GEOCHEMICAL MODELLING OF MAGMA MIXING IN THE MOUNT WALDO PLUTON, COASTAL MAINE

The Mount Waldo Pluton (MWP), 371 Ma, is one of the Late Devonian plutons that comprise the younger phase of magmatism of the Coastal Maine Magmatic Province. It is a coarse-grained, porphyritic to equigranular, biotite ± hornblende granite, that contains a range of enclaves from intermediate diorites and quartz diorites to more felsic compositions. Schlieren are common and they contain abundant accessory minerals (zircon, allanite and apatite). Plagioclase mantled K-feldspar (rapakivi), plagioclase zones with spikes in anorthite content, and other disequilibrium textures are also common and suggest that the MWP received inputs of more mafic magma as it crystallized. These thermal and physical additions produced a dynamic, convecting magma chamber in which magma mixing occurred.

Geochemical data for the MWP, including the granite and aplites, intermediate to felsic enclaves and a mafic dike, display linear trends on major element Harker diagrams, also indicative of magma mixing. To further test this hypothesis we utilized our extensive data set to model the composition of the intermediate hybrid enclaves. The data suggest that the compositionally intermediate enclaves were formed by mixing variable amounts of mafic magma, as represented by the mafic dike, with a more felsic magma compositionally similar to the leucocratic enclaves. These may in fact represent autoliths that formed early in the crystallization history of the MW magma, before the onset of magma mixing and/or fractionation.