Paul Karabinos

TACONIAN OROGENY IN THE NEW ENGLAND APPALACHIANS : COLLISION BETWEEN LAURENTIA AND THE SHELBURNE FALLS ARC

Tectonic models of the Ordovician Taconian orogeny in western New England usually invoke a collision between the Laurentian margin and a magmatic arc identified as the Bronson Hill arc. However, in central Massachusetts and southern New Hampshire, rocks in the Bronson Hill arc are 454 to 442 Ma and therefore younger than much of the Taconian deformation and metamorphism in western New England and eastern New York, which began by 470 Ma. U-Pb and single-grain evaporation zircon ages combined with geochemical analyses reveal the presence of an older magmatic arc, the Shelburne Falls arc, that formed west of the Bronson Hill arc at 485 to 470 Ma. The Shelburne Falls arc formed above an east-dipping subduction zone by the Early Ordovician. The Taconian orogeny was the result of the collision between Laurentia and the Shelburne Falls arc beginning ca. 475 to 470 Ma. The younger Bronson Hill arc formed above a west-dipping subduction zone that developed along the eastern edge of the newly accreted terrane during the final stages of and subsequent to the Taconian orogeny. The Taconian orogeny ended when plate convergence between Laurentia and Iapetus was accommodated by the newly developed west-dipping subduction zone instead of by crustal shortening in the Taconian thrust belt. The tectonic history of the New England Appalachians is inconsistent with a Middle Ordovician collision between Laurentia and the proto-Andean margin of Gondwana.

An evaluation of the single-grain zircon evaporation method in highly discordant samples

Comparison of results from the isotope dilution and the single-grain zircon evaporation methods on highly discordant samples shows that the evaporation method can yield accurate crystallization ages on some zircon populations that display complex discordant patterns. Three granite gneiss samples from the Western Gneiss Region, Norway, which crystallized at approximately 1660 Ma and experienced profound lead loss in zircon during Caledonian metamorphism at 400 Ma, were analyzed by both the isotope dilution (Tucker et al., 1990) and evaporation methods. 207Ph/206Pb evaporation ages on one sample agree with the isotope dilution crystallization age. Two other samples produced a range of anomalously young 207pb/206Pb evaporation ages. Analyses at the highest evaporation temperatures from one of these two samples were consistent with crystallization ages determined by the isotope dilution method, but it would be difficult to interpret them correctly without independent age information. The anomalously young results are generally easy to recognize by the pattern of increasing 207Ph/206Ph ages with increasing evaporation temperature. A more serious problem, however, is that some grains produced 207Ph/206Pb age plateaus over a range of increasing evaporation temperatures that cannot be correlated with any documented geologic event in the region. The evaporation analyses yielding the youngest 207Pb/206Pb ages also display the most common lead, suggesting that common lead may diffuse into the zircon grains during loss of radiogenic lead. A pegmatite from Massachusetts, USA, contains zircons with visible xenocrystic cores. The 207Ph/206Pb evaporation ages at lower temperatures agree well with the 299 Ma crystallization age determined by the isotope dilution method on the zircon rims (R. D. Tucker, unpubl. data). At higher evaporation temperatures, a range of older 207Pb/206Pb ages are produced by mixing Pb evaporated from both the zircon rims and xenocrystic cores. Large changes in the 206Pb/208Pb ratio accompany the increase in 207Pb/206Pb ages and indicate mixing of two distinct zircon domains. Thus, in addition to providing useful 207pb/206Pb age information, the evaporation method may help distinguish between the discordance produced by diffusional lead loss and the mixing of two domains of differing age.

Applications of single-grain zircon evaporation analyses to detrital grain studies and age discrimination in igneous suites

Abstract 207 Pb 206 Pb ages determined by evaporation of single zircon grains from rocks of the New England Appalachians in the USA permit an assessment of the applicability of this method to rocks that have been strongly disturbed by orogenesis. Tests conducted on highly metamorphosed and deformed orthogneisses of known protolith age, whose U-Pb upper intercept zircon ages obtained through isotope dilution methods show profound post-crystallization discordance, demonstrate that the evaporation method is successful in recovering protolith ages. Uncertainties are ±10–15 My for rocks of Paleozoic age or older. Preliminary results suggest that the method is particularly useful for provenance studies of sedimentary rocks and in the discrimination among groups of rocks that are known to differ in age but are otherwise difficult to separate on the basis of field or petrographic characteristics. 207 Pb 206 Pb ages of detrital zircon grains from two metasedimentary units from opposite sides of the New England Appalachians reveal the distinct provenances of these rocks. The Missisquoi Formation in southeastern Vermont, USA, is comprised, in part, of detritus derived from the 1.0–1.2 Ga Grenvillian province of Laurentia; whereas this component is conspicuously absent from the Plainfield Formation in easternmost Connecticut. The detrital zircon age data suggest that the Missisquoi Formation accumulated peripheral to the Laurentian craton during the early Paleozoic, whereas the Plainfield Formation detritus was supplied by non-Laurentian, ossibly West African, sources during the Late Proterozoic or early Paleozoic. In southern Vermont, the evaporation method successfully distinguishes between two groups of augen gneisses with ages of ~960 Ma and ~ 1120 Ma, which were first recognized through isotope dilution methods. Collectively, these results indicate that the dating of single zircon grains by the evaporation method can be widely exploited in the study of complexly deformed and strongly metamorphosed rocks.

Influence of deformation on pressure-temperature paths of metamorphism

Pressure-temperature ( P - T ) paths in structural terranes dominated by folds and thrust faults can be complex and can vary widely within a single metamorphic belt. The complexity and variability of P - T paths result from the advection of heat by folding and thrusting, conductive heat transfer during and after deformation, and the relatively short times for thermal relaxation within folds and thrusts. In fold terranes, contrasting P - T paths are observed between anticlines and adjacent synclines. Cooling paths are observed within anticlines, whereas heating paths are observed within synclines. These different P - T paths result from the folding of isotherms during deformation and the subsequent relaxation of isotherms after deformation. In polydeformed fold terranes, noncoaxial folding results in complex P - T paths over small distances (less than 15 km) within a metamorphic belt because of the repetition of the process of folding and relaxation of isotherms during each folding event In structural terranes dominated by thrust faults, P - T paths differ between major thrust sheets because heat is transferred from the hotter overriding sheet to the cooler lower plate during deformation. Heat transfer during thrusting causes the upper plate to cool and the lower plate to heat. In terranes with multiple thrust sheets, P - T paths can be complex and will depend upon the relative timing of thrust development and the rate at which heat can be transferred from one thrust sheet to the next The P - T paths observed in both the fold and thrust terranes discussed here record relatively short time intervals of 10 5 or 10 6 yr. Because the mineral equilibria respond to small-scale thermal perturbations, there may be a wide variability in P - T paths over a small distance in a metamorphic belt Therefore, before P - T paths are used to reconstruct tectonic histories, both structural and petrologic investigations must be carried out.

A newly identified Gondwanan terrane in the northern Appalachian Mountains: Implications for the Taconic orogeny and closure of the Iapetus Ocean

The Taconic and Salinic orogenies in the northern Appalachian Mountains record the closure of the Iapetus Ocean, which separated peri-Laurentian and peri-Gondwanan terranes in the early Paleozoic. The Taconic orogeny in New England is commonly depicted as an Ordovician collision between the peri-Laurentian Shelburne Falls arc and the Laurentian margin, followed by Silurian accretion of peri-Gondwanan terranes during the Salinic orogeny. New U-Pb zircon geochronology demonstrates that the Shelburne Falls arc was instead constructed on a Gondwanan-derived terrane preserved in the Moretown Formation, which we refer to here as the Moretown terrane. Metasedimentary rocks of the Moretown Formation were deposited after 514 Ma and contain abundant ca. 535–650 Ma detrital zircon that suggest a Gondwanan source. The Moretown Formation is bound to the west by the peri-Laurentian Rowe belt, which contains detrital zircon in early Paleozoic metasedimentary rocks that is indistinguishable in age from zircon in Laurentian margin rift-drift successions. These data reveal that the principal Iapetan suture in New England is between the Rowe belt and Moretown terrane, more than 50 km farther west than previously suspected. The Moretown terrane is structurally below and west of volcanic and metasedimentary rocks of the Hawley Formation, which contains Laurentian-derived detrital zircon, providing a link between peri-Laurentian and peri-Gondwanan terranes. The Moretown terrane and Hawley Formation were intruded by 475 Ma plutons during peak activity in the Shelburne Falls arc. We propose that the peri-Laurentian Rowe belt was subducted under the Moretown terrane just prior to 475 Ma, when the trench gap was narrow enough to deliver Laurentian detritus to the Hawley Formation. Interaction between peri-Laurentian and peri-Gondwanan terranes by 475 Ma is 20 m.y. earlier than documented elsewhere and accounts for structural relationships, Early Ordovician metamorphism and deformation, and the subsequent closure of the peri-Laurentian Taconic seaway. In this scenario, a rifted-arc system on the Gondwanan margin resulted in the formation of multiple terranes, including the Moretown, that independently crossed and closed the Iapetus Ocean in piecemeal fashion.

Garnet and staurolite producing reactions in a chlorite-chloritoid schist

During prograde metamorphism garnet and, in some higher grade samples, staurolite were produced in a chlorite-chloritoid schist, part of the Precambrian Z to Cambrian Hoosac Formation near Jamaica, VT. Garnet grew during two prograde events separated by a retrogression. This sequence resulted in distinctive inclusion textures and zoning anomalies in garnet produced by diffusive alteration. Textures, reaction space analysis, and mineral compositional variations constrain the possible sequence of reactions in these rocks. Below the staurolite isograd, and to some unknown extent above it, garnet grew by the reaction chloritoid+chlorite+quartz→garnet+H2O. With increasing grade the mineral compositions are displaced towards lower Mn/Fe and higher Mg/Fe ratios. The data are compatible with equilibrium with respect to exchange reactions for the matrix assemblages on a thin section scale and with minerals having closely followed equilibrium paths during reaction. The staurolite isograd coincides with the reaction chloritoid+quartz→garnet+staurolite+chlorite+H2O. This reaction is continuous and trivariant with ZnO becoming an additional component concentrated in staurolite. During this reaction both the Mn/Fe and Mg/Fe ratios of the phases appear to have decreased. This new chemical trend is recorded by garnet zoning profiles and is compatible with trends predicted from phase diagrams. Thus there are two distinct types of garnet zoning reversals in these samples. One is near the textural unconformity and is best explained by diffusive alteration during partial resorption of first stage garnet. The other occurs near the outer rim of garnet in staurolite zone samples and marks the onset of a new prograde garnet producing reaction.

Deformation and metamorphism on the east side of the Green Mountain massif in southern Vermont

An early deformation in the Jamaica, Vermont, area on the east side of the Green Mountain massif produced kilometre-scale, north-plunging F1 folds overturned to the west with an axial-planar foliation, S1. Motion on two major thrust faults juxtaposed three similar but distinctive cover-rock sequences after or late in F1 folding. Smaller-scale F2 folds deformed F1 fold limbs and the thrust faults and have an axial-planar crenulation cleavage, S2. In the structurally higher central and eastern cover-rock sequences, an early stage of garnet growth began late in S1 development or after it. A retrogression partially resorbed first-stage garnet and was followed by a second prograde stage of garnet growth late in (or after) S2 development. This metamorphic history resulted in distinctive textural unconformities and zoning anomalies in garnet. The western cover-rock sequence lacks evidence for a major retrogression. F1 folding and F2 folding approximately bracket both the motion on the thrust faults and the retrogression. The second prograde metamorphism and the younger deformation are Acadian. Tentative correlation of the first garnet growth stage with Ordovician metamorphism recognized in western New England and of motion on the thrust faults with Taconian synmetamorphic thrusts in the region suggests that these early thermal and tectonic events occurred in the Ordovician. The retrogression recorded in the central and eastern cover sequences furthermore may reflect thrusting of these rocks to a structurally higher, colder environment, followed by reburial and later prograde metamorphism.

Attracting Students to Science Through Field Exercises in Introductory Geology Courses

Field exercises in introductory geology courses allow students to be active and creative participants in their education. We bring small groups of students into the field and introduce them to the problem we will work on. Students work in groups of three or four, make observations, draw sketches, and take preliminary measurements. Then we regroup and discuss their observations, create a list of questions they would like to answer, and devise methods for answering their questions. They return to work. Then the students present their results in short papers that require them to synthesize what they discovered and to recognize the weaknesses in their data. Although this method is not efficient for transmitting information, it helps students: 1) understand the importance of separating observations from interpretations, 2) appreciate how difficult it is to ask good questions and gather data to answer them, and 3) to be creative. The instructor provides context for the problem to be investigated, asks questions...

Stratigraphic sequence of the Gile Mountain and Waits River Formations near Royalton, Vermont

The stratigraphic sequence of the Gile Mountain and Waits River Formations, two major Silurian-Devonian lithostratigraphic units in eastern Vermont, has long been controversial. This uncertainty has given rise to numerous difficulties in interpreting the regional structure of eastern Vermont. Extensive sequences of compositionally graded beds at 19 localities across the Gile Mountain belt near Royalton, Vermont, show that the Gile Mountain Formation is younger than the Waits River Formation, indicating that the belt is a syncline.

Quaternary Intertidal Deposits Intercalated with Volcanic Rocks on Isla Sombrero Chino in the Galápagos Islands (Ecuador)

Abstract A stratigraphic succession composed of limestone intercalated with volcanic ash and basalt capped by a conglomerate of mixed limestone and basalt cobbles was deposited in a trough-shaped depression approximately 25 m wide and 50 m long to a thickness of 1.62 m on the southwest side of Isla Sombrero Chino in the Galápagos Islands of Ecuador. Two layers of well-cemented calcarenite up to 20 cm thick accumulated as beach deposits with bioclasts of gastropods dominated by the Galápagos Periwinkle (Nodilittorina galapagiensis), a representative of the Beaded Hoofshell (Hipponix grayanus), broken crab fragments, and bird bones. Crustacean remains most likely belong to the Sally Lightfoot Crab (Graspus graspus). The bird bones are attributed to Audubons Shearwater (Puffinus iherminieri). Distinctly intertidal in origin, such a mixed assemblage of invertebrates and vertebrates is unusual, and the association with basalt flows is seldom met in the rock record. The pristine state of the volcanic cone on Sombrero Chino is consistent with a 3He exposure age of 13 ± 0.8 ka. The age of the basalt-limestone sequence is unknown but must be younger than the 3He exposure age. The basalt-limestone sequence is elevated approximately 3 to 4 m above current sea level. This implies that the intertidal limestone was deposited during an interval of higher sea level or, more likely, was uplifted by magmatic inflation. Such intertidal deposits, in conjunction with more precise dating, have the potential to constrain the history of relative sea-level change during island growth and isostatic subsidence related to volcanism and lithospheric cooling. Intertidal deposits of the kind reported here also help to distinguish between monogenetic as opposed to polygenetic history for volcanic islands.