“THE TALE OF 3,000 DETRITAL ZIRCONS”: AN INVESTIGATION OF GRENVILLE SEDIMENTATION IN AMAZONIA DURING RODINIAN ASSEMBLY

The continental collision between Laurentia and Amazonia that generated Rodinia and the Grenville Orogen is proposed to be one of the largest, hottest and longest-lived orogens in Earth history. Subsequent erosion and weathering led to a mountain range’s worth of sediment recycled into clastic systems across North America, otherwise known as the “Great Grenville Sedimentation Event”. The Sunsas orogeny in Amazonia is correlative to the Grenville in North America and is proposed to be the result of final Rodinian collision at 1200-1000 Ma. Despite the connection between the two ranges and the robust sedimentary record in North America, little work has been done to constrain sediment availability on the Amazonian side of the Grenville orogen. This study examined 16 samples collected from western Brazil including: quartz-arenite, diamictite, quartzite, and other metsedimentary units, with basement samples for age comparison. Detrital zircons were analyzed for U/Pb isotope ratios using LA-ICP-MS, resulting in approx. 3,000 analyses for the 16 samples. These analyses suggest only a minor sediment contribution to the Amazonian craton from Grenville-age terranes. Instead, most samples exhibit ages characteristic of Amazonian crustal growth. The dominance of Geons 15 and 17 indicates that Grenville-aged sediment does not constitute the main sediment-source region and with no significant increase in the number of Grenville zircons in samples collected proximal to the Sunsas orogenic front. Age modes are better explained from the Rio Negro-Juruena (1.8-1.55 Ga) to the northeast and the Rondônia-San Ignacio (1.5-1.3 Ga) provinces to the southwest. We interpret these results to mean that although Laurentian clastic sequences are perpetually dominated by Shawinigan and Ottawan sources, the Amazonian craton was not influenced by the same sediment influx. ⁴⁰Ar/³⁹Ar analyses from southwest Amazonian document a change in tectonic regime during the Grenville cycle, transitioning from collisional, thrust-dominated along the Laurentian margin to strike-slip in Amazonia. The lack of Grenville-aged sediment supports an asymmetric orogen in which changes in crustal thickness and exhumation prevented erosion and deposition on the Amazonian side making the primary source for sediments the Amazonian tectonic provinces