GEOCHEMISTRY OF ENCLAVES AND SCHLIEREN FROM THE MT. WALDO PLUTON, MAINE;IMPLICATIONS FOR MAGMA CHAMBER PROCESSES

Magmatic enclaves and schlieren are nearly ubiquitous in Siluro-Devonian felsic plutons from the Coastal Maine Magmatic Province. Although volumetrically insignificant, these rock types constitute important sources of information regarding the evolution of these granitic magma chambers. The Mt. Waldo granite is texturally variable and contains a diverse population of mafic to felsic, fine- to medium-grained magmatic enclaves along with planar mafic layers (schlieren).

Preliminary geochemical data for granitic rocks (including textural variants and aplite dikes), schlieren and enclaves reveal the following: 1) The granitic rocks have highly evolved chemistries with limited major and trace element variation (e.g. SiO₂ 70.1-76.1%, TiO₂ 0.16–0.61%, total Fe₂O₃ 1.2–3.7%, Zr 105–325 ppm, Sr 90–210ppm, Rb 190–270ppm). 2) Enclave compositions overlap those of the granites, but typically are less evolved and intermediate in composition (SiO₂ 60.5–70.4%, TiO₂ 0.5–1.2%, total Fe₂O₃ 2.9–7.6%, Zr 220-458ppm, Sr 147-284ppm, Rb 186-287ppm). 3) The most mafic enclave and a mafic dike within the pluton are geochemically similar. 4) Variation diagrams show nearly continuous trends of major and trace element variation from the least evolved enclaves, through the granites to highly evolved aplites. 5) Schlieren have intermediate compositions (SiO2 ~ 60 wt.%) and high trace and REEs abundance (Zr ~ 1000ppm, Nb ~70ppm, Ce ~700ppm and La ~300ppm) consistent with their distinctive accessory phase mineralogy.

Geochemical data, and field/petrographic evidence, support definite roles for magma mixing (involving mafic replenishments) and hybridization (enclave formation) during crystallization of the Mt. Waldo magma chamber. However, the close agreement between U-Pb zircon ages (371±2Ma; Stewart et al., 1995) and an Rb-Sr whole-rock isochron (376±4Ma; IR=0.70534±28) may limit the extent of such processes. The presence of cumulates (schlieren) suggests crystal-liquid fractionation also had a role in producing the geochemical variation. Detailed mineralogical studies (in progress) will be used to constrain contemporaneous fractionation and magma mixing/mingling processes necessary to model the evolution of this dynamic magma chamber.