and find homework help for other Science questions at eNotes. Another difference between basalt and gabbro has to do with their grains, or the size of the. Bowen's Reaction Series provides the basic relationships of some of the most common rock-forming . So, you probably have two questions at this point. Aphanitic Phaneritic Porphyritic Mafic Basalt Gabbro Basalt porphyry. The extrusive compositional equivalent of gabbro is ______. In the following list which term refers to the silica content typical of a basalt or gabbro? In the illustration for question 30, in nature, object D has a relationship to object C which in.
In recent years more light has been shed on the possible origin of the calc-alkaline suite. Perhaps the best evidence comes from experimental petrology and recent advances in experimental techniques.
First, however, consider experiments conducted at low pressure on tholeiitic basalt magmas. These experiments show that at low pressure Plagioclase and Olivine crystallize first, with proportion of plagioclase crystallizing being higher than that of olivine. Further crystallization of these phases will then drive the liquid composition along the cotectic to eventually crystallize pigeonite low Ca-pyroxene as shown by the light colored path on the diagram.
Analyses of the liquids produced in these experiments showed that, as expected the liquids would follow a trend of Fe-enrichment and thus the calc-alkaline trend could not be produced by fractional crystallization at low pressures.
Next, experiments were conducted at a pressure of 2 kb with enough H2O in the capsules to assure that the liquid would be H2O saturated at this pressure i. These experiments were conducted because it was known that H2O would lower the temperature of appearance of the silicate minerals, but would lower the temperature of appearance of oxide minerals, like magnetite to a lesser extent, and could stabilize a hydrous phases like hornblende at a higher temperature.
The proportion of Olivine relative to plagioclase becomes much higher than in the dry low pressure experiments. Magnetite becomes an early crystallizing phase and hornblende also crystallizes early if the liquids have a high enough concentration of Na2O.
Most importantly, analyses of the liquids produced in the experiments plot along the calc-alkaline trend in the AFM diagram. Furthermore, if subduction related arc rocks are plotted on the projection there are seen to lie in a field surrounding the 2 kb H2O saturated cotectic. This indicates that the calc-alkaline suite could be produced by fractional crystallization under moderate pressure water saturated conditions. From our previous discussion, we know that it is possible to introduce water into the subduction related environment by dehydration of the subducting lithosphere, whereas it is more difficult to envision a mechanism to add water to the source where tholeiitic magmas are generated.
Continental Igneous Rocks A wide variety of igneous rocks occur in the continental lithosphere, a reflection of its heterogeneous nature compared to oceanic lithosphere. In addition, because the continents are not subducted and are subject to uplift and erosion, older plutonic rocks are both preserved and accessible to study. We start with granitic rocks and their associated pegmatites, next consider large volume continental rhyolites and basalts, and finish with continental rift valleys.
Granitic Rocks Here we discuss a group of plutonic igneous rocks usually referred to as "granitic rocks", "granitoids", or loosely as "granites". Included are true granites, but our discussion will include all medium to coarse-grained rocks that are mostly felsic with a few mafic minerals. Classification A variety of classification schemes have been proposed for granitic rocks. The easiest to employ uses the modal mineralogy of the rocks, while others attempt classification on the basis of the tectonic setting, or type of source rock which melted to produce the granitic magma.
The IUGS mineralogical classification scheme shown here is based modal mineralogy. All rocks will likely contain mafic minerals such as biotite, hornblende, and perhaps pyroxenes, along with opaque oxide minerals. The base of the composition triangle is a thermal divide, that separates quartz-bearing rocks from feldspathoid-bearing rocks. The feldspathoid bearing rocks include the feldspathoidal syenites, which will not be considered to any large extent here.
Tectonic classification is more appropriately called a chemical classification, because, as we will see, the various chemical types are not necessarily restricted to certain tectonic environments.
S-type granites are thought to originate by melting or perhaps by ultrametamorphism of a pre-exiting metasedimentary or sedimentary source rock. These are peraluminous granites [i. Mineralogically this chemical condition is expressed by the presence of a peraluminous mineral, commonly muscovite, although other minerals such as the Al2SiO5 minerals and corundum may also occur. Many S-type granitoids are found in the deeply eroded cores of fold-thrust mountain belts formed as a result of continent-continent collisions, such as the Himalayas and the Appalachians, and would thus be considered orogenic granites.
I-type granites are granites considered to have formed by melting of an original igneous type source. These are generally metaluminous granites, expressed mineralogically by the absence of peraluminous minerals and the absence of peralkaline minerals, as discussed below.
Instead these rocks contain biotite and hornblende as the major mafic minerals. Mesozoic or younger examples of I-type granites are found along continental margins such as the Sierra Nevada batholith of California and Nevada, and the Idaho batholith of Montana. In these regions the plutonism may have been related to active subduction beneath the western U.
All the Answers are in Bowen’s Reaction Series | Paleopix
I-type granites are also found in the Himalayas, which are related to continent-continent collisions. Plutonic suites that were emplaced in convergent continental margin settings, show many of the same characteristics as the calc-alkaline volcanic suite that likely erupted on the surface above. The suites include gabbros, diorites, quartz monzonites, granodiorites, and granites. They show mild to no Fe-enrichment, similar to calc-alkaline volcanic rocks, and a range of isotopic compositions similar to the associated volcanic rocks.
Nearly all are I-type granitoids. An example of the a convergent margin plutonic suite is found in the Sierra Nevada Batholith and associated plutons in eastern California and western Nevada that were emplaced during the Mesozoic Era.
Exposed rocks are generally older toward the east and southeast. Kistler and Peterman showed that the Sr isotopic ratios vary across the batholith in a systematic way.
The younger rocks in the western portion of the batholith are mostly quartz diorites with Sr isotopic ratios less than 0. Plutons farther east are mostly quartz monzonites and granodiorites with ratios increasing along with age of the plutons toward the east and southeast.
One interpretation of the data is that the older rocks contain a higher proportion of older crustal material than the younger plutonic bodies. Minerals like the sodic amphiboles - riebeckite and arfvedsonite, and the sodic pyroxene - aegerine, are commonly found in these rocks. In addition, they tend to be relatively Fe-rich and thus fayalitic olivine sometimes occurs. They are considered anorogenic granites because they are generally found in areas that have not undergone mountain building events.
Instead, they appear to be related to continental rifting events wherein continental lithosphere is thinned as a result of upwelling asthenosphere. The upwelling raises the geothermal gradient resulting in melting.
Because the conditions under which a magma cools can play an important role in the texture and contact relationships observed in the final rock, plutons can be characterized by the depth at which they were emplaced.
Gabbro: Igneous Rock - Pictures, Definition & More
This is because depth, to a large extent, controls the contrast in temperature between the magma and its surroundings. The catazone is the deepest level of emplacement, considered to be at depths greater than about 11 km. In such an environment there is a low contrast in temperature between the magma and the surrounding country rock. These mafic minerals, though still having quite a lot of silica, have substantially less silica than do the minerals toward the bottom of BRS. This characteristic chemical composition tend to make mafic minerals less stable at the surface of the Earth.
At the bottom of BRS are the felsic minerals of quartz, muscovite, and potassium feldspar. Felsic minerals have high abundances of aluminum and potassium, and higher amounts of silica than mafic minerals. We know all the minerals on this series are plagioclase because they show the characteristic striations of plagioclase, but they differ in the relative amounts of certain elements, in particular calcium and sodium, which causes them to be different colors.
Thus, we have anorthite at the mafic end of the series, which is dark in color, and oligoclase at the felsic end which is light in color. At the dark end of the plagioclase scale is this mineral. It has lots of calcium and little sodium. This is probably Anorthite At the light end of the plagioclase scale is this mineral. Its rich in the element sodium. All right, so now we have all the minerals in place and understand something about their chemical composition.
So how do we use this? These minerals do not just form random associations. Those on the mafic end of the series only occur with other mafic minerals. See composition chart on this page. This mineral composition usually gives gabbro a black to very dark green color. A minor amount of light-colored mineral grains may also be present. Unlike many other igneous rocks, gabbro usually contains very little quartz.
You can see a close-up view of gabbro toward the bottom of this page. Gabbro and Basalt are Related Gabbros are equivalent in composition to basalts.
- All the Answers are in Bowen’s Reaction Series
The difference between the two rock types is their grain size. Basalts are extrusive igneous rocks that cool quickly and have fine-grained crystals. Gabbros are intrusive igneous rocks that cool slowly and have coarse-grained crystals. In the oceanic crust, basalt forms near the surface at a divergent boundary, but gabbro forms at depth from slow crystallization. Learn about teaching plate tectonics. Gabbro in Oceanic Crust It is often stated that Earth's oceanic crust is made up of basalt.
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The word "basalt" is used because the rocks of the oceanic crust have a "basaltic" composition. However, only a thin surface veneer of oceanic crust is basalt. The deeper rocks of the oceanic crust are generally coarser-grained gabbro.