GEOCHEMISTRY AND GEOCHRONOLOGY OF PSEUDOTACHYLYTES OF THE ALPINE FAULT, NEW ZEALAND

We obtained detailed geochemical and geochronological results for a suite of 30 pseudotachylyte samples collected from a single locality (Harold Creek), along the central section of the Alpine Fault Zone in New Zealand. This dextral strike-slip fault marks a portion of the boundary between the Pacific and Australian plates, which is characterized by high rates of oblique-slip displacement, exhumation and intense seismic activity. A biotite-rich, layered pseudotachylyte vein collected from this locality was earlier dated using the 40Ar/39Ar technique at ca. 1 Ma (Warr et al. 2003). With our more extensive sample suite we evaluate the relationships between rock composition, the degree and nature of melting, and the role of volatiles on the noble gas signatures of these veins. Inductively Coupled Plasma-Optical Emission Spectroscopy (ICP-OES) results reveal minor differences between wall rock schist and vein compositions. The veins are slightly, but systematically more volatile-rich than adjacent host rock, which reflects the preferential melting of hydrous phyllosilicates and amphiboles. However, the bulk composition of the veins does not strongly differ from that of the wall rock. To further constrain the character and history of the fluid, stable isotopes will also be reported for these pseudotachylytes. Fifteen new Ar ages across layered veins show a large range in ages, from 1Ma to 19Ma. Comparisons of these ages with elemental data show an excellent correlation with elemental composition. K-rich samples, reflecting large degrees of melting, reliably recorded the age of crystallization. In contrast, samples with lower K concentrations show systematically older Ar ages, reflecting either inherited or excess Argon. This study highlights the importance of tracking geochemical changes during frictional melting, and reveals that K composition and degree of melting are key to accurate dating of frictional melt events. The results place the generation zone of this pseudotachylyte sample suite at the base of the seismogenic zone.