AN ALTERNATIVE MODEL FOR THE ORIGIN OF THE BLAKE'S FERRY PLUTON, ALABAMA

Plutonism in the Eastern Blue Ridge (EBR) province of Alabama spans two major contractional orogenies associated with Appalachian mountain building. These plutons intruded the EBR prior to and following regional contraction, thus providing insights into the timing and processes involved in deformation and magmatism. The Blakes Ferry pluton is a post-kinematic pluton located in the northern Alabama Piedmont, north of the Brevard Zone. It is a leucocratic, medium-grained, hypidiomorphic equigranular trondhjemite composed of oligoclase, quartz, biotite, and muscovite. It is largely homogenous in composition with the exception of biotite schlieren dispersed throughout. Geochemically, trondhjemites of the Blake's Ferry pluton are magnesian, calcic to calc-alkalic and peraluminous with SiO2 contents ranging from ~69-77 wt.%. Trondhjemites are also classified as high Sr/Y, consisting of high Na (>4.0 wt.%), Al2O3 (typically >15 wt.%), and Sr (>900 ppm), but low Y (<2 ppm), Cr (< 1 ppm), Ni (< 3 ppm) and Rb/Sr (<0.1). Rare earth element patterns display steep fractionation (La/Lu >178), and little to no Eu anomalies. The composition of these trondhjemites is consistent with partial melting of a plagioclase-poor to absent, hornblende + garnet-bearing source at depths >40 km. Previous models have proposed that the Blake's Ferry pluton formed from partial melting of an ecolgitized subducting slab of MORB composition (Defant et al., 1988). We propose an alternative model in which the post-tectonic Blake's Ferry pluton formed after Acadian contractional deformation, perhaps due to thermal relaxation of orogenically thickened lower-arc crust. In this scenario, collision between the EBR and the Inner Piedmont resulted in crustal thickening of the EBR to >40 km depth. This fundamental change in crustal structure influenced subsequent post-orogenic magmatism, resulting in a suite of post-tectonic, high Sr/Y plutons and dikes which occur throughout the Southern Appalachians from Alabama to North Carolina.