Geology of the Bottle Lake Complex, Maine

Abstract

The Bottle Lake Complex is a Paleozoic composite granite emplaced near the core of the Merrimack synclinorium in east-central Maine. The two plutons exposed within the Complex are reversely zoned in their mineralogy, major and trace element compositions. Each intrusive exhibits characteristic features including the abundance of amphibole and mafic xenoliths as in the Passadumkeag River granite, and relative abundance of biotite, aplites, and pegmatites in the granite of Whitney Cove. Both display geochemical variations identifying the respective core facies as the least differentiated zones compared to the rims. On the basis of field relations the relative sequence of intrusion is established between the two intrusives, but geochronologic studies using Pb-Pb zircon ages do not display a clear age difference. The granite of Passadumkeag River is, as judged by the field relations, the youngest pluton in the Complex. This granite is also the most heterogeneous body as expressed by petrography, mineral chemistry, and bulk composition. The numerous mafic xenoliths enclosed by the Passadumkeag River pluton display textural, mineralogical, and geochemical variations distinct from those found in the host granitoid rocks.

The source of the Bottle Lake Complex was heterogeneous in composition as suggested by the petrology of the granites and xenoliths, and by the wide range in ²³⁸U/²⁰⁴Pb of the batholith. Fractional crystallization accounts for most of the variation within each pluton, although other processes such as mixing of liquids must also be postulated. In the case of the Whitney Cove pluton, mixing occurred prior to the major stage of fractionation. The granite of Passadumkeag River, however, records the mixing of a different batch of liquid during fractionation. Comparison of the geologic relations in the granites of east-central Maine, emplaced across the Merrimack synclinorium indicates that a major discontinuity in the sources of granitic liquid exists on either side of the Norumbega fault system. The Bottle Lake Complex is a distinctive batholith representative of the plutons emplaced in the core of the synclinorium. Together with the nearby Center Pond granite they form a distinct subgroup in comparison to the rest of the plutons on the northside of the Norumbega fault. The lead isotopic compositional variation across the synclinorium is unlike the documented changes present in complex subduction systems such as the Sierra Nevada. This is in agreement with previous geologic studies which suggested an absence of features characteristic of destructive tectonic margins in this area of New England. Granite generation across the synclinorium occurred in a short time interval probably by melting volcaniclastic materials on the north side of the fault, and by melting cratonic detritus on the opposite side.