Mechanisms and rate of emplacement of the Granodiorite

Kyle J. Krajewski

Chapel Hill 2019

Approved by: Allen F. Glazner

Introduction

Continental crust is largely composed of high-silica intrusive rocks such as granite, and understanding the mechanisms of granitic pluton emplacement is essential for understanding the formation of continental crust. With its incredible exposure, few places are comparable to the

Tuolumne Intrusive Suite (TIS) in for studying these mechanisms.

The TIS is interpreted as a comagmatic assemblage of concentric intrusions (Bateman and Dodge, 1970). The silica contents of the plutons increase from the margins inwards, transitioning from tonalities to granites, through both abrupt and gradational boundaries

(Bateman and Chappell, 1979; Fig. 1). The age of the TIS also has a distinct trend, with the oldest rocks cropping out at the margins and the youngest toward the center. To accommodate this time variation, the TIS is hypothesized to have formed from multiple pulses of magma

(Bateman and Chappell, 1979) . Understanding the volume, timing, and interaction of these pulses with one another has led to the formation of three main hypotheses to explain the evolution of the TIS. Figure 1. Simplified geologic map of the TIS. Insets of rocks on the right illustrate petrographic variation through the suite.

Bateman and Chappell (1979) hypothesized that as the suite cooled, it solidified from the margins inward. Solidification of the chamber was prolonged by additional pulses of magma.

These pulses expanded the chamber through erosion of the wall rock and breaking through the overlying carapace, creating a ballooning magma chamber. Compositional zoning of the suite occurred through crystal fractionation in the form of preferential nucleation of minerals at the margins of the magma chamber and downward settling. The subsequent enrichment of silica and volatiles in the core magma caused it to ascend and form the more silica-rich granitoids. These mechanisms drove concentric zoning in the TIS. Paterson and Vernon (1995) refuted the idea of a ballooning magma chamber, stating that country-rock deflections indicated that expansion was minimal. Instead, they hypothesized that a large magma chamber formed by ascent of magma diapirs. Each diapir is now represented by an individual pluton in the TIS. Each diapir intruded as an independent event and began to solidify before the next pulse. The next pulse intruded the previous pluton and hollowed it out through stoping and magmatic erosion, leaving the previous pluton only at the margins. Repeating this process produced the concentric pattern, and the compositional variation of the magma is interpreted as a function of changing source composition.

The third hypothesis, based on geochronology and thermal models, treats the TIS as an amalgamation of downward-stacking, domed laccoliths (Coleman et al., 2012). Thermal modeling of a single pulse of magma with the depth and dimensions of the TIS shows that the entire system would drop below 750°C (~50% melt) in 500 Ka (Glazner et al., 2004). This is contradicted by U-Pb geochronology of the TIS, which shows that it formed over a 10 Ma span, with the Khd forming in 4 Ma (Coleman et al., 2004). A model that accommodates this protracted age spectrum is incremental assembly. The emplacement of many small intrusions of magma allows for a longer period of formation because it is not restricted by the cooling time of a large, short-lived magma chamber. These increments are interpreted as downward-stacking laccoliths because the mirror image of the TIS and the outward dips of the contacts between units suggests doming of the formation with subsequent intrusion. Uplift and glacial erosion revealed the concentric map pattern of the TIS exposed today (Fig. 2).

Figure 2. Simple schematic the downward-stacking, domed laccolith model from Coleman et al. 2012. This model uses incremental assembly to better accommodate the geochronological data of the TIS.

The purpose of this study is to revisit old U-Pb analyses and contribute new ages across the Half Dome Granodiorite. Improvements in the methods of thermal ionization mass spectrometry (TIMS) zircon geochronology have improved precision and mitigation of Pb-loss.

Therefore, reanalyzing previous samples that showed significant Pb-loss using modern techniques could significantly improve their ages. New ages can be used to further our understanding of the processes that formed the Khd. For instance, the Khd has evidence of cyclic sheeting near Lake, where sheets have mafic granodiorite at the base and leucogranite at the top. These cycles are interpreted as a series of sheets intruded in a short enough timespan that they shared interconnected melt. Migration of pore melt through these interconnected sheets produces the silicic variation of the cycles (Coleman et al. 2012). With new high-precision of

TIMS analyses, there may be a resolvable difference in age between the sections, if the sheets had a prolonged magmatic history. Finally, with the additional data and the refined ages, we can take a transect across the Khd and calculate an average rate of emplacement for the Khd. This rate can be used to calculate rough timespans in which it took to emplace some of the

Tuolumne’s most famous landmarks, such as Half Dome itself.

Methods

The procedure below is adapted from Beck (2013). Seven whole-rock samples were collected across the Half Dome Granodiorite. Three of these samples were chosen for analysis: one from apron at the outermost contact of the Khd, one from the bottom of a cycle

(Coleman et al., 2012) at , and one from the top of this cycle. One additional sample of zircon separates, HD01-29, was chosen from Coleman et al. (2004) for reanalysis.

Zircons were extracted from whole-rock samples by mechanical separation (jaw crusher, disk mill, and water table). The heavy separates from the water table were run through a Frantz magnetic separator, removing magnetic minerals. Zircons were isolated from the remaining separates using a separation funnel and methylene iodide. Zircons from all samples were selected by hand-picking in ethanol under a binocular microscope. These grains were thermally annealed at 900°C for 48 hours and then chemically abraded in a mixture of HF+ HNO₃ to remove radiation damage and inclusions (Mattinson, 2005). All “HD17KK” samples were abraded at

220°C for 12 hours, whereas HD01-29 was abraded at variable temperatures to ensure that the remaining grains would survive the abrasion process. Fractions 1-17 were chemically abraded at

220°C for 12 hours, whereas fractions 18-20 were abraded at 180°C for 12 hours. After abrasion, grains were immersed in a solution of HCl and HNO₃ and then placed in a sonicator for ultrasonic cleaning. Grains were dissolved with a mixed ²⁰⁵Pb-²³³U-²³⁶U UNC spike and HF+

HNO₃ at 220°C for two to three days. This solution was then dried down to nitrates and converted to chlorides using 6M HCl (Parrish and Krogh, 1987; Krogh, 1973; Parrish, 1987).

Anion-exchange columns were used to isolate U and Pb from these chlorides (Krogh, 1973).

Uranium analyses were loaded onto 99.98% Re filaments, whereas Pb analyses were loaded onto

99.998% Re filaments. Both were loaded using colloidal silica gel.

Mass analyses were performed on the Phoenix X62 thermal ionization mass spectrometer at the University of North Carolina at Chapel Hill. Mass fractionation of U was calculated by measuring the ²³³U/²³⁶U of the spike, whereas fractionation of Pb was estimated to be

0.15%/amu. Uranium was measured on the mass spectrometer as UO₂ to increase ionization efficiency. The assumption was made that the zircons had no initial Pb and all Pb is attributed to lab blank. All fractions were corrected for Th/U disequilibrium (Schmitz and Bowring, 2001).

Samples were corrected using a Th/U ratio of 2.8, an average of the measured ratio of the Khd as reported by Gray, Glazner et al. (2008). Finally, raw data were processed using Tripoli and

Redux (Bowring et al., 2011; McLean et al., 2011).

Results

Four samples across the Khd were analyzed to obtain U-Pb geochronological data and

Th/U data. All analyses are single grain analyses, Th-corrected, and are reported as ranges and weighted mean averages. Analytical results are reported in Table 1.

Two samples, HD17KK-6 and HD01-29, are granodiorite, with abundant plagioclase, quartz and large euhedral hornblende phenocrysts. HD17KK-6 was collected from the outermost, western contact of Khd on Glacier Point apron. Six concordant analyses have a ²⁰⁶Pb - ²³⁸U age range of 93.3-92.7 Ma and a Th/U range of 0.33-0.50. HD01-29 (Coleman et al., 2004) was collected from the top of . It has 8 concordant analyses with a ²⁰⁶Pb - ²³⁸U age range of 92.1-91.0 Ma and a Th/U range of 0.19-0.43.

Sample HD17KK-4 was collected near Tenaya Lake at the base of a cycle. The base of this cycle is a more mafic granodiorite, with a high color index caused by an abundance of euhedral hornblende and biotite phenocrysts . This sample has 10 concordant analyses with a

²⁰⁶Pb - ²³⁸U age range of 101.3-90.2 Ma and a Th/U range of 0.38-0.64.

Sample HD17KK-2 was collected from the top of the same cycle as HD17KK-4 and has a luecogranitic composition. The sample has a very low color index and is instead dominated by quartz, plagioclase and K-feldspar. This sample has 6 concordant analyses, with a ²⁰⁶Pb - ²³⁸U age range of 91.2–87.3 Ma and a Th/U range of 0.45-0.54.

Discussion

Age Spectra Interpretation The age spectra of the samples analyzed have a greater range than the analytical uncertainty of any individual grain analysis. Several different techniques have been developed to interpret such spectra, which have become common as analytical techniques have improved

(Samperton, 2015). We will consider three. One technique is to consider the youngest grain in the spectrum as the time of crystallization, approximating for time of emplacement, and considering all older grains as antecrysts (Schaltegger et al. 2009). The second considers the whole spectrum as the time span for crystallization (von Quadt et al., 2011; Wotzlaw et al.,

2013). The final technique interprets the spectrum as a combination of antecrysts, autocrysts, and analyses that have experienced Pb-loss. This technique considers antecrysts and Pb-loss analyses as geologically insignificant, and takes a subset of the data to be the time of crystallization and emplacement. This subset is commonly a “plateau” or cluster of data points in the spectrum with the same age within uncertainty.

To assist in interpreting which grains are geologically significant, Th/U ratios of the grains were examined. Th/U of zircon can vary at the hand sample scale, due to in-situ magma evolution through time (Samperton 2015). Samperton (2015) found a negative trend within single samples and attributed this to the presence of allanite.

HDKK17-6 This sample has an age range of 93.29-92.65 Ma. The stepwise nature of the age spectrum for this sample makes it difficult to interpret. One could interpret the youngest grain as the time of emplacement at 92.65 ± 0.15 Ma, one could consider all grains as representing cooling time with a weighted average of 92.88 ± 0.07 Ma and a Δt= .64 Ma, or one could choose a cluster of data considered to be significant. Our preferred interpretation is to use all grains to estimate a cooling time due to the inability to confidently select a subset of data (Fig. 3).

Regardless of method of interpretation, this sample yields an age older than all other samples east of it on the western lobe of the Khd. This is consistent with incremental, outside-in, assembly of the pluton.

Figure 3. Weighted mean age plot of sample collected at the margin of the Khd. ²⁰⁶Pb - ²³⁸U ages are plotted. Each bar is an individual zircon grain with uncertainties. Bold gray line is the weighted mean average. Surrounding grayed out area is its corresponding uncertainty.

HD01-29 This sample was chosen for reanalysis because its original date was estimated with discordant analyses and a significant spread in ages. Original analysis yielded an age range of

86-90 Ma, with a cluster of ages around 89.5 Ma. Here we decided one grain that was ~1 Ma younger had suffered Pb-loss because it was not within uncertainty of the majority of the dataset.

Instead we selected 7 grains within uncertainty of each other to calculate the weighted mean age.

Reanalysis of this sample yielded a weighted mean age of 91.93 ± 0.30 Ma (Fig 4). The discrepancy in age between analyses is credited to lead loss, which was better mitigated by the improvement in analytical techniques of zircon geochronology, specifically chemical abrasion.

Original analysis of the sample utilized air abrasion, removing only the outer layer of the grain, while chemical abrasion preferentially removes areas prone to Pb-loss (Mattinson, 2005).

Figure 3. Weighted mean age plot of HD01-29. ²⁰⁶Pb - ²³⁸U ages are plotted. Each bar is an individual zircon grain with uncertainties. Bold gray line is the weighted mean average. Surrounding grayed out area is its corresponding uncertainty. 2004 analysis is plotted in red for comparison.

HDKK17-4 The preferred interpretation of the sample is that the three oldest ages ranging from 93.7-

101 Ma are antecrysts, because they predate all known ages in the Khd. The remaining seven grains have an age range greater than the analytical uncertainty of any single grain. Combining this with the stepwise nature of the data, we cannot confidently select a subset of the data to better represent the age and instead consider all data points when calculating the weighted mean.

This interpretation yields an age of 91.09 ± 0.09 with a Δt = 1.59 Ma (Fig. 5).

Figure 4. Weighted mean age plot of the bottom of the cycle. ²⁰⁶Pb - ²³⁸U ages are plotted. Each bar is an individual zircon grain with uncertainties. Unfilled bars are interpreted antecrysts. Bold gray line is the weighted mean average. Surrounding grayed out area is its corresponding uncertainty.

HDKK17-2 The preferred interpretation of this sample attributes the youngest grain in the sample to

Pb-loss because it is ~3 Ma younger than all other grains and not within uncertainty of any other data point. The remaining grains have a similar stepwise nature as previous samples, prohibiting a confident selection of points to best interpret the age. Considering all data points, the weighted mean is 90.83± 0.09 with a Δt = 0.69 Ma (Fig. 6).

Figure 6. Weighted mean age plot of top of the cycle. ²⁰⁶Pb - ²³⁸U ages are plotted. Each bar is an individual zircon grain with uncertainties. Unfilled bars are interpreted lead loss. Bold gray line is the weighted mean average. Surrounding grayed out area is its corresponding uncertainty.

Implications The above samples add ages across the TIS that are consistent with the model of incremental assembly. Additionally, HD17KK-6 provides a maximum age for the western lobe of the Khd. Its location right on the margin of the pluton provides an upper limit for the age of for the Khd in this area.

HD29-01 is a useful example of how improvements in U-Pb zircon geochronology can enable researchers to produce accurate ages for samples that were previously inconclusive. The first attempt to date this sample was complicated with too many discordant grains and Pb-loss to produce a conclusive age (Coleman et al., 2004), but with modern techniques these issues were alleviated. This suggests that modern techniques could be successful in analyzing samples that were previously too ill-behaved to produce a geologically significant date.

Ages and Th/U of HD17KK-2 and HD17KK-4 are consistent with the pore ascension hypothesis for the formation of the cycles. The cycles observed near Tenaya Lake are interpreted as a collection of sheets that were intruded rapidly enough to have interconnected melt, suggesting they should be related temporally and chemically. The bases of the cycles have a high color index and have a lower silica content. As you move up a cycle, mafic mineral proportion decreases and silica content increases, ending at a leucocratic granite at the top. The tops of the cycles usually have sharp contacts, and occasionally cut into the bases of the previous cycle. This gradation is attributed to the ascent of pore melt through the cycle, leaving early-forming mafic minerals at the base of the cycle and late-forming felsic minerals at the top. As more cycles intruded, doming occurred and the cycles were tilted to their current positions (Coleman et al.,

2012).

Based on this model, we hypothesized that the base of a cycle should be slightly older than the top, and if prolonged enough, may be resolvable with TA-CA-TIMS of zircon. The weighted means of HD17KK-4 and HD17KK-2 are 91.00 ± 0.52 Ma and 90.82 ± 0.32 Ma respectively, making the top and bottom of the cycle the same age within uncertainty (Fig.7). If there is an age difference, then we could not resolve it with current U-Pb zircon geochronology.

This outcome could still fit the pore melt ascension hypothesis, suggesting that zircons were crystallizing in the base and top of the cycle congruently, or that some zircons were transported from the base to the top via pore melt.

Figure 7. Weighted mean age plot of the entire cycle. ²⁰⁶Pb - ²³⁸U ages are plotted. Each bar is an individual zircon grain with uncertainties. Bold gray lines are the weighted mean averages. Surrounding grayed out area are their corresponding uncertainties. The top and bottom of the cycle are the same age within uncertainty.

Another hypothesis based off this model is that as the Th/U of the magma evolves during crystallization, the Th/U in zircons from the bottom and top of the cycle would fall along the same Th/U vs. time trend, with the top of the cycle plotting near end of the trend. This is because the top of the cycle was crystalized from the more evolved melt from the intrusions and so the

Th/U of the zircons from the top should lie on the more evolved end of the trend. The Th/U measured for HD17KK-4 and HD17KK-2 is consistent with this hypothesis (Fig. 8). Th/U from the base of the cycle has a positive trend, and the top of the cycle plots along this same trend, as hypothesized. Notably, the trend is positive contrary to that found by Samperton (2015), who attributed the negative trend in Th/U to the presence of allanite. However, allanite has not been represented in the Khd (Pabst, 1938). Without allanite, Th would remain in the melt while other elements were partitioned into crystals. Zircon also has a stronger affinity for U than Th and so over time the Th/U of grains crystallized from the melt would increase as U in the melt would decrease relative to Th.

Figure 8. Plot of Th/U vs ²⁰⁶Pb - ²³⁸U age for the whole cycle. Each data point represents an individual zircon grain. The bottom of the cycle plots has positive trend as time progress. The top of the cycle plots along the same trend.

Emplacement Rate Using two transects across the Khd, average rates of emplacement for the pluton can be calculated. However, assuming that the incremental assembly model is accurate, the laccoliths are tilted and must be accounted for when calculating a vertical accretion rate. The dips of contacts can be used to estimate the current tilt in the Khd, but they vary based on location in the pluton. In , at Glacier Point, the contact between the Glen Aulin tonalite and the

Half Dome Granodiorite is observed to dip ~45° to the west (Glazner, personal communication,

2019). In the northern sections of the pluton near May Lake, the pluton is much narrower and the dip is steeper. Here the dip of the contact is observed to be ~70° to the west (Glazner, personal communication, 2019).

The total range of ages of the Khd is 92.9-88.8 Ma, a total of 4.1 Ma across the pluton.

The two transects chosen were drawn perpendicular to strike of the contact, and placed close to clusters of data points (Fig. 9, Fig. 10). Transect A was measured to be a lateral distance of ~15.5 km and when adjusted for the 45° dip, yields an average lateral emplacement rate of ~4mm/a and an average vertical accretion rate of just under 3mm/a. Transect B was measured to be a lateral distance of ~5.5 km and when adjusted for the 70° dip, calculated to be an ~5 km vertical distance. Assuming the same time span for the narrower section of the pluton, this gives us an average emplacement rate of ~1.5mm/a. In this scenario, the lateral accretion rate and vertical accretion rate are so similar they are indistinguishable within these rough estimates. These emplacement rates are useful when considering the influx of magma in different sections of the pluton, such as the notable difference between the western lobe of the Khd and its much narrower northern section. These rates can also be used to make novel estimates about the time needed to form notable locations throughout Yosemite. For example, Half Dome proper is roughly 1 km laterally across its north face and can be estimated to have taken 250 Ka to accrete laterally, using the calculated emplacement rate. This formation time broadens our understanding about the park’s biggest attraction and one of the world’s most notable geologic features.

Figure 9. Geologic map of the western lobe of the Khd. Transect A-A’ is drawn close to the southern cluster of data and perpendicular to the strike of the western contact of the Khd. Transect B-B’ is also perpendicular to the contact and is drawn on the northern section of the Khd, where cycles have been observed. Figure 10. Elevation profile of Transect A-A’. Points plotted are samples plotted in Fig. 8. They have been translated down to the transect and plotted at their correct elevation. Dashed lines represent possible contacts between stacked laccoliths. Contacts were chosen to illustrate how the downward-stacked, domed laccoliths model can explain the phenomenon of young ages at low elevation and older ages at high elevation, despite breaking from the general west-east trend in age. Conclusions

U-Pb zircon geochronology continues to support a protracted formation of the TIS and

Khd, suggesting it was formed incrementally. Improvements in analytical techniques of U-Pb zircon geochronology may be presenting new opportunities to successfully date previously troublesome samples and opportunities to understand pluton formation at previously impossible timescales, such as the mafic-felsic cycles in the Khd. Though distinctly different physically, geochronological and Th/U data suggest that they are related and cooled from the same magma, though further research across multiple cycles needs to be conducted to support this data. These improved timescales have also enabled us to estimate the rate at which the Khd was accreted, helping to further our understanding of its formation and the formation of its many notable features.

References

Bateman, P.C., 1992, Plutonism in the central part of the Batholith, :

United States Geological Survey Professional Paper, v. 1483, 186 p. Bateman, P., and Chappell, B., 1979, Crystallization, fractionation, and solidification of the

Tuolumne Intrusive Series, Yosemite National Park, California: Geological Society of

America Bulletin, v. 90, p. 465–482. doi: https://doi.org/10.1130/0016-

7606(1979)90<465:CFASOT>2.0.CO;2

Bateman, P., and Dodge, F., 1970, Variations of major chemical constituents across the central

Sierra Nevada batholith: Geological Society of America Bulletin, v. 81, p. 409-420. doi:

https://doi.org/10.1130/0016-7606(1970)81[409:VOMCCA]2.0.CO;2

Beck, C., 2013, New insights into migration of the Cretaceous Sierran arc using high-precision U-

Pb geochronology [Master’s thesis]: Chapel Hill, University of North Carolina, 56 p.

Bowring, J., McLean, N., and Bowring, S., 2011, Engineering cyber infrastructure for U-Pb

geochronology: Tripoli and U-Pb_Redux: Geochemistry, Geophysics and Geosystems,

v.12, 19 p. doi:10.1029/2010GC003478

Coleman, D., Bartley, J., Glazner, A., and Pardue, M., 2012, Is chemical zonation in plutonic

rocks driven by changes in source magma composition or shallow-crustal

differentiation?: Geosphere, v. 8, p. 1568–1587. doi:

https://doi.org/10.1130/GES00798.1

Coleman, D., Gray, W., and Glazner, A., 2004, Rethinking the emplacement and evolution of

zoned plutons: Geochronologic evidence for incremental assembly of the Tuolumne

Intrusive Suite, California: Geology, v. 32, p. 433-436. doi: 10.1130/G20220.1 Glazner, A., Bartley, J., Coleman, D., Gray, W., and Taylor, R., 2004, Are plutons assembled over

millions of years by amalgamation from small magma chambers?: Geological Society of

America Today, v. 14, p. 4-11. doi: https://doi.org/10.1130/1052-

5173(2004)014<0004:APAOMO>2.0.CO;2

Gray, W., Glazner, A., Coleman, D., and Bartley, J., 2008, Long-term geochemical variability of

the Late Cretaceous Tuolumne Intrusive Suite, central Sierra Nevada, California:

Geological Society, London, Special Publications, v. 304, p. 183-201. doi:

https://doi.org/10.1144/SP304.10

Krogh, T., 1973, A low contamination method for hydrothermal decomposition of zircon and

extraction of U and Pb isotopic age determinations; Geochimica et Cosmochimica Acta,

v. 37, p. 485-494. doi: https://doi.org/10.1016/0016-7037(73)90213-5

Mattinson, J., 2005, Zircon U-Pb chemical abrasion (“CA-TIMS”) method: combined annealing

and multi-step partial dissolution analysis for improved precision and accuracy of zircon

ages: Chemical Geology, v.220, p. 47-66. doi:

https://doi.org/10.1016/j.chemgeo.2005.03.011

Mclean, N., Bowring, J., and Bowring, S., 2011, An algorithm for U-Pb isotope dilution data

reduction and uncertainty propagation: Geochemistry, Geophysics and Geosystems, v.

12, 26 p. doi: 10.1029/2010GC003479

Pabst, A., 1938, Heavy Minerals in the Granitic Rocks of the Yosemite Region: American

Mineralogist, v. 23, p. 46-53. Paterson, S., and Vernon, R., 1995, Bursting the bubble of ballooning plutons: A return to

nested diapirs emplaced by multiple processes: Geological Society of America Bulletin, v.

107, p.1356–1380. doi: https://doi.org/10.1130/0016-

7606(1995)107<1356:BTBOBP>2.3.CO;2

Samperton, K.M., Schoene, B., Cottle, J.M., Keller, C.B., Crowley, J.L., and Schmitz, M.D., 2015,

Magma emplacement, differentiation and cooling in the middle crust: Integrated zircon

geo-chronological-geochemical constraints from the Bergell Intrusion, Central Alps:

Chemical Geology, v. 417, p. 322–340. doi:

https://doi.org/10.1016/j.chemgeo.2015.10.024

Schaltegger, U., Brack, P., Ovtcharova, M., Peytcheva, I., Schoene, B., Stracke, A., Marocchi, M.,

and Bargossi, G.M., 2009, Zircon and titanite recording 1.5 million years of magma

accretion, crystal-lization and initial cooling in a composite pluton (southern Adamello

batholith, northern Italy): Earth and Planetary Science Letters, v. 286, p. 208–218, doi:

https://doi.org /10 .1016/j.epsl.2009.06.028

Schmitz, M., and Bowring, S., U-Pb zircon and titanite systematics of the Fish Canyon Tuff: an

assessment of high-precision U-Pb geochronology and its application to young volcanic

rocks: Geochemica et Cosmochimica Acta, v.65, p. 2571-2587. doi:

https://doi.org/10.1016/S0016-7037(01)00616-0

Stern, T.W., Bateman, P.C., Morgan, B.A., Newell, M.F., and Peck, D.L., 1981, Isotopic U-Pb ages

of zircon from granitoids of the central Sierra Nevada, California: United States

Geological Survey Professional Paper 1185,17 p. Parrish, R., 1987, An improved micro-capsule for zircon dissolution in U-Pb geochronology:

Chemical Geology, v.66, p. 99-102. doi: https://doi.org/10.1016/0168-9622(87)90032-7

Parrish, R., and Krogh, T., 1987, Synthesis and purification of ²⁰⁵Pb for U-Pb geochronology:

Chemical Geology, v. 66, p. 103-110. doi: https://doi.org/10.1016/0168-9622(87)90033-

9 von Quadt, A., Erni, M., Martinek, K., Moll, M., Peytcheva, I., and Heinrich, C.A., 2011, Zircon

crys-tallization and the lifetimes of ore-forming magmatic-hydrothermal systems:

Geology, v. 39, p. 731–734, doi: https://doi.org/10 .1130/G31966 .1

Wotzlaw, J.F., Schaltegger, U., Frick, D.A., Dungan, M.A., Gerdes, A., and Gunther, D., 2013,

Track-ing the evolution of large-volume silicic magma reservoirs from assembly to

supereruption: Geology, v. 41, p. 867–870, doi: https://doi.org/10.1130/G34366 .1