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7 Geochronology The proposed drill hole would provide a continuous unweath- ered section through many of the major tectonostratigraphic ter- ranes of the southern Appalachians and their contact zones. This would allow systematic determination of a variety of radiometric ages and would provide rigorous chronologic control for regional tectonothermal events. Samples from the hole also could be used for investigating diffusion-controlled radiometric systems in con- junction with companion isotopic and petrologic investigations (although if this were the primary objective, a site with a higher geothermal gradient would be chosen). REGIONAL GEOCHRONOLOGY The tectonothermal evolution of the southern Appalachians is complex, with effects of at least three major events locally recorded: Lower-Middle Ordovician (broadly termed "Taconic"), Lower-Middle Devonian (broadly termed "Acadian"), and Late Carboniferous-Permian (Alleghenian). The regional significance of these distinct events has not been clearly determined. Resolu- tion of the tectonothermal chronology of the crystalline terranes of the southern Appalachians is largely dependent on radiometric dating, but interpretation of results in such high-grade, potentially 28
29 polymetamorphic terranes is commonly equivocal. This, in part, is a result of the nature of many isotopic systems where mobile radiogenic daughter products (Ar, Sr, Pb) are lost by intracrys- talline diffusion until post-metamorphic or post-magmatic cooling through specific "closure" temperatures is achieved. Subsequent reheating may partially or completely reopen intracrystalline iso- topic systems. Because closure temperatures within various min- eral and whole-rock isotopic systems are different (Dodson, 1979), a spectrum of isotopic ages may be recorded within a single geo- logic unit. It is generally impossible to evaluate the geologic sig- nificance of individual age determinations, and a multidisciplinary approach is required. Available geochronological data from the southern Appalachi- ans show the expected divergence of ages recorded by different isotopic systems. Zircon U-Pb and whole-rock Rb-Sr data sug- gest widespread Ordovician and (or) Devonian tectonothermal activity (Tull, 1980; Glover et ai, 1983), whereas many K-Ar and Rb-Sr mineral ages are generally Late Carboniferous to Per- mian (Dallmeyer, 1978). 40Ar/39Ar incremental-release dating (Dallmeyer, 1979) has the potential of distinguishing gas-retention (cooling) K-Ar ages from disturbed K-Ar dates, which are likely in terranes affected by multiple thermal events. The drill hole would provide a complete profile through many of the southern Appalachian tectonostratigraphic terranes. Fresh continuous sections across tectonic contacts, usually deeply weath- ered in typically poor surface exposures, would be a major focus of geochronologic and petrologic research. U-Pb dating of zircon and monazite, together with Rb-Sr whole-rock analyses, would be employed to most closely date high-temperature events. The U-Pb system is particularly useful because two radioactive isotopes of U produce different Pb daughter product isotopes. For example, analysis of multiple fractions of zircon from the same sample has the power to date major thermal disturbances and to determine the original crystallization age of a rock. A companion study of relic detrital zircons may define their provenance and thus con- strain the origin of exotic terranes such as the inner Piedmont. About 2 kg of rock (150 cm of 2.5 cm diameter core or 16 cm of 7.7 cm diameter core) may yield sufficient zircon for analysis of several fractions; 10 kg samples are preferred, and up to 50 kg samples are commonly used in detailed studies of zircon-poor rocks (Table 2).
so Because recent work has shown that even Rb-Sr whole-rock ages may, in part, relate to diffusive loss of radiogenic Sr during prolonged post-metamorphic cooling or subsequent alteration, a variety of scales of whole-rock sample suites should be analyzed in- cluding contiguous, thin-slab suites. These requirements, together with the large sample sizes needed for adequate zircon extrac- tion, necessitate continuous core across all major tectonic contacts penetrated. With drill-core samples and the sophisticated min- eral separation and analytical techniques that are now available, it is possible to separate and analyze the rare minerals that are essential to our obtaining the complete range of age information available in the mineral assemblage of a rock. Mineral separates for Rb-Sr, K-Ar, and 40Ar/39Ar analyses and fission track dating may be prepared from the material processed for zircon and other accessory mineral concentration. It is anticipated that similar multifaceted geochronological investigations would be carried out on each major geologic unit. About 10 m of continuous core would be required from each unit selected (Tables 1 and 2). The total number of units chosen would be dictated by the overall geologic complexity of each terrane encountered. If core is not obtained for a substantial part of the hole, 40Ar/30Ar, K-Ar, and Rb-Sr mineral dates should be determined on mineral concentrates prepared from cuttings collected over very restricted depth intervals. These would give a more complete view of the time-temperature record within each terrane and thus help to define contrasts across tectonic contacts. However, isolated age information on a series of rock samples has limited usefulness, unless those samples can be placed into geologic context as can be done for drill-core samples. Without such ancillary geologic information, geochronological data will have only a fraction of their potential impact. DOWN-HOLE THERMAL STUDIES As noted earlier, radiogenic daughter products such as 87Sr and 40Ar produced within a mineral are not retained until tem- perature drops below a specific "closure" temperature. Assuming that the geochronological closure of a mineral isotopic system is in large part controlled by intracrystalline volume diffusion of ra- diogenic daughter products, Dodson (1979) derived an expression that related closure temperature to average cooling rate between
SI open and closed system behavior, and various solid-state param- eters that govern the diffusive mobility of daughter atoms during cooling. These solid-state diffusion parameters are known with varying degrees of certainty for the common rock forming miner- als. Combined with empirical field controls, the diffusion parame- ters suggest the following "closure" temperatures at cooling rates between 10 and 100°C/Ma: hornblende = 525 ± 25° C (K-Ar), muscovite = 400 ± 25°C (K-Ar), biotite 300 ± 25°C (K-Ar and Rb-Sr). Retention of fission tracks within minerals is also thermally dependent. These tracks are paths of damage produced within a crystal lattice by movement of heavy, charged particles which are liberated during spontaneous, intracrystalline fission of natu- rally occurring isotopes with high atomic numbers (mainly 238U). Although these tracks remain within most minerals for geologic times at low temperatures, the damage is annealed at higher tem- peratures and the tracks fade (Naeser and Paul, 1969). The rate of thermal annealing varies for different minerals, and therefore vari- ations in fission track mineral ages can be used to help resolve the thermal evolution of geologic units. The observed track density represents approximately the time elapsed since the temperature dropped below a value at which 50 percent of the tracks were retained. Experimental studies suggest the following 50 percent track retention temperatures at cooling rates of between 10 and 100°C/Ma: epidote = 550 ± 25°C, zircon = 425 ± 25°C, sphene = 375 ± 25°C, phlogopite = 175 ± 25°C, and apatite = 125 ± 25°C. Clearly, core from any deep drill hole will be useful for evalua- tion of temperature controls for both fission track annealing and mineral closure temperatures.
