EMPLACEMENT CONDITIONS OF A PORPHYRITIC FELSITE DYKE...

EMPLACEMENT CONDITIONS OF A PORPHYRITIC FELSITE DYKE AND

TIMING OF MOTION ALONG THE COOLIN FAULT AT BEN LEVY,

CO. GALWAY

ELIZABETH A. JOHNSON, CHRISTOPHER SUTHERLAND, M. AMELIA

V. LOGAN, SCOTT D. SAMSON and MARTIN FEELY

(Received 6 October 2010. Accepted 23 February 2011.)

Abstract

A NNE-trending porphyritic felsite dyke located on Ben Levy in Co. Galway cross-cuts the

E�W trending Coolin Fault. The porphyritic felsite is an altered rhyolite containing ocellarfeldspar with rare apatite inclusions, quartz phenocrysts as well as minor ilmenite, rutileand zircon, all within a ground-mass of feldspar and quartz. High whole-rock K/Na and

Rb/Sr ratios and textural evidence from cathodoluminescence (CL) imaging indicate that

the dyke underwent hydrothermal alteration; however, immobile Zr vs TiO2 for the Ben

Levy Dyke is consistent with the trend of values for Galway Batholith dykes. Magmatic

crystallization temperatures were calculated as 10679148C using Ti-in-zircon thermo-metry. Quartz phenocrysts may have been affected by diffusive loss of Ti post-emplacement,

and record an average temperature of B5748C based on Ti-in-quartz thermometry. An ageof 373.994.0Ma was determined from laser ablation inductively coupled plasma massspectrometry (LA-ICPMS) U�Pb isotopic analysis of zircon. The Ben Levy porphyriticfelsite is similar in age and orientation to other late-stage dykes which cross-cut the Galway

Batholith to the south, and may represent the northernmost extension of late-stage dyke

activity. Motion along the Coolin Fault occurred during the Caledonian Orogeny, between

deposition of the Lough Kilbride Formation (436�428.2Ma) and intrusion of the Ben LevyDyke at 373.994.0Ma.

Introduction

A felsic porphyritic dyke outcrops along the foot-

wall scarp of a NNE-trending normal fault,

which transects Ben Levy near Ballard, Co. Galway

(Fig. 1). The dyke has a strike of �0408, dips steeplyto the W, and is about 2�3 metres wide in locationswhere contacts can be clearly seen on both sides

of the porphyritic felsite (Locations A and C). The

porphyritic felsite extends through the Dalradian

Ben Levy Formation and into the Lower Silurian

Lough Mask and Kilbride Formations on the north

side of Ben Levy, cross-cutting the E�W-trending

Coolin Fault. The Coolin Fault lies within a thr-

ust belt associated with the Caledonian Orogeny

(Phillips 2001). Based upon field relationships in this

location, motion along the Coolin Fault must have

occurred after deposition of the Kilbride Formation.

The Kilbride is constrained to the Telychian Stage

(436�428.2Ma) of the Llandovery Series (Holland1985; Cocks 1989) by the presence of the brachiopod

Eocoelia curtisi curtisi within the Annelid Grits

member in the lower Kilbride (Harper et al. 1995).

In this study, the timing of motion along the

Coolin Fault is constrained using U�Pb age dat-ing. Cathodoluminescence (CL) imaging, major and

Irish Journal of Earth Sciences 29 (2011), 1�13

doi: 10.3318/IJES.2011.29.1

# 2011 Royal Irish Academy

1

trace element geochemistry, and Ti-in-zircon and

Ti-in-quartz thermometry are used to examine

emplacement conditions of the porphyritic felsite

and its relationship to the magmatism which pro-

duced the Galway Batholith to the south.

Regional geological context

The Ben Levy porphyritic felsite dyke is located NE

of the Galway Batholith (Fig. 1A). The Galway

Batholith was emplaced c. 423�380Ma based on

Clifden

Maam Cross

Figure 1B

CarnaSRF

53°20’N

10° W

Ordovician-Silurian Sediments and Volcanics

Oughterard Granite

Connemara Metamorphic Complex

Galway Batholith

Carboniferous

10 km

Skird Rocks

Aran Islands

Extent of Galway Granite

Spiddal

Galway

Coolin Thrust

370 m

300 m

Loc. A(106653,254311)

Loc. B(106699,254367)

NormalFaultContactInferredContact

Felsic Porphyry Dike

Kilbride Formation (Silurian)

Lough Mask Formation (Silurian)

Basal Member of Lough Mask Fm. (Silurian)

Ben Levy Formation (Dalradian)

N

×

Ben Levy

Faults

(A)

(B)

Loc. C(106897, 254649)

Fig. 1*(A) A simplified geological map of western Ireland showing the distribution of the major lithological units, i.e. Ordovicianand Silurian rocks; the Connemara Metamorphic Complex; the Oughterard Granite and the younger Galway Batholith; andthe Carboniferous limestones. The location of the mapped area in Fig. 1B is also shown. SRF: Skird Rocks Fault. Adapted fromLeake and Tanner (1994) and Feely et al. (2006); (B) Geological map of the eastern end of Ben Levy, near Ballard, Co. Galway. Theporphyritic felsite dyke is intruded along a normal fault. Both the porphyritic felsite and normal fault cross-cut the Coolin Fault,which marks the boundary between Silurian sediments and the overlying Dalradian Ben Levy Formation. Geological map of Ben Levyafter Whitmeyer et al. (2009).

2 Irish Journal of Earth Sciences (2011)

zircon age dating (Pidgeon 1969; Buchwaldt et al.1998, 2001; Feely et al. 2003) and Re�Os age datingof molybdenite (Selby et al. 2004; Feely et al. 2007;

Feely et al. 2010). The long axis of the batholith is

oriented WNW�ESE, and granite emplacementprogressed from NW to SE through time (Feely

et al. 2010). The timing and duration of magma

emplacement in the Galway Batholith are similar

to other regions of the Appalachian-CaledonianOrogeny, so by analogy slab break-off and resulting

asthenospheric flow are thought to be responsible

for the main pulse of magmatism in the Galway

Batholith (Feely et al. 2010). Late-stage magmatism

may have been produced by transtension and

decompression during the Devonian (Brown et al.

2008).

Tensional forces acting parallel to the longaxis of the Galway Batholith resulted in fractures

forming in a N�NNE direction perpendicular tothe batholith axis (El Desouky et al. 1996). Dykes

were emplaced along these fracture zones (Hunt

and Mohr 2007; Mohr 2003; Mohr 2004). The dykes

range in age from 400.594.4Ma to �37096Ma(Daly 1998; Jenkin et al. 1998; Hunt and Mohr

2007). Some of the dykes exhibit intriguing ocellartextures and other evidence for magma mixing and

mingling (Mohr 2004). Brecciated contact zones and

offshoots within surrounding country-rock indi-

cate paroxysmal emplacement (Hunt and Mohr

2007). Exposures of dyke swarms are concentrated

on the east and west blocks of the Galway Batholith

due to uplift and preferential erosion of the central

block of the Galway Batholith (Mohr 2003). Thedyke swarms can be followed northward from the

Galway Batholith into the Dalradian country-rock

(Friedrich et al. 1999; Mohr 2003).

Methods

Geochemical and petrographic analyses were com-

pleted on a sample of the porphyritic felsite taken

from Location B (Fig. 1).

Cathodoluminescence

CL images were obtained on the ELM-3R Lumino-

scope in the Department of Mineral Sciences,

Smithsonian Institution, Washington, D.C. General

imaging of thin sections was done at 20kV with a0.5mA source for exposures between 5sec and 1min

long. Zircon images were obtained at 15kV and

0.5mA with exposure times ranging from 8sec to

2min 31sec.

Bulk rock composition

Major and trace element compositions were ob-

tained with X-ray fluorescence spectroscopy on a

Philips PW1480 X-ray spectrometer in the Depart-

ment of Mineral Sciences, Smithsonian Institution

using the procedure outlined in Taggart Jr et al.

(1987). Major element analyses were run on a glass

bead at 40kV and 60mA; trace element analyses were

run on a pressed pellet at 80kV and 30mA. The ratio

of sample to Li tetraborate was 1:3 for major

elements and a 4:1 ratio of sample to cellulose was

used for trace elements. Fe3� /Fe2� was determined

by titration. Loss on ignition (LOI) analysis was

performed using a standard heating protocol. The

following standards were used: for trace elements,

RGM-1 and G-2; and for major elements, G-1,

BHVO-1 and RGM-1.

Electron microprobe analyses

Major and trace element analyses of zircon, quartz,

and feldspar were obtained using the JEOL JXA-

8900RWD/EDS microanalyser in the Department of

Mineral Sciences, Smithsonian Institution. Zircon

analyses were obtained at 20kV and 200nA with a

focused beam. Standards used for zircon analyses

were: Y, P �YPO4 (National Museum of NaturalHistory, Smithsonian Institution, Washington, D.C.

(NMNH) 168499); Na �omphacite (NMNH110607); Al, Fe, Ca �garnet (NMNH 110752);Ti �hornblende, Kakanui, New Zealand (NMNH143965); Th �Corning glass IR-W (NMNH117084); U �uraninite (NMNH unnumbered stan-dard); and Hf, Si, Zr �zircon (NMNH 117288�3).

Quartz analyseswere obtained at 20kVand 200nA

with a beam size of 40mm. Ti-in-quartz analyseswere 250sec on peak, 125sec off peak for standards

and unknowns. The following standards were used

for quartz analyses: Si �quartz, Hot Springs, AR(NMNH R17701);Ti �Corningglass IR-V (NMNH117083); Mn, Ba �Corning glass IR-W (NMNH117084); and Al, Fe, Na, K �hornblende, Kakanui,New Zealand (NMNH 143965).

Feldspar analyses were obtained at 15kV and

10nA with a beam size of 5mm. Standards used forfeldspar analyses were: Fe, Na, Mg, Ti �hornblende,Kakanui, New Zealand (NMNH 143965); Si, Al,

K �microcline (NMNH 143966); and anorthite(NMNH 137041). The anorthoclase standard

(NMNH 133868) was analysed as an unknown to

check the standardisation.

Johnson et al.*Emplacement conditions and timing of motion, Coolin Fault 3

U-Pb dating

The rock sample was crushed, sieved and split into

light and heavy fractions using sodium polytung-

state. Zircons were separated from the heavy frac-

tions by hand. Zircon crystals were inspected using

reflected light microscopy. Grains that appeared

clear and crack-free, and had minimal inclusions

were selected for analysis. The selected zircon grains

for each sample were embedded in epoxy and

polished to expose zircon cores. All individual

grains were imaged via CL using the JEOL 8600

electron microprobe at Syracuse University. Images

were used as guides during analyses to help to avoid

altered regions; regions with minor cracks not

observable through reflected light microscopy; and

areas with small inclusions; as well as to avoid

analysing potential overlapping petrogenetic zones.

U-Pb dating was carried out at the University of

Arizona Laserchron Geochronological Laboratory

using the Isoprobe-P LA-ICPMS. Mass spectro-metric methods followed those described by Gehrels

et al. (2006). Common lead corrections were based

on direct 204Pb measurement and estimates of initial

Pb composition were based on the model of Stacey

and Kramers (1975). 206Pb/207Pb and U/Pb were

calibrated relative to a zircon from Sri Lanka

standard dated at 56493.2Ma.

Results

Lithological description

The colour and texture of the ground-mass varies

along the length of the exposure. Samples were

collected at Location B for geochemical analyses. At

Location B (Fig. 1B), the very fine-grained ground-

mass is tan to pink in color, and flow banding is visible

locally (Fig. 2A). At Location A, the ground-mass is

white and very fine-grained to aplitic or gray and

4cm

1mm 1mm

ocellus

K feldspar

zircon

quartzquartz

K feldspar zircon

apatite

Na feldsparNa feldspar

(A)

(B) (C)

Fig. 2*Features of the Ben Levy porphyritic felsite at macroscopic and microscopic scales. (A) Localised sub-vertical flow banding inthe outcrop of the porphyritic felsite at location B. (B) An image taken under crossed polarised transmitted light of an Na-feldsparglomerocryst and the surrounding matrix of K-feldspar and quartz. The ocellus around the Na-feldspar glomerocryst composed ofquartz and K-feldspar is clearly visible. (C) CL image of the same Na-feldspar glomerocryst. Minor zircon (blue in CL) and rareapatite (greenish yellow) are found both as inclusions within phenocrysts and within the matrix. The matrix and ocellar K-feldsparappears yellow and quartz is a black to red colour.


massive, and alters to a yellow or brown colour. AtLocation C, the ground-mass is pale green-grey and

massive. At Location C, a pale green baked zone

�1cm wide is visible within the Kilbride Formationalong the contact with the dyke. Jointing sets occur

roughly parallel and perpendicular to the trend of the

dyke at Location A, and flattened vesicles indicating

flow banding are present in at least one outcrop.

Phenocryst modal abundances at all locations varyfrom 1�3%, with 1�4mm glassy colourless plagioclaseand pink K-feldspar phenocrysts most abundant

(�66% of total phenocryst volume), followed by0.5�3mm subhedral to euhedral quartz, and rare,opaque pink, fractured K-feldspar phenocrysts up to

5mm long. The phenocrysts have a distinctive ocellar

texture, with white rims B0.5mm enclosing bothfeldspar and quartz phenocrysts.

Fig. 2B is a crossed polarised transmitted light

image of a Na-feldspar glomerocryst and the

surrounding matrix. The ocellus around the Na-

feldspar phenocryst is composed of quartz and K-

feldspar, as is the ground-mass. The CL image in

Fig. 2C shows the ocellar texture as well as zircon

crystals within the Na-feldspar and matrix, and an

apatite crystal within the Na-feldspar phenocryst.Minor ilmenite and rutile were identified using

energy-dispersive X-ray spectroscopy (EDS) on the

electron microprobe and are present only in the

ground-mass.

Feldspar phenocrysts are almost pure end-mem-

ber Ab (Ab99.1An0.6Or0.3 as measured with the

electron microprobe, n �17), form glomerocrysts,and typically have multiple, complex embayments(Appendix 1). Quartz phenocrysts are anhedral to

embayed and compositions are given below. The

ocelli and ground-mass of the rock contain quartz

and K-feldspar (Ab1.3An0.2Or98.5, n �9; Appendix 1).

Major and trace element chemistry

Whole-rock major and trace element analyses are

compiled in Table 1. The Ben Levy porphyritic

felsite is plotted on a K/Na vs Rb/Sr discrimina-

tion diagram in Figure 3A along with representa-

tive data from Galway Batholith dykes (Mohr

2003). The Galway Batholith dykes fall in the

region of unaltered volcanics (Glazner 1988;

Hollocher et al. 1994; Rougvie and Sorensen2002). The Ben Levy porphyritic felsite lies out-

side this field with high K/Na vs Rb/Sr indicating

it has undergone alteration. Most of the major

and trace element concentrations have been al-

tered in the porphyritic felsite. Two of the least

mobile elements, Zr vs TiO2, are plotted in Figure

3B. The data for the Galway Batholith dykes form

a broad trend. The Ben Levy porphyritic felsite

plots along this trend and close to the field of

data for the Galway Batholith dykes.

Geothermometry

Crystallization temperatures were calculated using

Ti-in-zircon and Ti-in-quartz (TitaniQ) thermo-

meters (Ferry and Watson 2007; Hayden and

Watson 2007; Watson et al. 2006). Since quartz,

rutile, and ilmenite were present in the rock, it was

inferred that aSiO2 ¼ 1 and aTiO2 ¼ 1, so no adjust-ments were necessary for either thermometer. The

Table 1*Whole-rock chemical composition of the BenLevy porphyritic felsite. Fe2O3/FeO determined by

titration; b.d.l.�below detection limit. Sample ob-tained from Location B (Fig. 1).

wti% Split 1 Split 2 Average

SiO2 78.76 78.48 78.62

TiO2 0.07 0.08 0.08

Al2O3 12.09 12.05 12.07

Fe2O3 1.24 1.23 1.24

FeO 0.00 0.00 0.00

MnO 0.02 0.02 0.02

MgO 0.42 0.44 0.43

CaO 0.05 0.05 0.05

Na2O 0.47 0.49 0.48

K2O 6.47 6.47 6.47

P2O5 0.01 -0.01 0.00

LOI 1.21 1.21 1.21

Total 100.81 100.52 100.67

ppm Split 1 Split 2 Average

Rb 464 463 464

Sr 11 11 11

Cs b.d.l. b.d.l. b.d.l.

Ba 730 708 719

Sc 1 2 2

V b.d.l. b.d.l. b.d.l.

Ni b.d.l. b.d.l. b.d.l.

Cu 8 7 8

Zn 21 21 21

Ga 16 15 16

As 23 21 22

Y 43 42 43

Zr 123 124 124

Nb 28 28 28

Mo 1 2 2

Tl b.d.l. b.d.l. b.d.l.

Pb 18 16.5 17


average Ti concentration in the zircons was

135912ppm (n �34) with a range of Ti concentra-tions of 120�62ppm (Table 2; Appendix 2). Therewas no consistent variation of Ti concentration

from core to rim among the zircon grains. Based

on these Ti concentrations, the average crystal-

lization temperature calculated for zircon was

10679148C.Ti concentrations in quartz phenocrysts were

very low, B25ppm for all analyses, and were belowthe analytical limit for electron microprobe (Wark

and Watson 2006). Based on the average Ti con-

centration of about 17ppm, the quartz phenocrysts

record a temperature of B5748C (Hayden andWatson 2007; Wark and Watson 2006). It was not

possible to obtain a clean analysis on the fine-

grained quartz in the ocellar rims and ground-mass

that was intimately mixed with feldspar and otheraccessory minerals.

U-Pb dating206Pb/238U vs 207Pb/235U data are plotted in Fig. 4A.

Apparent ages range from 352.4�389.3Ma for con-cordant data (Table 3). Two analyses were obviously

discordant (Fig. 4A; Table 3). One discordant

analysis had a 206Pb/238U age date of 360.694.6Ma, falling within the range of concordant

analyses; the second discordant analysis had a very

poorly constrained 206Pb/207Pb age date of

1659.39112.0Ma. The age of the Ben Levy por-phyritic felsite was calculated to be 373.994.0Ma(n �12) excluding the discordant ages. Fig. 4B is aCL image of zoning within two representative

zircons showing concentric magmatic zoning.

Calculated ages are the same within error for

analyses obtained within approximately the same

zones, as shown in the zircon on the right. Ages are

also similar within error for points obtained in the

cores and rims of zircons, as shown on the left.Based on the analyses in this study, there is no

obvious major inherited zircon component.

Discussion

Emplacement conditions

A porphyritic texture with a fine-grained ground-

mass and the presence of vesicles both indicateshallow emplacement conditions for the Ben Levy

Dyke. It is likely that the dyke has undergone

hydrothermal alteration based on discoloration of

the matrix in hand sample and high K/Na and Rb/

Sr ratios that place it within values measured for

300

250

200

150

100

50

00

TiO2 wt%

Zr

(ppm

)

0.25 0.5 0.75 1

Ben Levy DykeGalway Batholith Dykes

100Ben Levy DykeGalway Batholith Dykes

Unaltered silicicvolcanic rocks

10

1

0.10.01 0.1 1

Rb/Sr

K/N

a

10 100

(A)

(B)

Fig. 3*(A) K/Na vs Rb/Sr discrimination diagram. The greyfield shows reported values for unaltered silicic volcanics(Glazner 1988; Hollocher et al. 1994; Rougvie and Sorensen2002); the Galway Batholith dykes lie within this unaltered field.The Ben Levy porphyritic felsite lies outside this field and isinterpreted to be metasomatized, resulting in addition of K andRb and removal of Ca and Sr. (B) Zr vs TiO2 diagram of the BenLevy Dyke and Galway Batholith dykes. Geochemical data forthe Galway Batholith dykes is taken from Mohr (2003).

Table 2*Ti concentrations and crystallization tem-peratures for zircon crystals. Temperatures were calcu-

lated using the thermometer of Ferry and Watson

(2007).

Grain

# of

analyses

Ti (ppm)

average

Ti (ppm)

range

T8C(Ferry &

Watson

2007)

Zircon 1 9 136 121�145 1068Zircon 2 8 148 139�162 1083Zircon 3 5 132 122�146 1064Zircon 4 2 121 121 1050

Zircon 5 5 132 120�144 1064Zircon 6 5 125 120�132 1055

Average 34 135912 120�162 1067914


metasomatised tuffs (Glazner 1988; Hollocher et al.

1994; Rougvie and Sorensen 2002). Preferential

addition of K and Rb and loss of Na and Sr during

infiltration of metasomatic fluids is thought to cause

K-metasomatism (Rougvie and Sorensen 2002).

Mass loss due to alteration may have resulted in

an artificially high SiO2 content for the bulk rock

(Table 1). The Ben Levy porphyritic felsite contains

78.62wt% SiO2, whereas the most felsic Galway

Batholith dykes contain 74�75wt% SiO2 (Mohr2003). Field evidence suggests that hydrothermal

activity was extremely localised within the dyke, as

visible alteration of the country-rock is limited to

a �1cm-wide baked zone where the contact isvisible.

The low temperatures (B5748C) recorded inquartz phenocrysts may be caused by resetting of

the Ti concentration in quartz during the high

quartz to low quartz transition, which occurs at

572.68C at 1bar (Koster Van Groos and Ter Heege1973). Hydrothermal or metasomatic conditions

may aid re-equilibration of Ti concentrations in

quartz during the high quartz to low quartz transi-

tion. For example, oxygen diffusion through quartz

is faster in the presence of water or CO2 than under

fluid-absent conditions (Cherniak et al. 2007), and

Ti diffusion in hydrous silicate melts is much faster

than Ti diffusion in dry quartz (Hayden and Watson

2007).

In contrast, Ti diffusion in zircon is very slow

under both dry and hydrothermal conditions (Cher-

niak and Watson 2007), so zircon crystals will

preserve crystallization temperatures (10679148C)even during hydrothermal or metasomatic events.

0.068 420

400

380

360

340

320

0.064

0.060

0.056

0.052

0.0480.2 0.4 0.6

207Pb/235U

206Pb238U

0.8 1.0

±

MSWD (of concordence) = 0.21, errors are 2σ

389.3±9.7 383.3±3.7

378.4±3.7

374.7±4.8

90µmCL

Concordia Age =373.9 ± 4.0 Ma(A)

(B)

Fig. 4*(A) Concordia diagram for zircons from the Ben Levy porphyritic felsite. All data are shown, but two obviously discordantpoints shown in light grey were not included in age determination. (B) CL image of representative zircons showing concentricmagmatic zoning and location of U�Pb analyses.


Cathodoluminescence imaging and concordance

confirm that the U�Pb age date obtained fromzircons was not affected by metasomatism.

Relationship to the Galway Batholith

Two lines of evidence support the idea that the Ben

Levy Dyke is genetically related to the late-stage

dykes of the Galway Batholith. Firstly, the age of

the Ben Levy Dyke, 373.994.0Ma, is similar in ageto the �37096Ma dykes which cross-cut the Gal-way Batholith directly (Daly 1998; Jenkin et al.

1998). Secondly, the �0408 trend of the dyke isconsistent with the predominant trend of late-stage

dykes in the Galway Batholith (Hunt and Mohr

2007). The Ben Levy Dyke could be considered a

northward extension of the dykes in the eastern

sector of the Galway Batholith since it is �15kmnorth of the surface exposure of the batholith.

Other late-stage dykes are found up to �20kmnorth of the easternmost side of the batholith near

Clifden; related dykes have previously been found

�6km north of the east-central portion of thebatholith (Mohr 2003). Unfortunately, metasomatism

has probably affected concentrations of many major

and trace elements in the whole rock analyses of Ben

Levy, and only the most immobile elements such as

Zr and Ti (Figure 3B) can be used to support

common petrogenesis of the Ben Levy and Galway

Batholith dykes. The Ben Levy Dyke is emplaced

along a normal fault which cross-cuts the Coolin

Fault. K-metasomatism of volcanic rocks associated

with detachment zones and normal faulting has

been widely documented (e.g. Brooks 1986). This

could explain why the dyke at Ben Levy exhibits

chemical and textural evidence for metasomatism,

while other dykes associated with the Galway Bath-

olith do not exhibit such textures.

In some granitoid intrusions and entrained

mafic enclaves of the Galway Batholith, quartz

crystals are rimmed by hornblende and/or biotite

and sphene crystals are surrounded by plagioclase,

quartz and K-feldspar (Baxter and Feely 2002). This

ocellar texture is interpreted as evidence of magma

mixing and mingling (Baxter and Feely 2002). It is

possible that the ocellar texture around the quartz

and Na-feldspar phenocrysts in the Ben Levy Dyke

is a preserved feature of magma mixing and

mingling, although no other textural or geochemical

evidence was found to support this hypothesis.

Alternatively, this texture could be the result of

metasomatism of the ground-mass surrounding the

phenocrysts, which themselves are less affected by

the metasomatism; feldspar phenocrysts retain a

pure end-member Ab composition and are glassy

and transparent in hand sample.

Age constraints on faulting

The age date of the Ben Levy Dyke constrains the

age of motion along the Coolin Fault. Since the

Coolin Fault thrust the Ben Levy Formation over

Table 3*U�Pb isotopic analyses of zircon.

Isotope ratios Apparent ages (Ma)

Analysis

U

(ppm)

206Pb204Pb U/Th

206Pb*207Pb*

9(%)

207Pb*235U*

9(%)

206Pb*238U

9(%)

error

corr.

206Pb*238U*

9(Ma)

207Pb*235U

9(Ma)

206Pb*207Pb*

9(Ma)

Zircon 7 spot 1 115 3550 1.0 18.5587 3.7 0.4270 4.1 0.0575 1.7 0.43 360.3 6.1 361.1 12.4 366.1 82.9

Zircon 7 spot 2 114 4655 1.5 18.2677 6.5 0.4284 6.7 0.0568 1.9 0.27 355.8 6.4 362.0 20.5 401.7 145.1

Zircon 8 spot 1# 419 2410 1.0 14.1171 21.6 0.5619 21.6 0.0575 1.3 0.06 360.6 4.6 452.7 79.2 952.7 446.7

Zircon 8 spot 2 90 2898 1.2 19.1713 4.6 0.4237 4.7 0.0589 1.0 0.21 369.0 3.6 358.7 14.3 292.5 105.6

Zircon 9 spot 1 132 4155 1.8 19.0269 4.1 0.4402 4.3 0.0607 1.0 0.23 380.2 3.7 370.4 13.2 309.7 94.5

Zircon 9 spot 2 218 6378 1.8 18.5631 1.6 0.4334 1.9 0.0583 1.0 0.54 365.6 3.6 365.6 5.8 365.6 35.5

Zircon 10 239 2938 1.2 16.7262 5.3 0.4631 6.2 0.0562 3.3 0.54 352.4 11.5 386.4 20.0 595.8 114.1

Zircon 11 spot 1# 140 495 0.7 9.8121 6.0 0.7943 6.9 0.0565 3.3 0.48 354.5 11.3 593.6 30.9 1659.3 112.0

Zircon 11 spot 2 292 4463 1.4 16.5802 10.0 0.4822 10.9 0.0580 4.4 0.40 363.4 15.5 399.6 36.1 614.8 216.4

Zircon 12 spot 1 261 3715 1.3 18.7376 7.3 0.4508 7.4 0.0613 1.0 0.14 383.3 3.7 377.8 23.4 344.5 166.2

Zircon 12 spot 2 188 4820 1.6 18.5409 2.1 0.4496 2.3 0.0605 1.0 0.43 378.4 3.7 377.0 7.4 368.3 47.9

Zircon 13 spot 1 127 3393 0.8 18.9643 3.0 0.4351 3.3 0.0598 1.3 0.40 374.7 4.8 366.8 10.1 317.2 68.7

Zircon 13 spot 2 347 8263 1.4 18.4097 2.4 0.4663 3.5 0.0623 2.6 0.73 389.3 9.7 388.6 11.4 384.3 54.6

Zircon 16 126 2833 1.3 19.0215 4.0 0.4399 4.2 0.0607 1.0 0.24 379.8 3.7 370.2 12.9 310.4 92.2

#Discordant analysis: not included in average age determination


the Lough Mask and Kilbride Formations, motion

must have occurred after deposition of the Kilbride

Formation during the Telychian Stage (436�428.2Ma), and before the Ben Levy Dyke cross-cut the

fault at 373.994.0Ma.

Conclusions

1. The macroscopic and microscopic textures

and chemical composition of the Ben Levy

porphyritic felsite indicate that the dyke

was emplaced shallowly and underwent

hydrothermal alteration. Despite the altera-

tion, phenocryst textures and zircon crys-

tallization temperatures of 10679148C arepreserved.

2. The age (373.994.0Ma), NNE-trendingorientation, and relative concentrations of

the immobile elements Zr and Ti are

consistent with a possible genetic relation-

ship between the Ben Levy porphyritic

felsite and the late-stage Galway Batholith

dykes (37096Ma).3. Motion along the Coolin Fault occurred

during the Caledonian Orogeny, between

deposition of the Kilbride Formation

(436�428.2 Ma) and intrusion of the BenLevy porphyritic felsite dyke at 373.994.0Ma.

Acknowledgements

The authors would like to thank the Alison

MacDonald Dougherty Foundation for providing

the money to fund C. Sutherland’s undergraduate

senior research project. We would also like to thank

Steven J. Whitmeyer and the James Madison Uni-

versity field course for aid with the geological map

of Ben Levy; and Jack Hietpas for help with the U�Pb age dating.

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ELIZABETH A. JOHNSON (corresponding author) andCHRISTOPHER SUTHERLANDDepartment of Geology and Environmental Science,MSC 6903 James Madison University,Harrisonburg, VA,USA.

E-mail: [email protected]

M. AMELIA V. LOGANDepartment of Mineral Sciences,National Museum of Natural History,Smithsonian Institution,Washington, DC,USA.

SCOTT D. SAMSONDepartment of Earth Sciences,Syracuse University,Syracuse, NY,USA.

MARTIN FEELYEarth and Ocean Sciences,Quadrangle Building,National University of Ireland,Galway,Ireland.


Appendix 1

Table A1*Feldspar compositions.

Na-feldspar phenocryst analyses (on the basis of 8 O)

1 2 3 4 5 6 7 8 9

Si 2.98 3.01 3.00 3.00 3.00 2.98 3.00 2.97 2.98

Ti � � 0.00 0.00 � 0.00 0.00 0.00 0.00Al 1.02 1.00 0.99 1.00 1.00 1.01 1.00 1.02 1.01

Fe 0.00 � � � � 0.00 0.00 0.00 0.00Ca 0.02 0.00 0.00 0.01 0.00 0.01 0.01 0.01 0.01

Na 0.99 0.96 1.01 1.01 1.00 1.03 1.00 1.02 1.02

K 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

Total 13.01 12.98 13.01 13.01 13.00 13.03 13.01 13.03 13.03

Na-feldspar phenocryst analyses (on the basis of 8 O)

10 11 12 13 14 15 16 17

Si 2.99 2.98 2.96 2.99 2.99 2.98 2.96 2.97

Ti � 0.00 0.00 0.00 0.00 � � 0.00Al 1.01 1.02 1.04 1.01 1.01 1.02 1.03 1.02

Fe 0.00 0.00 0.00 � 0.00 0.00 � �Ca 0.01 0.00 0.01 0.01 0.01 0.00 0.00 0.00

Na 0.99 1.04 1.04 1.02 0.98 1.03 1.06 1.03

K 0.00 0.00 0.00 0.00 0.00 0.01 0.00 0.00

Total 13.00 13.04 13.05 13.02 12.99 13.03 13.06 13.03

K-feldspar ground-mass and ocelli analyses (on the basis of 8 O)

1 2 3 4 5 6 7 8 9

Si 3.08 3.04 3.00 3.05 2.28 2.49 2.42 2.49 2.98

Ti 0.00 � 0.00 0.00 0.00 0.00 0.00 0.00 0.00Al 0.92 0.96 1.01 0.95 2.00 1.59 1.81 1.59 1.02

Fe 0.00 0.00 0.00 0.00 0.06 0.14 0.07 0.17 0.01

Ca 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

Na 0.01 0.02 0.02 0.02 0.00 0.01 0.00 0.01 0.01

K 0.91 0.93 0.96 0.92 0.68 0.61 0.60 0.58 0.99

Mg � � � � 0.03 0.19 0.09 0.18 �

Total 12.92 12.95 12.98 12.95 13.06 13.03 12.98 13.02 13.01


Appendix 2

Table A2*Zircon compositional analyses.

Zircon 1

Analysis# 1 2 3 4 5 6 7 8 9

SiO2 32.32 32.73 32.76 32.50 32.74 32.94 32.34 32.09 32.79

Al2O3 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

FeO 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

CaO 0.00 0.00 0.00 0.00 0.01 0.00 0.01 0.01 0.00

Na2O 0.02 0.03 0.02 0.02 0.03 0.03 0.02 0.02 0.03

ZrO2 65.92 66.06 65.42 65.58 65.65 67.12 65.91 65.60 66.04

HfO2 0.91 1.03 1.11 1.57 1.31 1.28 1.29 1.27 0.91

Y2O3 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

UO2 0.01 0.00 0.00 0.07 0.02 0.04 0.06 0.16 0.01

ThO2 0.02 0.02 0.02 0.04 0.01 0.04 0.06 0.19 0.03

TiO2 0.03 0.04 0.04 0.04 0.04 0.04 0.04 0.04 0.03

Total 99.23 99.90 99.36 99.82 99.80 101.48 99.74 99.36 99.84

Zircon 2

1 2 3 4 5 6 7 8

SiO2 32.82 32.88 33.05 32.88 33.09 33.48 33.00 32.62

Al2O3 0.00 0.00 0.00 0.00 0.00 0.09 0.05 0.00

FeO 0.01 0.01 0.00 0.01 0.01 0.01 0.01 0.01

CaO 0.00 0.00 0.00 0.00 0.00 0.00 0.01 0.00

Na2O 0.04 0.04 0.04 0.04 0.03 0.02 0.02 0.03

ZrO2 65.40 64.85 64.79 65.55 65.17 65.39 64.96 65.39

HfO2 1.30 1.28 1.26 1.25 1.19 1.23 1.39 1.24

Y2O3 0.00 0.00 0.00 0.00 0.08 0.20 0.00 0.01

UO2 0.10 0.07 0.05 0.05 0.08 0.12 0.03 0.06

ThO2 0.10 0.06 0.05 0.05 0.07 0.12 0.05 0.05

TiO2 0.04 0.04 0.04 0.04 0.04 0.05 0.04 0.04

Total 99.81 99.22 99.29 99.86 99.77 100.71 99.57 99.45

Zircon 3 Zircon 4 Zircon 5

1 2 3 4 5 1 2 1 2

SiO2 32.26 32.47 32.39 21.75 23.26 32.26 32.19 32.74 32.50

Al2O3 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

FeO 0.00 0.00 0.00 0.00 0.01 0.00 0.00 0.00 0.00

CaO 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

Na2O 0.05 0.04 0.04 0.04 0.04 0.03 0.02 0.08 0.07

ZrO2 66.23 65.65 65.75 65.17 65.89 66.01 66.59 66.18 66.79

HfO2 1.08 0.94 1.05 0.98 1.10 1.06 1.06 1.35 1.10

Y2O3 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

UO2 0.00 0.00 0.00 0.01 0.00 0.00 0.00 0.03 0.00

ThO2 0.01 0.01 0.01 0.02 0.01 0.00 0.00 0.01 0.00

TiO2 0.04 0.04 0.04 0.03 0.03 0.03 0.03 0.04 0.03

Total 99.67 99.16 99.28 88.01 90.33 99.40 99.90 100.42 100.51


Table A2 (cont.)*Zircon compositional analyses.

Zircon 5 Zircon 6

3 4 5 1 2 3 4 5

SiO2 32.71 32.93 32.74 32.82 32.00 32.84 32.53 32.43

Al2O3 0.00 0.01 0.00 0.00 0.00 0.00 0.00 0.00

FeO 0.01 0.01 0.02 0.00 0.00 0.00 0.00 0.00

CaO 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

Na2O 0.05 0.05 0.04 0.03 0.03 0.03 0.02 0.03

ZrO2 66.47 66.45 66.63 67.89 66.97 66.69 66.35 66.18

HfO2 1.13 1.04 1.14 1.03 1.03 1.02 1.02 1.02

Y2O3 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00

UO2 0.00 0.01 0.00 0.00 0.00 0.00 0.00 0.00

ThO2 0.00 0.01 0.00 0.00 0.00 0.00 0.00 0.00

TiO2 0.04 0.04 0.04 0.04 0.03 0.03 0.04 0.03

Total 100.40 100.55 100.60 101.81 100.06 100.61 99.97 99.70


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