TWO-STAGE DIKING AT THE DYING OF THE GALWAY BATHOLITH
by Paul Mohr
[“Igneous is irruptive, unlooked for and peremptory; sedimentary is steady-keeled, dwelt upon,
graduated” (Seamus Heaney)]
INTRODUCTION
Profuse diking is a feature common to many if not most granite batholiths. In the majority of cases
the diking peaked late in the formation of the batholith. That was the case for the Galway Batholith
in the West of Ireland.
Galway Batholith
The Galway Batholith is the westernmost exposed Caledonian batholith on the European
side of the Atlantic basin (Fig. 1),
It is located immediately north of the Iapetus suture which marks the Laurentia-Avalonia
collision line, and south of the allochthonous Connemara terrane (Dalradian crust that was strikeslipped and thrust south of the Highland Boundary Fault some 40 Ma before the intrusion of the
batholith).
The Galway Batholith is 90 x 35 km in elliptical plan, its long-axis aligned ESE-WNW, oblique
to the ENE-WSW Caledonian tectonic trend (Fig. 2). The southern half of the batholith is hidden
under Galway Bay and a cover of Carboniferous Limestone strata;
Emplacement of the batholith sutured the Ordovician Skerd Rocks Fault, a westerly
continuation from the Southern Uplands Fault in Scotland:
- to the north of the Fault, the batholith intruded the plutonic roots of a 470 Ma-old island-arc that
had been deformed into a metagabbro and granite gneiss complex;
- to the south of the Fault, the batholith intruded an Ordovician accretionary prism.
Disregarding its satellite plutons, the main batholith comprises an irregular pattern of a
spectrum of granodiorites and subordinate granites (Fig. 3).
Two major syn-late-intrusive fault zones, Shannawon (SFZ) and Barna (BFZ), divided the
batholith into three structural blocks, the central block the more uplifted and thus more deeply
unroofed.
Fig. 1 - Irish & Scottish Caledonian batholiths
Fig. 2 – The Galway Batholith
Fig. 3 - The component granites of the main Galway batholith
Summary of Intrusive History
The Galway Batholith was assembled in two major intrusive episodes (Table 1):
420-400 Ma - emplacement of the bulk of the granodiorite plutons/sheets, ending with highlevel microgranitic sills;
385-380 Ma - high-level tabular alkali granite intrusions.
Diking followed each of the two plutonic episodes:
1. Richly quartz-pllagioclase-microphyric dacite dikes, zircon dated at 405-400 Ma;
2. Composite dolerite-rhyolite dikes, dated at ca. 375 Ma.
Table 1 – Galway pluton and dike age-ranges
MAJOR EVENTS IN THE LIFE OF THE GALWAY BATHOLITH
LIFE I
420 – 400 Ma
405 – 400 Ma
LIFE II
385 – 380 Ma
ca. 375 Ma
Assemblage of perhaps 90% by volume of the
batholith:
axial granodiorite magma was charged with
abundant enclaves/dikes of appinite (hydrous
diorite).
Uplift, faulting and unroofing of the batholith
late in this period.
Diking by richly q-pl-microphyric dacitic
/granodioritic magmas.
Local intrusion of high-level tabular granites,
typically alkali granites.
Diking by coeval dolerite and subordinate
rhyolitic magmas.
[dates courtesy of Dr. Martin Feely and colleagues]
THE DACITE DIKING (The Earlier Dike Suite)
At the end of the first and most active episode of batholith formation, structural relaxation
perpendicular to the batholithic long-axis produced numerous N- to NE-trending fissure zones
cutting the batholith (Fig. 4).
Dike Fissures
Fissures were concentrated into sets (fissure numbers were too small to perhaps warrant the
term swarms), each set comprising up to 13 closely spaced and parallel fissures;
Fissure widths ranged 10 to 50 m.
Maximum crustal extension across any set was 30%.
Granodioritic magma ascended the fissures, carrying abundant microphenocrysts of Na-
plagioclase, K-feldspar, quartz and biotite – the liquid then crystallising to alkali feldspar and biotite
(see Mohr 2003 for detailed mineralogy).
Fig. 4 - Dacite dike zones in the Galway batholith. (The AGG and SCB dike sets are highlighted in
yellow)
The Ard Mhóir-Gabhla-An Gharmain (AGG) dike set
The prominent AGG dike set is situated in the western block of the Batholith (Fig. 4),
(n.b. The dense but only briefly exposed Salthill (SCB) dike set in the eastern block is now buried
under an explosively expanding Galway City!)
The AGG dikes can be traced 20 km north from the batholith axis, across the northern
exposed half of the batholith and on through the 70 Ma-older gneissic envelope (Figs. 4 and 5).
The number and widths of AGG dikes decrease northwards.
The AGG dikeset was no sooner emplaced than it was subjected to dextral displacements
along a grid of ENE-WSW oblique-slip faults (Fig. 5), the initial movement on which overlapped
with the terminal dike irruptions.
Fig. 5 - The AGG dike set, showing its individual dikes (simplified) and the ENE-trending faults that
displaced it soon after emplacement. (The individual sectors between the faults are named)
Geochemistry
Compared with the four major granitoid lithologies comprising the batholith, dike dacite most
closely matches the predominating megacrystic granodiorite in its geochemistry:
In both suites, SiO2 = 67  3% . Figure 6 confirms close similarities in the abundances of
FeO, MgO, Ni, TiO2 and most trace elements (green = dacite, red = megacrystic granodiorite).
Significant differences are restricted to the mobile elements: notably, K and Rb are reduced in
dacite cf. granodiorite, although K/Rb remains almost identical.
Low Nb (c.15 ppm) and high Ba/La values for the dacites are typical of subduction-related
silicic magmas.
Fig. 6 - Dacite geochemistry compared with that of the major granitoids of the Galway batholith
There is a further, both singular and striking geochemical feature of the AGG dikes (Fig. 7):
Fig. 7 - along-strike AGG geochemistry (see Fig. 5 for map)
Dike geochemistry (& mineralogy) reveals a progressive increase in maficity away from the
batholith axis. This lateral progression, from rhyodacite through dacite and micromonzonite to
trachyandesite, is undeflected where it crosses the batholith margin. From this it would appear that
the contrasting contaminations to be expected from granitic cf. amphibolitic inputs have not
operated significantly in the AGG dikes.
An alternative possibility, that of magma mixing, would require an unexposed mafic body in
the north. If such a body exists, one could speculate that the rise of mafic magma here followed
from an easier path of ascent outside of the zone of contemporary granitoid plutonism. Such mafic
magma is envisaged to have had the appinitic (water-rich dioritic) composition of the near-coeval
enclaves in the axial granodiorite of the batholith. Indeed, small such enclaves are present in the
root zone of the Spiddal dacite dike, deeply exposed in the central block of the batholith (Fig. 4).
A third possibility is that crystal fractionation in a single dacitic magma progressed northward
as the magma moved with a lateral component out from the batholith axis. This however would be
contrary to the flow direction well displayed in an AGG dike at An Maoileann (Fig. 8).
Fig. 8 - successive flows in the AGG dike at An Maoileann (the batholith axis lies southward to the
right. ‘mg’ = countryrock microgranite)
The indicators of flow direction here reveal a horizontal component directed southward
toward the batholith axis. How to reconcile this flow with the diminishment of diking intensity
northward is not yet answered, but the possibility of local convective cells in the cooling dike
requires further field examination.
Tectonics
The cross section provided in Figure 9 shows three profiles:
1. the original roof of batholith, after displacement by major syn-plutonic faulting (dashed line);
2. the topographic surface at the time of diking and presumed coeval volcanism (solid line);
3. the present topography can be represented by a roughly horizontal line through the subvolcanic magma chambers (shown as high-level sills).
The dacite dike sets in the batholith dip steeply in toward the N-S short-axis of the batholith,
as though toward a central magmatic focus.
The AGG & SCB dike sets each face a complementary normal fault-zone. The resulting
downward-converging dips have the form of an asymmetric graben. Such graben associated with
diking have been described from other batholiths, for example from mid-Eocene Washington
State, U.S.A.
Fig. 9 - Batholith long-axis cross-section at the time of diking
(SFZ = Shannawon Fault Zone, BFZ = Barna Fault Zone, MFZ = Maam Fault zone)
THE COMPOSITE DIKING (The Later Dike Suite)
The 2nd diking episode followed the 2nd plutonic phase in the building of the Galway
batholith. It provided the final Caledonide igneous activity in the West of Ireland.
Fig. 10 - Dolerite and composite dikes in the Galway batholith and its envelope. The major dikes are
named (‘NA hU’ = na hUilíInní). TD = Teach Dóite (Maam Cross). Dolerite dikes are indicated in green;
red termini signify that rhyolite members occur in the dike thus marked.
Pattern of composite diking
The Mid-Devonian, second episode of fissuring in Connemara had a wider extent and a
greater freedom from batholithic structural control than did the earlier, dacite diking episode (Fig.
10). Notable in this regard, however, is the northern barrier to the fissuring provided by the
Connemara Dalradian crustal terrane.
Single fissures, not associated in sets (cf. the dacite dikes), were subject to one of three
structural controls:
1. NNE trend: along now-extinct normal-fault zones active during the evolution of the batholith;
2. ENE trend: along oblique-slip fault zones initiated during the waning of the 1st dike episode,
and movement ending during this 2nd dike episode;
3. ESE trend: conformable in 3-D with the steep southern limb of the folded Connemara
Dalradian rocks.
Basaltic magma then took advantage of these three fissure trends.
Mineralogical summary (see Mohr 2004 for detailed mineralogic descriptions)
Dolerite: labradorite and augite (non-ophitic), subsequently hydrated in a greenschist-grade
metamorphism with some associated iron-enrichment. Rare serpentinised olivine
crystals are present in only a few samples.
Rhyolite: felsitic groundmass carries abundant resorbed quartz and albite microphenocrysts.
Some rhyolites evidence incomplete mixing of two magmatic inputs.
Mingled and mixed hybrid rocks have xenocrysts that circulated across the magmatic
interphase layer, and occasionally back again. There is a wide spectrum of mineralogy among the
rhyolites that attests to contamination from an iron-rich source, and this is confirmed in the
geochemistry (see below).
Table 2 - mean dolerite and rhyolite dike chemistries
Geochemistry
The basaltic magma had a MORB tholeiitic composition (Table 2), expressed in mean K2O =
0.25%. Dolerite samples from composite dike sectors show evident contamination (cf. from simple
dolerite dikes), revealed in significantly higher K2O and Rb contents.
The rhyolite magma was a high silica (76% SiO2), low-CaO (0.1%), low ITE melt, the
composition of which cannot be derived from any exposed batholithic rock by crystal fractionation
of granitoid phases.
Fig. 11 - Hybridisation among rhyolite components of nine widely distributed Galway composite
dikes. (The Rosmuc rhyolite plot, ‘rm’, is highlighted in red)
Hybridisation of rhyolite magma by coeval basalt magma is confirmed in linear two-element
plots from nine widely distributed rhyolite samples (Fig. 11). Rosmuc rhyolite provides the least
contaminated end-member in all plots.
An unexpected feature of the composite dikes is a sympathetic relationship of dolerite and
rhyolite geochemistry according to the nature of the countryrock (Table 3), following from the
geographical partitioning of the Connemara crust generated by the intrusion of the batholith.
Table 3 - dolerite and rhyolite share common geochemical fingerprints
Comparative geochemistry of composite-dike lithologies
from within and outside the Galway Batholith
Dolerite emplaced within cf. outside the batholith:
higher Mg, Li, (?Na)
lower K, Nb, Y, Zr (?Ti, V)
Rhyolite emplaced within cf. outside the batholith:
higher Mg, Li, (?Co, Sc, Sr, V, Zn)
lower K, Nb, Y, Zr, (?Ce, Cr, La, Ni)
[In each case, elements are listed in their order of decreasing differential]
Comparing the chemistry of dolerite and rhyolite from outside the batholith with dolerite and
rhyolite from inside the batholith reveals almost identical distinguishing fingerprints for the two
lithologies (Table 3). How might this be?
Accepting the hypothesis that the rhyolites had an anatectic origin (see argument in Mohr
2004), then it can be relevant that restitic lower crust under Ordovician island-arc Connemara had
been made even more refractory where the massive Galway Batholith had most recently been
sourced.
A refractory nature for the rhyolite magma source is consistent with higher Mg and Li in the
magma batches generated under and passing up through through the bathollith. The same
argument is supported by lower abundances of K and polyvalent elements (sourced from remnant
fusible components in restitic crust) in the intrabatholith dike magmas.
But why might this rhyolite geochemical division be shared by that of companion dolerite?
The geochemical 'fingerprints' common to dolerite and rhyolite, according to whether
emplaced inside or outside the batholith, are considered to express three factors linked in
succession:
1. ponding of basaltic magma in the lower crust;
2. partial melting of this crust, made more restitic under the batholith, yielded restricted batches
of silicic melt;
3. mutual contamination of locally aggregated rhyolitic magma with basaltic magma ascending
together up common fissures, leading to the shared geochemical signatures.
Mafic magma sources under Caledonian Connemara
The remnant subducting slab from the final closure of the Iapetus Ocean supplied mafic magma
that penetrated the Connemara crust during two episodes:
1. water-rich appinitic magmas rose from the still-dehydrating slab during the period of batholithic
plutonism which they likely facilitated;
2. at the end of that first, 40 Ma-long episode, MORB-type basaltic magma generation then
replaced the appinitic magmatism from a now dehydrated slab remnant.
SUMMARY
Two episodes of diking during the dying of the Galway Batholith expressed two late episodes
of upper crustal fissuring and magmatic ascent:
1.
N-NE crustal fissuring occurred in response to stress relaxation in the cooling batholith.
Residual granodioritic magma residing below the newly emplaced batholith then rose to supply the
dacite dikes. Meanwhile, mantle-derived magma reaching up in small volume outside the active
batholith that it was fostering, provided a mixing component into the more active and extensive of
these dikes.
2.
Late regional extension throughout Connemara produced isolated fissuring along preexisting lines of crustal weakness. The batholithic magmatism now extinct, mantle magmas rose
with equal ease through the rigid batholith and its envelope, remelting restitic crust en-route and
so producing high-level composite dolerite-rhyolite dikes.
Acknowledgments
Dr. Sadhbh Baxter kindly aided with the provision of Figures 1-3. Dr. Martin Feely gave
generous encouragement in facilitating the presentation of this paper to the GSA 2007
Northeastern Section Meeting in Durham, N.H.
n.b. detailed accounts of the Caledonian Connemara dikes are to be found in:
Mohr, P. 2003. Late magmatism of the Galway Granite batholith: I. Dacite dikes. Irish Journal of
Earth Sciences 21, 71-104.
Mohr, P. 2004. Late magmatism of the Galway Granite batholith: II. Composite dolerite-rhyolite
dikes. Irish Journal of Earth Sciences 22, 15-32.