THE SAGA OF A SINGLE SAMPLE: ATTEMPTS TO UNDERSTAND THE GEOLOGIC HISTORY OF A PORTION OF THE MAIDENS GNEISS, GOOCHLAND TERRANE, VIRGINIA

The Goochland terrane represents a puzzling tract of pre-Appalachian crust in the southern Appalachians. It contains two Mesoproterozoic units, the State Farm gneiss (1023-1046 Ma) and the Montpelier anorthosite (1045 Ma). The State Farm gneiss was later intruded by several granitoid plutons in the Neoproterozoic (~600 Ma). The heterogeneous Maidens gneiss, the most extensive unit in the terrane, was also long thought to be Mesoproterozoic, and to have experienced Grenvillian granulite-facies metamorphism. As part of an early attempt to date magmatism in the Maidens gneiss, zircons from a presumed felsic metavolcanic rock (now a pyx-granulite) were analyzed by Bob Tucker at Washington University in St. Louis. Results were too scattered to yield a meaningful date but hinted at a younger, sedimentary origin. Some time later, Bob Tracy at Virginia Tech (reported in Shirvell et al., 2004 GSA abstract) attempted to directly test the timing of high-grade metamorphism in this same sample, and several others, using in-situ microprobe Th-U-total Pb chemical dating of monazite. Results were surprising, yielding late Devonian ages (~373 Ma in this sample). Owens et al. (2010, GSA Memoir 206) then reported Devonian U-Pb zircon ages for several other metaigneous samples. Thus, the Maidens gneiss records mid-Paleozoic magmatism and high-grade metamorphism. To further refine the timing of metamorphism and potentially the maximum depositional of this sample (probable metagraywacke) additional zircons (n=50) were analyzed by LASS depth-profiling at UC Santa Barbara. A set (n=23) of concordant, low Th/U zircon rims yields an age of 359.2 ± 1.8 Ma (MSWD = 2.2), interpreted as the timing of granulite-facies (~700°C, Ti-in-zircon) metamorphism. Core ages span a range mostly from ~430-1800 Ma, but two are Archean (2580, 2791 Ma). Additional LASS analyses on polished grains (n=119) yield a similar range of dates, as well as a linear array (n=6) that yields an Archean upper intercept date of ~2700 Ma. Determining a maximum depositional age is challenging given the close overlap of youngest detrital and oldest metamorphic dates, but ~400-450 Ma is plausible. Regardless, this sample shows a strong Neoacadian metamorphic signature and evidence for diverse sediment sources; both have bearing on the overall history of Goochland terrane.