Mafic Silicates as Indicators of Intensive Variables in Granitic Magmas

Abstract

Reactions that involve biotites, amphiboles, pyroxenes and olivines place significant constraints on the intensive variables of magmas that crystallize to form granitic rocks. Fe₂+-bearing minerals such as magnetite, ilmenite, olivine, pyroxene, amphibole, biotite, and allanite constrain oxygen and sulfur fugacities. All (OH)- bearing phases place constraints on H₂O fugacities, especially if anhydrous reactants are present. Exchange reactions of (OH)- with halogens also place additional constraints on magma properties. Late stage subsolidus reactions commonly recrystallize these minerals and make interpretation more difficult.
Intensive parameters change during the history of a given magma. Sequences of crystallization, coupled with the composition of minerals, can record devolatilization reactions, increases in volatile constituents, and temperatures of magmas crystallizing to form plutons.
The intrinsic properties of a magma may reflect the source region, but may also change significantly during the history of a given magma. Reactions involving hydrous minerals in the source region place strong constraints on the initial water content of magmas. In regions of repeated crustal melting, early granitic magmas will be more hydrous than subsequent ones.