EMPA MONAZITE GEOCHRONOLOGY: INITIAL EXPERIENCE AND RESULTS FROM A TACONIAN AMPHIBOLITE-GRANULITE METAMORPHIC PROGRESSION, WESTERN NORTH CAROLINA BLUE RIDGE

Chemical dating of monazite (Mnz) by electron-microprobe analysis (EMPA), which assumes that Mnz incorporates no significant common Pb during growth, can resolve spatial resolution of complex zoning superior to that accessible via ion microprobe. U, Th, and Pb are measured, ideally along with a complete Mnz analysis. Background positions must be chosen carefully; wavescans are essential. Accurate on-peak interference corrections on Pb (by La, Th, Y) and U (by Th) also are critical to deriving meaningful dates. One test calculation of on-peak Pb corrections (@510 Ma) indicates that omitting only La and Y interferences may add over 130 Ma.

In the western North Carolina Blue Ridge, an amphibolite-granulite metamorphic progression crosses a premetamorphic-peak terrane boundary without significant disruption, from the western Blue Ridge (WBR) to the central Blue Ridge (CBR). EMPA data from Mnz in 7 samples, which range in grade from Ky zone to granulite facies (GF), require a Taconian metamorphic peak in both the WBR and CBR. Calculated mean ages for individual samples range from 449 to 510 Ma, all Ordovician. From 187 calculated Mnz dates (single spots), 8 outliers were excluded (>2s std dev from sample mean). Remaining dates range from 349-622 Ma (global mean = 476 Ma ±4 {2s std error, n = 179, 1s std dev = 53}). For individual samples, standard errors range from ±8 to ±16 Ma; conservative, counting-error based uncertainties range from 40-65 Ma. Although as yet this limited dataset restricts conclusions, no evidence for multiple populations or growth zoning seems indicated. Higher-grade rocks might be expected to show older dates, as conditions for Mnz formation likely would have occurred earlier, but these data indicate the reverse trend. Only uppermost-amphibolite or GF Mnz might be expected to show "cooling" ages. Perhaps irregular, high-surface-area Mnz may experience partial Pb loss at lower T than typically inferred. This apparent trend of younger dates with increasing metamorphic grade, if upheld, might indicate increased resetting of Taconian dates by younger events (Acadian and/or Alleghanian) to the SE.