Incipient oceanic accretion at the foot of the Gondwanian passive margin : the Neyriz ophiolites (Iran)

The Neyriz ophiolite is part of a 3000 km obduction belt that was thrust over the edge of the Arabian continent during the Late Cretaceous. ⁴⁰Ar/³⁹Ar ages from Neyriz ophiolite range from 83Ma to 98Ma, similar to the age of the Oman ophiolite. The Neyriz ophiolite is remarkable by the presence of marbles lenses in direct contact with the mantle harzburgites through a skarn interface. These marbles interpreted as Triassic extensional allochthons embedded in the peridotites along transform faults. The mineralogy of the skarns indicates strong Al and Si metasomatism, only compatible with the circulation of magmatic hydrothermal fluids. Decompression of the mantle during uplift is the source of magma that produced the large gabbroic dykes intruded in the peridotites up to the contact with marbles. At this contact, hot magmatic fluids interact with the Triassic limestones to produce successive paragenesis at decreasing temperature. The fabric of the harzburgites is dominated by a N20^o trending granoblastic foliation. The foliation locally shows circular structures, indicating diapiric intrusions (4.5 km diameter). The orientation of the foliation plane within the harzburgites turns to a N110^o trending transform direction marked by porphyroclastic foliation planes in the harzburgites and plastic flow within the marbles. The polyphased history of the harzburgites indicates a low degree second stage partial melting under hydrated condition from an already depleted mantle, followed by interaction with the mantle peridotites during percolation. This leads to wide range of REE patterns of lavas and dykes. Some of them are highly depleted. Gabbros are well developed at the top of the mantle sequence but are restricted to intrusive 250--1000m thick laccolitic bodies within harzburgites and dunites, hence they do not form a crust superposed to the mantle. No dyke complex is found. Basalts form only two isolated outcrops, probably directly resting upon the gabbros, even directly on the harzburgites. The structures and petrology of the Neyriz ophiolite reflect likely the initiation of a proto-ridge intruding an old oceanic lithosphere at the distal part of the pre-existing Triassic continental margin. Thus, Neyriz and Oman ophiolites that are issued from the same Cretaceous accretion system in the Tethys are different. The former attest from the incipience of an ultra low spreading ridge, the later from steady state processes at a fast to ultra fast spreading ridge.