ABSTRACT: Complex, and ocean floor basalts found in the

ABSTRACT: The abundance of volcanic rocks found in the
Ordovician period of Wales suggests widespread volcanism. A variation in
geochemical compositions, mainly basalt-andesite-rhyodacite typical of modern volcanic arcs
found in the Rhobell Volcanic Complex, and ocean floor basalts found in the
Fishguard Volcanic Complex of Wales suggest two distinct forms of volcanism
generally agreed upon in the literature. The causes of these two distinct types
of volcanism, arc and marginal basin volcanism, have been the subject of
ongoing debates. Arc volcanism is thought to have occurred in late Tremadoc-early Arenig times with a southerly subduction
of the Iapetus Ocean beneath Avalonian margins. Paleomagnetic data backed up by
faunal evidence place the rifting of Avalonia from Gondwana and its northward
migration in Arenig-Llanvirn times, which raises the question of
whether rifting was involved in the onset of arc volcanism. Predominantly
marine stratigraphic samples from Arenig times suggest a period of volcanic
quiescence with possible marine transgression. Geochemical data from magma
samples of mid to late Ordovician in the Fishguard Volcanic Complex point out
to a back-arc spreading, but the location of this back-arc in Wales still remains unclear.  To conclude, rifting of Avalonia-Gondwana might have happened in two-stages with different time intervals and the
proposed location of the back-arc near Lake District is tectonically not
possible and must have been closer to Wales.

 

1.INTRODUCTION

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The
geological evolution of Britain during the Ordovician is characterised by
periods of intense volcanic and tectonic activities. In Wales, the rocks
particularly record the evolution of arc volcanism and a subsequent transition
to marginal basin volcanism. Paleogeographic reconstructions (Fig. 2) place the position of Avalonia,
Gondwana as well as other continental fragments at Southern Hemisphere
latitudes during the early Ordovician (Pickering
and Smith, 1995; Torsvik et al.,1996). Throughout the Ordovician (Fig 1) Wales was part of Avalonia, a
microcontinent which was located on the northern margin of supercontinent Gondwana.

However,
Avalonia did not remain connected to Gondwana for much longer and instead may
have migrated northward after rifting. A time range for this separation at the
Avalonia-Gondwanan margin has been devised through the use of faunal contrasts
between continents by Cocks and Fortey.
(1990) and paleomagnetic data by the likes of Torsvik et al. (1996) and
Channell et al. (1992). Although this rifting and northward migration has
generally been agreed upon in the literature, certain details are still being
debated.

A
period of arc volcanism dominated the early Ordovician of Wales as proposed by Kokelaar (1979) and Koklaar et al. (1982), during the
closure and southerly subduction of an Iapetus ocean below the northern margin
of Avalonia. This arc volcanic episode was succeeded by marine transgression
and the development of marginal basin volcanism in Wales.

This
paper aims to review the existing evidence in the literature pointing out to
the events that might have resulted in the development of arc and marginal
basin-type volcanism in Wales during the Ordovician. In addition, the controversial
relationship between rifting and the onset of arc volcanism based on timing, as
well as the proposed location of a back-arc during the development
of a marginal basin in Wales are examined and alternative solutions proposed.

 

Figure 1: Chronostratigraphic chart
showing global series and stages in correlation with chronostratigraphic units
recognised in rocks of the UK. Source: Fortey et al. (2000).

 

 

Figure 2: Paleogeographic
reconstruction showing the position of Avalonia and Gondwana (South) as well as
other continental fragments during the Early Ordovician (Arenig). Source: Scotese and McKerrow (1990).

 

2.DATA & METHODOLOGY FROM WALES

 

 2.1 PALEOMAGNETIC DATA

Numerous attempts at palaeomagnetic data
compilation for the lower Paleozoic (Ordovician to Permian) of Southern and
Northern Britain have been made, notably by Torsvik et al. (1990a) are shown in Fig 3 and earlier by Briden
et al. (1984, 1988). Southern Britain, of which Wales was part, belonged to
the Avalonian continental fragment while Northern Britain belonged to the
Laurentian continent. Fig 3a comprises
three groups of poles corresponding to the Ordovician (squares),
Silurian-Devonian (diamonds) and Carboniferous-Permian (circles) time periods
and Fig 3b displays the revised
Apparent Polar Wander Paths (APWPs) indicated as stars on the Equal area plot.

Figure 3: (a) Palaeomagnetic poles
representing the Ordovician-Permian of Southern Britain (b) Revised model with new amended paleomagnetic poles (SH), (CA) and
(TV). Sources: Torsvik et al. (1990a), Trench & Torsvik (1990)

           

Three Ordovician poles appear to be off by
comparison of Fig 3a and Fig3b, namely (SH) obtained from
intermediate hypabyssal or volcanic intrusions of the Shelve Ordovician inlier
in mid-wales, (CA) obtained from the Carrock Fell Gabbro and (TV) from the
Tramore Volcanics of SE Ireland. A correct paleogeographic reconstruction (Fig 4) shows the position of Avalonian
continental fragments relative to Gondwana during late Tremadoc-early Arenig
(a) and Llanvirn (b).

 

 

 

 

Figure 4a: Paleogeographic
reconstruction of late Tremadoc-early Arenig.
Sources: Western and Eastern Avalonia Bullard et al. fit (1965), Gondwana Van
der Voo (1988)  (pole:  34″N, 007″E, African co-ordinates),
Laurentia and Northern Britain (Torsvik 
et al. 19906, pole: 13″S,29″E, European co-ordinates in a
Bullard  et al. fit (1965)), Baltica
(Torsvik er al. 19906,  pole: 31″,
086″E), Armorica (A) (Torsvik  et
al. 199oa, table 7,490 Ma  pole: 30″N,
334″E), Siberia with a mean pole of 30″N,  330″E (Torsvik  et al. 1990a)

 

Figure 4b: Paleogeographic
reconstruction of the Late Llanvirn (Llandeilo). Sources: Avalonia from Trench & Torsvik (1991, table 2, 470  Ma pole: 12″N,  23″E), 
Baltica  (Torsvik er al. 19906,
combined path X and Y, 470  Ma pole:  21″N, 
32″E), Armorica  (Torsvik er
al. 1990a, table 7,470 Ma pole: 33″N,  345″E), Laurentia and Northern Britain
(Torsvik er al. 19906, pole: 223, 19″E, European co-ordinates).

 

 

 

2.2 SUBSIDENCE ANALYSIS  

Stratigraphic sections have been sampled by Cowie et al. (1972) and Williams et al. (1972) in areas of
Wales to obtain well dated data for the Cambrian-Ordovician stratigraphic
profile of Eastern Avalonia (Fig 5),
the continental fragment of Avalonia that contained Wales.

 

Figure 5: Cambrian-Ordovician
stratigraphic sections of eastern Avalonia. Sources:  Cowie et al., (1972); Williams et al., (1972).

 

The sequences are mainly composed sedimentary
deposits of mudstones, sandstones, conglomerates, limestones and turbidites,
with volcanic deposits observed in the Tremadocian to Caradocian time interval.
Before obtaining a stratigraphic profile, the stratigraphic sections were back-stripped,
a process which removes the additional subsidence resulting from sediment loading and isolates the
subsidence due to tectonic forces (or decompacted) as first described by Steckler and Watts (1978). The
depositional thicknesses were then substituted by a same water thickness and used
to obtain a stratigraphic profile as shown in Fig 6. The
stratigraphic profile is a graphical representation of vertical movement which
can help us reconstruct the subsidence history of a basin and thus rifting (Van Hinte, 1978).

Figure 6: Cambrian-Ordovician
subsidence curves from localities shown in Figure 5. Main sources: Cowie et al.
(1972) and Williams et al. (1972).

 

 

2.3 FAUNAL EVIDENCE

The trilobite and brachiopod faunas found in
South Wales and elsewhere in Avalonia belong to the calymenacean-dalmanitacean
province typical of western Gondwana (Cocks
& Fortey 1982, 1988) as shown in Fig
7 below.

 

Figure 7: Early Ordovician (Arenig-Llanvirn)
paleogeographic reconstruction based on trilobite fauna, showing the
palaeolatitudinal relationship between Gondwana and Avalonia (South) as well as
other continental fragments. Avalonian trilobite fauna belong to the
Calymenacean-Dalmanitacean province. Source: Cocks and Fortey (1988).

A
remnant of the late tremadoc volcanic episode and ensialic destructive plate
margin volcanism in Wales, mainly inferred from basalt-andesite-rhyodacite, which survived
through Arenig time to the present is the Rhobell Volcanic Complex (RVC) as
suggested by Kokelaar (1977,1979,1986);
Kokelaar et al. (1982). The RVC is a series of N-S trending dykes and
intrusions which cut across uplifted and folded, broadly N-S trending areas
like the Harlech dome in Wales. The tectonic relations of the RVC and Harlech
dome is somewhat complex but can be summarised as shown in Fig 8.

 

Figure 8: Geological map summarising the
tectono-volcanic relationship between the Harlech dome and the Rhobell Volcanic
Complex (RVC) in Wales. All the dykes at the RVC site are shown and sediments
are presumably mainly composed of sediments derived from the Harlech dome.
Sources: Unpublished work of C.  A.
Matley (BGS archive); Matley & Wilson (1946); Kokelaar (1977); Institute of
Geological Sciences (1982); Allen & Jackson (1985); M.  Smith pers. comm. (1987).

 

 

 

 

 

Arenig
(non-volcanic) sediments have been sampled in part by Kokelaar et al. (1985)
and Craig (1985) in South Wales as shown in Fig 9, and their facies give the stratigraphic panel shown in Fig 10. These facies confirm
predominantly marine conditions in Wales during the Arenig and have been
proposed by Fortey (1984) to be
associated with an eustatic sea-level change that may have occurred worldwide
at the time.

 

 

 

 

Figure 9: Map showing locations for
the six areas studied (1- St David’s, 2- Llanferran, 3- Treffgarne Gorge, 4-
Whitland, 5- Llangynog inlier, 6- Carmarthen). L= Basement lineaments, RF= Ramsey
Fault. Fennian, Whitlandian and Moridunian are all subdivisions of the Arenig
time interval. Sources: Kokelaar et al. (1985); Craig (1985).

 

Figure 10: Stratigraphic panel
showing correlation of logs from areas studied. The difference members and formations
are numbered from 1 to 27 and a key is provided to help identify and correlate
facies. Sources: Kokelaar er al. (1985); Fortey and Owens (1987).

 

Other
samples collected by Bevins (1982) are volcanic deposits of basic to acid
composition, located to the northwest and northeast of the Fishguard, in the
Fishguard Volcanic Complex of Wales. To the northwest, the volcanic deposits
occur as basaltic pillow lavas and sheet flows as described by Kokelaar et al. (1984b) thus providing
evidence for a subaqueous setting. 

To
the northeast however, the rocks are predominantly thinner rhyolites and
rhyolitic tuffs that were in instances formed and deposited in a subaqueous
environment (Lowman and Bloxam, 1981).
The discriminant diagram in Fig 11 and
classification scheme in Fig 12
below help classify the provenance of magma in the Fishguard Volcanic Complex
and therefore can be used as evidence for a subaqueous environment in Wales and
perhaps marginal basin transition from early Llanvirn to later Ordovician
times. An explanation for the derivation of this discriminant diagram is also
provided in Fig 13.

 

 

Figure 11: Discriminant diagram for Titanium (Ti),
Yttrium(Y) and Zircon (Zr) of the Fishguard Volcanic Complex in Wales. LKT =
low-K tholeiite, CAB = calc-alkaline basalt, OFB = ocean floor basalt, WPB =
within-plate basalt. Source: Derived from Pearce and Cann (1973).

 

 

 

 

Figure 12: Proposed classification of magma provenance based on tectonic
plate motion. Source: Pearce and Cann (1973).

 

 

Figure 13: Diagrams explaining how discriminant
diagrams are derived. Source:
Pearce and Cann (1973).

 

 

 

3.DISCUSSION

3.1 INTERPRETATIONS

The existence of a proto-Atlantic (Iapetus) ocean is unquestionable based on evidence of a
faulted junction separating two faunal realms, ‘Pacific’ and ‘Atlantic, of
Scotland and Wales as proposed by J.
Tuzo Wilson’s (1966). The distribution of lower Ordovician graptolite
faunal provinces shows differences between wales in the South-eastern margin
with an Atlantic graptolite fauna (dominantly pendent Didymograptus species)
and southern Scotland in the North-western margin with a Pacific graptolite
fauna (no pendent Didymograptus species) (Skevington
I974). These differences progressively disappeared during the Ordovician,
thus proving the closure of an Iapetus ocean.

Arc
volcanism arises during the subduction of a denser plate beneath a plate of
lower density. The addition of fluids due to high temperatures and pressures
during subduction, lowers the solidus and causes partial melting of the crust
into less dense magma which rises up to the surface to form volcanoes. The palaeomagnetic
data showing palaeogeographic reconstructions (Figs 3, 4a and 4b), the subsidence analysis (Fig 6) showing a major subsidence event
in South Pembrokeshire on the Welsh basin margin which is backed up by the
increase in marine facies in the Arenig of Wales (Fig 10) and the faunal data (Fig
7), all point towards an Avalonia-Gondwana
rifting during Arenig-Llanvirn time and a
subsequent northward migration of Avalonia towards Laurentia. The closure of
Iapetus would therefore result in its subduction beneath Avalonia and the
formation of volcanic arcs which are survived today in North Wales as the
Rhobell Volcanic Complex (Kokelaar 1980)
as shown in Fig 8.

As
opposed to the Tremadoc, Arenig and later volcanic rocks found in Wales appear
to have mainly rhyolitic composition, typical of marginal basin volcanism (Kokelaar et al. 1984; Bevins et al. 1984).
In addition, the discriminant diagram (Fig
11) shows that magma samples from the Fishguard volcanic complex of Wales
concentrate in the 2+4 region corresponding to
OFB (Ocean Floor Basalt), further proving a marine type volcanism, and pointing
towards a back-arc spreading as the
mechanism for marginal basin formation.

3.2 CONTROVERSIES

According
to Kokelaar (1980), arc volcanism
occurred in Wales during Late Tremadoc time. On one hand in the
literature, a late Cambrian-Earliest Ordovician time for the rifting of
Avalonia-Gondwana is proposed by Cocks
and Torsvik (2002). On the other hand, all the evidence gathered in this
report suggests a later time for the rifting. This brings out two main controversies,
(1) whether arc volcanism was actually triggered by this rifting, and (2)
whether the timing for this rifting between Avalonia-Gondwana is correct. Regarding the first controversy, the
paleogeographic reconstructions based on paleomagnetic poles obtained by
sources listed in (Figs 4a and 4b)
show that Gondwana and Laurentia remained roughly in place as Avalonia was
migrating northward. The subduction of the Iapetus ocean beneath Avalonia must
have therefore been caused by the rifting between Gondwana and Avalonia.

Paleomagnetic
evidence (Fig 4) and faunal links (Fig 7) support a late Llanvirn time for
the initiation of rifting. In addition, any faunal contrast observed on Avalonia
earlier than Llanvirn times can potentially
be attributed to Avalonia being on a more northern latitude than Gondwana. Even
the stratigraphic evidence in Fig 6
may be interpreted as having a rapid period of subsidence between
Arenig-Llanvirn time but also during Late Cambrian-Early Tremadoc, and
therefore does not really constitute a solid evidence from which timing for the
rifting of Avalonia and Gondwana can be derived. It is therefore more likely
that the Arenig-Llanvirn timing for this
complete rifting is correct. One possible solution to the issue of late
tremadoc arc volcanism preceding the timing of rifting, is partial rifting.
Eastern Avalonia which contained Wales may have rifted-off Gondwana earlier and
thus may have observed destructive plate margin activity before Western Avalonia.

Another
controversy is the proposed back-arc marginal basin model, first put forward by
Kokelaar et al.(1984b) and which may
have formed behind the Lake District-Leinster terrane. It is important to
remember first here that the subduction of the Iapetus ocean occurred along and
below the Northern margins of Avalonia where any movement of an arc in Wales
would have therefore been seaward. Thus, the major problem with this model is that
the formation of a back-arc basin in Lake District during a southerly
subduction of Iapetus beneath Wales and the Lake district in their present
position and distance relative to each other is unlikely, mainly because this
implies the northward and so seaward migration of an original forearc from
Wales to the Lake District, a process known as trench roll-back, over present
distances. This is not supported by current tectonic models, see Dewey (1980). A distance is clearly
stated, over which trench roll-back may occur at current subduction rates, and
is lower than the one being considered here. A possible alternative might be
forearc migration from Wales over a shorter distance northward, but no evidence
supports this.

4.CONCLUSION

 The chronological causes for arc volcanism and
marginal basin transition in Wales can be assembled. The first proposed, is the
rifting between Gondwana and Avalonia during either the earliest or
mid-Ordovician. Given that Avalonia (comprising Wales) was located on the
south-eastern margin of the Iapetus ocean and that graptolite fauna show
progressive similarities between the south-eastern and north-eastern margins of
Iapetus, we can safely infer a southerly subduction of Iapetus below the
northern margin of Avalonia during closure.

This
subduction of the Iapetus ocean was followed by periods of uplift and magmatic
intrusions in Wales, subsequently leading to arc volcanism during late Tremadoc
times which is survived today by the Rhobell Volcanic Complex (RVC) in Wales (Fig 8). Marine transgression followed in
Wales during the Arenig, and is supported by the predominantly marine facies in
Fig 10. Spreading of a back arc in
the Lake District-Leinster terrane north of Wales followed this marine
transgression, in agreement with Kokelaar
et al.(1984b). Evidence in support of this transition is also given by the
Fishguard volcanic complex in Wales (Fig
11).