Crustal anatexis: An experimental review

Review of experimental studies in synthetic systems An—Ab—Or—Qz—H ₂O and K ₂O—Aℓ ₂O ₃—SiO ₂—H ₂O illustrates the dominance of feldspar—quartz assemblages in crustal anatectic processes, and the role of hydrous minerals such as muscovite. H ₂O-undersaturated liquids can exist with quartz—feldspar assemblages through hundreds of degrees with little change in composition in terms of anhydrous components. Review of the phase relationships in the rock series gabbro—tonalite—granodiorite—granite—H ₂O to 15 kbar is used as a basis for interpreting anatexis of deep crustal rocks, including metamorphosed grey wackes and pelitic rocks. For individual rocks, diagrams include PT projections with excess H ₂O, with hydrous minerals but no excess H ₂O, of H ₂O-undersaturated liquidus surfaces, and isobaric T-X _H2O sections. For the rock series, 10 kbar diagrams show the melting interval with excess H ₂O, the H ₂O-undersaturated liquidus surface, and the melting interval with 2% H ₂O. The normal product of regional metamorphism is H ₂O-undersaturated granitic liquid; H ₂O-saturated liquids exist only through narrow temperature intervals. Liquids of granite composition trend towards granodiorite with increasing temperature or pressure, but compositions do not reach tonalite unless metamorphic temperatures approach 1100°C. Tonalite plutons represent mushes of granitic liquid with refractory residual crystals, or recrystallized residual masses from which the granitic liquid escaped. Some tonalite plutons may be derived from magmas with source in subducted ocean crust or mantle.