Mountain systems typically contain large belts of regionally metamorphosed rock. These are often foliated metamorphic rocks developed under medium to high temperatures. They occur in belts of regional extent, from which the term regional metamorphism was originally derived. The accompanying pressures vary from low to high. Geothermal gradients, which are likewise moderate to high, produce Buchan and Barrovian Facies series. Because the pressures of Buchan and Barrovian Facies series are commonly higher than are those of Contact Facies Series, they may contain different sequences of minerals.
The presence of either andalusite or kyanite on metamorphosed shales and siltstones at the middle grades of metamorphism is one feature that distinguishes these facies series from one another.
BUCHAN FACIES SERIES
The Buchan Facies Series takes its name from a region in the Scottish Highlands. In general, the geothermal gradients that give rise to the low pressures and high temperatures of Buchan Facies Series may be attributed to (a) regional heating from intrusion of groups of plutons at shallow to moderate depths; (b) plate collisions at convergent margins; and (3) crustal thinning. Buchan metamorphism is common, and a number of Buchan belts have been described from various parts of the world, notably Spain and Japan. Other localities include Maine, New Hampshire, Colorado, Oregon, Alaska, Australia, India, and Ireland.
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
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
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.
Pelitic rocks in the Granulite Facies are distinguished by the general absence of white mica, by the presence of alkali feldspar + sillimanite or orthopyroxene, and by the occurrence of the assemblage cordierite + orthopyroxene.
Example: Buchan Metamorphism, Northern New England, U.S.A.
Perhaps the best-known Buchan Facies Series is that of northern New England. A line representing the aluminum silicate triple point extends through New England-from Rhode Island, through central Massachusetts, across western New Hampshire, and into northeastern Vermont-marking a change from a Barrovian Facies Series on the southwest to a Buchan Facies Series on the northeast (figure).
In the Buchan Facies Series of northeastern New England isograds have been mapped in the widely distributed pelitic rocks, including biotite, garnet, andalusite-staurolite, cordierite-staurolite, sillimanite, and K feldspar-sillimanite (Greenschist Facies). Locally, muscovite coexists with sillimanite and K feldspar in pelitic rocks of the uppermost zone; thus, the rocks containing these minerals belong to the Amphibolite Facies. Granulite Facies rocks are present only to the south, in New Hampshire, Massachusetts, and northern Connecticut. In northernmost Maine, Quebec, and New Brunswick, the Zeolite and Prehnite-Pumpellyite Facies are represented by analcite, prehnite-pumpellyite, and pumpellyite-epidote-actinolite zones in metaclastic and metavolcanic rocks.
Differences and Similarities Between Contact and Buchan Facies Series
Hi Grade Facies
BARROVIAN FACIES SERIES
The Barrovian Facies Series occurs in a number of Paleozoic mountain belts, as well as in some of Precambrian age. Notable are the Caledonides of northwestern Europe, including the classic region in the Scottish Highlands, and parts of the Appalachian Mountain System of eastern North America. Other belts with Barrovian rocks occur in Idaho, Colorado, British Columbia, Alaska, Venezuela, Spain, southern Europe and Asia and Japan. Precambrian belts of Barrovian rocks occur in the Black Hills of South Dakota, the Rocky Mountains, and Labrador, Quebec, and Ontario (Canada).
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)
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
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
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
The Granulite Facies is distinguished by the general absence of white mica and the presence of orthopyroxene and cordierite. Pelitic assemblages include
The quartzo-felspathic rocks differ from the pelitic rocks. Quartz and feldspar are the dominant phases, rather than the phyllosilicates, and calcium-bearing phases are common. Additional minerals that may occur include stilbite, calcite, stilpnomelane, actinolite and hornblende.
Example: Barrovian Metamorphism in the Southern Appalachian Orogen
The southern Appalachian Orogen extends from central Virginia to Alabama. It is a complex orogenic belt, parts of which have experienced regional metamorphism during four orogenic events. The ages of these events are Proterozoic, Ordovician (the Taconic Orogeny), Devonian-Mississippian (the Acadian Orogeny), and Pennsylvanian-Permian (the Alleghanian/Appalachian Orogeny).
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.
Because the overall metamorphic pattern in the Southern Appalachian orogen developed over a long period of time, it is difficult to discern the complete patterns of metamorphism associated with each orogenic event. In the western part of the orogen, that problem is increased where thrust faults have shortened the width of the orogen, concealing sections of the metamorphic belt. Nevertheless, the elongate metamorphic zones are typical of orogenic Barrovian Facies Series metamorphic belts.
BLUESCHIST FACIES SERIES
Glaucophane imparts an attractive blue hue to rocks. This feature undoubtedly accounts for the considerable interest given to the relatively uncommon glaucophane schists (the "blueschists") of the California Coastal Ranges. The blue color also serves as the basis for the name Blueschist Facies, even though this facies contains large volumes of rock that are neither blue nor schistose. It is also true that all rocks containing blue amphibole do not belong to the Blueschist Facies.
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
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
Three hypotheses for the origin of Blueschist Facies Series rocks are advocated by various geologists.
Example: Regional Hign P/T Metamorphism of the Franciscan Complex, CA
The Franciscan Complex forms the structurally complicated basement of much of the California Coast Ranges. It is composed of a wide variety of rock types, not all of which are metamorphosed. As a group, however, metamorphic rocks dominate. Graywacke and metagraywacke and associated shale and metashale are the most abundant rock types. Chert, pillow basalt, limestone, conglomerate, ultramafic rocks and the metamorphic equivalents of all of these also occur at numerous localities. Well known among the metamorphic rocks are eclogites, glaucophane schists and gneisses, and actinolite and hornblende schist and gneiss that occur in isolated blocks and sheets. The isolated masses most commonly occur in melanges. In addition, Eclogite, Blueschist, Amphibolite, and rare Greenschist Facies rocks form slabs and tectonic blocks along faults.
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.
Analyses of the conditions that produced the metamorphic rocks in the Franciscan Complex suggest metamorphism of Eastern Belt rocks occurred at P=6-10kb and T= 125-350 °C, whereas Central Belt melange metamorphism resulted from pressures of 2-6kb and temperatures of 125-300 °C. Zeolite Facies metamorphism of Coastal Belt rocks occurred at about P= 1-3kb and T=100-200 °C.
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