BREAKING UP IS HARD TO DO: A SUMMARY OF FIELD RELATIONS AND GEOCHRONOLOGY OF LATE TONIAN TO CRYOGENIAN INTRACONTINENTAL RIFTING OF RODINIA IN THE SOUTHERN APPALACHIAN BLUE RIDGE

The late Tonian Crossnore Complex in the Blue Ridge of VA, NC, and TN includes intrusive, extrusive, and sedimentary rocks that preserve evidence of intracontinental rifting, interpreted as the earliest rifting phase associated with the breakup of Rodinia. The Crossnore Plutonic Suite, composed of anorogenic granites (765–735 Ma) and gabbro (759­–757 Ma), intruded Mesoproterozoic basement rock. The Neoproterozoic Mount Rogers Fm. overlies Mesoproterozoic basement and consists of felsic and mafic volcanic rocks and coarse clastic sedimentary rocks. Clast lithologies and detrital zircon ages suggest intrabasin reworking and that volcanism initiated as early as ~780 Ma. The bimodal volcanic rocks are predominantly rhyolitic by volume and range in age from about 759–748 Ma. The glaciogenic Konnarock Fm. overlies and locally interfingers with the Mount Rogers Fm. and is overlain by the early Cambrian Chilhowee Group. The flow banded Ripshin Creek rhyolite (~751 Ma) erupted onto bedded glaciolacustrine rocks, constraining the minimum age of the Konnarock Fm. The Grandfather Mountain Fm. occurs unconformably on Mesoproterozoic basement and contains rift-related clastic sedimentary rocks and volcanic rocks of the Montezuma member and metarhyolite (~742 Ma). Clast provenance suggests rifting and intrabasin reworking of granitic rocks, rhyolites and greenstone; fine interbedded carbonate and phyllite are interpreted as lacustrine facies. Collectively, the Crossnore plutonic rocks represent the magmatic roots to some of the contemporaneous volcanic and sedimentary rocks that filled Neoproterozoic continental rift basins in the Blue Ridge. Clasts and detrital zircon provenance ages suggest intrabasin reworking of volcanics and plutonic roots, and significant tectonic uplift. Although these rift basins do not preclude the opening of a proto-Iapetus, there is little to no evidence that supports development into a rifted margin or ocean basin (e.g., marine facies, thick basalt flows, oceanic curst sensu stricto). Instead, we interpret the Crossnore Complex as related to intracontinental rifting that failed to break up Rodinia.