Regional metamorphism and hydrothermal alteration related to Miocene submarine volcanism (‘green tuff’) in the Yurihara oil and gas field, northwest Honshu Island, Japan

• Published in: Island Arc
• Main Author: Yagi, Masahiko
• Format: Article in Journal/Newspaper
• Language: English
• Published: Wiley 1993
• Subjects: Planktonic foraminifera
• Online Access: http://dx.doi.org/10.1111/j.1440-1738.1993.tb00091.x; https://api.wiley.com/onlinelibrary/tdm/v1/articles/10.1111%2Fj.1440-1738.1993.tb00091.x; https://onlinelibrary.wiley.com/doi/pdf/10.1111/j.1440-1738.1993.tb00091.x

Abstract Alteration of reservoir rocks in the Yurihara Oil and Gas Field, hereafter referred to as the ‘Yurihara field’, have been examined by using samples from six wells. These rocks are basalts in the lowermost part of the basin‐fills (‘green tuff’ Formation). These basalts were produced in many eruptions in a submarine environment during the early to middle Miocene, and they underwent continuous intensive alteration genetically associated with Miocene submarine volcanism. The alteration of the basalts is of two types: low grade metamorphism and hydrothermal. The former belongs to the type of ocean floor metamorphism and comprises two subgroups: zeolite (zone I) and prehnite‐pumpellyite (zones IIa: vein and amygdule occurrence, and IIb: replacing plagioclase). The latter is characterized by potassic metasomatism accompanied by adularia, quartz and calcite veins (zones IIIa: center and IIIb: margin of the metasomatism). This overprints the low grade metamorphic alteration. The central zone of hydrothermal alteration coincides with a major estimated fault, so that fluids probably assent along the fault. The basalts erupted during 16.5‐15.5 Ma, determined by planktonic foraminifera assemblages of inter‐bedded shales, then underwent successive low grade metamorphism. In time, the hydrothermal alteration that overprints low grade metamorphism occurred. Adularia veins of the altered rocks located in the hydrothermal alteration zones (zone IIIa and IIIb) have been dated as 9 Ma determined by the K‐Ar method. This fact indicates that the activity of low grade metamorphism had already crossed the peak before hydrothermal alteration occurred at 9 Ma. The shape of isotherms of fluid inclusion homogenization temperatures (Th) and that of isolines of apparent salinity (Tm) almost coincide with each other, and these also coincide with the distribution of hydrothermal alteration (zones IIIa and IIIb). This indicates that the fluid inclusions formed at the same time as ascending fluids produced the potassic metasomatism. The ...