Rb-Sr and K-Ar geochronology of turbidites and metavolcanics at Red Rocks, Wellington, New Zealand

Abstract

Rb-Sr and K-Ar dating of various lithologies at Red Rocks, Wellington coast, New Zealand, indicate a complex thermotectonic history. Turbidites west of Red Rocks yield a Rb-Sr whole-rock age of 216 Ma, interpreted to be the time of closure of the whole-rock system during burial and low-grade metamorphism of the sediments about 15–20 Ma after deposition. Turbidites east of Red Rocks yield internally concordant Rb-Sr ages of 188–190 Ma, which are similarly interpreted, and suggest a younger sequence, with a fault between the two turbidite units. The ages are consistent with a general eastwards-younging of the Torlesse accretionary prism in the Wellington region. Red/green argillites from Red Rocks itself define an imperfect Rb-Sr isochron of about 159 Ma. K-Ar ages for these rocks range from 147 to 163 Ma and reflect argon loss during a second localised thermal event, probably related to thrusting of the eastern over the western turbidite sequence during imbrication of the Torlesse accretionary prism.

Keywords:

• Torlesse
• Sr-Rb
• K-Ar
• geochronology
• whole-rock
• Red Rocks