REGIONAL METAMORPHISM 413 mineral assemblages of an early Alpine high-pressure/ low-temperature metamorphism (about 45 Ma) locally occur in south-eastern Naxos (zones I–II) and that part of the migmatite core (zone VI) is of pre-Alpine origin (from %300 Ma) Temperatures during the Barrovian-type metamorphism are estimated to have ranged from %700 C in the migmatite dome to %400 C in south-eastern Naxos at pressures of 0.5–0.7 GPa Six isograds mapped in metapelitic and metabauxitic rocks divide the metamorphic complex into seven Barrovian-type metamorphic zones (Figure 2; Table 1) The first three zones (I–III) belong to the greenschist facies: their subdivision is based on the first occurrence of corundum, by the reaction diaspore ẳ corundum ỵ H2O (in metabauxites), and of biotite, by the reaction chlorite ỵ muscovite ẳ biotite ỵ H2O (in metapelites) In mafic rocks, biotite already occurs locally in zone II, whereas the first biotite in bauxitic rocks appears in zone IV The presence of staurolite, both in pelitic and bauxitic rocks, indicates the beginning of amphibolite facies conditions Staurolite formation is related to complex breakdown reactions of chloritoid and approximately coincides with the disappearance of chloritoid Zones IV, VA, and VB belong to the amphibolite facies; whether zone VI, the beginning of which is defined by partial melting of metapelites, is amphibolite or granulite facies is a matter of debate The subdivision of the amphibolite facies zones of Naxos is based on the appearance of sillimanite and the disappearance of kyanite in metapelitic rocks The Naxos rocks clearly illustrate that bulk composition controls the formation of particular minerals or assemblages In common with other Barrovian terranes, the first kyanite in metapelitic rocks is found in staurolite-grade zone IV, whereas kyanite in bauxitic rocks is already formed in zone I by the reaction pyrophyllite ỵ diaspore ẳ kyanite ỵ H2O (Table 1) The upper amphibolite or granulite facies zone VI contains areas where partial melting of SiO2-rich rocks was intensive (melt phase has a granitic composition) and areas showing little evidence of melting Variations in the degree of migmatization may relate to local variations in water activity and rock composition The typical mineral assemblage in metapelitic composition is K-feldspar þ sillimanite þ biotite Muscovite often occurs, but textural evidence suggests it is mostly of retrograde origin Mafic rocks are devoid of orthopyroxene but the presence of sapphirine ỵ spinel þ corundum at a few localities in zone VI argues for granulite facies grade conditions See Also Diagenesis, Overview Metamorphic Rocks: Classification, Nomenclature and Formation; Facies and Zones Minerals: Other Silicates Plate Tectonics Tectonics: Mountain Building and Orogeny Thermal Metamorphism Ultra High Pressure Metamorphism Further Reading Barrow G (1893) On an intrusion of muscovite biotite gneiss in the S.E Highlands of Scotland and its ac companying metamorphism Quarterly Journal of the Geological Society, London 49: 330 358 Barrow G (1912) On the geology of lower Deeside and the southern highland border Proceedings of the Geologist’ Association 23: 268 284 Bucher K and Frey M (2001) Petrogenesis of Metamorphic Rocks, 7th edn Berlin Heidelberg New York: Springer Verlag Eskola P (1920) The mineral facies of rocks Norsk Geolo gische Tidsskrift 6: 143 194 Feenstra A (1985) Metamorphism of Bauxites on Naxos, Greece PhD thesis, Rijksuniversiteit Utrecht, Geologica Ultraiectina 39 Jansen JBH and Schuiling RD (1976) Metamorphism on Naxos: Petrology and geothermal gradients American Journal of Science 276: 1225 1253 Spear FS (1993) Metamorphic Phase Equilibria and Pres sure Temperature Time Paths Washington DC: Miner alogical Society of America Winter JD (2001) An Introduction to Igneous and Meta morphic Petrology Upper Saddle River, NJ: Prentice Hall Yardley BWD (1989) An Introduction to Metamorphic Petrology Burnt Mill, Harlow, Essex: Longman Group UK Limited