4. (10pts) Please discuss the origin and evolution of magmas in an arc setting (

Question

4. (10pts) Please discuss the origin and evolution of magmas in an arc setting (island arc and continental magmatic arc). How does the original magma form and what compositio How does the original magma evolve in each setting? W in an island arc? In a continental arc? Why the difference? hat is the most common magma type 5. (10pts) What is a layered mafic intrusion? What tectonic settings do they occur in? Using the given diagram, please explain the origin of the various features e.g. la repetitive layering, cryptic layering, graded beds, cross beds....? yering

Explanation / Answer

4. Continental arc- a type of volcanic arc that occurs as an "arc-shape" in a topographic high region along a continental margin. It is formed at an active continental margin where two tectonic plates meet and a subduction of one plate starts.

When two tectonic plates collide, relatively denser oceanic crust is subducted under relatively lighter continental crust. Because of this process, the relatively cooler oceanic crust, along with water, is subducted to the asthenosphere, where pressures and temperatures are much higher in comparision to the surface of earth. So,the downgoing plate releases volatiles such as H2O and CO2, which cause partial melting of the above asthenosphere.

This process creates a relatively buoyant magma, which subsequently forms a series of volcanoes at the surface along the subduction zone.Because the subduction zone (which is also the plate boundary) is generally an arc-shape, geologists named those volcanoes volcanic arcs. A volcanic arc built on continental crust is called a continental arc; when built on oceanic crust the volcanoes form an island arc.

The dehydration of the downgoing slab and the partial melting of asthenosphere together generate the primary magma of continental arcs. Primary magma is composed of olivine tholeiitic basalt because of mixture of peridotites from the mantle wedge.The larger thickness and lower density of the continental crust prevents the upwards rising of primary magma. Ascending primary magma ponds at the bottom of continental crust, forming a magma chamber. In this chamber an underplating process takes place, and the assimilation and fractional crystallization of primary magma and lower crustal rocks forms underplate at the bottom of crust.

Island arc - often composed of a chain of volcanoes, with arc-shaped alignment, situated parallel and close to a boundary between two converging tectonic plates.

These island arcs are formed when one oceanic tectonic plate is subducted under the another one this produces magma at depths below the over-riding plate.But, this is only true for those island arcs that are part of the group of mountain belts which are called volcanic arcs, a term which is used when all the elements of the arc-shaped mountain belt are composed of volcanoes.

In the subduction zone, the addition of volatiles such as water induces partial melting of the oceanic crust because the volatiles (such as water) boil faster than the surrounding rock, forcing the rock apart. This process is called flux melting, which generates a low-density calc alkaline magma which buoyantly rises to intrude and be extruded through the lithosphere of the overriding plate. The resulting volcano chain has the shape of an arc parallel to the convergent plate boundary and convex toward the subducting plate.

Magma is defined as molten rock that is found below the earth's surface. It's a Greek word which means 'thick ointment.' Magma is primarily a very hot liquid, which is called a 'melt.' It is formed from the melting of rocks in the earth's lithosphere that is the outermost shell of the earth wich constitutes crust and upper part of the mantle, and the asthenosphere, which is the layer below the lithosphere. It is composed of elements those made up the minerals in the source rocks.Most magma has other things mixed it in. For example, it contains bits and pieces of minerals that have not yet melted or have solidified (or crystallized) from the molten state as the magma cools. Many different gases dissolved in magma are also present. Water vapour, carbon dioxide and hydrogen sulphide are common. Even gaseous forms of hydrochloric or sulfuric acid might be present.

The difference between these type of magma is that:

Island arcs must be offshore, but they do not necessarily have to be volcanic (e.g. the non-volcanic Hellenic arc).

Oceanic arcs are volcanic arcs built on oceanic crust while continental arcs are built on continental crust.

• The composition of oceanic arc crust is different from that of continental arc crust. The oceanic arc crust is more mafic (basaltic/gabbroic) while the continental arc crust is prone to intermediate or felsic composition (andesitic/dioritic).

5. A layered intrusion is a large sill-like body of igneous rock which shows vertical layering or differences in composition and texture. These intrusions are many kilometres in area covering from around 100 km2 to over 50,000 km2 and several hundred metres to over a kilometre in thickness. Most layered intrusions are Archean to Proterozoic in age, for example, the Paleoproterozoic Bushveld complex.

In ancient cratons layered mafic intrusions are found and are rare but worldwide in distribution.The intrusive complexes exhibit evidence of fractional crystallization and crystal segregation by settling or floating of minerals from a melt.

These type of magma occur in mantle plumes and rifts.

Plume magmatism is an effective mechanism for the explaination of the large volumes of magmatism required to inflate an intrusion to several kilometres thickness. Plumes tend to create warping of the crust, weaken it thermally so that it is easier for them to intrude magma and create space to host the intrusions.

The large volume of magma which are created by the spreading mid-ocean ridge allow the accumulation of large volumes of cumulate rocks. The problem of creating space for such intrusions is easily explained by the extensional tectonics, extensional or listric faults which are operated at depth can provide a triangular space for keel-shaped or boat-shaped intrusions.The causes of layering in large ultramafic intrusions include convection, thermal diffusion, settling of phenocrysts, assimilation of wall rocks and fractional crystallization.

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