Buchan Facies Series, K feldspar and smectites are among the first minerals to disappear from aluminous rocks. Kaolinite also is commonly absent from Prehnite-Pumpellyite Facies rocks.

As the P-T conditions increase, Greenschist Facies assemblages with new minerals form. Typical assemblages in pelitic rocks include

white mica-chlorite-chloritoid-quartz-albite-magnetite-biotite-epidote-garnet-pyrophyllite

As is the case in the Buchan Facies Series, the Greenschist-Amphibolite Facies boundary is a broad zone. The disappearance of albite marks the maximum upper limit of the Greenschist Facies. Thus, albite, like pyrophyllite, is absent from Amphibolite Facies rocks. Staurolite, rather than chloritoid, occurs in the lower part of the Amphibolite Facies and the aluminum silicates kyanite (at lower grades) and sillimanite (at higher grades) characterize aluminous bulk compositions. Typical assemblages include

white mica-chlorite-biotite-quartz-plagioclase-garnet-magnetite-staurolite-ilmenite-kyanite-sillimanite

The Granulite Facies is distinguished by the general absence of white mica and the presence of orthopyroxene and cordierite. Pelitic assemblages include

biotite-garnet-sillimanite-K feldspar-andesine-quartz-cordierite-orthopyroxene-sillimanite-zircon

Example: Barrovian Metamorphism in the Southern Appalachian Orogen

While the Southern Appalachian Orogen is one of the major regions of Barrovian Facies Series rocks in North America, analysis of the metamorphism there has been confounded by several factors. First, the various tectonic belts (terranes) in the southern Appalachian Orogen have been juxtaposed by significant movements of various types along major faults—in several cases, after metamorphism had occurred. This problem is particularly significant in the central and eastern parts of the Orogen. Second, the thermal significance of various metamorphic zones is open to question.

A map of the orogen, showing the approximate positions of metamorphic facies of Paleozoic age, is presented below. A broad range of rock types exists in the region, but carbonate rocks, especially impure carbonate rocks, are relatively rare in the higher-grade parts of the metamorphic belt, whereas mafic and ultramafic rocks are rare to nonexistent in the low-grade zones. Rocks of the Zeolite and Prehnite-Pumpellyite Facies occur primarily in the Valley and Ridge Belt. At these lowest grades of metamorphism, the pelites are characterized by clays and the carbonate rocks by calcite and/or dolomite + quartz. Greenschist Facies assemblages are distributed in the western Blue Ridge Belt. Rocks of this grade consist of younger (Cambrian) sedimentary and igneous rocks and older (Proterozoic) polymetamorphic rocks. Quartz-rich metaclastic rocks typically contain the assemblage quartz-white mica-chlorite-alkali feldspar. Quartz-feldspar gneisses, probably products of retrograde metamorphism of Precambrian Amphibolite and Granulite facies rocks, contain similar assemblages. Pelitic rocks are composed of the assemblage chlorite-white mica-quartz-albite. In higher-grade assemblages, garnet is present. Metabasites contain assemblages such as chlorite-epidote- albite-quartz-actinolite.

Much of the eastern Blue Ridge Belt is composed of rocks of the Amphibolite Facies. Migmatites are common. Pelitic mica schists consist of various assemblages containing staurolite, kyanite, and sillimanite. Quartzo-feldpathic rocks are composed predominantly of the assemblage plagioclase-quartz-biotite-white mica-garnet. Mafic rocks are typical amphibole schists and gneisses, with hornblende and plagioclase as the dominant phases. Geothermometry and geobarometry indicate that the Amphibolite Facies rocks of the Blue Ridge were metamorphosed at temperatures between 500 and 850 °C at pressures of 5-11 kb.

Paleozoic Granulite Facies rocks have been recognized at only a few localities. Aluminous schist consists of biotite-garnet-sillimanite-K feldspar-andesine-quartz. Quartzo-feldpathic rocks contain assemblages such as andesine-quartz-K feldspar-biotite-garnet. A typical metabasite assemblage is hornblende-bytownite-biotite-orthopyroxene. Given that the estimated P-T conditions do not differ significantly from those for Amphibolite Facies metamorphism, the zones of Granulite Facies metamorphism probably represent local areas in which the rocks were dehydrated by previous metamorphic events.

The Blueschist Facies develops in terranes in which the geothermal gradient is low or the overall P/T is moderate to high. Two sub-types of facies series are recognized in such terranes: the Sanbagawa Facies Series and the Franciscan Facies Series. In the Sanbagawa Facies Series, the maximum temperatures are somewhat higher than in the Franciscan Facies Series. The facies sequence is Zeolite--Prehnite-Pumpellyite--Blueschist --Greenschist--Amphibolite. In the Franciscan Facies Series, the facies sequence is Zeolite--PrehnitePumpellyite--Blueschist--Eclogite.

Bluescist Facies series are widely distributed. They occur in North, Central, and South America, in the Caribbean region, in Europe, especially in the Alps, in the Middle East, in Asia, and in the circum-Pacific region (figure). Typically, these facies series form on the outer (trench) side of a paired metamorphic belt associated with a subduction zone. In some cases, high P/T (low-temperature) rocks form where subduction-induced collision between a continent and island arc or another continent is inferred.

Young mountain belts contain the majority of these rocks, but early Paleozoic and rare Precambrian Blueschist Facies rocks are known. The two sub-facies series of high P/T metamorphism take their names from well-studied examples on opposite sides of the Pacific Ocean. The Franciscan Facies Series is named for the Franciscan Complex of western California and southern Oregon. The Sanbagawa Facies Series takes its name from rocks exposed in southeastern Japan.

Mineral assemblages, facies, and textures set the high P/T facies series apart from those of lower P/T. Minerals such as lawsonite occur only at high P and low T. In general, the rocks in outer metamorphic belts are metamorphosed pieces of ocean crust and overlying sediments. The most common of the critical minerals that appear include laumontite, pumpellyite, glaucophane, lawsonite, aragonite, jadeitic pyroxene, and omphacite.

In the Zeolite Facies, common mineral assemblages are

heulandite-quartz-analcite-vermiculite-white mica-laumontite-calcite

These are replaced in the Prehnite-Pumpellyite Facies by assemblages such as

quartz-albite-prehnite-pumpellyite-white mica-chlorite- stilpnomelane-calcite

Blueschist Facies assemblages include

quartz-albite-lawsonite-pumpellyite-chlorite-white mica-jadeitic pyroxene-glaucophane-aragonite

Rare Eclogite Facies rocks contain

• Metasomatic Recrystallization Hypotheses - Blueschists result from low-pressure metasomatism induced by concentrated, saline pore fluids evolved during serpentinization.
• Tectonic Overpressure Model - This hypothesis argues that tectonic overpressures cause Blueschist Facies Series metamorphism. Tectonic overpressures develop below a regional thrust fault that is capped by serpentinite. Trapped water creates the overpressures.
• Burial Metamorphism Hypothesis - Deep burial may result from either sedimentation or tectonic thickening of the crust via faulting. The tectonic setting of high P/T metamorphic belts is consistent with this hypothesis. In particular, the presence of Blueschist Facies Series rocks in paired metamorphic belts suggests that subduction and associated accretion of subducted rocks, are generally responsible for Blueschist Facies and related rocks.

Example: Regional Hign P/T Metamorphism of the Franciscan Complex, CA

In the northern Coast Ranges, rocks of six metamorphic facies are distributed across three major, fault-bounded belts that are successively younger from east to west. High-grade schists and gneisses, in tectonic blocks and slabs, form a fourth unit that locally caps the Franciscan Complex along its eastern edge. Each belt is subdivided into several thrust sheets or fault blocks (commonly designated as terranes) that include various formations, broken formations, dismembered formations, and melanges. The Central Belt is largely melange. In contrast, the adjoining Eastern and Coastal belts, though locally containing melange, consist predominantly of rock bodies with greater internal coherence. In the area at the southern end of the Northern Coast Ranges, in the San Francisco Bay area and to the north for several tens of kilometers, the structural and metamorphic patterns are highly disrupted by Cenozoic faulting. The metamorphic patterns of the northern Coast Ranges are more regular than the patterns in the south.

In the north, the westernmost belt, the Coastal Belt, is a metawacke and metashale-dominated, Zeolite Facies metamorphic belt. The metawackes contain laumontite, prehnite, or pumpellyite. The Central Belt melanges structurally overlie the Coastal Belt rocks. Most rocks of the Belt are considered to belong to the Prehnite-Pumpellyite Facies. However, because the Central Belt consists primarily of an assemblage of melanges, rocks from Zeolite Facies to Eclogite and Amphibolite Facies are present. To the east and structurally overlying the Central Belt is a faulted Bluesehist Facies belt dominated by metasedimentary rocks and containing a variety of pumpellyite, lawsonite, and jadeitic-pyroxene-bearing assemblages.