Geochronologic constraints of the uplift and metamorphism along the Alpine Fault, South Island, New Zealand

Abstract

Geochronological studies of pegmatites and Alpine Schist exposed east of the Alpine Fault, South Island, New Zealand, reveal a complex history beginning with magmatism and metamorphism at c. 68 m.y. ago and ending with rapid uplift and exhumation in the last 5 m.y. Pegmatites exposed in the Mataketake Range give conventional U‐Pb monazite and SHRIMP ion‐probe zircon ages of 68 ± 2.6 Ma and 67.9 ± 2.5 Ma, respectively. Inasmuch as petrologic and isotopic data indicate that the Alpine pegmatites are melts derived from the Alpine Schist, the age of the pegmatites suggests that, at least locally, the high‐grade metamorphism is considerably younger than previously assumed. We tentatively suggest that metamorphism, in at least some areas of the Alpine Schist, may be associated with Late Cretaceous transtension rather than resulting from the consequences of collision during the Rangitata Orogeny.

⁴⁰Ar/³⁹ Ar studies of hornblendes from the Alpine Schist, collected from the Haast River to the Franz Josef Glacier area, reveal highly disturbed spectra. Despite this complexity, these analyses define a systematic decrease in ages both across‐strike toward the Alpine Fault (Haast River traverse) and northwards along‐strike towards Mt Cook. This pattern of decreasing ⁴⁰Ar/³⁹Ar hornblende ages is also observed in lower closure temperature systems such as zircon and apatite fission‐track ages. We interpret the decrease in ages toward the fault to be the result of deeper exhumation in the immediate vicinity of the Alpine Fault, whereas we interpret the northward younging of fault‐proximal samples to be a result of both more recent and possibly more extensive exhumation than occurred in areas to the south.

Keywords:

• geochronology
• Alpine Schist
• isotopes
• pegmatites