TIME SCALES OF EVOLVING MAGMA SOURCES DURING RIFT TO DRIFT TECTONICS ASSOCIATED WITH BREAKUP OF RODINIA: THE CENTRAL APPALACHIAN PERSPECTIVE

The breakup of supercontinents is accompanied by magmatism reflecting various geodynamic settings that enable emplacement of dike swarms, eruption of flood basalts, as well as development of upper and lower plate configurations associated with extending plates. The breakup of Rodinia, with respect to Laurentia, spans a time interval of 825 to 570 Ma, with a space-time progressive record of magmatism from China and Australia to eastern margin of North America, where a plume track with inferred plate motion rate of 2 cm/yr and, which corresponds in time to both Mackenzie and Franklin events. Although the global breakup of Rodinia was a slow process, magmatic history of rift to drift transition in central Appalachian region suggests a duration of approximately 30 m.y. We use mafic rocks in the Lynchburg basin, as well as overlying sub-areal flows of the Catoctin Formation to suggest a temporal change from lithosphere dominated melts (607 Ma) to plume (OIB) dominated melts by 575 Ma. In contrast to basalts in other Neoproterozoic basins, where OIB magmatism is prevalent, our data suggest that compositional changes occur late in basin evolution, and OIB magmas break through the thinned lithosphere late in the history of extension. Our data suggests basalts generated during incipient supercontinent extension (Stage I in the Lynchburg basin) have chemical/isotopic signatures dominated by sub-continental lithosphere mantle (SCLM) with TiO₂ < 2 wt%. Stage II basalts (amphibolites in Unicoi, Bassett, Sams Creek Formations, and sub-areal flows of Catoctin Formation mark transition to a rift stage where Ocean island basalt (plume) is dominant (TiO₂> 2 wt %), but often diluted by SCLM. Amphibolites hosted within Ashe and Alligator Back Formations belong to both Stages I and II, and perhaps are tectonically interleaved. Stage III amphibolites (Kennett Square) are related to a depleted asthenosphere source and represents Iapetus ocean floor basalts thrust on the continental margin during mid-Ordovician Taconic orogeny. Identification of compositional changes associated with well preserved rift/drift stages is critical in identifying temporal changes in mantle sources.