Ophiolite

Ophiolites are part of the oceanic lithosphere ( specifically the oceanic crust ), the basic and ultrabasic rocks, especially series of the ocean floor during an ocean - continent collision ( eg Andean ) pushed to the mainland ( " autopsy " ) were. The term ophiolithisch is however also used for ceilings complexes, which have no direct relation to the more oceanic crust but are just typical of ocean - continent collisions.

Etymology and history of the concept

The neologism ophiolite is derived from the ancient Greek and is composed of ὄφις ( ophis ) = snake and λίθος ( lithos ) = stone to denote green colored rocks with snake-like texture (mainly serpentinite, but also Spilite ). It was first used in 1813 by Alexandre Brongniart for a socialization of green rocks in the Alps ( for serpentine - containing rocks with diabase ). Later (1905 and 1927 ) modified the German geologist Gustav Steinmann the term to the effect that in addition to the serpentinites from now on pillow lava and radiolarites with included were ( the so-called Steinmann Trinity).

Structure and properties

A Ophiolithkomplex is ideally from the three major rock units of the oceanic crust. From the hanging wall to the footwall, these are:

• Marine sedimentary rocks
• Igneous rocks
• Rocks of the lithospheric mantle

However, most Ophiolithkomplexe are rarely complete and therefore have only parts of the classical sequence on.

For the sequence in detail:

The marine sediments are high for the marine deposits typical pelagic rocks - mainly Tiefseetone, fine lime sludge, cherts, radiolarites and turbidites. The velocities of seismic waves are low in these sediments. Longitudinal waves (p- waves) reached 1.6 to 2.5 km / s

Among the sediments are igneous rocks of the oceanic crust:

In the hanging wall of extrusive layers of pillow lava, whose interstices are ( the fragments of the glass skin of the individual pillows) and marine sediments filled by seawater contact with Hyalit or Hyaloklastit, the proportion of the sedimentary infill decreases toward the footwall back. The velocity of P-waves is here 2.8 to 4.5 km / s

In the footwall rocks followed by a package of vertical transition droves (English sheeted dykes ), by means of which the magma rose to the surface of the pillow lava. In this section, wave velocities from 4.5 to 5.7 km / s achieved.

Among them then connect intrusive gabbros, the plutonitischen equivalents of basalts. They are much more coarse-grained, since they had more time due to the slower solidification to form large crystals. They can be divided into two types: isotropic at higher altitudes, fractionated gabbros, which, in turn overlie layered gabbros ( engl. layered gabbros ) formed by Kumulatkristallisation a magma chamber. P-waves are here at speeds of up to 6.7 km / s A curiosity is in this area is the occurrence of individual acidic intrusives such as Plagiogranite, diorites or tonalites, especially since no intermediate rocks are present.

Mineralogical pass the basic rocks of the oceanic crust mainly of plagioclase and pyroxene ( clino - and orthopyroxene ).

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 referred to as lithological Moho.

The scoring for the lithosphere part of the jacket also includes two rock units, which are separated by the seismic Moho. The upper part consists of cumulate dunite - peridotites with rich layers. The underlying peridotites - mainly of olivine and pyroxene built harzburgites and lherzolites - have by tectonic movements in conditional shear structure. You can also mylonitisiert present (caused by very intense tectonic stress) and then are usually subject to a secondary water absorption, whereby the primary magmatic mineral composition is changed ( serpentinization ). Seismic waves propagate in the cumulate peridotites, with only slightly higher speeds compared to the igneous rocks. But this rise to the seismic Moho, which is below the sea floor at about seven kilometers deep, jumped to an average of 8.15 km / s. At mid-ocean ridge (MOR ), the value but go back to 7.6 km / s.

Problematic assignment of educational space

With the development of plate tectonics in the late 1950s and early 1960s, the idea prevailed that ophiolites used in 1963 underpinned by the magnetic work of Frederick Vine and Drummond Matthews principle of seafloor spreading are at mid-ocean ridges in the immediate connection. In addition to that, the investigations in 1968 by Ian Graham Gass at the vertical transition hosts of Troodos in Cyprus near Ophioliths put a seafloor spreading. This EM Moores and F. Vine 1971 newly raised conclusion was to the 1980s generally accepted.

More detailed geochemical and petrological studies had but in the meantime promoted problems with this somewhat simplistic assignment to days:

• The SiO2 content of ophiolitic basalts moves usually by 55 percent and their TiO2 content is always less than 1 weight percent, whereas basalts of oceanic ridges ( MORB ), only about 50 weight percent SiO2, however, quite high levels of 1.5 to 2.5 have weight percent TiO2.
• Trace elements by subduction zones or Inselbogenvulkaniten show as well as ophiolitic volcanic rocks compared with MORB usually elevated values ​​. Be enriched in particular Lithophile elements with large ionic radii (English LILE ) such as potassium, rubidium, cesium, and thorium and the light rare earth elements ( LREE engl. ).
• Compared with N- MORB depletion of high field strength elements ( HFSE engl. ) such as titanium (see above), niobium, tantalum and hafnium.
• The crystallization sequence in the Kumulatgesteinen ( gabbros, peridotites ) is clinopyroxene before plagioclase and is thus reversed from MORB, crystallized in the plagioclase before clinopyroxene.
• The mantle rocks in the footwall are refractory nature ( tectonically overprinted harzburgites and dunites ) as opposed to Spreizugszentren that are underlain by Lherzolithen.
• Higher chrome numbers and lower Mg / Fe ratios.
• Compared with MORB pull radiogenic isotope enrichment of strontium and lead in the volcanic rocks, the increased 87Sr/86Sr-, 206Pb/204Pb-, 207Pb/204Pb- and 208Pb/204Pb-Verhältnisse by itself. Neodymium is depleted at the same time, causing the 143Nd/144Nd-Verhältnis reduced.

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

The convergent Suprasubduktionsophiolithe can be divided into two types:

• Alpinotype ophiolites ( Tethys ophiolites ) emerged from the Tethysraum
• Cordilleran ophiolites of the Pacific Area

These two Ophiolithtypen differ fundamentally by the manner of their emplacement: the alpinotype ophiolites autopsies were performed on a passive, partly thinned continental margin, whereas a large part of the Cordillera ophiolites were passively pushed out by the under deferred accretionary prism (English accretionary uplift ).

Of course, ophiolites also form at divergent ocean spreading centers, at hotspots and seamounts, and in Interarc and in the backarc area, but they are autopsied only in rare cases and are thus also rarely receive. As examples of oceanic spreading centers of the Ligurian ophiolite and the Franciscan ophiolite can be viewed.

The key should ultimately lead to the degree of partial melting in the upper mantle be, ranging from a relatively low degree of melting at spreading centers ( Ligurian type with lherzolite as mantle rock ) at significantly higher levels in Suprasubduktionsophiolithen ( Yakuno - type with clinopyroxene - leading harzburgite and Papua - type with clinopyroxene -free harzburgite ) is sufficient. For this parallel is the development of separate basaltic magmas associated: it runs from alkali basalts or aluminum-rich basalts ( MORB ) on aluminum - poor basalts ( Inselbogentholeiite ) towards Boniniten and magnesium-rich andesites.

Temporal development

Throughout history, the subduction of oceanic crust and thus the formation of ophiolites was not uniform but in a pulsed manner. So come Ophiolithpulse statistically heaped in the Neoproterozoic ( Cryogenium ) to 750 million years BP, in the Paleozoic at the turn of the Ordovician / Silurian to 450 million years BP and the Mesozoic Era at the turn of Jurassic / Cretaceous to 150 million years BP.

Each of these Ophiolithpulse can be correlated with major orogenic phases. Thus, at the same time the maximum of the Pan-African orogeny occurred in the Cryogeniums about; the Early Paleozoic ophiolites appear to coincide with the Caledonian belts of the Appalachians, Caledonides and the Urals, whereas the Mesozoic ophiolites in the Alpine - Himalayan dominate belt. The circumpacific belt contain ophiolites belonging to the last two pulses. Be documented thus long-lasting, continuous mountain building processes in the Pacific region, based on subduction of oceanic crust with concomitant accretion. They stand in contrast to the relatively short-lived, episodic extending continent collisions.

Occurrence

Ophiolites can usually be found in suture zones. This subduction oceanic crust after completed sections between colliding continents, continent or island arc fragments are stored. However, based on a pure continent collision model concept has in the past in the case of the Yarlung Tsangpo suture (also Indus - Yarlung suture ) to misinterpretation, as it is likely to have well- acted here only a relatively small ocean basins (called backarc basin) and thus not to the expected large suture between Eurasia and India.

The most famous example of ophiolites ( Semail ophiolite - ) located in Oman and the United Arab Emirates, where the former oceanic plate of Neotethysraums was pushed to the Arabian plate. Other significant deposits are found in Cyprus, Spain, Switzerland, Morocco, Guinea, Newfoundland and California.

Localities in detail

• Australia - Macquarie Iceland
• France Massif Central - Merlis - serpentinite

• Sulawesi - Sulawesi Ultramafitgürtel

• Horokanai - ophiolite
• Oeyama - ophiolite
• Poroshiri - ophiolite
• Yakuno - ophiolite

• California: Coast Range ophiolite - forearc
• Franciscan ophiolite - spreading center

• (Found also in California ) Josephine ophiolite - backarc

Swell

• Shervais, JW: Birth, Death, and Resurrection: The Life Cycle of Suprasubduction Zone Ophiolites. In: Geochemistry, Geophysics, Geosystems. 2, Paper number 2000GC000080, 2001.