Abstract

For some plutons, periodic replenishment is required to explain trends in magmatic differentiation. The Houghton pluton, located in the northern Abitibi Greenstone Belt, contains breccias (interpreted as feeder dykes) that document the periodic replenishment of the chamber and the expulsion of the interstitial liquid between the enclaves. Interpretation of the textures in these breccias provides a mechanism for efficient magma extraction (which maybe applicable, for example, to partially molten rocks). There are two types of breccia: (i) polymictic breccias made of various enclaves with an interstitial leucotonalite of 5–20% and (ii) foliated block breccias made of foliated tonalite and deformed hornblendite enclaves with an interstitial leucotonalite of 0–5%. Dykes ending in enclaves, enclaves indented together and separated afterwards, and enclaves rimmed by a magma different from the interstitial one all argue in favour of periodic replenishment. Magma expulsion is explained by compaction of enclaves until they touch one another and deformation of some enclaves or borders of some enclaves. If the composition is appropriate and the nonhydrostatic stress is large enough, solution–precipitation and (or) stress-induced mass transfer can further increase the proportion of enclaves. It appears possible to expell all the liquid from an enclave–liquid mixture, contrary to the prediction of theoretical studies. These textures and their interpretation can also be applied to melt extraction in partially molten rocks.

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