Ben Johnston

Date of Award

12-2001

Level of Access Assigned by Author

Open-Access Thesis

Degree Name

Master of Science (MS)

Department

Geological Sciences

Advisor

Daniel R. Lux

Second Committee Member

David Gibson

Third Committee Member

Martin G. Yates

Abstract

The Coastal Maine Magmatic Province (CMMP) consists of over 100 post tectonic plutons with ages varying from Silurian to Carboniferous. Predominately, plutons are either felsic or mafic with little intermediate material. Several plutons within the CMMP show evidence for direct interaction of contemporaneous mafic and felsic magmas. These are classified as Mafic and Silicic Layered Intrusions (MASLI), a specific group of plutons with characteristics indicative of mafic magma replenishment into silicic magma chambers. While the Maine coast contains several MASLI type plutons, other plutons in the CMMP contain less definitive, more cryptic evidence that suggests interaction of mafic and silicic magmas during petrogenesis. One such pluton is the Oak Point Granite, Deer Isle, Maine. The Oak Point Granite (371± 2 Ma) is a coarse-grained, seriate, red-orange to salmon, hornblende-bearing, rapakivi granite. Evidence for MASLI type processes and mafic replenishment includes composite dikes and magmatic enclaves of variable composition. Magmatic enclaves are widely distributed throughout the pluton, varying in abundance at each outcrop. Three types of enclaves are present in the Oak Point Granite: medium-grain felsic, mediurn-grained dioritic, and fine-grained dioritic. All three groups of enclaves are more fine-grained than the host granite. Of the three, the fine-grain dioritic variety is the most abundant. These have relatively planar, disk shapes. Thickness can range from 5 cm to 60 cm but are most commonly 6-20 cm. Long dimensions are typically 10 to 25 cm, although some exceed 1 m. Field relations show that the enclaves and foliation defined by aligned feldspars have similar orientations, suggesting either magmatic flow or crystal settling within the magma chamber. Field and petrographic investigations conclude that the enclaves formed as a contemporaneous magma with the Oak Point Granite. The fine-grained textures and acicular apatite suggest quenching against the host granite. Inclusions of feldspar megacrysts derived from the Oak Point Granite, variations in textures and zoning of feldspars and relic clinopyroxene cores within some hornblende are consistent with the hypothesis that the enclaves are hybrids between silicic and mafic magmas. Enclaves, aplites, composite dikes and granite samples were analyzed for major and many trace elements. Bivariant variation diagrams produced highly correlated linear trends for major elements (excluding K20). This relationship is interpreted to reflect magma mixing. Interpretation of the data concludes that the mafic component of the composite dikes chemically represents a mafic injection during petrogenesis. This parental magma fractionated producing a residual magma that subsequently mixed with the granitic host. The resultant magma is the origin of the magmatic enclaves. Feldspar megacrysts were incorporated into the enclave magma during this mixing event. The aplite dikes, the felsic component of the composite dikes and the felsic enclaves have similar compositions and are interpreted to represent the felsic end member.

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