The role of cooling rate in the origin of high temperature phases at the chilled margin of magmatic intrusions

Abstract

Both large (i.e. from hundreds to thousands of metres thick) and small (i.e. from centimetres to a few metres thick) magmatic intrusions are characterized by mineral compositional variations proceeding from the outermost to the innermost part of the intrusive body. However, in the case of large intrusions, mineral compositions become progressively more primitive (e.g. An-rich plagioclases and En-rich pyroxenes) from the chilled margin towards the interior; whereas, the opposite occurs for small intrusive bodies. Since it is unclear to what extent variable cooling rate conditions may alter the phase compositions, we have performed isothermal and dynamic experiments within a temperature interval of 1250–1100 °C using four different cooling rates of 150, 50, 10 and 2.5 °C/h. Numerical simulations of thermal regimes in and around small and large magmatic intrusions have also been performed and compared with phase compositional variations observed in our laboratory experiments. Results indicate that, over rapid cooling rate conditions, the crystal compositions faithfully reproduce those of high-temperature formations, i.e. An-rich plagioclases, En-rich pyroxenes and Usp-poor spinels. However, such a process is limited to a maximum distance of 2–3 m from the margin of the intrusion. Moreover, in active volcanic systems, heat fluxes are released from the main regions of magma storage into host rocks; therefore, only magmas solidifying at the contact of cold wall rocks may develop chilled margins with features related to rapid cooling rate conditions. In the presence of hot host rocks, thermal gradients are significantly reduced and the role played by cooling dynamics on textural and compositional variations of minerals is practically negligible.