Abstract

Ion microprobe analyses of detrital zircons in the Te Akatarawa Terrane, New Zealand, reveal that the age of unfossiliferous turbidites overlying a fusuline- and coral-bearing limestone block olistostromal mélange is no older than 255 ± 4 Ma (Late Permian). This is approximately 15 m.y. younger than the Kungurian age of the fusulinid limestone. We interpret this to indicate collapse of a Permian oceanic seamount as it entered a subduction zone along the Pacific margin of Gondwana. These turbidites differ markedly in composition from adjoining Permian to Middle Triassic sandstones of the Torlesse Terrane. Detrital zircon age data indicate predominantly Permian and Carboniferous ages for source rocks supplying the Te Akatarawa turbidites, but also reveal significant earlier Palaeozoic and Proterozoic components, ranging back to 1.9 Ga. The warm-water setting of limestone blocks and the short 15 m.y. time period between sedimentation and accretion onto a continental margin require the limestone to have formed in a low-latitude position probably off the northeast Australian (New Guinea) margin of Gondwana. Zircons within the sample underwent re-crystallization at around 230 ± 11 Ma which may be related to alteration during accretion in a subduction zone environment. Over a period of 100 to 150 m.y. from 255 Ma the terrane underwent more than 5000 km translation along the continental margin southward to its current location as an exotic miniterrane enclosed within the New Zealand Torlesse Terrane.

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