The Sarn Complex is a group of closely related

igneous rock Igneous rock (derived from the Latin word ''ignis'' meaning fire), or magmatic rock, is one of the three main rock types, the others being sedimentary and metamorphic. Igneous rock is formed through the cooling and solidification of magma or l ...

s that intrude and cut through other rock

lithologies The lithology of a rock unit is a description of its physical characteristics visible at outcrop, in hand or core samples, or with low magnification microscopy. Physical characteristics include colour, texture, grain size, and composition. Lith ...

in the

Cymru Terrane The Cymru Terrane is one of five inferred fault bounded terranes that make up the basement rocks of the southern United Kingdom. The other notable geological terranes are the Charnwood Terrane, Fenland Terrane, Wrekin Terrane and the Monian Comp ...

Wales Wales ( cy, Cymru ) is a Countries of the United Kingdom, country that is part of the United Kingdom. It is bordered by England to the Wales–England border, east, the Irish Sea to the north and west, the Celtic Sea to the south west and the ...

.P. J. Brenchley, P. F. Rawson ''The Geology of England and Wales'', 2006, 2nd Ed The complex outcrops on the Llyn Peninsula in a variety of places including Mynydd Cefnamlwch and the flanks of Pen y Gopa.BGS Lexicon of Named Rock Units
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Geological information

The largest

pluton In geology, an igneous intrusion (or intrusive body or simply intrusion) is a body of intrusive igneous rock that forms by crystallization of magma slowly cooling below the surface of the Earth. Intrusions have a wide variety of forms and com ...

ic body in the terrane has limited outcrop and is sheared by the Llyn Shear Zone in the west and covered by later (

Arenig In geology, the Arenig (or Arenigian) is a time interval during the Ordovician period and also the suite of rocks which were deposited during this interval. History The term was first used by Adam Sedgwick in 1847 with reference to the "Areni ...

) sediments to the east. Altered to

greenschist facies Greenschists are metamorphic rocks that formed under the lowest temperatures and pressures usually produced by regional metamorphism, typically and 2–10 kilobars (). Greenschists commonly have an abundance of green minerals such as chlorite, ...

the

contains a bimodal suite of

gabbro Gabbro () is a phaneritic (coarse-grained), mafic intrusive igneous rock formed from the slow cooling of magnesium-rich and iron-rich magma into a holocrystalline mass deep beneath the Earth's surface. Slow-cooling, coarse-grained gabbro is ...

diorite Diorite ( ) is an intrusive igneous rock formed by the slow cooling underground of magma (molten rock) that has a moderate content of silica and a relatively low content of alkali metals. It is intermediate in composition between low-sil ...

monzogranite Monzogranites are biotite granite rocks that are considered to be the final fractionation product of magma. Monzogranites are characteristically felsic (SiO2 > 73%, and FeO + MgO + TiO2 < 2.4), weakly granodiorite Granodiorite () is a coarse-grained ( phaneritic) intrusive igneous rock similar to granite, but containing more plagioclase feldspar than orthoclase feldspar. The term banatite is sometimes used informally for various rocks ranging from gr ...

. The Sarn Granite is leucocratic and covers an expanse of about 6 km² in contrast to the gabbro and diorite that exist as small and scattered exposures. The dioritic component of the complex has been confirmed as having a

Neoproterozoic The Neoproterozoic Era is the unit of geologic time from 1 billion to 538.8 million years ago. It is the last era of the Precambrian Supereon and the Proterozoic Eon; it is subdivided into the Tonian, Cryogenian, and Ediacaran periods. It is ...

age of 614 ±2 Ma using U-Pb zircon dating.P. J. Patchett & J. Jocelyn ''U-Pb zircon ages for late Precambrian igneous rocks in South Wales.'', 1979, Journal of the Geological Society of London. 136 13–19 Therefore, the shearing of the Llyn is also temporally constrained by the date.

References

External links

The Geology of Wales – BGS

BGS Rock Lexicon
{{Coord missing, Gwynedd Precambrian Europe Geology of Wales Geological groups of the United Kingdom