Alternative TitleOrigin of feldspar veins and genesis of vesiculation in dolerite sills of the Shimokawa ophiolite in the northern Hidaka belt, Hokkaido
Note (General)Studies of the interaction of magma with sediments during shallow intrusion are scarce. This study will provide insight into the processes by which volatile components were added to the magma which intruded into shallow soft-sediment. Basalt and dolerite with N-MORB chemical features of the Shimokawa ophiolite in the northern Hidaka belt, Hokkaido, generally show poor vesiculation. However, some of dolerites are vesiculated up to 10 %, and these vesiculated rocks are exclusively found within sills thicker than 10 m, which intruded into clastic sediments. Non-vesiculated chilled margins suggest the vesiculation occurred after the magmatic emplacement. The vesiculated dolerite is richer in bulk-rock Na2O, K2O and Rb contents than poorly vesiculated basalt and dolerites. These observations suggest secondary enrichments of vapor and selective elements, and this paper tested this hypothesis by petrographic and petrochemical approaches. While looking for carriers of enriched components, I focused on plagioclase - K-feldspar veins 2 mm thick found in a vesiculated dolerite sill. The veins are irregularly bounded, and partly contain amygdules. Therefore, it is considered that the veins were formed before the solidification of the intruded magma. The examined vein shows trachytic bulk-rock major element composition and petrographically classified as monzonite. The multi-element patterns of the trace element composition shows that the incompatible elements are not enriched unlike trachytes from plume-related oceanic settings. The LREE-poor chemical feature is similar to that of normal to transitional MORBs. Both textures and chemical composition suggest that the veins originated from a trachytic melt. To seek the origin of the melt, the chemical compositions of the veins are compared with those of interstitial melt parts of the dolerite. The HREE-rich feature of the veins cannot be explained by the crystallization differentiation of the dolerite magma. Therefore, it is considered that the melt of the veins were not the residue of the host doleritemagma, and of an exotic origin. Although occurrences directly showing the vein melt have not been found yet, I assumed that the vein melt originated from mudstone to which the dolerite intruded. If mudstones are incorporated into dolerite magma and partially fused, the generated melt probably has a granitic composition on the three-phase eutectic point of quartz, albite, and K-feldspar, because the host mudstones are rich in these components. It is also considered that this granitic magma immediately loses silica, because it is surrounded by olivine tholeiite magma undersaturated in quartz. Losing silica, the granitic magma could thus transformed to monzonitic magma. The soft-sediment inclusions can provide water fluid and increase the volatile components in the dolerite magma. This mechanism can consistently explain the petrology of feldspar veins and vesiculation of the host magma together.
新大院博(理)第487号
Collection (particular)国立国会図書館デジタルコレクション > デジタル化資料 > 博士論文
Date Accepted (W3CDTF)2023-06-02T22:06:20+09:00
Data Provider (Database)国立国会図書館 : 国立国会図書館デジタルコレクション