BACKGROUND
In the lower crustal section of the Oman Ophiolite there are spectacular layered gabbros.  Dominating the lower part of the section there are gabbro and anorthosite layers with modal layering that is often graded as well.  These magnificent outcrops with distinct layers, make any inquisitive mind ponder their origins.  How did these layers come to be?

Over the past decade many improvements in understanding magma chamber geometry and dynamics at fast spreading centers have been made.  These developments enable us to answer the question on layered gabbro origination better.  In Pallister and Hopson's 1981 paper there are excellent petrologic descriptions of the Oman gabbro section, and in Smewing 1981, they both describe mineral composition variation and geochemical tests to conclude that the layering in this section resulted from crystallization along chamber walls and floors in a large magma chamber, without significant deformation.  It was not until later (Browning 1984) that the size of the magma chamber was disputed and in 1993 (Quick and Denlinger) seismic evidence of smaller magma chambers was reconciled with geologic field evidence. With these developments, two main theories dominated the current literature on layered gabbro formation.  Successive sill intrusions from a shallow magma lens (Hensock et al. 1993; Boudier 1996) and crystallization along the floor of the magma lens and subsequent subsidence (Quick and Denlinger 1993).  Nicolas et al. (1993), in a related model look at the subsidence and crystallization of the crystal mush component of the magma lens with the underlying mantle flow to explain the layering phenomenon.  These models are all mainly based on macro scale observation and analyses.

This project attempts to explain a smaller scale process, mineral orientation, to understand the gabbro layering.  How do the orientations of the olivines, clinopyroxnes, and plagioclase form?  Specifically, I will be analyzing the olivine orientation with respect to the foliation in the layered gabbros.  In the past many have looked at other ophiolite complexes such as Papau-New Guinea (Davies 1971, Jaques 1981), Bay of Islands (Casey and Karson 1981), Trodoos (George 1978), and Antalya (Juteau and Whitechurch 1980).  In general, most of the above authors have described petrologically, the layered gabbros in their respective ophiolites as having an associated mineral foliation which is considered to be parallel to the layering plane, which is also the case in Oman.  Some of these studies show plastic deformation as the developer of tectonite fabrics (George 1978, Kidd et al. 1978, Himmelberg and Loney, 1980, Girardeau and Nicolas 1981, Casey et al. 1983, and Misseri and Boudier 1985).  Others have shown that that the mineral lineations are of magmatic origin, hence due to magmatic flow (Thayer 1963, Jackson et al 1975, Girardeau and Nicolas 1981, Benn and Allard 1989).  In Oman we need to look at the petro and tectonite fabrics to distinguish the cause for the mineral orientations.  Benn and Allard (1989) using two olivine gabbro samples conclude that preferred mineral orientations of  olivine, plagioclase, and clinopyroxene, are "related" to magamatic flow in the Oman ophiolite, due to the analysis of Shape Preferred Orientations (SPO) and Lattice Preferred Orientations (LPO) of these minerals.  It seems that more samples from Oman need to be studied to make a confident determination. A recent study on the gabbro section, and associated minerals orientations by Boudier et al. (1996)  also agrees with the idea of magmatic flow and sill injection in the Oman Ophiolite
 

OBJECTIVES
In this study, Oman provided a unique opportunity to study the layered gabbro section of the ophiolite sequence.  The outcrops in this country were mainly non vegetated, accessible by off road vehicles, and very well preserved.  Also, published, comprehensive work on the fabrics of these gabbros has been limited to date,  and this provided an interesting avenue for Corvidea Wallace and myself under the supervision of I.S. McCallum to begin dissecting the layered gabbros of Oman.  Basically the project is divided into two realms.  The first part of the project involved two weeks in the field from February 22- March 4, 1998.  In Oman we collected large oriented samples of the compositionally layered gabbros from the lower crustal section of the ophiolite (see table below).

Once back in the laboratory, thin sections and petrologic microscopy will be performed on the many samples to evaluate the fabric and distinguish a relationship between the olivine orientation and the sample foliation.  We will also obtain an idea of local compositional variation as the many samples were taken over a large geographic range in Oman.  This work is scheduled to be completed by the end of summer 1998 and this page will be updated at that time.

Also in the laboratory we are hoping to corroborate with Peter Keleman, by using his digital texture program to conduct analyses on the sample sections as well.

 
 
OMAN...Beautiful Oman

ACKNOWLEDGMENTS:
I would like to express an incredibly huge and spectacular thank you to the entire Department of Geological Sciences, University of Washington, the School of Arts and Science, University of Washington, Bruce Nelson, Will Wilcock,  Ministry of Oil and Minerals of the Sultanate of Oman, and my geology and oceanography peers from the 1998 Oman class -U.W for making an incredible and magical event happen.

REFERENCES:
Benn, K. and B. Allard, Preferred mineral orientations related to magmatic flow in ophiolite layered gabbros, J. Petrol. 30, 925-946, 1989.
Boudier, F., A. Nicolas and B Ildefonse, Magma chambers in the Oman ophiolite: fed from the top and the bottom, Earth Planet. Let.144, 239-250, 1996.
Browning, P. Cryptic variation within the cumulate sequence of the Oman ophiolite: magma chamber depth and petrological implications, in: Ophiolites and
     Oceanic Lithosphere, I.G. Gass, S.J. Lippard and A.W. Shelton, eds., Geol. Soc. London. Spec. Publ. 13, 71-82, 1984.
Casey, J.F. and J.A. Karson, Magma chamber profiles from the Bay of Islands ophiolite complex, Nature, 292, 295-301, 1981.
Casey, J.F. et al. Reconstruction of the geometry of accretion during formation of the Bay of Islands ophiolite complex. Tectonics, 2,509-528, 1983.
Davies, H. L., Peridotite-gabbro-basalt complex of Eastern Papua: an overthrust plate of oceanic mantle and crust. Canberra: B.M.R Bull. no. 128., 1971.
George, R.P. Jr., Structural petrology of the Olympus ultramafic complex in the Troodos ophiolite, Cyprus, Geol. Soc. Am. Bull., 89,845-865, 1978.
Girardeau, J. and A. Nicolas, The structures of two ophiolite massifs, Bay of Islands, New Foundland: a model for the oceanic crust and upper mantle.
     Tectonophysics, 77, 1-34,  1981.
Henstock,T.J., A.W. Woods and R.S. White, The accretion of oceanic crust by episodic sill intrusion, J. Geophys. Res. 98, 4143-4161, 1993.
Himmelberg, G. R. and R. A. Loney, Petrology of the ultramafic and gabbroic rocks of the Canyon Mountain ophiolite, Oregon, Am, J. Sci. 280A, 232-268,
    1980.
Jackson, E.D. et al.,  The Vourinos ophiolite, Greece: cyclic units of lineated cumulates overlying harzburgite tectonite. Geol. Soc. Am. Bull., 86, 390-8, 1975.
Jaques, A.L., Petrology and petrogenesis of cumulate peridotite and gabbro from the Marum ophiolite complex, Northern Papau-New Guinea, J. Petrology,
    22, 1-40, 1981.
Juteau, T. and H. Whitechurch, The magmatic cumulates of Antalya: evidence of multiple intrusions in an ophiolitic magma chamber. In: Panayiotou, A(ed.) Proc.
    Intl. Ophiolite Symp., Nicosia, Geol. Survey Dept. Cyprus, 1980.
Kidd, W.S.F. et al., The Mings Bite ophiolite complex, New Foundland:Appalachian oceanic crust and mantle. Can. Jour. Earth. Sci., 15, 781-804, 1978.
Misseri, M. and F. A. Boudier, Structures in the Canyon Mountain ophiolite indicate an island arc intrusion. Tectonophysics, 120, 191-209, 1985.
Nicolas, A., C.L. Freydier, M. Godard and A. Vauchez, Magma chambers at oceanic ridges: how large?, Geology 21, 53-56, 1993.
Pallister. J. S. and C.A. Hopson, Samail ophiolite plutonic suite: field relations, phase variation, cryptic variation and layering, and a model of a spreading ridge
     magma chamber, J. Geophys. Res. 86, 2593-2644, 1981.
Quick, J.E.,  and R.P. Denlinger, Ductile deformation and the origin of layered gabbro in ophiolites, J. Geophys. Res. 98, 14,015-14,027, 1993.
Smewing, J.D. Mixing characteristics and compositional differences in mantle-derived melts beneath spreading axes: evidence from cyclically layered rocks in
     the ophiolite of North Oman, J. Geophys. Res. 86, 2645-2660, 1981.
Thayer, T.P., Flow-layering in  Alpine peridotite-gabbro complexes. Min. Soc. Am. Sp. Paper,1,55-61, 1963.