An episodic Cretaceous cooling model for the Otago‐Marlborough Schist, New Zealand, based on⁴⁰Ar/³⁹Ar white mica ages

Abstract

New 40 Ar/ 39 Ar 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.

U‐Pb and Sm‐Nd ages from the Alpine Schist, New Zealand

“…In contrast, the Alpine Schist is a steeply dipping, narrow belt with metamorphic grade increasing unidirectionally to the west (except where repeated by Oligocene-Miocene regional folds), culminating in oligoclase zone amphibolite facies rocks exposed in the hanging wall of the Alpine Fault (e.g., Cooper 1972Cooper , 1974. There is general agreement that the formation of the entire Haast Schist belt commenced in the deep parts of an accretionary prism in the Middle Jurassic, an age interpretation based on K-Ar and Ar-Ar profiles through the Otago Schist Little et al 1999). Stratigraphic and provenance data show that exhumation of the Otago Schist was complete by the late Early Cretaceous (c. 105 Ma, , but available evidence suggests that exhumation of the highgrade parts of the Alpine Schist is no older than Pliocene (e.g., Kamp et al 1989;Mortimer et al 2001).…”

Terrane affiliation and terrane boundaries of Mesozoic accretionary complexes, northeastern North Island, New Zealand: Some implications from recycled elastics

“…Graham (1985) dated schistose Rakaia rocks (Haast Schist) from the central North Island and could not verify whether a Late Jurassic to Early Cretaceous uplift event was of regional or only local significance. In contrast, Little et al (1999) distribution from the MTZ arc to the west, but uplift declined to the north (Fig. 7).…”

Cretaceous age, composition, and microstructure of pseudotachylyte in the Otago Schist, New Zealand

“…(1 (Little et al 1999). Uplift and exhumation of the schist was accompanied by regional extension and the development of low-angle ductile shear zones at 135Á105 Ma (Deckert et al 2002;Forster & Lister 2003).…”

“…Mortimer & Cooper (2004) suggest that the timing of highest grade metamorphic mineral growth in the Otago Schist was during the Jurassic. Little et al (1999) inferred that peak metamorphism in the Otago Schist occurred in the middle Jurassic (170Á180 Ma) and that the Otago Schist was held at mid-to-lower crustal depths until 135 Ma. Thereafter, exhumation occurred at 0.6Á1 mm yr…”

“…Little et al (1999) inferred that peak metamorphism in the Otago Schist occurred in the middle Jurassic (170Á180 Ma) and that the Otago Schist was held at mid-to-lower crustal depths until 135 Ma. Thereafter, exhumation occurred at 0.6Á1 mm yr(1 (Little et al 1999). Uplift and exhumation of the schist was accompanied by regional extension and the development of low-angle ductile shear zones at 135Á105 Ma (Deckert et al 2002;Forster & Lister 2003).…”