GEOCHRONOLOGICAL CONSTRAINTS OF THE UPLIFT AND METAMORPHISM ALONG THEALPINE FAULT, SOUTH-ISLAND, NEW-ZEALAND

Geochronological studies of pegmatites and Alpine Schist exposed east of the Alpine Fault, South Island, New Zealand, reveal a complex histo ry beginning with magmatism and metamorphism at c. 68 m.y. ago and end ing with rapid uplift and exhumation in the last 5 m.y. Pegmatites exp osed in the Mataketake Range give conventional U-Pb monazite and SHRIM P ion-probe zircon ages of 68 +/- 2.6 Ma and 67.9 +/- 2.5 Ma, respecti vely. Inasmuch as petrologic and isotopic data indicate that the Alpin e pegmatites are melts derived from the Alpine Schist, the age of the pegmatites suggests that, at least locally, the high-grade metamorphis m is considerably younger than previously assumed. We tentatively sugg est that metamorphism, in at least some areas of the Alpine Schist, ma y be associated with Late Cretaceous transtension rather than resultin g from the consequences of collision during the Rangitata Orogeny. Ar- 40/Ar-39 studies of hornblendes from the Alpine Schist, collected from the Haast River to the Franz Josef Glacier area, reveal highly distur bed spectra. Despite this complexity, these analyses define a systemat ic decrease in ages both across-strike toward the Alpine Fault (Haast River traverse) and northwards along-strike towards Mt Cook. This patt ern of decreasing Ar-40/Ar-39 hornblende ages is also observed in lowe r closure temperature systems such as zircon and apatite fission-track ages. We interpret the decrease in ages toward the fault to be the re sult of deeper exhumation in the immediate vicinity of the Alpine Faul t, whereas we interpret the northward younging of fault-proximal sampl es to Be a result of both more recent and possibly more extensive exhu mation than occurred in areas to the south.