CULMINATING PULSE OF PLUTONISM IN THE GRENVILLIAN BLUE RIDGE TERRANE OF SOUTHERN LAURENTIA: ZIRCON-AGES FROM THE ARCHER MOUNTAIN SUITE, ROSELAND ANORTHOSITE, AND ROSES MILL PLUTON OF THE LOVINGSTON MASSIF

The Archer Mountain Suite (AMS) is one of four, aerially extensive, younger suites of biotite-rich granitoids of the Lovingston massif (eastern flank of the Grenvillian Blue Ridge terrane, VA). The AMS, typically massive to slightly foliated biotite-quartz-monzonite, intruded an older deformed complex of foliated granitoids and both foliated and layered granulite-facies gneisses. Paleozoic retrograde metamorphism extensively altered parts of the AMS and associated charnockitic plutons to biotite augen-gneiss. Zircons from an outcrop along the Rockfish River of typical foliated, but igneous-appearing, AMS, with scattered xenoliths of older rocks and a younger granite dike, yielded a concordia upper intercept age of 1019+/-1.5 Ma. The Roses Mill Pluton (RMP) is one of many small charnockitic plutons that intruded the AMS and older rocks of the Lovingston massif. Zircons from RMP jotunite at the type locality yielded a concordia upper intercept age of 1011+/-1.5 Ma. Xenoliths of the Roseland Anorthosite (RA) are found in the RMP and dikes of RMP and nelsonite (a facies of the RMP) cut the RA, indicating that the RMP is younger than the RA. Two samples of the RA were analyzed: one from altered anorthosite in a roadcut near Roseland and one from fresh unaltered anorthosite from an active quarry near Piney River. Zircons from altered RA yielded a concordia upper intercept age of 1011+/-3.5 Ma; those from fresh RA yielded an upper intercept age of 1010+/-2.2 Ma. Titanite from this sample yielded an age of 682+/-2 Ma which is similar to the age of a nearby Iapetan granitoid and may represent crustal cooling after passage of a hotspot through this region. These zircon ages, coupled with a published age from the related Oventop Suite in northern VA and additional zircon data from the Montpelier Anorthosite in the Goochland terrane, VA (likely an extension of the Lovingston massif) suggest widespread AMS-related plutonism (ca. 1030-1015 Ma) throughout the Lovingston massif, followed by emplacement of alkalic anorthosites (ca. 1015-1010 Ma), followed by widespread intrusion of small charnockitic plutons like the RMP beginning ca.1010 Ma. Thus widespread AMCG plutonism (ca. 1030-1000 Ma) in the Lovingston massif characterizes the youngest phase of the Grenville orogeny along the eastern flank of the Grenvillian Blue Ridge terrane.