Geological Society of America Bulletin, published online on 13 February 2014 as doi:10.1130/B30935.1 Geological Society of America Bulletin Provenance variability along the Early Ordovician north Gondwana margin: Paleogeographic and tectonic implications of U-Pb detrital zircon ages from the Armorican Quartzite of the Iberian Variscan belt J. Shaw, G. Gutiérrez-Alonso, S.T. Johnston and D. Pastor Galán Geological Society of America Bulletin published online 13 February 2014; doi: 10.1130/B30935.1 Email alerting services click www.gsapubs.org/cgi/alerts to receive free e-mail alerts when new articles cite this article Subscribe click www.gsapubs.org/subscriptions/ to subscribe to Geological Society of America Bulletin Permission request click http://www.geosociety.org/pubs/copyrt.htm#gsa to contact GSA Copyright not claimed on content prepared wholly by U.S. government employees within scope of their employment. 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Notes Advance online articles have been peer reviewed and accepted for publication but have not yet appeared in the paper journal (edited, typeset versions may be posted when available prior to final publication). Advance online articles are citable and establish publication priority; they are indexed by GeoRef from initial publication. Citations to Advance online articles must include the digital object identifier (DOIs) and date of initial publication. Copyright © 2014 Geological Society of America Geological Society of America Bulletin, published online on 13 February 2014 as doi:10.1130/B30935.1 Provenance variability along the Early Ordovician north Gondwana margin: Paleogeographic and tectonic implications of U-Pb detrital zircon ages from the Armorican Quartzite of the Iberian Variscan belt J. Shaw1,†, G. Gutiérrez-Alonso2, S.T. Johnston1, and D. Pastor Galán3 1School of Earth and Ocean Sciences, University of Victoria, P.O. Box 1700 STN CSC, Victoria, BC V8W 2Y2, Canada 2Departamento de Geología, Universidad de Salamanca, 33708 Salamanca, Spain 3Department of Earth Sciences, Universiteit Utrecht, P.O. Box 80.021, 3508 TA Utrecht, Netherlands ABSTRACT of the then low-lying Saharan Metacraton (2) a swath of presumed-exotic continental crust and Arabian-Nubian Shield. Accepting paleo- independently sutured to the south of zone 4 (e.g., Detrital zircon laser ablation–inductively magnetic constraints, a 90° counterclockwise Martínez Catalán et al., 2007). coupled plasma–mass spectrometry U-Pb age rotation is required to reorient the Iberian The Armorican Quartzite is characteristic of data from the Lower Ordovician Armorican portion to a pre-oroclinal (Late Carbonifer- the lower Paleozoic Gondwana passive-margin Quartzite (deformed passive margin strata ous) north-south trend. The mechanisms for sequence of zones 1–3. A 5–40-km-wide fault- of Gondwanan affi nity) of the Iberian Massif accommodating such a rotation are unclear. bound band of rock at the boundary between are presented herein. The S-shaped coupled zones 3 and 4 has, on the basis of structural data Iberian oroclines defi ned within these zones INTRODUCTION and stratigraphic correlations, been interpreted palinspastically restore to a 2300 km linear as being transitional between them (Pereira Variscan orogen with a paleomagnetically Detrital zircon age dating is a powerful tool in and Silva, 2001; Solá et al., 2008). The Ossa- constrained Late Carboniferous north-south deciphering sedimentary provenance, and con- Morena–Central Iberian transition zone, here- trend. Detrital zircons are used to assess straining paleogeography and tectonic evolution after referred to as the transition zone (Fig. 1), paleo geog raphy and interpreted geometry of continental realms. Sampling within restricted extends north from the sinistral Badajoz-Cor- of the Iberian portion of the Gondwana pas- areas through thick stratigraphic successions can doba shear zone to the Los Pedroches batholith sive margin. A common signature is identi- address questions regarding local tectonic evo- (Linnemann et al., 2008, and references therein). fi ed by (1) Neoproterozoic (ca. 500–850 Ma), lution (e.g., May et al., 2013; LeMaskin, 2012). It is characterized by a zone 4–type lower Edia- (2) Stenian–Tonian (ca. 0.9–1.1 Ga), and Alternatively, sampling over a wide area within caran–Cambrian stratigraphic sequence overlain lesser (3) Paleoproterozoic and (4) Archean a single stratigraphic level (formation) can yield by a zone 3–type Ordovician sequence (Bandres populations (ca. 1.8–2.15 and 2.5–2.7 Ga, formidable constraints on a variety of spatial et al., 2002; Pereira and Silva, 2001; Pereira respectively). Minor site-to-site variation variables at the point in time of interest, from et al., 2010). Though it is generally believed in rela tive proportion of widely ranging relative location between continental masses to that no Armorican Quartzite equivalent exists age groups suggests near-uniform distribu- continental geometry and orientation. within the Ordovician stratigraphic sequence of tion of a highly varied detrital input. Prov- This study presents detrital zircon age data zone 4 (Robardet and Gutiérrez-Marco, 1990a, enance analysis reveals strong correlations sampled from the Lower Ordovician Armorican 1990b, 2004), the Sierra Albarrana Quartzites, with Cambro-Ordo vician clastic rocks from Quartzite (sensu lato) over a geographical area of exposed just south of the transition zone bound- northeast African realms. Similarity with roughly 150,000 km2 within the Variscan orogen ary and loosely constrained as early Paleozoic underlying sequences suggests a common of Iberia. The Variscan orogen of Iberia is charac- in age (Azor et al., 1991; Marcos et al., 1991), paleogeography from the Ediacaran through terized by the continental-scale S-shaped Canta- have been suggested as possible correlatives early Paleozoic and persistence of a prov- brian–Central Iberian coupled oroclines (Aerden, (Azor et al., 1991). The Cantabrian–Central enance distinction within the autochthonous 2004; Martínez Catalán, 2011; Shaw et al., Iberian coupled oroclines were defined in, Iberian Massif. Consistent northward paleo- 2012) (Fig. 1), and is divisible into a series of and affect that portion of, the Variscan orogen fl ow within widespread northeast African tectonostratigraphic zones. From the core of the located north of the transition zone; the nature lower Paleozoic sedi mentary cover suggests Cantabrian orocline south, four of these zones, of the relationship between the coupled oro- long-distance sedimen tary transport across a the (1) Cantabrian, (2) West Asturian–Leonese, clines and that portion of the Iberian Massif North African peneplain from outlying base- (3) Central Iberian, and (4) Ossa-Morena, are that lies to their south (zone 4 and the southern ment terranes. We propose that the 2300-km- considered parts of autochthonous Gondwana. allochthon) remains unclear. long Cantabrian–Central Iberian portion Allochthonous terranes of the Iberian Variscan Paleomagnetic and structural studies have of the early Paleozoic Gondwana margin belt include (1) an ophiolite-bearing structural demonstrated that the Cantabrian orocline stretched east-west along the northern limits stack that overrides the hinterland zone 3 at the formed by lithospheric-scale vertical-axis rota- core of the Central Iberian orocline (e.g., Pérez- tion of an originally linear Variscan orogen in the †E-mail: [email protected]. Estaún et al., 1991, and references therein), and Late Carboniferous–earliest Permian (Kollmeier GSA Bulletin; Month/Month 2014; v. 1xx; no. X/X; p. 1–18; doi:10.1130/B30935.1; 9 fi gures; 4 tables; Data Repository item 2014083. For permission to copy, contact [email protected] 1 © 2014 Geological Society of America Geological Society of America Bulletin, published online on 13 February 2014 as doi:10.1130/B30935.1 Shaw et al. –9° –8° –7° –6° –5° –4° –3° –2° –1° strained using zircon provenance analyses that WALZ-01 Ordovician focus on comparison of peri-contemporaneous Precambrian and clastic sequences rather than on the comparison Barrios undifferentiated Paleozoic of clastic sequence against potential basement 43° WALZ-02 ZONE 1 source. The detailed Early Ordovician “snap- CZ-02 Allochthonous terranes shot” of the Iberian Gondwana margin revealed GCZ-06 Transition zone through detrital zircon ages yields broad and 42° ZONE 2 Variscan strike-slip signifi cant implications for the pre-Ordovician GCZ-03 shear zones paleogeography and post-Ordovician tectonic Alpine deformation front evolution of the Variscan orogen. 41° IBR-02 SCS-05 Armorican Quartzite zircon GEOLOGIC SETTING sample sites PNC-4 ZONE 3 40° This study The Armorican Quartzite Fernandez-Suarez et al. (2002a) The Armorican Quartzite consists