Troodos ophiolite: formation and composition - Essay Example

?The primary lithological units found in the Troodos ophiolite in Cyprus, the tectonic, hydrothermal, and magmatic processes that formed this portionof oceanic lithosphere Introduction: Study of ophiolites helps in the development of concepts about the formulation of oceanic lithosphere. The main challenge encountered in the study of ophiolites is that they are no more active. Because of this fact, evidence collected from their study is disentagled by various processes which have played a role in modifying the new lithosphere. One of the most widely and deeply studied ophiolites is the Troodos massif. Study of this body has generated such data and information that has shaped human understanding of the concept of ophiolites and has developed a complete model to explain the structure and composition of oceanic lithospheres. This paper aims at discussing the main lithological units of Troodos ophiolite, and its development process. Main lithological units of the Troodos ophiolite: The Troodos ophiolite is situated in the East Mediterranean upon the Cyprus island. This island forms the end of a series of Cretaceous ophiolites towards the north-west. These ophiolites are located on the Arabian plate’s margin. Scientists mutually hold the consensus that the Troodos ophiolite is a Turonian. Many believe that it developed in the zone setting of supra-subduction in the Ocean of Tethys. With an anticlockwise rotation by an angle of 90? between early Eocene and Maastrichtian and the uplift terminating in the Quaternary, the oceanic crust of Troodos gained obduction (Gillis and Roberts, 1999). The field relationships depict momentary alterations in the supply of magma during formation of the ophiolite’s crust. The crust of Troodos was made as a result of numerous chambers of magma. Link between the plutonic sequence and the complex of sheeted dyke as well as the relationship between numerous plutonic bodies stay evidence to this. Also, many xenoliths pertaining to a specific lithology have occurred in different compositions which are indeed, the most objective proof of the multiple magmatic chambers in the crust of Troodos. The nature of relation between gabbros and the sheeted dykes is quite complex which is one more supportive factor for the existence of polyphase magmatism in Troodos ophiolite. Gabbroic plutons often intrude into the sheeted dykes when they are high in level. Dyke swarms may later intrude them. In fact, in some regions, dykes have emerged straight from pluton. Among all tectonic environments, maximum volume of magma is produced in the mid ocean ridge region. This trend has continued to be since early Precambrian and is a potential way of dissipation of the accumulated heat inside the earth. Plutonic sequence: Composition and structure: Scientists vary in their opinion about the traits of subrift magma chambers. A lot of debate has conventionally taken place regarding their form. Some scientists think that the chambers’ features are big and long lived and keep changing with the passage of time. Other scientists are of the view that their features are ephemeral and small and have their individualistic and independent patterns of melt evolution. The Deep Sea Drilling Project determined many ephemeral and distinct systems of magma supply (Robinson and Malpas, n.d.). Research done in the following years called the subrift chambers’ nature as the spreading rate function. With an increase in the rate of spread, the size and age of chambers increases. A number of models for the segmentation of ridge have been recently proposed that believe in the existence of a central magma chamber in a particular segment. Magma flows in both directions form that segment along the axis of the rift. Characteristics of lava in the Troodos ophiolite: The composition of lava in the Troodos ophiolite is much different from what it is in the mid-ocean ridge basalts (MORBs). The lava in Troodos ophiolite is rich in volatile content. Siliceous compositions are also more abundant in the lava of Troodos. These differences have made the volcanoes of Troodos rich in vesicularity that ranges from less than 5 per cent up to more than 15 per cent, and the viscosity at magmatic temperatures is also calculated at 50 to 50,000 Pa (Cann and Gillis, 2004). In contrast to the lava of Troodos, vesicularity of MORBs is quite lower i.e. less than 5 per cent and the range of their viscosity is also very narrow i.e. only between 50 Pa and 250 Pa (Cann and Gillis, 2004). The Troodos ophiolite was not covered uniformly by the Pelagic sediments. As soon as it was formed, the ophiolite was blanketed by the depth of carbonate compensation. Since then, the sediment preserved is largely siliceous. The Carbonate sediment started to move to the Cretaceous end. That was a point in time when the ophiolite’s western end was raised as compared to its eastern end because the oldest sediments of carbonate were more towards the eastern side. They were ponded kilometers deep all across the width and overlapped the basalts in the western region. The ophiolite remained uncovered from the carbonate sediment in the western region till Oligocene. There is variation in the Troodos’s volcanic portion in terms of thickness. The thickness ranges from 600 m to 1200 m at different points. A lot of extrusive lithologies combine to form the thickness of volcanic portion. Pillow lavas form a major part of the whole sequence of lava. Mineral composition of the Troodos ophiolite: Sulfide deposits in the Troodos ophiolite have different sizes and weights. A total of 50 Mt of sulfide deposits have been found in the Troodos ophiolite, which can all be classified into different orefields. About 75 per cent of sulfide mass in the Troodos ophiolite is found in 8 massive deposits, of which 40 per cent are in Solea orefield’s three greatest deposits. Likewise, 75 per cent of the 1 Mt of copper that has so far been found in the Troodos ophiolite is contained in Solea orefield’s three deposits. The copper grade of ores and the size of ores have no sequenced relationship with each other. Percentage of zinc in the Troodos ophiolite is very small. The ratio between amount of Cu and Zn in the Troodos ophiolite is no more than 7. Also, the zinc and copper grades in Troopos ophiolite have no well defined relationship with each other. At various levels in the depth of the lava pile, sulfide deposits are found. There is just one big sulfide deposit, called as Skouriotissa that forms the top-most layer of the pile of lava which remained uncovered by later flows of lava after it was created. Most of the sulfide deposits were deep down the pile of lava. They were mainly formed in close vicinity of the spreading axis. A vast majority of the sulfide are located at intermediate depths of the Troodos ophiolite, which essentially tells that they were made on the summit graben’s edge. They are hundreds of meters distant from the spreading axis. There are two basic types of sulfide deposits, namely massive ores and stockwork deposits depending upon the percentage of sulfide in the ores. The former are rich in sulfide while the latter is a mix of quartz and sulfide. Stages of hydrothermal circulation in the Troodos ophiolite: Three systematic phases of hydrogeological flow and hydrothermal circulation can be classified in the Troodos ophiolite. In many places, overlapping of the three stages can be found as a result of the strong exposure continued over kilometers in various structural levels of the Troodos ophiolite, though once can still see regions where some stages dominate others. Three episodes of the hydrothermal circulation are discussed below: 1. Near the spreading axis, there is hydrothermal circulation with a very high temperature. The hydrothermal circulation gave rise to metalliferous sediments, sulfide deposits, and the black smokers. All of these resemble the mid-ocean ridge venfields in their geological structures, textures, chemistry and mineralogy. Alteration pipes and stockwork zones are present under the top expressions in the sequence of volcanoes. In this sequence, sulfide-quartz veins intercept the hydrothermally changed basalt. There are concentric zones in these pipes. Extremely changed and fractures basalt lies in the core of tehse pipes. The conditions of greenschist facies modify the basalt chemically. There are quartz-epidote regions near the sheeted dike complex’s base. In this area, the temperature of about 350 C changes the dikes continuously. The dikes, here, undergo uninterrupted recrystallization and metasomatism. They lessen up in metals which contain heavy deposits of metalliferous sediments and sulfide, and are rich in fluid inclusions which contain a history of the interaction between fluid and rock. These epidosite regions can be referred to as the reaction areas where the underlying magma chamber changes the seawater with low temperature into a hydrothermal fluid with a considerably high temperature. The seawater’s reaction with rock walls also tends to raise its temperature. However, the axial circulation of the fluid terminates almost at 1 to 2 km from the spreading axis. 2. There is circulation of temperature at intermediate points inside the crust that is about millions of years old. Gold enrichment and silicification of basalts are the discharge sites for the temperature circulation. During silification, pyrite got precipitated in few discharge sites at increased temperatures. As the discharge took place, it changed the underlying extrusives into clays at cold temperature. As the hot axial circulation changed into off-axis circulation, it essentially terminated the active faulting and played a big role in making the fractures sealed. The process of sealing up of fractures may have been a big cause of the retardation of hydrothermal circulation in sheeted dikes. Thus, heat may develop and support the consistent process of change of the unit. 3. The third episode of hydrothermal circulation in Troodos ophiolite is the off-axis circulation in gabbros, lavas and sheeted dikes which sustains over a long period of time. As temperature in gabbros and sheeted dikes fell below the axial values, it led to the localization of fluid flow within the regions of fault. The migration of oxidative alteration in lavas to the volcanic sequence developed a region of carbonate cementation and consistent change. Factors that controlled the distribution and thickness of the very region included the open circulation duration, rate of sedimentation and its nature, as well as the initial morphology of the seafloor. Therefore, regions in which the synvolcanic umbers covered the lavas, it is not developed much. The rapid availability of the brecca units in inferred highs of topography are the conditions conducive for the development of oxidative alteration front. The brecca units remain exposed to seawater for long and are quite high in permeability. “Beneath this region, alteration proceeded at slightly higher temperatures but at more reducing conditions for at least 20–30 M.y” (Cann and Gillis, 2004). “Summary of structure and relationships in a high-temperature axial hydrothermal system” (Cann and Gillis, 2004). Review of debate regarding the hypothesis that the Troodos ophiolite developed in the center of mid-oceanic spreading: The study of Troodos ophiolite can be fundamentally classified into three stages. The first phase of study occurred between 1950s and 1960s when, the committee of Cyprus Geological Survey published a whole range of journals about the massif. This first stage of investigation played a big role in explaining the composition of the major units and relating them to one another structurally. This study resulted into the demonstration of the cogenetic nature of various igneous components. Also, the researchers realized the intrusive qualities of sheeted dykes. Followed by the first investigation was the research of Moores and Vine (1971) who studied the structure of massif and found it to be an ophiolite. Ophiolites developed in the spreading environment. That was why, the researchers assumed that the ophiolites were portions of ordinary oceanic lithosphere that were created at the ridges in the midst of oceans. However, this viewpoint was met with great controversy by other scientists like Miyashiro (1973) who believed in the compatibility of the main geochemistry history of the lavas of Troodos with their development in the arc of island. The philosophy presented by Miyashiro (1973) was altogether rejected by scientists in the following years on the basis of the historic alteration in the Troodos lavas which made their real geochemistry difficult to understand: The Sheeted Intrusive Complex, measured across strike, is 120 km of nothing but near vertical dykes. Screens of pillow lavas form less than 2% of the cross strike outcrop and then only at the top of the complex. Miyashiro dismisses this as ‘overlapping central vent volcanoes’. Having between us seen several hundred central vent volcanoes in various states of erosion, we venture to suggest that if Prof. Miyashiro were to visit Troodos he might change his opinion. (Gass et al., 1975 cited in Pearce and Robinson, 2010). The pervasive change in Troodos that has occurred frequently in the past has challenged the compatibility of the Cyprus’s regional geology with the environment of an oceanic arc. As a result of the conflicting philosophies and contradicting opinions, a lot of data has been collected regarding the geochemistry of Troodos but its origin has largely remained unidentified over the years. The third phase of research commenced in 1981. The Cyprus Geological Survey Department (CGSD) together with the International Crustal Research Drilling Group (ICRDG) furthered the research regarding identification of the origin of Troodos (Robinson and Malpas, n.d.). This research made use of a mix of laboratory studies, field mapping and comprehensive drilling which led to resolution of a lot of confusions of the past. In 1990, researchers argued that Troodos “had formed above a northward-dipping subduction zone during the earlier stages of convergence within an oceanic basin” (Robertson and Xenophontos, 1993). Gass (1968) originally identified the Troodos ophiolite as a piece of lithosphere of ocean. Troodos ophiolite shows up as an anticlinical dome that spreads from east to west. The dikes inside it and others that cut through the lava are spread from north to south. The spreading axis is essentially in the very direction. An east-west fault region is located near the southern end of the ophiolite that is known as the Arakapas Fault. It is a transform fault that maintains its consistency of orientation with respect to orientation of the dikes. The structure of Troodos ophiolite is fairly simple towards the Northern side of the transform fault. This feature is common in a lot of ophiolites. Plutonics are covered by sheeted dikes which are in turn, overlain by lavas. Lavas are blanketed by sediments from the seafloor. Outcrops of the plutonics in Troodos are restricted in the midst of ophiolite. In the southern region of the transform fault, a large extensional episode affects Troodos ophiolite that comes soon after the crustal construction in the ophiolite. It is largely believed that this crustal construction is the result of the tectonic activity which is related to the Arakapas fault region. The belongingness of the ophiolite to suprasubduction zone has been mutually shown by the plutonic petrography and the lava geochemistry. Development of the Troodos ophiolite is strongly linked with the subduction zone. However, one can not see any associated arc. Sediments that overlie the Troodos ophiolite are pelagic in nature, and there is no hint of land in the vicinity. References: Cann, J., and Gillis, K. (2004). Hydrothermal insights from the Troodos ophiolite, Cyprus. In Davis, E. E. and Elder?eld. H. (Eds.), Hydrogeology of the Oceanic Lithosphere, Cambridge University Press. Gass, I. G. (1968). Is the Troodos massif of Cyprus a fragment of Mesozoic ocean ?oor? Nature, 220: 39–42. Gillis, K. M., and Roberts, M. D. (1999). Cracking at the magma–hydrothermal transition: evidence from the Troodos Ophiolite, Cyprus. Earth and Planetary Science Letters 169: 227–244. Miyashiro, A. (1973). The Troodos ophilite complex was probably formed in an island arc. Earth and Planetary Science Letters. 19: 218-224. Moores, E. M., and Vine, F. J., (1971). The Troodos Massif, Cyprus and other ophiolites as oceanic crust; evaluation and implications. Philosophical Transactions of the Royal Society of London. pp. 433-466. Pearce, J. A., and Robinson, P. T. (2010). The Troodos ophiolitic complex probably formed in a subduction initiation, slab edge setting. Gondwana Research, 18: 60–81. Robertson, A., and Xenophontos, C. (1993). Development of concepts concerning the Troodos ophiolite and adjacent units in Cyprus. The Geological Society. Retrieved from http://sp.lyellcollection.org/content/76/1/85.abstract. Robinson, P. T., and Malpas, J. (n.d.). The Troodos ophiolite of Cyprus: New perspectives on its origin and emplacement. pp. 13-26. Read More