ABSTRACT The Mesoproterozoic southeastern margin of Laurentia, which consisted primarily of the ca. 1.5–1.35 Ga Granite-Rhyolite Province, was extensively reworked during ca. 1.3–0.9 Ga phases of the Grenville orogenic cycle. Questions remain for much of southeastern Laurentia regarding the transition from the Granite-Rhyolite Province to Grenville orogenic cycle, and for potential collisional interaction with Amazonia, due to Paleozoic sedimentary cover or tectonic reworking. Basement rocks sampled by drill core in the east-central United States include 1.5–1.35 Ga magmatic rocks, some overprinted by late Geon 10 (Ottawan) orogenesis, which are the most outboard evidence of Granite-Rhyolite Province crust. Newly recognized 1.35–1.30 Ga (pre-Elzevirian) granitic orthogneisses within the Mars Hill terrane of southeastern Laurentia (1) expand the along-strike distribution of the earliest crustal age components of the Grenville orogenic cycle in Appalachian basement inliers; (2) contain Geon 19–16 inherited zircons; and (3) were metamorphosed during late Ottawan to Rigolet tectonism. Paragneisses enveloping the Geon 13 orthogneisses are dominated by Geon 19–16 and Geon 13–12 detrital zircons overgrown by Geon 10–9 metamorphic zircon. The zircon age systematics require the paragneiss protoliths to be younger than orthogneiss protoliths and be partly sourced from the latter. Orthogneisses and paragneisses have Pb isotope compositions that overlap those of south-central Appalachian and southwest Amazonia basement, both of which are distinct from Laurentian Pb isotope compositions. The boundary between Amazonian (southern Appalachian) and Laurentian (northern Appalachian) Pb isotope compositions is thus a terrane boundary, with Geon 13 magmatic rocks being the youngest common crustal component. In comparison, the Paraguá block of the southwestern margin of Amazonia consists of a Geon 19–16 basement complex intruded by the batholithic-scale Geon 13 San Ignacio granite suite. The latter also contains inherited Geon 19–16 zircon and has Pb isotope compositions that help define the Amazonian trend. The correspondence of magmatic, inherited, and detrital ages and similarity in Pb isotope compositions are consistent with an origin for the exotic/orphaned Mars Hill terrane as an outboard sliver of the Paraguá block that developed before Grenvillian orogenesis (Geons 12–9). Manifestations of the latter are concentrated around the margins of the Paraguá block in the Sunsás (southwest), Nova Brasilândia (north), and Aguapeí belts (east). The Sunsás belt is a mostly low-grade metasedimentary belt with only minor Geon 10–9 magmatism and no Geon 12 or 11 magmatism, thus distinguishing it from the Mars Hill terrane. The Arequipa-Antofalla terrane, exposed in Andes basement inliers, lies outboard of the Sunsás belt and has Pb isotope and geochronologic characteristics that permit a correlation with the Mars Hill terrane and a paleogeographic position between the Mars Hill terrane and the Sunsás belt. The histories of the Mars Hill terrane, Arequipa-Antofalla terrane, and Paraguá block merge during Geons 10–9 and final collisional orogenesis between southeast Laurentia and southwestern Amazonia.

ABSTRACT The eastern Great Smoky Mountains basement complex consists of the following components: (1) ca. 1350–1325 Ma orthogneiss and mafic xenoliths that represent some of the oldest crust in Appalachian Grenville massifs (similar to “pre-Grenville” basement components in the Adirondack, Green Mountain, Hudson Highland, and Shenandoah massifs); (2) ca. 1150 Ma augen orthogneisses and granitic orthogneisses correlating with the Shawinigan phase of Grenville magmatism; and (3) paragneisses (cover rocks) that have either pre- or syn-Grenville (i.e., Mesoproterozoic) versus post-Grenville (Neoproterozoic) depositional ages, and that experienced Taconian metamorphism and migmatization. Mesoproterozoic paragneisses contain major zircon age modes that require a component of Proterozoic crust in the source region. The Neoproterozoic paragneisses exhibit the archetypical “Grenville doublet” in detrital zircon age distributions that matches the age distribution of Ottawan and Shawinigan magmatic/metamorphic events in eastern Laurentia. Most zircon U-Pb age systematics exhibit variable lead loss interpreted to result from high-grade Taconian (ca. 450 Ma) regional metamorphism and migmatization. Neodymium mantle model ages (T DM ) for ortho- and paragneisses range from 1.8 to 1.6 Ga, indicating that all rocks were derived from recycling of Proterozoic crust (i.e., they are not juvenile), which is consistent with Proterozoic detrital zircon ages in pre- to syn-Grenville paragneisses. Lead isotope compositions confirm the presence of an exotic (Amazonian) crustal component in the source region for the protoliths of the pre-Grenville orthogneisses and xenoliths, and that this exotic component was incorporated to varying degrees in the evolution of the basement complex. The oldest age component may represent an Amazonian pre-Grenville analog to the ca. 1.35 Ga native Laurentian crust present in Adirondack and northern Appalachian basement massifs.

Abstract The South American record of remagnetizations is linked to specific events of its tectonic history stretching back to Precambrian times. At the Ediacaran–Cambrian time interval (570–500 Ma), the final stages of the western Gondwana assemblage led to remagnetization of Neoproterozoic carbonates within the São Francisco–Congo Craton and at the border of the Amazon Craton, along the Araguaia–Paraguay–Pampean Belt. From the late Permian to early Triassic, the San Rafaelic orogeny and the emplacement of the Choiyoi magmatic province was responsible for widespread remagnetizations in Argentina and Uruguay. Cretaceous remagnetization has also been documented in Brazil and interpreted to result from magmatism and fault reactivations linked to the opening of the South Atlantic Ocean. We present a review of these widespread remagnetization events principally based on palaeomagnetic data and, when available, on rock magnetic and radiogenic isotope age data. This study gives an overview of the geographical distribution of the remagnetization events in South America, and provides important clues to better understand the geodynamic evolution of the South American plate at these times. In addition, magnetic mineralogy data for the different case studies presented here constrain the physical–chemical mechanisms that led to partial or total resetting of magnetic remanences in sedimentary rocks.