Petrogenesis of Almond Trondhjemite (Alabama): Implications for Southern Appalachian Tectonics.

In southern Appalachians, Paleozoic mountain building commenced at ca. 470Ma and continued until ca. 260Ma. Extensive plutonism accompanied regional deformation and metamorphism. These plutons are cropping out in Alabama and are grouped as Eastern Blue Ridge (EBR) intrusives and Inner Piedmont (IP) intrusives based on the tectonic units they intruded. EBR plutons are felsic in composition and lying in an orogen parallel (NE-SW) trend. Almond Trondhjemite is one of these plutons and contains important information for the tectonic evolution of southern Appalachians and for the crust-mantle interactions during late stages of Paleozoic Neoacadian Orogeny (349Ma).

Our preliminary results show that Almond Trondhjemite is an arc granite with calc-alkaline, peraluminous character and high SiO2 (avg. 72%) content. It consists of biotite, muscovite, quartz, plagioclase with minor k-feldspar and trace amount of garnet, zircon, topaz, magnetite and titanite. Loss on ignition for the whole rock analyses is less than 1% for all samples, indicating sample freshness, a notable challenge for robust Southern Appalachian studies. Major element data and presence of high Al phases suggest S type character. Our samples have strongly positive Pb and Sr, as well as negative Ta, Nb and Ti anomalies on primitive mantle normalized spider diagram. Moreover, they show elevated LREE patterns on chondrite normalized REE diagram and lack negative Eu anomalies. These are characteristics of continental arc magmatism with I type magma, which contradicts whole-rock major element data. High Al content might be explained by high plagioclase content of the source rock. Initial 87Sr/86Sr and ɛNd(T) values at 349 Ma were calculated as 0.7048-0.7054 and -1.9 to -1.2 (n=4), respectively, showing I-type Nd-Sr isotope characters. Electron microprobe analyses on biotite (n=192) yield average crystallization pressure of 3.94 kbar and temperature of 630C°.

These data will be complemented by more samples, 40Ar/39Ar dating of biotite-muscovite andzircon LA-ICP-MS U/Pb ages, as well as zircon trace element data and zircon-hornblende-titanite geothermometers, to better constrain chemical/isotopic character of this intrusion for a more solid understanding of Appalachian subduction dynamics in the area by exploiting tectonism-magmatism interplay.