ZIRCON and APATITE (U-Th)/He DATES and INTERPRETATION of HIGH-DAMAGE ZIRCON from the SOUTHERN ROCKY MOUNTAINS, FRONT RANGE, COLORADO By

Home , Lumpy Ridge, Mount Ida (Colorado), Never Summer Mountains

“INVERTED” ZIRCON AND APATITE (U-Th)/He DATES AND INTERPRETATION OF HIGH-DAMAGE ZIRCON FROM THE SOUTHERN ROCKY MOUNTAINS, FRONT RANGE, COLORADO by

JOSHUA E. JOHNSON B.A., Middlebury College, 2013

A thesis submitted to the Faculty of the Graduate School of the University of Colorado in partial fulfillment of the requirement for the degree of Master of Science Department of Geosciences 2015

This thesis entitled: “Inverted” zircon and apatite (U-Th)/He dates and interpretation of high-damage zircon from the Southern Rocky Mountains, Front Range, Colorado written by Joshua E. Johnson has been approved for the Department of Geosciences

(Dr. Rebecca Flowers)

(Dr. Kevin Mahan)

(Dr. Craig Jones)

Date

The final copy of this thesis has been examined by the signatories, and we Find that both the content and the form meet acceptable presentation standards Of scholarly work in the above mentioned discipline.

Johnson, Joshua E. (MSc., Geosciences) “Inverted” zircon and apatite (U-Th)/He dates and interpretation of high-damage zircon from the Southern Rocky Mountains, Front Range, Colorado Thesis directed by Associate Professor Rebecca Flowers

Radiation damage in zircon has a profound influence on helium retentivity. This study is aimed at understanding the He systematics of high-damage zircons since this end of the damage spectrum has received the least attention in prior work. We acquired 121 zircon (U-Th)/He (ZHe) dates from 29 samples from an ~50 km east-west transect across the Colorado Front Range that span the full range of alpha dosages (radiation damage) encompassed by previous diffusion experiments. Date-eU patterns within our ZHe dataset are broadly consistent with the expected influence of radiation damage, showing positive and then negative correlations. ZHe dates from the range core in Rocky Mountain N.P. record cooling to near-surface temperatures during the Laramide Orogeny (65-45 Ma). Closer to the range front, there is a sharp transition to Oligo-Miocene ZHe dates despite the presence of Laramide apatite He (AHe) dates in the immediate vicinity. Titanite He (THe) dates from the area record cooling through ~200 °C in the Neoproterozoic, precluding reheating above that temperature in the last 600 myr.

High-damage zircons (>1018 α/g) from Big Thompson Canyon have ~20 Ma ZHe dates that are “inverted” with respect to 65-45 Ma AFT and AHe dates from the same area. This inversion implies that these zircons are sensitive to temperatures of <70 °C, significantly lower than their nominal closure temperature of ~180 °C. At these high damage levels, there is a disconnect between the retentivity predicted by the current damage-diffusivity model, model ZHe dates, and relevant geologic and geochronologic constraints. Despite this disconnect, our results show that damaged zircons can serve as low-temperature chronometers. The utility of applying ZHe in this manner is demonstrated by the detection of a previously unrecognized reheating event on the order of ~50 °C in the Oligo-Miocene, implied by the ~20 Ma ZHe dates. Our preferred explanation for this event invokes the reburial of the range front under ~1 km of sediment derived from erosion of the high topography of the range core followed by the subsequent unroofing in the early Miocene, possibly recorded by the basal units of the Ogallala Formation on the High Plains.

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ACKNOWLEDGMENTS

I have many people to thank for helping me throughout the thesis process. First and foremost, I would like to thank my advisor Becky Flowers for her unwavering support and incredibly helpful feedback. Jim Metcalf provided incalculable analytical assistance – this project could not have happened without his help. David Liefert, a CU undergraduate, provided excellent field assistance and helped tremendously with the lab work that followed. Dr. Graham

Baird (University of Northern Colorado) and Dr. Shari Kelley (New Mexico Institute of Mining and Technology) graciously provided mineral separates that were used in this study. Kevin

Mahan (CU) provided valuable information on the geology of Big Thompson Canyon. Paul

McLaughlin and others at Rocky Mountain National Park assisted with the permitting process required to conduct research in the park. Nigel Kelley (CU) and Eric Ellison (CU) assisted with the collection and interpretation of Raman data. Ken Cochran (University of Northern Colorado) assisted with the collection of CL images. Lastly, thanks to all my fellow grad students and friends that let me bounce ideas off them these past two years.

This project would not have been possible without generous funding from the Rocky

Mountain Conservancy, the Geological Society of America, the Colorado Scientific Society, and the Dept. of Geosciences at CU.

CONTENTS

CHAPTER

I. INTRODUCTION ...... 1

II. GEOLOGIC SETTING ...... 4

2.1 Regional Geologic Setting ...... 4 2.2 Big Thompson Canyon ...... 9 2.3 Rocky Mountain National Park ...... 12

III. PREVIOUS THERMOCHRONOLOGY IN THE COLORADO ROCKIES ...... 13

IV. SAMPLES AND METHODS ...... 15

4.1 Samples ...... 15 4.2 Cathodoluminescence Imaging and Raman Spectroscopy ...... 15 4.3 (U-Th)/He Thermochronology ...... 18

V. RESULTS ...... 21

5.1 Zircon Characterization: CL Imaging and Raman Spectroscopy ...... 21 5.2 (U-Th)/He Thermochronology ...... 24

VI. DISCUSSION ...... 32

6.1 (U-Th)/He Systematics of High-Damage Zircons ...... 32 6.1.1 Alpha dose of analyzed zircons ...... 32 6.1.2 Reproducibility and temperature sensitivity of He data from high-damage zircons ...... 36 6.1.3 Testing the high-damage end of the damage-diffusivity model ...... 38 6.2 Geologic Implications ...... 41

VII. CONCLUSIONS ...... 49

REFERENCES….……………………..…………………………………………51

TABLES

Table

1. (U-Th)/He data ...... 25

FIGURES

Figure

1. Simplified geologic map of the northern Front Range ...... 5

2. Map of sample locations and plots of ZHe data ...... 16

3. CL and Raman data ...... 22

4. Age-elevation plot for Longs Peak ...... 29

5. Plots of AHe and THe data ...... 30

6. Simplified date-eU plot of all ZHe data ...... 31

7. Alpha dose comparison plot ...... 34

8. HeFTy simulation results ...... 40

SUPPLEMENTARY FIGURES

Figure

1. Results by individual sample and lithology ...... 50

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CHAPTER I

INTRODUCTION

Zircon’s impressive durability, longevity, ubiquity, and ability to incorporate uranium and other trace elements into its structure have enabled a wide array of geochronologic, trace element, and isotopic investigations with extensive insights into the geologic record. Zircon is now increasingly applied as a (U-Th)/He thermochronometer (ZHe), with initial diffusion work suggesting a nominal closure temperature of ~180 °C (Reiners et al., 2002; Reiners, 2005). It is well known that zircon’s typically high U-Th concentrations can cause significant radiation damage accumulation within its crystal structure (e.g., Meldrum et al., 1998). Recent work has shown that zircon He diffusion is strongly dependent on its accrued radiation damage (Guenthner et al., 2013). Recent publication of a zircon radiation damage-He diffusion kinetic model unlocks the potential to understand and exploit the ZHe data dispersion that is a consequence of this effect.

The effect of radiation damage on He diffusion kinetics was first recognized in apatite, where damage accumulation decreases the He diffusivity (Shuster et al., 2006; Flowers et al.,

2009). For some thermal histories, this phenomenon is evident as a positive correlation between date and effective uranium concentration (eU = U + 0.235 × Th). eU serves a proxy for the rate of alpha production, and hence, relative radiation damage in a suite of crystals that have experienced the same thermal history (Flowers et al., 2007). In zircon, progressive damage accumulation initially causes a decrease in He diffusivity that is interpreted to result from blocking of preferred diffusion pathways, but eventually a threshold is reached whereupon the damage zones become sufficiently interconnected to rapidly increase diffusivity (Guenthner et

al., 2013). These effects can lead to positive (at low damage) and/or negative (at higher damage) date-eU correlations in ZHe datasets. Because eU concentration (and thus the alpha dose) is typically an order of magnitude or more higher in zircon than apatite, in typical circumstances

ZHe dates are far more sensitive to radiation damage than apatite (U-Th)/He (AHe) dates. The low to moderate alpha dose end of zircon damage spectrum (1015-1017 α/g), where He diffusivity initially decreases, is much better calibrated (19 diffusion experiments) than the high-damage end (>1018 α/g) associated with the dramatic increase in diffusivity (Guenthner et al., 2013). The latter depends on two diffusion experiments that suggest low to negligible zircon He retentivity, with estimated closure temperatures of 50 °C to -50 °C (Fig. 8 in Guenthner et al., 2013). Initial applications of the damage diffusivity model have focused primarily on the lower dosage end of the spectrum (e.g., Guenthener et al., 2014b).

The new zircon damage model represents a significant advance in our understanding of zircon He retentivity, but it remains unclear whether metamict zircons are sufficiently retentive and if their He systematics are well-enough understood to be useful in (U-Th)/He studies. The model also raises interesting questions about whether one could reliably access temperatures as low or lower than those typically explored by AHe studies (<90 °C, depending on radiation damage). If so, the ubiquity of highly damaged zircons in Precambrian rocks would make them attractive targets for thermal history investigations in a wide range of settings.

To address these questions, we investigated a suite of zircons from Proterozoic basement rocks exposed in the northern Colorado Front Range that span the entire dosage range (1015-1019

α/g) of existing diffusion experiments. Our specific goal was to evaluate the He systematics of zircons at the high-damage end of the dosage spectrum, which has received the least attention in past work. The Front Range zircons accumulated substantial radiation damage owing to

residence at temperatures <300 °C since ~1300 Ma (Shaw et al., 1999; 2005). In addition, the relatively well-known Late Cretaceous-early Tertiary (Laramide) burial and unroofing history of these rocks (e.g., Raynolds, 1997; Kelley and Chapin, 2004) provides a framework for evaluating the ZHe results and zircon damage model predictions given the ZHe dates expected for this thermal history. We acquired 121 ZHe analyses for 29 samples from an ~50 km east-west transect across the range in the vicinity of Big Thompson Canyon and Rocky Mountain National

Park. Apatite and titanite (U-Th)/He data were obtained locally to evaluate the temperature sensitivities of these minerals relative to zircon in the same or nearby samples, because apatite kinetics are relatively well characterized (Shuster et al., 2006; Flowers et al., 2009) and titanite has kinetics previously interpreted to be similar to zircon (Tc = ~190-220 °C; Reiners and Farley,

1999). A subset of zircons was imaged by CL and analyzed with Raman spectroscopy to better characterize the zircon structure and damage range. We use the results to evaluate the reproducibility and general promise of utilizing highly damaged zircon crystals in He studies, assess the temperature sensitivity of the analyzed zircons relative to other He thermochronometers, and describe the unexpected insights that the ZHe dates provide into the

Cenozoic burial and unroofing patterns in the northern Front Range.

CHAPTER II

GEOLOGIC SETTING

2.1 Regional Geologic Setting

The southern Rocky Mountains consist of a series of north to northwest-striking ranges that stretch from northern New Mexico to southern Wyoming, part of the larger (~5000 km)

Rocky Mountain chain. The southern Rockies separate two distinctly different physiographic provinces – the Colorado Plateau to the west and the High Plains to the east – that have experienced comparatively less deformation in the Phanerozoic. The Rio Grande Rift bisects the southern Rockies from northern New Mexico to central Colorado, a product of the transition from a contractional regime in the Laramide to a more extensional regime in the late Cenozoic

(e.g., Chapin and Cather, 1994). The highest elevations within the range are found in central

Colorado, with peaks exceeding 4000 m.

The Colorado Front Range is the easternmost and largest uplift of the southern Rocky

Mountains, and is one of numerous ranges in the region where Precambrian rocks are widely exposed (Fig. 1, inset). At least three mountain-building events have affected this area, occurring in the Proterozoic (~1750 Ma), Pennsylvanian (~300 Ma), and Cenozoic (post-70 Ma)

(e.g., Tweto, 1979; Kluth and Coney, 1981; Bickford et al., 1986; McMillan et al., 2006). The metamorphic and plutonic core of the range formed in the Proterozoic, and represents the northernmost part of a >1000 km-wide orogenic belt that records the progressive accretion of distinct geologic provinces to the southern margin of the Archean Wyoming craton from ~1.8 to

1.0 Ga (e.g., Whitmeyer and Karlstrom, 2007). These terranes are distinguished primarily by the timing of major magmatism and deformation, as well as by their isotopic characteristics (e.g.,

Wooden et al., 1998). The majority of the basement rocks found in the Front Range are part of a

Figure 1. Simplified geologic map of the northern Front Range (adapted from Kelley and Chapin, 2004). Locations of existing thermochronologic data from the region are shown, including data collected in this study (black stars). Faults are shown as dark black lines, major rivers as dotted black lines. Inset: location of the study area within the Southern Rocky Mountains.

terrane commonly referred to as the Yavapai Province (also known as the Colorado Province after Reed et al., 1987; Sims and Stein, 2003), the accretionary product of juvenile island arc terranes colliding with the Wyoming craton between ~1.78 and 1.70 Ga (Condie, 1982; Reed et al., 1987; Bowring and Karlstrom, 1990).

Proterozoic basement rocks in the Front Range consist of a complex mélange of quartzofeldspathic gneiss, biotite schist, amphibolite, and migmatite intruded by larger plutonic bodies of granite, granodiorite, and tonalite. Within the metamorphic suite, km-scale metasedimentary and metavolcanic packages are commonly interlayered and were subjected to high-T/low-P upper amphibolite to migmatite conditions at ~1.7 Ga (Kellogg et al., 2004). Most rocks within the Front Range were intensely deformed, recrystallized, and partially melted with the notable exception of the Big Thompson Canyon area, where metamorphic isograds from biotite to migmatite grade are well preserved (Mahan et al., 2013 and references within).

Roughly coeval with ~1.7 Ga metamorphism and deformation in the Yavapai Province, there was extensive plutonism characterized by the intrusion of granodiorite and monzogranite batholiths and associated smaller bodies, referred to as the Routt Plutonic Suite (Tweto, 1987).

Prominent among these intrusions is the Boulder Creek granodiorite, which has a U-Pb zircon age of 1713 ± 4 Ma (Premo et al., 2010).

A second phase of tectonism and intrusive activity affected the region at ~1.4 Ga.

Granitic and monzogranitic plutons of this age (referred to as the Berthoud Plutonic Suite after

Tweto, 1987) are common in the Front Range and include the Mount Evans batholith (U-Pb zircon age of 1442 ± 2 Ma; Aleinikoff et al., 1993), the Silver Plume granite (U-Pb zircon age of

1422 ± 3 Ma; Graubard and Mattison, 1990), and the contemporaneous Longs Peak-St. Vrain batholith (Rb-Sr age of 1423 ± 30 Ma; Peterman et al., 1968). This extensive magmatism, which

occurred throughout the southwestern U.S. at ~1.4 Ga, reset the K-Ar isotopic system in these rocks, with cooling through 350-300 °C after ~1.4 Ga based on 40Ar/39Ar dates on muscovite and biotite (Shaw et al., 1999). Additional evidence for this tectonism in the Proterozoic rocks of the

Front Range is the presence of major ~1.4 Ga shear zones (e.g., Idaho Springs-Ralston SZ,

Moose Mountain SZ; Aleinikoff et al., 1993; Selverstone et al., 2000) and ~1.4 Ga monazite inclusions in porphyroblasts (Shah and Bell, 2012). The cause of this tectonism remains enigmatic, but existing observations suggest that it was a response to widespread thermal perturbation of the North American lithosphere at this time, resulting in crustal melting and accompanied by regional transpressional deformation (Selverstone et al., 1997; Sims and Stein,

2003). The final major rock-forming event in the Proterozoic was emplacement of the anorogenic Pikes Peak batholith at ~1080 Ma in the southern Front Range (Unruh et al., 1995).

There is little to no record of subsequent geologic activity in the region until emplacement of the State Line kimberlite field, a 200 km-long belt of ~600 to 350 Ma kimberlitic intrusions stretching from the northern Front Range to the suture with the Wyoming craton (Lester et al., 2001; Heaman et al., 2003). Thin early Paleozoic continental shelf sequences of quartz-rich sands and carbonates record the presence of shallow seas that covered the region at that time (Kellogg et al., 2004). Tectonic activity resumed in the Pennsylvanian with the formation of the Ancestral Rocky Mountains, a series of intracratonic basement block uplifts in Colorado and the surrounding area that resulted in north-northwest trending mountain ranges and basins (e.g., Mallory, 1958). In Colorado, the location of the Ancestral Rockies uplifts closely resembles that of the Laramide Front Range. These uplifts shed a considerable amount of arkosic clastic sediment, forming the Pennsylvanian Fountain Formation on the east flank of the Front Range and correlative units on the west flank that unconformably overly the

Precambrian basement (Kellogg, 2000). By the middle Jurassic, the Ancestral Rockies had been eroded to low relief and were subsequently buried by fluvial and lacustrine deposits of the

Morrison Formation (Kellogg et al., 2004). In the Late Cretaceous, the combination of major subsidence and sea level rise led to the submergence of the entire region under the western interior seaway, recorded by the shoreline sediments of the Dakota Group followed by over 2 km of marine shales and limestones.

The western interior seaway persisted until ca. 69 Ma (Cobban, 1993; Obradovich, 1993), whereupon it began to withdraw as renewed uplift occurred during the Laramide Orogeny, a ~20 myr period of mountain building, deformation, and igneous activity associated with flat slab subduction at the Pacific-Farallon plate boundary. The modern Front Range uplifted from sea level, with unroofing of over 2 km of sedimentary rock in only a few million years – the time span separating the Late Cretaceous marine deposits of the Pierre Shale and basement-derived conglomerate of the Arapaho Formation (Kellogg et al., 2004). This uplift resulted in more than

6300 m of structural relief between the highest basement rocks of the Front Range and the basement surface below the proximal (foreland) Denver basin (Haun, 1968). The Front Range uplift is bounded on both margins by thrust faults – the low-angle Williams Range thrust on the western margin and the high-angle Golden fault and related structures on the eastern margin

(Weimer and Ray, 1997; Erslev et al., 1999). Concurrent with the start of renewed uplift in the

Front Range was the development of the Colorado Mineral Belt, a northeast-trending series of

Late Cretaceous to Tertiary (68-27 Ma) calc-alkaline to alkaline stocks and dikes that extend from the eastern margin of the Front Range near Boulder to the San Juan Mountains in southwestern Colorado. Major tectonic activity associated with the Laramide orogeny ceased at

~45 Ma (Kellogg et al., 2004).

Following the Laramide Orogeny, a widespread late Eocene erosion surface formed across the Front Range, resulting in a bench-like surface of hilly topography that the high peaks of the current continental divide protruded through (Epis and Chapin, 1975). Intermittent volcanism continued throughout the Paleogene in portions of the Front Range, most notably the

~30-28 Ma Braddock Peak intrusive-volcanic complex exposed in the Never Summer Mountains on the western margin of Rocky Mountain National Park (Marvin et al., 1974). Renewed incision and erosion of the Front Range in late Miocene-Pliocene time (~8-4 Ma, perhaps as early as 19 Ma) led to the development of steep canyons in the eastern Front Range and the deposition of the Ogallala Formation gravels on the High Plains (Steven et al., 1997; Kellogg et al., 2004; McMillan et al, 2006). Gravel-infilled paleovalleys are found in the Front Range, including in the vicinity of Big Thompson Canyon, and are inferred to be Miocene in age and related to erosion during Ogallala deposition (Scott and Taylor, 1986; Cole and Braddock, 2009).

The cause of this episode of incision remains debated; some workers maintain that the Front

Range has been at roughly the same elevation since the Eocene on the basis of paleoelevation studies and link Neogene erosion to climate change (Molnar and England, 1990; Gregory and

Chase, 1992) while others favor an episode of Miocene uplift (up to 1000 m) centered on the Rio

Grande Rift that led to eastward tilting of the Front Range and subsequent canyon cutting (Eaton,

1986; Steven et al., 1997; Naeser et al., 2002; McMillan et al., 2006). The highest portions of the Front Range were extensively modified by repeated episodes of Pleistocene glaciation (e.g.,

Chadwick et al., 1997).

2.2 Big Thompson Canyon

Big Thompson Canyon (BTC) is a ~30 km-long canyon cut into the eastern margin of the

Front Range near Loveland, Colorado. Exposed within the canyon is the Big Thompson

Metamorphic Suite, a collection of layered metasedimentary and metavolcanic rocks that preserve the best (and possibly only) sequence of mappable metamorphic isograds in the

Colorado Rockies (Hutchinson and Braddock, 1987). Supracrustal rocks within the metamorphic suite have detrital components that are interpreted to be variably derived from the

Archean Wyoming craton, the ~1.8 Ga Trans-Hudson orogen, and/or the ~1.78 Ga Green

Mountain arc terrane that accreted to form the northern portion of the Yavapai Province (e.g.,

Reed et al., 1987; Aleinikoff et al., 1993; Reed, 1993; Selverstone et al., 2000; Hill and Bickford,

2001). Within the full sedimentary thickness of the suite, estimated at 3.65 km (Braddock,

1970), the well-developed metamorphic isograds define an arcuate pattern reflecting an increase in metamorphic grade from biotite+chlorite at the eastern mouth of the canyon to migmatite in the west. The presence of low-grade rocks in the eastern portion of the canyon preserves relict, primary sedimentary structures, which is unique amongst Proterozoic basement rocks in the

Front Range – most of which have been completely overprinted by upper amphibolite to migmatite grade metamorphism (Mahan et al., 2013). The maximum depositional age for the

Big Thompson Metamorphic Suite is ~1.76 Ga, the youngest detrital zircon age obtained from these rocks (Selverstone et al., 2000).

Several generations of intrusive rocks are also present in BTC. The first generation, the

~1.7 Ga Routt Plutonic Suite, is exposed in the canyon as the 1713 ± 4 Ma Boulder Creek granodiorite (Premo et al., 2010) and the 1726 ± 15 Ma Palisade tonalite/trondhjemite (Barovich,

1986), also known as the Big Thompson tonalite. The tonalite is exposed in BTC in several km- scale intrusions and numerous m-scale dikes within the supracrustal rocks. The ~1.4 Ga

Berthoud Plutonic Suite is also exposed in the BTC, manifested as plutons of “Silver Plume- type” granite found primarily in the western portion of the canyon (Boos and Boos, 1934; Cole,

1977). Muscovite-bearing granitic pegmatite dikes and sills also occur throughout the Big

Thompson Metamorphic Suite and are thought to be associated with the intrusive suites discussed above, but no definitive field relationships confirm specific associations to either the

1.7 Ga or 1.4 Ga intrusive rocks (Cole and Braddock, 2009). Most of the pegmatites appear largely undeformed and they host a variety of minerals, including garnet, tourmaline, and beryl

(Mahan et al., 2013).

The Big Thompson Metamorphic Suite preserves a record of multiple episodes of deformation and metamorphism. At least three generations of folding and fabric forming affected the suite, including km-scale isoclinal folds (D1) and crenulation cleavages (D2 and D3)

(e.g., O’Connor, 1961; Gawarecki, 1963; Curtin, 1965; Braddock and Cole, 1979; Cole and

Braddock, 2009). Two km-scale ductile shear zones bound the metamorphic suite on its northern and southern margins – the Buckhorn Creek shear zone (Cavosie and Selverstone, 2000) and the

Moose Mountain shear zone (Hodgins, 1997, Selverstone et al., 2000), the latter interpreted to be active at both ~1.7 Ga and ~1.4 Ga. Accompanying this deformational history is a complex metamorphic evolution. Monazite inclusions in porphyroblasts have been dated to ~1.7 Ga and

~1.4 Ga in different textural settings, providing evidence for two major metamorphic events in

BTC (Shah and Bell, 2012) that is further supported by 40Ar/39Ar data from the area (Shaw et al.,

1999). Existing P-T studies in BTC have not been definitive, but suggest conditions of 500-700

°C/0.4-0.6 GPa at ~1.7 Ga and a second P-T loop along a low-P trajectory reaching ~500 °C at

~1.4 Ga (Mahan et al., 2013).

2.3 Rocky Mountain National Park

Rocky Mountain National Park encompasses ~1000 km2 of high alpine terrain in the core of the Colorado Front Range. The majority of rocks exposed in the park are Precambrian basement, predominantly ~1.7 Ga biotite schist/gneiss and ~1.4 Ga Longs Peak-St. Vrain

(“Silver Plume-type”) granite. The metamorphic suite here is entirely migmatite grade, continuing the progression westward from BTC. Small pegmatite units are found throughout the

Park, and are linked to both the 1.7 Ga and 1.4 Ga intrusive events (Cole and Braddock, 2009).

Little of the geologic record from the Proterozoic onwards is preserved within the Park until the

Late Cretaceous (Braddock and Cole, 1990). The modern mountains exposed in the park initially underwent elevation gain during the Laramide Orogeny starting at ~69 Ma. Widespread volcanism occurred on the west side of the Park near the Never Summer Mountains and

Specimen Mountain in the Oligocene (Marvin et al., 1974; Braddock and Cole, 1990), followed by a period of erosion resulting in widespread fluvial deposition of volcanic clasts in the mid-

Miocene (14-11.5 Ma) upper Troublesome Formation (Steven et al., 1997). The park was extensively glaciated in the Pleistocene, resulting in the formation of large glacial valleys, cirques, and moraines (e.g., Madole, 1980).

CHAPTER III

PREVIOUS THERMOCHRONOLOGY IN THE COLORADO ROCKIES

Despite the utility of (U-Th)/He thermochronology for unraveling complex geologic histories, there is relatively little published work of this nature for the Colorado Rockies. There is only one published (U-Th)/He study in the region (AHe in the Gore Range; Landman and

Flowers, 2013), and no published He data from within the Front Range itself. Shaw et al. (1999;

2005) conducted 40Ar/39Ar thermochronology on muscovite, biotite, and hornblende to constrain the higher temperature cooling history of the region (Fig. 1). Muscovite and biotite have

40Ar/39Ar dates of ~1.4 Ga, whereas the hornblende dates are partially reset at this time. This data is consistent with a pervasive, regional, and short-lived thermal pulse reaching 500-550 °C at ~1.4 Ga, roughly coeval with the emplacement of the Longs Peak-St. Vrain granite (Peterman et al., 1968), monazite growth in porphyroblasts (Shah and Bell, 2012), and regional orogenic activity in the southwestern U.S. (e.g., Karlstrom et al., 1997). Clustering of mica data from the region suggests relatively rapid cooling though the temperature sensitivities of muscovite and biotite (~300-400 °C), with apparent cooling rates progressively diminishing westward towards deeper structural levels (Shaw et al., 1999; 2005). An age spectrum collected from microcline indicates slow cooling through the closure temperature range of this mineral (~200 °C) between

~1300 Ma and ~800 Ma (Shaw et al., 1999).

Kelly and Chapin (2004) conducted an apatite fission-track (AFT) study in the Front

Range that included a suite of 16 samples from Big Thompson Canyon and Rocky Mountain

National Park (Fig. 1). AFT dates from this region are between 65 and 45 Ma over an elevation range of ~3 km, which in conjunction with mean track lengths of 13.5-14.7 µm, is indicative of

rapid cooling through ~110 °C at that time. No clear east-west trend in the AFT dates is apparent. Younger AFT ages in the northern Front Range compared to the south are interpreted to reflect the effects of increased thicknesses of the insulating Pierre Shale and greater rock uplift to the north (Kelley and Chapon, 2004). Other AFT studies in the Colorado Rockies (Bryant and

Naeser, 1980; Kelley and Chapin, 1995) largely agree with this picture of Laramide cooling aside from complications associated with the Rio Grande Rift, which has led the Gore Range in central Colorado to record only middle to late Cenozoic exhumation (Naeser et al., 2002;

Landman and Flowers, 2013).

CHAPTER IV

SAMPLES AND METHODS

4.1 Samples

We obtained 29 samples for (U-Th)/He thermochronology from an ~50 km east-west transect across the northern Front Range from the mouth of Big Thompson Canyon (BTC) at the range front (~45 km NNE of Boulder, CO) to the range core along the continental divide in

Rocky Mountain National Park (RMNP) (Fig. 2A), spanning ~2700 m in elevation. Samples were collected from all major rock types in the region, including the ~1.7 Ga Big Thompson tonalite, ~1.7 Ga Boulder Creek granodiorite, ~1.7 Ga biotite schist, ~1.4 Ga Silver Plume granite, and Proterozoic pegmatites. The purpose of collecting samples in this manner was to span a large enough area to discern data trends within different portions of the range and to ensure that rocks with the desired accessory phases for (U-Th)/He dating were obtained.

Thirteen of the 29 samples were collected specifically during the course of this study; all others are from mineral separates provided by Graham Baird and Shari Kelley, the latter of which have published AFT data (Kelley and Chapin, 2004). Apatite He (AHe) and titanite He (THe) data were obtained locally to evaluate the relative temperature sensitivities of these chronometers relative to the ZHe system and provide additional information about the tT history of the Front

Range. Cathodoluminescence (CL) imaging and Raman spectroscopy were used to characterize the radiation damage in 22 representative zircon grains from three samples spanning nearly the full damage range.

4.2 Cathodoluminescence (CL) Imaging and Raman Spectroscopy

CL imaging can be used as a qualitative approach to characterize damage in zircon

Figure 2 (previous page). A) Simplified geologic map of the ~50 km sampling transect in Rocky Mountain National Park (RMNP) and Big Thompson Canyon (BTC). Sample locations are indicated by colored shapes. B) Plot of ZHe date versus eU for range core samples. C) Plot of ZHe date versus eU for the range front samples. D) Plot of ZHe dates versus elevation for analyses with >400 ppm eU (to minimize date-eU effects). E) Plot of ZHe dates versus distance from the range front for analyses with >400 ppm eU (to minimize date-eU effects). All error bars encompass full uncertainties (analytical and alpha-ejection correction).

because lower damage domains luminesce in CL more brightly than higher damage domains

(e.g., Hanchar and Miller, 1992). CL images also help constrain the nature and magnitude of zoning, which can introduce dispersion into ZHe dates (e.g., Hanchar and Miller, 1992;

Guenthner et al., 2013). We acquired CL images at the University of Northern Colorado on a

JEOL JSM-6610LV scanning electron microscope with a Gatan ChromaCL detector. Operating parameters were set at 20kV, a spot size of 69, and a pixel time of 300 µs. Images were collected and processed using Gatan DigitalMicrograph software.

Raman spectra of zircons show systematic changes with increasing metamictization due to the effects of radiation damage on the crystal lattice (e.g., Nasdala et al., 2001). The v3(SiO4)

Raman peak at 1008 cm-1 is the most sensitive portion of the spectra to radiation damage effects.

With increasing radiation damage, this peak will broaden (reflected in an increase in the full width at half-maximum, or FWHM) and a corresponding shift to lower wavenumbers (Nasdala et al., 1995; 1998). Thus, it is possible to use this technique to compare the relative degree of damage between samples.

Raman spectra were collected using a Horiba LabRAM HR Evolution confocal dispersive Raman spectrometer equipped with a 532nm frequency–doubled Nd:YAG laser

(Laser Quantum, Torus 532 + mpc3000) and coupled to an Olympus BXFM optical microscope with a 100x 0.90 NA dry objective lens. A 50 µm confocal pinhole, 1800 lines/mm diffraction grating, and a 1024 x 256 pixel thermoelectrically-cooled CCD detector were used, resulting in

~1 µm lateral spatial resolution and a spectral resolution of 1 cm-1 as measured by the full width

at half maximum of the 585 nm neon emission line. Raman shift calibration was performed daily prior to analysis using the 520.7 cm-1 Raman peak of silicon. We collected spectra from

-1 -1 -1 -1 605 cm to 1087 cm to capture the 1008 cm v3(SiO4) peak and complexities at ~800 cm .

Spectra were collected and processed with LabSpec version 6.3 software (Horiba Scientific).

Two sets of operating parameters were used based on grain characteristics. For small (<150 µm lengthwise) and/or cracked grains, the laser was set to 50% power and 4 seconds per analysis to avoid thermal disturbance of the grains. Large (>150 µm lengthwise), intact grains were analyzed with 100% power and 2 seconds per analysis. A lengthwise transect of Raman analyses was acquired for each zircon grain with a 2 µm step size (60-100 analyses per grain). Resulting spectra were corrected for instrument bias and then baseline corrected using a fifth degree polynomial function. Peaks were fit to the spectra using a Gaussian-Lorentzian fitting function.

Spurious data resulting from analyses taken within holes in the zircon were identified based on their anomalous peak widths and removed from the final dataset.

4.3 (U-Th)/He Thermochronology

Zircon, titanite, and apatite grains were isolated for (U-Th)/He thermochronology using standard mineral separation procedures. Individual mineral crystals were handpicked using a

Leica M165 binocular microscope capable of both reflected and transmitted polarized light and equipped with a calibrated digital camera. Grains were selected for analysis based on crystal form, size, clarity, and the absence of inclusions. Grain dimensions were measured using photographs to calculate an alpha-ejection correction (Ft). After characterization and measurement, all grains were placed into Nb tubes that were then crimped on both ends. Helium measurements were made at the University of Colorado in Boulder (CU). Nb packets containing

the selected grains were loaded into an ASI Alphachron He extraction and measurement line, placed under ultra-high vacuum (~3 X 10-8 torr), and heated with a diode laser to ~800-1100 °C for 5 to 10 minutes to extract the radiogenic 4He. The degassed 4He was then spiked with approximately 13 ncc of pure 3He, cleaned via interaction with two SAES getters, and analyzed on a Balzers PrismaPlus QME 220 quadrupole mass spectrometer. Zircon grains that did not fully degas during the initial laser heating were subjected to reextract steps (15A/10 min) until the resultant 4He /3He ratios were at blank levels. Fully degassed grains were then removed from the line, and taken to a Class 10 clean lab for dissolution.

Apatite grains, still enclosed in the Nb tubes, were placed in 1.5 mL Cetac vials, spiked

235 230 with a U/ Th mixture in HNO3, capped, and baked in a lab oven at 80 °C for 2 hours.

Zircon, titanite, and other more refractory phases were dissolved using Parr large-capacity dissolution vessels in a multi-step acid-vapor dissolution process. Grains (including the Nb tube) were placed in Ludwig-style Savillex vials, spiked with a 235U/230Th mixture, and mixed with

200 ml of Optima grade HF. The vials were then capped, stacked in a 125 mL Teflon liner, placed in a Parr dissolution vessel, and baked at 220 °C for 72 hours. After cooling, the vials were uncapped and dried down on a 90 °C hot plate until dry. The vials then underwent a second round of acid-vapor dissolution, this time with 200 ml of Optima grade HCl in each vial, and baked at 200 °C for 24 hours. Vials were then dried down a second time on a hot plate. Once dry, 200 ml of a 7:1 HNO3:HF mixture was added to each vial, which was subsequently capped and cooked on the hot plate at 90 °C for 4 hours. Following either the apatite or zircon/titanite dissolution process, the samples were diluted with 1 to 3 mL of doubly-deionized water and taken to the ICP-MS lab at the Institute for Arctic and Alpine Research at CU for analysis.

Sample solutions, along with standards and blanks, were analyzed for U, Th, and Sm content using a Thermo Element 2 magnetic sector mass spectrometer.

Once the U, Th, and Sm contents were measured, He dates and all associated data were calculated on a custom spreadsheet made by lab staff. Dimensional masses used to calculate the

U and Th concentrations were derived from the measured grain dimensions. Fish Canyon Tuff zircons and titanites and Durango apatites were analyzed as standards using the same procedures as for our samples. A mineral-specific alpha-ejection correction was applied to ZHe and AHe dates following Farley (2002); THe dates were adjusted using a standard zircon alpha ejection correction with a prismatic geometry.

CHAPTER V

RESULTS

5.1 Zircon Characterization: CL Imaging and Raman Spectroscopy

We acquired CL images and Raman spectra for 22 zircons from 3 samples (Fig. 3). We selected these samples to include igneous zircons at the relatively low (99CP02a, ~100-325 ppm eU), moderate (LR-6, ~325-1750 ppm eU), and high (BT-5, ~1870-3650 ppm eU) portions of the damage spectrum based on the eU concentrations of the zircons analyzed for (U-Th)/He.

Although several samples contained a small population of very low eU (<45 ppm) clear, rounded zircons that were interpreted to be metamorphic on the basis of morphology, we did not image any of these grains because of their rarity in the samples. The brightness of the CL images distinctly differs between the three imaged samples (Figs. 3A-C). As expected, the low to moderate damage zircons (samples 99CP02a and LR-6) luminesce more strongly than the higher damage zircons (sample BT-5). The lower damage zircons display complex zonation patterns that typically are oscillatory, with alternating thin (<5 µm) bands of high and low luminescence.

Several grains have a bright core and dark rim, or vice versa. The high-damage zircons are locally characterized by a bright, thin rim (<1 µm) but are otherwise dim, presumably due to radiation damage suppression of luminescence throughout most of the grain (e.g., Nasdala et al.,

2001). An igneous origin for these zircons is supported by the predominance of oscillatory zoning, their high eU contents (>100 ppm), and strongly metamict (brown, dull, and cracked) appearance.

The Raman data are presented in Figure 3D. Individual spot analyses are plotted as peak position versus width (FWHM), both of which vary predictably with increasing damage. All analyses (n = 1806) are shifted to lower wavenumbers relative to the 1008 cm-1 peak for

undamaged zircon, with a maximum of

1005.5 cm-1 and minimum of 981 cm-1.

Corresponding FWHM measurements

range from 6 cm-1 at high peak positions

to 41 cm-1 at low peak positions. Many

analyses fall along a linear array from

high peaks/small widths to low peaks/

high widths, though a large cluster of

analyses deviates from this trend at

approximately 1003 cm-1, showing an

increase in peak width without a

corresponding decrease in peak position.

Analyses from low damage sample

99CP02a cluster between peak positions

of 999 and 1003 cm-1 with very few

Figure 3. A-C) Representative transmitted light and CL images from the analyzed samples. D) Plot of all Raman data from the three analyzed samples. Individual analyses are plotted in terms of ZrSiO4 peak position versus full width at half-maximum (FWHM). Large arrow indicates that there is increasing radiation damage in analyses moving from the bottom right corner to the top left corner of the plot, corresponding to lower peak positions and wider peaks. Inset: theoretical prediction of how Raman peaks broaden and move to lower wavenumbers with increasing damage (after Nasdala et al., 2001). E) Results of a Raman grain transect (from A to B) are shown for LR-6_r2 (Fig. 3B). Changes in peak position are shown in dark gray, changes in FWHM are shown in light gray.

analyses at peak positions lower that 990 cm-1, consistent with it having the lowest eU values of the analyzed sample suite. Moderate damage sample LR-6 analyses form a well-defined cluster between peak positions of 996 and 1005 cm-1 with only two lower position outliers, indicative of relatively minor differences in damage between different zones within these grains. Analyzed zircons from high-damage sample BT-5 exhibit the widest spread of peak positions and widths with a significant number of analyses at low peak positions (<995 cm-1), consistent with it having the highest eU and most strongly zoned zircons of the analyzed sample suite.

In some cases, the Raman transects across individual grains exhibit significant intra-grain damage variability corresponding to the oscillatory zones that are evident in accompanying CL images (Figs. 3A-C). However, as the results from a transect across LR-6_r2 (a representative zircon from this dataset) show, peak position typically varies by <10 cm-1 and FWHM by 10-15 cm-1 between zones (ignoring the spurious data point in the center of that grain) while those parameters can vary by as much as 30 to 40 cm-1 between different samples (Fig. 3E). U and Th zonation can potentially affect (U-Th)/He dates by causing large discrepancies in 1) He concentration within a grain, 2) the alpha ejection correction (which assumes a homogenous

U+Th distribution) (Hourigan et al., 2005), and 3) He diffusivity through radiation damage effects (Farley et al., 2011). However, significant effects only arise with particular types of pronounced zonation – such as having a very high U core and very low U rim – rather than typical oscillatory zoning, where zoning effects tend to effectively cancel out one another and have no effect on bulk retentivity (Hourigan et al., 2005). Therefore, zonation may be responsible for some second-order dispersion within our ZHe dataset, but given the predominance of oscillatory zoning in the analyzed zircons and the inconsistency of core/rim

zoning where occasionally present, it is unlikely that zonation is systematically biasing the dates older or younger.

5.2 (U-Th)/He Thermochronology

We acquired He data for 121 zircons from 29 samples, 9 apatites from one sample (BT-

5), and 12 titanites from two samples (BT-1 & BT-13) (Table 1). Most aliquots contained single grains, but in a few cases, multigrain aliquots were used for the extremely low eU metamorphic zircons in order to ensure that U and Th contents were above the analytical detection limits. ZHe dates range from 6.7-190.8 Ma across a 0.1-3635 ppm span in eU. AHe and THe dates were only obtained for range front samples, where titanite and inclusion-free apatite were present.

Uncertainties on single grain (U-Th)/He analyses are reported at 1σ, include propagated analytical uncertainties from He and U+Th measurements in addition to an estimated alpha- ejection correction, and are identified as a [±] value in text or by error bars in plots and other figures.

To simplify discussion of this large dataset, we subdivide the results into two groups based on location and distinctly different data patterns for high eU (>400 ppm) zircons (Fig. 2A).

The range core is defined in this study as the area encompassing the 15 samples of Silver Plume granite, three samples of Boulder Creek granodiorite, two samples of pegmatite, and one sample of schist from RMNP and its immediate surroundings in the high elevations of the Front Range.

Of the 91 ZHe analyses from these samples, the 57 zircons with 400-2000 ppm eU yield generally uniform dates, with a mean of 53.7 ± 6.5 Ma (1σ). The results additionally display a relatively sparsely defined initial positive date-eU trend up to ~300 ppm eU (Chapin Peak and

Lumpy Ridge samples), and a sharp negative date-eU trend over the 300-400 ppm eU range

Table 1. (U-Th)/He data Raw Corr Sample* Mass Grains la ra Ftb U Th eUc He date dated 1σe (ug) (um) (um) (ppm) (ppm) (ppm) (nmol/g) (Ma) (Ma) (Ma) Range core: Mount Ida MI-1: granite (432774 E 4474513 N 13T / 3660 m) z2 19.8 1 304.1 74.4 0.84 290.9 39.0 300.1 123.6 75.9 90.1 6.4 z3 12.1 1 202.5 66.2 0.82 351.2 19.2 355.7 135.1 70.1 85.0 6.1 z5 19.0 1 371.5 69.3 0.83 230.3 102.8 254.5 95.0 68.8 82.9 6.0 z6 9.1 1 196.1 60.1 0.81 252.2 94.2 274.3 87.5 58.9 73.0 5.2 MI-2b: schist (432941 E 4474231 N 13T / 3555 m) z2 7.1 1 218.9 52.5 0.78 553.1 78.9 571.7 121.5 39.3 50.3 4.2 z3 3.6 1 154.1 43.0 0.74 217.3 85.0 237.3 71.7 55.8 75.7 10.1 z4 3.4 1 154.3 42.7 0.74 275.2 83.9 294.9 62.5 39.2 53.4 7.3 z5 2.8 1 144.5 39.3 0.72 460.3 43.4 470.5 105.7 41.6 58.0 9.9 z6 7.7 1 179.8 56.9 0.80 1180.0 29.9 1187.0 174.3 27.2 34.1 2.4 MI-4: granite (434398 E 4473940 N 13T / 3367 m) z1 29.7 1 316.7 86.6 0.86 604.4 78.6 622.8 137.1 40.7 47.1 3.3 z2 26.4 1 387.3 79.8 0.85 407.9 113.6 434.6 107.7 45.8 53.7 3.8 z3 7.9 1 230.5 54.5 0.79 1194.3 121.1 1222.8 258.0 39.0 49.5 3.9 z4 13.7 1 201.7 70.2 0.83 313.1 58.9 327.0 89.3 50.4 60.5 4.3 z5 9.4 1 224.7 58.7 0.80 559.6 254.8 619.5 132.6 39.6 49.3 3.5 Range core: Chapin Peak 99CP01: pegmatite (440600 E 4476145 N 13T / 3780 m) z1 19.9 1 228.6 78.4 0.85 199.1 20.9 204.0 88.8 80.2 94.3 6.8 z2 12.0 1 209.4 65.5 0.82 426.1 71.7 442.9 133.5 55.7 67.7 4.8 z3 4.8 1 129.1 48.7 0.77 132.4 5.9 133.8 19.5 27.0 35.2 2.7 z4 3.3 1 152.3 41.7 0.73 643.6 127.3 673.5 143.5 39.4 54.0 7.9 z5 4.5 1 130.8 48.7 0.77 164.3 17.4 168.4 103.4 112.8 147.1 11.4 z6 5.7 1 149.6 52.5 0.78 629.3 107.6 654.6 137.8 38.9 49.8 3.5 99CP02a: granite (438465 E 4476181 N 13T / 3463 m) z1 6.6 1 215.2 50.9 0.77 231.6 55.0 244.6 130.1 97.9 126.3 11.7 z2 4.8 1 197.4 46.0 0.75 258.6 46.0 269.4 100.9 69.1 91.7 11.3 z3 1.2 1 135.9 28.3 0.62 252.4 55.8 265.5 61.2 42.6 68.7 23.4 z4 4.1 1 190.2 43.4 0.74 223.4 56.0 236.6 109.2 85.0 114.9 16.6 z6 2.5 1 161.9 36.6 0.70 306.2 86.9 326.6 58.0 32.8 47.2 10.3 z7 1.1 1 111.1 29.2 0.63 93.0 20.5 97.9 5.0 9.6 15.2 4.7 Range core: Longs Peak 99LP01: granite - Summit (447701 E 4456261 N 13T / 4343 m) z1 4.1 1 232.8 40.9 0.73 371.4 95.1 393.7 213.3 99.7 137.4 25.3 z3 3.5 1 160.5 42.6 0.74 445.5 71.8 462.4 99.3 39.7 54.0 7.7 z4 2.2 1 189.9 33.3 0.67 1059.2 781.1 1242.7 342.1 50.8 76.2 21.1 99LP02: granite - Trough (447358 E 4456239 N 13T / 4121 m) z1 5.9 1 207.5 49.7 0.77 914.9 57.9 928.5 202.4 40.3 52.3 5.1 z3 4.6 1 182.0 46.0 0.75 1362.6 330.6 1440.3 252.5 32.4 43.1 5.1 99LP03: granite - Keyhole (447148 E 4456831 N 13T / 4023 m) z2 1.9 1 138.7 34.2 0.68 1103.1 229.5 1157.0 259.5 41.5 61.2 14.7 99LP04: granite - Granite Pass (448549 E 44584287 N 13T / 3688 m) z1 1.7 1 155.2 31.9 0.66 1341.3 354.5 1424.6 327.4 42.5 64.6 18.5 z2 2.2 1 116.1 37.7 0.70 371.8 46.9 382.9 82.0 39.6 56.2 9.5 99LP05: granite - Battle Mountain (450157 E 4458084 N 13T / 3383 m) z1 4.8 1 190.0 46.7 0.76 655.7 225.0 708.6 204.0 53.2 70.4 8.2 z2 4.9 1 185.7 47.4 0.76 596.5 84.4 616.4 166.2 49.8 65.5 7.0

al - length, r - radius bFt is alpha- ejection correction of Farley et al. (2002) , ceU - effective uranium concentration dcorrected using the alpha-ejection correction e1σ - includes propagated analytical + alpha-ejection correction uncertainty *z - zircon; a - apatite; t - titanite

Table 1 (cont). (U-Th)/He data Raw Corr Sample Mass Grains la ra Ftb U Th eUc He date dated 1σe (ug) (um) (um) (ppm) (ppm) (ppm) (nmol/g) (Ma) (Ma) (Ma) 99LP06: granite - Ranger Station (452828 E 4458014 N 13T / 2835 m) z1 7.6 1 135.8 59.1 0.80 164.9 22.6 170.2 50 .5 54.8 68.2 4.9 z2 2.9 1 140.6 40.2 0.72 361.2 41.1 370.9 75.3 37.6 52.0 8.2 99LP07: granite - Chasm Lake (448673 E 4456659 N 13T / 3599 m) z1 3.4 1 171.3 41.2 0.73 1991.5 306.7 2063.6 362.8 32.5 44.7 7.2 z2 1.6 1 139.1 31.1 0.65 1701.4 75.0 1719.0 328.8 35.4 54.4 15.9 Range core: Lumpy Ridge LR-2: granite (456157 E 4473287 N 13T / 2693 m) z1 7.2 1 244.7 51.4 0.78 1010.9 300.1 1081.5 230.0 39.3 50.6 4.9 z2 12.1 1 209.8 66.3 0.82 333.3 150.1 368.6 120.4 60.3 73.2 5.2 z3 5.5 1 215.1 47.2 0.76 511.5 209.2 560.6 145.1 47.8 63.1 7.4 z4 5.0 1 149.6 49.8 0.77 506.0 194.2 551.7 144.9 48.5 63.1 5.0 z5 8.4 1 243.8 54.8 0.79 456.8 151.0 492.3 124.3 46.7 59.1 4.6 z6 5.5 1 179.7 49.9 0.77 593.1 171.0 633.3 125.2 36.6 47.4 4.2 LR-3: granite (456311 E 4474005 N 13T / 2647 m) z2 5.8 1 181.2 51.0 0.78 875.9 283.0 942.4 174.2 34.2 44.1 3.8 z3 40.2 1 391.7 93.4 0.87 1965.2 236.3 2020.7 434.6 39.8 45.6 3.3 z4 8.4 1 246.8 54.7 0.79 1746.8 368.4 1833.4 388.4 39.2 49.6 3.9 z5 20.0 1 290.4 75.9 0.85 1383.6 291.8 1452.1 317.9 40.5 47.9 3.5 LR-4: granite (456616 E 4474083 N 13T / 2655 m) z2 7.0 1 169.8 55.8 0.79 633.9 214.9 684.4 158.5 42.8 54.0 3.8 z3 4.6 1 176.7 46.2 0.75 857.2 344.2 938.1 198.3 39.1 51.9 5.9 z4 19.2 1 260.9 76.5 0.85 812.2 307.4 884.5 201.7 42.1 49.8 3.5 z5 8.3 1 148.1 60.1 0.81 194.5 91.6 216.0 54.0 46.2 57.3 4.0 z6 4.6 1 189.4 45.9 0.75 1679.5 537.4 1805.8 437.5 44.8 59.6 7.2 LR-6: granite (453308 E 4476033 N 13T / 2700 m) z1 37.4 1 306.1 96.0 0.88 316.7 54.3 329.5 185.8 103.7 118.3 8.7 z2 16.4 1 264.1 73.6 0.84 915.0 119.6 943.1 242.8 47.6 56.6 4.0 z4 14.0 4 266.5 114.6 0.88 0.3 0.2 0.3 0.1 51.6 58.6 6.8 z5 13.3 4 257.9 110.6 0.88 1.5 26.8 7.8 1.2 28.6 32.6 2.3 z6 8.1 1 224.9 54.8 0.79 509.6 57.7 523.1 143.7 50.8 64.2 4.9 z8 8.6 1 256.8 54.2 0.79 1175.7 112.6 1202.2 314.4 48.3 61.3 5.1 z9 17.0 1 270.6 72.1 0.84 700.7 123.1 729.6 206.7 52.4 62.5 4.5 z10 6.0 1 206.9 49.8 0.77 1552.2 815.6 1743.9 266.6 28.3 36.8 3.5 Range core: Upper Big Thompson 96BT01: granite (458558 E 44693470 N 13T / 2300 m) z1 3.3 1 204.1 38.8 0.71 727.0 356.3 810.7 160.1 36.5 51.4 10.5 z2 3.2 1 152.2 41.0 0.72 314.4 148.7 349.3 132.4 69.9 96.5 15.0 z3 3.8 1 132.6 45.2 0.75 621.5 81.9 640.8 142.5 41.1 54.9 5.5 z4 1.8 1 154.1 32.7 0.67 1026.7 224.3 1079.4 153.6 26.3 39.6 10.8 z5 1.4 1 147.6 29.1 0.63 676.5 126.5 706.3 114.3 30.0 47.6 15.8 z6 1.9 1 125.5 34.8 0.68 488.4 252.7 547.8 81.0 27.4 40.2 8.9 96BT02: granite (460218 E 4470188 N 13T / 2250 m) z1 20.4 1 251.6 79.6 0.85 265.9 139.3 298.6 119.7 73.9 86.8 6.3 z2 7.4 1 179.1 55.9 0.79 405.5 174.3 446.5 108.6 44.9 56.7 4.1 z3 9.8 1 176.7 62.3 0.81 236.3 81.6 255.5 81.2 58.7 72.2 5.4 z5 3.8 1 228.7 39.6 0.72 539.9 163.5 578.3 128.8 41.2 57.5 11.5 z6 5.0 1 186.1 47.3 0.76 682.0 80.9 701.0 124.8 33.0 43.4 4.7 z7 3.7 1 180.2 42.1 0.73 366.4 55.2 379.3 289.3 139.8 190.8 29.8 z8 3.9 1 200.5 41.7 0.73 951.4 200.8 998.6 193.2 35.8 49.0 8.2

Table 1 (cont). (U-Th)/He data Raw Corr Sample Mass Grains la ra Ftb U Th eUc He date dated 1σe (ug) (um) (um) (ppm) (ppm) (ppm) (nmol/g) (Ma) (Ma) (Ma) BT-16: granodiorite (467076 E 4475240 N 13T / 2460 m) z1 21.4 1 526 64.5 0.82 991.4 195.4 1037.3 189.2 33.8 41.2 3.0 z2 7.3 1 283.1 49.5 0.77 459.3 26.5 465.6 144.6 57.4 74.5 8.4 z3 5.2 1 186.4 48.2 0.76 606.8 68.9 623.0 134.9 40.0 52.4 5.4 z4 5.8 1 183.2 50.3 0.77 462.4 99.5 485.7 123.2 46.9 60.7 5.3 z5 3.1 1 179.9 39.2 0.71 309.1 51.8 321.3 134.1 76.9 107.6 20.4 BT-17: granodiorite (464587 E 4472504 N 13T / 2152 m) z1 7.9 1 279.8 51.5 0.78 710.4 76.6 728.4 156.5 39.8 51.1 5.1 z2 10.0 1 285.8 56.5 0.80 650.5 41.4 660.3 132.2 37.1 46.5 3.6 z3 9.6 1 341.9 52.7 0.78 1499.9 175.0 1541.0 303.5 36.4 46.6 4.8 z4 12.0 1 401.6 54.6 0.79 990.2 97.1 1013.1 209.4 38.2 48.4 4.5 z5 22.7 1 587.7 63.5 0.82 1799.9 181.7 1842.6 264.3 26.6 32.5 2.3 BT-18: granodiorite (462001 E 4471578 N 13T / 2202 m) z1 12.4 1 344.7 58.5 0.80 614.0 227.8 667.5 243.5 67.3 83.9 6.3 z2 13.7 1 287.8 65.1 0.82 296.3 108.2 321.7 88.6 50.9 62.0 4.5 z3 52.9 1 503.4 98.0 0.88 318.5 129.0 348.8 102.2 54.1 61.6 4.4 z4 37.1 1 488.2 84.8 0.86 586.9 338.6 666.5 185.0 51.3 59.6 4.2 z5 13.8 1 328.7 62.5 0.81 449.9 101.4 473.7 116.0 45.3 55.6 4.0 Range front: Lower Big Thompson 96BT03: tonalite (472970 E 4475515 N 13T / 1900 m) z1 2.4 1 168.7 35.3 0.87 791.7 159.1 829.1 107.1 23.9 27.5 6.6 z2 1.8 1 208.7 29.3 0.63 1691.5 362.2 1776.6 106.0 11.1 17.5 6.1 BT-1: tonalite (482025 E 4475024 N 13T / 1605 m) t1 20.7 1 257.1 78.7 0.86 11.8 10.5 14.2 71.6 828.1 976.0 71.4 t2 13.8 1 239.6 68.4 0.84 41.0 18.2 45.3 63.3 508.9 614.2 44.0 t3 17.2 1 280.7 87.0 0.87 32.7 24.0 38.4 72.0 548.1 635.4 45.3 t4 10.8 1 238.2 68.4 0.84 17.8 16.2 21.7 71.6 726.3 878.4 64.0 t5 14.2 1 248.6 69.5 0.84 8.8 8.5 10.8 48.9 786.2 948.0 68.7 t6 19.4 1 293.4 75.1 0.85 14.0 12.8 17.0 80.4 817.8 971.9 69.5 t7 32.9 1 289.5 99.1 0.89 14.9 13.7 18.1 82.8 795.1 905.1 64.6 t8 17.7 1 264.1 80.5 0.86 25.9 17.0 29.9 97.2 574.6 674.1 48.7 BT-4: tonalite (473292 E 4475518 N 13T / 1784 m) z2 32.0 1 196.6 95.3 0.89 0.6 0.3 0.6 0.6 169.2 190.5 23.3 z4 13.5 1 163.9 72.7 0.85 1.1 0.5 1.3 0.7 106.4 124.7 16.2 z7 2.8 1 146.3 39.7 0.72 1769.4 367.9 1855.9 132.3 13.2 18.4 3.1 z9 10.2 1 160.3 64.4 0.82 3.5 3.6 4.4 2.0 85.7 104.9 10.8 z10 1.6 1 126.6 32.9 0.67 1390.8 227.7 1444.3 74.9 9.6 14.4 3.6 BT-5: tonalite (473810 E 4475525 N 13T / 1784 m) z2 1.2 1 132.2 28.8 0.63 1808.0 264.0 1870.1 121.6 12.1 19.2 6.3 z3 0.8 1 124.4 24.1 0.56 3172.3 1971.3 3635.6 73.8 3.8 6.7 2.8 z5 1.8 1 128.5 34.2 0.68 1887.4 259.7 1948.4 93.2 8.9 13.1 3.0 a3 4.1 1 194.7 59.8 0.73 3.4 1.9 3.8 1.1 48.8 67.1 5.5 a4 2.5 1 166.1 51.0 0.68 13.1 12.7 16.1 5.0 55.1 80.9 6.6 a5 8.0 1 245.1 75.1 0.77 8.0 12.8 11.0 2.3 38.0 49.1 3.5 a6 2.4 1 131.6 52.0 0.69 19.3 17.6 23.5 5.2 40.3 58.4 4.3 a7 2.7 1 196.0 51.7 0.68 11.5 16.9 15.4 3.5 40.9 60.0 5.3 a8 2.5 1 165.2 50.4 0.68 13.0 8.0 14.8 3.7 44.7 65.6 5.8 a9 4.8 1 234.8 61.7 0.73 11.8 9.8 14.1 4.0 51.3 70.1 5.2 a10 2.8 1 145.8 54.1 0.70 5.9 2.9 6.6 1.9 50.9 72.4 5.8 a11 10.8 1 309.6 80.3 0.79 8.3 5.6 9.7 3.2 59.3 75.0 5.5

Table 1 (cont). (U-Th)/He data Raw Corr Sample Mass Grains la ra Ftb U Th eUc He date dated 1σe (ug) (um) (um) (ppm) (ppm) (ppm) (nmol/g) (Ma) (Ma) (Ma) BT-6: tonalite (465979 E 4478529 N 13T / 2120 m) z1 69.6 1 253.2 121.8 0.91 21.0 52.5 33.3 10.8 60.0 66.0 4.7 z2 29.6 1 243.5 92.9 0.88 28.2 68.0 44.2 13.1 54.6 62.1 4.5 z3 24.0 1 189.6 86.0 0.88 12.8 0.4 12.9 4.6 66.2 75.6 5.8 z4 40.5 1 224.5 101.8 0.89 15.5 17.0 19.5 7.9 74.5 83.5 6.2 z5 55.1 1 254.7 113.2 0.90 18.9 85.7 39.0 20.0 94.2 104.6 7.5 z6 1.5 1 145.5 30.3 0.64 1659.6 723.7 1829.7 181.8 18.4 28.7 8.9 z7 0.8 1 140.2 23.8 0.56 1915.6 612.6 2059.6 120.3 10.8 19.3 8.4 BT-9: tonalite (473412 E 4480131 N 13T / 2154 m) z2 9.5 1 267.5 56.2 0.80 733.9 146.4 768.3 214.7 51.6 64.9 4.9 z3 7.3 1 190.1 54.8 0.79 917.7 274.7 982.2 50.3 9.5 12.0 0.9 z4 4.4 1 188.8 44.7 0.75 1496.7 93.7 1518.8 231.0 28.2 37.7 2.7 z5 6.7 1 153.8 54.9 0.79 911.9 452.8 1018.3 49.0 8.9 11.3 0.9 z6 6.4 1 208.9 51.0 0.78 1232.5 340.3 1312.4 78.7 11.1 14.3 1.0 z7 3.1 1 163.4 40.2 0.72 893.4 276.7 958.4 60.3 11.7 16.2 1.2 z8 8.0 1 193.4 57.2 0.80 757.4 225.0 810.3 28.0 6.4 8.0 0.6 BT-13: tonalite (482161 E 4474756 N 13T / 1605 m) z1 5.6 1 194.5 49.2 0.77 1117.7 158.9 1155.0 105.6 17.0 22.1 2.2 z2 10.3 1 254.5 58.7 0.80 800.2 116.4 827.6 74.3 16.7 20.7 1.5 z3 7.9 1 216.8 55.0 0.79 699.2 115.6 726.3 87.6 22.4 28.3 2.1 z4 9.6 1 208.1 60.4 0.81 871.5 70.0 888.0 230.3 47.9 59.3 4.2 z5 8.5 1 190.5 58.0 0.80 831.9 112.8 858.4 100.9 21.8 27.2 1.9 t1 360.7 1 672 210.8 0.95 11.0 9.8 13.3 69.1 894.7 950.0 67.7 t2 63.8 1 460.9 113.3 0.90 22.7 20.9 27.6 128.9 809.8 906.7 64.2 t3 45.0 1 402.7 97.8 0.88 19.6 18.1 23.8 105.8 773.3 881.8 62.8 t4 41.5 1 374.9 96.0 0.88 16.5 14.5 19.9 84.8 742.6 849.0 63.3 BT-15a: tonalite (473365 E 4475322 N 13T / 1784 m) z1 174.7 3 331 - 0.91 0.0 0.0 0.1 0.0 62.4 68.7 5.0

defined by all samples (Fig. 2B). Individual ZHe dates range from 137.4 ± 25.3 Ma to 32.6 ± 2.3

Ma. There is no correlation between ZHe date and elevation within the range core samples (Fig.

2D), as exemplified by an ~1500 m vertical transect from Longs Peak where all but one ZHe date overlap within error (Fig. 4).

We refer to the range front region as encompassing the 8 samples of the ~1.7 Ga Big

Thompson tonalite from the BTC and its immediate surroundings along the eastern margin of the

Front Range. Of the 30 ZHe analyses from this region, those with at eU values from 700-2000 ppm (igneous zircons) show relatively uniform dates, with a mean of 19.8 ± 3.4 Ma (1σ) for 18 zircons, excluding one younger and two older outliers. The results also include a cluster of older

Figure 4. Plot of ZHe date versus elevation for a ~1.5 km vertical transect on Longs Peak in RMNP. Error bars encompass full uncertainties (analytical and alpha-ejection correction). Apatite fission-track (AFT) data on these same seven samples is shown for comparison (from Kelley and Chapin, 2004).

dates at <50 ppm eU from metamorphic zircons, a gap in analyses over the 50-700 ppm eU range, and an isolated younger analysis (6.71 Ma) at very high eU (~3600 ppm). The eU gap

(45-700 ppm eU) for the range front samples in BTC is lithologically controlled (Supp. Fig. 1); the Silver Plume granite, which was sampled extensively in the range core and covers that eU range, is not present in the lower reaches of BTC. There is no correlation between ZHe date and elevation within this sample group (Fig. 2D).

AHe and THe data were also acquired for several range front samples. Nine AHe dates from BT-5 yield a mean date of 66.5 ± 9.6 Ma (1σ), and lack a date-eU correlation (Fig. 5A). In

general, the AHe dates are slightly

older than expected based on

existing AFT data; this may be due

to the presence of small U+Th-

bearing inclusions that do not get

dissolved along with the rest of the

apatite grain, resulting in a

‘parentless’ He problem that can

result in older than expected dates

(Vermeesch et al., 2007). Twelve

individual THe dates from two

samples at the mouth of BTC (BT-1

and BT-13) range from 976.0 ±

71.4 Ma to 614.2 ± 44.0 Ma across

the range of eU values of the

analyzed titanite grains (10-45 ppm)

(Fig. 5B). The three youngest THe

dates are from the highest eU grains

Figure 5. A) Apatite (U-Th)/He (AHe) data from Big Thompson Canyon. AFT and ZHe data from the vicinity is shown for comparison. B) Titanite (U-Th)/He (THe) data from Big Thompson Canyon. All error bars encompass full uncertainties (analytical and alpha-ejection correction). C) Comparison of THe and ZHe data from BT-13, highlighting their significantly different temperature sensitivities due to radiation damage effects.

(≥30 ppm), and there is no correlation between date and grain size.

Figure 6 shows the full Front Range ZHe dataset to facilitate comparison of data between the two regions. At low eU values (<400 ppm), both regions yield ZHe dates that are early

Tertiary and older. In contrast, at high eU values (>400 ppm), the ZHe dates from the range core flatline at ~55 Ma while dates from correspondingly high eU zircons from the range front flatline at ~20 Ma. Plots of ZHe dates versus elevation and distance from the range front indicate that this shift from Miocene to Eocene and older dates occurs between an elevation of 2100-2300 m and a distance of 17-23 km from the range front in the upper reaches of Big Thompson Canyon

(Fig. 2D & 2E). This discrete shift is defined by a sharp break in ZHe dates with no continuous date-distance or date-elevation trend to the east (lower elevations) or west (higher elevations).

Figure 6. Simplified date-eU plot showing all ZHe data acquired in this study.

CHAPTER VI

DISCUSSION

6.1 (U-Th)/He Systematics of High-Damage Zircons

6.1.1 Alpha dose of analyzed zircons

Given the first-order importance of radiation damage to He retentivity in zircon (e.g.,

Guenthner et al., 2013), it is of considerable interest to characterize this damage in our samples.

First, we perform an alpha dose calculation to allow for a quantitative comparison of radiation damage with previous ZHe work. We then employ a more qualitative approach, utilizing observed changes in CL intensity and Raman spectra to investigate the relative degree of damage seen across the range of our analyzed zircons.

The alpha dose is an estimate of the time-integrated accumulation of radiation damage in a sample since it was cooled below the threshold temperature for long-term damage annealing.

For zircon, this calculation is dependent on U and Th concentrations and an estimate of how long it has resided at temperatures below the damage annealing zone. While U and Th concentrations can be precisely measured, there is considerable uncertainty associated with the temperatures at which alpha damage anneals in zircon (Guenthner et al., 2013). At present, the kinetics of zircon fission-track annealing provide the only available approximations to explain damage annealing of any kind in zircon on geologic timescales (e.g., Rahn et al., 2004, Tagami, 2005; Yamada et al., 2007). In the apatite He system, fission-track annealing kinetics have been shown to provide a reasonable proxy for alpha damage annealing (e.g., Flowers et al., 2009; Shuster and Farley,

2009); however, this has not yet been experimentally demonstrated for zircon. Zircon fission- track (ZFT) annealing kinetics suggest that damage anneals between 223 and 310 °C (based on a

fanning curvilinear fit to ZFT data of Yamada et al., 2007 by Guenthner et al., 2013), though annealing temperatures have been shown to be as low as 180 °C in some heavily damaged zircons (Garver et al., 2005). ZFT kinetics were incorporated into the damage-diffusivity model of Guenthner et al. (2013) as the best available estimate for alpha damage annealing.

Alpha doses were calculated using the formulation of Nasdala et al. (2005) (Eq. 1). As discussed above, there is significant uncertainty associated with annealing temperatures and no single value is unanimously accepted. To account for this uncertainty, we selected two end- member threshold temperatures – below which long-term damage annealing did not occur – for our alpha dose calculations. We have chosen 200 °C and 300 °C for these end-member calculations because 1) they are the lower and upper bounds of most annealing temperature estimates and therefore our calculations are minimum and maximum estimates of damage accumulation, respectively, 2) 200 °C is slightly lower than the annealing temperatures used by

Guenthner et al. (2013) to reflect work that suggests highly damaged zircons can have annealing temperatures as low as ~180 °C (Garver et al., 2005), and 3) these temperatures are similar to the closure temperatures for the THe and biotite 40Ar/39Ar systems, which thus can provide the time at which cooling below each potential annealing threshold occurred.

Because the THe system has a nominal closure temperature of ~200 °C based on diffusion experiments on the Fish Canyon titanite (e.g., Reiners and Farley, 1999) and our grains have been relatively unaffected by radiation damage based on their low eU values, we use our

THe dates as a reasonable approximation of cooling to <200 °C – in this case, serving as a lower- bound alpha dose calculation for the reasons mentioned above. Our THe dates range from 1000

Ma to 600 Ma, so a midpoint of 800 Ma was used for this calculation. Biotite 40Ar/39Ar data from this region indicates that most rocks in the region cooled through ~300 °C by 1300 Ma

(Shaw et al., 1999; 2005), so this time was used for the upper-bound alpha dose calculation.

Alpha doses were calculated for representative eU values that span the entire spectrum of our analyzed zircons (1, 100, 1000, 2000, 3635 ppm eU), and are plotted on Figure 7 alongside previous alpha dose calculations for zircons subject to diffusion experiments with calculated closure temperatures (see Fig. 8 in Guenthner et al., 2013).

Figure 7. Plot of alpha dose versus closure temperature for previously conducted diffusion experiments on damaged zircons (circles, squares, and triangles) and samples from this study (vertical gray bars) (adapted from Fig. 8 of Guenthner et al., 2013). Alpha doses were calculated for five representative eU concentrations from our suite of zircons analyzed for (U-Th)/He dating for two end-member annealing temperatures (200 °C and 300 °C).

Zircons analyzed in this study span the entire dosage range of the samples used by

Guenthner et al. (2013) to calibrate their damage-diffusivity model, encompassing five orders of magnitude. The full alpha dose range is from 1015 α/g (for the very low eU metamorphic

zircons) to 1019 α/g (for very high eU igneous zircons) with a range of possible values for each individual calculation based on the annealing temperature used. The maximum damage estimates (using an annealing temperature of 300 °C) are ~70% higher than the minimum damage estimates (using an annealing temperature of 200 °C) (Fig. 7). Importantly, all analyzed grains with >1000 ppm eU from the study area have alpha doses greater than 1018 α/g and fall within the range of the two highest-damage experimental points of Guenthner et al. (2013) but at higher dosages than experimental points from other studies (e.g., Reiners et al., 2002; Reiners et al., 2004; Wolfe and Stockli, 2010) (Fig. 7). Therefore, the zircons analyzed in this study access the relatively unconstrained high-damage portion of the dosage spectrum, and provide some of the first He dates from zircons that are quantitatively constrained to have accumulated that much damage.

Alpha doses calculated in this study are qualitatively supported by other observations of radiation damage. The majority of analyzed zircons are strongly metamict and show conclusive evidence of radiation damage in the Raman analysis. For all ~1800 Raman spot analyses, the primary 1008 cm-1 Raman peak for undamaged zircon is shifted to lower wavenumbers with a corresponding broadening of the peaks, fully consistent with the progressive damage of the zircon crystal structure (Fig. 3D) (Nasdala et al., 1995; 1998). Zircons from sample BT-5 are the most damaged of the three samples analyzed based on the abundance of analyses with low peak positions and correspondingly high FWHM values. This interpretation is corroborated by the high eU values of the BT-5 grains and CL images that display negligible luminescence (Figs.

3A-C). In summary, the Raman and CL data confirm alpha dose calculations that indicate that a large subset of our analyzed zircons is at the high to very high end of the damage spectrum.

Initial applications of the damage-diffusivity model at the low to moderate portion of the

dosage spectrum were presented by Guenthner et al. for a previously uninterpretable dataset from the Longmen Shan in China (2014a) and a complex dataset from Sevier thrust sheets in central

Utah (2014b). The Longmen Shan study included samples with high eU grains (>1000 ppm) that likely spent ~500 myr accumulating radiation damage (Guenthner et al., 2014a). Although alpha doses were not calculated for these samples, they likely attained damage levels comparable to the highly damaged grains analyzed in this study based on the information provided. The authors utilized the thermal history modeling software HeFTy to identify viable tT paths for the high-damage zircons from the Longmen Shan with predicted closure temperatures of ~50 °C.

However, AHe and ZFT dates from the same samples could not be reproduced by those thermal histories given the currently accepted diffusion kinetics (Guenthner et al., 2014a), implying a problem with those kinetics at high damage. The thermal histories in both of these initial applications were unknown, in contrast to this study, where the thermal history is well known and therefore can be used to test the damage model.

6.1.2 Reproducibility and temperature sensitivity of He data from high-damage zircons

To a first order, the damage-induced patterns seen in our ZHe dataset are consistent with what is predicted by the Guenthner et al. (2013) damage-diffusivity model, including a slight positive date-eU correlation at low eU values (Fig. 2B) and a rollover to a sharply negative date- eU correlation at higher eU values (Figs. 2B and 2C). At even higher eU values, this negative trend flattens out and the ZHe dates remain roughly constant despite progressively increasing damage. A similar trend is apparent in the Longmen Shan dataset discussed above (see Fig. 2a in Guenthner et al., 2014a). The mechanism responsible for this particular trend at high damage levels is not explicitly addressed in the current damage-diffusivity model, but may be a product

of rapid cooling through the temperature sensitivities of those high-damage zircons. Within the eU range where the date-eU trend flattens (400-2000 ppm), our data is remarkably reproducible despite the high levels of damage and the significant physical degradation of the grains due to metamictization.

Understanding the temperature sensitivity of the damaged zircons analyzed in this study is critical to the interpretation of the ZHe dates and the evaluation of He systematics at high damage levels. The strongest constraint we have on this sensitivity is the comparison of ZHe dates with AFT and AHe dates from the same area. The AFT and AHe systems are sensitive to temperatures of ~110-70 °C and ~90-40 °C, respectively. In the range front region, the ZHe dates are considerably younger than the ZFT and AHe dates from the immediate vicinity (~20

Ma versus 65-45 Ma; Fig. 5a). Thus, the sensitivity of the ZHe system for these highly damaged grains must be lower than that of the AFT and AHe systems – in the range of 70-40 °C.

This conclusion is also supported by the comparison of the ZHe dates with THe dates from the same sample (Figs. 5B & C). Existing diffusion experiments suggest that the temperature sensitivity of the ZHe system for low to moderately damaged grains is roughly equivalent to that of the THe system (~190-220 °C; Reiners and Farley, 1999; Reiners et al.,

2002). However, the oldest ZHe dates in this region are ~200 Ma while the THe dates range from 600 to 1000 Ma, implying that the temperature sensitivity of the ZHe system, for even the most retentive grains, is still distinctly lower than that of the THe system. At the high damage end, the ZHe system is clearly sensitive to lower temperatures that the THe system, exemplified by the striking juxtaposition of ~20 Ma ZHe dates and 600-1000 Ma THe dates from the same sample in BTC (Fig. 5C).

The undeniable confirmation that this “inversion” of the ZHe and THe/AFT/AHe thermochronometers does occur and is directly linked to high levels of damage accumulation is a fundamental result of this study. This observation suggests that high-damage zircons can be used as a reproducible thermochronometer sensitive to temperatures lower than the AHe system

(i.e., the final few kilometers of exhumation), and encourages us to proceed to test the high- damage end of Guenthner et al.’s damage-diffusivity model.

6.1.3 Testing the high-damage end of the damage-diffusivity model

A new contribution of this work is the ability to explicitly test the high-damage end of the model against a well-constrained tT history (the Late Cretaceous-early Tertiary (Laramide) history of the northern Front Range). For this test, a thermal history simulation was run for the range front samples in lower BTC and vicinity using the HeFTy software. HeFTy is a popular thermal history modeling program that uses a Monte Carlo approach to test a large number of potential tT paths and rule out those that do not fit the inputted data and geologic constraints

(Ketchum, 2005). The program incorporates the kinetics of the zircon damage-diffusivity model of Guenthner et al. (2013) when testing for acceptable tT paths.

Data from the range front samples were separated into three eU bins – 0-500 ppm, 500-

1000 ppm, >1000 ppm – and averaged within those bins to leverage the date-eU relationships observed in the data rather than obscure them. Minimal geologic constraints were imposed upon the model: the age of crystallization of the Big Thompson tonalite (1.7 Ga by U-Pb;

Barovich, 1986), cooling of the tonalite through ~300 °C (1.4 Ga by 40Ar/39Ar; Shaw et al.,

1999), and near-surface temperatures at ~300 Ma associated with a well-known unconformity separating Pennsylvanian sedimentary rocks and the Proterozoic basement (Hubert, 1960;

Mallory, 1972). The time period of interest – Late Cretaceous-early Tertiary (Laramide) to present – was only constrained by maximum temperature of 350 °C (much higher than geologically reasonable to allow the model to explore a wide range of possibilities), a box forcing the model to consider reheating possibilities after the Laramide Orogeny, and surface temperatures at the present day.

The HeFTy simulation of the range front data produced only 155 ‘acceptable’ paths

(those not precluded by the data) and 32 ‘good’ paths (those supported by the data) (Ketchum,

2005) out of 100,000 attempted tT paths (Figs. 8A & B). All ‘good’ tT paths (shown in black) require a pulse of rapid cooling from ~150 °C to near-surface temperatures between 25 and 15

Ma to satisfy the Oligo-Miocene ZHe dates from the range front (Fig. 8B). Additionally, most of the ‘good’ tT paths do not include a pulse of rapid cooling during the Laramide Orogeny (65-45

Ma). These observations are inconsistent with all available geologic and thermochronologic constraints from the Front Range, including: 1) AHe data from this study, 2) AFT data from

Kelley and Chapin (2004), 3) peak temperatures at the top of the basement of ~135 °C in the northern Front Range (Kelley and Chapin, 2004) following burial beneath 2.5-4 km of Mesozoic sediment (Weimer, 1996), 4) initiation of Laramide uplift at ~69 Ma based on the withdrawal of the western interior seaway (Cobban, 1993) and deposition of synorogenic strata (Raynolds,

1997), and 5) the development of a the widespread erosion surface by the late Eocene (Epis and

Chapin, 1975).

All ‘good’ tT paths were exported from the simulation and imported into a HeFTy forward model to predict the AFT and AHe dates for those thermal histories. Since the Laramide history of the Front Range is well constrained, any reasonable tT history for the region must produce 65-45 Ma AFT and AHe dates, as well as satisfy the set of geologic constraints

39 12

discussed above. Figures 8C and 8D

show histograms of the distribution of

AHe and AFT dates predicted by the

HeFTy forward model using the 32

‘good’ tT paths from the thermal history

simulation of the range front data and

incorporating the radiation damage

kinetics of Guenthner et al. (2013). The

mean AHe date predicted by the model is

18.1 Ma with a maximum predicted date

of 24.4 Ma, which differs considerably

from the mean AHe date of 66.5 ± 5.3 Ma

from sample BT-5 in the range front

region (Fig. 8C). Likewise, the mean

AFT date predicted by the model is 23.3

Ma with a maximum predicted date of 27

Ma, which differs considerably from the

mean AFT date of 57.0 ± 6.5 Ma from the

range front region (Kelley and Chapin,

2004; Fig. 8D). These discrepancies are

Figure 8. A) Results of HeFTy simulation for range front samples. ‘Acceptable’ tT paths are shown as light gray lines, ‘good’ tT paths are shown as dark black lines. Model constraints are shown as thick dark gray boxes. B) Close-up of the last 100 myr in the simulation. All ‘good’ paths require rapid cooling from ~150 °C to near-surface temperatures between 25 Ma and 15 Ma. Minimal constraints were imposed on this portion of the model; see text for details. C) Comparison of model AHe dates (predicted by ‘good’ tT paths) with the mean measured AHe date from sample BT-5. D) Comparison of model AFT dates (predicted by ‘good’ tT paths) with the mean measured AFT date from Kelley and Chapin’s (2004) range front samples. 40

indicative of a conflict between the current damage-diffusivity model and the kinetics of these high-damage zircons, as has been cited for the previously discussed example from the Longmen

Shan (Guenthner et al., 2014a). However, in contrast to that example, the model’s preferred tT paths for the high-eU zircon dates for the Front Range conflict with all known geologic constraints from the region in addition to not satisfying the thermochronologic data.

Comparison of our ZHe dates with THe/AFT/AHe dates from the same area indicates that highly damaged zircons (>1018 α/g) are sensitive to temperatures of ~70-40 °C. However, the damage-diffusivity model requires temperatures of ~130-200 °C to reset the highly damaged

BTC zircons that have ~20 Ma ZHe dates, despite the model itself being based upon experimental data that implies closure temperatures of <50 °C for zircons at that dosage level

(Fig. 7). Thus, there appears to be a curious disconnect between the experimental results used to calibrate the model and how the model actually treats those highly damaged zircons. Additional work will be required to properly investigate the nature of these issues at the high-damage end of the spectrum; a likely possibility is that gaps remain in our knowledge of how radiation damage anneals in zircon, and how this annealing affects He diffusivity over geologic timescales. Our results may suggest that annealing of these highly damaged zircons is occurring at even lower temperatures than previously thought, allowing these grains to be more retentive than expected given their dosage levels.

6.2 Geologic Implications

The (U-Th)/He dataset presented here has implications for three portions of the ~1.7 byr geologic history of the Front Range. First, it allows us to gain new insights into the poorly constrained Proterozoic intermediate tT history of the area by utilizing a thermochronometer that

has been little used in the past (THe). Secondly, the relatively well-constrained Late Cretaceous- early Tertiary (Laramide) history of the Front Range provides a framework for evaluating the temperature sensitivity of damaged zircons for the (U-Th)/He system and demonstrates that highly damaged zircons can yield reliable, reproducible, and interpretable results. Lastly, ZHe is applied as a low-temperature thermochronometer to decipher a previously unrecognized reheating event in Oligo-Miocene time in this region.

While the higher temperature (>300 °C) history of the Front Range basement rocks is relatively well constrained through U-Pb and 40Ar/39Ar work, little is known about the intermediate temperature (300-110 °C) history of these rocks. Following cooling through the biotite 40Ar/39Ar closure (~300 °C) at ~1300 Ma (Shaw et al., 1999; 2005), few rocks of subsequent age are preserved other than the ~1080 Ma Pikes Peak batholith, and correspondingly little is known about geologic activity prior to the creation of the Ancestral Rockies in

Pennsylvanian time. Prior to this study, the only intermediate-T chronometer applied to this region was microcline 40Ar/39Ar, which produced a single spectrum from Big Thompson Canyon with a maximum age of 1312 Ma and minimum age of 814 Ma that was interpreted to indicate protracted cooling through ~200 °C during that 500 myr window (Shaw et al., 1999).

The 12 THe dates acquired in the study are Neoproterozoic (~1000 to 600 Ma, Fig. 5B;

Table 1). The effect of radiation damage on helium retentivity in titanite has not previously been identified (Reiners and Farley, 1999; Wolfe and Stockli, 2010; Cherniak and Watson, 2011), although the slight negative date-eU correlation present in our dataset suggests that it may play a minor role (Fig. 5B). If we assume that the effect of radiation damage is indeed minor for these low eU grains (<45 ppm) and that they have a temperature sensitivity similar to that previously estimated for titanite (e.g., Reiners and Farley, 1999; Wolfe and Stockli, 2010), our THe dates

suggest slow cooling of the basement rocks through ~200 °C in the Neoproterozoic. This interpretation is consistent with the 40Ar/39Ar data presented by Shaw et al. (1999), and implies that these rocks were not reheated above ~200 °C more recently in their history. This data therefore precludes the significant burial and reheating of the Front Range basement rocks during the Phanerozoic.

Although the Late Cretaceous-early Tertiary (Laramide) history of the Front Range has been investigated extensively, there are no previously published (U-Th)/He data from the area.

Existing AFT dates from the vicinity of RMNP and BTC are interpreted to record rapid cooling through ~110 °C in the Laramide (65-45 Ma) as the modern range was unroofed (Kelley and

Chapin, 2004). The majority of ZHe dates from the range core, including all dates from high-eU

(>400 ppm) grains, are also Laramide (~55 Ma). A similar spread of AHe dates (80-50 Ma) indicate that the basement rocks cooled rapidly below ~70 °C at this time, and likely to near- surface temperatures based on the Laramide ZHe dates for highly damaged grains. These dates are in agreement with the relatively well-understood history of the Front Range at this time given our previous demonstration that zircon is a reliable low-temperature thermochronometer when its kinetics are strongly influenced by radiation damage.

In contrast to the range core, ZHe dates from the range front (lower two-thirds of BTC, and vicinity) record a distinctly younger event than the Laramide Orogeny – an event that is lower temperature than that accessible to the AFT and AHe chronometers (Fig. 2C).

Importantly, it is only because of the damage-lowered temperature sensitivity of the ZHe system that we unexpectedly detected this younger event. It is likely that basement rocks throughout the northern Front Range, including those in the range front region, cooled to near-surface temperatures by the end of the Laramide Orogeny at ~45 Ma. This is evidenced by the Laramide

dates for highly damaged zircons in the range core and by the presence of a widespread late

Eocene erosion surface (Epis and Chapin, 1975) that unconformably overlies basement rocks in the vicinity of BTC (Scott and Taylor, 1986). Thus, reheating – either through burial and exhumation or proximity to a heat source – and subsequent cooling of the basement rocks in the range front region appears required to produce the observed Oligo-Miocene distribution of ZHe dates in that area. The magnitude of reheating required depends on the temperature sensitivities of these highly damaged zircons, which we showed to be <70 °C on the basis of “inverted” ZHe and AHe dates.

Additional evidence for a reheating event at this time in the Front Range is limited to the observation of shortened track lengths in apatites from samples collected in Poudre Canyon, about 25 km north of BTC along the range front (Fig. 1). The observation of 13 µm track lengths from these apatites were interpreted by Kelley and Chapin (2004) to reflect a reheating event in Oligocene to Miocene time with maximum temperatures of ~80 °C based on thermal history simulations using the program AFTSolve. Similar simulations could not be run using apatites from BTC because their low uranium content resulted in too few confined tracks

(Kelley, per. comm.).

Any plausible explanation for the anomalously young ZHe dates found in the range front region must satisfy both the timing and distribution of the dates – most notably, the lack of young dates in the range core and a sharp transition to young dates in the range front. This transition occurs abruptly at an elevation of ~2150 m in the upper reaches of BTC with no apparent relation to local geologic or geographic features (Fig. 2A & 2D). The discrete nature of this transition is illustrated by the >30 myr difference in ZHe dates between BT-16 and 96BT03, which are less than 5 km apart in BTC. The sharpness of this transition therefore must be

considered a fundamental constraint on the mechanism/event responsible for the observed distribution of ZHe dates. Lithology must be considered as a factor as well; all of the Oligo-

Miocene ZHe dates were acquired from the Big Thompson tonalite while all older dates were acquired from different rock units (Supp. Fig. 1). Thus, a chemical control on ZHe retentivity specific to zircons from the Big Thompson tonalite cannot be explicitly ruled out as the explanation for the Oligo-Miocene dates. However, such a mechanism has not previously been recognized, and we favor alternative geologic explanations.

The distribution of young ZHe dates effectively dismisses any explanation involving elevated regional heat flow due to Cenozoic volcanism because it would be expected to affect the entire study area, not just the range front. Furthermore, most volcanism in the northern Front

Range was small volume and related to the development of the Colorado Mineral Belt, which is too old and spatially restricted to reset the ZHe dates in the vicinity of BTC. Volcanism in the

Oligocene was limited to the Never Summer Mountains on the western margin of RMNP, which was not sufficient to reset the ZHe dates from damaged zircons in the Mount Ida area (<10 km away), let alone dates from the range front (>30 km away). Another set of possible explanations involves mechanisms specific to the canyon itself, such as the incision of the canyon, heating due to hydrothermal activity along major faults that run through the canyon, or some amount of movement on those faults. All of these explanations are explicitly precluded by the consistency of ZHe dates between samples collected within the canyon and one collected high above the canyon but in the immediate vicinity (BT-9; Fig. 2A). The limited relief of the canyon (<500 m) and lack of obvious hydrothermal features or post-Laramide fault scarps provide geologic evidence in support of that conclusion. Lastly, hypotheses involving late Miocene-Pliocene

tilting and uplift of the Front Range (e.g., McMillian et al., 2006) are untenable in this case due to the differences in timing (~5 Ma versus ~20 Ma).

The restriction of young ZHe dates to the range front, their notable absence in the range core, and lack of age-elevation relationships suggests that the reheating event is related to the position of the samples relative to the eastern flank of the Front Range. Our preferred explanation invokes the reburial of the range front under ~1 km of sediment following the

Laramide and subsequent unroofing of the basement rocks in late Oligocene to early Miocene time. Given a surface temperature of ~20 °C and a geothermal gradient of 25-30 °C/km, burial beneath ~1 km of sediment could possibly be enough to reset the highly damaged zircons from

BTC, assuming temperature sensitivities as low as 40-50 °C. Previous work suggests that the

500 to 1000 m escarpment along the east flank of the Front Range was created by the exhumation of the basement from beneath Paleogene sedimentary deposits that buried the flank associated with the development and carving of the late Eocene erosion surface (Leonard and

Langford, 1994; Chapin and Cather, 1994). These sediments are presumed to be derived from the post-Laramide topography of the basement rocks that were peneplained to form the widespread erosion surface (Steven et al., 1997). This overall interpretation is supported by: 1) the preserved remnants of Oligocene and Miocene deposits on the high topography of the Front

Range (Scott and Taylor, 1986; Steven et al., 1997; Cole and Braddock, 2009), 2) the absence of major late Cenozoic alluvial fans along the range front, and 3) the lack of AFT evidence for differential uplift in the late Cenozoic (Chapin and Kelley, 1997).

If one assumes that there was enough sediment available to rebury the basement rocks along the range front and reset their ZHe dates, then the timing of the subsequent unroofing becomes critical to the viability of this explanation. The ZHe dates imply unroofing and ensuing

cooling to <70 °C by ~10 Ma (the youngest ZHe dates here) in the range front region. The basal deposits of Ogallala Formation, found on the plains adjacent to the Front Range in northeastern

Colorado, are age-equivalent and spatially co-located with the unroofing of the sediments that are inferred have buried the range front. Deposition of the Ogallala began as early as 19 Ma and continued until ~5 Ma based on K/Ar dates (Izett, 1975), AFT dates (Naeser et al., 1980), and biostratigraphy (Reeves, 1984; Swinehart et al., 1985). The unit is interpreted to represent sediments that shed off of the Front Range in Miocene time due to its increasing grain size with proximity to the range (Izett, 1975) and a heavy mineral assemblage composed primarily of minerals derived from Proterozoic basement rocks (Sato and Denson, 1967; Denson, 1969).

Thus, the Ogallala may represent the potential remnants of the unroofed range-front sediment in this hypothesis. Most interpretations of the Ogallala invoke the tilting and subsequent erosion of the Front Range in response to the development of the northernmost segment of the Rio Grande

Rift (Easton, 1986; Chapin and Cather, 2004).

Our hypothesis of range-front burial could be tested in the future by determining whether similar ZHe date distributions – characterized by Laramide dates in the range core and younger dates near the range front – exist in other areas of the Front Range. Sampling along east-west sample transects through the major canyons to the north and south of Big Thompson Canyon

(e.g., Poudre Canyon, Boulder Canyon, Clear Creek Canyon) could be done to test whether the observations made in this study constitute a regional signal. However, if the young signal is confined solely to the Big Thompson area, a regional reburial of the range front would appear to be unlikely and more locally focused explanations would gain credence.

In summary, the basement rocks of the Colorado Front Range have experienced a complex, protracted history. Following well-documented tectonism at 1.7 and 1.4 Ga, the

Proterozoic basement cooled through ~200 °C by the end of the Proterozoic (U-Pb, 40Ar/39Ar,

THe). Tectonism was renewed during Pennsylvanian time with the uplift of the Ancestral

Rockies, though temperatures apparently did not exceed ~200 °C prior to this exhumational event based on the THe data. Basement rocks were then buried by up to 4 km of sediment prior to the uplift of the modern Front Range in the Laramide Orogeny. This event was characterized by rapid cooling to near-surface temperatures through the AFT (~110 °C), AHe (~70 °C), and damage-influenced ZHe (<70 °C) temperature windows. Following the Laramide Orogeny, the range core and range front histories diverged, reflected by the presence of Oligo-Miocene ZHe dates in the range front (Fig. 6). The ~20 Ma dates found in the range front region require a reheating event of 30-70 °C in order to reset the ZHe chronometer in heavily damaged grains but not influence the Laramide AHe and AFT dates from the same area. The explanation for this event remains open to interpretation, but likely relates to range core-range front dynamics, possibly a reburial of the range front under sediments shed off of the range core following the

Laramide Orogeny.

CHAPTER VII

CONCLUSIONS

Zircon (U-Th)/He dates from an ~50 km east-west transect across the Colorado Front

Range span the full range of alpha dosages encompassed by previous diffusion experiments on zircon, providing an opportunity to investigate recently parameterized damage-diffusivity relationships for the ZHe system. To a first order, our ZHe dataset supports the damage- diffusivity model proposed by Guenthner et al. (2013). However, at high damage levels, there exists a disconnect between the model-predicted retentivity, model ZHe dates, and known geologic and geochronologic constraints associated with the well-constrained Late Cretaceous- early Tertiary (Laramide) history of the Front Range. While the reason for this disconnect remains unclear, our results convincingly demonstrate that damaged zircons can serve as low- temperature chronometers sensitive to temperatures of <70 °C. The reproducibility of our data from high-damage zircons suggests that this technique holds considerable promise for investigating the final stages of exhumation in lithologies where suitable apatite is not available for AHe dating. The utility of using ZHe as a low-temperature thermochronometer is illustrated by the discovery of a previously unrecognized reheating event along the range front in the Oligo-

Miocene, which stands in contrast to the Laramide ZHe dates from the range core. It is speculated that this reheating event is associated with the preferential reburial and exhumation of the range front during this time period.

SUPPLEMENTARY FIGURES

Supplementary Figure 1. ZHe date-eU plots for the range core and range front with results delineated by individual sample (top) and lithology (bottom).

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