PETROLOGIC AND TECTONIC SIGNIFICANCE OF GRENVILLE-AGE BIOTITE BEARING GRANITOIDS, BLUE RIDGE PROVINCE, NORTHERN VIRGINIA

Detailed field mapping and petrologic studies of Mezoproterozoic biotite-bearing granitoids in the Woodville area indicate that post-orogenic rocks preserve evidence of divergent magmatic lineages and multiple sources associated with waning stages of Grenvillian orogenesis. Previously undifferentiated biotite granitoids are classified as two mappable lithologic units: Ylgr (leucocratic granitoid) and Ybg (biotite-rich granitoid). The Ylgr unit is characterized texturally by an interlocking framework of generally coarse-grained alkali feldspar with interstitial quartz, plagioclase, and biotite. The Ybg unit is a medium- to- course-grained granitoid composed dominantly of a matrix of biotite, quartz, and plagioclase with isolated alkali feldspar phenocrysts. These units collectively define a compositional continuum ranging from monzogranite to quartz monzodiorite with Ylgr rocks, generally containing greater than 65% SiO2, dominating more felsic compositions. The higher silica Ylgr rocks also include locally abundant pegmatites produced through internal fractionation, and these distinctive lithologies provide a reliable mapping aid. Compositional distinction between these units is further underscored by distinct differences in high-field-strength elements with Ybg characterized by higher Zn + Nb + Ce + Y concentrations and Ga/Al ratios. These A-type granitoid characteristics of Ybg are interpreted as being reflective of derivation from compositionally distinct magmatic sources. Textural characteristics and lack of correspondence in chemical composition indicate that the biotite granitoids were not produced through retrograde metamorphism of associated charnockites, as previously proposed by others. In contrast, the biotite granitoids are interpreted as representing a separate magmatic lineage of Mezoproterozoic age characterized by differences in intrinsic magmatic parameters such as water content and Fe/Mg. In this way, the petrologic diversity of Grenvillian Blue Ridge plutons can be explained by differences in primary igneous petrogenesis and thus need not result from a series of tectonic events.