The petrogenesis of Late Devonian peraluminous and metaluminous granitic plutons associated with these mafic rocks across central/coastal New England is problematic because the compositions of the mafic plutons and lavas from Maritime Canada to the Narragansett Basin indicate that the region was undergoing extension during this time period, yet the felsic rocks lack the within-plate compositions of granitic rocks of this age in central New England. A transtensional zone along dextral transcurrent faults allowed the formation of the Maritimes Basin of Nova Scotia and New Brunswick and the eruption of the 373 ± 4 Ma Fisset Brook Formation into the basin, as well as the eruption of similar-aged rocks in the Narragansett Basin. Coastal New England and Maritime Canada experienced extensional magmatism resulting from oblique convergence of Meguma accreting to Laurentia during the Neoacadian Orogeny. The mafic portions of these plutons are compositionally similar to within-plate tholeiites of the same age in Maritime Canada, the Narragansett Basin in Rhode Island, and the Coastal Maine Magmatic Belt. The ages of these plutons range from 380 to 360 Ma. Several Late Devonian plutons across southeastern New Hampshire, coastal Maine, and north-central Massachusetts have within-plate compositions distinct from the 400–410 Ma calc-alkaline plutons related to the Acadian Orogeny in the same region. The proposed model involving magma mixing at shallow crustal levels in a cryptic silicic-mafic magma chamber during post-Acadian extension is consistent with models for other better exposed occurrences of rapakivi granite in the northern Appalachian orogen. The chemical and isotopic (Sm-Nd) signature of the Margaree pluton is consistent with melting of pre-existing continental crust that was enriched in heat-producing elements, likely assisted by intrusion of mantle-derived mafic magma during Late Devonian regional extension. Magma evolution was controlled by fractionation of quartz, K-feldspar, and Na-rich plagioclase in molar proportions of 0.75:0.12:0.13. The megacrystic and rapakivi textures are attributed to thermal perturbation in the magma chamber caused by mixing of mafic and felsic magma, even though direct evidence of the mafic magma is mainly lacking at the current level of exposure. The rare earth elements display parallel trends characterized by enrichment in the LREE, flat HREE, moderate negative Eu anomalies, and in some cases positive Ce anomalies. The three rock types have similar U-Pb (zircon) ages of 363 ± 1.6 Ma, 364.8 ☑.6 Ma, and 365.5 ± 3.3 Ma, consistent with field and petrological evidence that they are coeval and comagmatic. The pluton consists mainly of coarse-grained megacrystic syenogranite, intruded by small bodies of medium-grained equigranular syenogranite and microgranite porphyry, all locally displaying rapakivi texture. The Margaree pluton extends for more than 40 km along the axis of the Ganderian Aspy terrane of northern Cape Breton Island, Nova Scotia. The thermal, chemical, and physical gradients initiated by the injection of mafic material produced not only the array of mixing textures observed in the MWG but also the large-scale convection needed to distribute these features throughout the pluton including its highest levels. Although very little mafic material is observed in the MWG at its present level of erosion, abundant textural evidence suggests that mixing occurred at depth in this intrusion. Specifically, abundant mafic clots, acicular apatite, inherited megacrysts of all the feldspar varieties common in the host granite, and titanite-plagioclase intergrowths are all observed. The MWG contains feldspars of various textures and composition, including abundant plagioclase-mantled K-feldspar (rapakivi texture), boxy and spongy cellular plagioclase, and plagioclase that show An spikes and/or central cores of higher Ca content, in addition, the magmatic enclaves also preserve textural evidence for mixing and hybridization.
It is a coarse-grained, seriate to porphyritic, biotite ± hornblende, titanite-bearing granite with abundant intermediate to felsic magmatic enclaves and less abundant mafic schlieren. The Mount Waldo granite (MWG), dated at 371 ± 2 Ma, is typical of the younger Devonian intrusions of the coastal Maine magmatic province.
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