DETAILED GEOLOGIC MAPPING, WHOLE ROCK GEOCHEMISTRY, AND U-PB ZIRCON GEOCHRONOLOGY IN THE HYLAS HIGH-STRAIN ZONE, VIRGINIA EASTERN PIEDMONT

New geologic mapping, geochemistry, and geochronology in the Hylas 7.5-minute quadrangle indicate that the Hylas high-strain zone (HHSZ) contains a suite of Silurian–Devonian meta-igneous to meta-volcaniclastic sedimentary rocks in the Virginia eastern Piedmont. This highly strained, dextrally translated package is bounded by the Mesoproterozoic–Paleozoic Goochland terrane on its west along the Hylas fault. On its east, the HHSZ is bounded by the Triassic Richmond basin along a major normal fault and by the mid to late Paleozoic Dinwiddie terrane. The map units of the HHSZ in the Hylas quadrangle include mylonitic metadacite (mym), mylonitic metafelsite (myf), ultramylonite/cataclasite (umc), and protomylonitic granitoid gneiss (myg). These lithologies have rhyolitic to dacitic compositions based on whole rock geochemistry, which may suggest that the HHSZ contains a suite of island arc rocks that are part of a previously unrecognized composite terrane in the eastern Piedmont.

Three samples were collected for LA-ICP-MS U-Pb zircon geochronology. Sample R-12028 (mym) yielded a unimodal age distribution of ca. 450–390 Ma and has a weighted mean age of 417.6 ± 5.0 Ma (MSWD = 0.65). This age represents the crystallization age for this sample. Whole rock geochemistry indicates a dacitic protolith. Sample R-12029 (umc) yielded a unimodal age distribution of ca. 440–402 Ma. The weighted mean age is 423.8 ± 3.7 Ma (MSWD = 1.26) and is interpreted to be the crystallization age of its granodioritic protolith. R-11989 (umc) yielded a multimodal age distribution of ca. 1880–370 Ma and may be a meta-sedimentary rock. This is a tentative interpretation given the older ages may be inherited zircon grains. The maximum depositional age is determined to be 435.7 ± 4.0 Ma (MSWD = 0.93). The R-11989 protolith may be a lithic wacke associated with the calc-alkaline magmatism that formed the other intermediate composition meta-igneous rocks in the HHSZ. The umc map unit remains undifferentiated given its structural complexity and limited exposure and is interpreted to be a package of meta-igneous and meta-volcaniclastic sedimentary rocks. The myf and myg map units were not dated in this study. Future geochronology and detailed mapping are needed to correlate or distinguish the HHSZ rocks from surrounding terranes in the eastern Piedmont.