Connected fluid evolution in fractured crystalline basement and overlying sediments, Pannonian Basin, SE Hungary

Abstract

Uplifted crystalline basement highs have played an important role in the hydrologic history of the Pannonian Basin providing conduits for fluid migration during basin evolution. Petrographic, stable isotope, electron microprobe and fluid-inclusion studies support a five–step basin evolution; (1) formation of crystalline core-complexes, (2) percolation of meteoric water through shear zones, (3) basinwide thermal subsidence, (4) hydrologic inversion, and (5) tectonic inversion. Brittle fractures in the basement are the results of uplift during core complex formation and contain pyrite, kaolin, illite, chlorite, calcite, and quartz. Hydrocarbon stains between calcite and quartz and yellow fluorescent, primary, hydrocarbon inclusions in quartz are indicative of migrations of hydrocarbon-bearing fluids during uplift from mid-crustal depth. Further uplift was associated with meteoric water infiltration along shear-zones as recorded by light δ 13C and δ 18O values, low trace element concentrations, all-liquid primary fluid inclusions and encased pollens and cuticules of terrestrial plants in a second fracture-calcite crystallised after quartz. A simultaneous meteoric influence in the overlying Pannonian (12.7–3 Ma) sediments is indicated by extensive plagioclase dissolution, calcite cement geochemistry and low-salinity, all-liquid fluid inclusions. The thermal subsidence of the basin led to isolation of the hydrologic system from the surface. Diminishing meteoric influence is recorded by increasing fluid inclusion salinities of secondary inclusions in calcite and quartz in the basement fractures and of primary inclusions hosted in late calcite and quartz in the sediments. A sudden decrease of the fluid inclusion salinities from 4.5 to 1 wt.% NaCl eq. at ∼130°C both in burial cements and fracture-filling minerals represents a newly established hydrological system, which redistributed the interstitial fluids, including hydrocarbons. This hydraulic inversion is indicated by abundant fluorescent, secondary fluid inclusions in fracture-filling minerals. Finally, the Pliocene to recent tectonic inversion reopened fractures in the crystalline basement, and induced a rapid crystallisation of laumontite. These results support a long-standing (late Miocene to recent) hydraulic connection between fractured crystalline basement and overlying sediments in the central part of the Pannonian Basin.