408 METAMORPHIC ROCKS/Facies and Zones grade cannot be related to a continuous process of metamorphism that has transformed all rocks of the area A good example of this is found along the west coast of Norway There are a large number of 100 m-sized lenses and blocks of eclogite or of UHP rocks (coesite eclogites) in an amphibolite facies gneiss terrain No relic assemblages of the eclogite facies are present in the gneiss The eclogite blocks have tectonic contacts with the gneiss and are characterised as allofacial relative to them Other examples are outcrops of garnet peridotite in the Central Swiss Alps that occur in amphibolite facies rocks These UHP rocks are also clearly allofacial An the other hand, serpentinites of the ZermattSaas ophiolite complex of the Central Swiss Alps occur together with metabasalts and metagabbros of the eclogite and blueschist facies The serpentinites represent hydrated mantle of the oceanic lithosphere and their assemblages antigorite ỵ forsterite ỵ diopside is isofacial with the eclogite facies metabasites The Metamorphic Facies Scheme, Additional Aspects and Facies Boundaries Features of the amphibolite–granulite facies boundary (line 3, in Figure 5) and the lower limit of the greenschist facies (line 1, in Figure 5) have been briefly discussed The greenschist–amphibolite facies boundary (line 2, in Figure 5) is characterized in mafic rocks by the relatively abrupt increase of the calcium (anorthite)content of plagioclase and by a simultaneous increase of the tschermak- and edenite-content of amphibole In metapelites, staurolite-bearing assemblages gradually replace chloritoid-bearing assemblages The boundaries 4, 5, and in Figure all mark the disappearance of plagioclase in mafic rocks towards higher pressures At low temperature, albite-bearing mafic rocks are replaced by assemblages with sodic amphibole (line 4, in Figure 5) Sodium is transferred from feldspar to amphibole and mica (paragonite) At higher temperature, amphibolite and granulite are directly replaced by eclogite (lines and 6, in Figure 5) The transition from blueschist to eclogite facies (line 7, in Figure 5) is a very gradual boundary In typical low-temperature eclogite, omphacite and garnet often coexist with glaucophane and clinozoisite over a fairly wide range of conditions This means that assemblages of both facies occur in the same metabasalt This is mostly due to the strong dependence of the stable assemblages on the redox conditions prevailing during high-pressure metamorphism Mineral Zones Characterizing the metamorphic grade can also be achieved by mapping in the field the occurrence of so called index minerals in a homogeneous rock unit with constant composition over a large outcrop area Index minerals have a PT sensitive distribution in the rock type studied In going up-grade, the first occurrence of an index mineral can be placed as a line on a map The line connecting all outcrops of the first occurrence of an index mineral is called a zone boundary of a mineral zone that is characterized by that index mineral The upper grade limit of the mineral zone is defined by the mineral zone boundary of the next index mineral with increasing grade The classic example for the use of this simple and straightforward technique is the map of mineral zones in metapelites (micaschists) in the Scottish Highlands (see Further Reading) At the lowest grade outcrops, the micaschists contain chlorite as a diagnostic mineral The first occurrence of biotite in the micaschists can be mapped as a line in the field It defines the zone boundary of the biotite zone The next diagnostic mineral that appears in the rocks is garnet Its first appearance defines the beginning of the garnet zone At still higher grade staurolite appears in the schist and the staurolite zone boundary marks also the upper end of the garnet zone Note that both biotite and garnet may still be present in the rocks Continuing upgrade, kyanite can be found in staurolite-garnet-biotite micaschists, defining a kyanite mineral zone Finally at the highest metamorphic grade but still within the amphibolite facies, sillimanite can be found in the rocks The complete zonal sequence of index minerals is known as the Barrovian sequence of metamorphism It is a typical pattern of metamorphism and it has been described from many orogenic belts See Also Metamorphic Rocks: Classification, Nomenclature and Formation; PTt - Paths Regional Metamorphism Thermal Metamorphism Ultra High Pressure Metamorphism Further Reading Austrheim H (1990) The granulite eclogite facies transition: A comparison of experimental work and a natural occur rence in the Bergen Arcs, western Norway Lithos 25 Barrow G (1912) On the geology of lower Deeside and the southern Highland border Proceedings of the Geologists Association 23: 268 284 Bucher K and Frey M (2002) Petrogenesis of Metamorphic Rocks Berlin, Heidelberg: Spring Verlag