IMPLICATIONS OF A TWO STAGE TECTONIC HISTORY OF THE SW AMAZON CRATON: THE NOVA BRASILANDIA METASEDIMENTARY BELT AS A LATE MESOPROTEROZOIC SUTURE ZONE IN RECONSTRUCTIONS OF RODINIA

Rocks exposed in the Nova Brasilândia metasedimentary belt (NBMB) of the SW Amazon craton are critical to Rodinia reconstructions, as they form the Mesoproterozoic margin of the Amazon craton. The P-T-t history of the belt reveals a one-stage orogenic evolution marked by the development of thrust faults and sinistral strike-slip faults that contrasts with the older, polycyclic history of the Early to Middle Proterozoic Amazon basement rocks to the north. Thermobarometric calculations for upper amphibolite to granulite facies rocks in the belt yield peak metamorphism of 8 kbar at 800°C. P-T paths determined from zoned minerals and overprinting relations demonstrate competing processes of crustal thickening, consistent with both granitic magma emplacement (counterclockwise loop of granulites) and crustal imbrication through thrust faulting (clockwise loop for amphibolites). New U-Pb ages from monazite (3 samples; 1090 – 990 Ma), titanite (3 samples; 1070 – 1020 Ma) as well as 40Ar/39Ar data from hornblende (2 samples; 980 - 960 Ma) and biotite (5 samples; 960 – 910 Ma) are used to constrain the cooling rate of the NBMB to 2-3°C/Ma from the peak of thermal metamorphism at ca. 1.09 Ga. The separate cooling trend of the belt, when compared with the early Grenville (ca. 1.2 –1.15 Ga) deformation of the Amazon basement to the north, signifies that two orogenic events characterize the Grenville history of this portion of the Amazon craton. We therefore prefer a model for the evolution of Rodinia in which initial collision of the Amazon craton with southern Laurentia (Llano region of Texas) was followed by progressive, along strike motion of the Amazon craton towards a more northerly position, such as near the Ontario-New York segment of the Grenville Province. This new transpressional model accounts for the polycyclic history of the Amazon craton and is supported with structural and geochronological evidence from the Grenville Province of eastern Laurentia.