1. metamorphic Geol., zyxwvutsrqp 1994, zyxwvutsrq 12, 429-444 Stable isotopic and fluid inclusion evidence for meteoric fluid penetration into an active mountain belt; Alpine Schist, New Zealand G. R. T. JENKIN,' D. CRAW2AND A. E. FALLICK' 'Scottish Universities Research and Reactor Centre, East Kilbride, Glasgow, Scotland, G75 OQU, UK 2 Geology Department, University of Otago, PO Box 56, Dunedin, New Zealand ABSTRACT zyxwvutsrq Calcite and quartz veins have formed, 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 H,O + NaCI-CO, mixtures at 200-350" C. Bulk fluid composition is 15-20 mol% CO, and <4.3 total mol CH, + N, + zyxwvut Ar/100 mol H,O. Water hydrogen isotopic ratio 6D,,,, in the fissure veins spans -29 to -68X, 6'xOH,o -0.7 to 8.5Ym, and bulk carbon isotopic ratio 6°C ranges from -3.7 to -11.70m. 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. Key words: Alpine Fault, New Zealand; fluid inclusions; meteoric fluids; stable isotopes. INTRODUCTION Veins that cross-cut metamorphic rocks attest to the presence of fluid during uplift of the metamorphic belt. These veins are commonly associated with retrogressive metamorphic reactions. The origin of the fluid that penetrates through fractures in the metamorphic rocks is a subject of debate. Evidence exists for partial or complete equilibrium among fluid, host rock, and veins, which is often interpreted as indicating an zyxwvutsrq in situ origin (Crawford et al., 1979; Craw & Koons, 1989; Hay & Craw, 1993). Conversely, there is evidence for incursion of surface waters to great depths in metamorphic belts (e.g. Nesbitt et al., 1986; Wickham & Taylor, 1987; Nesbitt & Muehlenbachs, 1989). Magmatic fluids derived from late-metamorphic granites may also make a contribution to the fluid flux (Burnham, 1979). Knowledge of the origin of post-metamorphic fluids can therefore contribute to our understanding of processes that occur in a metamorphic belt during uplift. The following study seeks to deduce the origin of fluids which deposited some post-metamorphic veins in the Alpine Schist belt (New Zealand). This belt provides an excellent area in which to study the origin of post-metamorphic fluids because it is currently undergoing rapid uplift, so that: (1) the veins are very young (<2 Ma) and we can eliminate the problem of overprinting of later tectonic events, a problem which plagues most studies of older metamorphic belts; (2) we can place good constraints on the tectonic setting, thermal profile and overall geometry of the belt; (3) there is no evidence for magmatism in the Alpine Schists, so that magmatic intrusions can be ruled out as a fluid source for the post-metamorphic veins. Study of the Alpine Schist is also pertinent, in that it is often cited as an area where the post-metamorphic veins (some of which contain significant gold) were formed by liberation of metamorphic fluids during uplift (Norris & Henley, 1976; Craw, 1988). However, more recent studies have suggested that meteoric fluids may also be important (Craw & Koons, 1989). In order to constrain the origin of these post- metamorphic fluids we have combined stable isotopic with fluid inclusion data. Crustal fluids are dominantly composed of 0, H and C, and their origin can often be indicated by the stable isotope ratios of these elements because fluids from different sources often possess distinctive stable isotope ratios (e.g. Sheppard, 1986). Fluid inclusions in late veins can also provide information on the pressure-temperature uplift trajectory of the metamorphic belt (Crawford et al., 1979; Roedder & Bodnar, 1980; Holm et al., 1989; Craw & Norris, 1993). Pressures derived from fluid inclusion studies are fluid pressures, not rock pressures. Hence, one of the problems in interpreting these data is a lack of knowledge of whether fluid pressure is due to lithostatic or hydrostatic load. The distinction is important when depth estimates are required for tectonic interpretations. 429