The Results of Magmatic Differentiation

If a parent rock is mafic in theory it will produce: peridotite at the base of the intrusion, rich in dense olivine gabbro forming the main part of the intrusion diorite may exist towards the top of the intrusion granite may form veins or a thin layer at the top of the intrusion In most cases intrusions are complex, with multiple injections of magma that may vary in composition. It is unsual to have a single volume of magma that crystallises in one period of cooling.Major Layered IntrusionsThese are large mafic or ultamafic igneous intrusions that cooled slowly below the surface so there was time for them to form distinct lyers. Most of the world's platinum and chlorium comes from layered intrusions, giving an added incentive to understand how they formed. Gravity settling is the key process in the formation of layers of magnetite, chromite and platinum.The Palisades SillThe Palisades Sill in New Jersey, USA outcrops along the Hudson River and is over 300m thick. It was intruded into Triassic sediments at a depth of about 3km, so that it cooled slowly and has now been exposed by erosion. The sill can be divided into three sections, each shows a different igneous process at work. the only parts of the sill to cool rapidly are the upper and lower edges, which were in contact with the cold country rock. These chilled margins have fine grained crystals and most importantly have the same composition as the original magma because they cooled before any differentiation took place. The rock is a basalt. as a main part of the intrusion began to crystallise, the early formed olivine crystals began to sink by gravity settling. Olivine has a density of 3.8gm/cm3 compared to 3.0gm/cm3 for a mafic magma. The olivine crystals form a layer 10m thick at the base of the intrusion - just above the chilled margin crystallisation was taking place from both the top and the bottom of the sill as the crystals grew in cooler areas. The main rock forming the intrusion is medium grained dolerite. The last part of the magma to crystallise is about 200m above the base of the intrusion and is coarse grained gabbro because it cooled slowly. As a result of fractionation, the composition is lower in mafic minerals than the original composition because the magma is depleted in iron and magnesium by the time this rock forms. It is richer in plagioclase as the last part of the magma to crystallise is enriched in silica. The Skaergaard IntrusionThe Skaegaard Intrusion in Greenland is one of the most studied in the world. It was intruded during the Tertiary, when the North Atlantic was opening, as a magma chamber for basalt volcanoes. Magma was probably intruded in a single injection into a huge conical intrusion.The Skaergaard is divided into three major units: the Marbginal Border Series is a fine grained chilled that cooled rapidly. It is no longer the same composition as the original magma, as it has been contaminated by country rock. Crystals grew inwards from the sides of the intrusion. The Layered Series shows rhythmic layering, most easily explained by crystal settling interrupted by periodic large-scale convection. A sequence of denser (olivine or pyroxene) crystals beneath the lighter plagioclases is deposited by gravity settling. Then filter pressing causes the expulsion of the differentiated liquid. Then convection mixes the magma, and the process is repeated. Each cycle is more evolved due to the removal of the crystals in the rhythmic layers, resulting in cumulates forming from the floor upwards. The Upper Border Series is thinner, but mirrors the 2500m Layered Series with layers that crystallized from the top down. Hekla VolcanoHekla is a large volcano in Iceland near the central rift zone. The magma should be entirely mafic as Iceland is on a divergent plate margin, yet it erupts intermediate lava. The longer the time between eruptions the more acidic the magma that is erupted. If there is a long interval between erutions, fractionation in the magma chamber causes more mafic minerals, which crystallise at high temperature, to be found at the bottom and felsic at the top. Each eruption takes magma from the top of the magma chamber and then as the eruption progresses from lower in the magma chamber. The 1947 eruption occurred after an interval of 102 years and initially erupted lava with a silica content of 63% , which formed a silicic rhyolite and as with all the eruptions ended as the intermediate andesite with a silica content of 54%.

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