Movatterモバイル変換


[0]ホーム

URL:


Jump to content
WikipediaThe Free Encyclopedia
Search

Trachyte

From Wikipedia, the free encyclopedia
Extrusive igneous rock
For other uses, seeTrachyte (disambiguation).
Trachyte
Igneous rock
A cut sample of trachyte, withphenocrysts ofsanidine, in a wall in Germany
Composition
ClassificationFelsic
PrimaryAlkali feldspar
SecondaryPlagioclase,quartz, orfeldspathoid
TextureAphanitic
EquivalentsIntrusive variant issyenite

Trachyte (/ˈtrkt,ˈtræk-/) is anextrusiveigneous rock composed mostly ofalkali feldspar. It is usually light-colored andaphanitic (fine-grained), with minor amounts ofmafic minerals,[1] and is formed by the rapid cooling of lava (or shallow intrusions) enriched with silica andalkali metals.[2][3][4] It is the volcanic equivalent ofsyenite.[5]

Trachyte is common wherever alkali magma is erupted, including in late stages of ocean island volcanism[6][7] and in continentalrift valleys,[8] abovemantle plumes,[9] and in areas ofback-arc extension.[10] Trachyte has also been found inGale crater on Mars.[11]

Trachyte has been used as decorative building stone[12] and was extensively used asdimension stone in theRoman Empire and theRepublic of Venice.[13]

Chemical composition

[edit]
TAS diagram with trachyte field highlighted

Trachyte has a silica content of 60 to 65% and an alkali oxide content of over 7%. This gives it less SiO2 thanrhyolite and more (Na2O plus K2O) thandacite. These chemical differences are consistent with the position of trachyte in theTAS classification, and they account for the feldspar-rich mineralogy of the rock type.Trachydacite occupies the same field in the TAS diagram as trachyte, but is distinguished from trachyte by anormative quartz content over 20%.[2] Trachydacite is not a recognized rock type in the QAPF classification, where rocks rich in alkali feldspar and with quartz over 20% would be classified as rhyolites.[4]

Mineralogy

[edit]
QAPF diagram with trachyte fields highlighted
Polished opal on trachyte

The mineral assemblage of trachytes consists of essential alkali feldspar. Relatively minorplagioclase andquartz or afeldspathoid such asnepheline may also be present.[7] This is reflected in the position of the trachyte fields in theQAPF diagram.Biotite,clinopyroxene andolivine are common accessory minerals. The plagioclase is typically sodium-richoligoclase. The alkali feldspar is typically also sodium-richsanidine (anorthoclase) and is oftencryptoperthitic, with alternating microscopic bands of sodium feldspar (albite) and potassium feldspar (sanidine).[14]

Trachytes are typically fine-grained and light-colored, but can be black if they consist mostly of glass.[15] They are often porphyritic, with large well-shaped crystals of sanidine in agroundmass containing much smaller imperfect sanidine laths.Rhomb porphyry is an example with usually largeporphyriticrhomb shapedphenocrysts embedded in a very fine-grainedmatrix. Some of the best known trachytes, such as the trachyte ofDrachenfels on the Rhine, show striking porphyritic character, having large sanidine crystals of tabular form an inch or two in length scattered through their fine-grained groundmass. In many trachytes, however, the phenocrysts are few and small, and the groundmass comparatively coarse. Theferromagnesian minerals rarely occur in large crystals, and are usually not conspicuous in hand-sized specimens of these rocks. Two types of groundmass are generally recognized: the trachytic, composed mainly of long, narrow, subparallel rods of sanidine, and the orthophyric, consisting of small squarish or rectangular prisms of the same mineral. Sometimes granular augite or spongy riebeckite occurs in the groundmass, but as a rule this part of the rock is highly feldspathic.[16]

Trachytes very often have minute irregularvesicles which make the broken surfaces of specimens of these rocks rough and irregular, and it is from this distinctive texture that they received their name. It was first given to rocks of this class fromAuvergne, and was long used in a much wider sense than that defined above, so that it included quartz-trachytes (now known asliparites andrhyolites) andoligoclase-trachytes, which are now classified asandesites.[17]

Quartz is rare in trachyte, buttridymite (which likewise consists ofsilica) is not uncommon.[14] It is rarely in crystals large enough to be visible without the aid of themicroscope, but inthin sections it may appear as small hexagonal plates, which overlap and form denseaggregates, like a mosaic or like the tiles on a roof. They often cover the surfaces of the larger feldspars or line the vesicles of the rock, where they may be mingled with amorphousopal or fibrouschalcedony. In the older trachytes, secondary quartz from therecrystallization of tridymite is not rare.[17]

Of themafic minerals present,augite is the most common. It is usually of pale green color, and its small crystals are often very perfect in form. Brownhornblende and biotite occur also, and are usually surrounded by black corrosion borders composed ofmagnetite andpyroxene; sometimes the replacement is complete and no hornblende or biotite is left, though the outlines of the cluster of magnetite and augite may clearly indicate from which of these minerals it was derived. Olivine is unusual, though found in some trachytes, for example those of the Arso inIschia. Basic varieties of plagioclase, such aslabradorite, are known also as phenocrysts in some Italian trachytes. Dark brown varieties of augite and rhombic pyroxene (hypersthene orbronzite) have been observed but are not common.Apatite,zircon and magnetite are practically always present as accessory minerals.[18][14]

Occasionally minerals of thefeldspathoid group, such asnepheline,sodalite andleucite, are present in trachytes,[16] and rocks of this kind are known as foid-bearing trachytes.[19] The sodium-bearingamphiboles and pyroxenes so characteristic of the phonolites may also be found in some trachytes;[14] thusaegirine or aegirine augite forms outgrowths ondiopside crystals, andriebeckite may be present in spongy growths among the feldspars of the groundmass (as in the trachyte ofBerkum on theRhine). Glassy forms of trachyte (obsidian) occur, as inIceland, andpumiceous varieties are known (in Tenerife and elsewhere), but these rocks as contrasted with the rhyolites have a remarkably strong tendency to crystallize, and are rarely to any considerable extent vitreous.[16]

Geographic distribution

[edit]
The Breadknife is aperalkaline trachytedike in theWarrumbungles of eastern Australia.
Iron Pot is one of several trachyte plugs in the Hedlow Creek region west of Yeppoon in Central Queensland.

Trachyte is the usual silica-rich end member of thealkaline magma series, in whichalkaline basaltic magma experiencesfractional crystallization while still underground. This process removes calcium, magnesium, and iron from the magma to give it a composition close to that of alkali feldspar.[6][7] As a result, trachyte is common wherever alkali magma is erupted, including late eruptions of ocean islands[6][7] and in continentalrift valleys andmantle plumes.[9] Only rarely does magmatic differentiation proceed beyond trachyte tophonolite or even more evolved alkaline magmas.[7] Trachyte also occurs in areas ofback-arc extension, such as the northernAegean Sea[9] and theAeolian arc of Italy.[20] The Aeolian back-arc includes theCampi Flegrei volcanic field,[21] where trachytes have been erupted.[22]

Trachytes are well represented among theCenozoic volcanic rocks of Europe. InBritain they occur inSkye aslava flows and asdikes or intrusions,[23] but they are much more common on the continent of Europe, as in the Rhine district and theEifel, also in Auvergne,Bohemia and theEuganean Hills. In the neighborhood of Rome,Naples and the island ofIschia trachyticlavas andtuffs are of common occurrence.[16] Trachytes are also found on the island ofPantelleria. In the United States, trachytes crop out extensively in theDavis Mountains,Chisos Mountains, andBig Bend Ranch State Park in theBig Bend (Texas) region, as well as southernNevada andSouth Dakota (Black Hills). There is one known voluminous flow from Puʻu Waʻawaʻa on the north flank ofHualālai in Hawaiʻi.[16] Here the trachyte is glassy and black in color.[15] In Iceland, theAzores,Tenerife andAscension there are recent trachytic lavas, and rocks of this kind occur also inNew South Wales (Cambewarra Range), Queensland (Main Range),[24] East Africa,Madagascar,Yemen and in many other districts.[16]

Among the older volcanic rocks trachytes also are not scarce, though they have often been described under the names orthophyre and orthoclase-porphyry, while trachyte was reserved forTertiary and recent rocks of similar composition. In England there arePermian trachytes in the Exeter district, andCarboniferous trachytes are found in many parts of the central valley of Scotland. The latter differ in no essential respect from their modern representatives in Italy and theRhine valley, but their augite and biotite are often replaced bychlorite and other secondary products. Permian trachytes occur also inThuringia and theSaar district in Germany.[16]

Alkaline rocks such as trachyte are rare in theArchean, but become common in theProterozoic. Alkaline rocks with an age close to 570 million years are common around the perimeters of manycontinental shields and are evidence of worldwide rifting at that time.[25]

Closely allied to trachyte is the rock type calledkeratophyre, which is the sodium-rich-plagioclase equivalent of trachyte.[26]

See also

[edit]

Notes

[edit]
  1. ^Macdonald, Gordon A. (1983).Volcanoes in the sea : the geology of Hawaii (2nd ed.). Honolulu: University of Hawaii Press.ISBN 0824808320.
  2. ^abLE BAS, M. J.; STRECKEISEN, A. L. (1991). "The IUGS systematics of igneous rocks".Journal of the Geological Society.148 (5):825–833.Bibcode:1991JGSoc.148..825L.CiteSeerX 10.1.1.692.4446.doi:10.1144/gsjgs.148.5.0825.S2CID 28548230.
  3. ^"Rock Classification Scheme - Vol 1 - Igneous"(PDF).British Geological Survey: Rock Classification Scheme.1:1–52. 1999. Retrieved23 September 2020.
  4. ^ab"CLASSIFICATION OF IGNEOUS ROCKS". Archived fromthe original on 30 September 2011.
  5. ^Philpotts, Anthony R.; Ague, Jay J. (2009).Principles of igneous and metamorphic petrology (2nd ed.). Cambridge, UK: Cambridge University Press. p. 140.ISBN 9780521880060.
  6. ^abcMacDonald 1983, pp. 51–52
  7. ^abcdePhilpotts and Ague 2009, pp. 369–370
  8. ^Kresten, Peter; Troll, Valentin R. (2018).The Alnö Carbonatite Complex, Central Sweden. GeoGuide. Springer International Publishing.ISBN 978-3-319-90223-4.
  9. ^abcPhilpotts and Ague 2009, pp. 390–394
  10. ^Pe-Piper, G.; Piper, D.J.W. (2005). "The South Aegean active volcanic arc: relationships between magmatism and tectonics".Developments in Volcanology.7:113–133.doi:10.1016/S1871-644X(05)80034-8.ISBN 9780444520463.
  11. ^Sautter, Violaine; Toplis, Michael J.; Beck, Pierre; Mangold, Nicolas; Wiens, Roger; Pinet, Patrick; Cousin, Agnes; Maurice, Sylvestre; LeDeit, Laetitia; Hewins, Roger; Gasnault, Olivier; Quantin, Cathy; Forni, Olivier; Newsom, Horton; Meslin, Pierre-Yves; Wray, James; Bridges, Nathan; Payré, Valérie; Rapin, William; Le Mouélic, Stéphane (June 2016). "Magmatic complexity on early Mars as seen through a combination of orbital, in-situ and meteorite data".Lithos.254–255:36–52.Bibcode:2016Litho.254...36S.doi:10.1016/j.lithos.2016.02.023.
  12. ^Troll, Valentin R.; Carracedo, Juan Carlos (2016). "The Geology of Fuerteventura".The Geology of the Canary Islands:531–582.doi:10.1016/B978-0-12-809663-5.00008-6.ISBN 9780128096635.
  13. ^Germinario, Luigi; Siegesmund, Siegfried; Maritan, Lara; Mazzoli, Claudio (November 2017). "Petrophysical and mechanical properties of Euganean trachyte and implications for dimension stone decay and durability performance".Environmental Earth Sciences.76 (21): 739.Bibcode:2017EES....76..739G.doi:10.1007/s12665-017-7034-6.S2CID 133942939.
  14. ^abcdBlatt, Harvey; Tracy, Robert J. (1996).Petrology : igneous, sedimentary, and metamorphic (2nd ed.). New York: W.H. Freeman. pp. 55–56.ISBN 0716724383.
  15. ^abMacDonald 1983, p. 128
  16. ^abcdefgFlett 1911, p. 117.
  17. ^abFlett 1911, p. 116.
  18. ^Flett 1911, pp. 116–117.
  19. ^Blatt and Tracy 1996, p.74
  20. ^Ruch, J.; Vezzoli, L.; De Rosa, R.; Di Lorenzo, R.; Acocella, V. (February 2016)."Magmatic control along a strike-slip volcanic arc: The central Aeolian arc (Italy): MAGMATISM AND STRIKE-SLIP FAULTING".Tectonics.35 (2):407–424.doi:10.1002/2015TC004060.hdl:10754/594981.S2CID 130297493.
  21. ^Vitale, Stefano; Isaia, Roberto (April 2014). "Fractures and faults in volcanic rocks (Campi Flegrei, southern Italy): insight into volcano-tectonic processes".International Journal of Earth Sciences.103 (3):801–819.Bibcode:2014IJEaS.103..801V.doi:10.1007/s00531-013-0979-0.S2CID 129909337.
  22. ^Troll, Margherita; Bouvet D.M, Caroline; Giordano, Daniele; Piochi, Monica; Mancini, Lucia; Degruyter, Wim; Bachmann, Oliver (1 April 2013)."Permeability of Campi Flegrei magmas and comparison with rhyolites and basalts"(PDF).Scientific Reports.272 (1):16–22.doi:10.1016/j.jvolgeores.2013.12.002.hdl:2318/149313.ISSN 0377-0273.S2CID 53633119.
  23. ^Troll, Valentin R.; Emeleus, C. Henry; Nicoll, Graeme R.; Mattsson, Tobias; Ellam, Robert M.; Donaldson, Colin H.; Harris, Chris (24 January 2019)."A large explosive silicic eruption in the British Palaeogene Igneous Province".Scientific Reports.9 (1): 494.Bibcode:2019NatSR...9..494T.doi:10.1038/s41598-018-35855-w.ISSN 2045-2322.PMC 6345756.PMID 30679443.
  24. ^Stevens, Neville (September 1996).The Main Range(PDF). Brisbane, Queensland: Geological Society of Australia. Archived fromthe original(PDF) on 19 July 2008.
  25. ^Philpotts and Ague 2009, pp. 390–391
  26. ^Schermerhorn, L. J. G. (January 1973). "What is keratophyre?".Lithos.6 (1):1–11.Bibcode:1973Litho...6....1S.doi:10.1016/0024-4937(73)90076-5.

External links

[edit]
Wikimedia Commons has media related toTrachyte.

 This article incorporates text from a publication now in thepublic domainFlett, John S. (1911). "Trachyte". InChisholm, Hugh (ed.).Encyclopædia Britannica. Vol. 27 (11th ed.). Cambridge University Press. pp. 116–117.

Types
Volcanic rocks
Lists and groups
Types of rocks
Igneous rock
Sedimentary rock
Metamorphic rock
Specific varieties
Authority control databases: NationalEdit this at Wikidata
Retrieved from "https://en.wikipedia.org/w/index.php?title=Trachyte&oldid=1269149436"
Categories:
Hidden categories:

[8]ページ先頭

©2009-2025 Movatter.jp