U-Pb zircon geochronology and field relations provide insights into metavolcanic and associated rocks in the Central Appalachian Piedmont of Maryland and northern Virginia. Ordovician ages were determined for volcanic-arc rocks of the James Run Formation (Churchville Gneiss Member, 458 ± 4 Ma; Carroll Gneiss Member, 462 ± 4 Ma), Relay Felsite (458 ± 4 Ma), Chopawamsic Formation (453 ± 4 Ma), and a Quantico Formation volcaniclastic layer (448 ± 4 Ma). A previously dated first phase of volcanism in the Chopawamsic Formation was followed by the second phase dated here. The latter suggests a possible source for contemporaneous volcanic-ash beds throughout eastern North America. Dates from the Chopawamsic and Quantico Formations constrain the transition from arc volcanism to successor-basin sedimentation. Ordovician metatonalites of the Franklinville (462 ± 5 Ma) and Perry Hall (461 ± 5 Ma) plutons are contemporaneous with the James Run Formation, whereas granitoids of the Bynum Run (434 ± 4 Ma) and Prince William Forest (434 ± 8 Ma) plutons indicate an Early Silurian plutonic event. The Popes Head Formation yielded Mesoproterozoic (1.0–1.25 Ga, 1.5–1.8 Ga) detrital zircons, and metamorphosed sedimentary mélange of the Sykesville Formation yielded Mesoproterozoic (1.0–1.8 Ga) detrital zircons plus a minor Archean (2.6 Ga) component. A few euhedral zircons (ca. 479 Ma) in the Sykesville Formation may be from granitic seams related to the Dalecarlia Intrusive Suite. A Potomac orogeny in the Central Appalachian Piedmont is not required, but the earliest Taconic orogenesis remains poorly constrained.

Abstract This field trip highlights the current understanding of the tectonic assemblage of the rocks of the Central Appalachians, which include the Coastal Plain, Piedmont, and Blue Ridge provinces. The age and origin of the rocks, the timing of regional deformation and metamorphism, and the significance of the major faults, provide the framework of the tectonic history which includes the Mesoproterozoic Grenvillian, Ordovician Taconian, Devonian to Mississippian Neoacadian, and Mississippian to Permian Alleghanian orogenies.

Parts of the central and southern Appalachian orogen appear to have evolved away from Proterozoic North America (Laurentia) and to have been accreted to it during the Paleozoic orogenies that collectively formed the orogen. Identifying each tectonostratigraphic terrane is a necessary step in understanding the evolution of the orogen. The terranes in the central and southern Appalachians are delineated, interpreted, and classified with varying degrees of confidence as: (1) Laurentian native terranes, (2) internal continental terranes of the Appalachian orogen, (3) disrupted terranes, (4) possible oceanic crustal remnants, (5) volcanic-arc terranes, (6) a continental terrane of Gondwanaland affinity, and (7) metamorphic complexes of undetermined affinity. The Laurentian native terranes consist of external massifs of Laurentian basement (Grenvillian and older), their rift- and shelf-facies cover rocks, and slope-rise prism deposits. External massifs are present in the Blue Ridge tectonic province, Reading Prong, and Honey Brook Upland. Rocks of the Talladega block are stratigraphically tied to Laurentia and, with the possible exception of the Hillabee greenstone, are also considered native. Offshore, deep-water, post-rift deposits of the Hamburg and Westminster terranes have no direct stratigraphic ties to Laurentia and are considered discrete native (not suspect) terranes. The internal continental terranes of the Appalachian orogen are isolated massifs of Middle Proterozoic (Grenvillian) continental basement and their cover sequences that occur within the metamorphic core of the orogen. These terranes, the Baltimore, Sauratown, and Pine Mountain terranes, could be either structurally isolated outliers of Laurentia or microcontinental fragments of Laurentian crust displaced by rifting or transcurrent faulting and later reassembled. Disrupted terranes in the central and southern Appalachians contain mélange complexes as well as more coherent terrane fragments (volcanic, ophiolitic, or continental) intermingled with the mélange complexes. Those identified include the Jefferson, Potomac, Smith River, Inner Piedmont, Falls Lake, Juliette, and Sussex terranes. The Bel Air–Rising Sun terrane (Baltimore Complex) in Maryland and Pennsylvania is the only terrane named separately as a possible oceanic crustal remnant. Similar mafic and ultramafic complexes are present in all of the disrupted terranes, but are too small to consider as separate terranes. Volcanic-arc terranes include the Chopawamsic, Carolina, Spring Hope, Roanoke Rapids, and Charleston terranes. The only terrane recognized as a continental terrane of Gondwanaland affinity is the Suwannee terrane, which contains rocks believed to correlate with those now exposed in west Africa. Metamorphic complexes of undetermined affinity are terranes that could not be clearly classified on the basis of available data. These include the Milton, Gaffney, Uchee, Crabtree, Goochland, Wilmington, and Hatteras terranes. The Penobscottian, Taconian, Acadian, and Alleghanian Paleozoic compressional events collectively assembled the various terranes into what is now the Appalachian orogen. Only the central and southern parts of the U.S. Appalachians are considered here. The Penobscottian orogeny, about 550 to 490 Ma, amalgamated the Potomac, the Chopawamsic, probably the Bel Air–Rising Sun, and possibly other exotic terranes at some unknown distance from Laurentia. This was followed by the Taconian orogeny, about 470 to 440 Ma, which accreted the previously amalgamated terranes and probably other terranes such as the Carolina terrane to Laurentia. The younger age limit for the Taconian event is partly constrained by Middle and Late Ordovician faunal assemblages in successor basin deposits of the Arvonia Slate and Quantico Formation. The significance of the Acadian orogeny, dated about 400 to 380 Ma in New England, is unclear in the central and southern Appalachians. In the Talladega block of Alabama and Georgia, an Early to Middle Devonian dynamothermal event is firmly bracketed between Early Devonian fossils and K-Ar ages that indicate a thermal peak no later than Middle Devonian time. A regional tectonothermal event and faulting of approximately this age are also suggested by isotopic studies in terranes to the east. The late Paleozoic (Alleghanian) continental collision between Laurentia and Gondwanaland, which formed the supercontinent Pangea, marks the final stage of accretionary history in the Appalachian-Caledonide orogen. Effects evident in the central and southern Appalachian region include: (1) the accretion of the Suwannee terrane and perhaps the Charleston terrane to what is now North America, (2) slicing and shifting of terranes along dextral strike-slip faults, particularly in the eastern Piedmont, (3) westward transport of native and previously accreted terranes in the western Piedmont and Blue Ridge as part of a composite crystalline thrust sheet, (4) deposition of clastic wedges in the Appalachian foreland, and (5) imbricate thrusting and folding of the resultant strata in the Valley and Ridge Province.

Abstract Until very recently, most geologists were conditioned to seek the effects of three major events–the Taconian, the Acadian, and the Alleghanian–within the Appalachian orogen. Things are not that simple, however, as the importance of older deformations is increasingly being recognized. Although this chapter is concerned primarily with the Taconic orogen (sensu stricto), two older deformational events are considered herein. These events are the Blountian and Penobscottian orogenies. The Penobscottian event has been recognized for some time (Neuman, 1967; Hall, 1969, 1970), but its importance in Appalachian geology has only recently become apparent by work in northern Maine (Osberg, 1983; Boone and others, 1984) and the Potomac Valley of Virginia and Maryland (Drake and Lyttle, 1981; Drake, 1987). In Maine, the Penobscottian can only be dated as pre-late Ibexian (pre-Arenigian), whereas in the Potomac Valley it is thought to be of late Middle Cambrian to early Late Cambrian (Dresbachian) age. Neither syn- nor post-orogenic sediments are recognized that could have resulted from the Penobscottian deformation. On the contrary, the Blountian event is recognized because of its syn-and post-orogenic sediment wedge, but deformational features related to the event have not as yet been recognized in the Blountian hinterland, although isotopic dating within the Blue Ridge is permissive of deformation at this time. The Blountian orogeny has been recognized for many years (Kay, 1942), and Rodgers (1953) has termed it the Blountian phase of the Taconicorogeny. In my opinion, it was a separate tectonic event that was completed prior to the Taconic (sensu stricto), as its uppermost molasse is overlain by distal Taconian syn- and post-orogenic deposits. It goes without saying that the effects of the Penobscottian and Blountian orogenies are difficult to recognize and separate from those of the Taconic orogeny. For this reason, the effects of the earlier events will be discussed with those of the Taconic where they are believed to be present.

The Proterozoic Y terrane of the Reading Prong of eastern Pennsylvania and New Jersey consists of light-colored, sodic-rich rocks containing intercalated amphibolite, the Losee Metamorphic Suite, calcarerous and quartzofeldspathic metasedimentary rocks, the intrusive Hexenkopf Complex, the Byram Intrusive Suite, and quartz-poor monzonite, syenite, and related pyroxene granite. The Losee consists of oligoclase-quartz gneiss and amphibolite that in places has been partly mobilized to form venite and albite oligoclase granite. Rocks of charnockitic affinity may be a partial melt of an amphbiolite-rich phase of the Losee. The Losee is thought to be metamorphosed quartz keratophyre and related sodic basalt. It is probably basement to the calcareous and quartzofeldspathic metasedimentary rocks. The calcareous rocks are mostly marble, amphibolite, pyroxene gneiss, and epidote- and scapolite-bearing gneisses. They are interlayered with quartzofeldspathic gneiss of two general types: biotite-quartz-feldspar gneiss and potassic feldspar gneiss. At places, the potassic feldspar gneiss has melted and has formed small bodies and layers of potassium-rich granite. The quartzofeldspathic rocks are of continental margin type and are thought to be a clastic wedge containing layers and lenses of calcareous rocks. The source of the clastic material was probably a granitic terrane because of the large amount of potassic feldspar. Some volcanic material may be present in this sequence, but the evidence is equivocal. The Hexenkopf Complex consists of severely altered mafic plutonic rock. It appears to lie beneath the Losee, and if so, is the oldest known rock in this part of the Reading Prong. The Byram Intrusive Suite consists of hornblende granite and alaskite that form syntectonic and conformable sheets within the metamorphic rocks, as well as scattered small bodies of biotite granite that resulted from the granitization of biotite-quartz-feldspar gneiss. Byram leucosome forms arterites from both biotite-quartz feldspar gneiss and amphibolite. The Byram probably results from the anatectic melting of older rocks, but at the present level of erosion, there is no evidence that this has taken place. The Byram probably had its origin in the source terrane of the quartzofeldspathic gneiss. The quartz-poor monzonite, syenite, and related pyroxene granite are not well understood. These rocks form syntectonic conformable sheets like the rocks of the Byram Intrusive Suite, with one exception, in which a sheet of quartz syenite appears to cut across the structure of hornblende granite. These rocks are also probably anatectic, perhaps originating at a lower level than the Byram. This place of origin is not certain, however, because the relation of the quartz-poor rocks to the Byram is not really known. The rocks in eastern Pennsylvania and New Jersey were metamorphosed in at least upper amphibolite facies, and most were probably metamorphosed in hornblende granulite facies. There is some evidence of polymetamorphism in northeasternmost New Jersey, but that concept needs further evaluation. The metamorphic, intrusive, and deformational event in the Reading Prong can be dated at about 1 b.y. ago, so it is clearly Grenvillian. The rocks in the Reading Prong are very much like those of the Adirondacks and probably like those of the Honey Brook Upland. They have similarities to rocks of the Green Mountains, Berkshires, and basement massifs of western Connecticut. They are not at all like the rocks of Avondale-West Chester Massif, the Baltimore Gneiss, or the rocks of the northern and southern Blue Ridge. In a few small areas, the Proterozoic Y rocks are overlain by a sequence of interlayered metasedimentary and metavolcanic rocks that are named the Chestnut Hill Formation. These rocks are at a lower metamorphic grade and are much less homogenized than the Proterozoic Y rocks and are thought to be of probable Proterozoic Z age.