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

Abstract

Monazite [(Ce,Th)PO 4 ] 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 204 Pb, is enriched in proximity to microcrack regions and correlates with 137 Ba. SIMS 232 Th-208 Pb analysis from grain regions with lowest contents of common Pb ( 208 Pb comprises >99% of Pb isotopes) yields two sets of ages: 263.5؎3.0 Ma (؎1; MSWD ‫؍‬ 1.7; n ‫؍‬ 11) and 234.1؎3.3 Ma (؎1; 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 (ϳ30 km) and age is the Petersburg granite (296.33؎0.11 Ma, zircon 238 U-206 Pb), the emplacement of which coincided with activity along the Hylas Fault. Because the fault experienced a brittle-ductile transition in the Late Permian (ϳ262 Ma), coeval with the older 232 Th-208 Pb 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.

Implications for Thrust‐Related Shortening Punctuated by Extension From P‐T Paths and Geochronology of Garnet‐Bearing Schists, Southern (Çine) Menderes Massif, SW Turkey

“…In general, Çine Massif monazites show elevated amounts of common Pb ( 204 Pb). Low amounts of radiogenic 208 Pb ( 208 Pb*) have been previously reported from Çine Massif monazites (Baker et al, ); this observation has recently been related to monazite Ba content and fluid interactions (Catlos & Miller, ). We include SIMS 137 Ba + /Th + ratios of these monazites from the analyzed spot, which are significantly elevated relative to the standard monazite 137 Ba + /Th + ratio (0.106±0.005 to 2.192±0.049 compared to the average standard value of 2.934×10‐04±1.579×10‐05).…”

“…Reported ages were corrected for common Pb using 204 Pb and an assumed 208 Pb/ 204 Pb=38.34 (Stacey & Kramers, ). In addition to the isotopes for geochronology, we measured 137 Ba + based on a suggested spatial covariation of 137 Ba + with 204 Pb + within regions of the Amelia monazite grain (see Catlos & Miller, ). These measurements were only made during day 2 of the calibration.…”

Speculations Linking Monazite Compositions to Origin: Llallagua Tin Ore Deposit (Bolivia)

“…The analyses were taken using the same standards and approach described in [28]. We followed a sample preparation protocol that would be typical for instruments that require polished monazite grains in a mounted environment (i.e., EPMA, ion microprobe, LA-ICP-MS).…”

“…The amount of F does not vary significantly across the monazite grains ( Figure 4A-C). We exclude the possibility that F is an analytical artifact, as a monazite grain from the Amelia pegmatite was analyzed using the exact procedure and did not detect F, which would be expected if problematic conditions arose with standards or choice of background and peak [28]. Note that using the TAP monochromator, the F Kα line is effectively free of any overlaps among the analyzed elements.…”

“…Because monazite can incorporate a wide variety of elements into its structure, and thus may have variable stoichiometry, generating high quality compositional data from monazite can be challenging [28][29][30]. We apply both electron probe microanalysis (EPMA) and laser ablation-inductively coupled plasma mass spectrometry (LA-ICP-MS) to constrain what elements are present in its structure.…”