Oxygen and hydrogen isotopic evolution of Variscan crustal fluids, south Cornwall, U.K.

Abstract

Structural analysis of quartz vein systems and fluid inclusion criteria were used to distinguish five different fluid types which flowed through a segment of Palaeozoic crust in southwest England during the Variscan orogeny. Mineralogical constraints in combination with fluid inclusion thermobarometry enabled the temperature of vein formation to be estimated, and isotopic compositions of fluids were determined by analysis of vein material and direct measurement on fluid extracted from inclusions. Peak, low-grade (pumpellyite-actinolite facies) metamorphic fluids had a high δD and δ 18O signature ( δD = −18 to −10% δ 18O = + 10.6 + 11.9%) which evolved to compositions in the range δD = −28 to −13%, δ 18O = +7.0 to +11.4% during later retrogression and uplift. Fluids in the contact aureole of the Cornubian batholith had δD-values intermediate between typical magmatic compositions and the regional metamorphic fluids (−23 to −43%), and a similar range of δ 18O-values to both magmatic and the regional metamorphic fluids (between + 5.6 and + 14.0%). These compositions are comparable with those of fluids responsible for SnW mineralisation in the province. Post-orogenic fluid chemical and isotopic compositions were exotic and indicate significant infiltration of externally-derived fluids during late- to post-orogenic brittle faulting. Low-temperature, low-salinity fluids which circulated in ENE-WSW-trending brittle normal faults had low δ 18O-values (−0.3 to + 7.4%) suggestive of a significant meteoric component. Low-temperature, high-salinity fluids, which flowed through N-S- to NNW-SSE-trending strike-slip faults and fractures and were responsible for PbZn mineralisation, had significantly D- and 18O-depleted compositions ( δD = −80 to −49%, δ 18O = −0.1 to 4.7%), typical of basinal brines. These data document the isotopic evolution of fluids in an external (Rhenohercynian) part of the Variscan orogen, through the complete cycle of foreland thrust-belt development and low-grade regional metamorphism, S-type granite emplacement and associated hydrothermal systems, post-orogenic collapse and low-temperature fluid flow in regional fractures. There is limited overlap in isotopic composition between the different fluid types, indicating that fluids flowing through the same host rocks at each stage of orogenesis may be distinguished on the basis of their oxygen and hydrogen isotopic compositions. These data provide a framework for future studies involving fluids of unknown origin in the Variscan and are a reference for comparison with the isotopic evolution of fluids in other orogenic belts.