Geochemical variability of the Oman ophiolite lavas: Relationship with spatial distribution and paleomagnetic directions

[1] The multielemental composition of 31 lavas sampled in the extrusive section of the Oman ophiolite was determined by inductively coupled plasma-source mass spectrometry (ICP-MS). This study allowed us to define clear geochemical criteria to characterize the different lavas types in Oman. Most of the Oman ophiolite extrusive sequence is composed of lavas of composition similar to present-day MORB with the exception of slight Nb-Ta negative anomalies (V1 magmatism). V1 lavas display REE patterns and Zr/Hf ratios in the range of MORB (Zr/Hf = 33.15-38.7). In contrast, the overlaying V2 lavas, which outcrop only in the northern part of the ophiolite, are REE depleted relative to V1 and display low Zr/Hf ratios (23.6-30.5). V2 lavas may be further classified into two sub-groups. V2 type I lavas display a continuous decrease from HREE to LREE. The upper V2 type II lavas are more depleted in HREE and MREE but they are enriched in LREE. They are also distinguished by their low Nb/Ta ratios (10.53-11.65) relative to other Oman basalts (V1: 12.5-14; V2 Type I: 12.35-14.4). As for N-MORB, melting of an upwelling mantle beneath an oceanic spreading centre formed V1 lavas. V2 resulted from fluid-enhanced melting of previously depleted mantle residual after V1 extraction. This process probably occurred at different melt/rock ratios thus resulting in the two V2 sub-groups, and was enhanced in the northern part of the ophiolite. V3 lavas overlie the V1-V2 sequence in the Salahi area. They display incompatible element rich patterns (LREE > MREE > HREE) and a high Nb/Ta ratio typical of within-plate basalts. In the different studied areas, the transition from one lava type to the other seems to correlate with the block rotations revealed by paleomagnetic data.