An investigation of the tectonic history of the SW Amazon craton in the late Mesoproterozoic: Implications for Rodinia reconstructions

The SW Amazon craton exposed in the western Brazilian states of Rondônia and Mato Grosso was studied to understand the tectonic history for the period 1.2–1.0 Ga, an interval of time that witnessed the construction of the pre-Pangaea supercontinent Rodinia. The amalgamation of Rodinia took place along suturing mountain belts that are collectively referred to as the Grenville orogeny, a name adopted from the deeply exhumed Grenville belt exposed along the eastern margin of Laurentia, the Precambrian nucleus of North America. A paleomagnetic and geochronological study suggests that the Amazon craton collided with southernmost Laurentia (Llano province, central Texas) at the beginning of the Grenville interval. The oblique collision caused extensive sinistral, strike-slip deformation of the Amazon craton from ca. 1.2 Ga to 1.12 Ga, constrained by 40Ar/39Ar geochronology and feldspar thermometry. A younger metamorphic history is preserved in the 1.09 Ga Nova Brasilândia metasedimentary belt, interpreted to represent the limit of cratonic Amazonia for preceding Mesoproterozoic times. Exhumation from mid-crustal conditions (peak metamorphism ca. 850°C at 800 MPa) following deformation indicates that substantial crustal thickening was accommodated in this collisional belt that marks the suturing of the Paragua craton to the Amazon craton during late Mesoproterozoic times. A comparison of time-temperature (T-t) paths constructed from U-Pb, 40Ar/39Ar, and Rb-Sr geochronology for the Amazon craton and the adjacent Nova Brasilândia metasedimentary belt indicates no overlap in the spatial or temporal occurrence of metamorphism. The separate T-t paths are used to reconstruct a two stage tectonic history for the orogenic interaction of the Amazon craton with Laurentia. Following the original collision at ca. 1.2 Ga, continued deformation of the Amazon craton under transpressive conditions resulted in significant strike-slip displacement from the original Llano position to a mid-Laurentian position. The suturing of the Paragua craton to the Amazon craton along the Nova Brasilândia metasedimentary belt took place in the final stages of the construction of Rodinia. In this model, the Grenville orogen is considered analogous to a Cordilleran type setting marked by both crustal thickening and orogen parallel strike-slip displacements.