In the North East corner of the Arabia, Peninsula lies the country of Iraq. The country is a land to different contrasting geography that consists of the arid desert in the west of mountains that are rugged of Taurus and Zagros in the northeast, the two regions are separated by the fertile depression of Mesopotamia. In geology, Iraq is said to lie in the transition between the Arabian Shelf and the damaged areas of Taurus and Zagros Zones in the North and North East. Moreover, the Arabian Shelf traces its evolution on the mobility of the basement of Precambrian and the process of tectonism along the margin of the Neo-Tethyan (Al-Juboury & Al-Hadidy, 2009).
Metamorphism is the change of the texture of mineral or geologic feel, which is the precise arrangement of minerals in the already existing rocks without the pre-existing rock melting off into liquid magma. Modifications of the metamorphosed rocks are due to heat, pressure and chemical processes, this usually happens when the rocks are buried deeply below the surface of the earth. Metamorphosed are divided into two distinct groups, the foliated metamorphic rocks which include gneiss, phyllite, schist among others. These rocks have layered appearance which is due to exposure to heat and the pressure directed to the stone. The next type metamorphosed rock is the non-foliated which include the hornfels, marbles, novaculite, and this category do not have a banded appearance.
The metamorphic change occurs basically due to changes in temperature; pressure exerted, and the introduction of fluids which are chemically active. For metamorphism to occur, there are some conditions which speed up the process that is the geologic events that happen on large scales such as the movement of the global lithospheric plate, the subduction of the lithosphere of the ocean, the collision of the continents and the spreading of the ocean floor. All the mentioned three have the consequence of rocks that are moving transport heat; these changes in pressure and temperature are the important variables in the changes in the rock texture (Owaid & Abed, 2015).
There are different types of the metamorphic process, but the most common in Iraq is contact metamorphism, this kind takes place in rocks within the locale of plutonic or the original bodies which are extrusive. The head that causes the metamorphic process come from the cooling effect of the natural bodies and the fluids and gasses released by the magma that is undergoing crystallization. Where metamorphic process cause the contact with the rock is known as the aureole and the affected rocks are the country rocks of the magma body. The width of the affected rock varies from several meters to even kilometers in some cases (Al-Juboury & Al-Hadidy, 2009). The width of the aureoles depends on several factors such as the composition of the stone, volume, the depth of intrusion of the magmatic body and finally the properties of the country rocks especially the amount of fluid the rocks contain and its permeability. The larger the volume of magma the higher the amount of heat it carries and vice versa, this increases the temperature of the rocks from the bordering country this last long enough to cause the mineral reaction. Rocks which are adjacent to small dikes, lava or sills flows are escaped metamorphism while the large natural bodies will give rise to the well-defined contact aureole around granite.
The paper will delve into different history and various types of foliated metamorphosed rocks found in Iraq. In geology, foliation is the repetitive layering in metamorphic rock, the thickness of each layer differ some are as thin as a piece of paper while others measure up to one meter in length. Foliated metamorphic rocks have their formation within the interior of the earth with the pressure that is high and extreme and unequal; the formation occurs when the pressure is greater in one direction than the other. The pressure causes the original rocks’ minerals to reorient themselves, the long and flat minerals align perpendicular to with the direction of the greatest force. The stripped shapes of the rock come as a result of the total pressure on it. The foliated metamorphosed rocks are identified with the different textures that they have; they include slate which is formed at a very low temperature, phyllite, trained at a low to intermediate temperature and pressure and lastly, schist which is formed in intermediate to high pressure and temperatures. Formed finally are gneiss, foliated rocks formed at high temperatures and pressure.
Different types of foliated metamorphic rocks are found in Iraq with different formation process; the most common one is the Marble, which a metamorphosed limestone or dolomite. It is commonly found in quarries in Kurdish Iraq; they are mostly metamorphic marbles. Marbles are located on the Thrust Zone, more accurately within the Penjween-Walash subzone of the central zone of Euogeosyncline. The rock is distributed along a belt near and parallels to Iran. Marbles are formed by regional metamorphism of different types of sedimentary rocks in an environment that does not exceed that of greenschist facies (Al-Juboury & Al-Hadidy, 2009).
Marbles have several variations depending on color and textures; the finale marble is influenced by the kind of parent limestone and the degree of metamorphism. The final marble has several colors like white, black, red and green, the economic importance of the last piece is highly regarded since it is used for the creating sculpture. Marble is also used as a building material due to its beauty and strength. The commercial marbles also include the use both the metamorphic and sedimentary rocks that are polished and the beautiful end product is used as ornamental stones (Afaj & Sultan, 2005).
The next example of the commonly found foliated metamorphosed rock in quarries in the Iraq is phyllite which is a finely grained metamorphic rock that has a laminar structure that is well developed. The parent rock of phyllite is shale, and it is made up of clay minerals. Depending on the degree of heat and pressure, shale rock can metamorphose into slate, phyllite among others. The final phyllite has undergone a greater degree of metamorphism than slate but lesser than schist. Phyllite has the property that is characterized by easily splitting with a surface that is slightly corrugated along the cleavage planes. Phyllite is composed of the quartz, sericite mica, and chlorite (Owaid & Abed, 2015).
The third example of the foliated metamorphosed rock is amphibolite which is frequent in the Penjween orphiolite complexes near the Iraqi Zagres Trust Zone, in the Northeast Iraq. The appearance of the rock in the region is on a discontinuous outcrop as pods and lenses in an apparent contact with the peridodite and serpentinite rocks. Amphibolite is a coarse-grained rock formed through the metamorphic process, and they are composed of green, brown, or black amphibole minerals and plagioclase group. Amphibolite is developed through convergent of boundaries of plates where heat and pressure cause regional metamorphism.
The rock is a product of metamorphism of natural mafic rocks like the basalt and gabbro or when clay-rich sedimentary rocks like marl and greywacke are metamorphosed. Amphibolite is commonly used in the construction industry with a variety of application; this is due to the property that is, and amphibolite is harder than limestone and at the same time heavier than granite (Afaj & Sultan, 2005). After being quarried and crushed, Amphibolite is cut and polished into for use in architecture; it is commonly used for facing stones on the outer side of the building and also as floor tile among other several purposes.
Gneiss is the next rock which falls on the foliated rock commonly found in Iraq; gneiss is unique in that one can identify it by bands and lenses which vary in composition while the other bands contain minerals reach in granular with a texture that is interlocking. The other bands have elongated minerals that form a plate-like structure; gneiss is defined by this lined structural formation rather than what it is composed of like a rock. The formation of gneiss is usually through metamorphism at the boundary of the connecting plates. Gneiss is a high-grade metamorphic rock in which mineral grains recrystallize under intense pressure and heat. Gneiss is formed in different ways, but the most common one in Iraq begins with a sedimentary rock called shale, this takes place when regional metamorphism takes place and transforms shale into slate, and then phyllite then moves into schist and finally into gneiss (Owaid & Abed, 2015).
The next process of formation of gneiss is through intense heat and pressure of the granite making it metamorphose into a lined rock called granite gneiss. This transformation of granite into granite gneiss is usually a structural change rather than mineralogical changes. Even though gneiss is not defined and categorized with the rocks’ texture, most of the specimen of these rocks have a band of feldspar and grains which are interlocking. The lines on gneiss are usually light in color with some alternate bands of darker minerals which are elongated. This dark mineral on gneiss is proof that of the pressure during the metamorphic process (Azizi., Hadi, Asahara & Mohammad 2013).
The mineral grains in gneiss rock are visible to the naked eyes, the lining of this rock is as a result of the segregation of minerals into separate dark and light colored layers. The thickness of the lined minerals on gneiss rock is usually around 1 to 10 millimeters if there is additional layering then it means that the rock has gained some new materials, such rocks are known as migmatites (Afaj & Sultan, 2005).
Orthogneiss is a protolith of gneiss that contains igneous rock; this rock is formed due to the shear in vicious granitic magma. The next in line is the paragneiss which is a variety of with a sedimentary protolith, in this case, the gneissic lining has nothing to do with the sedimentary rock layering at the original state. The features in the original rock are destroyed by the process of metamorphosis that is involved in this rocks formation.
There are various uses of gneiss; the common usage is based on the rocks property of not splitting along the planes of weakness like the other foliated metamorphic rocks. This property makes those in construction to use gneiss as a crushed stone in constructing roads, landscaping projects and many more. Others use on the durability of gneiss making them be used as a dimension stone (Bucher & Grapes, 2011). Gneiss is sheared into blocks and slabs that are used in different kinds of buildings, paving, and curbing. Some species of gneiss accept good polishing. Hence, the attraction makes the rock be used as an architectures stone. Polished gneiss is used in making tiles, facing stones, trades of stairs window sills, monuments among many others.
As an intermediate grade phyllite is a type of foliated metamorphic rock with the resemblance of sedimentary rock parents’ shale from the upper side and slate from the lower grade as far as formation is concerned. Phyllite just like slate can be distinguished from rock through its foliation also known as slaty cleavage, and the brittleness that it displays also called fissility. The natural colors of slate and Phyllis are dark with the most common color in Iraq being dark gray blue with the existence of dark red and green variety. The characteristic that distinguishes Phyllis from shale is the glossy sheen nature that is the foliation of Phyllis is slightly bent which makes it hard for the Rock to retain traces of the sedimentary beddings.
The large, visible mica crystal in the other rocks lack in phyllite, also lacking is the high-grade mineral index with the property of schist, nicknamed phyllite higher-grade metamorphic cousin. The texture of phyllite is due to the heating and compression of bedded sedimentary rocks which are rich in clay, also known as shale leads to the creation of series of rocks which have increased metamorphic grade that is slate, schist, phyllite, and gneiss (Bucher & Grapes, 2011). When shales are metamorphosing with the occasional volcanic ash layers, the process transforms platy clay minerals into sheets of mica which appear small. With the increase in the intensity of heat and compression, the metamorphic grade in the rock increases. This makes the mica rock to align themselves in a perpendicular direction where the stress comes from; hence growing larger. Phyllite has a crystal of sheen, these are silicate minerals such as chlorite, biotite and muscovite are so large that they the rock its distinctive satin sheen with slaty cleavage, the composition are not large enough to be seen with the naked eyes (Azizi., Hadi, Asahara & Mohammad 2013).
Phyllite has no organic sedimentary layering this is due to the amount of heat and pressure that is needed to transform shale into phyllite. When an additional process of metamorphism occurs, phyllite is turned into schist, at this stage, all the organic clay and the small mica crystals are converted into larger mica crystals with the remaining organic material being destroyed. The remaining minerals are the dominant minerals with the metamorphic index like garnet and staurolite.
Phyllite and slate are formed along the edges of the regional metamorphic belts, the place where marine sedimentary rocks are always caught between colliding continental plates or are scraped off the floor of the sea into a wage which is accretionary above the subduction zone.
The other foliated metamorphosed rock discussed above is gneiss which mostly consists of quartz and feldspar with clear layering. Gneiss’ layering may be weak or in some instances are well developed with varying concentration of biotite, garnet, and mica among other minerals. The structure of the gneiss has no record of layered disposition process, but they arise from preferential recrystallization which is along the flow of the stress line which occurs during metamorphosis of the parent rock.
Gneiss rock varies in groups which make their texture different too, like igneous rocks and the sedimentary metamorphosed rocks which fall into various categories like including the quartzofeldspathic, politics and hornblende gneiss. The quartzofeldspathic gneiss is formed by the either silicic igneous metamorphosis or rocks such as granite, rhyolite and rhyolitic or silicic sedimentary rocks such as sandstone. Quartzofeldspathic gneiss contains feldspar crystal with eye shape hence its nicknamed Augen gneiss. Pelitic rock is, on the other hand, formed when clay-textured sedimentary rocks are metamorphosed rocks rich in iron mostly (Bucher & Grapes, 2011).
The next category of gneiss with good texture is the calcareous gneiss which contains calcite; this rock is formed through the metamorphosis of limestone and dolomite which have a significant fraction of sand and clay. The calcareous gneisses that contain a large portion of calcite blur with the marbles, hence it’s unique texture.
The polished slabs and the fieldwork of the structure and texture of marble can be studied through the understanding of the three different types found in Iraq. The first is the black marble with white spots commonly found in Shalair Valley and areas of Penjween; it consist of the limestone contains spots nd extended patches of fractures filling recrystallized white calcite cement which is the guest material which forms the foreground texture of the rock.
The next type of marble is the gray with continuous lines and bands, the gray background contains a long continuous a lamina that is black with bands forming a well-developed distinct foliation. The disposition and the origin of the lamination are attributed to the relict of the original clay in the parent sedimentary rocks. This is possible since during metamorphism, buried sedimentary rocks clay and other materials which are insoluble are added to the bands. The result will be an increase in sizes and also the color will take more contrasts. These marbles are common in quarries of the Penjween and Shalair valleys.
Finally, the last type of marble common in Kurdish are is gray one with a black line which is discontinuous. The features of this kind of marble include the solution seam which is saturated, non-saturated and stylolites and the non-seam solution. When limestone is impure and dirty, then subjected to high pressure, different forms of small scale features are produced depending on the intensity of the pressure, the degree of impurity in limestone, and the content of the pressure resistive or non-resistive grains. Most of the texture of the marbles in Kurdish has either the diagenetic or sedimentary precursors. The structure of the marbles in Kurdish are have three main types of sedimentary rocks such as the fractured limestone, the stylolite limestone and finally the laminated limestone (Azizi., Hadi, Asahara & Mohammad 2013).
Ultimately, the foliated metamorphosed rocks form an enormous number of all the commonly found rocks in Iraq. The minerals that are extracted from these rocks have commercial importance to the people of Iraq; the formation of these minerals is due to the metamorphosis which might take long before materializing. Studies have indicated that rocks in Iraq have commercial values as discussed in the paper.
Afaj, A. H., & Sultan, M. A. (2005). Mineralogical composition of the urinary stones from different provinces in Iraq. The Scientific World Journal, 5, 24-38.
Al-Juboury, A. I., & Al-Hadidy, A. H. (2009). Petrology and depositional evolution of the Paleozoic rocks of Iraq. Marine and Petroleum Geology, 26(2), 208-231.
Azizi, H., Hadi, A., Asahara, Y., & Mohammad, Y. (2013). Geochemistry and geodynamics of the Mawat mafic complex in the Zagros Suture zone, northeast Iraq. Open Geosciences, 5(4), 523-537.
Bucher, K., & Grapes, R. (2011). Petrogenesis of metamorphic rocks. Springer Science & Business Media.
Owaid, M. N., & Abed, I. A. (2015). Mineral analysis of phosphate rock as Iraqi raw fertilizer. International Journal of Environment, 4(2), 413-415.
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