Petrology of basaltic sills from Ocean Drilling Program sites 794 and 797 in the Yamato Basin of the Japan Sea

Abstract

The basement rocks drilled at Ocean Drilling Program sites 794 and 797 in the Yamato Basin of the Japan Sea are composed dominantly of aphyric, plagioclase phyric, and olivine and plagioclase phyric, tholeiitic sills intruded into wet sediments. The rocks can be divided into an incompatible element depleted type, and an enriched type, based on their mineral and bulk rock compositions. Site 794 drilled an upper suite of dominating aphyric, enriched, basaltic sills; the upper part of which contains relatively calcic plagioclase phenocrysts (An85–90), and the lower part of which contains occasionally olivine phenocrysts. A lower suite at site 794 is composed of dominating aphyric, depleted sills, with plagioclase and olivine microphenocrysts occasionally in chilled margins. Site 797 recovered an upper suite of sparsely plagioclase and olivine phyric to aphyric, depleted sills, and a lower suite of sparsely plagioclase phyric and aphyric, enriched sills. The groundmass phases of all suites suggest a systematic enrichment in FeO, TiO2, Na2O, and K2O during solidification. In addition, there is evidence that olivine ceases to crystallize approximately when augite appears during groundmass crystallization. Irrespective of the enriched versus the depleted magma type, the rock compositions reveal two contrasting evolution trends, which can be related to fractionation processes in crustal magma chambers. A tholeiitic evolution trend, with increasing FeO and TiO2 and decreasing Al2O3, can be modelled by fractional crystallization of 40% to 50% plagioclase, olivine, and augite. A mildly calc‐alkalic evolution trend, with decreasing FeO, increasing Al2O3, and nearly constant TiO2, can be modelled by 8% to 12% olivine fractionation. One‐ atmosphere, anhydrous melting experiments on a basalt from hole 797C produced liquid compositions very similar to the basalts that exhibit the FeO enrichment trend. There is mineralogical evidence that the contrasting trends may be related to the effect of small amounts of water during crystallization. Relatively high water activity will suppress plagioclase crystallization and thus produce the iron depletion trend, while nearly anhydrous crystallization involves the early appearance of plagioclase, which is responsible for the iron enrichment trend. The tholeiitic suites occur in lower parts of the drill cores, while the calcalkalic suites occur in the upper parts. This suggests a magmatic and tectonic evolution of the Japan Sea, perhaps reflecting a transition between calc‐alkalic magmatism related to subduction zone activity and tholeiitic magmatism related to back arc spreading. Furthermore, any model must be able to account for the range in parental magmas from depleted to enriched types throughout the magmatic history of the Yamato Basin.