DATING MYLONITE FORMATION USING MICROPROBE MONAZITE DATA: EXAMPLES FROM THE SOUTHERN APPALACHIANS AND NEW ENGLAND, AND A TIMS-MICROPROBE COMPARISON

In complex orogenic terranes, constraining ages of metamorphic and deformational events by quantitative geochronology is typically difficult. Two examples illustrate the potential utility of microprobe Th-U-total Pb dating of monazites in constraining the ages of metamorphism and ductile deformation. The first is from western NC-eastern TN, where the Devil Fork fault juxtaposes distinct crystalline basement rocks of the western Blue Ridge, ~50 km west of the Grandfather Mountain Window. The fault zone itself is intensely mylonitic and of apparent greenschist-facies grade. The fault may be Taconic (480-460 Ma) or Alleghanian (325-265 Ma) in age, and our goal was to constrain its age. Large monazite (mnz) porphyroclasts (up to 300 microns) lie within mylonitic foliation and have prominent tails consisting of small mnz fragments in a matrix of allanite+apatite+thorite. Highly fractured porphyroclasts commonly show incipient alteration along fractures. Microprobe dating of mnz shows that porphyroclast cores are of Grenville age (ca. 1020 Ma) and ages decrease on grain edges and along fractures. Detailed analysis of small mnz in a mylonitic tail yields ages decreasing from >1000 Ma to 260 Ma, with >35% of the ages <350 Ma. Based on this we conclude that the fault is Alleghanian. Preliminary data from micromilling and TIMS analysis at UNC-Chapel Hill of the same grains dated by microprobe indicates discordant behavior due to lead loss, with a Grenvillian upper intercept and a Late Paleozoic lower intercept, consistent with microprobe dating and an Alleghanian age of the Devil Fork Fault. The second example is from a sil-grade ultramylonite near Sturbridge in central MA that cuts early Acadian (ca. 435 Ma) high-grade (grt-crd-sil-kfs) metapelite gneiss. Based on geology, the mylonite may be either “main-phase” Acadian (400 Ma) or neo-Acadian (360 Ma). Microprobe dating of small mnz “fish” in mylonite yields core ages of 435 Ma and ages of margins and growth tails of 400 Ma, with no younger ages, indicating that the mylonite formed in the “main-phase” Acadian and predated the widespread neo-Acadian event of central MA. Both examples indicate the utility of microprobe mnz dating in elucidating tectonic fine structure of orogenic evolution.