⁴⁰Ar/³⁹Ar and U-Pb geochronological constraints on the thermal and tectonic evolution of the Connemara Caledonides, Western Ireland

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"Ar/Ar and U-Pb Geochronological Constraints on the**

Thermal and Tectonic Evolution of the Connemara

Caledonides, Western Ireland

by

ANKE MARIA FRIEDRICH

M.S. University of Utah (1993)

B.S. University of Utah (1990)

'Vordiplom in Geologie' University of Karlsruhe, Germany (1988)

Submitted to the

Department of Earth, Atmospheric, and Planetary Sciences in Partial Fulfillment of the Requirements for the Degree of

DOCTOR OF PHILOSOPHY

at the

MASSACHUSETTS INSTITUTE OF TECHNOLOGY

September, 1998

I "U

O-'Dpartment of Earth, Atmospheric, and Planetary Sciences

Certified by Ce e bKip V. Hodges Thesis Supervisor Accepted by Ronald Prinn Department Head MASSACHUSETTS INSTJUTE

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"Ar/

9

_{Ar and U-Pb Geochronological Constraints on the}

Thermal and Tectonic Evolution of the Connemara

Caledonides, Western Ireland

ANKE MARIA FRIEDRICH

Submitted to the Department of Earth, Atmospheric and Planetary Sciences at the Massachusetts Institute of Technology on July 15, 1998 in partial fulfillment of the requirements for the degree of Doctor of Philosophy in

Geology

ABSTRACT

The Connemara region of the Irish Caledonides is a classical example of a regional-scale high-temperature metamorphic terrain. Its formation was related to intrusion of a compressional continental magmatic arc, for which a protracted thermal evolution was inferred based on a >75 Ma spread in U-Pb, Rb-Sr, and K-Ar mineral dates. Such a history is inconsistent with field observations which suggest a simple relationship between metamorphism and syntectonic magmatism. This study was designed to explore the significance of the large spread in apparent ages using higher resolution U-Pb and 4_"Ar/ 39_{Ar geochronometers. The results indicate that arc magmatism,} sillimanite-grade metamorphism, anatexis, and late fluid infiltration spanned only about 12 million years. Cooling following the metamorphic peak was actually relatively rapid at 35*C/Ma until about 460 Ma, then 214*C/Ma until 450 Ma. Regional differences in 40Ar/3 9_{Ar cooling ages of >15 Ma are related} to spatial and temporal variations in magmatism, metamorphism, and deformation, rather than differential unroofing of the orogen. 40Ar/ 39Ar dates older than the onset of magmatism or younger than a regional Silurian unconformity represent the combined effects of excess 40_{Ar contamination,}

metasomatism, thermal resetting or alteration related to post-orogenic pluton emplacement. This study shows that geochronologic data must be evaluated in the context of careful field mapping, structural and petrologic analysis.

Geochronological data from Connemara suggest that arc magmatism related to the Grampian orogeny in this region spanned a brief interval between 475 and 462 Ma and was followed by rapid cooling. The oldest recognized Grampian processes included high P/T metamorphism, followed

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by intrusion of the Connemara Gabbros into Dalradian metasedimentary

rocks, regional-scale ductile deformation, and sillimanite-grade meta-morphism between 474.5 and 470.1 Ma. Voluminous -467 Ma quartz diorites only intruded in southern Connemara associated with more localized deformation, anatexis and metasomatism between 468 and 462 Ma. Intrusion of the 462.5 Ma Oughterard Granite marks the end of arc magmatism and contractional deformation at Connemara. The compressional continental magmatic arc at Connemara (the Grampian orogeny) was coeval with continental arc magmatism in Scotland and Newfoundland, and postdates ophiolite formation and obduction along strike in the Appalachian-Caledonian orogen.

Thesis advisor: Dr. Kip V. Hodges Title: Professor of Geology

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ACKNOWLEDGMENTS

Financial Support for this dissertation was provided by the National Science Foundation through a NSF grant awarded to Kip Hodges and Samuel Bowring, a Geological Society of America Student Research Grant, and several SRFC grants from the Department of Earth, Atmospheric and Planetary Sciences. I would like to thank Drs. Cees van Staal, Greg Hirth, Sam Bowring and Kip Hodges for serving on my thesis committee, for reading the entire thesis and for very helpful comments and advice before, during and after the thesis defense.

The research related to this dissertation would have been impossible without the help, logistical support, field guidance, and friendship from many Irish and British colleagues, foremost Bruce Yardley (Leeds University), Paul Ryan (University College Galway), and Barry Long (Irish Geological Survey). The ideas I expressed in this dissertation benefited from lively discussions with Bruce Yardley, Paul Ryan, Bob Cliff, Geoff Tanner, Barry Long, Martin Feely, Bernard Leake, Jack Soper, Cees van Staal and with the MIT crowd including Meg Coleman, Nancy Harris, Audrey Huerta, Wiki Royden, Meg Thompson, Clark Burchfiel, Drew Coleman, David Hawkins, Chris Marone, Mark W. Martin, C.J. Northrup, Bill Olszewski, Jim Van Orman, Mark Schmitz, Gunter Siddiqi and of course Sam Bowring and Kip Hodges.

I would like to thank especially all of my friends in Galway,

Shanaheever, and Leenan for kindly hosting me and my field gear-including several hundred kilograms of rocks, sledge hammers, and a bicycle with no front wheel- and letting me in and out of their houses during any time of the day -and night: Paul Ryan, Martin Feely, Roisin Moran, the Hamiltons' from Lenaan and their Pub, and Frances, Thomas and Martin from the Shannaheever Campground. Thanks to Gunter Siddiqi for helping me explore Connemara during my first week in the field, for discovering the secret of Inishbofin, and for sharing the wettest and foggiest day of this planet on Cashel Hill, the Cashel Pub, the gabbros, and in a far-traveled R&V with homemade German Apfelkuchen.

I would have never ended up at MIT if it hadn't been for a series of larger or smaller coincidences involving Sonja Stotz, Lester Keller, Pat Miller, and Thor Kallerud from the University of Utah Ski Team, John Bartley (Ph.D. MIT 1981) my MS thesis advisor at the University of Utah, the Extensional Faults that overprinted Contractional Faults in Utah, the field-based Tectonics group at MIT that researched Extensional Faults in Contractional Orogens, and a new advisor-Kip Hodges. Among the countless formal and informal interactions with Kip, I enjoyed most the high level of scientific

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communication, advice, editorial work and last but not least the camaraderie. Thanks Kip!

The U-Pb lab at MIT, in short Sams' Lab, certainly is the scientific and social center of the 11th floor. Foremost, I would like to thank Sam Bowring for being a nonstop scientific inspiration by suggesting at least one Ph.D.-scale research project per day ("aren't you done yet?"); for instant judgment of my own ideas ("holy snipe"); for believing in the impossible ("those heat-lamps work 24 hours a day ...Let's do it!"). Thanks a lot to Kathy Davidek ala Keefe for teaching me the ropes of mineral processing and U-Pb clean lab techniques, and for knowing where all the tools are hiding in the lab. Pat Walsh deserves a special Dankeschdn for reducing MIT bureaucracy hurdles. Bill Olszewski was a most thorough, patient and forgiving teacher of Argon isotopic matters ("tape on the planchette-oh, well, you'll have to remove it. It would be best to start over..."). Drew Coleman and Mark Martin are two fine scientists who differ most in their musical choices for lab entertainment.

My fellow graduate students, the 1994 Wellesly field camp crowd

(especially Tracy Johnston), the Women of 2nd & 3rd west in McCormick Hall, the 'housemasters' Kathy Hess and Charles Stewart, and the MIT Ski Team contributed most to making my MIT experience a very pleasant and special one. I will always remember the IAP field camp fires; BlackForest Ham in Las Vegas, Kittery Shopping trips, Pizza dinners at the housemasters'; the many hours of stimulating scientific and less scientific discussions at the CJ-(Northrup)-and-DAVER-(Hawkins) Memorial Blackboard; watching the sunrise from the 11th floor because you're still there; soccer on the astro turf at 11:30 pm; Toscaninis' icecream. CJ Northrup, David Hawkins, Dawn Summer, Mousumi Roy, Steve Karner, Gunter Siddiqi, Nancy Harris, Jim VanOrman, Steve Parman, Erik Kirby, Mark Schmitz, Kirsten Nicolaysen, and new and old fellow Kip-mates Dave Applegate, Martha House, Meg Coleman, Audrey Huerta, Jose Hurtado, Arthur White, Julie Baldwin, Karen Viskupic (notice a new pattern?), thanks for sharing these moments! I am especially glad that Nancy 's and Audrey's final thesis push overlapped with mine!

I would like to announce that the Anke-McCormickHall-Boston-Hilton Hotel and Restaurant service is now closed. Please check all your belongings. A new hotel will soon open in another part of this hemisphere. I thank Kim Olsen, the founding father of the Boston-Hilton chain and Powerbar-Cafe, and all the customers over the years, Chris Carlson, Ebbe Hartz, Meg Coleman, David Hawkins, Kirsten Nicolayson, Audrey Huerta, Kalsoum Abassi, Reiner and Annette Haus and many more.

Thanks to Barb and Dave Noyse from Spruce Head Island, Maine, and Julchen, Lisa, Annette and Reiner Haus, for adopting me into their families. Finally I would like to thank my brother Heinz and my mother Uschi for their unconditional support and love without which I could have never stayed away from home for so long.

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To the Memory of My Dad

Dr. med. Kurt Friedrich

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CHAPTER 1: INTRODUCTION

Insight into the processes responsible for formation of the continental lithosphere can be gained by understanding the thermal evolution of continental crust. In stable continental regions heat transfer occurs mainly by conduction, but advective processes, such as faulting, plutonism, and fluid circulation, can result in high transient geothermal gradients in tectonically active areas (e.g. DeYoreo et al. 1991, Huerta et al. 1996). Such gradients are recognized today in metamorphic terrains through quantitative studies of pressure-temperature paths (e.g. England and Thompson 1984, Hodges 1991). Information about the timing of metamorphic events and the successive cooling history is of first-order importance in determining the thermal evolution of the crust and inferring rates of tectonic and surficial processes (Thompson & England 1984, Royden & Hodges 1984). Knowledge of the timing, intensity, and duration of events responsible for elevated transient geothermal gradients, however, depends on our ability to extract meaningful thermochronologic information from metamorphic terrains (e.g. Zeitler

1989).

40_Ar/3 9_{Ar and U-Pb geochronology of metamorphic minerals provide} powerful tools to track the thermal evolution of orogenic belts (McDougall & Harrison 1988, Heaman & Parrish 1991). Combined, these two methods can be used to reconstruct large portions of the thermal history of an orogenic belt between 760*C (the nominal U-Pb closure temperature for zircon) to roughly

170*C (4_Ar/39_{Ar in K-feldspar). As a result of recent improvements in}

analytical techniques, it is now possible to obtain U-Pb and 4_"Ar/ 3 9_{Ar dates} with an analytical uncertainty of less than 0.3% and 1% for most samples, respectively.

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However, even the most precise dates are not necessarily geologically meaningful. In some cases, excess "Ar or inherited acessory minerals can thoroughly complicate the interpretation of geochronologic data. Even if such factors are not important, the exact significance of a date depends on how a particular mineral-isotopic system responds to the changing thermal structure in an orogen.

Open-system behavior in mineral-isotopic systems is usually regarded as a thermally activated volume diffusion process (e.g. Everenden et al. 1960, Hart 1964, Hansen & Gast 1967, cf. Lee 1993). Thus, dates should correspond to the temperatures below which diffusive loss of the radiogenic daughter becomes effectively insignificant (Dodson 1973). Closure temperatures are different for each mineral-isotopic system because the diffusivity of the radiogenic daughter isotope depends on the crystal-chemistry of a mineral

(e.g. Giletti 1974a). Determining these parameters is difficult because, in natural samples, the diffusivity of the radiogenic daughter varies even within

a single mineral-isotopic system due to and differences in composition and effective diffusion dimension (Giletti 1974b, Harrison et al. 1985, Mezger et al.

1991, Scaillet et al. 1992). Variations in each of these parameters can change

the closure temperature of a mineral by up to 100*C (Harrison et al. 1985, Hames & Bowring 1994). Experimentally determined diffusion parameters are only available for a limited range of compositions and diffusion dimensions (e.g. Giletti 1974b, Harrison 1981, Harrison et al. 1985, Baldwin et

al. 1990, Foland 1993, Cherniak 1993); inappropriate application of these

parameters can lead to gross misinterpretation of geochronologic dates. Until a complete experimental database becomes available, these factors are best determined empirically in slowly cooled terranes, where the range in ages is

large enough to be resolvable by radiometric analysis.

The Connemara region of western Ireland was thought to be an ideal location to understand better factors that control ages in a slowly cooled metamorphic terrane. Slow cooling of Connemara had been assumed based on reconnaissance geochronologic data suggesting a >100 million year history

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of intrusive activity and subsequent cooling (e.g. Elias et al. 1988, Miller et al.

1991, Cliff et. al 1996). At Connemara, most of the commonly used

chronometers can be sampled in a variety of lithologies and metamorphic grade ranging from the upper greenschist to upper amphibolite facies. Each metamorphic zone experienced a distinct thermal history that can be evaluated as a function of composition and grain size.

The first part of my dissertation was designed to test the assumption that Connemara is a slowly (<5*C/Ma) cooled terrane by determining the age and duration of magmatism and high-temperature (>700*C) metamorphism (Chapter 2). The results show that magmatism and metamorphism occurred between 474 and 463 Ma. Cooling of the high-grade terrane from >600*C to <

200*C is restricted to between ~470 Ma, the timing of peak metamorphism,

and ~443 Ma, the age of the Silurian unconformity. Therefore, the thermal pulse associated with the Grampian orogeny lasted roughly 25 Ma, a far shorter interval than previously assumed (Chapter 3). The large spread in mineral-isotopic ages observed at Connemara cannot be explained by protracted cooling following a single orogenic event.

The second part of my dissertation was aimed at further determining the significance of the spread in published dates studies (e.g. Elias et al. 1988, Miller et al. 1991) using 40_Ar/ 39_{Ar thermochronology. The advantages of}

40_{Ar /}39_{Ar thermochronology over conventional thermochronologic methods} (K-Ar, Rb/Sr) are that (1) measurements of the parent and radiogenic daughter are made during a single analysis, resulting in a higher analytical precision and allowing in-situ intragranular analyses, and (2) that the presence of any excess radiogenic daughter isotope (excess 40Ar) can be recognized by incremental heating of the sample by a resistance furnace or a laser. My sampling focused on muscovite-, biotite-, and phlogopite-bearing metamorphic rocks from the garnet, staurolite, and sillimanite metamorphic zones, as well as from anatectic metasedimentary rocks and syn-and post-orogenic magmatic rocks. To place limits on the permissible range of 4_"Ar/ 39_{Ar (and therefore K-Ar and Rb/Sr) cooling ages, I combined}4_"Ar/ 39_Ar

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thermochronological results with geological field observations, thin section and electron microprobe analyses, high-precision U-Pb dating (Chapters 2, 3and 5), and results from paleomagnetic studies (Morris & Tanner 1977, Robertson 1984; Chapter 5).

The results show that the distribution of 4_"Ar/ 39_{Ar cooling ages is very} different from that observed in previous geochronologic studies (e.g. Elias e t

al. 1988). The large spread in ages (>50 Ma) appears to be restricted to areas

that have experienced the effects of post-orogenic magmatism (the Galway batholith) or brittle faulting (the Renvyle Bofin Slide). A much narrower age range (< 10 Ma) occurs in subareas of the metamorphic zones in the Dalradian fold and thrust belt. Most of this variation in ages is consistent with the different closure temperature for different minerals. The largest difference in cooling ages for a single mineral-isotopic system (c. 20Ma) occurs between the lowest (garnet) and the highest metamorphic ('migmatite') zone of northern and southern Connemara, respectively. On a scale of < 5 Ma, some of the

4 0_Ar/ 39_{Ar cooling ages appear to vary at random for minerals with similar}

compositions and grain sizes, indicating that the limit of resolution for 4 0_Ar/39_{Ar data may be c. 5 Ma (Chapter 4).}

This dissertation consists of four chapters that have been written for publication in international geologic journals. Chapter 2 presents new U-Pb geochronologic constraints on the age of the oldest and youngest intrusions of the continental magmatic arc at Connemara. The results show that the continental magmatic arc at Connemara was much younger and shorter lived that known previously, and help to resolve a long-standing controversy about the age and significance of the Grampian orogeny. This paper has been submitted to GEOLOGY with Samuel Bowring, Mark W. Martin, and Kip Hodges as co-authors. Chapter 3 presents additional U-Pb ages for

intermediate magmatism and coeval metamorphism in southern

Connemara, as well as 40_Ar/ 39_{Ar data which shows that Connemara was not}

a slowly cooled terrane. All high-precision geochronologic dates are remarkably consistent with their relative ages as inferred from field

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relationships. This paper was presented as an abstract at a "Discussion meeting on the Caledonides" in Glasgow, Scotland (September 9 & 10, 1997), and has been accepted for publication in the JOURNAL OF THE GEOLOGICAL SOCIETY OF LONDON; the co-authors are Kip Hodges, Samuel Bowring, and Mark W. Martin. Chapter 5 contains the main portion of the 4_"Ar/ 39_{Ar data obtained during the course of this dissertation. These}

data and their interpretation focused on the thermal history related to and immediately following emplacement of the Connemara magmatic arc, although the Connemara region preserved a long record of 4_"Ar/3 9_{Ar data} that are not related to cooling following the amphibolite-facies metamorphism recorded in the Dalradian country rocks. This chapter is co-authored by Kip Hodges and will be submitted for publication to TECTONICS. Chapter 4 is part of an attempt to determine the timing of peak metamorphism at each metamorphic grade. This chapter contains the preliminary results of U-Pb titanite analyses from calcsilicate rocks from all metamorphic grades. Some of the U-Pb titanite results for the staurolite and sillimanite zone titanites are surprisingly young relative to the expected age of peak metamorphism in northern Connemara, but are of the same age as the 462 Ma U-Pb titanite dates for the highest metamorphic zones of southern Connemara. We infer that these U-Pb titanite dates record the timing of fluid infiltration which occurred post peak metamorphism at the low metamorphic zones. This chapter represents research in progress, and will be complemented by U-Pb monazite data before submission for publication to the JOURNAL OF METAMORPHIC PETROLOGY, with co-authors Samuel Bowring and Kip Hodges.

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References

Baldwin, S. L., Harrison, T. M., & Fitz Gerald,

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D. 1990. Diffusion of 4_{"Ar in} metamorphic hornblende: Contributions to Mineralogy and Petrology, 105,

691.

DeYoreo,

J.,

Lux, D. R., & Guidotti, C. V. 1991. Thermal modeling in low-pressure/high-temperature metamorphic belts: Tectonophysics, 188, 209-238. Dodson, M. H., 1973, Closure temperature in cooling geochronological and petrological systems: : Contributions to Mineralogy and Petrology,40, 259-274. Elias, E. M., Macintyre, R. M., & Leake, B. E. 1988. The cooling history of Connemara, western Ireland, from K-Ar and Rb-Sr age studies: Journal of

the Geological Society, London, 145, 649-660.

England, P. C., and Thompson, A. B. 1984. Pressure-temperature-time paths of regional metamorphism I. Heat transfer during the evolution of regions of thickened continental crust: Journal of Petrology, 25, 894-928.

Evernden,

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F., Curtis, G. H., Kistler, R., & Obradovich,

J.

1960. Argon

diffusion on glauconite, microcline, sanidine, leucite, and phlogopite:

American Journal of Science, 258, 583-604.

Foland, K. A. 1993. Argon diffusion in feldspars. In: Parson, I. (ed), Feldspars

and their reactions: NATO ASI series, 421, 415-444.

Giletti, B.

J.

1974a. Diffusion related to geochronology. In: Hofmann, A. W., Giletti, B.

J.,

Yoder, H. S., & Yund, R. A. (eds), Geochemical Transport and

Kinetics. Washington, Carnegie Institute of Washington Publication, 61-76.

1974b, Studies in diffusion I: Argon in phlogopite mica. In:

Hofmann, A. W., Giletti, B.

J.,

Yoder, H. S., and Yund, R. A. (eds),

Geochemical Transport and Kinetics: Washington, Carnegie Institute of

Washington Publication, 107-115.

Hames, W. E., & Bowring, S. A. 1994. An empirical evaluation of the argon diffusion geometry in muscovite: Earth and Planetary Science Letters, 124,

161-167.

Hanson, G. N., & Gast, P. W. 1967. Kinetic studies in contact metamorphic

zones: Geochimica et Cosmochimica Acta, 31, 1119-1153.

Harrison, T. M. 1981. Diffusion of 40_{Ar in hornblende Contributions to}

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Duncan, I., & McDougall, I. 1985. Diffusion of 4_{"Ar in biotite:}

Temperature, pressure, and compositional effects. Geochimica et Cosmochimica Acta,49, 2461-2468.

Hart, R. S. 1964. The petrology and isotopic mineral age relations of a contact zone in the Front Range, Colorado: Journal of Geology, 72, 493-525.

Heaman, L., Parrish, R., 1990, U-Pb geochronology of accessory minerals. In: . Heaman, L. & Ludden,

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N. (eds), Applications of radiogenic isotope systems

to problems in geology. Mineralogical Association of Canada Short Course

Handbook, 59-102.

Hodges, K. V. 1991. Pressure-temperature-time paths. Annual Reviews of

Earth and Planetary Sciences, 19, 207-236.

& Bowring, S. A. 1995. 4_4Ar_/3 9_{Ar laser microprobe studies of slowly} cooled biotites: Insights from the Southwestern U.S. Proterozoic Orogen.

Geochimica et Cosmochimica Acta, 59, 3205-3220.

Huerta, A. D., Royden, L. H. & Hodges, K. V. 1996. The Interdependence of Deformational and Thermal Processes in Mountain Belts. Science, 273,

637-639.

Kelley, S. P., Arnaud, N. 0., and Turner, S. P. 1994. High spatial resolution 40_{Ar /}39_{Ar investigations using an ultra-violet laser probe extraction} technique. Geochimica et Cosmochimica Acta, 58, 3519-3525.

Leake, B. E., & Tanner P. W. G. 1994. The Geology of the Dalradian and associated rocks of Connemara, western Ireland. Royal Irish Academy,

Dublin, 96 pp.

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Mezger, K., Rawnsley, C. M., Bohlen, S. R., & Hanson, G. N. 1991. U-Pb garnet, titanite, monazite, and rutile ages: Implications for the duration of high-grade metamorphism and cooling histories, Adirondack Mtns., New York: Journal of Geology, 99, 415-428.

Miller, W. M., Fallick, A. E., Leake, B. E., Macintyre, R. M., & Jenkin, G. R. T.

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Connemara, western Ireland. Journal of the Geological Society of London,

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Robertson, D.

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1988. Paleomagnetism of the Connemara gabbros, western

Ireland. Geophysical Journal of the Royal Astronomical Society, 94, 51-64. Scaillet, S., Feraud, G., Ballevre, M. & Amouric, M. 1992. Mg/Fe and [(Mg,Fe)Si-A121 compositional control on argon behavior in high-pressure

white micas: A 4Ar /39_{Ar continuous laser-probe study from the Dora-Maira}

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56, 2851-2872.

Thompson, A. B. & England, P. C. 1984. Pressure-temperature-time paths of regional metamorphism II: Their inference and interpretation using mineral assemblages in metamorphic rocks. Journal of Petrology, 25, 929-954.

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S., Cliff, R. A., & Yardley, B. W. D. (eds), Evolution of metamorphic

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CHAPTER 2: A SHORT-LIVED CONTINENTAL MAGMATIC ARC

AT

CONNEMARA,

WESTERN

IRISH

CALEDONIDES:

IMPLICATIONS FOR THE AGE OF THE GRAMPIAN OROGENY.

Anke M. Friedrich, Samuel A. Bowring, Mark W. Martin, and Kip V. Hodges

submitted to Geology, March, 1998.

2.1. Abstract

New U-Pb data from the Connemara region of Ireland indicate that continental arc magmatism along the southern margin of Laurentia was short-lived, between c. 475 and 463 Ma. Field mapping demonstrates that intrusive activity at Connemara was synchronous with Grampian mid-crustal deformation and upper amphibolite-facies metamorphism. U-Pb zircon analyses indicate that the age of the two oldest intrusions, the Currywongaun and Cashel-Lough Wheelaun gabbros, are 474.5 ± 1.0 Ma and 470.1 ± 1.4 Ma, respectively. U-Pb analyses of xenotime from the postdeformational Oughterard granite constrain crystallization to 462.5 ± 1.2 Ma. The implied age of the Grampian orogeny at Connemara is substantially younger than generally acknowledged, but consistent with other age constraints for the 'evolution of the Laurentian-Iapetus plate boundary. Development of the 475-463 Ma magmatic arc at Connemara postdates Tremadoc-Early Arenig ophiolite obduction, and was broadly coeval with arc magmatism along strike of the Appalachian-Caledonian orogen. This implies that subduction beneath the Laurentian margin from the Appalachians to Scotland did not develop until after ophiolite obduction. The new data from Connemara imply that this subduction polarity reversal occurred at or just prior to 475 Ma, and was much less diachronous along strike than previously assumed. Major Laurentia-ward subduction did not develop until after the Grampian-Taconian orogeny, and is not recorded in the Grampian orogen, where plate convergence after c. 460 Ma was accommodated by strike-slip faulting.

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2.2. Introduction

The Appalachian-Caledonian orogen records destruction of the Laurentian passive margin during Iapetus ocean closure. The earliest orogenic events include widespread ophiolite obduction and accretion of oceanic terranes, which were followed by syn-tectonic arc magmatism in the Laurentian margin (e.g. van Staal et al. 1998). In the British and Irish Caledonides these events are attributed to the Grampian orogeny, which correlates with the Taconian/Humberian orogeny of the northern Appalachians (Figure 2.1A; e.g. Lambert and McKerrow 1976; Cawood et al.

1995; Karabinos et al. 1998, Swinden et al. 1997, Whalen et al. 1997). The

Grampian orogeny is recorded in Laurentian margin rocks (the

Neoproterozoic Dalradian Supergroup) by arc magmatism, contractional deformation, and metamorphism, but also by turbidite sequences, ophiolites and accreted island-arcs (Ryan and Dewey 1991). Correlating Grampian events between these two tectonic settings is difficult, partly due to later fragmentation of the Laurentian margin by transcurrent faulting, and requires knowledge of the precise timing of these events (Figure 2.1B; e.g. Hutton 1987). The timing of Humberian/Grampian ophiolite obduction and terrane accretion in Newfoundland and western Ireland is well established (e.g. Ryan and Dewey 1991, Cawood et al. 1995). A Late Cambrian to Lower Ordovician (c. 515 to 475 Ma) oceanic island-arc, which formed above an oceanward-dipping subduction zone, collided with the Laurentian margin in late Arenig/early Llanvirn time (c. 475 to 465 Ma). A new subduction zone

developed beneath the Laurentian margin, the ophiolites and accreted terranes during Middle Ordovician time (e.g. Dewey and Shackleton 1984; Tucker and Robinson 1990; van Staal 1994; Cawood et al. 1995, Swinden et al.

1997, Whalen et al. 1997, van Staal et al. 1998). However, the duration of

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>490 to 460 Ma based on published radiometric ages of continental magmatic arcs such as the one at Connemara. This implies that an active margin at

Connemara was contemporaneous with passive Laurentian margin

sedimentation and ophiolite obduction in Newfoundland and the British Isles. These timing relationships are inconsistent with field relationships which clearly show that ophiolites are cut by magmatic arc rocks (e.g. in Tyrone; Hutton et al. 1985), and pose a major problem for comprehensive models of the Grampian orogen, which currently are explained in two different ways. One model embraces significant along-strike variations in the Laurentian margin, with ophiolite obduction and terrane accretion at a long-lived and complicated Andean-type plate margin (e.g. Lambert and McKerrow

1976; Yardley et al. 1987; Dewey and Ryan 1990). The other model suggests that

the Dalradian block was exotic to the Laurentian margin, and did not accrete until after the Grampian orogeny (e.g. Bluck and Dempster 1991). The latter model is especially appealing because some exposures of the Dalradian Supergroup, for example at Connemara, are located in an unusual tectonic position with respect to the rest of the Laurentian margin (Figure 2.1B).

During the Grampian orogeny at Connemara, tholeiitic to calc-alkaline magmas of the Connemara Gabbro and Gneiss Complex (Connemara Complex) were emplaced into rocks of the Dalradian Supergroup (Fig. 1; Yardley and Senior 1982; Leake 1989). Field relations indicate that igneous activity began during and outlasted the main phase of Grampian deformation, locally referred to as D2 and D3; early mafic rocks exposed in the north and west of the complex are deformed, but felsic intrusions farther east are undeformed and cut all ductile structures (e.g. Leake 1989). The potential of these relationships to provide important constraints on the timing and duration of Caledonian orogenic processes was not fully realized in the past because reliable age constraints were not available for most intrusive phases at Connemara. The age of the youngest intrusions has been controversial with age estimates ranging from 407 to >470 Ma (Kennan et al. 1987; Tanner e t

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based on highly discordant U-Pb data for one pluton (Jagger et al. 1988). Along with U-Pb zircon data indicative of intrusive activity as young as 465 Ma (Cliff et al. 1996), this finding is commonly used to infer a duration of arc magmatism at Connemara of at least 25 million years.

Data presented in this study provide the first high-precision U-Pb ages of the earliest and latest intrusive phases at Connemara, placing constraints on the duration of arc magmatism and, hence, the Grampian orogeny. Our results indicate that the Grampian orogeny at Connemara was essentially coeval with the Humberian orogeny in Newfoundland, rendering complex models of the Laurentian margin unnecessary.

2.3. Regional Setting

Northwestern Ireland exposes a spectacular cross section through the Caledonian orogen from the Laurentian margin in the north to terranes accreted to its margin farther south (Figure 2.1B). The former Laurentian margin terminates at a major mid-Ordovician suture, the Fair Head-Clew Bay Line. Tectonic units south of this collisional suture include remnants of an intra-oceanic subduction zone and a juvenile island arc of Tremadoc-Arenig age, as well as a Tremadoc-Llanvirn forearc basin, the South Mayo Trough. Sedimentary rocks of the South Mayo Trough record erosion of an island arc, ophiolites, and a metamorphic terrane (e.g. Dewey and Ryan 1991). To the south, a Silurian unconformity conceals most of the boundary between the South Mayo Trough and the Connemara terrane. The Connemara Complex and its Dalradian country rocks were thrust onto Ordovician metavolcanic rocks during the Grampian orogeny (Figure 2.1B, e.g. Tanner et al. 1989). Farther south, rocks of oceanic affinity that contain Connemara-derived clasts and ophiolites, the South Connemara Group, probably formed as trench deposits over a Llanvirn-aged north-dipping subduction zone (P. Ryan, personal communication). Most of this contact is obscured by intrusion of the Devonian Galway batholith.

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2.4. Geology of the Connemara Complex

The Connemara Complex consists of a series of mafic and ultramafic intrusions, dominated by metagabbro throughout much of the region. In southern Connemara, a younger series of calc-alkaline intrusions, which range in composition from quartz diorite to granite are also included in the suite (Figure 2.2; e.g. Leake 1989). The metagabbros contain metamorphic amphibole which replaced primary pyroxene and plagioclase during post-crystallization hydration. In southern Connemara, the mafic bodies were intruded by quartz diorites. The youngest intrusive unit of the igneous complex is the undeformed Oughterard granite (Figure 2.2; e.g. Tanner et al.

1997).

The Connemara Complex intruded a variety of metasedimentary rocks of the Dalradian Supergroup. These rocks record three deformation episodes and two metamorphic events. The oldest is a Barrovian-type metamorphism (M2; >6 kbars, c. 550*C, Yardley et al. 1987) related to crustal thickening. This event is best preserved in northern Connemara where it is not overprinted by the regional upper amphibolite-facies metamorphism (M3). This latter metamorphic event (c. 5 kbars, 750*C, Yardley et al. 1987) culminated in the anatexis of some Dalradian units in southern Connemara.

The earliest fabric (Si) recorded in Dalradian rocks occurs only as inclusion trails within syn-D2 garnets and has unknown tectonic significance. Early main phase deformation, locally referred to as D2, is represented mainly

by a penetrative schistosity and isoclinal folds at a variety of scales. The folds

were refolded by large-scale, F3 fold nappes and cut by minor brittle faults. A penetrative schistosity, axial planar to F3 folds, developed synchronously with M3 sillimanite-grade metamorphism (e.g. Tanner and Shackleton, 1979). Emplacement of the Connemara gabbros occurred during contractional ductile deformation. The gabbros of northern Connemara intruded prior to the D3 deformation, possibly as early as the regional D2 deformation (Wellings 1998), and provide a maximum age for D3 deformation. In

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southern Connemara the age relationships of the metagabbros with respect to

D2 structures are obscured by the intense D3/M3 overprint. Emplacement

during early-D3 seems most likely, because the gabbros contain metamorphosed xenoliths, were deformed by F3 folds and predated quartz diorites (e.g. Tanner 1990) .

At a late stage of D3, the Connemara terrane was thrust southward over metarhyolitic rocks of the Delaney Dome Formation along the Mannin thrust (Leake et al. 1983). This thrust was folded by a F4 WNW-plunging antiform (Figure 2.2; Leake, 1986). Another macroscopic F4 fold, the Connemara antiform, deformed the Dalradian country rocks to the north and was intruded by the Oughterard granite. Thus the granite marks the end of magmatism and deformation in Connemara.

2.5. U-Pb Results

In order to determine the age and duration of magmatism at Connemara, we have conducted single-crystal U-Pb geochronologic studies of the oldest mafic intrusions and the youngest granite. Zircons were air-abraded for -40 hours until no crystal faces remained. All crystals were dissolved by standard anion-based liquid chromatography after Krogh (1982); details of the analytical protocol may be found in Bowring et al. (1993) and Hawkins and Bowring (1997).

We analyzed ten zircon fractions of the plagioclase-hornblende gabbro (sample AF47), which was collected from the same location as the gabbro dated by Jagger et al. (1988; National Grid Reference L 843.440). The zircons are typically clear, subhedral elongate crystals, roughly 200 Rm long. Eight of the zircon fractions define a discordia with an upper intercept date of 470.1 ± 1.5 Ma (MSWD = 0.53) and a weighted mean 27Pb/20'Pb date of 470.3 ± 0.5 Ma

(MSWD = 0.46; Table 1; Figure 2.3A). Two fractions were excluded from the regression because their 207_Pb/ 206_{Pb dates of 494.8 Ma and 472.8 Ma indicate the}

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this gabbro to be 470.1 ± 1.5 Ma. Jagger et al. (1988) chose to report their more precise 207_Pb/ 20_{'Pb date of 490 ± 1 Ma, which was based on the mean of six of} their more magnetic fractions, rather than their upper intercept date of 477 + 25 / - 6 Ma for all nine analyses, which overlaps within 2a with the 470 Ma

age of this study. We believe that their more discordant analyses resulted from analyzing multigrain fractions of unabraded zircons.

Single zircon U-Pb analyses of a basic pegmatite (sample AF124) are the first attempt to determine the radiometric age of the syn-D2 Currywongaun intrusion. All four crystals were clear, prismatic -250 gm crystals. The

analyses cluster near concordia with weighted mean

20

_Pb/

238_U,2 07_Pb/ 2 35

_{U, and}

207_Pb/ 206_{Pb dates of 472.0 ± 0.3 Ma (MSWD} ₌_{1.18), 472.5 ± 0.3 Ma (MSWD =} 0.25), and 474.5 ± 1.0 Ma (MSWD = 0.11), respectively (Table 2.1; Figure 2.3B).

We interpret the slight discordance between these three mean ages as a result of limited Pb loss. In this case, the best estimate of the age of the Currywongaun intrusion is the weighted mean 207_Pb/ 2 06_{Pb date of 474.5}

_±

_1.0 Ma.

Previous geochronologic investigations of the two-mica Oughterard granite were based on the Rb-Sr method, but age interpretation has been problematic because most of the major minerals are altered (e.g. Tanner et al.

1997). We analyzed zircon and xenotime from the Oughterard Granite

(sample AF97-0101; Table 2.1; Figure 2.3C). Zircon crystals from this sample are either small clear, prismatic crystals or larger, fractured crystals with clear cores and cloudy overgrowths. Single zircon analyses yield 20 7_Pb/ 206_{Pb dates of} 580 Ma and 2199 Ma, respectively, indicating that these zircons are inherited

grains. The granite also contains abundant bipyramidal xenotimes. Five single crystal analyses of clear yellow and some cloudy orange xenotimes have a weighted mean 20 7_Pb/2_{'Pb date of 462.2 ± 0.5 Ma (MSWD} ₌_{0.93) and} define a discordia with an upper intercept date of 462.5 ± 1.2 Ma (MSWD -1.16), which is the best estimate of the crystallization age of this granite.

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2.6. The Grampian Orogeny at Connemara

U-Pb geochronology of the Connemara gabbros and the Oughterard

granite shows that magmatism, mid-crustal deformation, and amphibolite-facies metamorphism occurred between 474.5 Ma and 462.5 Ma (Figure 2. 4). This rapid succession of events at Connemara is consistent with field relationships. In northern Connemara, mafic intrusions were intruded prior to D2, but D3 deformation occurred while the contact aureole of these intrusions still was hot (Wellings 1998). In southern Connemara, the hanging wall of the late-D3 Mannin thrust was chilled during emplacement against cold footwall rocks as they experienced prograde greenschist-facies metamorphism. This implies that the Connemara terrane was still relatively hot (>500*C) in late-D3 time (e.g. Leake 1986). Similarly, the post-D4 Oughterard Granite intruded country rocks that had not yet cooled after high-grade metamorphism, based on the lack of a contact aureole around the intrusion.

The Connemara Complex forms part of a contractional continental magmatic arc. The early stages of contractional deformation resulted in crustal thickening, recorded by M2 high P/T metamorphism (cf. Wellings

1998). A syn-D2 emplacement age of the 475 Ma northern Connemara

gabbros, as inferred by Wellings (1998), implies that arc magmatism above a subduction zone occurred during crustal loading. In Tyrone and Newfoundland crustal loading was related to ophiolite emplacement, but followed by intrusion of arc magmatic rocks into the Laurentian margin and the ophiolites. If ophiolite emplacement was the cause of crustal loading at Connemara, and the Currywongaun gabbro was emplaced during the regional D2/M2 event, the magmatic arc, i.e., a Laurentia-ward dipping subduction zone existed simultaneous with ophiolite emplacement (at 475 Ma). In light of existing regional tectonic models for the Grampian orogen these relationships can be reconciled only if it is assumed that the subduction polarity reversal was an extremely rapid event and began during ophiolite

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obduction. Alternatively, existing tectonic models for the Connemara orogen must be modified.

North-vergent folding and subsequent southward thrusting of the Connemara terrane could not have lasted long because the major structural, metamorphic and magmatic architecture of Connemara was assembled between c. 475 and 463 Ma. During this time exhumation of the orogen began and was followed by strike-slip faulting that prevented prolonged convergence. Removal of the Connemara terrane from the active margin shortly after orogenesis but prior to Silurian time explains the lack of igneous or metamorphic ages younger than c. 460 Ma.

2.7. Regional Implications

A younger age for the Grampian orogeny at Connemara helps to solve

a major conflict regarding the origin of the Dalradian terranes of Ireland and Scotland. A c. 475 Ma onset of the Grampian orogeny postdates carbonate platform sedimentation that is known to have characterized the southern Laurentian margin until Arenig/Llanvirn time (Figure 2.4; e.g. Soper and England 1995). This implies that the Dalradian rocks of Connemara could have been part of the Laurentian passive margin until the onset of the Grampian orogeny. An exotic origin of the Dalradian Supergroup rocks as proposed by Bluck and Dempster (1991) is not required.

The Grampian magmatic arc at Connemara postdates ophiolite obduction recorded along strike of the northern Appalachian-Caledonian orogen and was coeval with arc syntectonic arc magmatism of the Grampian/Humberian/Taconian orogeny (e.g. Tyrone, Hutton et al. 1985). This orogenic event occurred during a brief interval, between c. 475 and 463 Ma, along the Laurentian margin from the northern Appalachians to Scotland (Figure 2. 4; Figure 2.1B; cf. Rogers et al. 1994). In late Tremadoc to Early Arenig time the Laurentia-Iapetus plate boundary was dominated by obduction of supra-subduction zone ophiolites (e.g., van Staal et al. 1998).

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Subduction polarity reversal, which resulted in Late Arenig to Llanvirn contractional arc magmatism in the Laurentian margin, occurred at or just prior to 475 Ma.

There is no evidence for subduction beneath the southeastern Laurentian margin before Late Arenig time. Closure of the Iapetus Ocean before c. 475 Ma must have occurred within Iapetus or along its southern (Avalonian) margin. The earliest subduction beneath the Laurentian margin is recorded by short-lived magmatic arcs like the one at Connemara. However, major arc magmatism in the Laurentian margin did not occur until c. 454 to 425 Ma (e.g. Tucker and Robinson, 1990; Cawood et al. 1995; Karabinos et al. 1998), which is not documented in Ireland where intense strike-slip faulting prevented prolonged convergence. The Grampian orogeny at Connemara is consistent with arc magmatism along strike from at least Newfoundland to Scotland (e.g., Rogers et al. 1994; Karabinos et al. 1998, Swinden et al. 1997, Van Staal et al. 1998) and correlates with the Taconian/Humberian orogeny of the northern Appalachians. It does not support models of a continental collision between Laurentia and Avalonia/Gondwana for the Ordovician period.

Acknowledgments

This study was supported by a NSF grant awarded to KVH and SAB and a

GSA student grant awarded to AMF. We thank Barry Long from the Irish

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⁴⁰Ar/³⁹Ar and U-Pb geochronological constraints on the thermal and tectonic evolution of the Connemara Caledonides, Western Ireland

"Ar/**Ar and U-Pb Geochronological Constraints on the

Thermal and Tectonic Evolution of the Connemara

Caledonides, Western Ireland

at the

"Ar/

Ar and U-Pb Geochronological Constraints on the

Thermal and Tectonic Evolution of the Connemara

Caledonides, Western Ireland

ACKNOWLEDGMENTS

To the Memory of My Dad

Dr. med. Kurt Friedrich

TABLE OF CONTENTS

CHAPTER 1: INTRODUCTION

References

J.

J.,

J.

J.

J.

J.,

J.,

J.

J.

J.

CHAPTER 2: A SHORT-LIVED CONTINENTAL MAGMATIC ARC

AT

CONNEMARA,

WESTERN

IRISH

CALEDONIDES:

IMPLICATIONS FOR THE AGE OF THE GRAMPIAN OROGENY.

Anke M. Friedrich, Samuel A. Bowring, Mark W. Martin, and Kip V. Hodges

2.1. Abstract

2.2. Introduction

2.3. Regional Setting

2.4. Geology of the Connemara Complex

2.5. U-Pb Results

analyses cluster near concordia with weighted mean

Pb/

U, and

±

2.6. The Grampian Orogeny at Connemara

2.7.

Regional Implications

Acknowledgments

References Cited

J.

J.,

J. 1991. Exotic metamorphic terranes in the

J.

J.

J.

J.

J. C., Stoll, H. M. 1998. Taconian

J.,

J.,

J.,

J.

J.,

J.

J.

J.

J.,

J.

J.,

J.

J.,

J.

J.

J.

"Ar/Ar and U-Pb Geochronological Constraints on the**

_{Ar and U-Pb Geochronological Constraints on the}

_Pb/

_{U, and}

_±