Evaluation of the role of detachment faulting in the genesis of felsic melts in the Atlantis Bank oceanic core complex, Southwest Indian Ridge

Oceanic detachments are deep-rooted, long-lived structures at plate scale, acting as conduits for fluid introduction into the oceanic lithosphere. These processes impact plate rheology and potentially induce oceanic crustal anatexis. However, the mechanisms and extent of fluid ingress and crustal melting during detachment faulting remain poorly constrained. This study investigates felsic veins from the Atlantis Bank oceanic core complex (OCC) on the Southwest Indian Ridge to elucidate controls on crustal anatexis imposed by oceanic detachments.

We report systematic results for mineral chemistry, zircon U-Pb ages and Hf-O-Zr isotopes, and Nd-O isotopes of apatites from 23 felsic rocks retrieved from 50−800 meters below the seafloor in IODP Hole U1473A. Additionally, phase equilibria and zircon trace element modeling for three formation modes of oceanic felsic melts (hydrous partial melting of gabbros, fractional crystallization of MORB, and fractional crystallization of anatectic melts) were performed. These data and models consistently suggest that most U1473A felsic veins were products of advanced mid-ocean ridge basalt (MORB) differentiation.

Further examination of zircon trace element data for the Atlantis Bank OCC indicates that the felsic veins resulted from strong fractionation of either primitive basalts or magmas generated by hydrous melting of gabbros. The presence of anatectic felsic veins near the fault plane suggests that the detachment fault facilitated high-temperature (750–900°C) alteration and hydrous melting of gabbros. Additionally, analyses of felsic rocks from two OCCs on the Mid-Atlantic Ridge, based on published zircon trace element data and models, reveal distinct manifestations of the interplay among faulting, magmatism, and hydrothermal circulation across various OCCs. Our findings underscore the critical role of detachment faulting in fluid ingress and oceanic crust melting, with significant implications for chemical and thermal exchanges between seawater and the oceanic lithosphere.