Amphibole‐out phase boundary in partially melted metabasalt, its control over liquid fraction and composition, and source permeability

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A number of experimental studies in recent years have demonstrated that partial melting of hydrated basalt at lower crustal to upper mantle pressures is capable of generating up to 40% high‐SiO2 liquids, closely comparable in many geochemical respects to tonalitic‐trondhjemitic granitoids (TTG). From these studies it is apparent that the initiation of melting coincides with the beginning of amphibole dehydration but that a broad reaction interval over which amphibole coexists with high‐SiO2 liquids is defined by the wet basalt solidus on one side, and the amphibole‐out phase boundary on the other. The phase relations of an alkali‐rich tholeiitic metabasalt have been examined in the outer half of this region, up to and beyond the amphibole‐out phase boundary. Results indicate that amphibole exerts a strong control over the amount and composition of coexisting liquid over this interval. Melt fraction increases slowly and gradually (from 0% up to 20–30%) between the wet basalt solidus and up to amphibole‐out but increases substantially (from 30% to >50%) across and immediately beyond it. Liquids are strongly to moderately peraluminous (molar Al2O3/(CaO + Na2O + K2O), A/CNK, > 1.0) up to amphibole‐out, neutral at the boundary (A/CNK ∼ 1.0), and increasingly metaluminous (A/CNK < 1.0) beyond it. Up to amphibole‐out, potassium is strongly partitioned into liquid relative to amphibole (i.e., mineral melt kd ∼ 0.2–0.3), whereas titanium is strongly partitioned into liquid (kd ∼ 2.0–6.0). Physical and chemical criteria suggest that efficient melt segregation leading to TTG plutonism requires more than 20–30% batch melting of a garnet‐bearing basaltic protolith, near or beyond the amphibole‐out phase boundary, leaving dry residues of eclogite or garnet granulite and producing metaluminous liquids that are strongly depleted in heavy rare earth elements (HREEs) and yttrium (Y). Lower degrees of melting may involve critical melting, where melt is squeezed out of the residue by deformation of the crystalline matrix, resulting in disequilibrium melt compositions.