Date of Award
Program or Major
Doctor of Philosophy
Wallace A Bothner
The Mesozoic opening of the Atlantic Ocean was associated with the emplacement of subalkalic to alkalic complexes along a linear trend that extends southeasterly from Montreal, Canada, into the Gulf of Maine. The portion of this trend within southwestern Maine consists of Triassic and Cretaceouscomplexes. The central complexes were emplaced within three different lithotectonic blocks or terranes and thus provide a number of unique opportunities for the investigation of Mesozoic anorogenic magmatism.
Proposed terrane boundaries in southwestern Maine and adjacent regions were investigated by a detailed analysis of maps of regional aeromagnetic and gravity data. Significant aeromagnetic anomalies associated with lithologic units and fault zones onshore (i.e. the Nonesuch River, Calef, and Portsmouth faults) provide the basis for offshore fault zone indentification and the interpolation of coastal New England geology and terrane boundaries into the western and west-central portions of the Gulf of Maine. Within the western Gulf of Maine north-south aeromagnetic linears deflect westward, become segmented, and merge with the offshore extension of the Bloody Bluff and Clinton-Newbury Fault Zones. The regional pattern can be explained in terms of regional dextral transpression resulting from the late Paleozoic (Alleghanian (?)) deformation of previously accreted lithotectonic packages to the north and northwest of the Bloody Bluff and Clinton-Newbury fault zones.
Early Mesozoic rift-related magmatism was investigated through a detailed petrogenetic study of the Triassic Agamenticus Complex. Least squares models suggest that the aegirine granite and, possibly, the alkalic granite can be derived by fractional crystallization from a subsolvus augite syenite parent. Trace element modeling suggests that processes other than liquid-crystal equilibrium fractionation, such as volatile fluxing and deuteric alteration, also played a role in the development of the magmas. An alkalic syenite and biotite granite cannot be modeled as either cumulate or fractionate phases from this process and are considered to be separate magmatic pulses derived by partial melting of the lower crust.
The Triassic and Cretaceous felsic complexes in southwestern Maine can be separated on the basis of trace element and potential field data: eg. Triassic complexes are depleted in Sr and Ba and lack the positive aeromagnetic and gravity anomalies associated with the Cretaceous complexes. These data are interpreted to reflect the transition from dominantly crustally derived magmas in the Triassic to mantle derived melts in the Cretaceous. Significantly different petrogenetic processes operating at the beginning and end of the rifting cycle resulted in the emplacment of mantle derived melts at progressively higher levels within the crust as Mesozoic taphrogenesis evolved.
Brooks, John A., "The petrogenesis of the Agamenticus complex and late Paleozoic and Mesozoic tectonics in New England" (1990). Doctoral Dissertations. 1605.