THE DOVER CREEK SYENITE: A NEWLY RECOGNIZED NEOPROTEROZOIC PLUTON WITHIN THE MESOPROTEROZOIC STATE FARM GNEISS, GOOCHLAND, TERRANE, VIRGINIA

The Mesoproterozoic State Farm Gneiss was intruded at a variety of scales in the Neoproterozoic by several A-type granitoids. Owens and Tucker (2003 GSA Bulletin) reported two such small bodies about 8 km apart in the eastern part of the State Farm Gneiss that are mineralogically and compositionally similar, and whose U-Pb zircon ages overlap within the uncertainties (588 +9/-12 and 600 +7/-9 Ma, respectively). At the time, Owens and Tucker stated “The full extent of these intrusions is unknown at present.” More recent field work has revealed that the two bodies actually form the northern and southern portions of a single pluton of syenitic rocks, informally named here for numerous exposures in the Dover Creek drainage basin. The pluton is narrow (about 2 km wide at most), but highly elongate (about 10 km), and oriented approximately north-south. The rocks are characterized by crudely aligned, elongate clusters of amphibole + biotite that give them a distinctive spotted or streaky appearance in the field, and which define a foliation. Amphibole and biotite are both very dark in color in thin section, consistent with Fe-rich compositions. Typical accessories include magnetite, titanite, and garnet along with conspicuous dark yellow-brown allanite. Twelve samples that span the extent of the pluton range in composition from syenite to quartz-syenite (SiO₂ = 60.3-68.4 wt.%), and all are metaluminous. The rocks display all of the compositional characteristics of A-type granites, including high Fe/Mg, Ga/Al, and elevated concentrations of Zr (628-1173 ppm), Nb (51-104 ppm), and Y (44-159 ppm). In addition, overall concentrations of the REE are high (ΣREE = 565-1552 ppm) in all samples, and all are LREE-enriched with pronounced negative Eu-anomalies. All samples plot well within the “within-plate granite” field on standard tectonic discriminant diagrams for granitic rocks based on immobile trace elements, consistent with a rift-related origin. In combination with other km-scale plutons (e.g., Flat Rock, Fine Creek Mills), the Dover Creek intrusion reflects pervasive Iapetan rifting of the main State Farm Gneiss dome during the Neoproterozoic. The geographic position of the State Farm Gneiss at this time is unknown, and remains a key unresolved question.