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U-Pb Ages of Zircon from Selected Plutons
The mineral zircon (ZrSiO4) is an ideal geochronometer (Rollinson, 1993). During crystallization, zircon takes uranium into its crystal structure, which naturally decays to form its daughter product, lead, thus leading to the measurement of U/Pb ages. Zircons can have a complex history in the rock cycle, going from igneous to sedimentary to metamorphic, and back to igneous again. This complex history can lead to complex age structures within a single grain (e.g., an older core surrounded by new zircon growth) (Schuhmacher et al, 1994). Such small-scale heterogeneities in lead and uranium within a zircon crystal are best revealed by in-situ isotopic measurements (Schuhmacher et al., 1994).
Uranium and lead may be measured in situ, through the use of secondary ion mass spectrometry (SIMS). This technique uses a beam of ions (oxygen) to strike the zircon crystal (~30 micron spots), producing an ion bean that can yield uranium and lead isotopic ratios. The use of SIMS allows for complex ages to be resolved spatially, and for ages of the separate events that affected the zircon crystal to be obtained (figure 3).
Many of the plutons previously had been dated through Rb/Sr whole rock, and U/Pb TIMS methods (e.g. Pavlides et al., 1994, Sinha et al., 1989, Mose and Nagel, 1982; Hund, 1987; and others). These data are beneficial in providing time constraints for the crystallization ages of the plutons, but do not provide high-resolution ages of plutons, due to the heterogeneity within individual zircons, and zircon populations. This study employed the use of SIMS U/Pb mass spectrometry of zircons to obtain high precision ages for the plutons. A discussion of analytical techniques used in this study, as well as discussion of data analysis can be found in Appendix A.
Figure 3: Backscatter SEM image of a zircon grain from the Ellicott City pluton, Maryland. Small semi-circular pits are points where SIMS analyses were taken. Image of zircon grain displays zoning, and when paired with the age data from SIMS analyses shows complex age patterns that may occur in single zircon crystals. This study uses SIMS analysis of zircons in order to resolve core/overgrowth relationships.
Ages and Geochemistry of Plutons
The ages and geochemistry of the plutons provide us with data for creating a temporal and spatial framework related to the genesis of these rocks. All of the bulk rock geochemical data used in this study is given in table 1, and initial strontium ratios and SIMS ages can be seen in table 2. Detailed pluton descriptions can be seen in Appendix The ages, gathered on the UCLA Ion Microprobe (see Appendix A for description of analytical techniques), and the bulk rock geochemistry measured by Activation Laboratories (see Appendix D for an example of metadata associated with measurements), are described in Appendix C in more detail. Images of zircons used in this study can be seen in Appendix E.
Major Element and Strontium Isotope Geochemistry
The plutons within this study vary greatly in composition, initial Sr isotopic ratios, and ages. K2O vs SiO2 (figure 4a) highlight the bimodal nature of the plutons in the study area. Mafic plutons such as the Rich Acres, and the Green Springs show a range in silica values (~45 to ~58 Wt. %), with values in K2O which is comparable to those from the felsic plutons. The Lahore pluton shows the highest values of K2O (average 5 Wt. %) for the mafic rocks, and is comparable to the values exhibited in the felsic Leatherwood Pluton. Felsic plutons also display a range in K2O, from 1 to 6 Wt. 87Sr/86Sr initial vs SiO2 (figure 4b) show the variability of Sr isotopes for the plutons. Utilizing the average 87Sr/86Sr initial ratio for each pluton the contrast between mafic and felsic magmas is easily recognized. The mafic rocks have a range in isotopic values from 0.7034 to 0.7051 (average 0.7045), while the felsic rocks have a range from 0.7059 to 0.7074 (average 0.7066). These values reflect different source regions for the mafic and felsic rocks. The 87Sr/86Sr initial ratio shows no correlation to the presence or absence of inheritance in the ages of zircons (see next section for inheritance).
Table 2: Table showing Rb and Sr (ppm), measured value of 87Sr/86Sr, 87Sr/86Sr initial ratios and U/Pb SIMS age used in calculating 87Sr/86Sr initial ratio. Initial strontium ratio from Carysbrook calculated assuming an age of 457 Ma, based on field relations seen by Goodman et al., 2001.
Figure 4: a) Plot showing K2O vs SiO2. Plutons with large ranges of values are circled. Many plutons (Rich Acres, Lahore, Green Springs, Leatherwood, and Ellisville) show enrichment in K2O. b) Plot showing 87Sr/86Sr initial ratio vs SiO2. Those plutons with multiple analyses, are shown here with an average value.
NeU/Pw b SIMS Ages
The ages of the plutons (analytical techniques can be seen in Appendix A) are given in figure 5, Table 2 and in Appendix F, figures a-j. The analyses from the SIMS gave data showing both ages of crystallization, and ages of inheritance. The calculated ages (table 2) represent the best estimate for the age of crystallization of the pluton. Plutons which displayed inheritance were given an age of crystallization by removing those data points that showed clear evidence of older ages on both concordia and weighted average plots. Inherited ages were found in some of the plutons, e.g., Columbia, Diana Mills, Gretna body of Leatherwood, Melrose, Poore Creek and Green Springs (Appendix G, figures a-e). . Inherited ages ranged in age from 900 Ma to 1400 Ma. The Columbia pluton showed inheritance of 900 to 1000 Ma, the Diana Mills pluton showed inheritance of 1000 Ma, the Gretna body of the Leatherwood pluton showed 1200 and 1400 Ma inheritance, the Melrose pluton showed 1100 Ma inheritance, and the Poore Creek and Green Springs plutons showed 1200 and 1400 Ma inheritance. The inheritance seen in the plutons seems to have no association with lithotectonic belts (figure 5). The crystallization ages of the plutons range in age from 457 Ma to 429 Ma. When the ages are compared to the composition of the rock, a distinct trend is visible. The felsic rocks are predominantly older, ranging from 457 Ma to 441 Ma, while the mafic rocks are predominantly younger, ranging from 436 Ma to 429 Ma.
Geochemical and U/Pb SIMS analyses obtained for this study have large amounts of data associated with them. These data are essential to understanding the thermal processes occurring during the tectonic history of the central Appalachians. Therefore, making such data sets accessible on a regional scale is essential for further analysis and assessment of the geologic history of the region.
Abstract
Acknowledgements
Geologic Setting of Selected Plutons in the western Piedmont and Blue Ridge Provinces of Virginia
U-Pb Ages of Zircons from Selected Plutons
Ages and Geochemistry of Plutons
Major Element and Strontium Isotope Geochemistry
New U/Pb SIMS Ages
An Overview of GIS and its Application to Data in the Geosciences
Integration of Data and Plutons
GIS and Geosciences
Development of a GIS Pluton Database
GIS, Mapping and Accuracy
Geologic Discussion
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