Magmatic and tectonic controls on the evolution of oceanic magma chambers at slow-spreading ridges: Perspectives from ophiolitic and continental layered intrusions

Oceanic lithosphere formed at slow-spreading ridges shows pronounced lateral and vertical variations in crustal thickness and architecture as a result of a complex interplay between magmatic and tectonic processes. Emplacement of magma at slow-spreading centers is dominantly controlled by injection of sills, dikes, and ephemeral chambers. The internal evolution of such magma reservoirs is controlled by processes similar to those inferred to dominate the evolution of magma chambers at fast-spreading ridges. Magma chambers crystallize and undergo differentiation along their margins and floors by forming crystal-mush zones. Such zones solidify by compaction and vertical melt migration and by recycling of magma into the main reservoir. Compaction and plastic flow in the crystal mush result in banding and foliation and intensify preexisting modal layering and lamination. The presence of thick crystal-mush zones and their solidification by compaction and interstitial melt migration are common features of many continental, ophiolitic, and oceanic magma chambers. The magma-chamber development at slow-spreading centers, such as the Southwest Indian Ridge, may significantly be affected by tectonic uplift and withdrawal of the bulk of magma from the melt chambers. These processes collectively may result in cessation of compaction and a high concentration of trapped interstitial liquid in the partially solidified gabbro mush near the former segregation and crystallization front. This interstitial trapped liquid will crystallize to form evolved ferrogabbros partially as a result of lateral migration and syn- and post-tectonic channeled melt movement into uplift-induced pressure-release zones. Such fossil compaction profiles are not commonly recognized from layered gabbros of stable continental cratons and margins, but may be expected to occur more commonly in ophiolitic complexes. In general, magma-chamber processes that operate to develop gabbroic complexes at slow-spreading ridges are similar in many respects to those inferred front continental layered intrusions and from gabbros in fossil oceanic crust as preserved in ophiolites.