Architecture of the Oman-UAE ophiolite: Evidence for a multi-phase magmatic history

摘要

The Oman-United Arab Emirates ophiolite is the world's largest ophiolite. It is divided into 12 separate fault-bounded blocks, of which the northern three lie wholly or partly in the United Arab Emirates. Extensive mapping has shown that the United Arab Emirates blocks contain mantle and crustal sections which correspond to the classic 'Penrose conference' ophiolite definition but which are cut by a voluminous later magmatic sequence including ultramafic, mafic and felsic components. Samples from the later magmatic sequence are dated at 96.4 ± 0.3, 95.74 ± 0.3 and 95.2 ± 0.3 Ma; the early crustal section, which has not been dated directly, is thus constrained to be older than c. 96.4 Ma. Petrological evidence shows that the early crustal section formed at a spreading ridge, but the later magmatic sequence was formed from hydrous magmas that produced different mineral crystallisation sequences to normal mid-ocean ridge basalt (MORB). Mineral and whole-rock geochemical analyses show that the early crustal rocks are chemically similar to MORB, but the later magmatic sequence has chemical features typically found in supra-subduction zone (SSZ) settings. The ophiolite in the United Arab Emirates thus preserves clear evidence for two stages of magmatism, an early episode formed at a spreading centre and a later episode associated with the onset of subduction. Similar two-stage magmatism has been recognised in the Oman sector, but the United Arab Emirates contains the most voluminous SSZ magmatism yet described from this ophiolite.