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

Article

Reference

Constraints on the ages of the crystalline basement and Palaeozoic cover exposed in the Cordillera real, Ecuador:

⁴⁰

Ar/

³⁹

Ar 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 [...]

SPIKINGS, Richard Alan, et al . Constraints on the ages of the crystalline basement and

Palaeozoic cover exposed in the Cordillera real, Ecuador:

⁴⁰

Ar/

³⁹

Ar 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.

Constraints on the ages of the crystalline basement and Palaeozoic cover exposed in the Cordillera real, Ecuador:

⁴⁰

Ar/

³⁹

Ar analyses and detrital zircon U/Pb geochronology

R. Spikings

^a,

⁎ ^{, A. Paul}

^a

, C. Vallejo

^b

, P. Reyes

^b

aDepartment of Earth Sciences, University of Geneva, Switzerland

bFacultad de Geología, Minas y Petróleos, Escuela Politécnica Nacional, A.P. 17-01-2759, Quito, Ecuador

a b s t r a c t a r t i c l e i n f o

Article history:

Received 16 July 2020

Received in revised form 4 September 2020 Accepted 31 October 2020

Available online 06 November 2020 Keywords:

Pangaea Iapetus rift Palaeozoic Detrital zircon dates

40Ar/³⁹Ar dating

Gabbros and ultramafic rocks of the Huarguallá Gabbro unit exposed in faulted slivers along the western Cordil- lera 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 igne- ous record of Iapetus rifting north of the Huancabamba deflection (5°S) in South America. The age and composi- tion of the Huarguallá Gabbro unit is consistent with the reconstruction ofTegner et al. (2019), which juxtaposes Baltica and northwestern Gondwana within Panotia.²⁰⁶Pb/²³⁸U dates of detrital zircons combined with fossil as- semblages 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 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

The Cordillera Real of Ecuador forms part of the Northern Andes (north of the Huancabamba Deflection; ~5°S), and hosts metamor- phosed sedimentary and igneous rocks that span from the Early Creta- ceous to the Palaeozoic, and perhaps older. The Triassic – Late Cretaceous history has been extensively studied (e.g.Litherland et al., 1994;Cochrane et al., 2014a, 2014b;Spikings et al., 2015), and docu- ments the disassembly of Pangaea and subsequent evolution of the Pa- cific margin. However, a paucity of studies of the pre-Triassic units (e.g.

seeChew et al., 2008;Suhr et al., 2019) hinders geological models of the Pacific margin along southwestern Pangaea, which are currently heavily founded on the evolution of the Argentinian and Peruvian Andes (e.g.

Cawood, 2005;Chew et al., 2007;Miškovićet al., 2009). Here we pro- vide new geochronological constraints for the deposition of poorly stud- ied, Palaeozoic metasedimentary units of the Cordillera Real of Ecuador, and estimates of the crystallisation ages of metamorphosed ultramafic and mafic slivers that are entrained in the anastomosing Peltetec Fault Zone, which is located along the westernflank of the Cordillera Real.

Crystalline rocks of the Eastern Cordillera of Peru provide substantial evidence for continental arc magmatism between 474 and 442 Ma, and metamorphism at ~478 Ma (Famatinian Arc; Chew et al., 2007;

Miškovićet al., 2009) followed by a Devonian magmatic lull. Carbonifer- ous continental arc magmatism occurred during 333–313 Ma and ter- minated during high-grade regional metamorphism at 313–310 Ma (Chew et al., 2007). Migmatitic granitoids have been dated between 285 and 223, while the Triassic igneous rocks of the Mitu Group have been assigned to a continental rift setting during 245–220 Ma (Spikings et al., 2016). Within Colombia and Venezuela, remnants of Or- dovician continental arc magmatism are mainly preserved in the SantandeCawood et al., 2001r Massif and the Merida Andes, respec- tively, where zircon U\\Pb concordia ages of intrusions span between 500 and 415 Ma, and metamorphism is recorded at 477–472 Ma (Van der Lelij et al., 2016). Ordovician orthogneisses have also been recorded in the northern Central Cordillera of Colombia (La Miel Orthogneiss), where they yield zircon U\\Pb concordia ages ranging between 485 and 440 Ma (Villagómez et al., 2011;Martens et al., 2014), and in the Floresta and Quetame massifs, with ages of 520–420 Ma (Horton et al., 2010). Similar to Peru, these intrusions predate a Devonian magmatic hiatus, although in contrast to Peru there is no record of substantial Car- boniferous arc magmatism. Permian intrusions (278–253 Ma;Vinasco

⁎ Corresponding author.

E-mail address:richard.spikings@unige.ch(R. Spikings).

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.

Contents lists available atScienceDirect

Gondwana Research

j o u r n a l h o m e p a g e :w w w . e l s e v i e r . c o m / l o c a t e / g r

et al., 2006;Cardona et al., 2010;Cochrane et al., 2014b;Bustamante et al., 2017;Paul et al., 2018;Spikings and Paul, 2019) are geographically scattered and reveal a continental arc in a collisional setting, culminat- ing in regional metamorphism that is best preserved in the Sierra Ne- vada de Santa Marta of northern Colombia (Piraquive, 2017). Finally, S-type anatectites of the Cajamarca Group were emplaced during the evolution of the Palanda rift during 247–222 Ma, which was synchro- nous with the Mitu Rift in Peru (Spikings et al., 2016). These events have been previously used to construct a Palaeozoic history for the Pa- cific margin of the Central and Northern Andes of South America during the evolution of western Gondwana and the amalgamation of Pangaea (e.g.Chew et al., 2007). However, along-strike discrepancies such as a lack of Carboniferous arc magmatism north of Peru, have not been ad- dressed, partly due to a lack of data from the Pre-Triassic rocks in Ecuador.

We combine new U–Pb data from detrital zircons extracted from the pre-Triassic Chiguinda, and La Victoria (Amotape Complex) units with previous work to constrain their depositional ages and source regions.

The age of a newly defined basement to the Palaeozoic sedimentary units is constrained by⁴⁰Ar/³⁹Ar analyses of weakly metamorphosed and foliated gabbros and ultramafic rocks that were exhumed along the westernflank of the Cordillera Real within the Peltetec Fault Zone. The magmatic source regions and tectonic environment are investigated using bulk rock chemical and Nd isotopic compositions. These informa- tion are combined to constrain the Palaeozoic history of the Pre-Triassic units in Ecuador, and refine previous models for the Iapetus and Rheic margins of northwestern Gondwana.

2. Geological framework and previous work

Phanerozoic rocks north of the Huancabamba Deflection at 5°S (Fig. 1) can be separated into an allochthonous, oceanic Late Cretaceous sequence, which is faulted against older, differentiated crust via a Late Cretaceous suture (Vallejo et al., 2006;Spikings et al., 2015). Exhuma- tion of the continental crust during mainly compressive events in the Late Cretaceous–Cenozoic exposed Palaeozoic sequences within the Cordillera Real and the Amotape Block (e.g. Spikings et al., 2010;

Martin-Gombojav and Winkler, 2008;Gutierrez et al., 2019), which were mapped and differentiated by the British Geological Survey during 1986–1993 (Litherland et al., 1994). The Cordillera Real is an approxi- mate N-S trending topographic ridge, which is geographically and geo- logically continuous with the Cordillera Central of Colombia (e.g.

Villagómez et al., 2011;Spikings et al., 2015), and is oblique to the East- ern Cordillera of Peru across the Huancabamba Deflection.

Within Ecuador, evidence for Precambrian rocks is extremely cur- sory, and includes a single Rb–Sr data point of ~1600 Ma, which is re- ported as a personal communication in Litherland et al. (1994).

Equally as vague, rafts of migmatitic gneisses within the Jurassic Zamora Batholith (Fig. 1; easternflank of the Cordillera Real) are reported in Litherland et al. (1994)as Precambrian, although they remain undated.

Stratigraphic relationships suggest that black slates of the Pumbuiza Formation are pre-Upper Carboniferous and they were assigned to the Devonian byLitherland et al. (1994). Limestones, shales and sandstones of the Macuma Formation unconformably overly the Pumbuiza Fm., and have been assigned to the Lower Pennsylvanian–Permian on the basis of fossil assemblages (Tschopp, 1953;Litherland et al., 1994). The Pumbuiza and Macuma formations are exposed in small inliers in the Oriente retroforeland basin, and were not sampled in this study (they are exposed to the east of the map shown inFig. 1). Low grade phyllites, marbles and metamorphosed tuffs of the Isimanchi unit are exposed in the southern Cordillera Real (Fig. 1), where they are faulted against Tri- assic anatectites to the west, and Jurassic continental arc intrusions to the east (Fig. 1). Fossilisedfish remains loosely assign deposition to the Carboniferous to Late Triassic (British Geological Survey, 1989).

More recently, U–Pb concordia dates of detrital zircons from a black phyllite (368 ± 14 Ma) of the Isimanchi unit constrain its maximum

depositional age to the Late Devonian (Chew et al., 2008). No radiomet- ric dates have been obtained from the metavolcanic strata. Traversing westwards, the southern Cordillera Real is dominated by exposures of quartzites, phyllites and semi-pelites (Fig. 1; e.g.Litherland et al., 1994) of the Chiguinda Fm., which are fault bounded against Triassic anatectites to the west, and Lower Cretaceous metasedimentary rocks of the exhumed Salado Basin to the east. Depositional age constraints for the Chiguinda Fm. are sparse and include i) pre-Triassic on the basis of their correlation with metamorphic rocks in the Olmos Massif in Peru (Kennerley, 1973), which yield Ordovician–Silurian fauna (Mourier et al., 1988), and where they are overlain by the Triassic Mitu Group (Spikings et al., 2016), ii) fossilised microspores, which con- strain deposition to the post-Silurian (Owens, 1992), which collectively leadLitherland et al. (1994)to propose a Devonian–Permian deposi- tional age for the Chiguinda Formation. More recently,Chew et al.

(2008)report U–Pb ages of detrital zircons extracted from a quartzite, which yield a youngest age of 367 ± 12 Ma (Fig. 1), consistent with the interpretation ofLitherland et al. (1994).

The Peltetec Fault Zone (Fig. 2) is exposed along the westernflank of the Cordillera Real, where it juxtaposes Early Cretaceous continental arc rocks of the Alao Arc against parautochthonous Jurassic metasedimentary rocks of the Chaucha Block (the Chaucha Block hosts the Guamote Se- quence;Figs. 1 and 2). A majority of the fault zone is buried beneath Ter- tiary volcanic units, although it is interpreted to be geographically extensive (e.g.Litherland et al., 1994;Spikings et al., 2015), and may jux- tapose Early Cretaceous continental crust against allochthonous oceanic crust of the Pallatanga-Piñon terrane where the Chaucha Block is not present. Therefore, the Peltetec Fault is considered to be equivalent to the Cauca-Almaguer Fault along the westernflank of the Cordillera Cen- tral in Colombia (e.g.Villagómez et al., 2011). Anastomosed faulted blocks host numerous different lithologies, which are interpreted via lithological associations to include Palaeozoic metasedimentary units (Chiguinda Unit) and Triassic anatectites (Tres Lagunas Granite). Early Cretaceous metagabbros yield⁴⁰Ar/³⁹Ar plateau (plagioclase;Fig. 2) plateau dates of ~134 Ma, and are interpreted as transitional oceanic crust that formed in an Early Cretaceous marginal basin (Spikings et al., 2015). We provide new⁴⁰Ar/³⁹Ar dates from fault-bounded, metamorphosed mafic and ul- tramafic slivers within the Peltetec Fault Zone (Fig. 2) that reveal the pres- ence of late Neoproterozoic basement (see section 4.2).

TheE-W striking Amotape Complex is a large inlier of metamorphic rocks located in southwestern Ecuador (Fig. 1), and is considered to be a parautochthonous section of the Andean margin which decoupled from rocks of the Cordillera Real and rotated into the Andean forearc (e.g.

Mourier et al., 1988; Mitouard et al., 1990) after ~115–110 Ma (Jaillard et al., 1999;Spikings et al., 2005). Weakly metamorphosed ar- koses, wackes and quartz arenites of the El Tigre Formation are consid- ered to be turbiditic and are devoid of volcanic debris (Litherland et al., 1994). These sedimentary rocks are unconformably overlain by late Aptian sedimentary rocks of the Celica Lancones Basin along the south- ern exposure of the Amotape Complex (Fig. 1;Jaillard et al., 1999;

Valarezo et al., 2019), and are intruded by the Marcabeli Pluton (U–Pb zircon ages 227–238 Ma;Aspden et al., 1995;Cochrane et al., 2014b;

Paul et al., 2018) and host pre-Devonian acritarchs and spores (Zamora and Pothe de Baldis, 1988). U–Pb concordia dates of the youn- gest detrital zircons within a sandstone of the El Tigre Unit (sample EO4) constrain its maximum depositional age to 512 ± 21 Ma (Suhr et al., 2019). These constraints suggest a Cambrian–Silurian age for the El Tigre unit. The El Tigre Unit is fault bounded against higher grade paraschists and paragneisses of the La Victoria Unit along its northern boundary (Fig. 1), which is considered byLitherland et al.

(1994)to be the metamorphosed equivalent of the El Tigre Unit, via el- evated temperatures during the intrusion of the Moromoro granites (228–237 Ma;Aspden et al., 1995;Cochrane et al., 2014b;Spikings et al., 2015;Paul et al., 2018).Suhr et al. (2019)report overlapping U– Pb concordia ages of detrital zircon cores from the La Victoria Unit of 365 ± 14 Ma and 357 ± 15 Ma, with typically magmatic Th/U ratios

Fig. 1.Geological map of the southern Cordillera Real and Amotape Complex of Ecuador (afterLitherland et al., 1994,Aspden et al., 1995). New and previous (Chew et al., 2008;Suhr et al., 2019) U\\Pb concordia dates of the youngest detrital zircons are shown, along with new⁴⁰Ar/³⁹Ar dates of gabbros and ultramafic rocks of the Huarguallá Gabbro Unit, within the Peltetec Fault Zone. The red box highlights the location ofFig. 2. F: Floresta Massif, M: Merida Andes, Q: Quetame Massif, PF: Peltetec Fault, S: Santander Massif. (For interpretation of the references to colour in thisfigure legend, the reader is referred to the web version of this article.)

of 0.68 and 0.52, respectively. Therefore, it is likely that the La Victoria Unit has a younger depositional age than the El Tigre Unit. The Bocana Unit is defined by Aspden et al. (1995) as a series of anatectic metasedimentary rocks that form part of the Triassic (see above) Moromoro Granite Complex (Fig. 1).Suhr et al. (2019)constrain its maximum depositional age to 367 ± 20 Ma (sample EO6) on the basis of U–Pb concordia ages of detrital zircons. U–Pb monazite dates of gar- net bearing migmatites constrain the timing of metamorphism of the sedimentary protolith to 229–223 Ma (Riel et al., 2013). These Palaeozoic sedimentary units, which underwent Triassic anatexis (Spikings et al., 2015), are bound to the north by a highly sheared zone that separates them from medium grade semi-pelites and schists of the Palenque Mélange (Aspden et al., 1995;Fig. 1).Suhr et al.

(2019)report U–Pb concordia ages of detrital zircons extracted from a sandstone within the Palenque Mélange, which yield a maximum depo- sitional age of 391 ± 17 Ma.

This summary highlights the lack of currently exposed Precambrian source regions within Ecuador. However, within South America, Pre- cambrian rocks are exposed within Colombia (E.g. Cordani et al., 2005), Peru (Miškovićet al., 2009), and within the Amazonian craton that is exposed to the east (e.g.Chew et al., 2011). Cambrian - Ordovi- cian zircons may be derived from the Famatinian Arc, which is exposed within Peru (e.g.Miškovićet al., 2009), Colombia (e.g.Van der Lelij et al., 2016) and Venezuela (e.g.Van der Lelij et al., 2016), while Carbonifer- ous zircons may be derived from the Carboniferous arc of the Eastern Cordillera of Peru (e.g.Miškovićet al., 2009). The relict conjugate mar- gins to South America within the Mixteca Terrane and the Maya Block (Spikings and Paul, 2019) are also potential sediment source regions for Gondwanan basins within Ecuador, and these are discussed in detail in section 5.3.

3. Analytical methods 3.1. U–Pb analyses of zircons

The U–Pb isotopic composition of zircons was obtained using Laser Ablation Inductively Coupled Mass Spectrometry (LA-ICP-MS) at the University of Lausanne. Polished zircons (in epoxy) were ablated with an UP-193FX ArF 193 nm excimer ablation system (ESI) using the fol- lowing parameters: 35μm beam size, 5 Hz repetition rate, 30 s signal and a beam energy density of 2.2–2.5 J/ cm2. Isotopic intensities were measured using an Element XR single-collector sector-field ICP-MS (Thermo Scientific). GEMOC GJ-1 zircon (CA-ID-TIMS²⁰⁶Pb–²³⁸U age of 600.5 ± 0.4 Ma;Boekhout et al., 2012;Ulianov et al., 2012) was used as a primary standard. Secondary standards used to monitor con- sistency in the measured U–Pb dates were either Harvard 91,500 (1065.4 ± 0.3 Ma; Wiedenbeck et al., 1995) zircon, or Plešovice (337.13 ± 0.37 Ma;Sláma et al., 2008) zircon. Dates (Table 1) were cal- culated using LAMTRACE (Jackson, 2008). More details regarding the spectrometer setup and data reduction can be found inUlianov et al. (2012).

3.2.⁴⁰Ar/³⁹Ar analyses

Two gabbros and an ultramafic rock were crushed and sieved to

≤300μm, and plagioclase was extracted using conventional magnetic and gravimetric methods. Translucent-transparent, inclusion free pla- gioclase grains were hand-picked using a binocular microscope, and washed using ultra-sound in di-ionized water for ten minutes. Plagio- clase separates were irradiated at the shielded CLICIT position at the Or- egon State University TRIGA reactor for 15 h, along with evenly spaced aliquots of Fish Canyon Tuff sanidine (28.201 ± 0.046 Ma;Kuiper et al., 2008) to track neutronfluences. The irradiated plagioclase sepa- rates were degassed using a 50 W, CO2-IR laser (Photon Machines Inc.), and the gas was cleaned via a coldfinger held at−130 °C, and hot GP50 (S101) and AP10 getters. Ar isotopes were measured on a

multi-collector Argus V mass spectrometer, equipped with one Faraday with a 1E11 ohm feedback resistor (⁴⁰Ar), and four Faradays with 1E12 ohm feedback resistors (^39-36Ar). Data were reduced using ArArCalc (Koppers, 2002), and baseline and blank corrected data are presented inTable 2.

3.3. Whole Rock Geochemistry

Representative whole rock powders were prepared using an agate mill and major and trace elements were measured using a Philips PW2400 X-Ray Fluorescence (XRF) spectrometer at the University of Lausanne, Switzerland. The NIMN, NIMG, BHVO and SY2 standards were used for quality control. Glass fused disks prepared for XRF analy- ses were fragmented and mounted for additional analyses of trace and rare earth elements (REE). Measurements were made using a Perkin Elmer ELAN 6100 DRC quadrupole ICP-MS, and depending on the ex- pected enrichment within samples, either NIST SRM 610 or 612 fused glasses were used as external standards. The laser settings used for anal- yses were 10 Hz frequency, 140 mJ energy and 80–120μm spot size.

Blanks were measured for ~90s, after which the laser was switched on and the signal was measured for 45 s. The Sr or Al2O3concentrations (previously determined by XRF) were used as an internal standard.

Each sample was ablated 3 times, and average concentrations were cal- culated offline using LAMTRACE (Jackson, 2008). The uncertainties of 3 spots per sample are ±10% for rare earth elements (REE), and ± 5% for other trace elements. Whole rock compositions (Table 3) have been normalised to an anhydrous state in the graphical representations.

3.4. Nd bulk rock isotopes

100 mg of whole rock powder was dissolved in 4 ml of concentrated HF and 1 ml of 15 M HNO₃in closed Teflon vials at 140 °C for seven days.

The samples were dried down and re-dissolved in 3 ml of 15 M HNO3

before being dried down again. Sr–Nd chemical separation followed the methods described inPin and Zalduegui (1997)andChiaradia et al. (2011). Radiogenic isotopes of Sr and Nd were analysed at the Uni- versity of Geneva using a Thermo Neptune PLUS Multi-Collector ICP-MS following the methods described byChiaradia et al. (2011).

Isotopic ratios were corrected for internal fractionation using

146Nd/¹⁴⁴Nd = 0.7219 for the¹⁴³Nd/¹⁴⁴Nd ratio. JNdi-1 (¹⁴³Nd/¹⁴⁴Nd = 0.512115; Tanaka et al., 2000; long-term external reproducibility:

10 ppm) was used as an external standard. Due to a systematic difference between measured and accepted standard ratios, Nd isotope ratios were further corrected for external fractionation by a value of +0.047 and + 0.5 amu, respectively. The interference of ¹⁴⁴Sm on ¹⁴⁴Nd was monitored on¹⁴⁷Sm and corrected with a value of 0.206700 (¹⁴⁴Sm/¹⁴⁷Sm). The data are presented inTable 4.

4. Results

In-situ (LA-ICPMS) U–Pb dates have been obtained from detrital zir- cons extracted from nine, weakly metamorphosed sedimentary rocks that were mapped as the Chiguinda (Cordillera Real) and La Victoria (Amotape Complex) units by the British Geological Survey (Litherland et al., 1994). These data have been combined with previous U–Pb anal- yses of detrital zircons from the same, and other Palaeozoic sedimentary sequences within Ecuador (Table 1;Chew et al., 2007;Suhr et al., 2019;

raw U and Pb isotopic data are presented in Supplementary Table 5).

Representative cathodoluminescence images of detrital zircons are pre- sented as a supplementaryfigure. We also present new⁴⁰Ar/³⁹Ar, step- heating analyses of plagioclase extracted from exhumed mafic and ul- tramafic rocks within the Peltetec Fault Zone (Table 2).

4.1. U–Pb analyses of detrital zircons

Prior palaeontological constraints on the depositional ages of the sampled units are poor, and thus we rely heavily on the youngest zircon ages to provide maximum depositional age constraints. However, the low precision of LA-ICPMS dates can result in inaccuracies related to un- identified discordance. We have attempted to mitigate low-precision spot analyses by applying a similar approach toDickinson and Gehrels (2009), and derive temporal depositional constraints with several dif- ferent statistical tolerances that trade-off precision and accuracy, while only considering grains that are concordant within 2σ(Fig. 3).

These different dates are i) the youngest single grain (YSG), which is particularly prone to inaccuracy as a result of unidentifiable lead loss within the precision of LA-ICPMS analyses, and a potential lack of repre- sentation, ii) weighted mean of the youngest three (Y3) grains, which mitigates the effect of lead loss, but can group dates that clearly do not define a single date population, iii) weighted mean of the youngest clus- ter of 2 or more grains with overlapping dates at 1σ(YC2 + (1σ); e.g.

Jones et al., 2009), iv) weighted mean of the youngest cluster of 3 or more grains with overlapping dates at 2σ(YC3 + (2σ)), and v) the youngest detrital zircon (YDZ) calculation ofLudwig (2012), which is determined by a Monte Carlo analysis of the youngest subset of detrital zircon dates in a detrital zircon population. All dates are reported with 2σuncertainties (Table 1).

The YSG, Y3 and YDZ dates overlap within uncertainty in all 9 newly studied samples (Fig. 3;Table 1), and define the youngest possible detri- tal zircon dates that mainly span the Devonian - Carboniferous. The Mean Squared Weighted Deviate (MSWD; equivalence) vales for the Y3 dates range from 8 to >300, suggesting they do not define a single date population. The YC2 + (1σ) and YC3 + (2σ) dates are more con- servative and statistically robust, and yield older dates that are consis- tently older than the YSG.Dickinson and Gehrels (2009)showed that the youngest single grain age was equivalent to the depositional age in 90% of their sedimentary rocks from the Colorado Plateau, which probably reflects the proximity of their sampling sites to contempora- neous volcanic activity. Similarly, Carboniferous arc magmatism was abundant in Peru, and crust that was outboard of Ecuador (see sec- tion 5), and thus it is reasonable to suggest that there was a abundant supply of Carboniferous zircons into the Chiguinda and La Victoria units. Therefore, we consider the YSG dates to be the best estimates of the minimum detrital zircon age of the sedimentary rocks, while we also consider the YC2 + (1σ) dates to be the best conservative estimates of the detrital zircon ages, given that the YC3 + (2σ) dates were typi- cally >10 Ma older than the timing of deposition in the study of Dickinson and Gehrels (2009).

Metasedimentary rocks 99RS38, 99RS39, 11RC20, 13AP53 and 13AP54 were all sampled from low grade, gently folded strata that are mapped as the Chiguinda Unit (Litherland et al., 1994). The rocks

0774 0770

R. Alao

Peltetec

78°33’

78°34’

9794

1°53’

1°51’

1°52’

9790

2 km

Guamote Sequence Alao Arc

Yunguilla Unit

Tres Lagunas Unit Metagabbro Metasedimentary rocks

Peridotite

P e lt e tec Unit

Metabasalts

Chiguinda Unit Bayo Pungu Unit

Road and track 04PR98, 566±35

Pr

Pr Previous analyses (Spikings et al., 2015)

Huarguallá

04PR68, 582±41 04PR116, 586±14

09PR48, 135±1 09PR47, 134±13

Metagabbro

Huar guallá Unit

Late Neoproterozoic Carboniferous Triassic

Jurassic - Lower Cretaceous

Pr

Fig. 2.Geological map of the Peltetec Fault Zone afterReyes (2006), showing the location of new⁴⁰Ar/³⁹Ar dates (plagioclase) of the Huarguallá Gabbro Unit, and previous⁴⁰Ar/⁴⁰Ar analyses (Spikings et al., 2015). Outcrops are accessible along the River Huarguallá. Universal Transverse Mercator coordinates are provided (zone 17 M).

Table 1

Summary of new zircon (detrital) U-Pb dates obtained from the Chiguinda and La Victoria Fms., including previously published data from the Cordillera Real and the Amotape Complex.

Sample Lithology Unit Latitude Longitude No. of YSG Y3 YDZ YC1σ

(2+)

YC2σ (3+)

Prominant age peaks Chord Intercepts

analysed ²⁰⁶Pb/²³⁸U ²⁰⁶Pb/²³⁸U ²⁰⁶Pb/²³⁸U ²⁰⁶Pb/²³⁸U ²⁰⁶Pb/²³⁸U ²⁰⁶Pb/²³⁸U (Ma)* (Ma)ǂ spots$ ±2σ(Ma) ±2σ(Ma)^# ±2σ(Ma) ±2σ

(Ma)^#

±2σ (Ma)^# This study

Cordillera Real

99RS38 Quartzite Chiguinda 03° 59′

38.0”

79° 08′

21.9”

119 (90) 441.9

± 9.1

446 ± 32 (8.3)

423 + 10/−8

506 ± 3 (1.2)

508 ± 6 (2.2)

480–620, 720–780, 1000–1080

195–700, 1080.

555–1080 435–1080, 2130, 2800 99RS39 Quartzite Chiguinda 03° 59′

15.2”

79° 05′

37.9”

119 (91) 348.6

± 5.5

419 ± 87 (279)

349 + 6/−7

484 ± 10 (1.8)

517 ± 6 (1.4)

520–680, 1040–1100 500–1210, 2740 11RC20 Paraschist Chiguinda 03° 59′

15.2”

79° 06′

24.0”

109 (82) 358.1

± 4.9

401 ± 55 (228)

358.3 + 5/−6

451 ± 7 (1.3)

449 ± 6 (3.8)

480–640, 700–720, 880–1020

400–1010 13AP53 Metasandstone Chiguinda 03° 59′

26.2”

79° 06′

27.4”

100 (88) 313.7

± 2.6

336 ± 38 (178)

314 + 3/−3

511 ± 3 (0.6)

510 ± 4 (1.3)

310–370, 480–580, 960–1040

525–1010 13AP54 Metasandstone Chiguinda 03° 59′

32.4”

79° 06′

25.2”

88 (68) 317.8

± 7.8

373 ± 100 (323)

318 + 9/−9

467 ± 5 (0.38)

501 ± 13 (3.3)

320–380, 460–560, 700. 720, 940–1080

325–945, 1050 99RS53 Black quartzite Chiguinda 03° 00′

45.4”

78° 35′

56.2”

120 (92) 346.5

± 10.6

366 ± 28 (9.2)

347 + 11/−12

394 ± 15 (0.38)

496 ± 20 (3.2)

560–640, 840–960, 1040–1100, 2020–2100

515–1110, 2090 99RS55 Bt paraschist Chiguinda 03° 00′

09.4” 78° 39′

14.9”

108 (83) 324.7

± 4.7

339 ± 17 (56)

325 + 5/−5

459 ± 3.3 (0.20)

489 ± 3 (0.79)

440–680, 960–1080,

2660–2720 270–600

515–1100, 1890 11RC27 Hbl quartzite Chiguinda 02° 12′

15.4”

78° 21′

57.4”

118 (81) 334.9

± 7.3 391

± 100,263) 336 + 8/−8

498 ± 11 (0.01)

488 ± 26 (2.4)

480–620, 940–1060, 1740–1860, 2500–2580

515–1090, 2130, 2750 Amotape Complex

13AP33 Bt paraschist La Victoria

03° 42′

52.6”

79° 51′

10.1”

120 (84) 327.5

± 10.5

372 ± 76 (38)

327 + 12/−12

448 ± 13 (0.64)

456 ± 17 (1.3)

480–640, 760–780, 980–1040, 1700–1760

110–655 485–1140 Previous Work

Cordillera Real Chew et al.(2008)

99RS28 Quartzite Chiguinda 04° 24′

38.3”

79° 09′

32.4”

49 (42) 367 ± 12 400 ± 80 (39)

377 + 43/−45

365 ± 9 (0.25)

529 ± 26 (3.6)

500–800, 900–1100, 2500–2700

N.D.

99RS65 Phyllite Isimanchi 04° 49′

42.3”

79° 06′

59.9”

50 (41) 368 ± 14 387 ± 40 (0.78)

409 + 19/−20

448 ± 39 (0.04)

460 ± 9 (2.0)

480–620, 920–1180, 1800–2000

N.D.

Amotape Complex N.D.

Suhr et al.(2019) N.D.

EO2 Sandstone Palenque 03° 37′

7.50”

80° 03′

19.5”

38 (31) 391 ± 17 474 ± 100 (69)

396 + 18/−18

561 ± 10 (0.53)

568 ± 13 (1.6)

540–640, 880–1060, 2020–2040

N.D.

EO4 Sandstone El Tigre 03° 51′

49.8”

80° 05′

56.7”

40 (37) 512 ± 21 516 ± 12 (0.19)

512 + 12/−16

521 ± 9 (0.59)

528 ± 11 (1.5)

512–640, 680–720, 960–1100

N.D.

EO5 Paragneiss La

Victoria

03° 44′

01.7”

79° 50′

07.7”

38(36) 357 ± 15 390 ± 65 (33)

366 + 15/−13

361 ± 10 (0.61)

539 ± 18 (3.6)

500–640, 778–971, 1039–1149

N.D.

EO6 Sandstone La Bocana 03° 45′

25.0”

79° 38′

36.2”

70 (63) 367 ± 20 390 ± 21 (4.3)

394 + 9/−17

399 ± 7 (0.07)

399 ± 7 (0.07)

500–620, 900–1060 N.D.

$ Values in parentheses are the number of grains remaining afterfiltering with a discordance of ±5%

# Values in parentheses are the MSWD of equivalence

* Discordia ±5%

N.D.: Not Determined

ǂEach row should be read as lower intercept - upper intercept1, upper intercept2 etc. (i.e. a common lower intercept). Multiple rows per sample indicate more than one lower intercept.

Table 2

Summary of 40Ar/39Ar (plagioclase) dates, and whole rock Nd and Sr compositions of the Huargualla Gabbro unit

Sample Lithology Unit Latitude Longitude ⁴⁰Ar/³⁹Ar ⁴⁰Ar/³⁹Ar (⁴⁰Ar/³⁶Ar)i ENdi 87

Sr/⁸⁶Sr plateau date inverse isochron

±2σ(Ma) date ± 2σ(Ma)*

04PR68 Metagabbro Huarguallá Gabbro 01° 52′35.85” 78° 33′45.37” 581.8 ± 41.1 582.1 ± 497.1 (0.12) 298.4 ± 169.5 2.40 0.706097 04PR98 Metagabbro Huarguallá Gabbro 01° 51′19.39” 78° 33′30.27” 565.5 ± 34.4 230.2 ± 234.5 (1.22) 172.6 ± 238.4

04PR116 Peridotite Huarguallá Gabbro 01° 51′19.37” 78° 33′30.26” 585.5 ± 14.1 519.4 ± 135.8 (0.74) 321.3 ± 51.3 −0.57 0.704296 Plateau dates defined as three contiguous heating steps that account for 50% or more of the total³⁹Ar released

*Numbers in parentheses are the MSWD

were sampled along the road between Loja and Zamora that transects the southern Cordillera Real (Fig. 1) and are deformed by regional scale N-S fold axes. 119 spot analyses of zircon cores and rims from quartz arenite 99RS38 yielded 90 concordant analyses with²⁰⁶Pb/²³⁸U concordia dates ranging between 442 ± 9 Ma and 1939 ± 25 Ma (Fig. 4a). The youngest spot age, YDZ and Y3 are indistinguishable, and thus the youngest detrital zircon age is considered to be 442 ± 9 Ma, while the YC2 + (1σ) and YC3 + (2σ) dates overlap, with a YC2 + (1σ) age of 506 ± 3 Ma. Kernel Density Estimates of the

206Pb/²³⁸U dates with ±5% discordance yield prominent peaks at 480–620 Ma, 720–780 Ma and 1000–1080 Ma (Fig. 4a). Discordant

206Pb/²³⁸U and²⁰⁷Pb/²³⁵U dates yield a complex array and statistical

analysis yields numerous best-fit chords (Fig. 5a;Reimink et al., 2016).

A lower intercept of ~195 Ma is younger than all of the concordant dates, and is common to upper intercepts at ~700 and ~ 1080 Ma. A lower intercept at 435 Ma is common to upper intercepts at 1080, 2130 and 2800 Ma, and a statistically likely chord has an older lower in- tercept at 555 Ma and an upper intercept at ~1080 (Table 1). Quartz arenite 99RS39 yielded 91 concordia dates from zircon cores and rims with dates ranging between 349 ± 6–2627 ± 77 Ma (Fig. 4b). The youngest spot age, YDZ and Y3 are indistinguishable, and thus the youn- gest detrital zircon age is considered to be 349 ± 6 Ma, with a YC2 + (1σ) age of 484 ± 10 Ma. Kernel Density Estimates of the

206Pb/²³⁸U dates with ±5% discordance yield prominent peaks at 520–680 and 1040–1100 (Fig. 4b). Bestfit analyses of the discordant data yielded two chords with a common lower intercept at ~500 Ma, and upper intercepts of ~1210 and ~ 2740 Ma (Fig. 5b). 109 spot analy- ses of zircon cores and rims from paraschist 11RC20 yielded 82 concor- dant analyses with²⁰⁶Pb/²³⁸U concordia dates ranging between 358 ± 5 and 2599 ± 16 Ma (Fig. 4c). The youngest spot age, YDZ and Y3 are in- distinguishable, and thus the youngest detrital zircon age is considered to be 358 ± 5 Ma, while the YC2 + (1σ) and YC3 + (2σ) dates overlap, with a YC2 + (1σ) age of 451 ± 7 Ma. Kernel Density Estimates of the

206Pb/²³⁸U dates with ±5% discordance yield prominent peaks at 480–640 Ma and 880–1020 Ma, and a weakly defined peak at 1980–2140 (Fig. 4c). Bestfits to the discordant²⁰⁶Pb/²³⁸U and²⁰⁷/²³⁸U dates yield a single chord with intercepts at ~400 and ~ 1010 Ma (Fig. 5c). 100 spot analyses of zircon rims and cores from quartzite 13AP53 yielded 88 ²⁰⁶Pb/²³⁸U concordia dates ranging between 314 ± 3–3128 ± 23 Ma (Fig. 4d). The youngest spot age, YDZ and Y3 are indistinguishable, and thus youngest detrital zircon age is consid- ered to be 314 ± 3 Ma, while the YC2 + (1σ) and YC3 + (2σ) dates overlap, with a YC2 + (1σ) age of 511 ± 3 Ma. Kernel density estimates of the²⁰⁶Pb/²³⁸U dates with ±5% discordance yield prominent peaks at 310–370 Ma, 480–450 Ma and 960–1040 Ma (Fig. 4d). Statistical analy- sis of the discordant dates yielded a single chord with lower and upper intercepts of 525 Ma and 1010 Ma, respectively (Fig. 5d). Finally, quartz- ite 13AP54 yielded 68²⁰⁶Pb/²³⁸U concordia dates from 88 spot analyses of rims and cores, which span between 318 ± 8 Ma and 3045 ± 20 Ma (Fig. 4e). The youngest spot age, YDZ and Y3 are indistinguishable, and thus the youngest detrital zircon age is considered to be 318 ± 8 Ma, with a YC2 + (1σ) age of 467 ± 5 Ma. Kernel Density Estimates of the

206Pb/²³⁸U dates with ±5% discordance yield prominent peaks at 320–380 Ma, 460–560 Ma and 940–1080 Ma (Fig. 4e). Bestfits to the discordant²⁰⁶Pb/²³⁸U and²⁰⁷/²³⁸U dates yield a chord with a lower in- tercept at ~325 Ma, with upper intercepts at ~945 Ma and ~ 1050 Ma (Fig. 5e).

Farther north within the Cordillera Real (roads between Riobamba and Macas, and Cuenca and Indanza), metasedimentary rocks 99RS53, 99RS55 and 11RC27 were also sampled from low grade strata that pre- serve original bedding and are deformed by low amplitudeE-W fold axes (Fig. 1). These strata were mapped as the Chiguinda Fm. by Litherland et al. (1994). Black quartzite 99RS53 yielded 92²⁰⁶Pb/²³⁸U concordia dates from 120 spot analyses of rims and cores, which span between 347 ± 11 Ma and 2656.8 ± 129.7 Ma (Fig. 6a). The youngest spot age, YDZ and Y3 are indistinguishable, and thus the youngest detri- tal zircon age is considered to be 347 ± 11 Ma, with a YC2 + (1σ) age of Table 3

Whole rock geochemistry of the Huargualla Gabbro unit

04PR68 04PR116

Gabbro Peridotite

wt% wt%

SiO2 48.94 47.83

P2O5 0.19 0.17

TiO2 0.77 0.49

Al2O3 11.32 8.93

CaO 8.49 14.91

Na2O 3.03 0.83

K2O 0.26 0.00

MnO 0.16 0.17

Fe2O3 11.19 9.11

Cr2O3 0.13 0.14

MgO 12.08 14.96

ppm ppm

Rb 3.53 0.31

Ba 85.72 11.02

Th 1.07 0.47

U 0.11 0.04

Nb 3.40 2.00

Ta 0.32 0.60

Pb 0.54 0.40

Sr 75.00 30.00

Zr 41.78 16.33

Hf 1.27 0.57

Ti 4634.07 2915.46

Tb 0.43 0.30

Y 15.30 10.30

Tm 0.26 0.16

Yb 1.64 1.00

Co 57.86 60.44

La 5.60 2.16

Ce 12.13 5.20

Pr 1.80 0.84

Nd 7.98 4.09

Sm 2.36 1.47

Eu 0.80 0.44

Gd 2.67 1.70

Tb 0.43 0.30

Dy 2.78 1.85

Ho 0.60 0.38

Er 1.77 1.21

Yb 1.64 1.00

Lu 0.24 0.16

V 349.00 303.00

Table 4

Sm and Nd isotopic compositions of the Huargualla Gabbro Unit

Sample Age ± 2σ Sm Nd 147Sm/144Nd 143Nd/144Nd (143Nd/144Nd)i εNd εNdi 2σ

(Ma)^# (ppm) (ppm) (±2σ)* (±2σ)

585 ± 14 2.36 7.98 0.182140 0.512692 (21) 0.511994 1.05 2.16 (0.4)

585 ± 14 1.47 4.09 0.217060 0.512686 (36) 0.511854 0.94 −0.57 (0.69)

# 40Ar/39Ar age of 04PR116, which is assumed to be valid for 04PR68.

*Numbers in parentheses are the 5th and 6th decimal place.

394 ± 15 Ma. Kernel Density Estimates of the²⁰⁶Pb/²³⁸U dates with ± 5% discordance yield prominent peaks at 560–640 Ma, 840–960 Ma, 1040–1100 Ma and 2020–2100 Ma (Fig. 6a). Bestfits to the discordant

206Pb/²³⁸U and²⁰⁷/²³⁸U dates yield two chords with a common lower intercept of ~515 Ma, and upper intercepts of ~1110 Ma and ~ 2090 Ma (Fig. 7a). 108 spot analyses of zircons within biotite bearing paraschists 99RS55 yielded 83 ²⁰⁶Pb/²³⁸U concordia dates spanning between 325 ± 5 Ma and 2647 ± 21 Ma (Fig. 6b). The youngest spot age, YDZ and Y3 are indistinguishable, and thus youngest detrital zircon age is considered to be 325 ± 5 Ma, while the YC2 + (1σ) yields an of 459 ± 3 Ma. Kernel density estimates of the²⁰⁶Pb/²³⁸U dates with ± 5% discordance yield prominent peaks at 440–680 Ma, 960–1080 Ma and 2660–2720 Ma (Fig. 6b). Discordant²⁰⁶Pb/²³⁸U and²⁰⁷/²³⁸U dates yield a complex array (Figs. 7b) and statistical analysis yields numerous best-fit chords. Thefirst chord has lower and upper intercepts at

~270 Ma and ~ 600 Ma. Two chords have a common lower intercept at ~515 Ma, with upper intercepts at ~1100 Ma and ~ 1890 Ma. The youngest chord intercepts are younger than all of the ²⁰⁶Pb/²³⁸U concordia dates. Finally, hornblende bearing quartzite 11RC27 yielded 81²⁰⁶Pb/²³⁸U concordia spot dates from zircon rims and cores, which span between 335 ± 7 Ma and 2677 ± 58 Ma (Fig. 6c). The youngest spot age, YDZ and Y3 are indistinguishable, and thus youngest detrital zircon age is considered to be 335 ± 7 Ma, while the YC2 + (1σ) and YC3 + (2σ) dates overlap, with a YC2 + (1σ) age of 498 ± 11 Ma. Ker- nel Density Estimates of the²⁰⁶Pb/²³⁸U dates with ±5% discordance yield prominent peaks at 480–640 Ma, 940–1060 Ma, 1740–1860 Ma and 2500–2580 Ma (Fig. 6c). Bestfits to the discordant²⁰⁶Pb/²³⁸U and

207Pb/²³⁵U dates yield three chords with a common lower intercept at

~515 Ma, and upper intercepts of ~1090 Ma, ~2130 Ma and ~ 2750 Ma (Fig. 7c).

Biotite bearing paraschist 13AP33 was sampled from the La Victoria unit within the Amotape Complex, along the road between La Bocana and Balsas (Fig. 1). 120 spot analyses of zircon rims and cores yielded 84 ²⁰⁶Pb/²³⁸U concordia dates that span between 328 ± 11 and 2671 ± 63 Ma (Fig. 6d). The youngest spot age, YDZ and Y3 are indistin- guishable, and thus youngest detrital zircon age is considered to be 328 ± 11 Ma, while the YC2 + (1σ) and YC3 + (2σ) dates overlap, with a YC2 + (1σ) age of 448 ± 13 Ma. Kernel Density Estimates of the²⁰⁶Pb/²³⁸U dates with ±5% discordance yield numerous prominent peaks at 480–640 Ma, 980–1040 Ma and 1700–1760 Ma (Fig. 6d).

Best-fit analysis of the discordant dates yields two chords, with lower –upper intercept pairs at ~110 Ma and ~ 655 Ma, and ~ 485 Ma and ~ 1140 Ma (Fig. 7d).

Chew et al. (2007)andSuhr et al. (2019)published U–Pb dates of detrital zircon grains extracted from six rocks with the Eastern Cordil- lera and the Amotape Complex, and they assumed the youngest single grains were consistently the best estimates of the maximum time of de- position. In all cases, the youngest spot age, YDZ and Y3 are indistin- guishable, and thus the youngest grain ages (Table 1;Fig. 3) are also considered here to represent the statistically youngest zircon detrital ages. However, more conservative estimates provided by the YC2 + (1σ;Table 1) statistic are 365 ± 9 Ma (99RS28, Chiguinda Fm.) and 448 ± 39 Ma (99RS65, Isimanchi Fm.), for the Eastern Cordillera.

YC2 + (1σ;Table 1) dates for the Amotape Complex are 561 ± 10 Ma (EO2, Palenque Fm.), 521 ± 9 Ma (EO4, El Tigre Fm.), 361 ± 10 Ma (EO5, La Victoria Fm.) and 399 ± 7 Ma (EO6, La Bocana Fm.).

260 280 300 320 340 360 380 400 420 440 460 480 500 520 540 560 580 600

99RS38 99RS39 11RS20 13AP53 13AP54 99RS53 99RS55 11RC27 99RS28 99RS65 EO5 EO2 EO4 EO6 Youngest Single Grain (YSG)

Youngest 3 zircons (Y3) Youngest Detrital Zircon (YDZ)

Youngest ≥2 zircons (1V overlap; YC2+1V) Youngest ≥3 zircons (2V overlap; YC3+2V)

13AP33 238U/206Pb ±2V date (Ma)

Chiguinda Fm. Isimanchi

Fm.

La Victoria Fm.

Palenque Fm.

El Tigre Fm.

La Bocana Fm.

C O R D I L L E R A R E A L A M O T A P E C O M P L E X

CarboniferousDevonianSilurianOrdovicianCambrian

Fig. 3.Summary of youngest detrital zircon U\\Pb dates for Palaeozoic metasedimentary rocks of the Cordillera Real and Amotape Complex of Ecuador, including new and previous (Chew et al., 2008;Suhr et al., 2019) work. Definitions of each date are provided in the text.

206Pb/238U

99RS38, micaceous quartz arrenite, Chiguinda Unit

11RC20, paraschist, Chiguinda Unit

0.054 0.058 0.062 0.066 0.070 0.074 0.078 0.082

0.38 0.42 0.46 0.50 0.54 0.58 0.62 340

380 420

460 0.055

0.065 0.075 0.085

0.4 0.5 0.6 0.7

380 420

460 500

540

0.045 0.055 0.065 0.075 0.085

0.3 0.4 0.5 0.6 0.7

300 340

380 420

460 500

540 206Pb/238U206Pb/238U

99RS39, micaceous quartz arrenite, Chiguinda Unit

C A

B

0.045 0.055 0.065 0.075 0.085

0.3 0.4 0.5 0.6 0.7

300 340

380 420

460

13AP53, metasandstone, Chiguinda Unit

0.045 0.055 0.065 0.075 0.085

0.3 0.4 0.5 0.6 0.7

300 340

380 420

460

207Pb/²³⁵U 206Pb/238U206Pb/238U

540 500

540 500

E D

13AP54, metasandstone, Chiguinda Unit

n=90 (±5% discordance) Youngest zircon 441.9±9.1 Ma

4 2 6

0 8 10 12

0 500 1000 1500 2000 2500 3000

206Pb/238U age (Ma) 420 460 500

0.0 0.2 0.4 0.6

0 4 8 12 16 20

1000 1400

1800 2200

2600

3000 0.0

0.2 0.4

0 4 8 12 16 20 24

1400 1800

2200 2600

3000

0.0 0.1 0.2 0.3 0.4 0.5

0 2 4 6 8 10 12 14

1000 1400

1800 2200

2600 C2

1000 C3

A2

0.0 0.2 0.4 0.6

0 4 8 12 16 20 24

1000 1400

1800 2200

2600 3000

0.0 0.2 0.4 0.6

0 4 8 12 16 20 24

1000 1400

1800 2200

2600 3000

207Pb/²³⁵U 0.6

n=91 (±5% discordance) Youngest zircon 348.6±5.5 Ma

4

2 6

0 8 10

0 500 1000 1500 2000 2500 3000

320 360 400 440 480

n=82 (±5% discordance) Youngest zircon 358.1±4.9 Ma

0 500 1000 1500 2000 2500 3000

5 4 3 2 1 0 6

360 400 440

n=88

Youngest zircon 313.7±2.6 Ma

10

5 15

0

0 500 1000 1500 2000 2500 3000 3500 300

320 340 360 380

2 1 3

0 4 5 6

n=68

Youngest zircon 317.8±7.8 Ma

0 500 1000 1500 2000 2500 3000 7

3500 206Pb/²³⁸U age (Ma)

N

N

N

N

N

Fig. 4.Wetherill concordia and frequency histograms (only spots with ±5% discordance) showing the Kernel Density Distributions of spot U\\Pb analyses of detrital zircons, acquired using LA-ICPMS. The²⁰⁶Pb/²³⁵U ages of the youngest three zircons are shown in the inset of the histograms. Labels in the concordia plots highlight best-fit chords that are also depicted inFig. 5.

4.2. Mafic and ultramafic rocks entrained within the Peltetec Fault Zone The Peltetec Fault Zone separates parautochthonous continental crust of the Chaucha Block (Guamote Sequence) from the Early

Cretaceous continental Alao Arc (Fig. 2). The 1–2 km wide fault zone hosts a series of N-S oriented litho-tectonic slices ranging in thickness from one to hundreds of meters (e.g.Litherland et al., 1994). We present

40Ar/³⁹Ar, geochemical and isotopic data from metagabbros and an 99RS38, Chiguinda Unit, n=119

0 5 10 15 20 25 30 35 40

99RS39, Chiguinda Unit, n=119

0 10 20 30 40 50 60 70 80 90

99RS53, Chiguinda Unit, n=120 Age (Ma)

0 500 1000 1500 2000 2500 3000 3500

Age (Ma)

0 500 1000 1500 2000 2500 3000 3500 0

50 100 150 200 250 300

Likelihood

350

45 50

LikelihoodLikelihood

Age (Ma)

0 500 1000 1500 2000 2500 3000 3500

0 1000 2000

1000

0 A

A1 A2, B1, C1 B C

C2 C3

0 1000 2000 3000 0

1000

A A1

A2

0 1000 2000

0 1000

A A1

A2

0 50 100 150 200 250 300

Age (Ma)

Likelihood

0 500 1000 1500 2000 2500 3000 3500 11RC20, Chiguinda Unit, n=109

A A1

0 1000

0 1000

2000 500

Upper Intercept (Ma)

Lower Intercept (Ma)

Upper intercept Lower intercept

A B

C D

Age (Ma)

0 500 1000 1500 2000 2500 3000 3500 0

50 100 150 200 250

0 1000 2000

0 1000 13AP54, Chiguinda Unit, n=88

A A1

A2

B B1

Likelihood

E

Fig. 5.Highest likelihood upper and lower intercepts of chords defined by discordant U\\Pb data obtained from detrital zircons of the Chiguinda unit of Ecuador. Likelihoods were deter- mined using the algorithm described inReimink et al. (2016). Insets show the same data, with darker blue colours depicting the highest likelihood. Labels, A, B, C are intercept values of distinct chords, where, for example, A depicts a lower intercept, while A1, A2 etc. depict different upper intercepts with the same lower intercept. (For interpretation of the references to colour in thisfigure legend, the reader is referred to the web version of this article.)

0.045 0.055 0.065 0.075 0.085

0.3 0.4 0.5 0.6 0.7

340 380

420 460

500 540 99RS55, biotite paraschist, Chiguinda Unit

0.0 0.1 0.2 0.3 0.4 0.5

0 2 4 6 8 10 12 14

1000 1400

1800 2200

2600

0.045 0.055 0.065 0.075 0.085

0.3 0.4 0.5 0.6 0.7

300 340

380 420

460 500

540 13AP33, biotite paraschist, La Victoria Unit

206Pb/238U206Pb/238U

324.7±4.7 Ma (1 grain) 0.045

0.055 0.065 0.075 0.085

0.3 0.4 0.5 0.6 0.7

300 340

380 420

460 500

540

0.0 0.1 0.2 0.3 0.4 0.5

0 2 4 6 8 10 12 14

1000 1400

1800 2200

2600 0.0

0.1 0.2 0.3 0.4 0.5

0 2 4 6 8 10 12 14

1000 1400

1800 2200

2600 99RS53, metasandstone, Chiguinda Unit

11RC27, hornblende quartz arrenite, Chiguinda Unit

0.045 0.055 0.065 0.075 0.085

0.34 0.38 0.42 0.46 0.50 0.54 0.58 0.62 340

380 420

460 500 206Pb/238U

334.9±7.3 Ma (1 grain)

206Pb/238U

346.5±10.6 Ma (1 grain)

A2 A3 A2

B A

C

D

0.0 0.1 0.2 0.3 0.4 0.5

0 2 4 6 8 10 12 14

1000 1400

1800 2200

2600

B2

n=83 (±5% discordance) Youngest zircon 324.7±4.7 Ma

4

2 6

0 8

0 500 1000 1500 2000 2500 3000 315

325 335 345 355

n=92 (±5% discordance) Youngest zircon 346.5±10.6 Ma

4

2 6

0 8 10

0 500 1000 1500 2000 2500 3000 320

360 400 440

n=81 (±5% discordance) Youngest zircon 334.9±7.3 Ma

4

2 6

0 8 10

0 500 1000 1500 2000 2500 3000 300

380 460

4

2 6

0 8 10

0 500 1000 1500 2000 2500 3000 n=84

Youngest zircon 327.5±10.5 Ma

300 380 460

206Pb/238U age (Ma)

206Pb/238U age (Ma)

207Pb/235U 207Pb/235U

N

N

N

N

Fig. 6.Wetherill concordia and frequency histograms (only spots with ±5% discordance) showing the Kernel Density Distributions of spot U\\Pb analyses of detrital zircons, acquired using LA-ICPMS. The²⁰⁶Pb/²³⁵U ages of the youngest three zircons are shown in the inset of the histograms. Labels in the concordia plots highlight best-fit chords that are also depicted inFig. 7.