THE END OF AN ERA: HOW GEON 14 CAME TO A CLOSE DURING GEON 13 WITH THE APPROACH OF AMAZONIA AND RODINIAN COLLISION – EVIDENCE FROM SOUTHEASTERN LAURENTIA AND SOUTHWESTERN AMAZONIA

The Mesoproterozoic southeastern margin of Laurentia includes the Geons 14-13 Granite-Rhyolite provinces that were extensively reworked during the Geon 11 Shawinigan and Geon 10 Ottawan phases of the Grenville orogenic cycle (GOC). Questions remain regarding the extent of GRP and transition from the GRP to Grenville orogen for much of southeastern Laurentia due to sedimentary cover and tectonic reworking during Paleozoic orogenesis. Scattered basement cores, detrital zircon ages from Meso-/Neoproterozoic to Phanerozoic Laurentian margin clastic sequences, new basement geochronology, and expanded Nd-Pb isotope systematics nail the proposed piercing point for the conjugate SE Laurentian and SW Amazonian margins in the Mesoproterozoic. Basement rocks in eastern OH-KY-TN include Geon 14 magmatic rocks, some overprinted by Geon 10 granulite facies metamorphism, and with early Neoproterozoic Ar-Ar cooling ages. These characteristics mirror those along the exposed Grenville Front. These rocks are the most outboard tangible evidence of (Grenville modified) Geon 14 crust. Newly recognized Geon 13 crust in southeastern Laurentia, the earliest component of the GOC as exposed in Grenville Blue Ridge inliers (NC to VT), formed outboard of and is reworked along with Geon 14 crust by Geons 12, 11 and 10 tectonism. In comparison, the southwestern margin of Amazonia consists of Geon 18-14 belts (younging outward similar to southeastern Laurentia), with Geon 14 crust being reworked during the Geon 13 San Ignacio orogeny. The correspondence of ages and whole rock Pb isotope compositions support the Laurentian-Amazonian piercing point. In contrast to Laurentia, where the Grenville is one of Earth’s largest, hottest, longest-lived orogens, and where the Appalachian orogen exposes Grenville crust, there is little exposed Grenville basement along the modern Andean convergent margin. The Sunsas belt, the proposed Grenville orogen-equivalent in western Amazonia, is a mostly low-grade metasedimentary belt with only minor magmatism at ca. 1050-950 Ma. It is hardly a match for the magnitude of crustal reworking exhibited by the Laurentian Grenville. However, detrital zircon ages from Neoproterozoic and Paleozoic rocks in the Andes, and from Pleistocene to modern Amazon river sediments exhibit major Grenville (Shawinigan and Ottawan) age modes, hinting at either a greater Grenville orogen at depth or spillover from the “Great Grenville Sedimentation Episode” onto Amazonia as the Grenville orogen was slowly exhumed in the Neoproterozoic.