MAGMA INTERACTIONS IN A CENTRAL MAINE PLUTON

The Passadumkeag River pluton is a reverse zoned, Devonian granitic pluton located in east-central Maine. Ayuso (1984) divided the pluton into a rim facies (71 to 77 % SiO2) and core facies (67 to 72 % SiO2). The pluton contains dark ovoid 5 to 50 cm enclaves of medium grained diorite (62-63% SiO2) that are more abundant at the core and become less abundant towards the rim.

The host granite contains abundant plagioclase phenocrysts in a matrix of biotite, hornblende, quartz, and K-spar, with accessory sphene, zircon, and Fe-oxides. The enclaves contain more abundant hornblende and biotite phenocrysts in a matrix of plagioclase, and Fe-oxides, with accessory quartz, K-spar, zircon, and sphene. Electron Microprobe analyses of plagioclase grains in the host rock show increases in Ca content from core to rim, whereas in the enclaves they show decreases in Ca content from core to rim.

Whole rock major and trace element abundances were determined via XRF analysis. Major element mass balance calculations show that the enclaves are not genetically related to the host granite. However, a simple mixing model using major elements and trace element partitioning shows that mixing between the enclaves and the most felsic granitic rocks can generate rock of the same composition as the core of the pluton.

Field relations show that some enclaves have sharp contacts with the host rock, while others have gradational boundaries showing almost complete assimilation into the host. The ovoid rather than elongate shapes of the enclaves suggest a minimum of convection in the pluton, further suggesting that diffusion was responsible for enclave-granite homogenization. The range of contact relationships suggests that the enclaves were injected continuously or in multiple stages.

A geologic history of the pluton can be constructed wherein: (1) the granitic body of the pluton was emplaced; (2) an injection of a smaller volume of dioritic magma in the center of the pluton occurred; (3) sufficient time for chemical diffusion in the pluton caused the diorite and host rock to partially homogenize, with the core being more mafic than the rim due to a higher concentration of enclaves, and; (4) cooling of the pluton caused younger injected diorite enclaves to crystallize before homogenization with the host.