Deep penetration of sedimentary fluids in basement rocks from southern Norway: Evidence from hydrocarbon and brine inclusions in quartz veins

Abstract

This paper presents evidence for fluid flow and fluid-rock interaction at upper crustal levels within the crystalline basement of southern Norway. In the high-grade Modum Complex postmetamorphic veins of quartz occur in association with albitisation of metagabbros and metasediments. Pressure-temperature conditions for the formation of these veins are in the range 250–300°C and ca. 1–2 kbar. Primary and pseudosecondary fluid inclusions in the quartz veins show two fluids: (1) hydrocarbon ± C0 2 inclusions and (2) aqueous inclusions with variable salinities. Dark carbonaceous solid inclusions are also present. The hydrocarbon inclusions are methane dominated (ca. 80–100 mot%), and the presence of higher, complex hydrocarbons is demonstrated. The aqueous inclusions in the metagabbro-hosted veins show more saline compositions than the metasediment-hosted veins. A salinity range of ca. 23-0 wt% NaCl eq is found. Some of the aqueous inclusions may contain NaHCO 3. Crush-leach analyses of inclusion fluids show Na-Ca-K-CI dominated compositions, with Na⪢Ca>K. Hydrocarbon-poor concentrates yield Br/Cl ratios close to seawater, while hydrocarbon-rich concentrates show higher values. The types and chemistry of the hydrocarbons in these veins indicate a biogenic origin of the hydrocarbon fluids. The inclusion fluids are interpreted as derived from an overlying sedimentary basin in late Precambrian or Permian times. The veining is interpreted as the deep expression of the percolation of basinal fluids into the metamorphic basement during crustal extension.