Oceanic plagiogranites as a result of interaction between magmatic and hydrothermal systems in the slow-spreading mid-ocean ridges

The paper considers some petrological and geochemical aspects of the formation of oceanic plagiogranites (OPG)—felsic intrusive rocks, which were found in the plutonic complexes of modern mid-ocean ridges (MOR) and ophiolites of paleo-collisional zones. Based on the multi-equilibrium clinopyroxene-orthopyroxene-amphibole-plagioclase geothermobarometry, typical OPG found in gabbros and peridotites were formed at temperatures of 820–850°C and pressure of 2–2.5 kbar. Close temperature estimates (825 ± 50°C) were obtained from literature data on Ti content in zircon, with allowance for lowered TiO₂ activity in the rock. Under these P-T parameters, OPG can be generated only in the presence of fluid of water activity $ \left( {a_{H_2 O} } \right) $ \left( {a_{H_2 O} } \right) close to 0.9. OPG and associated recrystallized gabbroids contain high-temperature hornblende with significant Cl content (0.5–2 wt %). In addition, the plagiogranites are characterized by particular geochemical features such as extremely high Na₂O/K₂O (up to 135), sharp LREE enrichment ((Ce/Yb)_cn and (La/Sm)_cn up to 10 and 4, respectively), and elevated ⁸⁷Sr/⁸⁶Sr ratio relative to DMM. All these facts point to the key role of hydrothermal fluid, the seawater derivative, in the OPG formation. The fluid with $ a_{H_2 O} = 0.9 $ a_{H_2 O} = 0.9 (approximately 28 wt % NaCl) could be produced from seawater due to hydration reactions at the higher lower temperature horizons of oceanic crust in the course of its percolation to the OPG generation areas. The formation of plagiogranites in the MOR oceanic core complexes possibly reflects the fundamental feature of oceanic accretion: practically simultaneous (at the geological time scale) proceeding of exogenic (neptunic) and endogenous (plutonic) processes.