Mantle driven cretaceous flare-ups in Cordilleran arcs

Mantle driven cretaceous flare-ups in Cordilleran arcs

Accepted Manuscript Mantle driven cretaceous flare-ups in Cordilleran arcs Ana María Martínez Ardila, Scott R. Paterson, Vali Memeti, Miguel A. Parad...

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Accepted Manuscript Mantle driven cretaceous flare-ups in Cordilleran arcs

Ana María Martínez Ardila, Scott R. Paterson, Vali Memeti, Miguel A. Parada, Pablo G. Molina PII: DOI: Reference:

S0024-4937(18)30469-9 https://doi.org/10.1016/j.lithos.2018.12.007 LITHOS 4898

To appear in:

LITHOS

Received date: Accepted date:

9 July 2018 7 December 2018

Please cite this article as: Ana María Martínez Ardila, Scott R. Paterson, Vali Memeti, Miguel A. Parada, Pablo G. Molina , Mantle driven cretaceous flare-ups in Cordilleran arcs. Lithos (2018), https://doi.org/10.1016/j.lithos.2018.12.007

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ACCEPTED MANUSCRIPT Mantle driven Cretaceous flare-ups in Cordilleran arcs Ana María Martínez Ardila1,2, Scott R. Paterson1, Vali Memeti2, Miguel A. Parada3 and Pablo G. Molina3 1

Department of Earth Sciences, University of Southern California, Los Angeles, 3651 Trousdale

Department of Geological Sciences, California State University, Fullerton, 800 N State College

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Parkway, California 90089, U.S.A. email: [email protected]

Blvd, California 92831, U.S.A.

Departamento de Geología, Centro de Excelencia en Geotermia de los Andes (CEGA),

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Universidad de Chile, Plaza Ercilla 803, Santiago, Chile.

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ABSTRACT

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Continental arcs display episodic magmatism characterized by flare-ups and lulls. Models

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published to explain these patterns invoke (1) upper plate crustal processes driven by internal feedback; (2) episodic mantle melting processes, or (3) external lower plate tectonic events.

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This study addresses the role of mantle magmas during flare-ups in Mesozoic Cretaceous continental arcs using geochronological and geochemical data for three Cretaceous arc segments:

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the western Peninsular Ranges Batholith (wPRB), the Peruvian Coastal Batholith (PCB), and the

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Chilean Coastal Batholith (CCB). In all three arc segments, bedrock zircon age patterns defining a flare-up from ~125 to 90 Ma characterized by gabbro to granite units with Sri <0.705, ƐNd from 0 to +7.5, 208Pb/204Pb from 38.2 to 38.7, and 206Pb/204Pb from 18.3 to 18.7. These values project well towards a depleted mantle source. Areal measurements show that gabbro forms ~18% (wPRB), ~24% (PCB), and ~10% (CCB) of exposed plutonic material. AFC modeling indicates that these magmas have experienced fractional crystallization with only minor crustal assimilation (<201

ACCEPTED MANUSCRIPT 30%), implying that the great majority of these magmas are mantle-derived. Thus the cause of these flare-ups must be episodic mantle processes: crustal melting was not required for triggering the flare-up, and only played a secondary role in modifying melt compositions. It remains unclear if the episodic mantle processes reflect internal feedback(s) or external tectonically

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1. INTRODUCTION

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Keywords: mantle, crust, flare-up, arcs, isotopes, assimilation

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driven processes.

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Studies of continental magmatic arcs in the North and South American Cordilleras have recognized patterns of episodic magmatism. Episodicity is characterized by periods of high

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magma addition rates (MARs) called flare-ups that are separated by periods of low MARs, called

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lulls (Ducea, 2001; DeCelles et al., 2009; de Silva et al., 2015; Paterson and Ducea, 2015; Kirsch

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et al., 2016). Episodic patterns are documented either by (1) the relative abundance of igneous bedrock and their ages (Condie et al., 2011; Ducea et al., 2015; Paterson and Ducea, 2015;

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Kirsch et al., 2016), (2) the relative abundances of different detrital zircon ages in sediments

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derived from the arc, or (3) volume estimates of plutonic and volcanic rocks produced during a flare-up versus a lull (Paterson and Ducea, 2015; Kirsch et al., 2016).

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Why magmatism in arcs is episodic remains an unresolved and exciting question. Episodicity may reflect a combination of internal feedback processes in the upper plate, mantle, or crust within the arc (Ducea, 2001; DeCelles et al., 2009; Chin et al., 2015; DeCelles et al., 2015; Cao et al., 2017), and/or external forcing by tectonic events outside the arc (Hughes and Mahood, 2008; Zellmer, 2008; de Silva et al., 2015). Large, linked geochemical and geochronological data sets are needed to expand our knowledge about the non-steady-state

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ACCEPTED MANUSCRIPT magmatic activity of continental arcs, to test the validity of the proposed models, and to better evaluate the role that mantle-crust interactions play. Mantle melting drives magmatism at mid-oceanic ridges, hot spots and ocean island arcs, but its role in driving magmatism in continental margin arcs has remained contentious. In this

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paper, we first briefly introduce proposed models that attempt to explain continental arc flare-ups and then explore in more detail the role that mantle magma input plays. We examine the

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geochemical and geochronological evolution of mafic versus felsic plutonic rocks formed during

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the Late Cretaceous, the volumetrically largest Mesozoic Cordilleran flare-up in the western Peninsular Ranges Batholith (wPRB) in California, the Peruvian Coastal Batholith (PCB) and the

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Central Chilean Coastal Batholith (CCCB). Chemical data suggest that mantle-derived magmas

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drive flare-ups in these arc segments with moderate to little amounts of crustal materials involved, implying that mantle processes drove these flare-ups. A comparison to other

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subduction-related arc segments suggests that mantle processes may commonly be the dominant

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driver of flare-ups and that crustal cyclic (?) processes largely play a second-order effect in the

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degree to which the mantle signature is modified. 2. REVIEW OF FLARE-UP MODELS

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Several models have been published in the last decade to explain cyclic and episodic patterns of magmatism and deformation in arcs. Models invoke either internal feedback driven by intra-arc cyclic processes fairly independent of plate motions, or external controls caused by events outside the arc and driven by plate tectonic processes (Haschke et al., 2006; DeCelles et al., 2009; Chapman et al., 2013; Ducea et al., 2015; Paterson and Ducea, 2015; Kirsch et al., 2016).

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ACCEPTED MANUSCRIPT A popular crustal “internal feedback” model originally described by Ducea (2001), Ducea and Barton (2007) and DeCelles et al. (2009, 2015) involves periodic episodes of high flux arc magmatism fueled by underthrusting foreland lithosphere. In this model, underthrusting drives increased melting and arc magmatism and, along with associated tectonism, produces thick crust,

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mountains, and dense melt residues that during crustal thickening change to eclogite (Ducea,

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2001; Sobolev et al., 2006). When the eclogite residues reach a critical mass, they founder into

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the mantle causing isostatic adjustment of the topographic surface and rapid regional elevation gain in response to removal of the dense root (Garzione et al., 2006; Pelletier et al., 2010).

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Increased topography drives renewed rapid foreland underthrusting that recharges the magmatic

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system, and the cycle is rejuvenated.

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External control models involve events outside the crustal arc column, such as plate reconfigurations and changes in mantle flow and magma production (e.g., Hughes and Mahood,

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2008; Zellmer, 2008; de Silva et al., 2015). Specifically, if arc systems are controlled by

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parameters of the down-going plate, such as convergence rate, age, and subduction angle, then flare-ups and lulls would likely be widely distributed along the arc and occur as events that

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coincide with periods of global plate reorganization (Kirsch et al., 2016).

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Mantle melt input has also been cited as playing an important role in driving flare-ups in which the mass transfer and heat input of mantle-derived magmas promotes fusion of crustal rocks (Gaetani and Grove, 1998; Ulmer, 2001). Evidence supporting this idea comes from studies of hybrid intermediate and felsic plutonic rocks in the Famatinian arc (Otamendi et al., 2012) and mafic and felsic magmatism with isotopically primitive values and their respective contemporaneity (Condie, 1998; Pietranik et al., 2008; Sun et al., 2010; Condie et al., 2011; Kimbrough et al., 2015; Schwartz et al., 2017). 4

ACCEPTED MANUSCRIPT Other potential processes, such as episodic volatile fluxing into the mantle wedge and episodic melting and ascent scenarios within the mantle wedge may drive episodic mantle melting (Sekine and Wyllie, 1982; Scott and Stevenson, 1984; Nicolas, 1986; Billen and Arredondo, 2018). Research conducted at subduction zones by Peacock (2001) and Obara (2002)

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suggested that episodic volatile flux causes intermediate-depth earthquakes, and instances of

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episodic volatile fluxing into the mantle can be indicated by slab seismicity (Manning, 2004).

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Episodic melt extraction events suggested by Nicolas (1986) trace successive stages of melt extraction from mantle diapirs and explain how diapiric rise and melting can generate the

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discontinuous and episodic processes of melt extraction. The periodicity of such events depends

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on the spreading rate. Scott (1984) studied the process of melt migration in the Earth's mantle by applying a numerical experiment and suggested that episodic melt ascent processes would lead to

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episodic eruptions. Most recent studies of mantle flow and subduction zone dynamics conclude

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that episodic motion of the slab and subduction rates are a consequence of episodic mantle flow which depends on the viscosity of the asthenosphere. Changes in mantle viscosity might thus

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result in the variation of subduction rates that could contribute to the episodic patterns observed

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in arc magmatism (Jadamec and Billen, 2010; Billen and Arredondo, 2018).

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Also, studies of the timing and magnitudes of magmatism in oceanic arcs show that magmatism is mantle-derived, sometimes episodic, and that flare-ups are driven by mantle processes. The evidence from oceanic arcs strongly supports the idea that flare-ups can be driven by mantle processes without crustal involvement (Ishizuka et al., 2011; Reagan et al., 2013; Jicha and Jagoutz, 2015). 3. GEOLOGICAL SETTING

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ACCEPTED MANUSCRIPT The Cretaceous wPRB, PCB, and CCCB, arc segments of the North and South American Cordillera provide examples of continental margin arc flare-ups. In the following sections, the tectonic and magmatic history of the selected Cordilleran arc sections is briefly summarized. Isotopes and age compilations of the three arc segments presented in this paper (Figures 1 and 2)

signature and reservoirs.

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3.1 Western Peninsular Ranges Batholith (35-33° N)

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are focused on the Late Cretaceous with particular emphasis on documenting the mantle

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The northern part of the Cretaceous Peninsular Ranges Batholith between about 33° and 35°N is divided into western and eastern zones with distinct ages and chemistry (Baird et al.,

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1979; Kistler et al., 2014; Kimbrough et al., 2015). The numerous western PRB plutons (and

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some volcanic rocks) emplaced in an oceanic arc setting have ages of ca. 134-108 Ma. The many

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central PRB plutons have ages of ca. 111-93 Ma and record the west to east progression of subduction transitioning from an oceanic to a continental arc setting. The few large eastern PRB

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plutons were emplaced in a continental arc setting at ca. 98-91 Ma (Hildebrand and Whalen,

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2014b; Morton et al., 2014). The western zone (wPRB) is characterized by gabbro (~14%) and a wide range of more siliceous compositions with primitive island-arc geochemical affinities

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(Silver and Chappell, 1988; Todd et al., 2003; Kimbrough et al., 2015). In the eastern zone, gabbros are rarely exposed (~1%) and are dominated by a tonalite-trondhjemite-granodiorite (TTG) suite that composes ~50% of the exposed rocks (Kimbrough and Grove, 2006).

3.2 Peruvian Coastal Batholith (6-18° S) Between 6° S and 18° S, the Cretaceous marks the intrusion of the Peruvian Coastal batholith (PCB), a linear belt of calc-alkaline granitoids emplaced from 130 to 60 Ma (Pitcher et 6

ACCEPTED MANUSCRIPT al., 1985; Mukasa, 1986; Hildebrand and Whalen, 2014a). The PCB consists of plutons with bimodal magma compositions, exhibits a west to east change in chemical and isotopic signatures and increase in the thickness of the continental crust (Atherton and Petford, 1996), potentially due to subduction shallowing (Coira et al., 1982). Five segments are recognized, but only two

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mapped in any detail: the 400 km long Lima segment and the 1000 km long Arequipa segment

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(Pitcher et al., 1985). The southern Arequipa segment is influenced by the rise of magmas

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through thicker Precambrian and Paleozoic basement with evidence of higher crustal contamination. The northern Arequipa segment is characterized by a larger proportion of gabbro

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at the present erosion level (~30%) and by a diversity of more siliceous rocks including quartz

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diorite, tonalite, granite, and monzogranite all with primitive geochemical affinities. The gabbro plutons occur over the entire width of the batholith, but most of them along the western flank

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(Regan, 1985).

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3.3 Chilean Coastal Batholith (18°– 35°S)

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In the Chilean Andes, the Cretaceous batholith is located in the western belt and divided

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into two tectono-magmatic domains: central Chilean Coastal Batholith (18°– 35°S) and Patagonian Batholith (42°– 55°S). The central Chilean Coastal Batholith (CCCB) is

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characterized by Mesozoic plutonic belts with eastward decreasing ages and isotopic enrichments that culminate with primitive Late Cretaceous plutons dominated by mafic compositions (~60%) hosted in Early Cretaceous volcanic-sedimentary successions (Parada et al., 1988). Emplacement started in the Early Mesozoic at the end of an extensional tectonic regime (Parada et al., 1988, 1999; Hervé et al., 2007; Parada et al., 2007; Parada et al., 2005a). Gabbro, diorite, granodiorite, granite, and monzogranite are the dominant lithologies. Very low initial 87Sr/S6Sr ratios and positive ƐNd for these CCCB plutons and associated volcanic rocks 7

ACCEPTED MANUSCRIPT (mostly ca. 0.7035 and between +4.0 and +5.0) confirm derivation of the magmas from the upper mantle with little to no continental crustal involvement (Parada et al., 1988). Isotopic variations recorded in Cretaceous igneous rocks are thought to be a consequence of continuous lithospheric delamination and associated asthenosphere upwelling and melting that started in the Early

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Jurassic (Parada et al., 1999; Parada et al., 2005).

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4. METHODS AND RESULTS

The role of the mantle for triggering flare-ups was examined using geochronological and

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geochemical datasets from all three arc segments: wPRB, PCB, and CCB. The compiled geochemical and geochronological data (see the supplementary data) are mainly derived from

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published data and supplemented by the Central Andes geochemical and geochronology database

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(http://andes.gzg.geo.uni-goettingen.de). Data for the PCB are also supplemented with new data

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from the lead author and from Ben Clausen at Loma Linda University.

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4.1 New U-Pb zircon geochronology U-Pb single zircon ages were obtained for the PCB from nine plutonic samples at the

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Arizona Laserchron Center, University of Arizona. Measured spots (20 µm size) included cores

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and rims in all samples. All U-Th/Pb isotopic measurements were performed by Element2 single-collector laser ablation-inductively coupled plasma mass spectrometry (LA-ICP-MS) following the procedure outlined by Gehrels et al. (2008) and Johnston et al. (2009). The spectrometer is coupled to an Excimer laser system operating at a wavelength of 193 nm. This method does not involve chemical abrasion pretreatment on the zircons. Final ages reported and discussed throughout this paper are concordia ages calculated using the Isoplot Excel® macro of Ludwig (2003). Ages given in the text and figures are quoted at a 2σ confidence level. All 8

ACCEPTED MANUSCRIPT geochronological data are included in supplementary material, and errors are reported at ± 2σ (see the supplementary data). 4.1 New whole rock element and isotope geochemistry We present whole-rock geochemical data of major and trace elements, and Sr, Nd, and Pb

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isotope ratios from 31 samples of the PCB (see the supplementary data). The samples used for

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geochemical analysis had no visible inclusions. Whole rock samples were analyzed for major element chemistry at the SGS laboratories in Canada using a Thermo Jarrell Ash Enviro II

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simultaneous and sequential ICP with a detection limit from 0.001 to 0.01% for major elements, from 0.002 to 0.05 ppm for REE, and from 0.01 to 20 ppm for other trace elements. Two

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instrumentation techniques were used by SGS Laboratories to obtain the chemical data: ICM90A

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using sodium peroxide fusion analyzed via ICP-MS for trace elements, and ICP95A using lithium metaborate fusion analyzed via ICP atomic emission spectroscopy (AES) for major and

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some trace elements.

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The isotopic ratios of 87Sr/86Sr and 143Nd/144Nd and the trace element concentrations of Rb, Sr, Sm, and Nd were measured by thermal ionization mass spectrometry in the

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Geochronology and Thermochronology Lab of the University of Arizona and were performed on

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a VG Sector TIMS instrument using the techniques described by Ducea (1998) and Otamendi et al. (2009). The common isotopes of lead were analyzed on separate batches of dissolved samples. Lead was extracted using an anion exchange procedure modified after Chen and Wasserburg (1981).

4.3 Databases for Cretaceous Arcs

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ACCEPTED MANUSCRIPT We synthesized U-Pb zircon age data from 333 plutonic and volcanic samples with an age spectrum of 150-60 Ma including analyses determined by thermal ionization mass spectrometry (TIMS), laser ablation-inductively coupled plasma-mass spectrometer (LA-ICPMS), sensitive high-resolution ion microprobe (SHRIMP), and secondary ion mass spectrometry

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(SIMS) analyses. Areal measurements (km2) were made of outcrop exposures for mafic and

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felsic rocks within the study areas and calculated using ArcMap 10.6 by compiling geological

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map data and attributing polygons to all plutonic bodies. The purpose of this analysis was to determine the area percentage represented by mafic and felsic compositions formed during the

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Cretaceous magmatic flare-up in each arc segment. In this case, calculations of the area rather

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than the volume of the plutons in the three arc segments is more appropriate, as the compositional variation beneath the exposed surface of the intrusive units is unknown. The

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presented compositional features and areas calculated for mafic and felsic intrusions agree with

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previously published research (Pitcher et al., 1985; Parada et al., 2002; Kimbrough et al., 2015). We rule out tilting because there is little evidence in the arcs examined and we estimate the

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respective volume of magma added during each pulse by using the present-day exposures.

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However, this simplification may introduce a significant bias because tilting could increase or

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decrease apparent sizes (Petford et al., 2000; Karlstrom et al., 2017; Takasuka et al., 2018). The selected whole rock geochemical dataset includes data from 222 samples of the Late Cretaceous flare-up accompanied by U-Pb zircon data. SiO2, MgO, and Mg# are used as proxies to evaluate the degree of differentiation and to provide a filter for the least differentiated mantlederived rocks. Samples with SiO2 values < 45 wt% were classified as ultramafic, 45-55 wt% classified as mafic (gabbro), and > 55 wt% classified as felsic rocks. Using MgO and Mg# the group of mafic rocks is defined by Mg# >52, MgO > 3.5 wt%, and SiO2 < 55 wt%. These 10

ACCEPTED MANUSCRIPT categories are consistent with the observed compositional groups defined by the SiO2 proxy. Sr, Nd, and Pb isotopes were used to evaluate the role of mantle versus crust for triggering flare-ups, and to identify mantle reservoirs (e.g., DePaolo, 1981a; Zindler and Hart, 1986; Chapman et al., 2017).

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4.2 Flare-Up and Bedrock Ages

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Figure 2 shows the age histograms from compiled geochronological dataset for ages between 150 and 60 Ma for the wPRB, PCB, and CCCB. Bedrock zircon age patterns from the

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wPRB area define a flare-up from ~125 to 90 Ma and with a peak at ~108 Ma. Zircon age patterns from the PCB area define a flare-up from ~110 to 87 Ma. Smaller peaks are observed in

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the bedrock age histogram plot, two older maxima at 130 Ma and 115 Ma, and one younger

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maxima at 68 Ma. Age patterns for the CCCB define a flare-up from ~105 to 90 Ma. Three older

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peaks are seen at 138 Ma, 128 Ma, and 117 Ma, and two younger maxima at 85 and 66 Ma. The timing of the Late Cretaceous flare-up thus can be defined from ~125 to 90 Ma collectively for

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the three arc segments.

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The histogram plot includes zircon U-Pb crystallization ages from mafic and felsic compositions and shows that at the beginning, during, and at the end of the flare-up mafic and

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felsic magmatism is coeval for the three arc segments.

4.3 Radiogenic Isotopes In Sri versus εNd, and Pb isotope plots, both mafic and felsic compositions of the wPRB, PCB, and CCCB lie along the mantle array trending towards a depleted mantle (DM) signature, and show minor systematic changes in Sri (Figure 3). The mafic compositions with restricted Sri 11

ACCEPTED MANUSCRIPT values ranging from 0.703 to 0.705, ƐNd values between 0 and +6, 208Pb/204Pb from 38 to 39.2, and 206Pb/204Pb from 18.2 to 19. The felsic compositions have Sri values ranging from 0.703 to 0.706, ƐNd values between -2 and +7, 208Pb/204Pb from 38 to 39.6, and 206Pb/204Pb from 18 to 19.4. These values are attributed to a depleted mantle source with no to an increasing amount of

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4.4 Assimilation and Fractional Crystallization (AFC)

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crust involved.

Isotope AFC modeling after DePaolo (1981) was used to estimate the potential crustal

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contribution to the isotopic signatures for mafic and felsic rocks. The first step is the selection of the input parameters that involve the end members and the bulk partition coefficient (D). The

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primary concern for this modeling was the characterization of the chemistry of the end members,

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the parental mantle melt composition (C0) and the assimilant (Ca) (Powell 1984).

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In general, the most primitive sample in the dataset is selected as the C0, while the average composition of the crustal basement units is regarded as the Ca (Keskin 2012). In this

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study, for all three arc segments, depleted mantle (DM) values (Stracke et al., 2003) are

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interpreted to approximate C0 representing the most primitive mantle source for the modeling. To characterize Ca, two different values were considered to estimate a likely maximum amount of

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crust assimilated: (1) the felsic sample with the most isotopically evolved Sri value from our dataset and (2) the respective Sri value for the crustal basement in each area. Whereas one AFC model (Figure 4) assumes that the most isotopically evolved felsic sample is the crustal contaminant assimilated during fractional crystallization (model 1), another model (Figure 5) assumed that the basement crustal values are the most likely contaminants (model 2). For model 2, the appropriate Sri (~0.708) crustal basement value for the wPRB is from Early Jurassic to Triassic metavolcanic and metasedimentary rocks (Kistler et al., 2014), for the PCB the Sri 12

ACCEPTED MANUSCRIPT (~0.710) value is from our own Precambrian and Paleozoic samples, and the Sri (~0.708) value for the CCCB is from Paleozoic rocks (Parada et al., 1999; Gonzalez et al., 2017). The Sr bulk partition coefficient (D) used in the modeling is from Rollinson (1993) and the GERM Kd database for basalts (https://earthref.org/GERM/).

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The modeled curves correspond to different r-values; r being the ratio of mass assimilation rate to fractional crystallization rate. The input parameters control the positions and

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shapes of the modeled curves, and the model is modified if any parameter is changed. In this

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way, we can run the two models using different parameters for Ca and observe the results (Figure 4 and 5).

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After running models, we consider that the most representative results from AFC

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modeling that fit the isotopic data are from model 2 which indicate that most of the mafic and

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felsic compositions have experienced fractional crystallization (r=0) combined with variable degrees of assimilation (with r>0). However these likely maximum amounts of needed crustal

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assimilation are low: wPRB rocks incorporated about 30% crustal materials (Figure 5a), the PCB

5c).

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rocks about 25% in average (Figure 5b), and the CCCB rocks about ca. 30% in average (Figure

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For the gabbro compositions used in the AFC modeling, we are concerned about the chemical changes as a result of partial melting of DM and mafic crust, crustal assimilation, and fractional crystallization. For this reason, we calculated the final SiO2 wt% compositions assuming values for an initial melt ranging from 45 to 54 wt% and for a crustal assimilant ranging from 55 to 68 wt% of SiO2 and a maximum of crustal assimilation of 25%. The data suggest that most of the resulting compositions (~85%) can be classified as gabbros with final compositions having < 55 wt% of SiO2. 13

ACCEPTED MANUSCRIPT Because each data point on the AFC plots has an associated age, these data can be used to broadly estimate the relative amounts of mantle and crustal magmas through time (Fig 2). We have matched ages to mapped units to roughly predict the area of units of different ages, compositions, and isotopic values. If the AFC results are accepted, then these results indicate that

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mantle magmas dominated at the initiation of the flare-up and the amounts of mantle magma

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continued to increase up to the peak of the flare-up. And since we used conservative input

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mantle magma input dominated throughout the flare-ups.

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values, we would argue that these present maximum amounts of crustal magmas implying that

5. DISCUSSION “TESTING MANTLE DRIVEN FLARE-UPS”

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A Late Cretaceous flare-up event was characterized using zircon bedrock ages for the

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wPRB from 125 to 90 Ma, in the PCB from 110 to 87 Ma, and for the CCCB from 105 to 90 Ma.

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In the three arc segments, most mafic and felsic rocks are contemporaneous, and the isotopic compositions of the mafic rocks do not change much through time (Figure 3). These

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observations are supported by previous studies conducted in other continental magmatic arcs

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(Sun et al., 2010; Kimbrough et al., 2015; Kirsch et al., 2016; Schwartz et al., 2017). Almost all samples from the three arc segments have Sri < ~ 0.705 and higher values of

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Sri are rare, with relatively few data points >0.706, and εNd values mostly from -2 to +7 (Figure 3a). In the Sri versus εNd plot, both mafic and felsic compositions lie along the mantle array trending towards a depleted mantle (DM) signature and show minor systematic changes in Sri. The Pb isotopes plot shows all samples in the DM reservoir field and mafic and felsic compositions have the following ranges: 206Pb/204Pb ratios from 18 to 19.5 and 208Pb/204Pb rations from 38 to 39.5 (Figure 3b). The Sr, Nd, and Pb isotope values correspond to mantlederived magmas source defining a trend from the DM towards the older crust emphasizing the 14

ACCEPTED MANUSCRIPT idea of a mantle source. Since the associated felsic granitoids and volcanic rocks have overlapping or slightly more crustal isotopic signatures (Fig. 3, 4) and are temporally associated with gabbros maintaining primitive isotopic signatures (see also Kimbrough et al., 2015), we conclude that these felsic units are the result of similar mantle magmas that incorporated some

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crustal rocks. Our interpretation of existing isotopic data is complemented with AFC modeling

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which indicates that both mafic and felsic compositions consist of ~85% to 70% mantle-derived

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magmas and no more than ~25 to 30% recycled crust in all three arc segments.

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If 70% to 100% of these Late Cretaceous arc rocks originated from a DM reservoir and contain only 0% to 30% crust, it implies that crust is not required for triggering nor playing a

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dominant role in an arc flare-up. These conclusions, based on the Late Cretaceous flare-up events

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documented here, are remarkably different from the internal feedback model for the evolution of the Mesozoic North American and South American Cordilleras presented by Ducea (2001),

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Ducea and Barton (2007) and DeCelles et al. (2009, 2015). In their model, upper plate materials

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play a significant role in triggering and driving flare-ups and the Sr, Nd, and Pb isotope values correspond to crustal-derived magma sources. Although our isotope data in this study are limited

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to three arc segments, our conclusion is in agreement with research done in other magmatic arcs

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like the Median Batholith in New Zealand (Decker et al., 2017; Schwartz et al., 2017). In the Median Batholith, zircon chronology and isotope data strongly support an externally triggered, mantle-generated process such as ridge subduction or a slab-breakoff event, leading to the surge of mafic and intermediate magmatism from 128 to 114 Ma with only limited contributions from evolved lithospheric sources. Their data suggest that crustal signatures might reflect intracrustal partial melting due to elevated geothermal gradients resulting from increasing mantle melt influx

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ACCEPTED MANUSCRIPT to the base of the crust and/or assimilation of mantle-derived and hybrid magmas during ascent through the crustal column (Schwartz et al., 2017). The wPRB and the PCB arc segments exhibit the typical spatial isotopic trend for continental arcs, with magmatism closest to the trench having an isotopic composition

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comparable to the depleted mantle and felsic magmatism increasingly evolved landward (Chapman et al., 2017). More efficient assimilation as result of hotter lower crust or a more

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prolonged crustal assimilation during magma ascent, lead to more evolved isotopic compositions

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towards the continent (Farmer and DePaolo, 1983; Hildreth and Moorbath, 1988). In the CCCB, the isotopic compositions are less evolved landward and explained by the progressive crustal

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delamination and tectonic extension during that period (Parada et al., 1999; Parada et al., 2005).

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However, the more evolved magma signature is linked to crustal thickness, and thus the result of crustal assimilation from evolved lithospheric sources needs consideration. Under this scenario

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the internal feedback model presented by DeCelles et al. (2009) may play an important role in

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magmatic contamination, but not for triggering the flare-ups.

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Our data strongly suggest that during the Late Cretaceous flare-up the mantle-derived magmas are dominant over the crustal-derived magmas and therefore the episodic patterns of arc

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magmatism originated from episodic mantle processes (e.g., melting, ascent, and volatile fluxing) as described above. Important evidence for episodic melting in the mantle is presented in studies of subducting slab and mantle flow dynamics suggesting that episodic folding of the slab is a consequence of increasing viscosity in the asthenosphere. Billen and Arredondo (2018) presented a 2D dynamic model of subduction to model the time-evolution of slab deformation and thermal structure. They found that rapid sinking of the slab and folding causes a reduction in asthenosphere viscosity, which allows the overriding plate to move in the opposite direction of 16

ACCEPTED MANUSCRIPT the asthenosphere. Increase in viscosity leads to episodic folding of the slab and causes episodic motion of the trench and plates in which velocities increase and then decrease during each episodic folding event. Convergence rate has been proposed to influence the extent of melting beneath arcs

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(Hughes and Mahood 2008; Zellmer 2008; Hebert et al. 2009; England and Katz 2010; Turner and Langmuir 2015a, 2015b). However, some studies have rejected such a relationship,

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providing evidence that flare-up events in some parts of the Cordillera are seemingly out of sync

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with peaks in convergence rates (e.g., Ducea 2001; DeCelles et al., 2009, 2015; Cao et al., 2016). Higher convergence rates have been shown to: (1) lead to more vigorous hydration of the

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mantle wedge causing increased melting (e.g., Cagnioncle et al., 2007; Plank et al., 2009), and/or

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(2) increase the flux of hot mantle into the wedge corner, raising the temperature and causing

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increased melt formation beneath the arc (England and Wilkins 2004; England and Katz 2010; Turner and Langmuir 2015a, 2015b). Convergence rates during the Cretaceous for the Farallon

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and Nasca plates are between 100-200 mm/y and 60-100 mm/y respectively. These relatively

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high convergence rates may be related to (1) the successive emplacement of a sequence of large igneous provinces during 140 and 120 Ma and (2) a major plate reorganization event occurred at

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ca. 100 Ma (Müller et al., 2016; Shephard et al., 2013; Matthews et al., 2012). For our three arc segments, we consider a possible link between arc-external events and magmatic episodicity because the correlation between plate convergence rate and episodicity observed for the selected time scale about ca. 90-120 Ma. In summary, MOR’s and island arc magmatism are clear examples where mantle melts are the source of sometimes episodic crustal magma additions. Synchronous ages of fairly primitive gabbros and felsic granitoids and isotopic ƐNd, Sri, and Pb data from Cretaceous 17

ACCEPTED MANUSCRIPT Cordilleran continental arc segments discussed herein support the idea that episodic mantle processes, leading to the episodic development of mantle melts, are playing the major role for triggering and driving continental arc flare-ups. Upper plate cyclic crustal processes may play a secondary role in modifying these mantle magmas. The potential for episodic mantle processes is

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well supported by published models of mantle flow near subduction zones. An implication of

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this observation is the need to further examine the potential links between oceanic plate motions

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and episodic mantle melting above subduction zones, which in turn lead to episodicity in continental arc magmatism. A more extensive linked geochemical and geochronological data

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compilation, complemented with plate reconstructions and numerical simulations of mantle

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processes are needed to test the mantle-trigger model for other arc segments.

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ACKNOWLEDGMENTS

The authors acknowledge the financial support of this study from Loma Linda University and the

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Geoscience Research Institute. We want to thank Orlando Poma, Italo Payacan, and Fabian

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Figueroa for their valuable help during fieldwork activities. We also would like to thank the editor Xian-Hua Li and reviewers Emily J. Chin, Tetsuo Kawakami, and anonymous reviewer

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for their useful comments that helped improve the quality of the manuscript.

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FIGURE CAPTIONS 23

ACCEPTED MANUSCRIPT Figure 1. Schematic representation of the Cretaceous arc system along the western margin of North and South America and the locations of samples and distribution of U–Pb ages of intrusive rocks. Inset maps of the selected arc sections and distribution of U–Pb ages and isotope data. Abbreviations are as follows: WPRB = Western Peninsular Ranges Batholith, PCB = Peruvian

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Coastal Batholith, CCCB = Central Chilean Coastal Batholith. See the supplementary material

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for data sources.

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Figure 2. Age histograms and magmatic flare-ups for different domains of Cretaceous arcs in

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North and South America. Mafic magmatism is indicated only for the selected flare-ups. Total number of samples with U-Pb zircon ages (n). See the supplementary material for data sources.

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Figure 3. Magma sources and compiled isotopic data from mafic and felsic rocks for three

Pb/204Pb versus 208Pb/204Pb. Color fields represent data for felsic rocks (wPRB = yellow, PCB

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Cretaceous arc segments: wPRB, PCB, and CCCB. (a) Isotope ratios of Nd and Sr. (b)

= red, CCCB = blue). For comparison, the field end members are included, and these are

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depleted mantle (DM), high µ (HIMU), enriched mantle with low initial 87Sr/86Sr (EM1), and

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enriched mantle with high initial 87Sr/86Sr (EM2). Other end member fields are from Ducea (2001), and these are C—old lower crust; M—mantle; S—sedimentary.

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Figure 4. AFC modeling for Sri vs. Sr plot. The initial magma composition uses a Sri and Sr values suggested for DM (Gale et al., 2003; Faure, 2009). Color fields represent the distribution of the felsic samples, and the star symbol indicates their respective averages. The maximum amounts of crust assimilated to form the gabbros are: (a) an average of 30% for the wPRB; (b) an average of 25% for PCB, and (c) an average of 30% for CCCB.

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Isotope values of mafic and felsic rocks correspond to mantle-derived magmas



AFC indicates that these magmas have minor crustal assimilation (0-30%)

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Figure 1

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Figure 4