Question

regional metamorphism of quartzo felspathic rocks

Answer 1

The Dalradian and Moinian Series of Scotland
The classic example of a regionally metamorphosed area is the Dalradian series of Scotland. The Dalradian Series occurs in a zone 50 to 80 km wide, north of the Highland Boundary Fault.  A similar group of metamorphic rocks occurs to the North of the Great Glenn Fault (a strike-slip fault) and is called the Moinian Series. 

The rocks were originally shales, limestones, diabase sills, and basalts  that had been emplaced in the Precambrian to early Cambrian. In this area, Barrow (1893) mapped metamorphic zones in pelitic rocks based on mineral assemblages he observed in a small part the area.  This mapping was later extended across the Scottish Highlands to cover most of the Dalradian and Moinian Series as shown in the map.  The series of metamorphic zones mapped by Barrow has since become known as the Barrovian Facies Series (At the time Barrow did his mapping, the facies concept had not yet been developed). In pelitic rocks, Barrow recognized 6 zones of distinctive mineral assemblages, which he recognized as representing increasing grade of metamorphism. The boundaries for his zones were based on the first appearance of a particular mineral, called an index mineral, which is characteristic of the zone.  These boundaries were later called isograds (equal grade) and likely represent surfaces in a three dimensional sense. He called the zone of lowest grade rocks the "zone of digested clastic mica," but Tilley, mapping the area in 1925, renamed this zone the chlorite zone.Zone 
(textural type)Mineral Assemblage in Pelitic RocksChlorite (slates & phyllites)quartz, chlorite, muscovite, albiteBiotite (phyllites & schists)biotite begins to replace chlorite, quartz, muscovite, albiteGarnet (phyllites and schists)quartz, muscovite, biotite, almandine, albiteStaurolite (schists)quartz, biotite, muscovite, almandine, staurolite, oligoclaseKyanite (schists)quartz, biotite, muscovite, oligoclase, almandine, kyaniteSillimanite (schists & gneisses)quartz, biotite, muscovite, oligoclase, almandine, sillimanite

 

 

Mineral assemblages for pelitic rocks of the Barrovian Zones are listed in the table above.  Note the following important points: The index mineral that defines a zone, does not necessarily disappear when entering the next higher grade zone.  For example the first appearance of biotite is at the biotite isograd where chlorite is seen to be reacting to produce biotite.  Biotite does not disappear at the garnet isograd, and, in fact continues to be seen though the garnet, sillimanite, staurolite, kyanite, and sillimanite zones.

Staurolite, kyanite, and sillimanite only occur in the staurolite, kyanite, and sillimanite zones, respectively.

The composition of the plagioclase changes with increasing grade of metamorphism.   It is nearly pure albite in the chlorite and biotite zones, and becomes somewhat more calcic (oligoclase) in higher grade zones.

The texture of the rocks changes from slates and phyllites in the chlorite zone to schists in the staurolite and kyanite zones, to schists and gneisses in the sillimanite zone.

 

 

As mentioned above, Tilley mapped the area in 1925, and extended the zones across the area between the Highland Boundary Fault and into the Moinian Series, and renamed the chlorite zone.  Later, Wiseman (1934), mapped the metabasic rocks and Kennedy (1949) mapped the meta calcareous sediments.  Mineral assemblages in these rocks and their correlation with the metamorphic zones in the pelitic rocks are shown in the table below.

 

  

  

 

Index Mineral
(Pelitic Rocks)Basic RocksCalcareous RocksFaciesChloriteChlorite, albite, epidote, sphene, ± calcite
± actinoliteGreenschistqtz, muscovite, biotite, calciteBiotitegarnet, zoisite, sodic plagioclase, biotite or hornblendeGarnetHornblende, plagioclase, 
±epidote, ±almandine, ±diopsideAmphiboliteStaurolitegarnet, anorthite or bytownite, hornblendeKyaniteSillimanitegarnet, anorthite or bytownite, pyroxene 
Note that the metabasic rocks only define two zones, one corresponding to the chlorite and biotite zones in the pelitic rocks, and the other corresponding to the staurolite, kyanite, and sillimanite zones.  The calcareous rocks define four zones.  The lowest grade rocks are only metamorphosed in the higher grade parts of the pelitic chlorite zone, another set of minerals occurs in the calcareous rocks throughout the biotite and garnet zones, another mineral assemblage is characteristic of the staurolite and kyanite zone, and a fourth mineral assemblage is found in calcareous rocks of the sillimanite zone.Note how the Anorthite content of plagioclase increases with increasing grade of metamorphism in the basic rocks and the calcareous rocks.The