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

New Ar-40/Ar-39 ages of 12 white mica samples from deep levels of the Otago and Marlborough Schists, together with previously published whole-rock K-A r ages yield a nonlinear age-depth profile suggesting a fossil partial rete ntion 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 uniqu ely but support a model where the schist was held at mid to lower crustal d epths 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 Z ealand. 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 invo lved crustal reheating. Post-mid Cretaceous phases of argon loss may have c oincided with periods of heat input into the crust, including metamorphism of the Alpine Schist at deep levels of the Haast Schist.