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Marcus C. Tate


Ph. D. Thesis

The Relationship Between Late Devonian Mafic Intrusions and Peraluminous Granitoid Generation in the Meguma Lithotectonic Zone, Nova Scotia, Canada

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Current theories for the generation of voluminous granitoid intrusions state that mafic magmas can provide a localized, external source of heat for crustal melting. Regardless of their age or tectonic setting, many alkaline, metaluminous, and peraluminous granitoid bodies occur with contemporaneous satellite mafic intrusions and show spatial relationships with mafic magmas in the form of synplutonic mafic bodies and mafic igneous enclaves. Physical juxtaposition of the intruded mafic magmas and resulting anatectic granitoid melts also allows for the transfer of chemical components to produce hybridized lithologies by mechanical interaction and/or chemical diffusion. In this multidisciplinary study, identification of all these established criteria for mafic-granitoid genetic relationships, in combination with one-dimensional thermal models of mafic intraplating, allows the assessment of a role for intruded mafic magma as a source of chemical components and, ultimately, heat in a suite of granitoid rocks.

Late Devonian (385-372 Ma) peraluminous granitoid plutons and batholiths in the Meguma Zone of southwestern Nova Scotia crop out with 14 volumetrically minor mafic dykes and plugs that have contemporaneous ages (380-370 Ma 40Ar/39Ar; 376 Ma U-Pb). Four of these mafic bodies are synplutonic intrusions into the Barrington Passage, Shelburne, and Port Mouton plutons, where they formed mingled and commingled mafic pillows at their contacts or produced hybrid tonalites with metaluminous tendencies by chemical diffusion of mobile elements (alkalies, Rb, Ba, Sr) and exchange of Ti, Fe, Ca, and V in plagioclase and ferromagnesian phenocrysts. No granitoids contain enclaves with microtextures resembling the known mafic intrusions or hybrids, but the Barrington Passage, Shelburne, Port Mouton, and Canso plutons have enrichments of Ti, Fe Ca, Cr, and V, coupled with low values for d18O (8.3-10.4‰), d34S (0-5‰), and 87Sr/86Srt=380-370 (0.705-0.710) that suggest mantle chemical input.

These small granitoid intrusions peripheral to the South Mountain Batholith, therefore, show all of the required evidence to support a temporal and spatial genetic relationship between mafic and granitoid magmas, except enclaves. Poor exposure (<2%), and the epizonal exhumation of the Meguma Zone, probably explain the small number of mafic intrusions at the current exposure level. Emplacement of the synplutonic mafic bodies after the cessation of active granitoid convection may account for the lack of enclaves. Numerical models of heat advection above mafic sills intruded into the sub-Meguma basement allow heating from intruded mafic magmas in the protoliths for granitoid plutons occupying peripheral onshore areas of the MZ between circa 385 Ma and 372 Ma, and perhaps also to a lesser extent in centrally located intrusions at circa 372 Ma. Mafic magmatism possibly augmented heat produced by a variety of tectonic processes after the onset of Acadian terrane accretion.

Supervisor: D. Barrie Clarke