410 REGIONAL METAMORPHISM depth (Figure 1) They are typically formed during subduction of a cold oceanic plate underneath a continent Mafic protoliths best document such high-pressure conditions by their diagnostic mineral assemblages in blueschist (sodic amphibole bearing; the term ‘blueschist’ derives from the blue sodic amphibole, glaucophane) and eclogite (dominated by garnet ỵ omphacitic clinopyroxene) Metapelitic rock compositions may develop diagnostic minerals or assemblages such as carpholite, phengite, jadeite ỵ quartz, and talc ỵ kyanite Facies of Low Pressure Low-pressure facies are in many cases typical for contact metamorphism (see Thermal Metamorphism), but especially in cordilleran-type mountain belts this contact metamorphism can be of regional extent due to long-lasting intrusive activity over a large area, producing a transition to regional metamorphism (Buchan-type in Scotland, Abukuma-type in Japan) It also may follow the medium-pressure facies Metapelites are characterized by andalusite and cordierite Facies of Medium Pressure Most metamorphic terranes, especially in the internal parts of continents, are composed of greenschist, amphibolite, and granulite facies rocks, including migmatites at the high metamorphic grades Such a facies series is related to a geothermal gradient of 20–35 C km 1, typical of the Barrovian-type metamorphism as defined by Barrow in south-east Scotland A well-documented example of a Barrovian-type metamorphic zoning is exposed on the island of Naxos, Greece (see later) As already discussed, medium-pressure regional metamorphic zones are best developed in Al-rich metapelitic rocks, which may contain either kyanite or sillimanite, or both Al-silicates The formation of staurolite and the breakdown of chloritoid are generally considered to reflect the greenschist–amphibolite facies transition in pelitic rocks In mafic rocks, the greenschist–amphibolite facies transition is designated by the appearance of hornblende and Ca-rich plagioclase and the disappearance of actinolite and Na-rich plagioclase In metapelites, the amphibolite–granulite transition is considered by some workers to be indicated by the assemblage K-feldspar ỵ Al-silicate forming at the expense of muscovite ỵ quartz Other workers consider the formation of orthopyroxene to be typical of granulite facies conditions, although this occurs at significantly higher temperatures (100–150 C higher) than does formation of K-feldspar ỵ Al-silicate In mafic rocks, the presence of orthopyroxene, which forms at the expense of hornblende and/or biotite, is considered indicative of granulite facies conditions The role of water activity, however, complicates characterization of the amphibolite–granulite facies transition If rocks are water saturated, they will, with a eutectic or eutectoid chemical composition near to granite, partially melt at PT conditions in the muscovite ỵ quartz stability field When water activity is low, pervasive partial (dry) melting will occur at much higher temperatures and hydrous minerals (muscovite, biotite, and amphibole) may decompose to anhydrous ones by local dehydration melting Facies of (Very) Low Grade Facies of very low grade are present in the transition zones of an orogenic mountain belt to the sedimentary foreland (e.g., Swiss Alps), but are often missing in regional metamorphic terranes, where the metamorphic rocks are overthrusted onto the foreland They are typically best developed in regions with burial or hydrothermal metamorphism (e.g., New Zealand, Central Andes) The Barrovian-Type Metamorphic Complex of Naxos The island of Naxos is part of the Attic–Cycladic Metamorphic Complex that stretches from mainland Greece to south-western Turkey Isoclinally folded sequences of metasediments (pelitic and psammitic schists and gneisses, quartzites, and calcitic and dolomitic marbles) and metavolcanics (amphibolites and mafic schists) are the dominant lithologies on Naxos (Figure 2) Ultramafic rocks occur in a few stratigraphic horizons and as isolated lenses Metacarbonate units are widespread and contain in certain stratigraphic horizons diaspore and/or corundumbearing lenses (up to %8 m thick; Figure 2), for which protoliths are bauxite formed in a karst soil These metabauxite occurrences offer a unique opportunity to investigate the prograde metamorphism of a rock composition with excess Al The metamorphic pattern of Naxos was shaped during a late Alpine medium-pressure event that culminated 15–20 Ma; it consists of concentric zones of decreasing metamorphic grade, outward from a highgrade migmatitic core (Figure 2; Table 1) In the western part of Naxos, a %13-My-old granodiorite intruded the metamorphic complex and induced andalusite–sillimanite-type contact metamorphism in about a 1-km-wide zone In this same region of Naxos, a post-intrusive upper unit, comprising Tertiary sediments and ophiolite-suite rocks, tectonically overlies the metamorphic complex and granodiorite Petrological and geochronological studies indicate that relict