A tectonic model for the juxtaposition of granulite- and amphibolite-facies rocks in the Eburnean collision in the orogenic belt (Sassandra-Cavally domain, Côte d’Ivoire)

25 The Sassandra-Cavally (SASCA) domain (SW Côte d’Ivoire) marks the transition between the 26 Archean Kenema-Man craton and the Paleoproterozoic (Rhyacian) Baoule-Mossi domain. It is 27 characterized by the tectonic juxtaposition of granulite-facies and amphibolite-facies rocks. 28 Migmatitic grey gneisses, garnet-cordierite-sillimanite migmatitic paragneisses and garnet29 staurolite-bearing micaschists reached peak pressure conditions ranging from ~6.6 kbar at 30 620°C to ~10 kbar at 820°C. These conditions are associated with the first recorded deformation 31 D1 and correspond to a Barrovian geothermal gradient of ~25°C/km. Subsequent exhumation, 32 associated with a second deformation D2, was marked by decompression followed by cooling 33 along apparent geothermal gradients of ~40°C/km. A D3 deformation phase is marked by 34 folding and local transposition of the regional S1/S2 foliation into E-W trending shear zones. 35 LA-ICP-MS U-Pb dating of monazite, which displays complex internal structures, reveals four 36 age groups correlated to textural position of monazite grains and analytical points : 1) Rare 37 relictual zones yield dates at the Archean-Paleoproterozoic transition (c. 2400–2600 Ma); 2) a 38 cluster of dates centered at c. 2037 Ma on grains aligned along the S2 foliation of the migmatitic 39 grey gneiss, attributed to D2; 3) a cluster of dates centered at c. 2000 Ma, and 4) dates spreading 40 from c. 1978 to 1913 Ma, documented for the first time in the West African Craton monazites. 41 The ages of the latter two groups are similar to the ones identified in the Guiana Shield, and 42 could be attributed to a disturbance by fluids, to a periodic opening of U-Pb system or to an 43 episodic crystallization of monazite during slow cooling lasting several tens of Myrs. These 44 data allow to propose a model for the tectonic evolution of of the SASCA domain at the contact 45 between the Rhyacian Baoule-Mossi domain and the Archean Kenema-Man nucleus whereby 46 crustal thickening is achieved by crustal-scale folding and is followed and concomitant to lateral 47 flow of the thickened partially molten crust accommodated by regional transcurrent shear 48 zones. This combination of crustal thickening controlled by tectonic forces and gravity-driven 49 flow leads to the juxtaposition of granuliteand amphibolite-facies rocks. 50 51


INTRODUCTION
aplites and pegmatites (Papon, 1973), and cross-cut by large ductile structures such as the 229 Greenville-Ferkessédougou-Bobo-Dioulasso shear-zone (Lemoine et al., 1985; Baratoux et al.,    particles ablated from the sample were mixed with Ar before entering the plasma. Laser spot 297 size was 5 μm for monazite and the laser was operated at a repetition rate of 2 Hz using a 12 298 J/cm 2 energy density. Total analysis time was 60 s with the first 15 s used for background 299 measurement which was subtracted from the sample signal. Before each analysis the surface of 300 the targeted zone was cleaned with 10 pulses using a spot size larger than the size used for U-

314
High spatial resolution mapping at 15 keV and 20 nA for 5-10 s per pixel on the CAMECA 315 SXFive electron microprobe of Archean grey gneisses monazites (SP3B) larger than 100 μm 316 was performed at a resolution of ca 200x120 pixels per grain to visualize their complex 317 chemical zonations. Chemical maps of Ce, Pb, Th, U, and Y were used to guide subsequent to the fold axes and plunges between 80° and 30° to the NNE. D2 is interpreted as recording 330 continuous NNW-SSE shortening under a transpressive regime. In the San Pedro area, D1 and 331 D2 structures are overprinted by the deformation phase D3 that is expressed as F3 folds and a 332 penetrative subvertical E-W axial planar schistosity S3 associated with E-W-striking dextral 333 shear-zones suggesting a WNW-ESE shortening (Fig. 4). Geological cross-section and equal-334 area, lower hemisphere stereoplots document the structures related to the D2 deformation phase 335 ( Figs. 4b and 4c, respectively). The relationship between structural observations and mineral 336 assemblage will be discussed in the petrography and mineral chemistry section.  Table 1). Representative mineral 341 compositions are reported in Table 3.

502
Migmatitic grey gneiss (SP3B) 503 Monazite in SP3B has a variable size, ranging from 10 to 100 μm. It commonly forms clusters 504 aligned parallel to S2 (Fig. 11b). Under BSE imaging, some grains reveal contrasting areas 505 showing complex zonations (Figs 9d-g). X-ray maps were performed on three grains (≥50 µm) 506 in order to assess the internal distribution of the chemical elements and target U-Pb analyses.  Eighty-one analyses were carried out on twenty-eight grains located in different textural 511 positions in the thin section, included in either biotite, plagioclase or garnet (Table S2, (Fig. 12a). The main group of 46 analyses 517 provides a 207 Pb/ 206 Pb weighted mean age of 2037 ± 2 Ma (MSWD = 0.79; n = 46) whereas the 518 second group yields a distinctly younger age of 2003 ± 3 Ma (MSWD = 0.45; n = 28). It is 519 noteworthy that the 2037 ± 2 Ma-old group corresponds to monazites that are aligned parallel 520 to the S2 foliation (Fig. 11b).

522
Staurolite-bearing micaschist (SAN) 523 Monazite is small (c. 10 µm) and either included in andalusite and biotite, or located along grain 524 boundaries. Eighteen analyses have been performed on twelve monazites grains. All analyses 525 cluster close to Concordia (≥95% concordance) and define three groups of ages yielding   . The latter is adopted as our best estimate for the age of monazite in the 547 micaschist KOU7. One analysis is significantly younger and plots concordantly at 1948 ± 33 548 Ma (Table S2, see supplementary materials, Fig. 12f).

675
The metamorphic pressure peak is first followed by a nearly isothermal decompression,