ION MICROPROBE 232TH/208PB AGES FROM THE HIGH COMMON PB AMELIA PEGMATITE MONAZITE, VIRGINIA: IMPLICATIONS FOR ALLEGHENIAN TECTONICS

Monazite from a pegmatite in the Amelia mining district of the eastern Piedmont of central Virginia has unusually large and variable amounts of common Pb, leading to problematic interpretation of U-Th/Pb ages and speculation regarding its relationship to nearby granite intrusions and faults. Ion microprobe ²³²Th/²⁰⁸Pb analysis from regions on a large, single Amelia monazite grain where ²⁰⁸Pb comprises >99% of Pb isotopes yields two sets of ages: 263.5±3.0 Ma (±1σ; MSWD=1.7; n=11) and 234.1±3.3 Ma (±1σ; MSWD= 0.4; n=13). The monazite has compositional discontinuities along microcracks consistent with secondary zones of recrystallization. These results, interpreted with available regional geochronological and structural constraints, suggest the Hylas Fault, a NE-striking Alleghanian shear zone, played a role in Permian emplacement and Triassic deformation of the Amelia pegmatite. Our Amelia monazite ages are broadly consistent with youngest Appalachian pegmatite bodies, interpreted as intrusions during the final (Alleghanian) stages of Laurentia-Africa collision. However, among locally surrounding dated intrusives, only the large (~1400km²) Petersburg granite could be coeval. Because Petersburg granite emplacement occurred in pulses that coincided with Hylas Fault activity, pegmatite belt development may have been syntectonic with fault motion. The fault experienced a brittle-ductile transition reported elsewhere coeval with the oldest Amelia Th-Pb monazite ages, consistent with a switch in plate motion as the structure changed from transpression to transtension, or an extensional overprint. During the structural transition, the Hylas Fault may have operated as a fluid migration system from the Petersburg granite to the Amelia pegmatite belt. The younger Amelia monazite age is consistent with Triassic normal (or possible sinistral) faulting linked to lower temperature development and deformation of local rift basins.