ELECTRON MICROPROBE AND ISOTOPIC GEOCHRONOLOGY OF SHEARED MONAZITE PORPHYROCLASTS: DOCUMENTATION OF VARIABLE LEAD LOSS AT GREENSCHIST-FACIES CONDITIONS

The Alleghanian Devil Fork Fault in westernmost NC juxtaposes two Grenville-age blocks of granulite-facies rocks. At the time of faulting/shearing, the ambient temperature has been estimated to have been at greenschist facies conditions (<500 °C) and presence of abundant hydrous silicates indicates local availability of aqueous fluid. There is no evidence of any significant thermal disturbance between the Grenville and Alleghanian events. One sample of intensely sheared mafic gneiss in this shear zone contains abundant large (up to 300 micrometer) monazite porphyroclasts with mylonitic tails that align with the mylonite fabric. These tails consist of small (5-50 micrometer) broken monazite fragments surrounded by a reaction mass of allanite, and apatite, with minor thorite or huttonite (ThSiO₄), xenotime (YPO₄) and galena. Monazite Chemical Age (MCA) analysis by electron microprobe of porphyroclast cores, rims, areas adjacent to cracks, and fragments in tails reveals a range in calculated ages from 1070 Ma down to <300 Ma for monazites that contain up to 100,000 ppm Th and 10,000 pm U (and up to 6000 ppm Pb). Pristine cores (away from cracks and rims) typically yield ages from 1050 - 1080 Ma. Among randomly selected tail fragments, about 1/3 of all calculated ages are <350 Ma, indicating substantial lead loss from these fragments. Within larger porphyroclasts, areas adjacent to fractures yield widely variable ages ranging from about 1000 Ma down to about 500 Ma. Contouring of ages calculated from microprobe spots near cracks shows systematic partial lead loss and apparent-age reduction closest to cracks. ID-TIMS analysis of five diamond-milled aliquots of monazite from a large porphyroclast in the polished probe section yields a Discordia line with upper intercept of 1077 Ma (+/- 29) and a lower intercept of 356 Ma (+/- 92). 206/238 ages of the five discordant separates range from 1003 Ma to 805 Ma. The microprobe results from most pristine monazite core areas (mean age of 1075 Ma +/- 6) are in excellent agreement with the upper intercept of the Discordia line. We interpret the results to indicate that monazites can undergo significant recrystallization and at least partial lead loss along both fractures and margins at relatively low temperatures (500 °C or below) in the presence of aqueous fluid.