The distribution of Fe and Mg between biotite and amphibole in granitic rocks: Effects of temperature, pressure and amphibole composition

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The compositional systematics of biotites and hornblendes from the batholiths of California have been examined for the purpose of assessing the effects of temperature and pressure of crystallization on the distribution of Fe and Mg among coexisting mafic silicates in granitic rocks. Thermodynamic calculations indicate that at magmatic conditions, Fe-Mg exchange between biotite and amphibole is probably not a strong function of crystallization temperature. However, theoretical predictions and natural mafic phase compositions show that the Fe-Mg distribution is dependent upon amphibole composition. Specifically, decreases in amphibole total Al and A-site occupancy lead to increases in the Mg/(Mg+Fe^T) of amphibole relative to the Mg/(Mg+Fe^T) of coexisting biotite. In magmas containing the appropriate igneous buffering assemblage, amphibole Al content is positively correlated with crystallization pressure (cf. Hammarstrom and Zen, 1986). In these magmas, the distribution of Fe and Mg between coexisting amphibole and biotite is primarily a function of consolidation pressure, owing to the dependence of Mg/(Mg+Fe^T) on Al content in amphibole. Variations in natural amphibole chemistry as a function of crystallization pressure indicate that the dominant pressure-sensitive coupled substitution in amphibole in the batholiths of California is probably of the form: (A-site)+(Mg+Fe²⁺)+2Si⇔(Na+K)(A-site)+2Al^IV+(Al+Fe³⁺)^VI