Ion microprobe 232Th-208Pb ages from high common Pb monazite, Morefield Mine, Amelia County, Virginia: Implications for Alleghanian tectonics

Abstract

Monazite [(Ce,Th)PO4] from a pegmatite in the Morefield Mine of the eastern Piedmont of central Virginia has unusually high and variable amounts of common Pb, leading to problematic interpretations of its U-Th-Pb ages and how the monazite relates to nearby granite intrusions and faults. To address these issues, we analyze a single large monazite grain from the pegmatite using electron microprobe analysis (EMPA, n = 64), laser ablation-inductively coupled plasma mass spectrometry (LA-ICP-MS, n = 58), and secondary ion mass spectrometry (SIMS, n = 59). The monazite study grain exhibits compositional variations in proximity to microcracks consistent regions of secondary alteration and recrystallization. Although the compositions of these regions fit the ideal stoichiometry of monazite, they have lower Si, Th, U, and Y, and higher P, rare earth element (REE), and Ca concentrations compared to visibly unaltered portions of the grain. LA-ICP-MS and SIMS analyses demonstrate that common Pb, as proxied by 204Pb, is enriched in proximity to microcrack regions and correlates with 137Ba. SIMS 232Th-208Pb analysis from grain regions with lowest contents of common Pb (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). Regionally, the ages are similar to the youngest Appalachian pegmatite bodies emplaced during the terminal (Alleghanian) Laurentia-Africa collision. However, the monazite ages are younger than locally surrounding intrusions. The closest intrusive in distance (∼30 km) and age is the Petersburg granite (296.33±0.11 Ma, zircon 238U-206Pb), the emplacement of which coincided with activity along the Hylas Fault. Because the fault experienced a brittle-ductile transition in the Late Permian (∼262 Ma), coeval with the older 232Th-208Pb monazite age group, we postulate that the Hylas Fault may have operated as a fluid migration system between the Petersburg granite and Morefield Mine pegmatite. The younger monazite age coincides with Triassic normal and/or sinistral faulting linked to the development and deformation of local rift basins.