Lewisian gneiss synonyms, lewisian gneiss pronunciation, lewisian gneiss translation, english dictionary definition of lewisian gneiss. The torridonian and moine sequences represent at least two main. Difference between schist and gneiss difference between. Weaver and john tarney department of geology, university of leicester, leicester lei 7rh england received february, 1981 revised version received may 14, 1981 the geochemistry. Formed originally as igneous rocks in the earths crust, the gneiss was later buried over 25km below the surface, where it was heated, squeezed and stretched.
The tectonic evolution and geochemistry of the lewisian complex of. The lewisian gneiss complex of northwest scotland is a classic precambrian basement gneiss complex. The rocks found on the isle of tiree are part of the lewisian metamorphic suite also found on the western isles, coll, tiree, iona, islay and a mainland scotland coastal strip of. This booklet does a remarkable job in explaining the dramatic forces of nature that have shaped the outer hebrides over millions of years. These rocks are of archaean and palaeoproterozoic age, ranging from 3. The lewisian gneiss complex of the scottish highlands is of great historical significance and remains a textbook example of how lowercrustal processes can operate, and of how sound field observations can unravel the geological. Earth and planetary science letters, 55 1981 171180 elsevier scientific publishing company, amsterdam printed in the netherlands 2 lewisian gneiss geochemistry and archaean crustal development models barry l. Most of the outer hebrides of scotland have a bedrock formed from lewisian gneiss. Lewisian gneiss from northwest scotland, formed during the precambrian period, making it among the oldest rocks in the uk, pushed to the surface during an oregenic event, lewisian gneiss exhibits banded like most gneisses but is comprised of more dark, dense minerals, often igneous in origin. Weaver and john tarney department of geology, university of leicester, leicester lei 7rh england received february, 1981 revised version received may 14, 1981 the.
The outer hebrides are exposed to the extremes of the elements. The glencoul thrust, brings the older precambrian gneiss up over the cambrian quartzite along loch glencoul. The lewisian gneiss complex of mingulay, outer hebrides. Media in category lewisian gneiss the following 114 files are in this category, out of 114 total. The schist is type of mediumgrade metamorphic rock that contains flat, sheet like grains in a pattern. Lewisian gneiss, one of the oldest rocks in great britain, three billion years old, two thirds the age of the earth read about the geology of the outer hebrides the isle of lewis and the isle of harris rocks and the callanish stones. A banded or foliated metamorphic rock, usually of the same composition as granite. This latter is a highly dense rock that doesnt break or weather easily. The minerals are arranged into layers that appear as bands in cross section. It has gradually passed into acceptance as a generic term signifying a large and varied series of metamorphic rocks, which mostly consist of quartz and felspar orthoclase and plagioclase. Lewisian gneiss article about lewisian gneiss by the free. Lewisian gneiss article about lewisian gneiss by the. The rocks that form gneiss are exposed to extreme pressures and temperatures of between 600 and 700 degrees celsius.
Lewisian gneiss definition of lewisian gneiss by the free. In episode one, deep time, ian shows how lewisian gneisses are britains oldest rocks using radioactive isotope dating methods. Turn right for the oldest highgrade metamorphic rocks. Sterling igneous suite granite gneiss pale pink to gray, mediumgrained granite gneiss, commonly with small porphyroclasts of microclineorthoclase. The lewisian complex or lewisian gneiss is a suite of precambrian metamorphic rocks that outcrop in the northwestern part of scotland, forming part of the hebridean terrane and the north atlantic craton. The lighter bands contain mostly quartz and feldspar, the darker often contain biotite, hornblende, garnet or graphite. The lewisian gneiss complex contains a record of igneous, depositional and orogenic events that.
Bgs staff provided archive images, scientific equipment and their expertise, as advisors and presenters, in the making of the bbc series men of rock dr ian millar and dr tom bradwell were in front of the camera in episodes one and three. It was used by the megalithic cultures to construct monuments such as the callanish stone circle on the isle of lewis. The foreland is largely made up of lewisian gneiss, which at 3000 million years old is the oldest rock type in britain. The geology of the lewisian gneisses is complicated, but is so much more. Other bands contain platy or elongate minerals with evidence of preferred orientation. The lewisian gneiss is an extension of the canadian shield with which it was in contact until about 200 million years ago. Gneiss, a term long used by the miners of the harz mountains to designate the country rock in which the mineral veins occur.
Very similar rocks exist in east greenland, to which scotland was once joined, before the. Generally speaking, felsic gneisses form from igneous rocks of acid composition, mafic gneisses from basic. Lewisian gneiss geochemistry and archaean crustal development. Mineral exploration for gold and base metals in the lewisian.
It is this banded appearance and texture rather than composition that define a gneiss. Rocks of nw scotland scourie oxford earth sciences. The rocks at scourie are mostly composed of threebillionyearold lewisian gneiss. These rocks are of archaean and paleoproterozoic age, ranging from 3.
Lewisian gneiss rock of the outer hebrides western. Lewisian gneiss rock of the outer hebrides western isles. Gneiss is characterized by alternating light and dark bands differing in mineral composition and having coarser grains than those of schist. Named after the hebridean isle of lewis, these colourful, banded rocks are the oldest in western europe. These rocks are of archaean and palaeoproterozoic age, ranging. An area of nw highlands geopark known as the foreland lies to the west of the moine thrust zone and includes most of the coastal townships. Orthogneiss is gneiss derived from igneous rock such as granite. New upb singlezircon geochronology undertaken on tonalitic gneisses, granite sheets, migmatites and metasediments from the lewisian gneiss complex on the mainland and the northern part of the outer hebrides, nw scotland, have been used to test the correlation of socalled laxfordian events across the complex from the outer hebrides to the mainland, and the current model for the evolution.
The archaean tonalitetrondhjemitegranodiorite ttg gneisses of the lewisian gneiss complex are cut by a suite of palaeoproterozoic c. Gneiss gneiss is a high grade metamorphic rock, meaning that it has been subjected to higher temperatures and pressures than schist. Norway 361 v books and reports may also be obtained from hmso publications centre or from hmso bookshops and agents. The rocks are moine schist with cambrian quartzite in the bottom right hand corner. New upb singlezircon geochronology undertaken on tonalitic gneisses, granite sheets, migmatites and metasediments from the lewisian gneiss complex on the mainland and the northern part of the outer hebrides, nw scotland, have been used to test the correlation of socalled laxfordian events across the complex from the outer hebrides to the mainland, and the current model for the evolution of. Gneiss definition of gneiss by the free dictionary. Comparison of median composition of eulysites from the project area with those of quartz magnetite schists qms from gairloch and average compositions of banded ironstones. Gneiss, on the other hand, is formed in the pattern of layers of sheetlike planar structures. The geological society of london lewisian, torridonian. Lewisian gneiss from northwest scotland is a metamorphic rock formed. Buried in the earth were remnants of fragments of the same type of rock that makes up the callanish stonescalled lewisian gneisswhich.
Lewisian complex, also called lewisian gneiss, major division of precambrian rocks in northwestern scotland the precambrian began about 4. A typical sample of laxfordiandeformed gneiss from the csz contains 55% quartz, 20% feldspar, 10% hornblende, 10% biotite and muscovite, and 5%. Sutherland2 1 department of geology and mineralogy, the university of adelaide, adelaide, south australia 2 department of geology, the university, leicester, england. Lewisian gneiss western isles rocks and geology photos. The light bands of gneiss are generally composed of quartz quartz, one of the commonest of all rockforming minerals and. Gneiss is a foliated metamorphic rock identified by its bands and lenses of varying composition, while other bands contain granular minerals with an interlocking texture. These are highly metamorphosed igneous rocks of both acid and basic compositions.
Effects of lithology, cataclasis and melting on the. At the time of their formation, the rocks were buried in the earths. Turn left for rocks deformed and heated again in the later laxfordian episode. Structural characteristics and reos dating of quartz. Geochronological evidence for its tectonic assembly from disparate terranes in the proterozoic. Structural characteristics and reos dating of quartzpyrite. November 21, 2014 january 14, 2015 christopher spencer. The lewisian is divided into a number of terranes on the basis of structural, metamorphic and. The geochemistry of lewisian amphibolitefacies gneisses from northwest scotland. Lewisian complex, major division of precambrian rocks in northwestern scotland the precambrian began about 4. Composition is granite with generally less than 3% dark minerals. Parts of the lewisian gneiss named after the island of lewis in northwest scotland are thought to be almost 3000 million years old. Location of graphite gneiss samples in the eastern lewisian 67.
Gneiss displays distinct foliation, representing alternating layers composed of different minerals. The lewisian complex or lewisian gneiss is a suite of precambrian metamorphic rocks that outcrop in the northwestern part of scotland, forming part of the hebridean terrane. The moine thrust separates relatively undeformed rocks to the west from those more severely deformed to the east. Summary statistics for the talcsilicate gneiss samples summary statistics for samples of graphitic gneiss total carbon and sulphur contents of graphitic gneisses comparison of gold and base metal contents of veins in the ratagain complex and the moine and lewisian country rocks figures. Sedimentary protolith transforms in a stepwise manner according to the level or grade of foliation. Similar to alaskite gneiss, but with more than 3% dark minerals. Difference between schist and gneiss schist vs gneiss. It is a unique composition of many minerals including feldspar, epidote and quartz, all of which are part of the lewisian gneiss complex. Gneiss is formed by high temperature and highpressure metamorphic processes acting on formations composed of igneous or sedimentary rocks. High grade metamorphic rocks of the scourian episode. The origin of the lewisian gneisses of northwest scotland, with. Both schist and gneiss are thus known as foliated metamorphic rocks. Dykes as physical buffers to metamorphic overprinting.
It is characterized by alternating light and dark bands differing in mineral composition coarser grained than schist. These temperatures cause the individual minerals to migrate, forming distinct bands through the rock. It is formed by the metamorphosis of granite, or sedimentary rock. The lewisian complex of north harris is composed of homogeneous acid. Paragneiss is gneiss derived from sedimentary rock.
This is perhaps unsurprising since the lewisian gneiss complex contains a record of sedimentary, igneous and metamorphic events that occurred over a period of 1400 million years that length of time covers almost one third of the earths existence. The lewisian complex or lewisian gneiss is a suite of precambrian metamorphic rocks that. Gneiss from evje, norway, containing more feldspar and quartz than lewisian gneiss from scotland, giving it a pink colour, these specimens show excellent banding and text book crystal structure, supplied in 3 sizes, ideal for educational use. The light bands of gneiss are generally composed of quartz quartz, one of the commonest of all rockforming minerals and one of the most important constituents of the earths crust. A comparison of crustal components, tectonic evolution and amalgamation history, tectonics recent advances, evgenii sharkov, intechopen, doi. Was the precambrian basement of western troms and lofoten. Granite gneiss definition is a rock consisting of an orthogneiss or paragneiss having the composition of a granite. The geochemistry of the lewisian gneisses of the drumbeg area, which are. Lewisian has been treasured for thousands of years by the clans of scotland.
Gneiss simple english wikipedia, the free encyclopedia. Read, highlight, and take notes, across web, tablet, and phone. The lewisian, torridonian and moine of scotland are basement lewisian and cover rocks formed on the eastern margin of laurentia between about 3000 ma and 900 ma i. Scourie dykes of the scottish highlands with tom baker. The geological society of london lewisian, torridonian and. Every year we see small changes to familiar scenes as a result of the power of wind and wave. Gneiss is a medium to coarsegrained, semischistose metamorphic rock. Schist and gneiss are two different types of rocks that look remarkably like. In the region where they occur, lewisian rocks form the basement, or lowermost, rocks. Scourie dykes of the scottish highlands with tom baker november 21, 2014 january 14, 2015 christopher spencer the lewisian gneiss complex of the scottish highlands is of great historical significance and remains a textbook example of how lowercrustal processes can operate, and of how sound field observations can unravel the geological history. The foreland is largely made up of lewisian gneiss, which at 3000 million years old is the oldest. The banded lewisian gneisses of mingulay, with minor amphibolites, show the effects of at least five phases of folding and related deformation. The moine schist is thrust onto the lewisian gneiss in places, e.
Temperaturetime evolution of the assynt terrane of the. Compositions range from quartz monzonite to granite. Wholerock analyses of pseudotachylytes and their gneissic hostrocks from the lewisian complex of northwest scotland indicate that the composition of the pseudotachylyte is sensitive not only to the bulk hostrock composition but also to its mineralogy and fabric. Lewisian gneiss definition of lewisian gneiss by the. Island arc models and the composition of the continental crust. Gneiss definition is a foliated metamorphic rock corresponding in composition to a feldspathic plutonic rock such as granite. Granitegneiss definition of granitegneiss by merriamwebster. The lewisian is exposed on the outer hebrides and also along the northwest coast of the scottish mainland and is composed of a variety of gneisses, including the oldest rocks in the britain, at c. An illustrated magazine swept the young boys imagination from the familiar domesticity of his english home to the wild islands that rise in jagged ranks off scotlands northwest coast. Gneiss is a common and widely distributed type of metamorphic rock. Stack of glencoul, assynt district, nw highlands of scotland. Formation of early scourian badcallian highgrade gneiss complex. Lewisian gneiss, one of the oldest rocks in great britain, three billion years old, two thirds the age of the earth read about and view photos of the rocks of the outer hebrides the isle of lewis and the isle of harris rocks and the callanish stones.
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