Abstract Two volcanogenic massive sulfide deposits in northern Maine were dated by zircon U-Pb SHRIMP-RG geochronology. Most zircons from the Bald Mountain and Mount Chase deposits are light brown, equant, euhedral to subhedral, multifaceted, and contain multiple, euhedral growth zones; cores or inclusions indicative of inherited domains are absent. For Bald Mountain the best age estimate is 467 ± 4 Ma, and for the Mount Chase deposit, 60 km to the south, the best age is 467 ± 5 Ma. 206 Pb/ 238 U ages (49 zircons) at Bald Mountain range from 446 to 1335 Ma, with a major peak from 446 to 490 Ma. Zircons younger than 467 Ma were affected by Pb loss; four zircons have 206 Pb/ 238 U ages older than 490 Ma due to inheritance of older zircons. At Mount Chase, 206 Pb/ 238 U ages (40 zircons) range from 418 to 1384 Ma with a major peak from 450 to 485 Ma. Again, zircons younger than 467 Ma were affected by Pb loss and eight zircons having 206 Pb/ 238 U ages older than 548 Ma, up to 1384 Ma, are due to inheritance. Pb isotope compositions of sulfide minerals (galena, pyrite, chalcopyrite, pyrrhotite) at Bald Mountain representing all major hypogene paragenetic stages of mineralization range in 206 Pb/ 204 Pb from 18.048 to 18.264, 207 Pb/ 204 Pb from 15.535 to 15.655, and 208 Pb/ 204 Pb from 37.803 to 38.160. Galena from the earliest exhalative stage of mineralization is less radiogenic than chalcopyrite, pyrrhotite, and pyrite, which were deposited by later replacement. Hydrothermal siderite and calcite from late parageneses range in 206 Pb/ 204 Pb from 18.131 to 22.519, 207 Pb/ 204 Pb from 15.555 to 15.856, and 208 Pb/ 204 Pb from 37.831 to 37.957. Pb isotope compositions of the sulfides plot along a narrow band on standard uranogenic and thorogenic plots. Calculated values of μ representing the sources contributing to the sulfides (9.5–10) are mostly higher than the average crustal Pb evolution curve and attest to a crustal influence on the Pb isotope compositions. Sulfides reflecting μ values lower than the average crustal curve point to the contribution of mantle isotopic compositions. The galenas are more radiogenic than hydrothermal sulfides in sediment-free modern environments, such as primitive arcs and sulfides associated with rifted arc basins, but resemble sulfides from sedimented ridges. The intrinsically high 207 Pb/ 204 Pb values of many of the Bald Mountain sulfides reflect radiogenic Pb derived from continental crustal basement or deep footwall sedimentary rocks. We conclude that Bald Mountain formed during rifting of a continental block (peri-Gondwanan) or was formed near crustal rocks characterized by high U/Pb values that provided the radiogenic Pb source for the sulfides. Sulfides from the Bald Mountain deposit are less radiogenic than the massive sulfide ores of the world-class Bathurst Mining Camp in New Brunswick and the Mount Chase deposit in the Central Maine terrane. Sulfides from the Bald Mountain deposit have a range of Pb isotope compositions identical to those of massive sulfides from Early to Middle Ordovician island-arc sequences from the Exploits subzone in the Central mobile belt of Newfoundland and some deposits from coastal Maine. Massive sulfides from the Notre Dame subzone in the Central mobile belt of Newfoundland, from the Eastern Townships of Quebec, and from the Vermont Cu belt are less radiogenic than the Bald Mountain deposit.

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.

Along a transect across the central Virginia Blue Ridge complex, the terrane is divisible into two discrete massifs—the Pedlar and the Lovingston, which are separated by the mid-Paleozoic Rockfish Valley fault. The Pedlar Massif is characterized by two major lithologies—an older volcanic sequence (1130 m.y.) intruded by the younger Pedlar River Charnockite Suite (1070 m.y.). The Lovingston Massif has a more complex association of rock types that include an older paragneissic sequence (Stage Road Layered Gneiss) with approximately 1870 m.y. old detrital zircons and intruded by quartz monzonite, pegmatite and charnockite of the Archer Mountain Suite, as well as anorthosites (1100 m.y.). The entire Blue Ridge complex terrane was subjected to granulite grade metamorphism approximately 920 m.y. ago. Later igneous activity is recorded at ∼7 00 m.y. as the Rockfish River granodiorite and Catoctin greenstones.