OPHIOLITES

Presented By :- Ajoy Saikia M.Sc. Sem.-1, 2013

Dept of Earth & Environmental Science KSKV Kachchh University

What are Ophiolites ??? Ophiolites are techtonicaly emplaced successions of mafic and ultramafic rocks that are considered to represent fragments of oceanic or back-arc basin crust.

An ideal ophiolite includes from bottom to top the following units :

1. Ultramafic tectonite (generally harzburgites)

2. Layered cumulate gabbros and ultramafic rocks

3. Non-cumulate gabbros, diorites and plagiogranites

4. Sheeted diabase dykes

5. Pillowed basalts

An idealizedophiolite succession compared

with various exposedophiolites. Modified after Moores (1982).

Tectonic Settings:

Ophiolites have been described from three oceanic tectonic settings :

Mid ocean ridges

Back-arc basins

Immature island arcs

( in some instances )

Emplacement of Ophiolites:

Ophiolites are emplaced in arcs or collisional oregenes

by three major mechanisms.

I. Obduction or thrusting of oceanic lithosphere onto a passive continental margin during continental collision

II. Splitting on the upper part of a descending slab and obduction of a thrust sheet onto a former arc

III. An addition of a slab of oceanic crust to accretionary prism in an arc system.

Continue…

The metamorphic complex at the base of ophiolites may play a major role in ophiolite emplacement.

These metamorphic soles, as they are often called, have many features in common:

1. Thickness generally range from 10-500 m

2. They extend laterally for tens to hundreds of kilometers

3. Most show a sharp decrease in metamorphic grade from top to bottom

4. They are highly deformed and have a pronounced tectonic foliation

5. They are composed of mixed mafic volcanics, serpentine, and metasediments.

These data are consistent with an origin for soles of successive underplating and welding onto the base of an ophiolite as it moves from the upper mantle to the surface.

OphioliteSubduction realated ophiolitesSubduction unrealated ophiolites

Subduction related ophiolites…

Subduction related ophiolite include suprasubduction zone and volcanic arc types.

Whose evolution is governed by slab dehydration and accompanying metasomatism of the mantle.

Melting of the subducting sediments repeated episodes of partial melting of metasomatised peridotites

Subduction unrelated ophiolites…

It include continental margin mid-ocean ridge and plume type ophiolites.

That generally have MORB composition.

Its developed during the closure of the ocean basin whereas involved during rift drift and sea floor spreading

Difference BetweenOphiolites & Oceanic Crust

The density of the oceanic crust is low as compare to the ophiolite sequence

Ophiolites do not have any contrast thickness. They may have even more or less thickness while oceanic crust have 5-7 km thickness.

Ophiolite lavas are dominated by arc tholeiites, back arc basalt, andesite etc which are different from oceanic crust lavas

An Examples…

Semail Ophiolite in Oman One of the best of the exposed ophiolite

sequences in the world.

600 km long, 100-150 km wide.

Tethysian oceanic lithosphere obduted on the Arabian continental margin.

Ophiolites in India Naga hills ophiolites

at the India-Burma plate boundary.

It’s a convergent plate boundary.

India plate boundary had subducted eastward below the Burma micro plate at early cretaceous.

Conclusion:

Ophiolites are slabs of ancient oceanic crust obducted/preseved onto the continental crust/earth surface.

They are located in collisional boundaries.

Their compostion is sediments, lavas, sheeted dikes, gabbros, and ultramafic rocks.

Have similarity with oceanic crust.

Reference :• Plate tectonics and crustal evolution (kent c. condie)

And from many website.

Thank You !