THE ROLE OF MAFIC INPUTS IN THE EVOLUTION OF FELSIC MAGMA CHAMBERS, EVIDENCE FROM THE DEER ISLE AND MT WALDO PLUTONS, COASTAL MAINE

The episodic injection of mafic magma into crystallizing felsic magma chambers is well documented from the coastal Maine magmatic province. For example, the Pleasant Bay and Vinalhaven plutons display the spectacular field relations typical of magma mingling of compositionally diverse, but coeval magmas. Such physio-thermal perturbations played a major role in setting up dynamic conditions within these magma chambers and greatly influenced the nature of the resultant intrusions. We examine here the role mafic injections may have had in the evolution of the Deer Isle (DI) and Mt. Waldo (MW) plutons. They are representative of the younger granitic suite of the province. Mafic rocks are important although volumetrically minor in both plutons at their present erosion level. The DI and MW plutons display internal textural variations, which enable them to be subdivided into distinct textural facies. Their field relations document the evolving nature of these magma chambers. In addition, both granites display convincing evidence that mafic injections also interrupted their crystallization history with significant influence on magma chamber dynamics. These are: 1) The juxtaposition of a compositionally diverse feldspar population. 2) Disequilibrium textures that indicate magma mixing/mingling processes (Hibbard, 1995). 3) Abundant mafic microgranular enclaves of intermediate composition, which are distributed throughout the plutons and represent hydrids. 4) Mafic schlieren, composed of ~ 10% accessory phases, which often display trough structures. 5) Linear trends on major and trace element Harker diagrams. The spatial distribution of the facies in the DI pluton suggests they originated within a zoned magma chamber with the development of basal cumulates, a transition area of texturally variable granite and an evolved felsic cap. The MW pluton lacks direct evidence of basal cumulates but its textural facies correlate with the transition zone and felsic cap in the DI pluton. The thermal, chemical and physical gradients initiated by mafic injections created not only the conditions necessary for mixing textures to form but also the large scale convection needed to disrupt and distribute cumulate fragments, feldspar megacrsysts, schlieren and the microgranular enclaves around the magma chamber.