Geochemistry of metacherts associated with continental margin volcanic seamount hydrothermal activity: metamorphic sole rocks of the UAE

The metacherts of the Asima Window, in northeastern U.A.E. at the southeastern corner of the Arabian Peninsula, are the dominant metamorphic sole lithologies of the Semail ophiolite that was obducted onto the Arabian continental margin at the end of the Cretaceous. There have been no previous geochemical studies of these metacherts. The Asima metachert samples of this study form four groups based on geochemistry, petrography and field association: (1) black hematitic, (2) green epidote-chlorite rich, (3) pink piedmontite-bearing and (4) grey micaceous metacherts. A continental margin setting for the parent cherts is indicated geochemically by Al₂O₃ to Fe₂O₃ and SiO₂ ratios, low ΣREE, low La, moderate positive Ce/Ce* and La_n/Ce_n ~ 1. A significant terrigenous fraction was composed of illite, chlorite and Fe hydroxides. A hydrothermal component can be recognized in all of the metachert groups but is particularly evident in the black and green metacherts by low Al/(Al + Fe + Mn), high (Fe + Mn)/Ti, high Ba and Ba/Sr ratios, positive Eu/Eu* and left-leaning REE profiles. Most silica was biogenic. A hydrogenous component included REE’s, Cu, Ni, Zn, V, Ba and Co that were adsorbed onto clays, Fe hydroxide and Mn oxide particles. The Asima cherts were deposited in a marine basin adjacent to but separated from the continental margin by seamounts of the Al Hala Volcanics. These volcanics provided Fe,Mn-rich hydrothermal fluids that mixed with the oxygenated seawaters, leading to rapid hematite deposition (black metacherts) and slow Mn oxidation to form Mn oxide particles that accumulated in the pink metacherts. Grey metacherts were deposited farther from the vents. Green metacherts have similarities to the hydrothermal black metacherts. Graphical abstract: [Figure not available: see fulltext.].