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Chemical Geology 417 (2015) 322–340

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Chemical Geology

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Magma emplacement, differentiation and cooling in the middle crust:Integrated zircon geochronological–geochemical constraints from theBergell Intrusion, Central Alps

KyleM. Samperton a,⁎, Blair Schoene a, JohnM. Cottle b, C. Brenhin Keller a, James L. Crowley c, Mark D. Schmitz c

a Department of Geosciences, Guyot Hall, Princeton University, Princeton, NJ 08544-1003, USAb Department of Earth Sciences & Earth Research Institute, Webb Hall 2028, University of California, Santa Barbara, CA 93106-9630, USAc Department of Geosciences, 1910 University Drive, Boise State University, Boise, ID 83725-1535, USA

⁎ Corresponding author.E-mail address: [email protected] (K.M. Sampe

http://dx.doi.org/10.1016/j.chemgeo.2015.10.0240009-2541/© 2015 Elsevier B.V. All rights reserved.

a b s t r a c t
a r t i c l e i n f o
Article history:Received 1 August 2015Received in revised form 15 October 2015Accepted 15 October 2015Available online 20 October 2015

Keywords:Zircon geochronologyMagma evolutionTrace elementsCooling ratesPlutons

U–Th–Pb zircon geochronology is an essential tool for quantifying the emplacement, differentiation and thermalevolution of crustal magmatic systems. However, the power of U–Pb zircon dates can be enhanced through com-plementary characterization of mineral texture and geochemistry, as this permits more detailed interpretationsof geochronological datasets than conventionally achieved. Our approach to better relating zircon dates andgeological processes consists of a multi-method analytical workflow, including cathodoluminescence imaging(CL), in situ LA-ICPMS/EPMA zircon geochemistry, U–Pb zircon ID-TIMS geochronology, and solution ICPMSzircon Trace Element Analysis (U–Pb TIMS-TEA). These methods are here applied to zircon from the BergellIntrusion, a composite Alpine pluton preserving a ~10 kmmid-crustal transect. Hand samples of tonalite, grano-diorite and hybridized granitoid each record 250–700 kyr of autocrystic zircon growth. Bergell zircons areubiquitously zonedwith ca. 104–106 yr growth histories, as evidenced by ID-TIMS analysis ofmicrosampled frag-ments from single crystals. U–Pb TIMS-TEA data exhibit compositional trends onmultiple spatiotemporal scales,including the handsample-scale, representing in situ differentiation at the emplacement level (e.g., Th/U);lithology-scale, defining trajectories corresponding to the production of tonalitic versus granodioritic magmas(Lu/Hf); and pluton-scale, indicating increasingly-evolved melts over ~1.6 Myr of pluton assembly (Zr/Hf).These absolute TIMS-TEA temporal trends are corroborated by relative LA-ICPMS/EPMA core-to-rim geochemis-try. We compare records of trace element evolution from TIMS-TEA, Bergell whole-rock geochemistry, and aglobal compilation of whole-rock geochemical data. These findings support zircon compositional evolution as arobust indicator of differentiation at local and crustal scales, and provide key empirical constraints onmelt differ-entiation and cooling timescales in the middle crust.

© 2015 Elsevier B.V. All rights reserved.

1. Introduction

One of the central goals of igneous petrology is to understand thediversity of crustal magmatic systems by determining the rates andmechanisms of melt production, differentiation and emplacement.Efforts have been made to reconcile interpretations of these processesbased on different lines of evidence from the igneous rock record,including field/structural observations, petrography, geochronology,geochemistry, and numerical models (Petford et al., 2000; Glazneret al., 2004; Annen et al., 2006; Bachmann et al., 2007). Integratingthese data is essential to quantifying the crust's modern thermochemi-cal structure and reconstructing lithospheric evolution throughoutEarth history. However, uncertainties persist as to where in the crust,by which mechanisms, and over what timescales igneous geochemical

rton).

and textural heterogeneities are developed. To explain the productionand evolution of magmas, a range of models have been proposed thatinvoke different sites of differentiation (e.g., upper vs. lower crust),emplacement processes (dikes vs. diapirs), and corresponding rates.

For example, classically-rooted perspectives on crustal magmatismhave posited that geochemical and textural variability are largelyproduced by fractional crystallization operating at the melt emplace-ment level, including the shallow crust (i.e., b10 km depth; Bachmannet al., 2007; Dufek and Bachmann, 2010; Gualda and Ghiorso, 2013).Popular for explaining the evolution of volcanic systems, such modelsemphasize the role of in situ crystal–liquid separation and interpretplutonic rocks as the residual intrusive material left behind after volca-nic melt extraction (Lipman, 2007; Deering and Bachmann, 2010).Significantly, implicit in such models are relatively large volumes ofmobilizable magma at some point in the history of a magmatic system.An alternative to this view is that geochemical heterogeneity observedin the upper crust is a function of evolving source melt composition

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323K.M. Samperton et al. / Chemical Geology 417 (2015) 322–340

and/or changing melting conditions in the lower crust (Eichelbergeret al., 2000; Glazner et al., 2004; Clemens et al., 2009; Annen, 2011;Coleman et al., 2012). These models emphasize the assembly of crustalmagmatic reservoirs by discrete, pulse-wise intrusion, and transport ofmelts from deep crustal source regions as dikes (Petford et al., 1993).In one endmember model, researchers have proposed the assembly ofsome batholiths by magmatic “crack-seal”, wherein the instantaneousratio of melt to final intrusion volume is vanishingly small (Bartleyet al., 2006; Stearns and Bartley, 2014). Cumulatively these models pre-dict a spectrum of melt volume–time relationships that can be tested invarious geologic settings. Specifically, they 1) emphasize differentmechanisms in generating evolved felsic crust from primitive mantle-derived melts, 2) imply contrasting degrees of thermal and composi-tional stratification in the crust, and 3) yield contradictory predictionson the genetic relationship between volcanic and plutonic rocks.

A key motivating paradigm that must be satisfied by all of thesemodels is the incremental nature of magmatic processes, a phenome-non scaling from whole arcs to single intrusions (Ducea, 2001;Coleman et al., 2004; Paterson et al., 2011; de Silva et al., 2015). Gener-ally, this theory asserts that magmatic systems are characterized bypunctuated intervals of high melt flux separated by periods of relativequiescence. While “incremental assembly” in plutonic systems hasfound support from various data, quantitative constraints have largelyoriginated from high-precision U–Pb zircon geochronology by isotopedilution-thermal ionization mass spectrometry (U–Pb zircon ID-TIMS).As such, U–Pb zircon ID-TIMS dates have been used to infer the time-scales of fundamental melt-generating and emplacement processes(Coleman et al., 2004; Matzel et al., 2006; Michel et al., 2008; Memetiet al., 2010; Tappa et al., 2011; Rioux et al., 2012; Davis et al., 2012;Barboni and Schoene, 2014), and considerable advances have beenmade in exploring the thermal, rheological and genetic implicationsof incremental emplacement in plutons and subvolcanic magmareservoirs (Glazner and Bartley, 2006; Annen, 2009, 2011; Gelmanet al., 2013; Caricchi et al., 2014; Menand et al., 2015). Importantly,the observation that instantaneous magma fluxes can far exceed asystem's long-term flux is central to understanding melt volume–timehistories. For example, thermal modeling has shown that the long-term magma emplacement rates of most plutons, ranging on the orderof 10−3–10−4 km3/yr (as inferred from zircon geochronology), are toolow to result in the accumulation of large magma reservoirs (Annen,2009). Comparatively, some large volcanic systems have rates between10−2–10−1 km3/yr, permitting the formation of large magma chamberscapable of undergoing differentiation. Thermal models also demon-strate the important insulating effects of country-rock temperature(TCR) on magma residence timescales. In shallow crustal systems withrelatively low TCR, magmas cool rapidly and residence is relativelybrief; at greater emplacement depths, both TCR and magma residencetime increase (Annen et al., 2006; Menand et al., 2015). Accordingly,for a given emplacement rate that would produce only ephemeralmagma “chambers” in the shallow crust, substantial coherent meltsystems can be incrementally assembled at deep crustal levels. More-over, suchmid- to lower-crustal systemsmay act as important interme-diary reservoirs that feed plutons and volcanoes in the shallow crust.Quantifying the timescales ofmelt production, emplacement, differenti-ation and cooling in the deep crust is therefore essential to informingtrans-crustal models of crustal magmatism. In this contribution weattempt to constrain these timescales by applying U–Pb zircon geochro-nology to a mid-crustal intrusive suite.

2.Workflows for integrated zircon geochronology and geochemistry

Conventional U–Pb zircon geochronology has enriched our under-standing of magmatic processes. However, methodological advancesnow permit the detection of previously unresolvable temporal hetero-geneity in U–Pb datasets, a finding that requires explanation. For exam-ple, ID-TIMS analytical precision of ≤0.1% on individual zircon analyses

(2σ) routinely resolves dispersed zircon dates at the handsamplescale, thus complicating the once-straightforward use of pooledweighted-mean dates as accurate proxies for the ages of plutonic andvolcanic rocks (e.g., Rioux et al., 2012). This finding is also consistentwith U-series dating of young volcanic rocks revealing ~105 yr growthhistories within single zircon crystals (Claiborne et al., 2010). Whilethis dispersion may reflect protracted, geologically-meaningful time-scales of zircon growth, such interpretations must be tempered by thepotential for zircon inheritance in contributing additional age disper-sion to a crystal population (Miller et al., 2007), as well as residual Pb-loss following chemical abrasion (e.g., Mattinson, 2005; Ovtcharovaet al., 2015). Given these complications, novel analytical protocolsintegrating zircon geochronologywith zircon compositional and textur-al information are needed to interpret U–Pb systematics.

In an effort to address interpretative ambiguity, researchers haveinnovated multi-step analytical workflows capable of quantifyingzircon age, composition and texture. These workflows includetandem in situ+bulk zircon analyses (e.g., LA-ICPMS+ID-TIMS;Rivera et al., 2013) and simultaneous in situ, high-spatial resolutiongeochronology+geochemistry (e.g., LA-ICPMS split-stream petro-chronology; Kylander-Clark et al., 2013). Similarly, Schoene et al.(2010b) introduced a protocol pairing ID-TIMS geochronology andsolution ICPMS trace element characterization of the same mineraldomain, thereby enabling direct volumetric coupling of geochrono-logical and compositional information. This method, U–Pb TIMS-TEA, has since been applied to track magma evolution in severalgeological localities, including the Adamello batholith and FishCanyon Tuff (Schoene et al., 2012; Wotzlaw et al., 2013). Here wepresent a new workflow consisting of 1) high-spatial resolution geo-chemistry by electron-probe microanalysis (EPMA) and laser abla-tion–inductively coupled plasma mass spectrometry (LA-ICPMS),2) high-temporal resolution geochronology by ID-TIMS, and 3) bulksolution ICPMS geochemistry (TIMS-TEA). These techniques aresuccessively applied to zircons with textures characterized bycathodoluminescence imaging (CL). As a proof-of-concept, weapply this workflow to zircon from the mid-crustal Bergell Intrusionin the Swiss-Italian Alps (Fig. 1). By integrating high-precision ID-TIMS zircon dating with in situ and bulk crystal trace elementanalysis, we demonstrate the ability to: 1) directly quantify single-zircon temporal and compositional heterogeneity; 2) trace magmaevolution over an intrusion's emplacement history; 3) distinguishautocrystic from antecrystic zircon domains; 4) resolve protractedzircon crystallization spectra produced by magma cooling, permit-ting estimation of post-emplacement cooling rates; 5) detectincremental assembly during pluton formation; and 6) estimateintegrated magma emplacement rates in the middle crust.

3. Bergell Intrusion, Central Alps

3.1. Geologic overview and background

The Bergell Intrusion is a classic calc-alkaline intrusive suite locatedin the Southern Steep Belt of the Swiss-Italian Alps (Fig. 1B; Schmidet al., 1996). It is one of the largest Alpine intrusions (~250 km2), secondonly to the extensively-studied Adamello batholith (~670 km2; Brack,1983; John and Blundy, 1993; Schaltegger et al., 2009). The Bergellconsists of lithologies that typify normally-zoned intrusions and is dom-inated by three phases that encompass N90%of the exposed pluton area:1) an outer unit of tonalite (samples BR10-03 and -04 in this study), 2) acore of K-spar megacryst-bearing granodiorite (BR10-08 and 11-03),and 3) compositionally- and texturally-hybridized granitoid, largelyinterpreted as the product ofmixing between tonalitic and granodioriticmelts (“Übergangszone” of Moticska (1970); BR10-05, 11-09 and 11-14). Hybridized granitoids form a heterogeneous transition zone be-tween tonalite and granodiorite along the western and southern mar-gins of the main intrusion body, varying in thickness from ~10–103 m.

Fig. 1. (A) Rank-order plot of new ID-TIMS 206Pb/238U zircon dates from the Bergell Intrusion, Central Alps (n = 135), including data from tonalites (Δ), hybridized granitoids (◊), andgranodiorites (○). Colored rectangles are single-grain/-fragment analyses with vertical lengths corresponding to 2σ confidence intervals (analytical uncertainties only). Asterisks (*) in-dicate “microsampled” zircon fragments withmultiple ID-TIMS dates from a single crystal (see Figs. 2, 5) and pluses (+) indicate zirconwith LA-ICPMS REE+Hf geochemistry (Fig. 3). AllU–Pb analyses have corresponding TIMS-TEA trace element data (Figs. 6, 7). Δth is the difference between the oldest and youngest zircon dates in a handsample; Δta is the difference be-tween the oldest and youngest autocrystic zircon dates as determined from CL imagery and geochemistry. Antecrystic analyses are transparent and red arrows indicate the oldestautocrystic zircon. U–Pb weighted-mean (WM) emplacement ages for the tonalite and granodiorite reported by von Blanckenburg (1992) and Gianola et al. (2014) are shown for com-parison. Zircon and allanite dates from the extreme western tonalite “tail” (not shown) spread between 33–32 and 32–28 Ma, respectively (Oberli et al., 2004). (B) Generalized geologicmap adapted from Schmid et al. (1996) with new and literature sample locations indicated. Representative Al-in-hornblende barometric data in red from Davidson et al. (1996, units:kbar). Key lithologies are tonalite (purple), granodiorite (pink), hybridized granitoid (stippled pink), distinct basement units (gray), and alluvium (“a”). PFS: Periadriatic Fault System(Insubric Line). N: Novate granite. See Schmid et al. (1996) for details of basement formations. (C) Kernel density estimations of U–Pb distributions illustrating unique zircon age spectrafor each handsample. Note: Lithology symbols and sample colors here are used in all subsequent figures.

324 K.M. Samperton et al. / Chemical Geology 417 (2015) 322–340

The hybridized samples analyzed here were collected from near thetonalite–Übergangszone contact (BR11-14), in the center of theÜbergangszone (BR11-09), and near the granodiorite–Übergangszonecontact (BR10-05). Tonalites were sampled near the southeast andsouthwest margins of the main intrusion body (BR10-04 and -03, re-spectively), and granodiorites were sampled at the northeast contactand core of the pluton (BR11-03 and BR10-08, respectively).

The Bergell Intrusion is bounded to the south by the Insubric Line,itself part of the greater Periadriatic Fault System (PFS), a dextral-transpressive, 700-km mylonite belt that parallels the entire Alpineorogen (Rosenberg, 2004). The most important Tertiary structure inthe Alps, the PFS generally demarcates the extensively deformedCentral Alps from the relatively undeformed Southern Alps (Schmidet al., 1989). The close temporal and spatial correspondence of the PFSand Alpine plutons is well documented: excluding the southernAdamello, Alpine plutonism occurred over ~6 Myr between 34 and28 Ma, with all intrusions immediately abutting the PFS (Rosenberg,2004). This observation led to the interpretation of the PFS as asynmagmatic, crustal-scale shear zone along which deeply-sourcedmagmas were vertically transported to emplacement levels in the mid-dle to upper crust (Rosenberg, 2004). Isotopic constraints (εNd, 87Sr/86Sr,δ18O) indicate fractional crystallization of mixed partial mantle meltsand assimilated crustal material to produce the evolved lithologies ofthe Bergell and other Alpine plutons (von Blanckenburg et al., 1992,1998; Gregory et al., 2009). Melting of lithospheric mantle at the baseof the thickened continental crust (40–50 km depth) is thought to

have been initiated by breakoff of a subducting, Tethyan oceanic slabduring Eurasia–Africa collision (Coward and Dietrich, 1989; Daviesand von Blanckenburg, 1995; Rosenberg, 2004).

Rosenberg et al. (1995) and Berger et al. (1996) synthesized anumber of key field and geochemical observations to develop thecurrent model of Bergell emplacement, including the following:

1. Al-in-hornblende barometry from a westward transect through theBergell documents a systematic increase in amphibole crystallizationpressures from ~4.5 to 8.5 kbar, equivalent to a mid-crustal depthrange spanning ~10 km (Fig. 1B; Reusser, 1987; Davidson et al.,1996).

2. 40Ar/39Ar and K–Ar thermochronological data demonstrate youngingbiotite and hornblende dates westward through the Bergelltonalite from ~26 to 21 Ma, consistent with tilting, cooling, andlater exhumation of the more deeply rooted pluton “tail” (Villa andvon Blanckenburg, 1991).

3. Country rock metamorphic grade during emplacement increasesfrom greenschist facies in the Malenco serpentinite at the easternBergell margin (~350 °C; Trommsdorff and Connolly, 1996) toupper amphibolite facies in the Gruf Complex at the western Bergellmargin (~700–750 °C; Bucher-Nurminen and Droop, 1983; Galliet al., 2011, 2012).

4. Paleomagnetic data indicates ~10–15° of E-dipping, post-emplacement tilting of the entire intrusion and Lepontinearea (Rosenberg and Heller, 1997).


5. Structural observations of syn- and post-intrusive deformation atthe eastern Bergell margin, including flattened enclaves, boudinagedsills, contact–parallel magmatic foliations, folded dikes, andstrain concentration toward the Bergell–country rock contact(Conforto-Galli et al., 1988; Berger and Gieré, 1995), are interpretedas evidence of emplacement by ballooning (e.g., Paterson andVernon, 1995).

6. Structural observations at the western Bergell margin indicatingconcordant folding of the pluton and country rocks duringregional N–S shortening/E–W stretching. Synmagmatic folds withnear-vertical E–W striking axial planes, migmatization along theBergell–country rock contact, and E-plunging stretching lineations(Davidson et al., 1996) were interpreted as resulting fromsynmagmatic deformation (e.g., Hollister and Crawford, 1986).

7. The western Bergell tonalite “tail” asymptotically terminating alongthe Insubric Line with an E–W-striking, subvertical magmaticfoliation presently extending to a 20 km depth (Holliger andKissling, 1991). This structure is interpreted as the feeder zonethrough which melt was transported as dikes to the main Bergellpluton (Rosenberg et al., 1995).

Rosenberg et al. (1995) concluded that these constraints are parsi-moniously explained by the preservation of a ≥10 km crustal sectionthrough the Bergell, continuously increasing in depth from E to W(Reusser, 1987). Notably, the highest structural level currentlypreserved is the side of the pluton near Val Forno at the NE margin,with the top of the intrusion lost to erosion (Berger et al., 1996).Given this range in Bergell emplacement depths, the authors empha-sized the stark contrast in deformation recorded in the shallow plutonside (ballooning in E) compared to the deep floor (synmagmatic foldingin W). To explain this, Rosenberg et al. (1995) proposed a model offorceful emplacement in which deep-crustal shortening along thepluton floor was coeval with shallow-crustal expansion, driving thepartially-molten Bergell mush upwards through the crust via verticalescape. These authors further suggested that this model may be impor-tant in explaining the formation of ballooned plutonsmore generally, asmost intrusion floors (and, by extension, the deep-crustal deformation-al structures recorded there) are not exposed in outcrop. Understandingthe unique trans-crustal perspective of the Bergell may thereforeprovide fundamental insights into the processes governing plutonassembly and modification.

3.2. Geochronology of the Bergell: previous work

Initial attempts to date the Bergell by U–Th–Pb yielded mixedresults. Pervasive inheritance in zircon impeded precise dating, with adiscordia lower-intercept of ~30.3 Ma frommonazite and zircon analy-ses providing the best estimate of emplacement (Gulson and Krogh,1973). Subsequent efforts by von Blanckenburg (1992), who investigat-ed in detail the accessory phases from the easternmost Bergell tonaliteand granodiorite, proved more fruitful. ID-TIMS analysis produced apreciseweighted-mean 206Pb/238U date of 31.88± 0.09Ma, interpretedas the intrusion age. Comparatively, granodioritic zircons exhibitedcomplex dispersion attributed to Pb-loss, with 206Pb/238U datesspanning ~26–31 Ma. Instead, a joint 208Pb/232Th allanite and206Pb/238U titanite age of 30.13 ± 0.17 Ma was reported for the grano-diorite (Fig. 1; von Blanckenburg, 1992). A subsequent geochronologicalstudy of the extremewestern tonalite feeder zonebyOberli et al. (2004)reported several interesting accessory phase data. First, zircons herecrystallized over 1 Myr between ~33–32 Ma. Second, titanites recordclosure dates of ~29.8 Ma. Third, zoned allanites yielded 208Pb/232Thdates over 4 Myr between ~32–28 Ma. Trends in allanite chemistrywith time and numerical diffusionmodeling lead these authors to inter-pret heterogeneous allanite dates as reflecting protracted crystallizationin a fractionating, deep crustal magma reservoir over ~5 Myr. Similarly,a LA-ICPMS study by Gregory et al. (2009) demonstrated compositional

trends in titanite and allanite (decreasing Eu/Eu* and εNd), consistentwith continued fractionation and assimilation over the duration ofBergell magmatism. Gregory et al. (2009) dated samples of tonaliteand granodiorite fromwithin the hybridized zone at the floor of the plu-ton, for which they reported equivalent weighted-mean 206Pb/238U zir-con ages of 31.22 ± 0.04 and 31.13 ± 0.10 Ma, respectively (Fig. 1).These dates are intermediate between the tonalite and granodioriteages of von Blanckenburg (1992), broadly consistent with expectationsfrom field relations for the relative age of the transition zone.

3.3. Quantifying Bergell magmatism: a new approach

As summarized above, the Bergell Intrusion preserves a three-dimensional section of a normally-zoned intrusive suite, offering anexcellent opportunity to study crustal magmatic processes over arange of mid-crustal emplacement levels. Given this, we propose tocharacterize the U–Pb zircon systematics of the Bergell at an unprece-dented scale. In contrast to previous geochronological studies of theBergell, which focused the scope of their analytical efforts on specificparts of the intrusion, we present data from a suite of samples thatcover a range of lithologies and paleo-crustal depths in order to assesspluton-scale emplacement and differentiation dynamics (Fig. 1). Thisstrategy mitigates several potential issues, including those arisingfrom comparing data from different studies, dates from minerals withdifferent closure/saturation conditions (e.g., zircon vs. allanite vs.titanite), and different decay systems (206Pb/238U vs. 208Pb/232Th).Moreover, using the multi-method analytical workflow describedbelow, we attempt to quantify the evolution of individual Bergell zirconcrystals and magma pulses.

4. Analytical methods and workflow

4.1. Zircon separation, preparation and imaging

Zircons were extracted from their host rocks at Princeton Universityby standard crushing, gravimetric- andmagnetic-separation techniquesusing a Bico Braun “Chipmunk” Jawcrusher, discmill, Wilfley table,methylene iodide, handmagnet, and Frantz isodynamic separator. Zir-conswere picked in reagent-grade ethanol using a LeicaM125 binocularmicroscope from the least magnetic mineral separate. Between 100–200 zircons were selected to represent the range of grain sizes andmorphologies observed in a sample while avoiding crystals with visibleinclusions, cracks and cores. These zircons were transferred in bulk toquartz crucibles, loaded in a Fisher Scientific Isotemp Muffle Furnace,and annealed at 900 °C for 48 h after the procedure of Mattinson(2005). Post-annealing, grainsweremounted in EpoFix Resin andHard-ener and polished halfway through to expose a cross-section throughindividual crystals. Cathodoluminescene (CL) images were obtainedwith a JEOL T300 scanning electron microscope (BSU) and FEI XL30FEG-SEM (PU), both with a Gatan MiniCL. CL images were used to char-acterize zircon growth and disequilibrium textures and to target singlegrains and sub-grain fragments for subsequent geochronological andgeochemical analyses.

4.2. LA-ICPMS & EPMA: in situ core-to-rim zircon geochemistry

Zircons were analyzed for major and trace elements in situ usinga laser ablation-inductively coupled plasma mass spectrometer (LA-ICPMS) and an electron probe microanalyzer (EPMA) at the Universityof California–Santa Barbara. LA-ICPMS analyses were performed withan Analyte 193 nm ArF excimer laser attached to a Nu AttoM HR SC-ICPMS with the following operating conditions: 10 μm spot size,~5 μm pit depth, 4 Hz repetition rate, 2.3 J/cm2 fluence and 30 s perpoint analysis time. The instrument was operated in E-Scan mode andcollected masses 140Ce–179Hf (inclusive). Elemental abundances andtheir uncertainties were calculated using a matrix-matched sample-


standard bracketing approach in Iolite v. 2.1.2 (Paton et al., 2010), andinclude corrections for baseline and instrumental drift. “GJ-1” zircon(utilizing the reference values reported in Liu et al. (2010)) served asthe primary reference material and “91500” zircon as a secondarystandard. Full LA-ICPMS analytical details are described by Kylander-Clark et al. (2013) and Lederer et al. (2013). Quantitative major andtrace element analyses by EPMA were performed on a Cameca SX-100with the following operating conditions: 40° takeoff angle, 20 keV accel-erating voltage, 200 nA beam current and 5 μm beam diameter.Unknownswere corrected for deadtime, drift and interferences. Accura-cy of unknown analyses was checked routinely using in-house naturalzircon reference materials and NIST SRM 610. Full EPMA analyticaldetails, including interference corrections, are described by Cottle(2014).

4.3. U–Pb ID-TIMS: high-precision geochronology

Zircons previously characterized by LA-ICPMS and/or EMPA werechosen for U–Pb ID-TIMS dating in addition to grains without suchgeochemical data, with the objective of directly comparing in situ andbulk crystal compositional information. Zircons were removed fromgrain mount, and a further subset was fragmented (“microsampled”)using stainless steel picking tools before being transferred in distilledacetone to separate 3-ml Savillex PFA Hex beakers for clean labgeochemical analyses. Both whole grains with complete in situ tran-sects, as well as grains fragmented along in situ transects, were targetedto assess the timescales of potential compositional heterogeneitypreserved in single zircon crystals.

Zircons were rinsed in Hex beakers using distilled 6 N HCl and MQH2O and fluxed in 6 N HCl at 100 °C for 12 h. These grains were loadedinto 200 μl Savillex “micro”-capsules with 100 μl 29 M HF + 15 μl 3 NHNO3 for chemical abrasion in high-pressure Parr bombs at 185 °C for12 h to remove crystal domains affected by Pb loss (Mattinson, 2005).Grains were rinsed post-leaching with 6 N HCl, MQ H2O, and 3 NHNO3 prior to spiking with the EARTHTIME (202Pb–)205Pb–233U–235Utracer and addition of 100 μl 29 M HF + 15 μl 3 N HNO3 (Condonet al., 2015; McLean et al., 2015). Zircons were then dissolved to com-pletion in Parr bombs at 210 °C for 48 h. Dissolved zircon solutionswere subsequently dried down, redissolved in 100 μl 6 N HCl andconverted to chlorides in Parr bombs at 185 °C for 12 h, after which so-lutions were dried again and brought up in 50 μl 3 N HCl. The U–Pb andtrace element aliquots were then separated by anion exchange columnchromatography using 50 μl columns and AG-1 X8 resin (200–400mesh, chloride form (Eichrom); Krogh, 1973).

The U–Pb aliquot was loaded in a silica gel emitter (Gerstenbergerand Haase, 1997) to an outgassed, zone-refined Re filament. Isotopicdeterminations were performed using an IsotopX Isoprobe-T thermalionization mass spectrometer (TIMS) at Boise State University and anIsotopX PhoeniX-62 TIMS at Princeton University, with Pb analysesperformed in peak-hopping mode on a Daly-photomultiplier ioncounting detector. A correction for mass-dependent Pb fractionationwas applied in one of two ways. For double-Pb spiked analyses(202Pb–205Pb, ET2535), a cycle-by-cycle fractionation correction wascalculated from the deviation of measured 202Pb/205Pb from theknown tracer 202Pb/205Pb (0.99924 ± 0.00027 (1σ)). For single-Pbspiked analyses (205Pb, ET535), a Pb fractionation of 0.16 ± 0.02%/amu (1σ) was used, as determined by repeat measurement ofNBS981 at Princeton. A Daly-photomultiplier Pb dead time of 21.2 and40.5 ns was used at BSU and Princeton, respectively, as determined byrepeat measurement of NBS standards. UO2 measurements wereperformed in static mode on Faraday cups with a cycle-by-cycle Ufractionation correction calculated from the deviation of measured233U/235U from the known tracer 233U/235U (0.995062 ± 0.000054(1σ)). All Pbc was attributed to laboratory blank with a mean isotopiccomposition determined by total procedural blank measurements (seeSupplemental data Table 1 for values). Data reduction was performed

using the programs Tripoli and U–Pb Redux (Bowring et al., 2011;McLean et al., 2011) and the U decay constants of Jaffey et al. (1971).Uncertainties in reported U–Pb zircon dates are at the 95% confidencelevel and exclude tracer calibration and decay constant uncertainties.Correction for initial 230Th disequilibrium in the 206Pb/238U systemwas made on a fraction-by-fraction basis by estimating (Th/U)melt

using (Th/U)zircon determined by TIMS and a mean (Th/U)zircon–melt

partition coefficient ratio of 0.117 from Nardi et al. (2013, Group 2+3granitoids). This subset was selected because the constituent samplesinclude I-type, metaluminous calc-alkaline granodiorites+tonalitesfrom magmatic arcs, and are hence reasonably good compositionalapproximations of Bergell lithologies. A blanket uncertainty for theresulting (Th/U)melt of ±0.5 (2σ) was then applied.

4.4. TIMS-TEA: bulk solution geochemistry

The trace element compositions of the same zircon fragments datedby ID-TIMS were characterized following the analytical protocol ofSchoene et al. (2010b) at Princeton University. Trace element washesisolated during U–Pb column chemistry were dried down in pre-cleaned 2.0 ml polypropylene vials and redissolved in 1.0 ml 1.5 MHF + 0.1 M HNO3 + 1 ppb Ir. Measurements were performed on aThermo Fisher ELEMENT2 sector field–inductively coupled plasma–mass spectrometer (SF-ICP-MS) with a sample introduction systemconsisting of a CETAC Aridus II desolvation nebulizer + ASX-100autosampler with an uptake rate of 100 μl/min. Measured elementsincluded Zr, Hf, Sc, Y, Ti, Nb, Ta, REEs and Ir, with iridium monitored asan internal standard during mass spectrometry. The instrument wastuned in medium resolution mode with an optimal signal intensity of0.5–2 Mcps for 1 ppb Ir. A matrix-matched, gravimetric external cal-ibration solution was prepared with the relative abundance oftargeted elements representing those observed in natural zircon(e.g., Zr/Hf = 50). A dilution series was generated using this solutionto cover the range of concentrations observed in unknowns(e.g., [Zr] = 101–104 ppb solution), which was then used to generatea concentration–intensity calibration curve for each trace element atthe beginning of the analytical session. Samples and interspersed in-strumental and total procedural blanks were analyzed in sets of 24over 3 h, with a line washtime of 120 s and uptake time of 90 s.Following data acquisition, solution concentrations were convertedto stoichiometric concentrations in zircon by normalizing solutionconcentration data assuming that all trace elements partitioninto the Zr4+ site in ZrSiO4, where Σ Zr+Hf+… +REEs =497,646 ppm.

5. Results

5.1. Cathodoluminescence imaging

CL imaging revealed zoning patterns specific to each Bergell litholo-gy. Zircons from tonalite samples BR10-04 and -03 are characterized bysimple oscillatory and/or sector zoning; occasional homogenous CLbrightness; and an absence of obvious xeno-/antecrystic cores (Fig. 2a,see Supplemental material). Comparatively, hybridized granitoid sam-ples BR10-05, BR11-09 and -14 revealmore variable zircon architecture,including: oscillatory-zoned whole crystals with or without subtlecore–rim CL contrast (Fig. 2b, e, f, g); oscillatory-zoned crystal rimscored by resorbed, convolutely zoned interior domains (Fig. 2d); andan absence of sector and homogenous zoning. Granodiorite samplesBR10-08 and BR11-03 zircons have similar textures as the hybridizedgranitoid zircons, with both oscillatory-zoned and mixed oscillatoryrims-convolute cores observed (Fig. 2c). Additionally, many zirconsfrom BR10-08 exhibit thin (≤10 μm) CL-bright rims.

Each crystal domain analyzed by U–Pb TIMS-TEA was coded forboth crystal geometry and texture in order to provide semiquantitativecriteria by which to assess subsequent U–Pb geochronological

Fig. 2. Representative cathodoluminescence images of Bergell tonalite (A, BR10-03), hybridized granitoid (B, BR10-05), and granodiorite zircons (C, BR10-08). Panels D–G show progres-sively younger zircons from hybridized sample BR11-09: note that the oldest crystal (z2) has a large, convolutely-zoned core, while the younger crystals (z1, z10 and z13) resolve ca.250 kyr of oscillatory-zoned zircon growth. Numbered green circles are core-to-rim LA-ICPMS transect spots (~10 μmdiameter), and dashed red lines are boundaries of crystal fragmentsanalyzed by U–Pb TIMS-TEA. Italicized numbers are fragment 206Pb/238U dates and 2σ analytical uncertainties (units: Ma). Δtz denotes intra-crystal U–Pb date dispersion (see Fig. 5 andtext for details). Images of all zircons analyzed in this study are available in the Supplemental data file. NA: non-analyzed fragment.


and geochemical data (Supplemental Table 1). Categories for crystalgeometry include tips of crystals (T), cores (C), mixed tips+cores (M),and whole crystals (W). Categories for crystal texture includeoscillatory zoning (O), sector (S), homogeneous (H), and convolute/re-sorbed (C).

Fig. 3. Compilation of Bergell zircon LA-ICPMS core-to-rim trace element transects (younging ftom). The vertical scale on the left is for all panels.Mean 2σ analytical uncertainties of all LA-ICPMSupplemental data file.

5.2. LA-ICPMS and EPMA zircon geochemistry

In situ zircon analyses were performed across a range of grain sizes,morphologies and CL textures in order to quantify the degree of compo-sitional heterogeneity in each zircon population (Fig. 2). A total of 735

rom left-to-right), showing results for Yb/Dy (top row), Lu/Hf (middle) and Hf (ppm; bot-S analyses are displayed inpanels at the far left. Complete LA-ICPMSdata table available in

Fig. 4.Box-and-whisker plots of zircon geochemistry fromsamples BR10-04, -03, -05 and -08 determined by electron-probemicroanalysis (EPMA). Shifts in the distributions of HfO2

(higher; B) and Zr/Hf (lower; C) from tonalites to granodiorites are consistent withtemporal trends resolved by U–Pb TIMS-TEA. Th/U (A) has broadly overlapping distribu-tions with more dispersed and greater values for tonalites, also consistent with TIMS-TEA data. Numbers of EPMA point analyses per sample are given in panel A. Heavyhorizontal lines within boxes are data medians and box edges are the 25th and 75thdata percentiles (Q25 and Q75, respectively). Red crosses are outliers beyond an envelopedefined as less than Q75 + [1.5 × (Q75 – Q25)] or greater than Q25 − [1.5 × (Q75 − Q25)].Horizontal whiskers are the endmember analyses that fall within that envelope.


LA-ICPMS point measurements were made over 102 core-to-rim zircontransects from seven Bergell handsamples, and 90 EPMA point mea-surements were made over 16 core-to-rim zircon transects from fourof these samples (BR10-04, -03, -05 and -08). LA-ICPMS analyses mea-sured REE+Hf concentrations, and from EPMA analyses we report Si,O, Zr, Hf, Th, U, Sc and Y concentrations. Corresponding data tables areavailable in the Supplemental data file (Tables 3 and 4).

LA-ICPMS zircon data are summarized in Fig. 3, inwhich zircon Yb/Dy(top row), Lu/Hf (middle) and Hf concentration (ppm; bottom) are plot-ted as core-to-rim transects. Individual panels have normalized pointlocations on the horizontal axis and geochemical data on the verticalaxis. Lithology progresses from tonalites at left, hybrid granitoids atcenter, to granodiorites at right. This visualization allows assessment ofrelative temporal trends in crystal chemistry from the zircon core torim (i.e., oldest crystal domain at the left panel, youngest at right), aswell as comparison of data variance within and between samples. Singletransects are indicated by points joined by a line.

In situ LA-ICPMS data permit key first-order observations. First,Bergell zircons demonstrate compositional variability well beyond theanalytical uncertainties of single LA-ICPMS spotmeasurements. Second,zircons from different samples exhibit similar core-to-rim trends fora given trace element proxy, with increasing-to-constant Yb/Dy,decreasing-to-constant Lu/Hf, and increasing Hf concentration. Thesegradual compositional changes indicate that zircons are not composi-tionally uniform at the crystal-scale, but are instead zonedwith distinct,sub-grain growth domains. Third, for a given lithology, some samplesshow resolvable offsets in geochemistry (e.g., Yb/Dy in tonalites BR10-04 and -03) while others have statistically-equivalent trends (Lu/Hf ingranodiorites BR11-03 and BR10-08).

EPMA zircon data are summarized in Fig. 4, in which zircon Th/U,HfO2 (wt.%) and Zr/Hf are displayed as box-and-whisker plots (seeFig. 4 caption for details). While EPMA grain transects did not resolveobvious core-to-rim variations in zircon composition, shifts in datadistributions are observed in comparing handsample populations. Forexample, both themedian and variance of zircon Th/U decrease succes-sively as lithology transitions from tonalites to granodiorites (Fig. 4a).Similarly, a systematic increase in median HfO2 from ~1.2 wt.% (tonalitesample BR10-04) to 1.6wt.% (granodiorite sample BR10-08), equivalentto a decrease in median Zr/Hf from ~54 to 42. These systematic shifts incomposition indicate lithology and pluton-scale changes in zirconchemistry, consistent with finer crystal-scale heterogeneity revealedby LA-ICPMS analyses. Together, LA-ICPMS and EPMA analyses ofBergell zircon reveal compositional variations at the crystal-,handsample-, lithology-, and pluton-scales.

5.3. U–Pb zircon ID-TIMS geochronology

As with our sampling strategy for LA-ICPMS/EPMA, a range of zirconsizes, morphologies and textures were selected for U–Pb dating so asnot to bias our consequent high-precision age dataset by producingapparently homogenous U–Pb zircon systematics. We also analyzed arelatively large number of zircons per handsample (n = 15–30; 135total) to maximize resolution of the full duration of zircon crystalliza-tion in each sampling location. Full data table is available in theSupplemental data file (Table 1).

The results of U–Pb dating are displayed in Fig. 1. All 206Pb/238Uzircon dates reported in this study span a ~1.6 Myr interval from~31.9–30.3 Ma, with a mean 2σ analytical precision of ±0.03 Myr onindividual zircon analyses. Zircon dates span from older/exterior/rela-tively mafic Bergell tonalite to younger/interior/relatively felsic grano-diorite, as described by von Blanckenburg (1992) for the Bergell andobserved in other normally-zoned intrusive suites (e.g., Coleman et al.,2004). Handsamples are characterized by 250–1050 kyr of dispersionin zircon dates (Δth), with variably overlapping, unique U–Pb zircondistributions for samples with the same lithology (e.g., tonalite samplesBR10-04 and -03; Fig. 1c). Moreover, no sample contains an obvious

cluster of 206Pb/238U dates amenable to calculation of a precise,weighted-mean “intrusion age”. We discuss potential interpretationsof this geochronological dataset after presenting TIMS-TEA geochemis-try of zircons dated here (see below).

The crystallization times of individual zircons can be estimated bycomparing U–Pb dates of multiple fragments subsampled from thesame grain. To quantify the degree of crystal-scale age heterogeneityin Bergell zircon, we conducted a microsampling experiment in which38 zircons were each broken into 2–5 zircon fragments. These werethen analyzed separately for U–Pb age and geochemistry. From thesedata, a minimum duration of zircon growth (Δtz) was calculated asthe difference between the oldest and youngest fragment 206Pb/238Udates from the same crystal, with uncertainties determined by summingthe U–Pb uncertainties of those same fragments in quadrature.Fragment pairs were subdivided into two groups, symmetrically-microsampled (Group #1; Fig. 5b) and asymmetrically-microsampledfragments (Group #2; Fig. 5c). Group #1 pairs encompass equivalentor near-equivalent volume domains, while Group #2 pairs integratedifferent volume domains. Importantly, Group #1 pairs are anticipatedto yield statistically-indistinguishable U–Pb dates while Group #2pairs should resolve potential age heterogeneities. The results of theexperiment, summarized in Fig. 5, comply with these expectations:

Fig. 5. (A) Rank-order summary of “microsampling” experiment designed to assess intra-zircon age heterogeneity. Zircons were broken into ≥2 fragments each that were analyzed sep-arately for age and geochemistry. Single-zirconΔtz: difference between the oldest and youngest fragment 206Pb/238U dates (kyr±2σ). Group#1 analyses consist of symmetrically-sampledfragments that should yield the same age (B), andGroup#2are asymmetrically-sampled fragments that should producedifferent ages (C). Zircons that intersect the age-equivalency line at0 kyr (blue) exhibit no resolvable age zoning at the 2σ-level. All Group #1 zircon pairs satisfy this criterion; conversely, all Group #2 zircons are temporally zoned from core-to-rim. Blackand white stars are the mean and median, respectively, of Group #2 Δtz values. Grain BR11-14 z1 is omitted from plot for clarity (Δtz = 857 ± 35 kyr). CL image scale bars are 50 μm.


Group #1 pairs exhibit no resolvable age differences (Δtz ~ 0). Compar-atively, Group #2 Δtzs range between ~50–850 kyr, with a mean andmedian of 205 and 150 kyr, respectively. Critically, because fragmentsampling invariably incorporates relatively large crystal volumes withmultiple growth domains, Δtz values for Group #2 represent minimumzircon growth timescales.

5.4. TIMS-TEA zircon geochemistry

The trace element aliquot of each zircon dated by U–Pb ID-TIMSwasdetermined by solution ICPMS analysis, allowing direct coupling of bulkzircon compositional data to our high-precision U–Pb age dataset(Schoene et al., 2010b). TIMS-TEA data table is available in theSupplemental data file (Table 2).

U–Pb TIMS-TEA data are plotted as both time-series (Fig. 6) andcompositional crossplots (Fig. 7). Fig. 6 displays stacked records ofvarious zircon compositional proxies, including Th/U (a), Yb/Dy (b),Lu/Hf (c), Hf (ppm; d), and Zr/Hf (e), as a function of 206Pb/238U zircondate (note: U–Pb dates young from left-to-right and correspond to thesame analyses as in Fig. 1). Several immediate observations can bemade from Fig. 6. First, Bergell zircon TIMS-TEA geochemistry is notuniform in handsample zircon populations, with dispersion wellbeyond analytical uncertainties. This is consistent with results of insitu LA-ICPMS and EPMA analyses. Second, zircon composition changesat different timescales (colored arrows in Fig. 6). For example, linearly-decreasing Th/U is recorded in handsamples over ca. 500 kyr timescales(Fig. 6a), with simultaneously increasing Yb/Dy (Fig. 6b). Comparative-ly, Lu/Hf shows a continuously-decreasing trajectory between tonalitesamples, with more diffuse trending observed in granodiorites(Fig. 6c). Finally, Hf concentration and Zr/Hf display mirrored pluton-scale increasing and decreasing trends, respectively, which are in turncomposed of more diffuse handsample-scale trends. This mirroringeffect is a result of the stoichiometric normalization performed onTIMS-TEA data: while Zr/Hf trends are unaffected by this procedure,Hf concentrations obtained in this manner will inversely covary withZr/Hf. In general, zircons from hybridized granitoids form scatteredarrays between tonalitic and granodioritic endmembers, although forsome proxies well-developed trends are observed in the youngestzircon analyses (e.g., Th/U; Fig. 6a).

The geochemical relationships of zircons from different samples andlithologies can be further assessed in crossplot space (Fig. 7). Plots of Cevs. Zr/Hf show strong lithology-dependent clustering, with granodiorit-ic zircons having high-Ce/low-Zr/Hf and tonalitic zircons low-Ce/high-Zr/Hf (Fig. 7a). Zircons from hybridized granitoids scatter betweenthese two groups, consistent with their formation in variably mixedtonalite–granodiorite magmas. A similar relationship is displayed inTh/U vs. Hf space, with granodiorites having relatively low-Th/U/high-Hf and tonalites extending to high-Th/U/low Hf (Fig. 7c). These plotscan also be used to identify potential granodioritic antecrysts by findingthose analyses that groupwith or trend onmixing lines toward tonaliticcompositions, although it should be cautioned that such correlations donot uniquely pinpoint inheritance. In contrast, Lu/Hf vs. Yb/Dy demon-strates sample-dependent clustering of tonalitic zircons with distinctfields corresponding to each handsample (Fig. 7b). The implications ofthese findings will be discussed in detail below.

In summary, the observed crystal-, handsample-, and lithology-scaledispersion in both zircon dates and zircon composition far exceedsanalytical uncertainties, necessitating a geological explanation.

6. Discussion

6.1. Reassessing interpretations of high-precision U–Pb dates, part I

The spread in U–Pb zircon dates from Bergell handsamples is consis-tent with recent geochronological studies of both plutonic and volcanicrocks, reflecting a trend of increasingly-resolved dispersion in suchhigh-precision datasets (e.g., Schoene et al., 2012; Wotzlaw et al.,2013). These findings require a reassessment of how tomost accuratelyinterpret U–Pb zircon dates. To that end, a set of possible interpretationsof 206Pb/238U dates from handsample BR10-08 is displayed in Fig. 8.While we explicitly treat the zircon dataset from BR10-08 because it isthe best characterized and has the largest number of U–Pb dates(n = 30), our preferred interpretation for all samples is given in Fig. 1.In introducing potential interpretations we consider the conventionaltreatment of U–Pb geochronological data alone before subsequentlyintegrating the perspective provided by complementary textural andgeochemical information.

Fig. 6. U–Pb TIMS-TEA record of Bergell zircon compositional evolution as expressed by Th/U (A), Yb/Dy (B), Lu/Hf (C), Hf concentration (D), and Zr/Hf (E). Colored arrows highlight theyoungest, autocrystic trends in zircon geochemistry at the handsample-scale, interpreted as evidence of protracted zircon crystallization in discrete, chemically-fractionating hostmagmas.Antecrystic zircon analyses are transparent for clarity. Gray arrows indicate progressive pluton-scale trends. Mean 2σ analytical uncertainties for each ratio/concentration are displayed.Full geochronological/geochemical data tables are available in the Supplemental data file.


In Interpretation #1 (Fig. 8A), a conventional treatment of the dataidentifies a subpopulation of U–Pb dates amenable to calculation of aweighted-mean “emplacement age” for the host rock (templacement). Inthe scenario depicted in Fig. 8A, nine analyses near the middle of thezircon distribution yield a precise weighted-mean age of 30.803 ±0.007Ma, with a statistically-acceptable mean square of weighted devi-ates (MSWD = 0.71; York, 1969; Wendt and Carl, 1991). Inclusion ofadditional U–Pb dates in the weighted-mean calculation results in anMSWD≫ 1 (i.e., outside the 95%-level acceptance criterion), such thatthe resulting age is not statistically robust. Because of this, the 21analyses not included in the weighted-mean calculation are dismissedas either antecrystic/inherited (in the case of older grains) oraffected by residual Pb-loss (for younger grains). Determining thesubset of analyses on which to calculate a weighted-mean age isarbitrary, as older and younger clusters of U–Pb dates also yieldstatistically-equivalent age clusters. There are four assumptions in thisinterpretation: 1) the timescales of autocrystic zircon crystallizationare shorter than the absolute precision of individual zircon analyses,thereby justifying calculation of a more precise “mean” age, 2) resolveddispersion in 206Pb/238U dates is not a geologically-meaningful signal

relative to theprocess of interest (e.g., magmaemplacement), 3) residu-al Pb-loss is pervasive even in chemically-abraded zircons, and4) autocrystic zircon crystallization is synchronous with magmaemplacement (tsaturation ~ templacement). This interpretation has beenused in studies of ashbed geochronology (e.g., Davydov et al., 2010;Machlus et al., 2015; Ovtcharova et al., 2015) and plutonic and volcanicrocks (e.g., Mills and Coleman, 2013).

In Interpretation #2 (Fig. 8B), all zircons are deemed antecrystic(i.e., pre-emplacement) with the exception of the youngest analysis,which is used as the best constraint on templacement. This view results intemplacement at 30.37 ± 0.04 Ma, ~500 kyr younger than that of Interpre-tation #1. It is assumed that Pb-loss has been fully mitigated bychemical abrasion and that the majority of zircons are inherited.While this approach produces a unique answer for a given dataset, theinterpretation is dependent on a single analysis (e.g., Schaltegger et al.,2009). As with Interpretation #1, autocrystic zircon crystallization isassumed to be synchronous with emplacement.

In Interpretation #3 (Fig. 8C), dispersion is attributed to protracted,autocrystic zircon crystallization in a cooling magma reservoir. In thismodel, the oldest zircon indicates the time at which zircon saturates

Fig. 7.Crossplots of TIMS-TEA zircon geochemistry, including Ce vs. Zr/Hf (A), Lu/Hf vs. Yb/Dy (B), andTh/U vs. Hf (C). Top panels are colored by sample, lowerpanels are same data coloredby lithology. Ellipses highlight granodiorite vs. tonalite fields (A) and tonalite handsample-scale subfields (B). Mean 2σ analytical uncertainties for each ratio/concentration are displayed.


and beings to crystallize (tsaturation), zircons of intermediate age growduring cooling, and the youngest zircon indicates cessation of crystalli-zation at the solidus (tsolidus) or eruption (e.g., Wotzlaw et al., 2013).Antecrysts and Pb-loss are not invoked to explain the observed disper-sion in U–Pb dates. In stark contrast to Interpretations #1 and 2, thisassumes the preservation of a protracted time–temperature history ofmelt emplacement, cooling, and solidification over 650 kyr. Crucially,this interpretation does not produce a strict expectation of templacement

except that emplacementmust precede or be synchronouswith the sol-idus (tsolidus ≤ templacement). As all grain domains/dates are interpreted asautocrystic, it is anticipated that zircons should be relatively simple bothtexturally (e.g., oscillatory-zoned with no growth discontinuities) andgeochemically (e.g., monotonic time–compositional–(temperature)trends).

Fig. 8. Endmember interpretations of high-precision U–Pb geochronological data with the samzircon date calculated from a subset of data yields a statistically-acceptable mean square of weiOlder/younger zircons are attributed to inheritance/Pb-loss, respectively, and autocrystic zircongest zircon recordsmagma emplacement and older zircons are attributed to inheritance. Reliesdifferent ages record autocrystic mineral growth over a span of time (Δt), with the oldest zircooling, and the youngest zircon at near-zero melt fraction (tsolidus). Emplacement can occur athe oldest zircons are attributed to inheritance and the remaining zircons record autocrystic sa

Finally, in Interpretation #4 (Fig. 8D) the same assumptions aremade as Interpretation #3 except that a component of the oldest zirconsis attributed to inheritance (antecrysts). While a time–temperaturehistory is still preserved, the time between autocrystic zirconcrystallization and the solidus is less (e.g., 450 kyr vs. 650 kyr forInterpretation #3). In contrast to Interpretation #3, zircons should ex-hibit more complicated textures and compositions (e.g., resorbedcores/oscillatory-zoned rims with more diffuse geochemical trends),consistent with mixing of distinctly sourced intra- and inter-zircongrowth domains.

The interpretations described above for a given geochronologicaldataset differ greatly in both the information inferred from U–Pb datesas well as their underlying assumptions. This discussion illustratesthat, given sufficient analytical precision, the once-straightforward

e dataset in all panels (BR10-08 zircon, n = 30). (A) Interpretation #1: a weighted-meanghted deviates (MSWD), resulting in a precise, averaged age of emplacement (templacement).crystallization (tsaturation) is synchronous with emplacement. (B) Interpretation #2: Youn-on a single zircon analysis to estimate templacement/tsaturation. (C) Interpretation #3: Zircons ofcon indicating tsaturation, intermediate zircons recording protracted crystallization duringny time before tsolidus.(D) Augmented form of Interpretation #3 wherein a component ofturation and cooling to the solidus.


interpretation that zircon dates always record the age of their host rockis no longer valid. In the absence of independent complementary infor-mation with which to further probe these data, it is essentiallyimpossible to discern which interpretation (or combination thereof) ismost robust.

6.2. Zircon growth model: comparing in situ and bulk crystal geochemistry

Integrating U–Pb zircon dates with geochemistry potentially allowsfor more detailed interpretation of geochronological data. As demon-strated above, Bergell zircon composition is variable over a range ofscales, from single crystals to the entire intrusion, as determined byboth in situ and bulk crystal characterization (Figs. 3, 4, 6, 7). In orderto validate cross-comparison of in situ and bulk crystal measurementsof zircon composition,we developed a zircon growthmodel thatweighsLA-ICPMS data, producing predicted TIMS-TEA values that can becompared to measured TIMS-TEA data (Fig. 9).

The model assumes that zircons consist of symmetrically-zonedsubdomains younging from core to rim (Fig. 9C).While no assumptionsare made about the thickness, volume or composition of these crystalsubdomains, the same cross-sectional sampling area is assumed for insitu point measurements progressing from zircon core to rim (e.g., aLA-ICPMS point transect). If this is true as in the current study, in situdata can be used to predict the volume-integrated geochemistry of theentire crystal (or grain fragment) by applying a nonuniform weightingfactor to each point measurement. The weighting factor Vi,norm of eachpoint measurement is determined by the quadratic equation

Vi;norm ¼ 3i i−1ð Þ þ 1n3 ; ð1Þ

for the ith pointmeasurement progressing from core to rim and n num-ber of point measurements (i.e., icore = 1, irim = n; Fig. 9D).This nonuniformweighting scheme results from the significantly great-er contribution of more exterior crystal domains to the volume-integrated, whole crystal signal. For example, in the case of n = 2 boththe core and rim point measurements integrate 50% of the grain radius;however, because of the cubic scaling of volume as a function of radius,the rim point measurement represents 87.5% of the crystal volume and

Fig. 9. Schematic of zircon growth model for comparing in situ and bulk crystal trace element dpyramids (four-sided) with characteristic length-scale ω. (B) Opaque zircon morphology of Aconcentric growth zones (light/dark banding) produced during successive core-to-rim crystallin situ microanalytical point measurements across a core-to-rim transect, projected as the y-data frommeasured in situ data. Total crystal volume (Vz) is proportional toω3 (upper equationgreen circles) increases quadratically from core-to-rim (lower equation). n: number of in situm(colored lines/circles) with their modeled bulk crystal value (colored stars) normalized to theirdomain values. While the model is developed for single crystal in situ/bulk comparisons, the pricrystals with both in situ and bulk geochemistry (e.g., LA-ICPMS and TIMS-TEA, respectively). Tpoints.

the core only 12.5%. As such, bulk zircon composition is stronglyweight-ed toward the composition of the outermost grain domains (Fig. 9E). Byextension, whole-crystal zircon U–Pb dates can also be weightedtoward their rims depending on subdomain U concentrations. Whilethe model here is developed explicitly for whole-crystals with both insitu and bulk geochemistry, the principle of quadratic weights favoringouter grain domains is equally valid for grain fragments. The approachoutlined here is generalized to accommodate any symmetrical crystalgeometry and transect length.

Direct comparison of modeled LA-ICPMS and TIMS-TEA geochemis-try from Bergell zircon demonstrates excellent cross-method agree-ment. Predicted Yb/Dy, Lu/Hf and Hf concentrations covary withmeasured values along a 1:1 equivalence line as anticipated for 44zircons with both LA-ICPMS and TIMS-TEA data (Fig. 10). In order toremove systematic methodological biases for this comparison, we stan-dardized the data and model outputs by subtracting the correspondingmean: this procedure is permissible as in the current study we are pri-marily interested in relative compositional trends and not absolutegeochemical values. Modeled LA-ICPMS data plotted as a time-series,with grain ages from ID-TIMSdating, corroborates first-order consisten-cy between methods and exhibits comparable variance to TIMS-TEA(Fig. 11). Notably, second-order deviations between modeled andmeasured geochemistry are observed (Fig. 10). These differences arelikely due to nonuniform partial dissolution of zircon subdomainsduring chemical abrasion (Mattinson, 2005), which is performed be-tween LA-ICPMS and U–Pb TIMS-TEA in our workflow. For example,some TIMS-TEA analyses may deviate from their modeled LA-ICPMSvalues because the in situ analyses include U-rich domains (with theirown trace element profiles) that are lost during grain leaching: thesedomainswill therefore be excluded from the resulting TIMS-TEA record.Divergence between modeled LA-ICPMS and TIMS-TEA may in someinstances also be due to an in situ sampling bias introduced by imperfectgrain mount polishing: if zircons are over- or under-polished (i.e., notexposing a perfect core-to-rim cross section), then LA-ICPMS transectswill not capture the true intracrystal compositional variability. Giventhese caveats, we assert that the generally good reproduction of bulkcrystal geochemistry with modeled in situ data validates comparisonof zircon composition determined by different analytical methods andconfirms the fidelity of trends in zircon composition.

ata. (A) Modeled three-dimensional zircon morphology as a tetragon and two terminatingwith orientation of bisecting plane projected in C. (C) Internal zircon structure exhibitingization. Details of modeled crystal geometry are given in red. Numbered green circles areaxes in D and E. (D) Volume-weighting model used to predict bulk crystal trace element) and the normalized volume-integrated contribution of each in situmeasurement (Vi,norm,easurements (e.g., here n = 7). (E) Comparison of various in situ trace element transectsmaximum value. Modeled bulk crystal values generally are strongly weighted toward rimnciple of quadratic weighting toward outer grain domains is equally valid for fragments ofhis approach can accommodate any symmetrical crystal geometry and number of transect

Fig. 10. Comparison of modeled LA-ICPMS and TIMS-TEA Yb/Dy (A), Lu/Hf (B) and Hf concentration data (C) demonstrating excellent first-order agreement betweenmodeled in situ andbulk fragment trace element measurements. Datasets have been demeaned to remove systematic methodological biases and allow for comparison of relative geochemical trends. 2σ an-alytical errors are displayed. Dashed purple line: 1:1 equivalence line. See main text for discussion of potential causes of LA-ICPMS and TIMS-TEA differences.


6.3. Reassessing interpretations of high-precision U–Pb dates, part II

Differentiating autocrystic and antecrystic zircon domains isparamount to accurately interpreting our U–Pb dates in the context ofthe models depicted in Fig. 8 (Miller et al., 2007). On the basis of inte-grated geochronology, geochemistry and texture, we present our pre-ferred interpretations of each lithology's U–Pb spectra. For tonalites(BR10-04 and -03), the following features are observed: 1) simple CLzoning patterns (oscillatory/sector/homogeneous zoning, no cores),2)well-behaved LA-ICPMS profiles with gradual core-to-rim variability,3) consistent, monotonic TIMS-TEA trends, and 4) 150–500 kyr ofintragrain U–Pb variability. Comparatively, for hybrid granitoids and

Fig. 11. Overlay of demeaned modeled LA-ICPMS and TIMS-TEA data as time-series fromID-TIMS zircon dates. Same modeled LA-ICPMS data as shown in Fig. 10 (large red circleswith 2σ error bars), same TIMS-TEA plots as in Fig. 6 (small blue circles). Mean 2σ analyt-ical error bars for TIMS-TEA data shown for comparison. Modeled LA-ICPMS data resolvessame relative compositional trends and variance as TIMS-TEA.

granodiorites (BR10-05, -08, BR11-03, -09, -14), the following areobserved: 1) complex CL zoning patterns (oscillatory zoning ±resorbed/convolute cores), 2) LA-ICPMS profiles with both gradualand abrupt core-to-rim variability, 3) scattered TIMS-TEA arrays withtrends developed in the youngest zircons, and 4) 50–850 kyr ofintragrain U–Pb variability.

Given the straightforward zoning and geochemistry of tonaliticzircons, these populations are best described by Interpretation #3: allzircons are autocrystic and record the interval between magma satura-tion and cooling to the solidus (Fig. 8C). In contrast, the complex zoningand geochemistry of hybrid and granodioritic zircons suggests that acomponent of the U–Pb dispersion is caused by the presence ofantecrystic grain domains. These populations are therefore bestdescribed by Interpretation #4: antecrysts comprise the older portionof the zircon spectra, and the younger portion consists of autocryststhat record zircon growth betweenmagma saturation and solidification(Fig. 8D). Through combined CL imaging and geochemistry, we identi-fied primarily oscillatory zoned grain fragments and whole grains thatmade up the youngest parts of each sample's zircon spectra (Fig. 1;Fig. 2d–g; Supplemental Tables 1, 2). Autocrysts identified in thismanner encompass 30–50% of the total U–Pb dispersion in hybridgranitoids and 40–70% in granodiorites, emphasizing the nontrivial re-tention of antecrystic zircon in these rocks. Moreover, exclusion ofantecrysts parsimoniously results in similar autocrystic growth dura-tions (~240–315 kyr) and termination times (30.8 Ma) for hybridgranitoids (Fig. 1). For granodiorites, the exclusion of antecrystsproduces offset autocrystic U–Pb spectrawith themarginal granodioriterange (BR11-03) about ~400 kyr older than the interior granodiorite(BR10-08), consistent with findings from other zoned intrusions(Fig. 1; Coleman et al., 2004).

6.4. U–Pb TIMS-TEA: multiple scales of Bergell zircon evolution

Heterogeneous TIMS-TEA zircon compositions are indicative of oneor a combination of the following options: 1) changing magma compo-sition with time (dX/dt) coeval with zircon crystallization, 2) variationsin zircon–liquid partition coefficients (dD/dt) during cooling, and3) mixing of two endmember zircon compositions to produce apparentage–compositional trends. As described above, zircon texture andgeochemistrywere used to distinguish the youngest, autocrystic zirconsfromolder antecrysts in each handsample population: as such,we arguethat mechanical mixing of distinct zircon subdomains is not the sourceof trends in the youngest U–Pb TIMS-TEA analyses (Fig. 6). While inthe following discussion these trends are interpreted solely in the


context of evolving magma composition, it is possible that changes inpartition coefficients during cooling also contributed to producing theobserved TIMS-TEA record (Blundy and Wood, 2003; Rubatto andHermann, 2007).

In order to semiquantitatively assess the relative importance ofobserved Bergell minerals in driving magma compositional changes,we compiled mineral–melt partition coefficients for REE, Hf, Th and Ufrom the Geochemical Earth Reference Model (GERM; Fig. 12A; http://earthref.org/GERM/http://earthref.org/GERM/). Because trace elementpartitioning is sensitive tomelt composition, temperature and pressure,the values presented in Fig. 12 are not meant to be interpreted sensustricto (e.g., Rubatto and Hermann, 2007). Instead, we refer to Fig. 12only as a first-order guide to understand the phases capable of control-ling a given geochemical proxy (e.g., Th/U) in a magmatic system. Wenow discuss the autocrystic trends in U–Pb TIMS-TEA (except wherenoted), scaling fromhandsamples to the pluton,within a differentiationframework (Fig. 6).

6.4.1. Th/U & Yb/Dy: emplacement-level differentiation indicatorsZircon Th/U linearly decreases at the handsample-scale through

time (Fig. 6A). Tonalite handsamples decreasemonotonically in a singlearray; notably, these arrays are parallel. Hybridized granitoids andgranodiorites generally develop a younger, autocrystic array with a tailof older, diffuse antecrysts. On the pluton-scale these trends produce a“sawtooth” pattern, with each handsample evolving from high- tolow-Th/U. Similarly, zircon Yb/Dy linearly increases at the same scale(Fig. 6B). However, the repeated pattern observed in Th/U is notpresent: tonalite BR10-04 Yb/Dy ranges from ~2.5–4 and BR10-03zircons span 4–6. Hybridized granitoids and granodiorites also evolveto progressively higher Yb/Dy through time.

We interpret trends in Th/U and Yb/Dy to reflect handsample-scalechanges in magma conditions (i.e., ΔX–T–P) during zircon crystalliza-tion. Moreover, because the ratios of these trace element partitioncoefficients are relatively insensitive to changes in temperature andpressure (Rubatto and Hermann, 2007), we believe these trends arebest explained by evolving magma compositions. In the case of Bergelltonalites, similar Th/U trends and different ranges of Yb/Dy indicatethat tonalitic zircons crystallized from two distinct magma reservoirs.Furthermore, given Th/U periodicity wherein handsamples with thesame lithology have similar trends offset by hundreds of thousands ofyears,we also interpret these data as evidence for incremental assemblyof the Bergell Intrusion (Coleman et al., 2004). This conclusion is consis-tent with the model of von Blanckenburg et al. (1992) that attributedthe geochemical heterogeneity of Bergell tonalites to emplacement of

Fig. 12. (A) Compilation ofmineral–melt partition coefficients for REE, Hf, Th andU.Data is fromorg/GERM/) for 65 wt.% whole-rock SiO 2. (B) Thin section photomicrograph of large, euheallanites exhibit optical evidence for metamictization, suggesting high Th and U contents. (C(sample BR10-05).

multiple, discrete magma batches with varying degrees of crustalcontamination.

That handsamples exhibit the same apparent differentiation trends(i.e., decreasing Th/U and increasing Yb/Dy) suggests a common frac-tionating agent for all Bergell lithologies. These trends are likely theresult of zircon crystallization synchronous with the growth of allanite(Fig. 12A). This REE-enriched accessory mineral is present in Bergelltonalites and granodiorites (Fig. 12B), strongly fractionates Th relativeto U (e.g., (Th/U)allanite N 100; Gregory et al., 2009), and thus regulatesthe Th/U of its host magma (Klimm et al., 2008). Prior dating of Bergellallanite yielded ages comparable to the zircon ages reported here,corroborating coeval zircon–allanite crystallization as the primary driv-er of melt Th/U evolution (von Blanckenburg, 1992; Oberli et al., 2004;Gregory et al., 2009). Moreover, these zircon Th/U trends are consistentwith decreasing allanite Th/U through time reported by Oberli et al.(2004). Alternatively, Th/U fractionation could be fully or partially driv-en by titanite ± thorite crystallization as both are present in Bergellrocks (Fig. 12C).

6.4.2. Lu/Hf: lithology-dependent fractionation signatureIn contrast to Th/U and Yb/Dy, zircon Lu/Hf is characterized by

lithology-defined trajectories in addition to handsample-scale trends(Fig. 6C). Tonalite Lu/Hf decreases progressively between handsamplesfrom ~0.008–0.003; comparatively, more scattered granodioritic Lu/Hfdecreases from ~0.015–0.005.

Similar Lu/Hf trends suggest common fractionation mechanisms fortonalites and granodiorites. One process that could produce decreasingtonalite Lu/Hf is preferential incorporation of Lu into restitic amphiboleduring Bergell magma generation in the deep crust (Dostal et al., 1983).Continued amphibole crystallization duringmelt extraction, transporta-tion and emplacement would then drive Lu/Hf down. This mechanismexplains the continuous fractionation trend defined by two distincttonalite emplacement pulses resolved by Th/U. Alternatively,apparently-decreasing tonalite Lu/Hf could be caused by zircon crystal-lization at higher temperature in the younger, deeper tonalite (sampleBR10-03; P=6.9 kbar) relative to the older, structurally-higher tonalite(BR10-04; P=6.0–6.6 kbar; Davidson et al., 1996). Unlike the partitioncoefficient ratios of Th/U and Yb/Dy, DLu

Zrn/melt decreases more rapidly athigher temperatures thanDHf

Zrn/melt. For example, at 800 °C the ratio of Luand Hf partition coefficients is about 0.13; at 950 °C, this ratio decreasesby nearly an order ofmagnitude to 0.021 (Rubatto andHermann, 2007).Subtle changes in crystallization temperature can therefore affect zirconREE partitioning. However, while differences in crystallization tempera-ture address the relative Lu/Hf values of tonalite handsamples

theGeochemical Earth ReferenceModel (GERM;http://earthref.org/GERM/http://earthref.dral allanite under cross-polarized light (granodiorite sample BR10-08). Many Bergell) Thin section photomicrograph of subhedral titanite wedge under cross-polarized light

http://earthref.org/GERM/http://earthref.org/GERM/





(i.e., lower Lu/Hf in deeper rocks), amechanism is needed to explain thedecreasing Lu/Hf trend within handsamples. Allanite could drive Lu/Hfdecrease through preferred uptake of Lu. Alternatively, the observed de-crease in Lu/Hf could result from zircon crystallization at progressivelylower melt fractions with greater Hf enrichment, with modally-significant amphibole (22.9%), epidote (3.4%), and apatite (0.5%) prefer-entially incorporating Lu (Fig. 12; von Blanckenburg, 1992).

Similarly, hybridized granitoid and granodiorite samples exhibit Lu/Hftrends with noisier fractionation signals (Fig. 6C). For example, the youn-gest zircons from granodiorite BR10-08 develop a steep, decreasing Lu/Hftrend, also potentially reflecting zircon growth under lowermelt fraction,higher Hf concentration conditions.

6.4.3. Hf concentration and Zr/Hf: progressive pluton-scale evolutionZircon Hf concentrations and Zr/Hf increase and decrease, respec-

tively, on the scale of the intrusion (Fig. 6D, E). These pluton-scaletrends consist of diffuse, handsample-scale trends that are controlled lo-cally by zircon saturation. Experiments on metaluminous melts(i.e., Bergell compositions) yield DHf/Zr

zircon between 0.2–0.5 (Linnen andKeppler, 2002): zircon crystallization could therefore drive melt Zr/Hfdown and Hf concentration up as observed here. While the pluton-scale trend could be generated by zircon crystallization at depth, weconsider such a scenario unlikely as 1) magmas in the lower crust areunlikely to reach zircon saturation (Boehnke et al., 2013), and 2) Th/Utrends suggest zircon did not saturate until after emplacement inthe middle crust. Alternatively, other important deep-crustal phaseshave DZr/HfN1 and will fractionate this ratio during melt generation(e.g., clinopyroxene, amphibole and garnet; Linnen and Keppler,2002). Cross-sample, pluton-scale trends in Hf and Zr/Hf are thereforeattributed to both fractional crystallization of a deep crustal reservoirsourcing all Bergell rocks and increasing crustal contaminationof melts during transportation and emplacement. This model is consis-tent with Nd, Sr and O isotopic data that preserve the full range ofmantle-to-crust values in Bergell whole-rocks+accessory phases andnecessitate the operation of both fractionation and assimilation toexplain the geochemistry of Bergell magmas (von Blanckenburg et al.,1992; Gregory et al., 2009). Moreover, the Hf and Zr/Hf variability ofzircons from the same lithology is consistent with a recent compilationof Bergell whole-rock geochemistry showing extensive heterogeneitywithin tonalites, granodiorites, and hybrid granitoids (Gianola et al.,2014). Decreasing Zr/Hf has previously been shown to be a robustproxy for increasing differentiation with a magmatic system (Davidet al., 2000; Bea et al., 2006; Claiborne et al., 2006). Our Zr/Hf and Hfrecords are therefore consistent with the production of increasinglyevolved Bergell magmas through combined fractional crystallizationand crustal assimilation.

6.5. Comparison of U–Pb TIMS-TEA to whole-rock geochemistry

In addition to providing criteria bywhich to evaluate U–Pb geochro-nological data, characterization of zircon geochemistry also allows forcomparison of U–Pb TIMS-TEA with more conventional geochemicalrecords. As illustrated above, U–Pb TIMS-TEA provides absolute tempo-ral constraints on magma differentiation: such information is notaccessible from standard whole-rock (WR) geochemical data, whichoften projects various geochemical proxies as a function of wt.% SiO2

(i.e., Harker diagrams). Bridgingwhole-rock geochemistry and geochro-nology with zircon geochemistry can therefore provide information asto the timescales over which differentiation occurs more broadly. Tothat end, we present records of both Bergell and global-scale plutonicWR geochemistry from Gianola et al. (2014) and Keller et al. (2015),respectively (Fig. 13).WRTh/U, Lu/Hf and Zr/Hf variability are displayedover 50–70wt.% SiO2: this range encompassesmost observed Bergell li-thologies. For the globally-averaged plutonic values we present meansand 2σ standard deviations of 2 wt.% SiO2 bins.

As described above, we interpret the “sawtooth” variability in zirconTh/U as an emplacement-level differentiation signal (Fig. 6A). Compar-atively, WR Th/U displays no consistent trend with increased differenti-ation (Fig. 13A). The absence of a Th/U fractionation signature in bothBergell WR data and the plutonic database suggests a generalizedmechanism wherein melt Th/U is largely controlled by late-stageaccessory mineral saturation (e.g., allanite) under highly crystallineconditions.Magmas thus retain their “parental” Th/U signature generat-ed during melt formation in the deep crust. WR Lu/Hf decreases withsuccessive differentiation (Fig. 13B), with relatively little Bergell WRvariability compared to the observed range of database WR Lu/Hf. Thisfinding is broadly consistent with the handsample- and lithology-scaletemporal trends in Bergell zircon, which decrease successively throughtime (Fig. 6C). This correspondence between relativeWR and TIMS-TEALu/Hf trajectories therefore supports our interpretation of apparentzircon Lu/Hf evolution as reflecting protracted differentiation.Similar to Lu/Hf, WR Zr/Hf decreases with successive differentiation(Fig. 13C), consistent with the observed pluton-scale trend from Bergellzircon TIMS-TEA (Fig. 6E). Our interpretation of decreased Bergell TEAZr/Hf with time being driven by a combination fractional crystallizationand assimilation is therefore supported by similar processing occurringon the scale of the continental crust.

Whole-rock geochemistry and integrated geochronological–geochemical workflows such as that presented here provide comple-mentary records of crustal evolution. Comparison of U–Pb TIMS-TEA,Bergell WR and database WR geochemistry demonstrates how coupledgeochronological–geochemical data can be used to constrain the time-scales of differentiation processes. In demonstrating magma evolutionover a range of spatiotemporal scales, we infer 1) which elements arefractionated, 2) the location and timescales of differentiation, and3) minerals responsible for driving fractionation. Elements are fraction-ated over different crustal intervals, indicating that the crust will bestratified to varying degrees based on element compatibility in crystal-lizing assemblages (e.g., Rudnick and Gao, 2003; Jagoutz and Schmidt,2012).While futureworkwill more thoroughly explore the relationshipof these datasets, we assert here that zircons at the scale of an intrusioncan be used to understand the timescales of crustal-scale differentiationprocesses.

6.6. Long-term magma emplacement rate in the Bergell

The samples presented here encompass themain core of the BergellIntrusion (i.e., N90% of the pluton area), and new U–Pb zircon datesrecord ~1.6 Myr of zircon crystallization. If the pluton volume can beestimated, we can calculate the long-term magma emplacement rateas done in numerous previous studies (e.g., de Saint Blanquat et al.,2011). Crucially, this rate estimate is a minimum value integrated overthe entire history of the intrusion: instantaneous rates during times ofpulsed intrusion would have been much greater.

The thickness of the intrusion is estimated from Al-in-hornblendebarometry by using the pressure difference between the structurally-highest pluton side (Val Forno at NE, P = 4.5 kbar) and thestructurally-lowest location in the main intrusion body (Val Mera atSW, P = 6.9 kbar). Assuming a mean crustal pressure gradient of 3kbar/km, we calculate a thickness of 7.2 km. The diameter of the Bergellis then estimated at 14 km from the distance between Passo deTrubinasca (NW Bergell contact with Gruf Complex) and Rifugio Ponti(SE Bergell contact). Simplifying the pluton to a cylindrical geometry,these dimensions equate to a volume of ~1100 km3. Crucially, this is aminimum volume estimate due to erosive loss of the intrusion roof.Dividing this value by 1.6 Myr, we determine a mean Bergell magmaemplacement rate of 0.7 × 10−4 km3/yr. This is similar to values calcu-lated from other large plutons, which yield long-term rates on the orderof 10−3–10−4 km3/yr (Coleman et al., 2004; Matzel et al., 2006; Tappaet al., 2011; Davis et al., 2012; Mills and Coleman, 2013).

Fig. 13. Harker diagrams of whole-rock Th/U (A), Lu/Hf (B) and Zr/Hf (C) evolution as a function of SiO2 (50–70 wt.% range). Included are a compilation of published and unpublishedBergell whole-rock geochemical data from Gianola et al. (2014, “Bergell comp.”), new whole-rock geochemistry from this study, and averages from a database of global plutonicwhole-rock geochemistry from Keller et al. (2015, “Database WR”). Insets in panels A and C show the full range of Bergell whole-rock geochemical variability, which is much greaterthan DatabaseWR. Mean 2σ standard deviations for DatabaseWR are displayed. Arrows in panels B and C correspond to trend of zircon TIMS-TEA evolution at the handsample- and plu-ton-scales, respectively (Fig. 4).


6.7. Inferring magma cooling rates from U–Pb zircon geochronology

Our Bergell U–Pb dataset can be used to estimate high-temperaturecooling rates (dT/dt), assuming zircon age–compositional trends reflectprotracted mineral growth as described above. For example, if U–Pbdates are interpreted as a cooling signal (Fig. 8D), then the oldestautocrystic zircon records when the host magma reached its zirconsaturation temperature (Tsat; Boehnke et al., 2013). Zircon will thencontinue to crystallize as the magma cools, with the youngest zirconindicating the cessation of crystallization at the solidus (Tsolid). Whole-rock Tsat (Tsat

WR) for average Bergell tonalitic and granodioritic composi-tions are ~710 and 770 °C, respectively. However, zircons likely beganto crystallize 50–100 °C above T sat

WR due to earlier saturation of zirconin a magma undergoing fractional crystallization (Harrison et al.,2007). Approximating that zircon crystallization began ~75 °C N Tsat

WR,we calculate Bergell tonalite and granodiorite Tsat of 785 and 845 °C, re-spectively. We used the same whole-rock compositions and therhyolite-MELTS thermodynamic software package to calculate Tsolid ofeach lithology (Gualda et al., 2012). Because Tsolid is sensitive to the

poorly-constrained magmatic water content, we calculated Tsolid at~0.75 and 2.5 wt.% H 2O, yielding Tsolid ranges of 650–730 °C (tonalite)and 625–665 °C (granodiorite). These ranges are consistent with exper-imental and thermodynamic constraints on the wet granite solidus at4–7 kbar (~650 °C; Holland and Powell, 2001). The differences betweenTsat and Tsolid (ΔT) are therefore 55–135 °C (tonalite) and 180–220 °C(granodiorite). We emphasize that these temperature ranges areestimates for obtaining order-of-magnitude cooling rates only; howev-er, given more detailed petrologic constraints, these estimates could behoned.

These temperature ranges can be divided by the difference of theoldest and youngest autocrystic zircons in a handsample (Δth, Fig. 1)to produce a cooling rate. As described above, we interpret the spreadin tonalitic zircon dates to reflect protracted, autocrystic growthwith no discernible ante-/xenocrystic components. Use of tonalitehandsample Δths of 609 and 706 kyr (BR11-04 and -03, respectively)therefore yield cooling rates of ~90–220 and 80–190 °C/Myr. Compara-tively, geochemical and textural evidence suggests an antecrystic com-ponent in the granodioritic zircon populations (Figs. 6, 7): cooling


rates calculated using granodioriticΔths shown in Fig. 1would thereforeyield minimum values, and could be as much as a factor of two greater.Calculation using autocrystic granodiorite durations of 331 and 449 kyr(BR11-03 and 10-08, respectively) yield minimum cooling rates of~540–660 and 400–490 °C/Myr. For Bergell rockswe therefore calculatepost-emplacement cooling rates on the order of 102–103 C/Myr.

Interestingly, tonalites have apparently slower cooling ratescompared to granodiorites. One potential explanation for this differenceis lithological variation in magmatic H2O content, with tonalites havingmore water compared to granodiorites: this would result in highertonalite/lower granodiorite cooling rates. Another possibility could bedifferences in the relative size of magma increments, with largertonalitic/smaller granodiorite pulses. Alternatively, synmagmatic shearheating along the Insubric Line could have provided additional thermalenergy to the more proximal tonalites, permitting the protracted time-scales of zircon growth observed in our samples (Leloup et al., 1999).Comparatively, the more distal granodioritic melts would be lessbuffered by this heat source due to their emplacement further fromthe feeder zone, being injected into (relatively) cool country rocksnorth of the Insubric Line. Deformation-induced viscous heating, arguedto have been active in the formation of the Lepontine Dome duringorogenesis, may have also thermally-buffered the tonalites, although itis not obvious why this heat would be preferentially partitioned intotonalites only (Burg and Gerya, 2005). The source of this lithology-dependent variability in cooling rates will be more thoroughly exploredin future work.

In summary, cooling rates inferred from U–Pb zircon data mayprovide powerful constraints for reconstructing the thermal historiesof crustal magmatic systems, especially when used in concert withU–Pb thermochronometers (e.g., titanite and apatite).

6.8. Implications of protracted zircon growth for high-precision U–Pbgeochronology

Moving forward, the complexity of zircon dates both within andamong crystals highlights several important lessons for high-precisiongeochronology, many of which have been noted in previous studies:

1. Zircon dates zircon (so date more zircon!): The timescales of processesinferred from geochronological data (e.g., magma differentiation)can in some geological settings far exceed the absolute precision ofsingle U–Pb analyses; moreover, analytical resolution will onlyincrease with subsequent methodological developments in U–Pbgeochronology. As such, zircon dating will be increasingly capableof resolving the initiation, duration and termination of such process-es. While the general ages of igneous rocks can be inferred fromrelatively few analyses (i.e., 2–5 U–Pb zircon dates), full resolutionof the extent of zircon crystallization requires analyzing more zirconper sample. In the present study we found between 15–30 analysesper sample provided sufficient temporal and geochemical informa-tion to quantify the emplacement, differentiation, and coolinghistories of Bergell magmas, and recommend a similar number ofanalyses for assessing such processes.

2. Sub-sample zircon fragments: Sampling strategies that restrict the scaleof U–Pb zircon analyses to whole crystals will result in mixing of sub-grain growth domains, producing volume-weighted, average zircondates. As demonstrated above, sub-grain sampling of Bergell zirconsresolves minimum crystallization times on the order of 104–106 yrfor single zircons from slowly-cooled, mid-crustal plutonic rocks(Fig. 5). By comparison, dating of whole grains will mask such disper-sion in zircon dates. In the extreme case, a resulting population ofwhole-grain U–Pb dates will be amenable to calculation of a preciseweighted-mean age and an acceptable MSWDwith minimal data ex-clusion: such a statistically-valid age will be of questionable geologicsignificance. Relatively coarse sampling can thus oversimplify the U–Pb systematics of a zircon population, leading to erroneous data

interpretation. Geochronologists interested in quantifying variouspluton emplacement phenomena (e.g., melt residence timescales,cooling rates, ante- vs. autocrystic domains) should therefore exploresub-crystal sampling strategies in applications where the precision ofU–Pb analyses is better than potential zircon growth timescales.Similarly, integration of our workflow with ultra-high spatial resolu-tion analysis (e.g., NanoSIM, SIMS depth profiling) may detect evenfiner-scale compositional heterogeneity than that described here(Hofmann et al., 2009; Storm et al., 2014). Linking micrometer-scaletrace element variations to LA-ICPMS and TIMS-TEA data couldprovide further constraints onmagma differentiation and zircon crys-tallization dynamics, given that the latter techniques will tend to “av-erage-out” signals at such fine spatial scales.

3. Characterize zircon texture+geochemistry: As discussed at lengthabove, characterization of zircon texture and geochemistry providesa powerful tool for interpreting U–Pb geochronological data. Whilean integrated in situ+bulk crystal workflow such as that presentedhere is optimal, stand-alone geochemical analysis by either optionis likely sufficient for most applications. Zircon geochemistry maybe deemed unnecessary in systems with simple, uniform zirconsystematics: however, such populations may yield heterogeneousgeochemistry despite homogeneous U–Pb dates, potentially indicat-ing thatmagmatic processes are occurring at timescales toofine to beresolved with geochronology.

4. Integrate geochronological–geochemical workflows with complementa-ry data: The geochemical signal recorded by an arbitrary zircon pop-ulation is directly connected to system-specific intensive variables,including: 1) magma emplacement depth/temperature, 2) zirconsaturation temperature, 3) magma composition/volatile content,4) modal accessory phase makeup, 5) cooling rate, 6) detailed ther-mal history (e.g., simple cooling vs. repeated recharge), and 7) thedegree of xenocrystic/antecrystic zircon incorporation. Zircongeochronological–geochemical spectra can be contextualized withthin section petrography, whole-rock geochemistry and isotopictracer data to reconstruct more robust interpretations of a magmaticsystem's emplacement, differentiation and cooling history. Thesecomplementary data should be used in conjunction with U–Pbdates+geochemistry if available.

5. Beyond plutons: accuracy in ashbed geochronology: The resolution ofzircon-scale age heterogeneity in the Bergell provides an importantendmember perspective on geochronological studies of volcanicsystems and Earth history events. Dispersed zircon dates in ashbedsare becoming increasingly common, and discussion is ongoing ashow to best interpret high-precision U–Pb data to determine themost accurate and precise eruption/depositional ages (Simon et al.,2008; Schoene et al., 2010a; Schmitz and Kuiper, 2013; Sagemanet al., 2014; Schoene et al., 2015; Ovtcharova et al., 2015). The resultsof this study support an interpretation of geologically-meaningfuldispersion in slowly cooled plutonic rocks, as opposed to this obser-vation resulting from residual Pb-loss. Similarly, increased scrutiny ofashbed zircons by workflows such as that described here willundoubtedly continue to resolve ever-finer compositional and tem-poral heterogeneity in such populations. We challenge workers toappreciate the resolving power of high-precision geochronologyrelative to zircon-forming processes, and to adopt more dynamicinterpretations of zircon geochronological data than previouslypossible.

7. Summary & conclusions

Cumulatively, our integrated U–Pb TIMS-TEA, LA-ICPMS and EPMAworkflow reveals a dynamic history of Bergell magmatism. Distinct yetoverlapping timescales ofmelt residence for different samples and lithol-ogies are consistent with field observations of magma mixing and hy-bridization along the western pluton margin (Davidson et al., 1996).Additionally, Bergell TIMS-TEA data demonstrate an emplacement level


differentiation signal, the feasibility of which has been challenged by thediscrepancy between rapid cooling timesmodeled for shallow intrusionsand the range of zircon ages observed in upper crustal igneous rocks(Annen, 2011; Coleman et al., 2012). In situmelt evolution in the Bergellmay be due in part to hotter ambient thermal conditions arising fromdeeper emplacement levels compared to shallower systems like the Tu-olumne Intrusive Suite and Adamello batholith (b10 km), thereby em-phasizing the sensitivity of initial country rock temperature in dictatingsubsequent differentiation pathways (Coleman et al., 2004; Schoeneet al., 2012; Menand et al., 2015). These data also place constraints onthe potential for larger scale post-emplacement differentiation, withmagma residence timescales in the order of 105–106yr for Bergell granit-oids permittingmelt hybridization and extraction in themiddle crust. Ev-idence for extended magma residence is consistent with the Bergellemplacement model of Rosenberg et al. (1995), which requires a cohe-sive, Bergell-sized reservoir of partially-molten magma capable of beingtransported en masse through the crust and converting deformation atdepth into strain at higher crustal levels. Integration of coupledgeochronological–geochemical records with detailed field observations,thermochronology and thermalmodelingwill permitmore thorough as-sessment of the emplacement histories of the Bergell Intrusion and otherintrusive suites. Finally, we believe the application of combined high-precision geochronological–geochemical workflows provides an excitingpath to understanding crustal magmatic processes at finer spatial andtemporal scales.

Acknowledgments

This study was supported by 2012 GSA Research Grant No. 9762-12(Samperton), a 2012 GSA MGPV Division Student Research Grant(Samperton), and U.S. NSF grant No. EAR1219766 (Petrology andGeochemistry Program; Schoene). Formal reviews by Axel Schmitt andan anonymous reviewer are greatly appreciated. Reviews of an earlierversion of the manuscript by Brendan Murphy, Fernando Corfu andCalvin Miller, and subsequent conversations with Oliver Jagoutz, BruceWatson, Catherine Annen and Judy Swan, about the data and interpreta-tions presented herein, helped us to clarify our arguments and approach.We thank Eric Reusser for his guidance and hospitality during a Summer2010 tour of the Bergell; Gareth Seward for conducting EPMA analyses;Jeff Gronewold and Andrew Gregovich for enthusiastic field assistance;and Mélanie Barboni, Jon Husson, Elizabeth Lundstrom and John Higginsfor TIMS and ICPMS lab support.

Appendix A. Supplementary data

Supplementary data to this article can be found online at http://dx.doi.org/10.1016/j.chemgeo.2015.10.024.

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