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U/Pb Zircon Evidence for a Cryptic Suture in the Goochland Terrane, Virginia
Duke, Hope
Duke, Hope
Abstract
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 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 with discontinuous bands of amphibolite and a widespread but heterogeneous gneissic unit (Maidens Gneiss Group), 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 and U-Th/He zircon ages from four of those five samples in the northern GT using LA-ICP-MS at the UTChron laboratory. Zircons from the Montpelier 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 core of the North Anna dome yields 1.0-1.1 Ga core ages and ~300 Ma rim ages. A paragneiss in the cover sequence to the 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. We propose the high-grade cover sequence was emplaced over Mesoproterozoic rocks during the Acadian orogeny, and a previously unrecognized suture frames the GT domes.
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2019-04-01
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Geology