Geology of Anglesey: A
journey through time
Part
1 Part 2
Part 3 Part
4
Part 3: Painting the
picture:
A well-travelled island....
Beginnnings - the
destruction of Rodinia
The oldest rocks in Britain, cropping out in
North-west Scotland, had already undergone two
major episodes of mountain-building and
metamorphism by the time that a multiple series
of continental collisions threw together the
Rodinia supercontinent around 1,300 million years
ago. At the time of the formation of Rodinia,
England and Wales simply didn't exist at all! The
supercontinent, straddling the equator and
stretching nearly from pole to pole, was a
relatively long-lived affair that only broke up
about 750 million years ago into a series of
smaller continents.
By late Precambrian times, 600 million years ago,
these were bunched together again, this time
close to the South Pole as the supercontinent
Pannotia. Because so much land was near the
poles, the Late Precambrian was an Ice House
World with extensive glaciation. However, this
time, supercontinent breakup occurred relatively
rapidly at the beginning of the Cambrian Period,
540 million years ago. Three of the four major
fragments - Laurentia (North America, but
including at that time Scotland and Greenland),
Baltica (Scandanavia and Eastern Europe) and
Siberia (Russia east of the Urals) drifted
northwards; the fourth and by far the largest,
Gondwana, remained well to the south.
Volcanic roots....
England and Wales first came into being as a
group of active volcanic islands situated along
the northern margin of Gondwana, over 650 million
years ago. These Older Arcs, as they are known,
included what is now the Malvern Plutonic Complex
of Central England and, some distance away on
Anglesey, the high-grade gneisses surrounding the
Coedana Granite.
From about 630-570 million years ago, much more
extensive arc-type volcanic activity took place,
further adding to the crust already built,
expanding in size to cover much of an area from
Wales across to Eastern England, forming part of
the land of Avalonia. On Anglesey, evidence for
activity at this time comes from radiometric
dates for the Coedana Granite, which intruded and
hornfelsed the Older Arc gneisses.
The relationship of Avalonia to Anglesey at this
time, in terms of their distance apart, remains
unknown: however what we do know is that in the
latest Precambrian, Anglesey was approaching
Avalonia, with the oceanic crust in between them
being subducted southeastwards beneath the
latter.
Down the trench....
Subduction occurred, as with modern equivalents,
in a deep oceanic trench into which the oceanic
rocks overlying the lithosphere tumbled into
chaotic debris flows, resulting in thick, poorly
stratified deposits in which are juxtaposed
kilometre-scale rafts, huge boulders, cobbles and
pebbles of oceanic ridge-type pillow basalt,
shallow-water quartzite and algal limestones,
bedded cherts, red mudstones and, locally,
granite debris eroded from the Older Arc which
can be matched with the Coedana Granite. Thus was
formed the famous Gwna Mélange.
Some of the mélange was subducted with the
underlying oceanic lithosphere to great depths -
tens of kilometres - beneath the north-west
margin of Avalonia, where its properties,
including its mineralogy, were drastically
altered under the great pressure prevailing.
....and back up again!
Finally, subduction slowed down and then ceased.
Under its own buoyancy, the upper part of the
highly pressurised slab of oceanic lithosphere
and trench-fill broke free and slowly began to
rise up out of the much denser upper mantle,
creeping back towards the surface until it found
physical equilibrium with its surroundings, to be
exhumed many millions of years later following
uplift and erosion, as the Blueschist Belt.
Metabasites within the Blueschist Belt contain
sodic amphibole (glaucophane) together with
epidote, hematite and quartz. The mineral
assemblage is believed to have developed firstly
during low-grade ocean-floor metamorphism 590-589
million years ago, and then secondly during the
high-pressure, low-temperature metamorphism
within the subduction zone itself, no more than a
few tens of millions of years later.
Meanwhile, on the passive
margin....
Seaward from the subduction zone, with its at
times violent geological processes, things were a
little quieter. Here, in shallow marine basins,
clastic sediments were being deposited from
undersea turbidity currents. In some areas,
clean, fairly coarse quartz-dominated sands,
eroded from nearby land, were deposited. In
others, there was a greater input of silt and mud
resulting in layered but generally finer-grained
deposits.
Thus were the strata that make up the South Stack
Group and the New Harbour Group formed. After
their deposition, a number of igneous bodies
became emplaced within the New Harbour Group
turbidites. These include gabbros and
serpentinised ultrabasic rocks (primarily dunites
and harzburgites). These are believed to
represent parts of a tectonically emplaced
ophiolite sequence including slivers of oceanic
crust - the term "dismembered
ophiolite" is often applied to them. Some of
the serpentinites were quarried for ornamental
serpentine whilst others were worked as a source
of chromite, often a common mineral in such
rocks.
At some point following their deposition, an
episode of compressive deformation affected the
South Stack and New Harbour groups, which, along
with all other Precambrian units, were uplifted
and folded. Folding in the thinly-bedded strata
of the New Harbour Group is especially
spectacular. The deformation event is recognised
in some other parts of Wales and also in New
England and Newfoundland - see Part 4 to find out
why! The whole area appears to have remained as
land from then right through to early Ordovician
times.
Avalonia goes North
In early Cambrian times, Avalonia began to rift
away from Gondwana and begin its slow drift in a
generally northwards direction, so that by
Silurian times it was making its way through the
tropics. Lying between it, Laurentia and Baltica
there lay a wide sea - the Iapetus Ocean - and,
as in the past, subduction of the ocean floor was
ongoing, with island-arc volcanoes especially
active during the Ordovician Period across Wales
and Cumbria. So what was going on on Anglesey,
now very much a part of Wales?
The beginning of the Ordovician Period ushered in
the transgression of the sea over the deformed
Precambrian rocks of Anglesey. As this process
advanced, wave action severely eroded the
Precambrian strata and thus spectacular
conglomerates form the earliest Ordovician
deposits, of Arenig age (about 480 million years
old), above the striking unconformities seen on
the north coast. Most of the later Ordovician and
overlying lower Silurian deposits consist of
sandy and silty facies and in some places
graptolitic shales appear, indicating the
sporadic existence of areas of deeper water.
Volcanic activity was localised, compared to the
major events going on on the mainland.
The Black-Smokers of
Amlwch.....
Silurian strata are poorly represented on
Anglesey, and comprise a relatively small area of
graptolitic shales outcropping at Parys Mountain.
However, the chief interest of that locality is
its world-famous and complex mineral deposits.
Parys Mountain was a historically famous source
of copper and in the late 18th Century it was
Europe's biggest producer of that metal.
In the early 1960s, the first of several major
exploratory drilling programmes commenced at
Parys Mountain. Successive companies were
attracted to the area with the most recently
active being Anglesey Mining plc, and as time has
gone by, a much better geological picture of the
stratigraphic sequence and the nature of the
mineralisation has emerged. The drilling has also
resulted in the discovery of stratiform lenses of
massive sulphide mineralization, containing
percentage levels of copper, lead and zinc, with
noteworthy concentrations of silver and gold,
both to the west and the north of the old mine.
Several million tonnes of ore have now been
demonstrated to be present in these areas.
The mineralisation is accompanied by intense
silicification and pyritisation of the
surrounding sedimentary and igneous rocks. It is
dominated by quartz and pyrite, with important
quantities of chalcopyrite and, in some sulphide
lenses (the "bluestone"), galena and
sphalerite. Numerous uncommon compounds of lead,
bismuth, arsenic, antimony, silver and gold are
also present. The ore deposit is thought to
belong to the Volcanogenic Massive Sulphide (VMS)
class, deposited on the late Ordovician or early
Silurian sea-bed from heated solutions
percolating up through the rocks and exiting into
the water above as "black-smokers".
The destruction of Iapetus
Meanwhile, if we zoom back out in our view of
things, by the end of Silurian times we can see
that ‘proto’ Anglesey had reached 20o
south of the equator. By the end of the Silurian
Period, virtually all of Iapetus had disappeared,
and in the final collision, England and Wales
were, at last, welded to Scotland as Avalonia
collided with Laurentia. The stitch-marks, if any
were visible, would follow a line running ENE
from the Solway Firth.
These events left their mark in the form of
folding of the Lower Palaeozoic rocks and the
already deformed Precambrian strata; in the case
of the latter, successfully complicating the
structural picture even more! This deformation is
today referred to as the Acadian Phase of the
Caledonian Orogeny, the latter being recognised
as a multiphase cycle of progressive deformation
related to Lower Palaeozoic interactions between
Avalonia, Laurentia and Baltica as they drifted
northwards and eventually came together.
The early Devonian Period, therefore, saw this
part of the story come to an end. Sat in a newly
formed continent and drifting towards the
equator, Anglesey's maritime life had apparently
come to an end and a harsh, arid landscape
emerged in its place. Now go to Part 4 to find
out about the final stages of the island's
development up until modern times, and the new
extremes it faced along the way.
Go
to Part 4
Go
to Top
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Above: in the NW Scotland Geopark are found the
oldest rocks in the UK. Darker, Proterozoic
sediments lie upon a paler, ancient eroded
landscape of crystalline "basement",
dating back to the Neoarchaean. Photo: John
Mason.
Above: that part of the geological time-scale
pertaining to the Precambrian, consisting of
three eons - the Hadean, Archaean and
Proterozoic, which are subdivided into eras, such
as "Neoproterozoic" and then into
periods, such as "Ectasian". A small
bit of the Phanerozoic Eon, the Lower Palaeozoic
Era which began with the Cambrian Period, 540
million years ago, is also shown at the top. The
older rocks on Anglesey are late Cryogenian to
Cambrian in age. Graphic: John Mason.
Above: palaeogeographical map for 650 million
years ago, after the Rodinia supercontinent had
started to break up. Image: Christopher R.
Scotese, Palaeomap Project.
Above: well-bedded red cherts of ocean-floor
origin, now part of the Gwna mélange, exposed at
Llanddwyn Island. Photo: Brian Windley.
Above: palaeogeographical map well into the
Cambrian Period. Image: Christopher R. Scotese,
Palaeomap Project.
Above: strong pre-Arenig deformation is recorded
by intense folding within the rocks of the South
Stack Group, on the coast near Holyhead. Photo:
Stewart Campbell.
Above: polished section of ore from Parys
Mountain, viewed under the reflected light
microscope. The field of view is 1mm across.
Highly reflective pyrite (pale yellow) with
chalcopyrite (deeper yellow) and sphalerite
(grey) - typical of the fine-grained sulphide
intergrowths that make up the orebody here.
Photo: Rob Ixer.
Above: palaeogeographical map of the Mid-Silurian
world when Anglesey was, as part of Avalonia,
moving north into the tropics. Image: Christopher
R. Scotese, Palaeomap Project.
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