Wadi Abyad Ophiolite and Moho

    Elsewhere on this website i have indicated that the term ‘ophiolite‘ is commonly misused as if it was a rock type. Its correct useage is to describe a whole sequence of rock types. Check out this ophiolite link for more details. So, the ophiloite sequence is a section of the earths crust extending from the uppermost part of the mantle through the oceanic crust and up to the sea bed. This section of mantle and oceanic crust seen in Wadi Abyad was obducted onto contiental crust around 95ma ago (which makes it around the early part of the Late  Cretaceous – i.e. Cenomanian). The Wadi Abyad  example, has been described by many, as one of the worlds best exposures/ outcrops of the lower part of the ophiolite sequence. Basically we can see the uppermost mantle and the lowermost oceanic crust and of course, the moho separating the two. (Moho being the Mohorovicic discontinuity named after Croatian seismologist Andrija Mohorovičić who made the first observations of its importance.) The Mantle Sequence: The photograph above has the mantle component of the ophiolite sequence labelled as ‘peridotite‘ which is a group rock type here comprised largely of a rock called ‘harzburgite‘ with minor amounts of ‘lherzolite‘ and ‘dunite’. These rock types are referred to as being ‘ultramafic‘ due to the high abundance of magnesium and iron rich minerals (hence ‘ma – fic’). The minerals are largely ‘pyroxenes and olivine’. Harzburgite is dominated by olivine and orthopyroxene, while lherzolite is similar to harzburgite but has more clinopyroxene. Dunite is dominated by olivine which may impart a green colour to freshly broken rock surface (olivine ..olive green). Apart from the green colour, none of this can be seen without the use of thin sections of rock and a polarising microscope. It has been suggested that this section of the ophiolite may be up to 12km in thickness. It also contains a number of dikes with various ultramafic rock types and locally we see chromite veins in the olivine rich dunites. Check the photo. The Crustal Sequence: Remember that here we are talking about the oceanic crust that has been obducted onto the continental crust. We are not talking about the continental crust, its an important point. The sequence above the moho, the obducted oceanic crust is comprised of more peridotites and gabbros overlain by massive non layered igneous rocks. At high levels these include intrusive sheeted dikes and extrusive pillow lavas interbedded with oceanic sediments. This sequence has been estimated to be up to 9 km in thickness, so already including the mantle sequence we may be looking at rocks that should be around 21 km below us. About 25% of this sequence is layered peridotite (wherlite and dunite), with the rest (75%) being gabbro.   MORE IN PREP