The Neyriz ophiolites: a nascent ridge located in the closeVicinity of the margin of Gondwana
The Neyriz ophiolite lies to the south of Iran. Tectonic emplacement of the complex onto the Arabian platform occurred in Cenomanian times by means of radiolarite and colored melange thrusts. Relative to the Oman and Taurus ophiolites, albeit of the same obduction age, in an identical structural position, and originated from the same Tethian ocean, the Neyriz ophiolite displays an original feature consisting of marble panels that are thought to be the result of high temperature metamorphism affecting platform limestone of Upper Triassic age. Mantle harzburgite induced contact metamorphism, transforming limestone into skarn with well-developed pyroxenes. In summing up, the marbles represent extensional allochtonous formations located in the outermost part of the continental margin and submitted to the intrusion of mantle diapirs in the Cretaceous. The cross-section of the ophiolite complex reveals significant differences with that of Oman, located 300km to the south of Neyriz:
- A set of mantle rocks which are highly depleted in incompatible elements and intruded by thick dikes of dunite. Occasionally the latter contain lenses of foliated gabbros and pods of chromite;
- Absence of a sheeted dike complex but occurrence of dike swarms of diabase that crosscut the peridotite-gabbro contact;
- Gabbros with encased structures, the viscous foliation of which are of variegated orientation. The gabbro panels alternate with dunite-harzburgite ones and are intruded by wehrlite dikes;
- Lavas, 1.2 km thick, lie directly on the gabbros;
- Absence of developed infra-ophiolitic sole but occurrence of a 2m thick amphibolite layer.
The granoblastic harzburgite display a high temperature fabric with N-S foliation and essentially west dips. The spreading zones were, accordingly, situated to the east of the investigated massifs but with an eastward migration. Three important transforming tracks of N110-130E direction have been evidenced. They are characterized by a rotation of the high temperature foliation and occurrence of porphyroclastic harzburgite of low temperature and foliated gabbros with low pitch lineation. Shear direction, based on various criteria, is right handed. Accordingly, Neyriz ophiolite represents the western side of several modest accretion zones of on the average N-S direction separated by dextral N110-130E transform zones, located in the outermost part of the Gondwana margin.
The magmatic history of the ophiolite is rather complex. The thick dikes of residual dunite are subsequent to the high temperature foliation. They correspond to channels taken by the magmas in their ascent to the surface. Rare earth elements (REE) suggest a genetic link between gabbros depleted in REE, likely produced by a second fusion of the mantle, and depleted harzburgite. Diabase, which crosscut the gabbros, is viewed as differentiated liquids originated from a mantle already depleted in incompatible elements. Again a second fusion is required but the genetic link to the gabbros depleted in REE is hardly established. Nevertheless, diabase is probably related to the gabbros enriched in REE and to the lavas depleted in REE. Tentatively, they may reflect fractional melts in the vicinity of transform faults. In contrast, no genetic link is evidenced between the differentiated lavas rich in REE, marked by the precipitation of magnetite and plagioclase, and diabase.
Isotopic dating shows that the essential of the magmatic history is Cretaceous in age. The residual mantle testifies to the existence of two episodes of melting the later of which is Upper Cretaceous. The history of the Tethys indicates that the break-up between the margin of Gondwana and the block of Central Iran occurred in the Triassic for the Neyriz region and in the Permian for the Oman Mountains, creating the original oceanic lithosphere. It is supposed that the mantle beneath Neyriz underwent a first fusion at that time. Plate tectonics reconstruction predicts that ophiolite of Neyriz and Oman originated from the same accretion zone but with numerous transform faults of N120E direction for the Neyriz region. Consequently, the Oman ophiolite would mirror a young and hot lithosphere, created by tectonic inversion at a fast spreading ridge. In contrast the Neyriz ophiolite took place in a nascent ridge located in the close vicinity of the margin of Gondwana.