ZONED (CRETACEOUS AND CENOZOIC) GARNETS IN ALPINE FAULT MYLONITE, SOUTHERN ALPS, NEW ZEALAND

Results of new Sm-Nd (garnet) and ⁴⁰Ar/³⁹Ar (hornblende) age determinations, together with metamorphic fabric and garnet compositional zoning studies, show for the first time that garnet growth in the Alpine Schist belt, Southern Alps orogen, New Zealand was markedly diachronous (Cretaceous and Cenozoic).

Cretaceous metamorphism and isograd development are recorded by: a Sm-Nd age of 97.8 ± 8.1 Ma for the core of a multiply-zoned garnet that records at least diachronous garnet growth in a sample of the Alpine Fault mylonite; a minimum Sm-Nd age of 77.9 ± 3.6 Ma for early growth of zoned Mn-rich garnets; and a ⁴⁰Ar/³⁹Ar age of ~110 Ma for hornblende. The growth of randomly-oriented hornblende ± garnet in mafic rocks, garnet in Mn-rich cherts, and rare garnet (cores) in the abundant quartzofeldspathic Alpine Schist occurred at relatively low temperatures and pressures (probably near 420-550 °C and ~6-7 kbar for most samples). Surprisingly, the Cretaceous ages obtained in this study correlate well with a period of widespread extension that preceded the opening of the Tasman Sea at c. 84-82 Ma, raising the possibility that initial garnet growth was triggered by crustal extension during continental break-up, rather than collision or orogenesis.

Much later, renewed metamorphism, garnet growth, and fabric development occurred in the Cenozoic. Peak metamorphic temperatures and pressures (~600 °C/~11 kbar) were attained in the late Cenozoic, when a narrow outer rim of Ca-rich garnet formed, overgrowing the Alpine Fault mylonite foliation in the deepest crustal part of the orogenic root of the Southern Alps (~41 km depth). The immediately adjacent inner garnet rim gave a Sm-Nd age of 12 ± 37 Ma (MSWD=0.89). These results establish for the first time that garnet growth and ductile deformation in the Alpine Schist are partly Cenozoic in age. Both the inner and outer rims of the garnet probably grew near the start of the modern phase of oblique collision and associated rapid uplift and exhumation of the Southern Alps orogen, which began c. 5-6 Ma. A consideration of regional tectonics and P-T paths indicates that the inner garnet rim grew after 45 Ma, almost certainly after c. 25 Ma, and probably much more recently. Calculations based on P-T data and uplift rates indicate that the outer garnet rim grew between about 8.7 and 4.0 Ma. Like the garnets, the dominant, subvertical "Alpine" foliation fabric in the Southern Alps is a strongly diachronous composite fabric element that was initiated in the Cretaceous but "reused" in the Cenozoic.

Our data indicate that prograde metamorphism and dehydration are occurring in the crust under the present Southern Alps orogen, accompanying a Cenozoic phase of renewed garnet growth. Additional support for this interpretation is provided by the occurrence of rare large quartz veins that cut the late Cenozoic mylonitic foliation of the Alpine Fault, and by geophysical evidence of an extensive low seismic P-wave velocity zone and a roughly coincident zone of low resistivities, obtained in the recent South Island GeopHysical Transect (SIGHT) programme. The geophysical signals may reflect both fluid release by prograde metamorphism and changes in the composition and wetting characteristics of the fluid, with a coexisting C-O-H metamorphic fluid becoming less water-rich as the rocks pass through the root zone of the orogen and up along the Alpine Fault.