PINE MOUNTAIN TERRANE: GEOCHEMISTRY AND GEOCHRONOLOGY OF BASEMENT GNEISSES AND XENOLITHS

The basement of the Pine Mountain terrane (PMT) is characterized by quartzofeldspathic gneisses and metagranitoids, including charnockites.  The relationship of this basement to the Laurentian Grenville provinces has been debated for decades, in part because a comprehensive geochemical and geochronologic database for these rocks has not been developed.  New elemental and Pb and Nd isotopic analyses on whole rock samples of the Whatley Mill gneiss, Woodland gneiss, and several foliated xenoliths drilled from the Woodland gneiss were performed, along with U-Pb and Lu-Hf analyses on individual zircons from these rocks. The data yield important information for deciphering the Mesoproterozoic history of the PMT and its relationship to Laurentian Grenville terranes. U-Pb zircon ages for the Whatley Mill gneiss range from 1024 ± 13 Ma to 1063 ± 14 Ma (2σ).  One sample of Woodland gneiss yielded an age of 1011 ± 12 Ma (2σ).  Zircons from two xenoliths within the Woodland gneiss were analyzed via SHRIMP and yielded two main age populations (²⁰⁷Pb/²⁰⁶Pb ages).  The first is equivalent within error to the age of the Woodland gneiss itself (average of multiple grains = 1018 ± 13 Ma for one xenolith; 1024 ± 10 Ma for another). The second is slightly older (average of nine grains = 1136 ± 13 Ma). A few older single zircons were also identified (e.g., 1678 ± 68 Ma; 1242 ± 62 Ma; 1238 ± 62 Ma (2σ)).  The younger ages (ca. 1020 Ma) are younger than the most recent Grenville-age magmatism in the southern Appalachian Blue Ridge and distinguish the PMT from that region. Whole-rock common Pb data for PMT rocks plot within fields for Appalachian Blue Ridge and Amazonian basement, but above fields for Grenville-age rocks from the Llano region and the Laurentian Grenville sensu stricto. Whole-rock Nd depleted mantle model ages are 1.52 Ga to 1.59 for the Woodland gneiss, 1.32 to 1.39 Ga for the Whatley Mill gneiss, and 1.35 to 1.97 Ga for xenoliths from the Woodland gneiss. Lu-Hf data obtained by laser ablation MC-ICP-MS on zircons also reveal evidence of a limited prior crustal history. There is no evidence in the current data set that any lithospheric component older than Paleoproterozoic contributed to the PMT basement gneisses.