Unravelling the origins and PT-t evolution of the 1 allochthonous Sobrado unit ( Órdenes Complex , NW 2 Iberia ) using combined UPb titanite , monazite and zircon 3 geochronology and REE geochemistry 4

José Manuel Benítez-Pérez, Pedro Castiñeiras, Juan Gómez-Barreiro , José Ramón 5 Martínez Catalán, Andrew Kylander-Clark, Robert Holdsworth 6 7 8 Centro de Ciências e Tecnologias Nucleares. Instituto Superior Técnico. Universidad de Lisboa. Estrada 9 Nacional 10 (km 139,7), 2695-066, Bobadela LRS, Portugal 10 Departamento de Mineralogía y Petrología. Facultad de Ciencias Geológicas, Universidad Complutense 11 de Madrid. C/ José Antonio Novais, 12. Ciudad Universitaria, 28040, Madrid, Spain 12 Departamento de Geología, Universidad de Salamanca, Pza. de los Caídos s/n, 37008, Salamanca, Spain 13 Department of Earth Science, University of California, Santa Barbara, CA 93106, United States 14 Earth Science Department, Durham University, Science Labs, Durham DH1 3LE, United Kingdom 15 16 17 Correspondence to: J. Gómez Barreiro (jugb@usal.es) 18 19


Introduction
1 Zircon, monazite and titanite are accessory mineral phases found in rocks with a very wide range 2 of compositions. These minerals can resist numerous sedimentary, igneous and metamorphic events 3 across a wide range of temperatures, pressures and strains, even when fluids are present. Frequently, 4 compositional domains can be defined in these minerals that record changes in different parameters (e.g. 5 Castiñeiras et al., 2010;Hacker et al., 2015;Stearns et al., 2016;Stipska et al., 2016;Storey et al., 2007; 6 Stübner et al., 2014). These minerals additionally provide several closed decay chains or disintegration 7 systems ( 238 U → 206 Pb, 235 U → 207 Pb y 232 Th → 208 Pb), because they hold variable concentrations of 8 uranium (U) and/or thorium (Th) in their crystal lattices. Such variations in concentration allow accurate 9 dating using microscopic scale analysis (tens of microns size).

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The Upper Allochthon is further subdivided into high-P/high-T and intermediate-P units (Gómez 32 Barreiro et al., 2007). The present study focuses on two of the high-P/high T upper units. The origin of 33 the high-P event recorded in these units is controversial, but might reflect the accretion of the units into

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After the resin was cured, the surface was eroded using a wet abrasive silicon carbide abrasive paper   Cathodoluminescence images are useful to relate the crystallization of parts of zircon crystals to 10 specific igneous, metamorphic or deformational events (Corfu et al., 2003, Nasdala et al., 2003, Zeck et 11 al., 2004. Zircon grains from the paragneises display a wide variety of external morphologies (   8 Fifty-one titanite analyses were projected onto a Tera-Wasserburg concordia plot (Tera and 9 Wasserburg, 1972) ( Fig. 3a). After a preliminary evaluation, twelve analyses were rejected due to either 10 high common Pb or high discordance (>10%) and were considered no further (

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Data from seventy-six U/Th-Pb monazite analyses are shown in Table 2 and displayed using a 29 Tera-Wasserburg concordia plot (Fig. 5a). Four of these analyses, not related to chemical zoning, were 30 discarded due to common Pb loss and were considered no further. The remaining analyses form a single 31 population (mean square of weighted deviation; MSWD = 0.48) centered on a concordia age of 382.5 ± 32 1.0 Ma (2σ). Monazite geochemistry is shown in Figure 5b. REE patterns analyzed show an LREE 33 enrichment, HREE depletion and negative Y anomalies with respect to chondrite with little variation 34 within or between samples. The profiles suggest simultaneous crystallization of monazite with garnet 35 (Holder et al., 2015, Mottram et al., 2014, Rubatto, 2002, Rubatto et al., 2006

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Eighty-three analyses were performed on eighty zircon grains from the Sobrado paragneiss 42 (Table 3). Five analyses were rejected for age calculation due to high contents of common Pb (grain 43 numbers 7, 29, 64, 69, 73) and two others were rejected due to analytical errors (grain numbers 8, 36).
age range are likely due to inheritance (Fig. 7). It is also possible that there is some inheritance older than 1 600 Ma, but the register is discontinuous and it is difficult to distinguish between protolith ages and 2 inherited ages.

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There is a general correlation between zircon grain texture in cathodoluminescence and 206 U 4 / 238 U ages. Two groups are recognized (Fig. 7b). The first group range in age from 380 to ca. 500 Ma and 5 share sub-rounded or fragmentary grain morphologies. Sixteen spots were performed on rims or poorly 6 luminescent homogeneous cores (Fig. 2)

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The chondrite-normalized REE patterns of the Sobrado zircons giving ages less than 600 Ma are 16 shown in Figure 9. In general, this group has variable REE patterns with higher ∑REE values compared 17 to older zircons. Low La contents (0.01-0.38 ppm) make it unlikely that metamictization of the analyzed 18 zircons has occurred (e.g. Belousova et al., 2002, Castiñeiras et al., 2010, Hoskin, 2005

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Eu 2+ available in the system, although the Eu anomaly could also be conditioned by oxygen fugacity (e.g.

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Ma zircons aliquot has a much greater variation in HREE patterns. Most of the population shows a 30 negative slope or flat HREE pattern (nº 15, 59, 76, 79, 47, 67, 75, 53, 70)  theses zircons (Fig. 10b). Nevertheless, high Ce/Sm values indicate an oxygen fugacity in the system or a 1 magma fractionation (Belousova et al., 2002, Castiñeiras et al., 2010. Additionally, in the bi-logarithmic 2 plot of the U/Ce ratio versus Th concentration, a 1:1 line can be used to separate magmatic from 3 metamorphic zircons (Fig. 10c) (Bacon et al., 2012). This is because metamorphic zircon has higher U 4 concentration compared to igneous zircon, whereas Ce is higher in magmatic zircon (e.g. Hoskin and 5 Schaltegger, 2003). Noticeably, the 500-600 Ma zircon population entirely fits within the magmatic field 6 whereas 380-500 Ma zircon aliquot, except three atypical analyses (grain numbers 10, 11, 26), shares a 7 metamorphic origin. On a Eu/Eu* versus Hf concentration plot, the Hf homogeneity, ranging from 70000 8 to 110000 ppm, in the 500-600 Ma group suggests that fractional crystallization of the magma that 9 formed those zircons did not occur (Fig. 10d). The Eu anomaly seen in the Sobrado zircons is interpreted 10 to be a consequence of coeval plagioclase growth and has no clear association with any age group.

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In

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The best estimate age obtained is 530.37 (+7.60, -7.46) Ma, using the TuffZirc algorithm on a 29 group of eighteen analyses ranging from ca. 500 to 600 Ma (Fig. 11b). This age is obtained by pooling

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and is related to a magmatic arc creation around the periphery of Gondwana (Abati et al., 2007, 1999.

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Data availability

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The data are not publicly accessible 32 33

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There is no supplement related to this article.

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The authors declare that they have no conflict of interest.