Genesis of vesiculation in dolerite sills of the Shimokawa ophiolite in the northern Hidaka belt，Hokkaido and origin of albite - K-feldspar veins

Basaltic rocks with N-MORB features of the Shimokawa ophiolite in the northern Hidaka belt, Hokkaido, generally show poor vesiculation. Some of dolerites are vesiculated up to 10% that are exclusively found within thick sills (> 10 m) interlayered between sediments. The thick dolerite sill about 22 m thick was studied in detail. The chilled margin is not vesiculated, but the inside of the sill is vesiculated from 5-10%, indicating that the magma was not vesiculated at the intrusion.Albite -K-feldspar veins about 2 mm wide are found within the dolerite sill. The veins are irregularly bounded and cut by other veins, and contain relatively large rounded vesicles. Therefore, the veins were formed before the solidification of the sill. The bulk composition of the vein shows trachytic composition. It’s difficult that trachyte melts are produced by fractional crystallization of MORB magma. Mineral compositions of the veins show that they were crystallized below solvus temperature. The bulk composition of dolerite is higher in K2O and Rb than poorly vesicularity basalt.The sediments are silicified along the contact to the dolerite sills. LIL elements are highly depleted, while Na2O contents are enriched in the silicified sediments. Thus, intrusion of magmas into the thickly accumulated soft sediments resulted in induration of the sediments from which the trapped seawater (H2O and NaCl), LIL elements and CO2 were expelled due to the dissolution of organic carbon. The expelled Na2O partly precipitated albite in the silicified part. Thus, vast fluids rich in Na2O and K2O, caused by an intrusion of magmas into soft sediments, were discharged into the magmas and resulted in vesiculation of the magmas. Finally, the fluids were solidified as the albite -K-feldspar vein.