The Middle Pennsylvanian Fire Clay tonstein, mostly kaolinite and minor accessory minerals, is an altered and lithified volcanic ash preserved as a thin, isochronous layer associated with the Fire Clay coal bed. Seven samples of the tonstein, taken along a 300-km traverse of the central Appalachian basin, contain cogenetic phenocrysts and trapped silicate-melt inclusions of a rhyolitic magma. The phenocrysts include beta-form quartz, apatite, zircon, sanidine, pyroxene, amphibole, monazite, garnet, biotite, and various sulfides. An inherited component of the zircons (determined from U-Pb isotope analyses) provides evidence that the source of the Fire Clay ash was Middle Proterozoic (Grenvillian) continental crust inboard of the active North American margin. 40 Ar/ 39 Ar plateau ages of seven sanidine samples from the tonstein have a mean age of 310.9 ± 0.8 Ma, which suggests that it is the product of a single, large-volume, high-silica, rhyolitic eruption possibly associated with one of the Hercynian granitic plutons in the Piedmont. Biostratigraphic analyses correlate the Fire Clay coal bed with a position just below the top of the Trace Creek Member of the Atoka Formation in the North American Midcontinent and near the Westphalian B-C boundary in western Europe.

Over the past 30 years, both isotope geochronology and plate tectonics grew from infancy into authoritative disciplines in the geological sciences. Previously, mountain systems like the Appalachians had been viewed almost entirely in the context of the classical geosyncline, implying a gradualism in stratigraphic and structural change throughout the orogen. Age control, determined largely from distant fossiliferous strata, was unabashedly carried to high-grade metamorphic rocks based only on lithological correlations. With the new concepts in tectonics came the realization that abrupt breaks in stratigraphy and structure occur in many cases at the boundaries of lithotectonic zones. Fortunately, the new techniques of isotope geochronology could be brought to bear directly on the rocks of the immediate study area. This paper chronicles some of the major contributions to the geology of the New England Appalachians that resulted from these efforts during the past three decades. In tracing the history of geochronologic research, one encounters an increasingly sophisticated approach to the analytical and interpretive aspects of the discipline. Today, the geochronologist can, under optimum conditions, constrain the age of stratigraphic units, igneous activity, deformation, and metamorphism with accuracy that is capable of resolving fine structure within individual orogenic pulses. He participates in full partnership with other colleagues of the science in unravelling the mysteries of mountain building. Several of the topical problems of New England geology in which geochronology played a key role include (1) the recognition and delineation of Avalonia as a Late Proterozoic eastern basement distinct from more western terranes, (2) the dating of the White Mountain Plutonic-Volcanic Suite, a Mesozoic igneous event spanning 100 m.y., and (3) the temporal and spatial separation of structural and metamorphic features imprinted by the Taconic and Acadian orogenies. The existing geochronology is summarized into a map and table emphasizing the temporal construction of the New England Appalachians. By using lithotectonic zones as the building blocks of the orogen, seven such zones are defined in terms of pre-, syn-, and post-assembly geologic history. From west to east, these lithotectonic zones are (1) Berkshire-Green Mountain, (2) Rowe-Hawley, (3) Connecticut Valley, (4) Bronson Hill, (5) Kearsarge-Central Maine, (6) Tatnic Hill-Nashoba, and (7) Avalonia. Avalonia is further divided into three subzones, Hope Valley, Esmond-Dedham, and Penobscot Bay, which themselves may have had distinct origins and assembly histories. The boundaries between these zones are faults in most cases, some of which may have had recurring movement to further complicate any plate-tectonic scenario. A delineation of underlying Grenvillian, Chain Lakes, and Avalonian basement is also attempted, which now can make use of isotopes in igneous rocks as petrogenetic indicators to supplement the rare occurrences of basement outcrop within mobile zones of the orogen. The belt of Permian thermal disturbance within the Kearsarge-Central Maine zone is hypothesized to reflect rapid rebound following compressional thickening of underlying Avalonian basement during the Alleghanian orogeny.

Euhedral zircon crystals several millimeters in size were collected from feldspathoidal syenite constituting the central part of the Potash Sulfur Springs igneous complex, Garland County, Arkansas. Found loose in the overlying residual soil, the zircons displayed simple tetragonal dipyramid habits with small prism faces. The largest crystal weighed 78 mg and was internally adamantine except for some secondarily(?) corroded and iron-stained domains. Following crushing, a sample containing only completely clear fragments (A-l) and one containing some incipiently corroded surfaces (A-2) were picked from this crystal. Selected fragments from two other somewhat smaller, more translucent crystals constituted a third sample (BC-1). Ages (Ma) Sample 206 pb /238 U 207 Pb /235 U 207 pb /206 pb 208 pb /232 Th A–l 98±1 98±1 102±4 99±1 A–2 92±1 93±1 99±6 92±1 BC–1 90±1 91±1 101±7 90±1 Sample A-l yields concordant U-Th-Pb ages, demonstrating that it has behaved as a closed system subsequent to crystallization. Good agreement in 207 Pb/ 206 Pb ages of all samples further confirms this result, although some internal discordance within samples A-2 and BC-1 reveals them to be slightly open systems from which radiogenic lead recently has leaked. Current time scales place our preferred concordia intercept age of 100 ± 2 Ma for the Potash Sulfur Springs igneous complex near the end of the Albian Stage of the Early Cretaceous. Previously published ages for the nearby Magnet Cove Complex are in good agreement with the results of this study, which attest to the contemporaneity of the two plutons.