K–Ar mineral ages and thermal history of magmatic and metamorphic Palaeoproterozoic units from the northern part of Kedougou Kenieba Inlier, West African Craton (Eastern Senegal)

Abstract

AbstractThe northern part of the Kedougou Kenieba Inlier (Eastern Senegal) is composed of Palaeoproterozoic volcano‐plutonic and volcano‐sediment areas cross‐cut by differentiated plutonic bodies of gabbroic to granitic composition. They are emplaced in an arc‐type environment and accreted during the Eburnean Orogeny. We present here new K–Ar ages obtained on several plutons from different mineralogical phases, each characterized by a different closure temperature for the K–Ar system. Ages from hornblende, between 2013 ± 29 and 2141 ± 30 Ma, constrain the cooling history of the magma bodies through the 550 °C isotherm. They are slightly younger than U–Pb ages previously obtained from the same units, attesting of a rapid cooling, tectonically controlled, of about 7 °C/Myr. Ages obtained on biotite minerals range from 1955 ± 28 to 1893 ± 27 Ma, suggesting that from 550 to 325 °C, cooling rate decreased to about 3 °C/Myr. Significantly younger ages ranging from 1704 ± 24 to 1615 ± 23 Ma have been obtained on plagioclase and microcline. They can be interpreted in terms of either a slow cooling with a rate of only 1 °C/Myr, from about 1.9 to 1.6 Ga, or either reflecting the occurrence of a rather limited thermal event younger than 1.7 Ga. Since no age younger than 2.0 Ga has previously been obtained for plutonic rocks from the early Birimian in the West African Craton, ages of 1.7–1.6 Ga obtained here from feldspars might be the first evidence for a regional re‐heating event having occurred in the East–North Kedougou Kenieba inlier. Copyright © 2015 John Wiley & Sons, Ltd.