PETROLOGIC AND GEOCHEMICAL CHARACTERISTICS OF MESOPROTEROZOIC BASEMENT ROCKS ALONG A PORTION OF THE BLUE RIDGE PARKWAY, CENTRAL VIRGINIA: REGIONAL COMPARISONS AND IMPLICATIONS FOR PRECAMBRIAN TERRANE ACCRETION

New geochemical and petrologic data from Mesoproterozoic rocks in the Shenandoah massif (SM) along a portion of the Blue Ridge Parkway (BRP) in central Virginia indicate considerable compositional variation and suggest similarities with basement rocks of the Shenandoah National Park (SNP) area. Oldest rocks (ca. 1150-1140 Ma) include amphibole syenogranite gneiss and garnetiferous syenogranite that are characterized by fine-scale layering, but differ geochemically. Younger rocks include unfoliated leucocratic monzogranite and pyroxene-bearing granodiorite(ca. 1080 to 1027 Ma) that are distinct mineralogically and geochemically. Compositionally, most rocks are broadly silicic with SiO₂ contents of 61-78 wt%. All rocks exhibit Fe enrichment typical of tholeiites, and range from peraluminous to metaluminous. High Ga/Al and elevated concentrations of high-field-strength elements (HFSE) Zr, Nb, Y, Yb, Ta, and LREE indicate A-type affinity with HFSE enrichments suggesting derivation from heterogeneous, orogen-related crust. Similarities between BRP and SNP lithologies include coeval timing of magmatic activity, tholeiitic and A-type affinity, dominance of silicic compositions, and presence of igneous charnockites. Moreover, the presence of ca. 1080 Ma leucocratic granitoid suggests that fractionation was a significant petrologic mechanism leading to compositional diversity in both regions. Nevertheless, BRP and SNP lithologies are significantly different from rocks in the French Broad massif (FBM) of SW Virginia, where the oldest rocks are migmatites and amphibolites. Furthermore, the youngest rocks in the FBM and SNP include biotite-rich granites, whereas youngest rocks of the BRP contain only minor biotite. The differences in mineral assemblages and geochemistry imply that magmatism in the two areas involved different petrologic mechanisms, and possibly sources, and evolved along different lines of descent. Data indicate that the SM and the FBM have distinct histories prior to ca. 1080 Ma, consistent with models suggesting that the massifs are comprised of exotic crustal blocks possibly derived from different areas of Amazonia during Grenville-age orogenesis.