GeoMôn

Anglesey Geopark

 

Above: palaeogeographical map for 650 million years ago, after the Rodinia supercontinent had started to break up. Image: Christopher R. Scotese, Palaeomap Project.

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: 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: 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.

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.

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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.

parys_ore

 

Part 03: 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.

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Above: well-bedded red cherts of ocean-floor origin, now part of the Gwna mélange, exposed at Llanddwyn Island. Photo: Brian Windley.

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Above: palaeogeographical map well into the Cambrian Period. Image: Christopher R. Scotese, Palaeomap Project.

Geology of Anglesey: A journey through time