From basin to basement: the movement of surface fluids into the crust

Quartz veins from Precambrian, high grade, basement rocks in Southern Norwa y contain both aqueous and biogenically sourced hydrocarbon fluid inclusion s. Aqueous fluids show a wide range in salinities (0-40 wt% NaCl eq.) and d ensities (0.80-1.16 g/cm(3)) but the ultimate source of this salinity is un clear. The oxygen isotopic composition of most of the veins is dominated by the host-rock compositions. Surface derived fluids migrated downwards as a result of extension of dry crystalline basement rocks with pore fluid pres sures less than hydrostatic. Quartz precipitation occurred in the veins ini tially at near hydrostatic pressures but later quartz generations were prec ipitated at pressures closer to lithostatic. (C) 2000 Elsevier Science B.V. All rights reserved.