A Two-stage Thermal Evolution Model of Magmas in Continental Crust

Abstract

When a basaltic magma is emplaced in a continental crust, a INTRODUCTION silicic magma is generated by melting of the crust. The light silicic When magma is emplaced into continental crust, it is magma forms a separate magma layer with little chemical interaction believed to be stored in a ‘magma chamber’ for a certain with the underlying dense basaltic magma layer. Extensive melting period. Extensive studies have revealed how petrological occurs at the boundary between the silicic magma and the crust and geochemical features of magmas are controlled by while the basalt acts a heat source. The mass and heat transfer at processes in magma chambers such as crystallization of the boundary between the silicic magma and the crust controls the magma, melting of crustal materials, and mixing of thermal evolution of the silicic magma. The thermal evolution of magmas (e.g. McBirney, 1980; Huppert & Turner, 1981; the silicic magma after the basalt emplacement is divided into two Sparks et al., 1984; Koyaguchi & Blake, 1991). This paper stages. In the first stage, the temperature in the silicic magma rises discusses how petrologic processes such as assimilation above and then decays back to the melting temperature of the crust and fractional crystallization affect the thermal evolution on a short timescale (10 years). The results of fluid dynamics and the residence times of magmas in magma chambers experiments suggest that the silicic magma generally has a lower in continental crust. melting temperature than the crust because of fractional crystallization Some exploratory models for the residence time of a and mixing of partial melts during the first stage, and that it can magma chamber in continental crust have been proposed be effectively liquid at the end of the first stage. In the second stage, on the basis of the physics of heat and mass transfer the silicic magma cools slowly by heat conduction on a much longer (Spera, 1979; Huppert & Sparks, 1988a; Marsh, 1989). timescale (10 years). Petrological features of the magma in the To formulate the present problem it is helpful to introduce second stage are strongly constrained by petrological features of the the model by Huppert & Sparks (1988a), in which the surrounding crust as well as those of the supplied magma itself; its physics of crystallization and melting at the chamber roof temperature remains at or just below the melting temperature of the as well as the effects of thermal convection in the magma crust for a long time because of the slow cooling rate; its phenocryst are taken into account. According to their model, a content reflects the difference in the melt fraction vs temperature basalt emplaced into continental crust results in extensive relationships between the magma and the crust. Judging from the melting of the crustal materials at the chamber roof to distinct cooling rate between the two stages, erupted magmas are form a silicic magma. The temperature in the silicic statistically more likely to reflect the characteristics of magmas in magma rises above and then decays back to the fusion the second stage. temperature of the crust (in general ‘the effective fusion