What is an ophiolite in geology ?
A piece of oceanic crust
Ophiolites are fragments of ancient ocean floors that have been displaced and attached to continents during the geological process called "obduction". Obduction occurs when rocks from the oceanic crust are forced to slide onto a continent or another tectonic plate, often as a result of plate collisions. This phenomenon occurs mainly when a continent meets an island arc or between two continental plates. Ophiolites are the result of these events and offer valuable insight into the mechanisms of tectonic plate dynamics and the evolution of the Earth's crust.
The concept of an ophiolite complex :
The term "ophiolite" is often replaced by "ophiolite complex" to emphasize the complexity and diversity of the rock complexes that compose them. Indeed, ophiolites are not limited to a single rock, but are made up of a series of geological formations originating mainly from the oceanic crust and the upper mantle. These complexes can reach a thickness of 4 to 5 km, sometimes up to 15 km, and are generally divided into several distinct geological units which, together, reflect the structure of an ancient ocean floor.
Typical structure of an ophiolitic complex :
- Foliated peridotites (Harzburgites) and chromite
The deepest layer of an ophiolite complex consists of foliated peridotites, especially harzburgites, which are rich in olivine and pyroxene. These rocks form the upper part of the Earth's mantle and are evidence of the high pressure and temperature conditions found in subduction zones. Depending on the local geology, chromite deposits may also be present, indicating mantle differentiation processes. - Gabbros and layered cumulate peridotites
Above the peridotites are gabbros, rocks rich in plagioclase and pyroxene. The latter form from magma that has crystallized at moderate depths. The transition from gabbros to layered cumulate peridotites, such as dunites and lherzolites, is also common in ophiolite complexes. These rocks are the result of the fractional crystallization of oceanic magma and provide evidence of magmatic processes in the oceanic crust. - Basalts and sedimentary layer
The upper layers of ophiolitic complexes are dominated by basalts, often in the form of pillow basalts (or "pillow lavas"). These formations are characteristic of submarine volcanic eruptions, where magma, upon contact with seawater, rapidly solidifies into spherical or pillow formations. Pillow basalts represent the most superficial layer of the oceanic crust. Above this is a thin layer of oceanic sediments, which can consist of various sedimentary rocks such as clays, carbonates and silts, reflecting depositional conditions in shallow oceanic environments.
The obduction process : witness to plate dynamics
The obduction of ophiolites occurs during large movements of tectonic plates, when subduction zones or collisions between continents cause a piece of oceanic crust to rise towards the continental surface. This phenomenon occurs in plate collision zones, where an island arc or oceanic crust is "pushed" onto a neighboring continent. These geological events are crucial for understanding plate tectonics and the evolution of the Earth's crust.
Ophiolites provide a unique insight into what happens in the deep oceans, where oceanic crust forms and is then modified or obducted. They also provide insight into the dynamics of the Earth's mantle, magmatic differentiation processes, and the formation of oceanic crust.