T he Errol quadrangle, New Hampshire–Maine (44° 45′–45° N. lat., 71°–71° 15′ W. long.) is underlain by approximately 27,000 feet of stratified metamorphic rocks that form the core of the Boundary Mountain anticlinorium. Approximately 22,000 feet of these rocks are probably Cambrian and Ordovician, overlain unconformably in the northwest corner of the area by a Siluro-Devonian sequence. The metamorphic rocks were intruded by a batholith of hornblende-biotite granodiorite (Taconic or Acadian) and several stocks of biotite-muscovite adamellite (Acadian). The pre-Silurian stratified rocks, in which the metamorphic grade ranges from the biotite to the sillimanite zone, are divided into three thick formations (from oldest to youngest): the Cambro-Ordovician(?) Aziscohos, the Ordovician (?) Albee, and the Ordovician(?) Dixville Formations. Most of these rocks are metasedimentary, but important metavolcanic units are present in the Dixville Formation and in the Siluro-Devonian sequence. The Aziscohos Formation is tentatively correlated with the Stowe and Ottauquechee Formations of Vermont and with the Mansonville of the Eastern Townships, Quebec. The Albee is traceable along strike to the type Albee in western New Hampshire and is correlated with the lower part of the Moretown Formation of Vermont and the lower part of the Beauceville Formation of the Eastern Townships. The Dixville is correlated with the Partridge and Ammonoosuc Formations of northwestern New Hampshire, the Cram Hill and upper Moretown Formations of Vermont, and the upper Beauceville of the Eastern Townships. The Siluro-Devonian rocks are correlated, at least in part, with the Seboomook Slate of northwestern Maine and the Eastern Townships. The stratified rocks in general trend northeast to north, but a major fold, the Diamond Peaks anticline, dominates the eastern and northeastern part of the area. It is believed to plunge south or southwest. Minor folds and other b lineations throughout most of the area plunge steeply with a strong vertical maximum and are believed to have been formed during a second stage of deformation. Because of the angular unconformity beneath the Siluro-Devonian rocks, it is inferred that broad folding occurred during the Taconic disturbance, although the major orogeny was Acadian.

Ages from 17 new graptolite localities have made possible refinements in Silurian and Silurian-Devonian stratigraphy in the Kingsbury and Skowhegan quadrangles and indicate major facies changes in the Silurian metasedimentary rocks of central Maine. Northwest of the Currier Hill syncline, the observed sequence is: Sangerville Formation (upper Llandovery through lower Wenlock)—variably bedded meta-graywacke and pelite with minor polymict granule conglomerate, ribbon limestone, and carbonaceous pelite; Eddy Formation (lower Ludlow)—rusty weathering sulfidic carbonaceous pelite and non-carbonaceous quartzose sandstone and granule conglomerate; Brighton Formation [Silurian-Devonian(?)]—massive biotite-quartz granulite with minor calc-silicate granulite beds and pods (lower member) and thinly interbedded biotite-quartz granulite and biotite-muscovite phyllite (upper member); Solon Formation [Lower Devonian(?)]—nearly homogeneous slate with occasional rhythmically interbedded quartzose sandstone and slate. Southeast of the Currier Hill syncline, the sequence is similar but there is a decrease in grain size in most units accompanied by an increased amount of pelite and a drastic thinning of Sangerville bedding to produce a pinstriped unit (transitional Sangerville). My interpretation of the facies relationships suggests a sequence that becomes finer to the east with a source of sediments to the northwest. Correlations based on projection of these facies changes and on fossil ages wherever possible yield the following: West Seboomook Formation Madrid Formation Smalls Falls Formation Perry Mountain Formation Rangeley Formation Solon Formation Brighton Formation Eddy Formation Sangerville Formation East Vassalboro Formation Waterville Formation Mayflower Hill Formation

Abstract The Bald Mountain volcanogenic massive sulfide (VMS) deposit of Early Ordovician age in northern Maine contains 30 million metric tons (Mt) of Cu-Zn-Au-Ag sulfides. It is exceptionally well preserved, lacking penetrative deformation, and having experienced only prehnite-pumpellyite–grade regional metamorphism. The deposit occurs within a homoclinal west-dipping volcanic sequence that consists of, from bottom to top, basalt and basaltic andesite, crystal-poor rhyolite ignimbrite, massive sulfide and related units, crystal-rich rhyolite ignimbrite and intercalated andesite, carbonaceous argillite, and rhyolitic volcaniclastic rocks. Basalts stratigraphically below the massive sulfide are intruded by an elongate body of tonalite-plagiogranite; gabbros intrude rocks both above and below the massive sulfides. The basal contact of the host volcanic sequence is believed to be a thrust with underlying Middle Ordovician clastic sedimentary rocks; the upper contact is depositional with the Middle to Upper Ordovician Winterville Formation and, in places, with Silurian conglomerates. Ordovician synvolcanic faults that predominantly strike 025°, 050° to 060°, 325° to 335°, and 350° formed a small (320 × 275 m) synvolcanic graben in which as much as 215 m of massive sulfide accumulated. Hydrothermal solutions utilized these faults as fluid conduits, causing structurally controlled epidote and silica alteration in the deep footwall. Structurally controlled alteration is also indicated by the presence of magnetic low areas in mafic rocks up to 1 km below the deposit. Movement of zinc- and copper-rich fluids was controlled by the location of the Ordovician faults. Zinc-rich fluids were concentrated along faults that bound the northern, western, and southern sides of the small graben; copper-rich fluids moved along faults that define the eastern side of the graben. Rocks overlying the massive sulfide body show little evidence of the growth faulting that occurred within and below the deposit, indicating that most extensional deformation ceased shortly after exhalative sulfide deposition. Synvolcanic Ordovician faulting and graben formation are the principal causes for the small lateral dimensions of the Bald Mountain deposit relative to those of most VMS deposits of comparable tonnage. Postsulfide deformational events occurred in the Late Ordovician to Early Silurian when rocks hosting the Bald Mountain deposit were thrust over Ordovician clastic sedimentary rocks and in the Early Devonian when Acadian faulting and folding segmented the deposit and tilted it to the west.

Recognition of the timing of peak metamorphism in the eastern Blue Ridge (ca. 460 Ma), Inner Piedmont (ca. 360 Ma), and Carolina terrane (ca. 540 Ma) has been critical in discerning the history of the collage of terranes in the hinterland of the southern Appalachian orogen. The Inner Piedmont consists of two terranes: the Tugaloo terrane, which is an Ordovician plutonic arc intruding thinned Laurentian crust and Iapetus, and the Cat Square paragneiss terrane, which is interpreted here as a Silurian basin that formed as the recently accreted (ca. 455 Ma) Carolina terrane rifted from Laurentia and was transferred to an oceanic plate. The recognition of an internal Salinic basin and associated magmatism in the southern Appalachian hinterland agrees with observations in the New England and Maritime Appalachians. Structural analysis in the Tugaloo terrane requires the Inner Piedmont to be restored to its pre-Carboniferous location, near the New York promontory. At this location, the Catskill and Pocono clastic wedges were deposited in the Devonian and Mississippian, respectively. Between the two wedges, an enigmatic formation (Spechty Kopf and its correlative equivalent Rockwell Formation) was deposited. Polymictic diamictites within this unit contain compositionally immature exotic clasts that may prove to have been derived from the Inner Piedmont. Following deposition of the Spechty Kopf and Rockwell Formations, the Laurentian margin became a right-lateral transform plate boundary. This continental-margin transform was subsequently modified and translated northwest above the Alleghanian Appalachian décollement. Thus, several critical recent observations presented here inspire a new model for the Silurian through Mississippian terrane dispersal and orogeny that defines southern Appalachian terrane geometry prior to emplacement of the Blue Ridge–Inner Piedmont–Carolina–other internal terranes as crystalline thrust sheets.

ABSTRACT This three-day field trip focuses on the stratigraphy and the structural characteristics of the late- and post-Taconian sedimentary basins of the southern Québec Appalachians, with a particular emphasis on N-to-S and W-to-E structural and lithological variations. In order to discuss various aspects of the regional structural evolution of these basins, we will visit a series of key outcrops following three sections, the Beauce/Thetford-Mines sections, the Sherbrooke section, and the Coaticook section. RÉSUMÉ Cette excursion de trois jours se concentre sur la stratigraphie et les caractéristiques structurales des bassins sédimentaires tardi- et post-Taconien des Appalaches du sud du Québec, en mettant l’accent sur les variations structurales et lithologiques du nord au sud et d’ouest en est. Afin de discuter des divers aspects de l’évolution structurale régionale de ces bassins sédimentaires, nous visiterons une série d’affleure ments clés en suivant trois sections, soient les sections de Beauce/Thetford-Mines, de Sherbrooke, et de Coaticook.

Abstract The present study was undertaken to document the lithogeochemistry of the principal volcanic units hosting the Early Ordovician Bald Mountain Cu-Zn-Au-Ag massive sulfide deposit in northern Maine as well as that of volcanic units from the surrounding region. Results document several distinct petrochemical associations that reflect variations in the tectono-magmatic evolution of an intraoceanic arc to continental arc–back-arc complex along the convergent Ordovician margin of Gondwana. Footwall and immediate hanging-wall rocks at Bald Mountain are composed of tholeiitic basalt-andesite massive to pillowed lava flows and hyaloclastite breccias, and felsic aphyric, pumice- and crystal-rich ignimbrites. Mafic rocks are characterized by low incompatible element contents, flat to slightly light rare earth element-depleted patterns, and prominent negative anomalies for Nb, Ta, Zr, Hf, and Ti on chondrite-normalized trace element plots. The felsic ignimbrites are tholeiitic dacite-rhyodacite having trace element characteristic similar to that of the associated mafic rocks. The footwall and immediate hanging-wall rocks define a bimodal tholeiitic, mafic-dominated intraoceanic arc volcanic suite similar in composition to recent volcanic rocks from the Kermadec arc in the southwest Pacific. In contrast to the lower, mafic-dominated section, the upper hanging-wall section at Bald Mountain is composed of dominantly felsic volcaniclastic rocks and associated sediments with only minor andesite flows and/or sills. The volcanic rocks are transitional to calc-alkaline with the felsic rocks showing high Th (˜3–18 ppm) and light REE (La ˜11–68 ppm) contents, and negative Nb, Ta, P, and Ti anomalies. Their geochemistry and isotopic signatures indicate significant involvement of an enriched continental crust component in their source region. The upper hanging-wall rocks are similar in composition to calc-alkaline suites from mature arc and continental margin arc–back-arc settings like the Taupo Volcanic Zone of New Zealand. Volcanic rocks from the surrounding Munsungun-Pennington Mountain and Weeksboro-Lunksoos Lake anticlinoria in northern Maine include basalts having arc, continental back-arc, and within-plate non-arc (no negative Nb-Ta anomalies) petrotectonic affinities. Associated felsic volcanic rocks have enriched calc-alkaline compositions with continental crustal geochemical and isotopic signatures. The various volcanic units partially overlap in composition with some volcanic rocks from the hanging-wall portion of the Bald Mountain sequence. Volcanic rocks hosting the Mount Chase Zn-Pb-Cu-Ag-Au massive sulfide deposit in the Weeksboro- Lunksoos Lake anticlinorium, southeast of Bald Mountain, are coeval with upper hanging-wall units at Bald Mountain and have continental back-arc basin compositional affinities. The Bald Mountain sequence is interpreted to reflect the progression from an oceanic to transitional continental crustal setting as part of the evolving Popelogan Arc–Tetagouche-Exploits back-arc basin system. The setting may have been analogous to that observed along the present Kermadec-Havre Trough-Taupo Volcanic Zone arc and back-arc system in the southwest Pacific. The Bald Mountain deposit most likely formed in a submarine oceanic arc volcano caldera located proximal to, but offshore from, the continental back-arc basin in which the Mount Chase deposit developed.