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

The northern part of the Kedougou Kenieba Inlier (Eastern Senegal) is composed of Palaeoproterozoic volcano-plutonic and volcanosediment 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.

Proterozoic magmatic events recorded in 40Ar/39Ar data from the northern part of the Kedougou Kenieba Inlier (eastern Senegal)

“…New 40 Ar/ 39 Ar ages obtained here on hornblendes and biotites range within 2028 ± 14 and 2055 ± 10 Ma (Table 1), with a mean age of about 2040 Ma. Comparison with previous K-Ar ages realized on the same samples (Sagna et al, 2017) shows that the 40 Ar/ 39 Ar ages obtained here are more clustered and some are significantly older. This could be explained by the fact that the K-Ar technique is unable to detect loss of radiogenic argon from the less retentive sites during thermal events (e.g., McDougall and Harrison, 1999).…”

“…Recent K-Ar ages, performed on hornblende, biotite and K-feldspar minerals from the same samples as those investigated in the present study, have shown that plutonism in the Kedougou Kenieba Inlier occurred between 2.0 and 2.1 Ga, and suggested either a slow cooling rate for hundreds of Ma, or that a thermal event partially re-opened the K-Ar system of K-feldspars from this area (Sagna et al, 2017).…”

“…They were emplaced in an arc-type setting (Dia et al 1997;Sagna et al, 2017) and accreted during the Eburnean Orogeny (Baratoux et al, 2011;Begg et al, 2009;Feybesse et al, 2006).…”

Exhumation history of the Variscan orogen in western Iberia as inferred from new K-Ar and 40Ar/39Ar data on granites from Portugal

“…One Ar determination was conducted in K-feldspar of the samples for preliminary purposes, as K-Ar ages on such phases generally provide inconclusive information regarding granite crystallization age at depth due to their poor Ar retentivity at high temperature (e.g. Sagna et al, 2017). However, K-feldspar can be used to achieve a thermochronological analysis through step-heating 40 Ar/ 39 Ar (see next section, and Lovera et al, 1997;Sagna et al, 2021).…”

In-situ K-Ar dating on Mars based on UV-Laser ablation coupled with a LIBS-QMS system: Development, calibration and application of the KArMars instrument

“…The air aliquot used to calibrate the 40 Ar measurements is calibrated by routine measurements of the interlaboratory standard HD-B1 (Fuhrmann et al, 1987). This technique is suitable for dating into the last millennia (Gillot et al, 2006;Quidelleur et al, 2001), as well as into the pre-Cambrian (Sagna et al, 2017). Note that for old samples determination, the total relative age uncertainty is 1.4% and is calculated as the quadratic sum of the 1% uncertainties attached to both the K content determination and the 40 Ar calibration, the radiogenic argon content uncertainty being negligible.…”