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A Tall Tale of a Tiny Pluton: The L’Enfer Norite, Grenville Province, Québec
Redefinition of the Petersburg batholith and implications for crustal inheritance in the Dinwiddie terrane, Virginia, USA
Ferruginous Quartzites in the Chopawamsic Terrane, Piedmont Province, Virginia: Evidence for an Ancient Back-Arc Hydrothermal System
Abstract The 295-300 Ma Petersburg batholith in east-central Virginia forms one of the largest and northernmost of the Alleghanian plutonic complexes in the southern Appalachian Piedmont. The batholith is primarily composed of granite including massive and foliated (both magmatic and solid-state fabrics) varieties. The plutonic complex intruded medium-grade metamorphosed volcanic/plutonic rocks of the Roanoke Rapids terrane. The western edge of the batholith experienced right lateral transpressional deformation associated with movement on the Hylas fault zone during the Alleghanian orogeny; this was followed by normal faulting and exhumation during the development of the Triassic Richmond basin. Much of the batholith was buried by a thin veneer of primarily Cenozoic siliciclastic sediments at the western edge of the Atlantic Coastal Plain. Granite rocks of the Petersburg batholith have long been quarried for both dimension and crushed stone. The purpose of this trip is to discuss the age, origin, and tectonic significance of the Petersburg batholith.
Comparative Petrology of the Montpelier and Roseland Potassic Anorthosites, Virginia
Traversing suspect terranes in the central Virginia Piedmont: From Proterozoic anorthosites to modern earthquakes
Abstract The central Virginia Piedmont is underlain by complex igneous and metamorphic rocks, including: Paleozoic, Neoproterozoic, and Mesoproterozoic rocks of the suspect Goochland terrane; Early Paleozoic rocks of the suspect Chopawamsic arc terrane; Mid-Paleozoic successor basin deposits; and a suite of Taconic and Alleghanian plutons. Terranes are juxtaposed along a network of Late Paleozoic dextrally transpressive high-strain zones. The origin and significance of both the Goochland and Chopawamsic terranes remain a source of debate. The central Virginia Piedmont includes a distinct suite of commercial-grade mineral deposits including rutile-rich anorthosite, pegmatite, kyanite, and slate. The widely felt 2011 Virginia earthquake (M = 5.8) occurred along an unrecognized fault in the central Virginia seismic zone and demonstrates that old Appalachian structures are still active in eastern North America's modern stress field.
The Goochland terrane in the central Piedmont Province of Virginia contains two documented units of Mesoproterozoic age, the State Farm Gneiss (ca. 1023–1046 Ma) and the Montpelier Anorthosite (ca. 1045 Ma). The heterogeneous Maidens gneiss, the most extensive unit in the terrane, has previously been assumed to be Mesoproterozoic. This inferred age was based primarily on the presence of locally preserved granulite-facies assemblages (particularly in metapelites), with granulite conditions assumed to reflect metamorphism accompanying the Grenville orogeny. However, we report new U-Pb zircon results for meta-igneous varieties of Maidens gneiss that indicate Paleozoic (Devonian) crystallization ages. Whole-rock major- and trace-element compositions of these and additional samples suggest intermediate igneous protoliths of high-K, calc-alkaline affinity. Our age determinations clearly demonstrate that the protoliths for these samples were not Mesoproterozoic, and could not have been affected by Grenvillian metamorphism. As an alternative, we suggest that the protoliths were plutonic igneous rocks, reflecting deep-seated emplacement of magmas approximately coincident with high-grade metamorphism in associated rocks during the Acadian orogeny. Whether our age results can be applied to the entirety of the Maidens gneiss is uncertain, but if so, a large portion of the Goochland terrane as currently defined is not Mesoproterozoic.
ÅKERMANITE BREAKDOWN TO A CUSPIDINE-BEARING SYMPLECTITE IN A CALC-SILICATE XENOLITH, KIGLAPAIT INTRUSION, LABRADOR, CANADA
Kyanite Quartzites in the Piedmont Province of Virginia: Evidence for a Possible High-Sulfidation System
Rediscovery of the Mattawa Anorthosite Massif, Grenville Province, Quebec
Nd isotopic compositions for ten samples from the State Farm Gneiss (ca. 1046–1023 Ma), the Montpelier Anorthosite (1045 Ma), and several Neoproterozoic Atype granitoids (ca. 600 Ma) in the Mesoproterozoic Goochland terrane range in initial ε Nd from −0.4 to +1.3. The A-type granitoids reflect Neoproterozoic rifting of the Goochland terrane, and their isotopic compositions are consistent with a substantial contribution from the State Farm Gneiss, or equivalent crust at depth, in their petrogenesis. Protoliths of the State Farm Gneiss and Montpelier Anorthosite were emplaced at approximately the same time, and their Nd isotopic compositions are the same. Based on our results and age, bulk compositional, and isotopic similarities between the State Farm Gneiss and Mesoproterozoic rocks (Pedlar River and Archer Mountain suites) in the Blue Ridge Province, we suggest that the gneiss and anorthosite represent a coeval anorthosite-charnockite suite. The unusually potassic Montpelier Anorthosite is also isotopically similar to, and the same age as, the alkalic Roseland Anorthosite in the Blue Ridge Province. Depleted mantle model-ages for the State Farm Gneiss and Montpelier Anorthosite range from 1.38 to 1.43 Ga. These ages are similar to those of many other Grenvillian crustal blocks (e.g., Adirondacks, Blue Ridge, Llano uplift) along the eastern and southern margins of Laurentia, which show a strong peak from 1.3 to 1.5 Ga. Petrological, geochronological, and geochemical data obtained thus far from the Goochland terrane are consistent with the view that it represents a fragment of Laurentia.