Petrogenesis and Tectonic Implications of Almond Trondhjemite of Southern Appalachians in Alabama

Abstract

In the southernmost Appalachians, Devonian-Carboniferous Eastern Blue Ridge (EBR) syn/post-orogenic granitoids are examined in two groups, high Sr/Y and low Sr/Y granitoids. The Almond Trondhjemite (AT) is a part of these Alleghenian granitoids, and it shows high Sr/Y ratio, as well as other adakitic geochemical characteristics, and intrudes Wedowee Group metasedimentary rocks. Its parent melt was proposed to result from heating of lower crust by upwelling asthenospheric mantle via delamination. The existing data on AT is neither complete nor statistically robust to support this interpretation. New whole rock geochemistry data was complemented by critical novel data such as Sr-Nd isotope systematics, single mineral chemistry and 40Ar/39Ar thermochronology to solidify interpretations on its source, emplacement, and post crystallization history in the context of Paleozoic-Mesozoic tectonism in Southern Appalachians. The AT is a weakly foliated, I-type peraluminous granitoid that consists of quartz+plagioclase (mostly oligoclase)+muscovite+biotite with trace amounts of orthoclase+zircon+apatite and secondary epidote+calcite. Sr-Nd isotope systematics, whole rock geochemistry and mineral chemistry support previous interpretations of a lower crustal source. Grenville orogenic affinity of this crustal source was proved with Sr-Nd isotopes. However, the lack of associated mantle related rocks in the vicinity argues against asthenospheric involvement in generation of the AT. Biotite crystallization temperatures show that the magma was fluxed with water during final stages of crystallization. Muscovite 40Ar/39Ar thermochronology suggests that cooling of the AT is controlled regional cooling, possibly on the footwall of a core complex-style exhumation.