Petrologic and geochronologic evolution of the Grenville orogen, northern Blue Ridge Province, Virginia.

Abstract

Basement rocks in the northern Virginia Blue Ridge include petrologically diverse granitoids and granitic gneisses that collectively record over 100 m.y. of Grenville orogenic history. New U-Pb sensitive high-resolution ion microprobe (SHRIMP) isotopic analyses of zircon indicate igneous crystallization ages of 1159 ± 14 Ma (high-silica charnockite), 1078 ± 9 Ma (leucogranite gneiss), 1060 ± 5 Ma (Old Rag magmatic series), and 1050 ± 8 Ma (low-silica charnockite). These ages, together with SHRIMP and thermal ionization mass spectrometry (TIMS) ages from previous studies, define three intervals of Grenville-age magmatic activity: Ca. 1160-1140 Ma (Magmatic Interval I), ca. 1112 Ma (Magmatic Interval II), and ca. 1080-1050 Ma (Magmatic Interval III). Field relations and ages of crosscutting igneous units indicate that a high-grade deformation event, likely associated with Ottawan orogenesis, occurred between 1078 and 1050 Ma. All rocks display tholeiitic affinity and trace-element concentrations indicative of derivation from heterogenous sources. The low-silica charnockite exhibits A-type geochemical affinity; however, all other meta-igneous rocks are compositionally transitional between A-types and fractionated I-types. Similar ages of magmatism in the Blue Ridge and Adirondacks indicate that meta-igneous rocks in both massifs define age clusters that both predate and postdate the main pulse of local Ottawan orogenesis. Late- to postorogenic A-type magmatism is represented by the 1050 Ma low-silica charnockite in the Blue Ridge and the 1060-1045 Ma Lyon Mountain granitic gneiss in the Adirondacks. Zircons from Blue Ridge granitoids emplaced during Magmatic Interval III preserve evidence of thermal effects associated with waning stages of Ottawan orogenesis at ca. 1020 Ma and 980 Ma. Show more