ELECTRON MICROPROBE U-TH-PB GEOCHRONOLOGY OF MONAZITES IN CONTACT METAMORPHOSED ALUMINOUS ROCKS OF THE MARTINSVILLE INTRUSIVE COMPLEX, SMITH RIVER ALLOCHTHON, VIRGINIA

A dataset of approximately one hundred fifty microprobe analyses has been collected from monazite grains in two samples from the contact aureole of the Rich Acres Gabbro of the Martinsville Intrusive Complex (MIC), southern Virginia. Sample 1 is a dense oxide-rich rock with the assemblage monazite-zircon-magnetite-ilmenohematite-quartz- sillimanite-corundum, and may be either a metamorphosed heavy-mineral sediment or a contact-metasomatized emery occurrence. Sample 2 is from a Fork Mountain Formation locality a few meters from gabbro which has been heated to T well in excess of 700 C. Formerly a pelitic schist, this rock is now a coarse migmatitic gneiss with a contact-metamorphic assemblage of quartz- Kfeldspar-muscovite-plagioclase-biotite-garnet-cordierite-monazite. Our monazite U-Th-Pb analytical program was aimed at two goals: (1) To assess degree of preservation of any pre-contact, Taconic regional- metamorphic ages (ca. 475 Ma); and (2) To determine the age of contact metamorphism, and thus of intrusion, of the MIC. Analyses were done on a Cameca SX50 using 15kV and 200 nA conditions on 1-2 micron spots with REE and Pb phosphates and U and Th oxides as standards. Counting times were 750 sec on peaks and 350 sec on backgrounds. In analyzed monazites, mean Th content was 56000 ppm (49000-65000), mean U was 1900 ppm (930-4200) and mean Pb was 1208 ppm (900-1350). Analyses were collected in traverses across 50-100 micron monazite grains.

We found little or no evidence of any preserved Taconic ages, although about five to ten analyses from the dataset yielded ambiguous ages older than 450 Ma. However, most analytical points yielded calculated contact-metamorphic ages averaging 433 Ma, with a 1-sigma of 12.1 Ma based on population statistics and a 2-sigma of 8 Ma based on analytical counting statistics. We therefore conclude: (1) Contact heating in both samples was sufficient to produce virtually 100% recrystallization and age-resetting of monazites to a contact-metamorphic age; (2) the age of the contact-heating event is 433 (+/- 12) Ma, and this is likely the time of intrusion of the MIC.