Residual Melt after the Crystallization of Oxide Gabbro: An Example of a Silicic Product of MORB Differentiation in the Modern Oceanic Crust, Ashadze Complex, Central Atlantic

The plutonic rocks studied in the Ashadze Complex contain four groups of mineral assemblages: (I) coarse-grained gabbronorite, which is dominant; (IIa) micrograined differentiated gabbro (oxide microgabbro) enriched in ilmenite and apatite and sporadically containing zircon grains smaller than 70 µm; (IIb) local oxide microgabbro aggregates with zircon grains as large as 200 µm and quartz; and (III) biotite- and orthopyroxene-bearing plagiogranite veinlets. As an interpretation, a model of sequential differentiation of MORB-type melt is proposed. The compositional variations of the plagioclase and orthopyroxene are consistent with the general sequence of crystal fractionation and show partial overlap of assemblages (II) and (III). In the sequence of zircon-bearing assemblages (IIa), (IIb), and (III), zircon shows a systematic enrichment in Hf. Zircon of assemblage (IIb) hosts melt inclusions. The inclusions were completely remelted at 910°C and quenched into homogeneous glass. The analyzed granitoid (75–76 wt % SiO₂) water-bearing (H₂O ~ 3 wt %) composition of the inclusions was interpreted as a residual melt after crystallization of the evolved vein gabbro rich in ilmenite. The results offer an example of an evolved granitoid product of MORB differentiation and are consistent with high-degree magmatic differentiation in the oceanic crust, a model according to which the crystallization of evolved oxide gabbro results in a residual silicic melt.