Ophiolite

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Scheme

Ophiolites are components of the oceanic lithosphere (specifically the oceanic crust ), whose mainly basic and ultra- basic rock series of the ocean floor were pushed (" autopsied ") onto the mainland in the course of an ocean-continent collision (e.g. Andes ) . However, the term ophiolithic is also used for ceiling complexes that no longer have any direct reference to the oceanic crust, but are only typical of ocean-continent collisions.

Etymology and conceptual history

Bay of Islands Ordovician Ophiolite in Gros Morne National Park , Newfoundland

The word ophiolite is derived from ancient Greek and is composed of ὄφις (ophis) = snake and λίθος (lithos) = stone, to designate green-colored rocks with a snake-like texture (mainly serpentinites , but also spilites ). It was used for the first time in 1813 by Alexandre Brongniart for a socialization of green rocks in the Alps (for serpentine rocks with diabases ). Later (1905 and 1927) the German geologist Gustav Steinmann modified the term in such a way that, in addition to the serpentinites, it also contained pillow lavas and radiolarites (the so-called Steinmann trinity ).

Structure and properties

An ophiolite complex ideally consists of the three large rock units of the oceanic crust. From hanging to lying are:

Most ophiolite complexes, however, are rarely complete and therefore only show parts of the classical sequence.

The sequence in detail:

The marine sediments consist of the pelagic rocks typical of high- sea deposits - predominantly deep-sea clays , fine limestone sludge , cherts , radiolarites and turbidites . Seismic wave velocities are low in these sediments. Longitudinal waves (p-waves) reach 1.6 to 2.5 km / s.

Under the sediments lie igneous rocks of the oceanic crust:

Pillow lavas from an ophiolite sequence in the northern Apennines , Italy

In the hanging wall, extrusive layers of pillow lavas , the spaces between which are filled with hyalite or hyaloclastite (the fragments of the glass skin of the individual pillows) and marine sediments through contact with seawater , whereby the proportion of sediment filling decreases towards the lying surface . The speed of the P waves is 2.8 to 4.5 km / s here.

In the footwall a rock package of vertical follows transition hosts ( engl. Sheeted dykes ), by means of which the magma of the pillow lava rose to the surface. Wave speeds of 4.5 to 5.7 km / s are achieved in this section.

This is followed by intrusive gabbros , the plutonite equivalents of the basalts . They are much coarser-grained as they had more time to form large crystals due to the slower solidification. They can be classified into two types divide isotropic at higher altitudes, fractional gabbros, which in turn layered gabbros (engl. Layered gabbros ) formed by Kumulatkristallisation a magma chamber overlap. The p waves come here at speeds of up to 6.7 km / s. A curiosity in this area is the occurrence of individual acidic intrusives such as plagiogranites , diorites or tonalites , especially since there are no intermediate rocks.

From a mineralogical point of view, the basic rocks of the oceanic crust consist mainly of plagioclase and pyroxenes (clino- and orthopyroxes).

Among the igneous rocks , which still belong to the lithological oceanic crust, follow the rocks of the lithospheric mantle . The boundary between the two units is called the lithological Moho .

The part of the mantle belonging to the lithosphere also comprises two rock units, which are separated from each other by the seismological Moho . The upper part consists of cumulative peridotites ( dunite- rich layers). The underlying peridotites - mainly Harzburgites and Lherzolites composed of olivine and pyroxenes - show shear structures caused by tectonic movements . They can also be mylonitized (caused by very intensive tectonic stress) and are then mostly subject to secondary water absorption, which changes the primarily igneous mineral stock ( serpentinization ). Seismic waves propagate in the cumulative peridotites with only slightly higher velocities than in the igneous rocks. However, at the seismic Moho, which lies below the sea floor at a depth of about seven kilometers, these rise by leaps and bounds to an average of 8.15 km / s. At mid-ocean ridges (MOR), however, the value can drop to 7.6 km / s.

If the former continental margin was also autopsied, turbidite sequences can also be added to the geological-genetic unit of the ophiolite .

Problematic allocation of the educational area

In the course of the development of plate tectonics in the late 1950s and early 1960s, the idea prevailed that ophiolites are directly connected to the principle of ocean floor spreading on mid-ocean ridges, which was underpinned by the magnetic work of Frederick Vine and Drummond Matthews in 1963 . In addition, Ian Graham Gass's investigations in 1968 on the vertical veins of the Troodos ophiolith in Cyprus also suggested an ocean floor spreading. This conclusion, taken up again by EM Moores and F. Vine in 1971, was generally accepted until the 1980s.

However, more precise geochemical and petrological studies had in the meantime uncovered problems with this somewhat simplified assignment:

  • The SiO 2 content of ophiolitic basalts is usually around 55 percent by weight and their TiO 2 content is always below 1 percent by weight, whereas basalts of the oceanic ridges ( MORB ) only around 50 percent by weight SiO 2 , but quite high contents of 1.5 to 2.5 percent by weight of TiO 2 .
  • Trace elements from subduction zone or island arc volcanites, as well as ophiolithic volcanic rocks, usually show increased values ​​compared to MORB. In particular, lithophilic elements with large ionic radii ( LILE ) such as B. potassium , rubidium , cesium and thorium as well as the light rare earths ( LREE ).
  • Compared to N-MORB, depletion of elements of high field strength ( HFSE ) such as titanium (see above), niobium , tantalum and hafnium .
  • The crystallization sequence in the cumulative rocks (gabbros, peridotite) is clinopyroxene before plagioclase and is thus the reverse of MORB, in which plagioclase crystallizes before clinopyroxene.
  • The mantle rocks in the lying area are of a refractory nature (tectonically overprinted Harzburgite, as well as Dunite) in contrast to spreading centers, which are underlain by Lherzolites.
  • Higher chromium numbers and lower Mg / Fe ratios.
  • Enrichment of radiogenic isotopes of strontium and lead in the volcanic rocks, which leads to increased ratios of 87 Sr / 86 Sr, 206 Pb / 204 Pb, 207 Pb / 204 Pb and 208 Pb / 204 Pb compared to MORB. Neodymium is depleted at the same time, reducing the 143 Nd / 144 Nd ratio.

All these geochemical differences can only be explained if it is assumed that a formation of the ophiolite not divergent, but mainly at convergent oceanic crust areas (subduction zones) - the so-called Suprasubduktionszonen-ophiolites in forearc area ( English suprasubduction zone ophiolites or SSZ ophiolites ) .

The convergent suprasubduction phioliths can be divided into two types:

  • Alpinotype ophiolites (Tethys ophiolites) emerged from the Tethys space
  • Cordilleras ophiolites of the Pacific region

These two types of ophioliths differ fundamentally in the way they are seated: the alpinotype ophiolites were autopsied on a passive, partly thinned continental margin, whereas a large part of the cordillera ophiolites were passively pushed out by the accretionary wedge placed under them ( accretionary uplift ).

Of course, ophiolites also form at divergent oceanic spreading centers, at hotspots and on deep-sea mountains , as well as in the interarc and backarc area, but they are only rarely autopsied and are therefore rarely preserved. The Ligurian ophiolite and the Franciscan ophiolite can be seen as examples of oceanic centers of expansion .

Ultimately, the decisive factor is the extent of the partial melting in the upper mantle, with the spectrum ranging from a relatively low degree of melting at spreading centers (Ligurian type with lherzolite as mantle rock) to significantly higher degrees in suprasubduction phiolites (Yakuno type with clinopyroxene-containing resin burgite and Papua type with clinopyroxene-free Harzburgite) is sufficient. Parallel to this, the development of the separate basaltic magmas goes hand in hand: it runs from alkali basalts or aluminum- rich basalts (MORB) via aluminum-poor basalts (island arc holeiites) to boninites and magnesium-rich andesites .

Development over time

In the course of the earth's history , the subduction of oceanic crust and thus the formation of ophiolites did not take place uniformly, but in a pulsed manner. Ophiolite pulses occur statistically in the Neoproterozoic ( Cryogenium ) around 750 million years BP , in the Paleozoic at the turn of the Ordovician / Silurian by 450 million years and in the Mesozoic at the turn of the Jurassic / Cretaceous around 150 million years BP.

Each of these ophiolite pulses can be correlated with major orogeny phases . Pan-African orogenesis occurred roughly at the same time as the maximum of the cryogenium ; the early Paleozoic ophiolites appear at the same time as the Caledonian belts of the Appalachians , Caledonids and the Urals , while the Mesozoic ophiolites dominate the Alps - Himalayan belt. The Circumpacific Belts contain ophiolites belonging to the last two pulses. Long-lasting, continuous ore formation processes in the Pacific region based on subduction of oceanic crust with accompanying accretion are thus documented . They stand in contrast to the relatively short-lived, episodic continental collisions.

Occurrence

Ophiolites can mostly be found in suture zones. Here, after subduction is complete, oceanic crust sections are stored between colliding continents, continent fragments or island arcs . However, the model concept based on a pure continent collision led in the past to a misinterpretation in the case of the Yarlung-Tsangpo-Suture (also Indus-Yarlung-Suture), since it was probably only a relatively small sea basin (a so-called backarc Basin ) and thus not the expected great suture between Eurasia and India .

Probably the most famous example of ophiolites (semail ophiolite) is in Oman and the United Arab Emirates , where the former oceanic plate of the Neotethys area was pushed onto the Arabian plate. Other significant deposits can be found in Cyprus , southern Spain , Switzerland , Morocco , New Guinea , Newfoundland and California .

References in detail

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  • John W. Shervais: Birth, death, and resurrection: The life cycle of suprasubduction zone ophiolites . In: Geochemistry, Geophysics, Geosystems . tape 2 , no. 1 , January 31, 2001, ISSN  1525-2027 , p. 1010 , doi : 10.1029 / 2000GC000080 .

Individual evidence

  1. ^ A. Brongniart: Essai d'une classification minéralogique des roches mélangées . In: Journal des Mines . tape 34 . Paris 1813, p. 5-48 .
  2. ^ Lexicon of Geosciences . 4: north to syllable . Spektrum Akad. Verl., Heidelberg Berlin 2001, ISBN 3-8274-0420-7 , pp. 36 .
  3. ^ Gustav Steinmann: Ophiolite concept and the evolution of geological thought . English: The ohiolitic zones in the Mediterranean mountain chains. In: Yildirim Dilek, Sally Newcomb (Ed.): Geological Society of America - Special Papers . tape 373 . Geological Society of America, 2003, ISBN 0-8137-2373-6 , ISSN  0072-1077 , pp. 77-91 , doi : 10.1130 / 0-8137-2373-6.77 (German: The ophiolitic zones in the mediterranean chain mountains . 1927. Translated by Daniel Bernoulli, Gerald M. Friedman, reprint).
  4. ↑ keckgeology.org (PDF)
  5. ^ FJ Vine, DH Matthews: Magnetic anomalies over ocean ridges . In: Nature . tape 199 , 1963, pp. 947-949 .
  6. ↑ IG Gass: Is the Troodos massif of Cyprus a fragment of Mesozoic ocean floor? In: Nature . tape 220 , 1968, pp. 39-42 .
  7. ^ EM Moores, F J. Vine: The Troodos massif, Cyprus, and other ophiolites as oceanic crust: Evaluation and implications . In: Philosophical Transactions of the Royal Society of London . 268A, 1971, p. 443-466 .
  8. ↑ JA Pearce: Trace element characteristics of lavas from destructive plate boundaries . In: JS Thorpe (Ed.): Andesites . John Wiley, New York 1982, pp. 525-548 .
  9. ↑ JW Shervais: Ti-V plots and the petro genesis of modern and ophiolitic lavas . In: Earth and Planetary Science Letters . tape 59 , no. 1 , 1982, pp. 101-118 .
  10. ^ R. Hebert, R. Laurent: Mineral chemistry of the plutonic section of the Troodos Ophiolite: New constraints for genesis of arc-related ophiolites . In: J. Malpas, EM Moores, A. Panayiotou, C. Xenophontos, Geol. Surv. Dep., Nicosia, Cyprus (Ed.): Ophiolites . Oceanic Crustal Analogues: Proceedings of the Symposium Troodos 1987 . 1990, p. 149-163 .
  11. ↑ HJB Dick: Abyssal peridotites, very slow spreading ridges and ocean ridge magmatism . In: Magmatism in the Ocean Basins, Geol. Soc. Spec. Publ. Volume 142 , 1989, pp. 71-105 .
  12. ↑ Barry B. Hanan, Jean-Guy Schilling: Easter microplate evolution: Pb isotope evidence . In: Journal of Geophysical Research (Ed.): Solid Earth . tape 94 , 1989, pp. 7432-7448 .
  13. ↑ a b John W. Shervais: Birth, death, and resurrection: The life cycle of suprasubduction zone ophiolites . In: Geochemistry, Geophysics, Geosystems . tape 2 , no. 1 , January 31, 2001, ISSN  1525-2027 , p. 1010 , doi : 10.1029 / 2000GC000080 .
  14. ↑ É. Bédard, R. Hébert, C. Guilmette, G. Lesage, CS Wang, J. Dostal: Petrology and geochemistry of the Saga and Sangsang ophiolitic massifs, Yarlung Zangbo Suture Zone, Southern Tibet: Evidence for an arc-back-arc origin . In: Lithos . tape 113 , no. 1-2 , November 2009, ISSN  0024-4937 , pp. 48-67 , doi : 10.1016 / j.lithos.2009.01.011 .
  15. ↑ Peter M. Kelemen: Fire under the water. In: Spectrum of Science. January 2010, pp. 82-87.
  16. ↑ Ophiolites. In: Mineralienatlas.
  17. ^ Macquarie Island. on environment.gov.au
  18. ↑ J. Berger, inter alia: A Variscan slow-spreading ridge (MOR-LHOT) in Limousin (French Massif Central): magmatic evolution and tectonic setting inferred from mineral chemistry . In: Mineralogical Magazine . tape 70 , no. 2 , 2006, p. 175-185 .
  19. ^ EA Silver: Gravity results and emplacement geometry of the Sulawesi ultramafic belt, Indonesia . In: Geology . tape 6 , 1978, p. 527-531 .
  20. ↑ AW Ruttner: Southern borderland of Triassic Laurasia in north-east Iran . In: Geologische Rundschau . tape 82 , 1993, pp. 110-120 .
  21. ^ S. Kojima, inter alia: Mesozoic radiolarians from the Bagh Complex in the Muslim Bagh area, Pakistan: Their significance in reconstructing the geologic history of ophiolites along the Neo-Tethys suture zone . In: Geol. Surv. Mon. Band 45 , no. 2 , 1994, p. 63-97 .
  22. ↑ JL Smellie, P. Stone: Geochemical control on the evolutionary history of the Ballantrae Complex, SW Scotland, from comparisons with recent analogues . In: LM Parson, BJ Murton, P. Browning, Geol. Soc. Spec. Publ. (Ed.): Ophiolites and Their Modern Oceanic Analogues . tape 60 , 1992, pp. 171-178 .
  23. ↑ Y. Dilek, et al: Structure and petrology of Tauride ophiolites and mafic dike intrusions (Turkey): Implications for the Neotethyan ocean . In: Geol. Soc. At the. Bull. Band 111 , no. 8 , 1999, p. 1192-1216 .

Web links

  • Ofioliti , an international English-language journal on ophiolites and related subjects, published by the Istituto di geoscienze e georisorse , Pisa