Phase equilibrium controls on the tholeiitic versus calc‐alkaline differentiation trends

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Abstract
This paper discusses some of the factors that allow tholeiitic and calc‐alkaline differentiation trends to be generated from a common basaltic parent melt, and calculates liquid lines of descent, for the two divergent trends. Phase proportions and compositions measured in 1‐atm experiments on natural basalts are used as input to calculate the tholeiitic trend, and high‐pressure phase proportions, inferred from phase equilibrium studies on natural compositions, are used to calculate the calc‐alkaline trend. An important control on the development of the contrasting tholeiitic versus calc‐alkaline trends is the proportions of olivine, plagioclase, and pyroxene that crystallize from the basaltic parent melt. The tholeiitic trend is produced by fractional crystallization of a basalt magma at low pressures. The crystallization sequence is olivine, followed by plagioclase, followed by augite. Plagioclase dominates the assemblage, and the three‐phase crystallization continues to a reaction point where olivine+liquid react to augite, plagioclase, and pigeonite. Total iron in the liquid increases throughout this crystallization process, and a dramatic increase in iron and a mild decrease in SiO2 occur at the reaction point. The calc‐alkaline trend develops when olivine, calcic plagioclase, and augite crystallize in nearly equal mass proportions. This phase assemblage precipitates under conditions of moderate pressure and water undersaturation in the middle to upper crust and produces a gentle increase in total iron, an increase in SiO2 and a decrease in MgO in derivative liquids. Liquids derived by such a process can evolve to andesites and dacites by continued crystallization involving plagioclase, orthopyroxene, pigeonite, augite, and magnetite. The assimilation of a crustal component by a fractionating basaltic melt and the mixing of basaltic liquids with siliceous residual liquids produced by either fractionation or fractionation aided by assimilation are additional processes that assist in the production of the calc‐alkaline trend.