GRENVILLE-AGED PALEOGEOGRAPHY AND DEFORMATION OF THE AMAZON CRATON: EARLY COLLISION WITH LAURENTIA?

A paleomagnetic and geochronologic study was undertaken on the Nova Floresta Formation of western Brazil, which is a large, undeformed mafic sill in order to constrain the paleogeography of the Amazon craton in Middle Proterozoic times. The paleomagnetic pole established from this formation (n=16 sites, Plat.=26N, PLon.=163E, A95=5.9) demonstrates that the Amazon craton was rotated 90 degrees from its present orientation and occupied an intermediate (c.45 degrees) latitude. New 40Ar/39Ar ages from biotite in the sill are 1198 +/- 3 Ma. Comparison of this position with the drift of Laurentia in late Mesoproterozoic times permits the geographic proximity of these two cratons, suggesting that the Amazon craton may have collided with Laurentia in early Grenville times. Deformation of the western Amazon craton is preserved in an extensive network of sinistral strike-slip shear zones in the western Brazilian state of Rondonia. Preliminary 40Ar/39Ar ages on the timing of this deformation from single grains of hornblende and biotite within this zone are ~1100-1150 Ma. Microprobe analysis coupled with BSE image analysis were used to determine the reintegrated compositions of exsolved feldspars, allowing the calculation of the temperatures at which feldspars recrystallized during this deformation. Results from this method suggest deformation at 600-700C. The observed sinistral sense in the shear zone network of Rondonia and the Nova Floresta paleopole are interpreted as recording Amazonia’s collision and strike-slip accommodation in the final Rodinia geometry around 1.1 Ga.