Polyphase deformation and deformation-magmatism interaction in the Pan-African 4°50′ shear zone (Hoggar, Southern Algeria)

Abstract

The 4°50′Shear zone (SZ), one of the major structures of western Gondwana, formed in Hoggar during the Pan-African orogeny is still poorly studied; especially ages and deformation characteristics are lacking. We performed microstructural analysis and EBSD measurements on nine selected mylonites from the 4°50′SZ, and zircon U-Pb dating on two granitoid samples. The protolith of these mylonites are granitoids. Most samples display segregation of dominant feldspar and minor quartz in separate layers that underline the N-S subvertical mylonitic foliation. Feldspar grains display evidence of intracrystalline deformation, supporting dextral shear-sense. Plagioclase and K-feldspar are frequently corroded with embayments infilled by secondary K-feldspar and plagioclase respectively. Interstitial quartz grains disseminated in feldspar aggregates may infill fractures in feldspar grains. Quartz ribbons, usually parallel to the foliation, locally crosscut feldspar layers. Quartz is partially recrystallized and elongate crystals form a secondary foliation at ∼ 30° to the main foliation, supporting a later sinistral shearing. In some samples, amphibole is associated with quartz and oriented parallel to the foliation. In all samples, quartz CPO suggests a dominant prism-<a > slip-system, activated at medium/high temperature (∼500-700°c), but stress-induced oriented crystallization may have contributed to this CPO. K-feldspar CPO supports the activation of the [100](010) slip system while the plagioclase CPO points to activation of [100](001) system. These CPO support dislocation creep under amphibolite facies conditions. These new data, in addition to U-Pb dating, suggest the following evolution: 1) successive intrusions of granitoids in the SZ (∼661-∼639 Ma) due to partial-melting of the lower-crust and, possibly also of the lithospheric-mantle, 2) the 4°5 0′SZ was rooted in the lower-crust or the upper-mantle, and 3) dextral shearing lasted over ≥ 20My. This was followed by successive magma batches that intruded these rocks during a late migmatitic episode (∼623-∼609 Ma). After ∼ 609 Ma, a moderate sinistral reactivation of the 4°50′SZ occurred and was accommodated through dislocation creep mainly localized in quartz ribbons. During accretion of the Western Gondwana, the 4°50′SZ was active during subduction. During ≥ 20My, it was successively intruded by granitoids that cooled down slowly, and have been deformed in the submagmatic state then in the solid-state.