Article Constraints on the ages of the crystalline basement and Palaeozoic cover exposed in the Cordillera real, Ecuador: 40Ar/39Ar analyses and detrital zircon U/Pb geochronology SPIKINGS, Richard Alan, et al. Abstract Gabbros and ultramafic rocks of the Huarguallá Gabbro unit exposed in faulted slivers along the western Cordillera Real of Ecuador crystallised between 623and531 Ma (40Ar/39Ar dates), were derived from asthenospheric sources with minor crustal contamination, and form part of the Central Iapetus Magmatic Province. These rocks formed in an early rift environment during the opening of the Iapetus Ocean, and represent the only igneous record of Iapetus rifting north of the Huancabamba deflection (5°S) in South America. The age and composition of the Huarguallá Gabbro unit is consistentwith the reconstruction of Tegner et al. (2019),which juxtaposes Baltica and northwestern Gondwana within Panotia. 206Pb/238U dates of detrital zircons combinedwith fossil assemblages shows that the Chiguinda unit of the Cordillera Real, and La Victoria Unit of the Amotape Complex were deposited during the Carboniferous. These new data, combined with previous studies of magmatism and sedimentation from southern Peru, Colombia and Venezuela, imply that the rocks of the Cordillera Real were in the Ordovician and Carboniferous back-arcs, while [...] Reference SPIKINGS, Richard Alan, et al. Constraints on the ages of the crystalline basement and Palaeozoic cover exposed in the Cordillera real, Ecuador: 40Ar/39Ar analyses and detrital zircon U/Pb geochronology. Gondwana Research, 2021, vol. 90, p. 77-101 DOI : 10.1016/j.gr.2020.10.009 Available at: http://archive-ouverte.unige.ch/unige:146042 Disclaimer: layout of this document may differ from the published version. 1 / 1 Gondwana Research 90 (2021) 77–101 Contents lists available at ScienceDirect Gondwana Research journal homepage: www.elsevier.com/locate/gr Constraints on the ages of the crystalline basement and Palaeozoic cover exposed in the Cordillera real, Ecuador: 40Ar/39Ar analyses and detrital zircon U/Pb geochronology R. Spikings a,⁎,A.Paula,C.Vallejob,P.Reyesb a Department of Earth Sciences, University of Geneva, Switzerland b Facultad de Geología, Minas y Petróleos, Escuela Politécnica Nacional, A.P. 17-01-2759, Quito, Ecuador article info abstract Article history: Gabbros and ultramafic rocks of the Huarguallá Gabbro unit exposed in faulted slivers along the western Cordil- Received 16 July 2020 lera Real of Ecuador crystallised between 623and531 Ma (40Ar/39Ar dates), were derived from asthenospheric Received in revised form 4 September 2020 sources with minor crustal contamination, and form part of the Central Iapetus Magmatic Province. These Accepted 31 October 2020 rocks formed in an early rift environment during the opening of the Iapetus Ocean, and represent the only igne- Available online 06 November 2020 ous record of Iapetus rifting north of the Huancabamba deflection (5°S) in South America. The age and composi- Keywords: tion of the Huarguallá Gabbro unit is consistent with the reconstruction of Tegner et al. (2019), which juxtaposes 206 238 Pangaea Baltica and northwestern Gondwana within Panotia. Pb/ U dates of detrital zircons combined with fossil as- Iapetus rift semblages shows that the Chiguinda unit of the Cordillera Real, and La Victoria Unit of the Amotape Complex Palaeozoic were deposited during the Carboniferous. These new data, combined with previous studies of magmatism and Detrital zircon dates sedimentation from southern Peru, Colombia and Venezuela, imply that the rocks of the Cordillera Real were 40Ar/39Ar dating in the Ordovician and Carboniferous back-arcs, while the arcs occur in conjugate margins that separated during the Triassic rifting of Pangaea. Faulted remnants of Ordovician arc rocks in the Cordillera Central of Colombia are probably allochthonous, and have been displaced from an Ordovician margin that did not face the rifted crustal sections. © 2020 Published by Elsevier B.V. on behalf of International Association for Gondwana Research. 1. Introduction Crystalline rocks of the Eastern Cordillera of Peru provide substantial evidence for continental arc magmatism between 474 and 442 Ma, and The Cordillera Real of Ecuador forms part of the Northern Andes metamorphism at ~478 Ma (Famatinian Arc; Chew et al., 2007; (north of the Huancabamba Deflection; ~5°S), and hosts metamor- Mišković et al., 2009) followed by a Devonian magmatic lull. Carbonifer- phosed sedimentary and igneous rocks that span from the Early Creta- ous continental arc magmatism occurred during 333–313 Ma and ter- ceous to the Palaeozoic, and perhaps older. The Triassic – Late minated during high-grade regional metamorphism at 313–310 Ma Cretaceous history has been extensively studied (e.g. Litherland et al., (Chew et al., 2007). Migmatitic granitoids have been dated between 1994; Cochrane et al., 2014a, 2014b; Spikings et al., 2015), and docu- 285 and 223, while the Triassic igneous rocks of the Mitu Group have ments the disassembly of Pangaea and subsequent evolution of the Pa- been assigned to a continental rift setting during 245–220 Ma cific margin. However, a paucity of studies of the pre-Triassic units (e.g. (Spikings et al., 2016). Within Colombia and Venezuela, remnants of Or- see Chew et al., 2008; Suhr et al., 2019) hinders geological models of the dovician continental arc magmatism are mainly preserved in the Pacific margin along southwestern Pangaea, which are currently heavily Santande Cawood et al., 2001r Massif and the Merida Andes, respec- founded on the evolution of the Argentinian and Peruvian Andes (e.g. tively, where zircon U\\Pb concordia ages of intrusions span between Cawood, 2005; Chew et al., 2007; Mišković et al., 2009). Here we pro- 500 and 415 Ma, and metamorphism is recorded at 477–472 Ma (Van vide new geochronological constraints for the deposition of poorly stud- der Lelij et al., 2016). Ordovician orthogneisses have also been recorded ied, Palaeozoic metasedimentary units of the Cordillera Real of Ecuador, in the northern Central Cordillera of Colombia (La Miel Orthogneiss), and estimates of the crystallisation ages of metamorphosed ultramafic where they yield zircon U\\Pb concordia ages ranging between 485 and mafic slivers that are entrained in the anastomosing Peltetec Fault and 440 Ma (Villagómez et al., 2011; Martens et al., 2014), and in the Zone, which is located along the western flank of the Cordillera Real. Floresta and Quetame massifs, with ages of 520–420 Ma (Horton et al., 2010). Similar to Peru, these intrusions predate a Devonian magmatic ⁎ Corresponding author. hiatus, although in contrast to Peru there is no record of substantial Car- E-mail address: [email protected] (R. Spikings). boniferous arc magmatism. Permian intrusions (278–253 Ma; Vinasco https://doi.org/10.1016/j.gr.2020.10.009 1342-937X/© 2020 Published by Elsevier B.V. on behalf of International Association for Gondwana Research. R. Spikings, A. Paul, C. Vallejo et al. Gondwana Research 90 (2021) 77–101 et al., 2006; Cardona et al., 2010; Cochrane et al., 2014b; Bustamante depositional age to the Late Devonian (Chew et al., 2008). No radiomet- et al., 2017; Paul et al., 2018; Spikings and Paul, 2019) are geographically ric dates have been obtained from the metavolcanic strata. Traversing scattered and reveal a continental arc in a collisional setting, culminat- westwards, the southern Cordillera Real is dominated by exposures of ing in regional metamorphism that is best preserved in the Sierra Ne- quartzites, phyllites and semi-pelites (Fig. 1;e.g.Litherland et al., vada de Santa Marta of northern Colombia (Piraquive, 2017). Finally, 1994) of the Chiguinda Fm., which are fault bounded against Triassic S-type anatectites of the Cajamarca Group were emplaced during the anatectites to the west, and Lower Cretaceous metasedimentary rocks evolution of the Palanda rift during 247–222 Ma, which was synchro- of the exhumed Salado Basin to the east. Depositional age constraints nous with the Mitu Rift in Peru (Spikings et al., 2016). These events for the Chiguinda Fm. are sparse and include i) pre-Triassic on the have been previously used to construct a Palaeozoic history for the Pa- basis of their correlation with metamorphic rocks in the Olmos Massif cific margin of the Central and Northern Andes of South America during in Peru (Kennerley, 1973), which yield Ordovician – Silurian fauna the evolution of western Gondwana and the amalgamation of Pangaea (Mourier et al., 1988), and where they are overlain by the Triassic (e.g. Chew et al., 2007). However, along-strike discrepancies such as a Mitu Group (Spikings et al., 2016), ii) fossilised microspores, which con- lack of Carboniferous arc magmatism north of Peru, have not been ad- strain deposition to the post-Silurian (Owens, 1992), which collectively dressed, partly due to a lack of data from the Pre-Triassic rocks in lead Litherland et al. (1994) to propose a Devonian – Permian deposi- Ecuador. tional age for the Chiguinda Formation. More recently, Chew et al. We combine new U–Pb data from detrital zircons extracted from the (2008) report U–Pb ages of detrital zircons extracted from a quartzite, pre-Triassic Chiguinda, and La Victoria (Amotape Complex) units with which yield a youngest age of 367 ± 12 Ma (Fig. 1), consistent with previous work to constrain their depositional ages and source regions. the interpretation of Litherland et al. (1994). The age of a newly defined basement to the Palaeozoic sedimentary The Peltetec Fault Zone (Fig. 2)isexposedalongthewesternflank of units is constrained by 40Ar/39Ar analyses of weakly metamorphosed the Cordillera Real, where it juxtaposes Early Cretaceous continental arc and foliated gabbros and ultramafic rocks that were exhumed along the rocks of the Alao Arc against parautochthonous Jurassic metasedimentary western flank of the Cordillera Real within the Peltetec Fault Zone. The rocks of the Chaucha Block (the Chaucha Block hosts the Guamote Se- magmatic source regions and tectonic environment are investigated quence; Figs.