Chronostratigraphic framework and provenance of the 1 Ossa-Morena Zone Carboniferous basins ( SW Iberia ) 2 3

Carboniferous siliciclastic and silicic magmatic rocks from the Santa Susana-São 17 Cristovão region contain valuable information regarding the timing of synorogenic processes in 18 SW Iberia. In this region of the Ossa-Morena Zone (OMZ), Late Carboniferous terrigenous 19 strata (i.e. the Santa Susana Formation) unconformably overlie Early Carboniferous marine 20 siliciclastic deposits alternating with volcanic rocks (i.e. the Toca da Moura volcano21 sedimentary complex). Lying below this intra-Carboniferous unconformity, the Toca da Moura 22 volcano-sedimentary complex is intruded and overlain by the Baleizão porphyry. Original 23 SHRIMP and LA-ICP-MS U-Pb zircon are presented in this paper, providing 24 chronostratigraphic and provenance constraints, since available geochronological information is 25 scarce and only biostratigraphic ages are currently available for the Santa Susana-São Cristovão 26 region. Our findings and the currently-available detrital zircon ages from Paleozoic terranes of 27 SW Iberia (Pulo do Lobo ZonePLZ, South-Portuguese ZoneSPZ, and OMZ), were jointly 28 analyzed using the K-S test and MDS diagrams to investigate provenance. The marine 29 deposition is constrained to the age interval of c. 335-331 Ma (Visean) by new U-Pb data for 30 silicic tuffs from the Toca da Moura volcano-sedimentary complex. The Baleizão porphyry, 31 intrusive in the Toca da Moura volcano-sedimentary complex, yielded a crystallization age of c. 32 317 Ma (Bashkirian), providing the minimum age for the overlying intra-Carboniferous 33 unconformity. A comparison of detrital zircon populations from siliciclastic rocks of the 34 Cabrela and Toca de Moura volcano-sedimentary complexes of the OMZ suggests that they 35 https://doi.org/10.5194/se-2020-26 Preprint. Discussion started: 11 March 2020 c © Author(s) 2020. CC BY 4.0 License.

. In the Santa Susana-São Cristovão region, Late 73 Carboniferous siliciclastic strata of the Santa Susana Formation unconformably overlie: i) the 74 poorly-dated Baleizão volcanic-subvolcanic suite, and ii) the Early Carboniferous Toca da 75 Moura volcano-sedimentary complex, which includes volcanic rocks that have never been 76 dated. This intra-Carboniferous unconformity was generated as consequence of regional uplift 77 and falling sea level, leading to a change in depositional environment from Early Carboniferous 78 marine to Late Carboniferous terrestrial (Gonçalves and Carvalhosa, 1984 In this paper, SHRIMP and LA-ICP-MS U-Pb analyses were performed on zircon grains from 83 silicic volcanic, subvolcanic, and siliciclastic rocks sampled in the Santa Susana-São Cristovão 84 region (OMZ, SW Iberia). The aim of this geochronology study is to establish the 85 chronostratigraphic framework of the Carboniferous strata in the Santa Susana-São Cristovão 86 region and to discuss their provenance using a statistical approach (Kolmogorov-Smirnov test 87 and Mutiscaling diagrams). Thus we pay tribute to J.R. Martínez-Catalán, who devoted part of 88 his career to investigating the Carboniferous synorogenic basins of NW Iberia. 89 90

Geological setting 91
In SW Iberia, the tectonic limit between the OMZ (Gondwanan-side) and the PLZ and SPZ 92 (Laurussian-side) has been regarded as constituting the tectonically reworked suture zone of the 93 Rheic the Toca da Moura volcano-sedimentary complex (Santos et al., 1987, and references therein) 116 (Fig. 2). 117 The Toca da Moura volcano-sedimentary complex is mainly composed of pelites (i.e.

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"Xistinhos"; Teixeira, 1944 alternating beds of pelite and sandstone interbedded with coal seams, and few beds of 144 conglomerate (Fig. 2). These terrestrial deposits were most probably deposited in an 145 alluvial/fluvial-to-fluvial/lacustrine (floodplain lakes and/or abandoned channels with abundant 146 https://doi.org/10.5194/se-2020-26 Preprint. Discussion started: 11 March 2020 c Author(s) 2020. CC BY 4.0 License. Formation indicates denudation and recycling of a crystalline basement involving granite whose 167 age is unknown. The dating of the granite cobble (sample SCV-7) is useful for discussing 168 provenance and estimating the maximum depositional age of the Santa Susana conglomerate. In The new U-Pb results obtained in the present study are compared with previously-reported age 179 spectra for pre-Kasimovian siliciclastic rocks from the OMZ, PLZ and SPZ siliciclastic 180 sequences of SW Iberia, using statistical tools.

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Zircon grains for U-Pb geochronology were selected using traditional techniques: density 182 separation using a wilfley table (Universidad Complutense de Madrid, Spain) and also using 183 https://doi.org/10.5194/se-2020-26 Preprint. Discussion started: 11 March 2020 c Author(s) 2020. CC BY 4.0 License. granulometric separation using sieves with a mesh size of less than 500 microns, density 184 (panning) separation procedures, and mineral identification using a binocular lens and 185 preparation of epoxy resin mounts with zircon grains ( The probability of the observed maximum vertical difference between the cumulative 206 probability curves (D-value; Fig. 7c) being unrelated to age differences between the two detrital 207 zircon populations is given by a P-value corresponding to a confidence interval of 95% Sample SCV-2 is a fine-grained banded rhyolitic tuff consisting of variable size and shape 223 quartz and K-feldspar phenocryst and lithoclasts (less than 1mm in diameter) dispersed in ash 224 matrix (Fig. 5a). Zircon grains appear as stubby-to-elongated euhedral prisms (50-150 μm in 225 diameter), mostly showing oscillatory concentric zoning growing on distinct cores or as simple 226 crystals. There are some dark inclusions, unzoned patches and transgressive variably 227 luminescence embayments. A total of 44 U-Th-Pb SHRIMP analyses of 44 grains yielded U 228 content ranging from 262 to 628 ppm, and Th/U ratios ranging from 0.17 to 0.95 (mean = 0.42).

Siliciclastic rocks from the Santa Susana Formation 315
Sample SS-2 represents medium-to-coarse grained poorly-sorted sandstone. It is mainly 316 composed of lithoclasts (siltstone, mudstone, quartzite, phyllite, rhyolite, basalt) and quartz 317 grains, but also includes muscovite and feldspar grains (Fig. 5g). The zircon population is 318 mostly characterized by stubby to prismatic, subrounded to subangular, grains (120-300 μm in 319 diameter). Morphologically were found simple and composite grains. Cathodoluminescence 320 imaging shows that most zircon grains have concentric oscillatory zoning, irregular zoning and 321 are banded or unzoned. A total of 153 U-Th-Pb LA-ICP-MS analyses were performed on 322 detrital zircon grains. They show U content ranging from 15 to 6158 ppm, and Th/U ratios 323 ranging from 0.02 to 3.57 (mean = 0.66). A population with 51 grains yielding U-Pb ages with 324 90-110% concordance (Fig. 8b)  pre-Carboniferous ages-P-value = 0.879) at the 5% confidence level (Fig. 9a). A comparison of 355 samples SS-1 and SS-2 reveals that they are "significantly different" (P-value ≤ 0.01). Unlike 356 sample SS-2, the sample SS-1 detrital zircon population is "significantly different" (P-value < 357 0.01) from the SS upper population (Fig. 9a), indicating distinct sources. Besides this, sample 358 SS-1 is much closer to that of the SS upper (D-value = 0.323), and more distant from sample 359 SS-2 (D-value = 0.465) as regards the distance between cumulative probability curves (Fig. 8c). 360 In Figure 9b 0.165) at the 5% confidence level (Fig. 9a). This similarity is also illustrated in the 370 approximation between SS-2, P-G-R-A-R and OMZ populations in the MDS diagrams (Figs.  371 9b, c).

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K-S test results for the comparison between samples SS-2 and TM-3 indicate that they present 373 'not significantly different' detrital zircon populations (all ages-P-value = 0.399; pre-374 Carboniferous ages-P-value = 0.0.411) at the 5% confidence level (Fig. 9a). Furthermore, their 375 cumulative probability curves are much closer (Fig. 8d): D-values are 0.195 (all ages) and 0.203 376 (pre-carboniferous ages) (Fig. 9a). The close relationship of the two detrital zircon populations 377 suggests that the Toca da Moura volcano-sedimentary complex directly supplied sediment to the 378 Santa Susana basin. However, the relationship described above does not extend to the entire 379 Santa Susana basin since sample SS-1 presents a greater degree of similarity with the Cabrela 380 detrital zircon population as regards the proximity between cumulative probability curves (Fig.  381 8d) and MDS diagrams (Figs. 9b, c). 382 In addition, Cabrela siliciclastic rocks are 'significantly different' at the 5% confidence level 383 from sample TM-3 (P-values < 0.01) as regards the significant distance between them on the 384 MDS diagram (Figs. 9b, c), and the significant distance between cumulative curves (Fig. 8d), 385 with a D-value interval of 0.712-0.731 (Fig. 9a). The difference found in the detrital zircon 386 populations suggests that Cabrela and Toca da Moura siliciclastic rocks probably derived from 387 different sources. 388 As result of the K-S test and MDS analyisis, the Horta da Torre Formation is 'significantly 389 different' (Fig. 9a) provides evidence that they derived from different sources. The TM-3 population presents 64% 425 Precambrian detrital zircon grains, while the CB population contains only 10% (Fig. 8a). Toca 426 da Moura siliciclastic rocks have a greater affinity with the Phyllite-Quartzite, Tercenas, Santa 427 Iria and Represa formations (Fig. 9), indicating that detrital zircon populations were reproduced 428 faithfully in SPZ and PLZ (Laurussian-type) sources. A contribution from the oldest siliciclastic 429 sequences of PLZ (Pulo do Lobo, Atalaia, Gafo and Ribeira de Limas formations) and OMZ 430 (Gondwanan-type) sources cannot be ruled out for sample TM-3 (Fig. 9). The number of Late-431 Middle Devonian zircon grains in sample TM-3 (6%) is smaller than that of the CB population 432 (68%) (Fig. 8a), suggesting that Cabrela siliciclastic rocks were most likely derived largely from 433 a Devonian source consistent with a limited contribution from recycled ancient rocks. This 434 indicates that the origin of the Visean Toca da Moura and Cabrela basins is most likely more 435 closely linked to sources located in the SPZ and PLZ (Laurussian-type) than in the OMZ 436 Phyllite-Quartzite, Tercenas and Ronquillo formations) and OMZ sources cannot be ruled out 474 (Figs. 9a, c). The zircon age population of sample SS-1, which is distinct from the SS-2 475 population, presents a great degree of affinity with the CB population, suggesting lateral Carboniferous plutons were emplaced at shallow crustal levels consistent with the low 499 assimilation of country rocks and the sharp contacts, and therefore, they may have experienced 500 denudation shortly after its crystallization without being required unrealistic uplift rates. 501 In Kasimovian-Ghzelian, sedimentation probably occurred through the opening of the pull-apart 502 terrestrial basin related to the movement of major strike-slip faults (i.e. Porto-Tomar fault zone, 503 Machado et al., 2012, and references therein) during the waning stages of oblique continental 504 collision between Laurussia and Gondwana, simultaneously with the progressive uplift of the 505 Appalachian-Variscan orogenic belt (i.e. OMZ, PLZ and SPZ; Fig. 10b). 506 507 7. Conclusions 508 The main conclusions of this study are the following: