Mafic Silicates as Indicators of Intensive Variables in Granitic Magmas

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, il menite, 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 tempera tures 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.  The relationship of mineral assemblages in granitic rocks to the intensive variables of the original magma is not always obvious. Many granitic plutons react with the magmatic fluids during cooling (e.g., CZAMANSKE and MIHALIK, 1972; CZAMANSKE and WONES, 1973), while others clearly react with metamorphic fluids (FERRY, 1979). The mafic silicates record much of the crystallization and cooling history of a given pluton, and can, used in conjunction with the oxides, sulfides, and feldspars, provide information about the original magma. TAKAHASHI et al. (1980) review the comparison of magnetiteand ilmenite-series granitoids (ISHIHARA, 1977) and Iand S-type granitoids (CHAPPELL and WHITE, 1974). EWART (1979), in a review of volcanic rocks and tectonic settings, pointed out that the relative oxidation states of volcanic rocks are related to tectonic setting. ISHIHARA et al. (1979) have stressed the importance of oxidation state on the origin of Sn-bearing granites. This paper reviews the relative stabilities of biotites and amphiboles in granitic magmas and suggests specific observations to be made on granitic rocks that aid in the interpretation of the initial magma. Stabilities of Hydrous Silicates The relative positions of dehydration reactions of amphiboles and biotites are shown in Figures 1 and 2 (see also ERNST, 1968). These data are taken from the references listed in Table 1, and are rephrased in log fH2O and 1/K in Figure 1, using the data of BURNHAM et al. (1969) for the properties of H2O. The fundamental reaction for each of these curves is: R2+(OH)2(amphibole, biotite) =R2+(silicate)+H2O As pointed out by HAZEN and WONES (1972, * Paper presented in the Symposium on Granitic Magmatism and Associated Mineralization in Japan and Korea, held in Tokyo on January 27, 1981. The manuscript received on March 3; accepted on June 29, 1981. ** Department of Geological Sciences, Virginia Polytechnic Institute and State University, Blacksburg, Virginia 24061 U.S.A.

Field of study: Geology