Crustal anatexis: An experimental review

Abstract

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_(H_2O) 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.