Thermal and tectonic evolution of the paleoproterozoic Transamazonian orogen as deduced from 40Ar/ 39Ar and AMS along the Oyapok river (French Guyana)

Abstract

New 40Ar/ 39Ar dating of hornblende and biotite, and measurement of anisotropy of magnetic susceptibility (AMS) have been performed on a 150 km section along the Oyapok river in order to better constrain the timing of cooling of the Transamazonian orogen in the south of French Guyana. From north to south, hornblende cooling ages range from 2052±4 to 1993±7 Ma, and corresponding biotite cooling ages vary from 1995±4 to 1928±4 Ma. This suggests a uniform cooling rate (3–5 °C/Ma) from 550 to below 250 °C. The South Guyana Complex (SGC) cool down later than the Southern Guyana Belt (SGB). The SGC displays high temperature metamorphism and higher magnetic anisotropy degree ( P′:1.18–1.44). We interpret the high P′ values as the result of the development of a ductile fabric consecutively to transcurrent tectonic. The regular southward decrease of the cooling ages and the temperature profile determined at 1995 Ma along the studied 150 km section, indicates that no significant differential vertical movements affected the SGC. These results imply that the regional cooling was not associated with a rapid exhumation process as found in modern orogens. Instead, we propose a model of exhumation with a constant speed and later exhumation of the southern Oyapok zone belonging to a lower structural level and/or retarded by the transcurrent tectonics during the last Transamazonian magmatic event at 2100 Ma.