TRACES OF GRANITOID MAGMATISM AND THE COMPOSITION OF THE SILICIC MELT IN GABBROIC ASSEMBLAGE OF THE MID-ATLANTIC RIDGE AT 13°N: NEW DATA ON MELT INCLUSIONS
Despite the local occurrence of silicic magmatism during the formation of the oceanic crust, the nature of felsic granitoid veins (“oceanic plagiogranites”) remains an important debatable problem of chemical differentiation in the global system of mid-oceanic ridges. To date, actual compositions of granitoid melts in the modern oceanic crust are still poorly constrained, and this prevents petrogenetic interpretations. Structural features and mineral chemistry of plutonic rocks from the oceanic core complex at 13°N on the Mid-Atlantic Ridge show the sequence of magmatic differentiation: coarse-grained gabbronorite → vein microgabbro enriched in Fe-Ti oxides and apatite → biotite-plagiogranite veins. The transition to silicic magmatism corresponds to the local crystallization of quartz and zircon in the vein microhabbro with further Zr/Hf fractionation by zircon. Experimental study of melt inclusions in zircon demonstrated homogenization at 850–910°C and the granitoid composition of the melt with K2O > 1.8 wt.% and SiO2 75–76 wt.%. Such K/Si ratio of the silicic melt, along with the genetic connection to highly differentiated gabbroids allows to prefer the model of a high degree differentiation of MORB-type magma in oceanic core complex for the studied plagiogranite-gabbro association.