OphioFact

*There are approximately 150 ophiolites which rest on the continental crust.

OphioFact

*Until the mid-1960's, two opposing theories of ophiolites prevailed. Continental European geologists believed that there was a close relationship of serpentines, pillow lavas, and radiolarites ("Steinmann's Trinity") which resulted from outpourings of mafic/ultramafic magma in the deep troughs of "eugeosynclines". English speaking geologists argued that orogenic belt serpentines represented intrusions, either solid or magmatic, into "eugeosynclines, and they had no genetic connection to other parts of the "Trinity".

OphioFact

*The world's largest and most complete ophiolite is the Semail Ophiolite in Oman. Below is an infared picture of the composition of the Semail Ophiolite.

OphioFact

*The Oman Tehtys (Neo-Tethys) appears to have formed in the Early Triassic by the rifting off and migration of small continents (e.g., Iran, Tibet, Afghanistan) northward from the remainder of Gondwanaland.

OphioContraversy

*The proposal that ophiolites are on-land fragments of oceanic lithosphere has been with us for over 20 years and has been widely accepted by the Earth Sciences' community. Despite; this, many oceanographers are reluctant to use ophiolite data. Their argument (and it comes largely form the geological rather than geophysical oceanographers) is that 'Even if ophiolites are on-land fragments oceanic lithosphere, they must be atypical for otherwise they would have been subducted and not obducted.' So, they conclude that, although the study of ophiolites is perfectly acceptable in its own right, its results should not be used in the investigation of present-day oceanic, in-situ oceanic lithosphere- i.e. the invoking of 'reversed uniformitarianism' is not acceptable. (2)

I guess there's drama between some branches of the geosciences! Interesting.

Here's some more ophiolite and oceanic crust history and theory for you to sink your teeth into.

*Current understanding of the processes that construct and modify the oceanic upper crust has been the result of nearly 50 years interplay between concepts derived from ophiolites and those derived from active mid-ocean ridges. The Troodos ophiolite in Cyprus has played a central role in this exchange, since there the upper ophiolitic crust is preserved for over 100 km across strike, and has scarcely been deformed or heated more than a few tens of degrees since it formed in an arc environment over 90 million years ago. Early in the plate-tectonic debate, the presence of the sheeted dyke complex in Troodos, demonstrating 70 km of 100% extension, was crucial in allowing acceptance of the new ideas. Later, the recognition of the exhalative nature of the ore deposits within the extrusive unit prepared the way for the discovery of black smokers, and then the oceanic observations in turn brought new insights into the structure and activity of the hydrothermal systems that had fed the ophiolitic deposits. These insights are now being returned to the oceans. Demonstration of the narrowness of the zone of crustal construction in the oceans, and the structure of upper crust at spreading axes, led to a reassessment of the structure of the upper crust in Troodos, in turn generating new approaches to the construction of the upper crust in the oceans. This two-way traffic seems set to continue, fuelled by the contrasting types of information that can be gained from each environment. In the oceans, rates of processes can be measured, and the spatial distribution of current activity determined. In ophiolites one can walk around on cross-sections of the fully-formed crust, unhampered by the limited exposure available on the ocean floor. Because of chemical differences between even the most oceanic of ophiolitic crust and that of open ocean basins, parallels must be drawn with care; it is naturally safer to compare physical processes than petrological ones. But creative interplay between the two environments, so crucial early in the plate tectonic revolution, shows no sign of abating. This topic is not just history; it is also active science in the making.
The information above can be found at: http://gsa.confex.com/gsa/2001AM/finalprogram/abstract_22203.htm

Literature and Web page citation

1. Ophiolites and Oceanic Crust, Edited by Yildirim Dilek, Eldridge Moores, Don Elthon, and Adolphe Nicolas. Geological Society of America. Boulder;2000 (Preface, pp. 1-12).

2.Ophiolites and Oceanic Lithosphere, Edited by I.G. Gass, S.J. Lippard, and A.W. Shelton; Published for the Geologic Society by Blackwell Scientific Publications, Oxford. 1984 (Preface, pp. 1, 34, 67-69).

3. [Journal] Petrology and the Structure of the Vourinos Ophiolitic Complex of Northern Greece, Eldridge M. Moore; Published by the Geological Society of America, Inc., Special Paper 118, Boulder. 1970 (pp. 9-13, 20-22, 25-31).

4. [Journal] Structure of the Canyon Mountain (Oregon) Ophiolite complex and its implication for sea-floor spreading, Hans G, Ave Lallemant; Published by the Geological Society of America, Inc., Special Paper 173, Boulder. 1976 (pp. v-7, 16-21, 39-43).

5. The Earth's Mantle: Composition, Structure, and Evolution, Edited by Ian Jackson; Published by Cambridge University Press, Cambridge. 1998 (pp. 358-63)

6. Plate Tectonics and Crustal Evolution third edition, Kent C. Condie; Pergamon Press; New York. 1989 (pp. 57-58, 115-116, 166-167, 177-179).

7. The Earth's Crust and Upper Mantle, Edited by Pembroke J. Hart; National Academy of Sciences-National Research Council Publication, Willima Byrd Press; Richmond, VA. 1969 (pp. 480-501, 513-519).

Internet Sources and Photo attributions
http://www.angelfire.com/ms/snasir/CARB.htm
http://www.uni-wuerzburg.de/mineralogie/links.html
http://online.redwoods.cc.ca.us/depts/science/earth/smith/smithop.htm
http://gsa.confex.com/gsa/2001AM/finalprogram/abstract_21799.htm
http://www.angelfire.com/ms/snasir/page14.html
http://gore.ocean.washington.edu/classpages/oman/smenden/geopic.htm
http://www.eas.slu.edu/People/Students/BLuetkemeyer/Page2.html
http://gsa.confex.com/gsa/2001AM/finalprogram/session_761.htm

*This site is dedicated to my sister Maria Bustamante, my mother Theresa Garcia, and my father Arturo Bustamante. I love you. There's nothing I want more than to make you proud. I hope you enjoy this webpage.

The mosaic of Oman Mountains courtesy of http://gore.ocean.washington.edu/classpages/oman/lawson/

An ophiolites is a fragment of an oceanic plate that has been obducted onto the edge of continental plates. Ophiolites provide examples for the processes at mid-ocean ridges.

Ophiolites are an assemblage of mafic and ultramafic lavas and hypabyssal rocks found in association with Sedimentary rocks like greywackes and cherts. They are found in areas that have complex structure.

Ophiolites have been found in Cyprus, New Guinea, Newfoundland, California, and Oman. The Samail ophiolite in southeastern Oman has probably been studied in with the greatest frequency. Most likely, the rocks formed during the Cretaceous not far from what is now the Persian Gulf. At a later time the rocks were thrust (pushed uphill at a low angle) westward onto the Arabian shield.

By definintion, an ophiolite is merely the rock sequence. The majority of geologists interpret these sequences as representing oceanic crustal and upper mantle material that has been pushed up onto continents when slivers of the sea floor are caught between converging plates.

Photo courtesty http://volcano.und.nodak.edu/vwdocs/vw_hyperexchange/ophiolites.htm

One of the most significant reasons for the study of ophiolites is the restructuring of ancient plate boundaries, ever since their discovery as on-land fragments of oceanic lithosphere. The construction of well intact ophioite complexes reveals ophiolites to be sound structural analogues for oceanic crust. Specifically, these complexes allow three-dimensional exposures and age relationships that can be used to examine the nature of extensional tectonics and magmatic construction in oceanic spreading spreading environments. (1)

The following list of rocks are taken from an ophiolite in Oman. Ophiolites are quite common in the Middle East. They occur in elongated belts that make up an integral part of the Alpine mountain chains. These ophiolites proceed eastward and southward from Cyprus into Syria, the Turkey-Iran border fold belt, through Neyriz in Iran, then across the Arabian Gulf into Oman. The Semail ophiolite, Sultanate of Oman, is part of these ophiolites. It provides the best exposure in the world to study oceanic lithosphere. The Semail ophiolite crops out in a belt 600km long and 150 know wide and between 5 and 10km thick. The Sultanate of Oman forms the southeastern corner of the Arabian Plate. It can be divided into five structural elements. The Arabian Platform; the Huqf-Haushi Uplift the Oman Mountains; the Masirah Ophiolite Uplift and the Gulf of Oman.

The Arabian Peninsula- formerly attached to Africa, comprises a Precambrian basements (Proterozoic) that is overlain by a thick Phanerozoic shelf succession (5500m thick).

The Huqf-Haushi Uplift- southeastern margin of Oman, trending NE-SW, formed due to the subsidence of the Ghaba Salt Basin to the west and the Masirah Trough to the east. Uplifting started in the Infra-Cambrian and continued until the late Cretaceous.

The Oman Mountains- represent the margin of the Arabian continental platform. They stretch from the Strait of Hormuz in the northwest to the Arabian Sea in the southeast. The mountains contain a great thickness of authochthonous shelf carbonate rocks. The mountains are geologically distinct from the rest of the Arabian Peninsula because of the presence of extensive nappes of pleagic sedimentary deposites and ophiolites which overly the shelf carbonate rocks tectonically.

Pillow Basalts: Pillow lavas are usually basaltic in compostition but have been pervasively altered to mineral assemblages in the zeolite and greenschist metamorphic facies. Thus, the plagioclase has been converted to albite, and the mafic minerals have been transformed mainly to chlorite. These basaltic pillow lavas with albitic plagioclase were originally thought to be a special type of magma and were given the name spilite. Today, however, their particular compostion is recognized to be the product of hyrothermal alteration.

Sheeted Dikes: This unit consists of 100% dikes, that is, dikes intruding dikes. with no intervening screens of other rocks. Most dikes are less than 5m wide. Many, in ophiolites appear to have only one chille margin. This is a consequence of repeated intrusions in the center of a single open fissure. By studying statisically the assymetry of chilled margins, the position to man axis of spreading can be determined. The dikes preserve an ophitic texture, but most of their minerals have been hydrothermally altered. They were intially aphyric subalkaline diabases composed of plagioclase and augite; olivine and orthopyroxene are not reported. Although proably related to the underlying gabbros, thery are not derived directly form them. The dikes cut the underlying gabbros but pinch out downward.

Gabbros: These cumulates are generall mafic at the base and grade upward to more feldspathic rocks. Olivine becomes progressively more iron-rich, reaching Fo70 in the uppermost gabbros. Orthopyroxene is present but far less abundant than in the underlying deformed ultrmafic rocks. Diopside augite is the principle pyroxene. Toward the top of the layered gabbros, irregular intrusive bodies of plagiogranites occur. These are thought to be the final differentiation product of the gabbroic magma, which gave rise to layered rocks.

Ultramafic Cumulates:Below the lowest layered mafic, the transition to the harzburgitic mantle section os usually marked by the occurence of plastically deformed dunites. the deformation, which is characteristic of the underlying harzburgite, dies out in this transition zone. In terms of mineral composition, the mafic-ultramafic transition is smoothed by the presence of ultramafic layers, gabbro dikes, sills and impregnations in the dunites and the uppermost harzburgites. The amount of clinopyroxene and plagioclase increase upward through the zone. Thus dunite (ol) at the base passes up through wherlite (ol+cpx) to clinopyroxenite and troctolite (plag+ol). Near the base of the transition zone (lherzolite) olivine grains exhibit kink bands, but the chrome diopside and plagioclase that grow interstitially, show no signs of deformation. With the decrease in the amount of olivine, then, the degree of deformation decreases.

Metamorphic Sole: Metamorphic rocks welded to the base of ophiolites (metamorphic soles or dynamothermal aureoles) are widely documented and are generally believed to have formed during detachment and/or emplacement of oceanic lithosphere into orogenic belts. The rocks found here range from mylonitic peridotites, garnet amphibolites, epidote amphibolites, and greenschists.

The photos above are shown with courtesy at http://www.eas.slu.edu/People/Students/BLuetkemeyer/Page2.html

The Semail Ophiolite can generally be divided into two major units.

1) The Mantle Sequence; This sequence represents the upper sub-oceanic mantle. It is composed of tectonized harzburgites (85-95%), lherzolites and dunites(5-15%). The sequence may reach a thickness of 10-12km. The sequence is being cut by numerous dukes, veins and bodies of ultramafic and mafic rock types. The primary silicates are usually altered to lizardite and chrysotile. The dunites locally contain chromatite pods. The contact between the mantle sequence and the overlying crustal layers in marked by a structural and petrological boundary which is taken to represent the sub-oceanic "petrological Moho."

2) The Crustal Sequence; Consisting of a layered series (cumulate peridotites and gabbros), which is overlain by non-layered plutonic rocks (high-level Intrusives), a sheeted Dyke Complex and an extrusive sequence of lavas which is interbedded with and overlain by pelagic sediments. The sequence varies in thickness between 4 and 9km. The layered series of the Semail Ophiolites consists of layered peridotites (dunite and wehrlite 25%) and gabbros (75%). The series vary in thickness between 0.5-6km. They rest disconformably on the petrologic Moho. They are characterized by rhythmic layering on a scale of 0.5cm to 2m. The cyclic layering points to open system fractionation. They overlying high level gabbros are characterized by the absence of layering and variable texture. They form a discontinuous unit up to 700m thick. They are characterized by medium-grained hypidiomorphic to ophitic textures. The gabbros grade turns into diorites and trondhemites. Late intrusive complexes composed of peridotites. gabbros, diorites and plagiogranites intrude up into the upper crustal units of the ophiolite. These complexes were subdivided into an older series of differentiated gabbro to plagiogranite plutons and a younger group of peridotite-gabbro intrusions. The older group composed mostly of layered and massive gabbros and diorites with few plagiogranite. The upper part of the Semail Ophiolite, that is on top of the sheeted dyke complex, consists of up to 2000m of pillowed basaltic lavas. The metalliferous and associated pelagic sediments found in the upper parts of the ophiolite sequence, provide strong evidence for a deep-water origin for the lavas.

Well, that's my ophiolite website. I hope you learned a thing or two about these interesting crustal wonders. If you have any questions, comments, or concerns with regard to any of the information on this page, please contact me at bustaca@earlham.edu.

Link to all the class Webpages

This website is part of a Geology 211 project in Geological Processes

Copyright © 20031 Earlham College. Revised 25 February 2003. Send corrections or comments to parkero@earlham.edu