A COMPARISON OF ZIRCON AND MONAZITE U-PB AGES FROM THE ROLESVILLE BATHOLITH, NC: LESSONS FROM MISBEHAVING MINERALS

Among the NE-trending discontinuous chain of Pennsylvanian-Permian granites that stretch along the Appalachians is the Rolesville batholith. Ironically, the batholith comprises the largest group of Alleghanian plutons (1710 sq km) and yet has remained undated. The Rolesville is a composite body of texturally, mineralogically, and geochemically distinct facies (from N-S): 1) Louisburg pluton, a medium- to fine-grained biotite granite; 2) Rolesville main phase, a medium- to fine-grained biotite granite; 3) Archer's Lodge pluton, a megacrystic biotite granite. Monazite and zircon U-Pb geochronology was conducted on these three phases of the undeformed batholith. From the batholith's main phase, four zircon fractions of 5-6 crystals each were analyzed and yielded an upper intercept age of 297.6±3.2 Ma (MSWD=0.8). Eight single monazite crystals were also analyzed from the phase and yielded a range of 207Pb/235U dates from 286-270 Ma, clustering at ca. 286 Ma. For the Louisburg pluton, a concordant zircon analysis and three discordant analyses yielded an upper intercept age of 298.8±4.3 Ma (MSWD=0.4), indistinguishable from the main Rolesville phase. Five single monazite crystals yielded a weighted mean 207Pb/235U date of 294.6±1.0 Ma. In both phases and mineral systems, no evidence for inheritance exists. However, three zircon fractions of the Archer's Lodge pluton lie on a chord between 301 Ma and ~650 Ma, suggesting a significant xenocrystic component is present in the granite. Together with earlier published geochemical data, these results indicate that the Rolesville batholith was derived from variable source materials but constructed during a single pulse of magmatism at ca. 298 Ma, likely due to deformation-driven anatexis during Alleghanian activity. In both phases in which monazite was analyzed, the dates are erroneously younger than the true crystallization age, in some instances by as much as 30 m.y. Although the Louisburg pluton yields consistent monazite dates, these are still apparently problematic. These results further demonstrate the need for caution when interpreting monazite crystallization dates, even from undeformed igneous rocks. Elemental mapping of single monazites may provide useful information as to the nature and extent of mineral alteration.