PALEOZOIC (NOT MESOPROTEROZOIC) HIGH-GRADE METAMORPHISM IN THE GOOCHLAND TERRANE, VIRGINIA: NEW RESULTS FROM ELECTRON MICROPROBE DATING OF MONAZITE

The heterogeneous Maidens Gneiss forms the largest component of the Goochland Terrane in the central Piedmont Province of Virginia. Although strongly overprinted by younger amphibolite-facies metamorphism, it locally preserves evidence for an older granulite-facies event. This high-grade event has long been assumed to have occurred during the Grenville Orogeny, and that the entire terrane is therefore Mesoproterozoic. To test this assumption, we analyzed monazite grains from three widely-spaced samples of Maidens Gneiss using in-situ electron microprobe Th-U-total Pb chemical dating. This technique is ideally suited to address this question because monazite can preserve a record of multiple metamorphic events, and is difficult to completely reset. Two of our analyzed samples preserve their high-grade assemblages, including: 1) a sill + K-spar metapelite; and 2) a bio-bearing pyx-granulite. The third sample is a heterogeneous bio-rich gneiss, possibly migmatitic. A total of 223 analyses were obtained on multiple traverses across three grains (~100-300 microns diameter), one grain from each sample. Results for two samples are similar, with cores that yield ages from ~360-420 Ma, and thin, younger (~260-320) rims. Statistical evaluation of results indicates core ages of 387 ± 2 Ma and 377 ± 5 Ma, and rim ages of 293 ± 5 Ma and 289 ± 23 Ma, respectively. Results for the third sample are more scattered, but most core ages span the same age range (~350-416 Ma). This grain contains numerous cracks, which may have facilitated later disturbance of the U-Th-Pb system. Significantly, no ages older than ~420 Ma were found in any grain. Collectively, these results strongly suggest that the granulite-facies event in the Goochland Terrane was not Grenvillian, unless the U-Th-Pb system was completely (and similarly) reset in each monazite grain. We consider the latter interpretation highly unlikely, especially because two samples show minimal evidence for retrogression. Rather, the core ages seem to reflect high-grade “Acadian” metamorphism, with rims recording recrystallization and overgrowth during the Alleghanian. The location of the Goochland Terrane during this high-grade event is uncertain, but we speculate that it may have been much further to the north (where Acadian effects were more pronounced), and was subsequently transported to the south during the Alleghanian.