STABLE ISOTOPIC AND FLUID INCLUSION EVIDENCE FOR METEORIC FLUID PENETRATION INTO AN ACTIVE MOUNTAIN BELT - ALPINE SCHIST, NEW-ZEALAND

Calcite and quartz veins have tormed, and are forming, in steeply dipping fissures in the actively rising Alpine Schist metamorphic belt of New Zealand. The fluids that deposited these minerals were mostly under hydrostatic pressure almost down to the brittle-ductile transition, which has been raised to 5-6 km depth by rapid uplift. Some fluids were trapped under lithostatic pressures. Fluids in the fissure veins were immiscible H2O + NaCl-CO2 mixtures at 200-350-degrees-C. Bulk fluid composition is 15-20 mol % CO2 and <4.3 total Mol CH4 + N2 + Ar/100 mol H2O. Water hydrogen isotopic ratio deltaD(H2O)) in the fissure veins spans -29 to -68 parts per thousand, deltaO-18(H2O) -0.7 to 8.5 parts per thousand, and bulk carbon isotopic ratio deltaC-12 ranges from -3.7 to -11.7 parts per thousand. The oxygen and hydrogen isotopic data suggest that the water has a predominantly meteoric source, and has undergone an oxygen isotope shift as a result of interaction with the host metamorphic rock. Similar fluids were present during cooling and uplift. Dissolved carbon is not wholly derived from residual metamorphic fluids; part may be generated by oxidation of graphite.