U–Pb dating and Sm–Nd isotopic analysis of granitic rocks from the Tiris Complex: New constaints on key events in the evolution of the Reguibat Shield, Mauritania

Abstract

New U-Pb geochronology indicates that the main intrusive events, broadly 27 associated with formation of dome-shaped structures, occurred at around 2.95 to 2.87 28 Ga and 2.69 to 2.65 Ga. This study also recognises younger regional metamorphism largely Palaeoproterozoic granitic and volcano-sedimentary rocks (Bessoles, 1977; 46 Dillon & Sougy, 1974). In Mauritania, these crustal domains were juxtaposed during 47 the, c. 2.10-2.00 Ga, Eburnean Orogeny (Schofield et al., 2006). et al., 1992; Potrel, 1994; Potrel et al., 1996) 104 including orthogneiss dated by the U-Pb method at 3515 ± 15 Ma and 3422 ± 10 Ma 105( Potrel et al., 1996). Incorporation of an even older crustal component is suggested by 106Nd T DM ages of around 3.60 Ga (Potrel et al., 1996). A subsequent event has been 107 proposed based on U-Pb dating of charnokite at 2986 ± 8 Ma (Potrel et al., 1998 to 3.1 Ga (Potrel, 1994; Potrel et al., 1996;1998 The Amsaga Complex is juxtaposed against the Tiris Complex along a broad 122 belt of aluminous gneisses (Bronner, 1977;Rocci et al., 1991; O'Connor et al., 2005). 123To the northeast, these pass into an extensive complex dominated by foliated and planar fabrics (Fig. 4c). These are unaffected by earlier fold phases and are interpreted 269 as recording the youngest deformation episode episode in this part of the study area. 270The northern margin of this domain is defined by the El M'dena Fault Zone (Fig. 2 This sample was collected from the Mirikli Formation within the high strain looking crystals, suggesting a lower uranium concentration than the first population. 360Of twenty-seven zircon analyses from this second population six provided near 361 concordant data. The age spread suggests crystallization of the granite at 2487 ± 8 Ma 362(2σ, Fig. 8d) with inheritance at c.2550 and 2700 Ma (Fig. 8d). This sample yielded 363 an ε Nd value of -5.7 at t = 2487 Ma and T DM 3.14 Ga. good quality titanite and allanite (Fig. 7e) and good quality bipyrimidal and elongate 372 zircons (Fig. 7f). Thirty one analyses from twenty two zircons were performed on this 373 sample. Four analyses were excluded from a calculation giving a Concordia age of 374 2472 ± 6 Ma (2σ, Fig. 8e). This is interpreted to represent the crystallization age of 375 the rock. This sample yielded an ε Nd value of -2.1 at t = 2472 Ma and T DM 2.9 Ga. Geological development of the Tiris Complex 388The oldest rocks preserved in the Tiris Complex, pre-dating the main phase of 389 granite magmatism at around 2.95 Ga, comprise metasedimentary ferruginous 390 quartzite and aluminous and hypersthene-bearing paragneiss of the Mirikli Formation. 391These may be equivalent to similar assemblages preserved in the El Khadra and el 392Geicha formations. The presence of bands and pods of metamafite intercalated with 393 the metasedimentary host indicate that there was also an early phase of mafic dyke 394 emplacement, although the age of this also remains unconstrained. 395Early tectonism, also pre-dating the main intrusive episode, is recorded by 396...

Archaean crustal evolution in West Africa: A new synthesis of the Archaean geology in Sierra Leone, Liberia, Guinea and Ivory Coast

“…U-Pb zircon studies also indicate the formation of granitic crust at 3.1 Ga in the Tasiast-Tijirit terrane (Montero et al, 2014) and between 3.0-2.9 Ga in all other parts of the Reguibat Shield (Potrel et al, 1998;Schofield et al, 2012;Montero et al, 2014;Heron et al, 2015). This would appear to be the main crust forming event in this region.…”

“…The Archaean rocks of the Reguibat Shield have been subdivided into four terranes (Schofield et al, 2012). In the southwest is the Tasiast-Tijirit granite greenstone belt terrane, east of which is the Amsaga granitic gneiss complex.…”

“…In the southwest is the Tasiast-Tijirit granite greenstone belt terrane, east of which is the Amsaga granitic gneiss complex. To the northwest of the Tasiast-Tijirit terrane is the Tiris Complex a granite gneiss complex with intercalated sediments, and further to the northeast is the Ghallaman granite gneiss and granulite complex (Schofield et al, 2012;Montero et al, 2014), see Figure 1.…”

“…There are high grade, BIF-hosted iron-ore deposits in the Tiris Complex and disseminated chromite in banded serpentinites in the Amsaga complex (Markwitz, 2016a) Unfoliated granites, post-dating the formation of the TTG gneisses formed at 2.83 and 2.92 in the Ghallaman Complex (Lahondere et al, 2003) and between 2.71-2.73 Ga in the Amsaga Complex (Potrel et al, 1998). There is also late Archaean granitic magmatism at 2.65-2.69 Ga in the Tiris Complex but Nd model ages indicate involvement of older granitic crust (Schofield et al, 2012).…”

“…Whilst the geological histories of the Archaean part of the Reguibat Shield and the Archaean rocks in Sierra Leone, Liberia, Guinea and Ivory Coast are not well known, on the basis of our current knowledge of the geology and geochronology there appear to be some significant similarities between the two regions (Schofield et al, 2012). Both have remnants of 3.5 Ga crust and both cratons appear to have grown significantly between 3.25-2.85 Ga with the main crust-forming event probably between about 3.0-2.9 Ga.…”