U-PB ZIRCON EVIDENCE FOR A CRYPTIC SUTURE IN THE GOOCHLAND TERRANE, VIRGINIA

The Goochland Terrane (GT) forms a distinctive and enigmatic sequence of igneous and metamorphic rocks in the east-central Piedmont of Virginia. Some workers argue that the GT is a block of Laurentian crust, whereas others posit it to be an exotic terrane with a Gondwanan affinity. The composite terrane includes Mesoproterozoic granitoid gneiss and meta-anorthosite exposed in a series of elongate domes. Neoproterozoic A-type granitic plutons intrude the older granitoid gneiss. The domes are structurally overlain by a cover sequence of discontinuous bands of amphibolite and a widespread but heterogeneous gneissic unit (Maidens gneiss), some of which preserves a granulite facies assemblage. Previous workers infer the contact between the domes and cover sequence to be an unconformity or an intrusive contact.

We obtained U/Pb zircon ages from five samples in the northern GT using LA-ICP-MS at the UTChron laboratory. Zircons from the Montpelier meta-anorthosite indicate a crystallization age between 1.0-1.1 Ga with metamorphic pulses at 600-650 Ma and 275-350 Ma. Granitic gneiss in the North Anna dome yields 1.0-1.1 Ga core ages and ~300 Ma rim ages. A kyanite/K-feldspar-bearing paragneiss in the cover sequence, just west of the North Anna dome, includes two populations: a detrital component with ages from 770 Ma to 1.7 Ga and metamorphic rims with ages of 400-450 Ma. A felsic gneiss in the Hylas Zone at the eastern edge of the GT yields two age populations at ~450 Ma and ~300 Ma. A deformed pegmatite i­n the Spotsylvania high-strain zone, at the western margin of the GT, yields two age populations at 400-475 Ma and 300-375 Ma.

Based on these zircon age populations, we argue that Mesoproterozoic rocks in the GT could not have been in contact with the Maidens cover sequence until after 400 Ma. We propose that the high-grade cover sequence was emplaced over Mesoproterozoic rocks during the Acadian orogeny. The GT domes are framed by a previously unrecognized suture, and they form tectonic windows that developed during transpressional deformation of the entire GT in the Alleghanian.