Gabbroic rocks from the Mathematician Ridge failed rift

Abstract

Plutonic rocks from the ocean crust provide direct evidence on the petrological processes leading to its formation and evolution. The nature of such rocks is important for interpreting the origin of on-land ophiolites. So far, oceanic plutonic rocks have been mainly recovered from slow-spreading ridges and their associated fractures^1–13 and from trenches^14,15. Here we report the results of petrological studies of a suite of gabbros and related plutonic and volcanic rocks dredged from the Mathematician Ridge (MR), which was a fast-spreading ridge (43 mm yr⁻¹) until it was abandoned ∼3.5 Myr ago¹⁶. Textures and mineral chemistry of the rocks together with structural considerations indicate that before abandonment the MR produced fractionated mid-ocean ridge basalt (MORB) and fractionated gabbros. The gradual decrease of spreading rate during abandonment resulted in morphological change, eruption of more primitive MORB and possibly tectonic emplacement of the older ferrogabbros. Alternatively, the latter could have been tectonically emplaced after abandonment.