WAS THERE A “GREAT GRENVILLE SEDIMENTATION EPISODE” (GGSE) ACROSS AMAZONIA FOLLOWING RODINIAN ASSEMBLY? EXPLORING THE CLASTIC RECORD IN SOUTHWESTERN BRAZIL USING DETRITAL ZIRCON GEOCHRONOLOGY

The Grenville Orogen, resulting from the continental collision of Laurentia and Amazonia, is proposed to be one of the largest, hottest longest-lived orogens in Earth history. Syn- to post-Grenville exhumation generated a pancontinental clastic wedge across Laurentia (the “Great Grenville Sedimentation Episode" [GGSE]: Rainbird et al., 2012). The Sunsas orogen of western Amazonia is purportedly correlative to the Grenville in southeastern Laurentia and related to final Rodinian assembly at 1200-1000 Ma (Shawinigan and Ottawan phases). Despite the paleogeographic correlation and robust detrital zircon (DZ) record in North America, no systematic DZ provenance analysis has been conducted on the Amazonian side of the Grenville orogen to test for a GGSE. Sixteen Proterozoic, Cambrian, and Devonian clastic units (~3000 DZ grains) from SW Brazil were dated via U-Pb LA-ICP-MS. Results were compared to an extensive Mesoproterozoic through Phanerozoic DZ record from eastern Laurentia.

Paleo- and Mesoproterozoic samples are dominated by Geons 20, 18, 17, 15, 14, and 13 age modes correlated to SW Amazonian basement terranes. Neoproterozoic samples are also dominated by Amazonian sources but with minor input from early Grenville orogenic events (Elzevirian). Ottawan and Shawinigan age modes appear by the late Neoproterozoic and then dominate the early Cambrian spectrum in the same manner observed in eastern Laurentian basal Cambrian strata. Amazonian Paleozoic samples become increasingly complex, exhibiting multiple age modes from sources within Amazonia, Laurentia, and Gondwana. In the Paleozoic samples the Ottawan- and Shawinigan-aged DZs become a prominent but not dominant component in DZ age spectra, similar to what is observed in Paleozoic samples from eastern Laurentia. Late Neoproterzoic Andean basement metaclastic units and sediment from Pleistocene-Holocene Amazon River drainages both show a dominant Grenville doublet despite no Grenville-aged basement exposed in the Amazon basin - the doublet must be recycled from Andean basement. Although deemed a “Grenville orogen” the dominantly low-grade metamorphic Sunsas belt exhibits a weak Ottawan magmatic component and lacks Shawinigan magmatism. The preferred explanation for the presence of the Ottawan-Shawinigan DZ age doublet in Amazonia is that some sediment from the GGSE spilled onto Amazon during Neoproterozoic exhumation of the Grenville orogen and was redistributed along the Neoproterozoic-Cambrian “rift to drift” margin of southwestern Amazonia.