Deformation and related radiochronology in a late Pan-African mylonitic shear zone, adrar des Iforas (Mali)

Abstract

The influence of a deformation on the zircon U/ Pb system was studied using the example of the western margin of the Iforas granulitic unit (Mali) where undeformed sub-alkaline leptynites have evolved to strongly foliated mylonites and then to ultramylonites. The deformation episod was late Pan-African and occurred at low temperature. Zircons show evidences of a brittle behavior with microcracks development, then, fracturation of the crystals and dispersion of the fragments in the ductile matrix. An equilibrium grain size was reached which probably depended of the stress, but was independant of the initial size of the zircons and of the strain.

In order to establish an accurate radiochronology of the studied area before the deformation, three rocks, not affected by the late Pan-African shear zone, have been dated by U/Pb method on zircons. In agreement to previous U-Pb, Rb-Sr and Sm-Nd data, the present results show that in the Iforas as in the western Hoggar, a main granulite facies metamorphism occurred 2,120±20 Ma ago. U/Pb data on zircons with inherited cores were interpreted by a multistage model with two episodic radiogenic lead losses, the first one at 2,120 Ma and the second one during Pan-African events. Then, a systematic investigation of the behavior of two chronometric systems (U/Pb on zircons and Ar³⁹-Ar⁴⁰ on feldspars) during a strong deformation episod at low temperature, was carried out. For the mylonitic episod, an upper limit of 566±8 Ma, is deduced from U/Pb data on non magnetic, coarse zircons of a late Pan-African granite, intruded in the western margin of the Iforas granulitic unit and slightly deformed by the mylonitic episod. Furthermore inherited cores are commonly present in these zircons, thus, the U/Pb data also indicated that this granite was formed 566±8 Ma ago, by partial melting of rocks with an apparent age of 2,609±200 Ma. A lower limit for the mylonitic episod was defined at 535±6 Ma by Ar³⁹-Ar⁴⁰ on K-feldspars extracted from a mylonite and an ultramylonite. An Ar³⁹-Ar⁴⁰ age of 900 Ma obtained on K-feldspars from an undeformed leptynite has no clear geological significance; the disturbing effect of the very closed shear zone, was probably not strong enough to reset totally the Ar chronometer. Finally, U/Pb data on zircons extracted from an ultramylonite indicated that the U-Pb system of these zircon fragments, remained closed during the deformation episod at low temperature.

The distinct behavior of the two radiochronological systems (Ar³⁹-Ar⁴⁰ on feldspars and U-Pb on zircons) is emphasized. For ultradeformed samples at low temperature, the age of the deformation and the age of the rocks which have undergone this deformation, can be determined using the two complementary chronometers.