REU: Geologic Evolution of the Robertson River Suite: Implications for the Timing, Petrogenesis, and Thermal History of the Late Proterozoic Rifting of Laurentia

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The Robertson River Suite (RRS, a.k.a. Robertson River Formation) encompasses a series of metaluminous to peralkaline granitoids and felsites exposed within the core of the Blue Ridge anticlinorium (BRA) in northern and central Virginia. Broad- scale reconnaissance mapping by previous workers has established that the RRS is intrusive into Grenville age (1.0-1.1 G.a.) gneisses and is intruded by numerous greenstone dikes, some of which may have served as feeders to the Catoctin metavolcanics of the BRA cover sequence. The age of the RRS is thus only broadly constrained by field evidence as younger than Grenvillian metamorphism and older than at least most of the late Precambrian Catoctin Formation. Previous attempts using radiometric techniques to determine a more exact timing for emplacement of the suite have produced mutually contradictory data spanning the range 730 to 570 M.a. Much of the reason for these inconsistent data lies in the fact that these studies have either 1) assumed the formation to be homogeneous, and thus accurately characterized by multiple samples collected from throughout the outcrop belt, or 2) acknowledged the apparent heterogeneity of the suite and nevertheless attempted to extrapolate data from one location to the entire body. Neither approach is appropriate, however, in light of new evidence from more detailed field, petrologic, and geochemical studies recently completed in the northern portion of the suite by this research group. These data demonstrate that the RRS encompasses a series of distinct plutons that records an obviously complex petrogenetic history spanning a time interval which may have been considerable. This study proposes to build on this work by 1) completing 1:24,000-scale mapping of the southern portion of the suite, 2) using petrologic and geochemical data to define the nature of the constituent plutons, and 3) obtaining precise and accurate isotopic data bearing on the timing and duration of RRS emplacement. Furthermore, the petrochemical and isotopic data to be obtained in this study will be applied to refine models for the petrogenetic evolution of this compositionally distinctive suite and the isotopic characteristics of probable source regions. In summary, the results of this stud will 1) provide a firm foundation for appropriate petrogenetic models concerning emplacement of this anorogenic suite, 2) place important constraints on the maximum age of prelapetan rifting, 3) establish a database on which to evaluate the relationship of this plutonism to the voluminous felsic lavas included within the Catoctin Formation and felsic intrusives of similar (?) age and composition occurring elsewhere in the central Appalachians, 4) define specific isotopic characteristics of the source areas of magma generation, and 5) lead to the development of geologically reasonable models concerning the petrogenetic evolution of such compositionally unique anorogenic suites.