An episodic Cretaceous cooling model for the Otago‐Marlborough Schist, New Zealand, based on ⁴⁰Ar/³⁹Ar white mica ages

Abstract

New ⁴⁰Ar/³⁹Ar ages of 12 white mica samples from deep levels of the Otago and Marlborough Schists, together with previously published whole‐rock K‐Ar ages yield a nonlinear age‐depth profile suggesting a fossil partial retention zone for argon. In contrast to earlier studies, we interpret (1) the peak of Otago Schist metamorphism to have occurred in the Middle Jurassic (170–180 Ma) rather than Early Jurassic; and (2) subsequent cooling to have been episodic, not slow and continuous. These data cannot be modelled uniquely but support a model where the schist was held at mid to lower crustal depths until c. 135 ± 5 Ma, after which it was rapidly unroofed at 0.6–1.0 mm/yr during regional crustal thickening along the eastern margin of New Zealand. We infer that there were also one or more younger periods of argon loss affecting deep levels of the Otago/ Marlborough Schists in a spatially heterogeneous way after c. 120 Ma. Late Cretaceous argon loss at <75–84 Ma coincided with seafloor spreading offshore of New Zealand and probably involved crustal reheating. Post‐mid Cretaceous phases of argon loss may have coincided with periods of heat input into the crust, including metamorphism of the Alpine Schist at deep levels of the Haast Schist.

Keywords:

• Cretaceous tectonics
• Ar‐Ar geochronology
• Haast Schist
• Otago Schist
• Marlborough Schist
• cooling history