A JURASSIC CHERT-LIMESTONE-SPILITE ASSOCIATION NEAR EKETAHUNA, NORTH-ISLAND, NEW-ZEALAND

Bedded chert and associated bioclastic limestone and spilite near Eket ahuna are locally in depositional contact with enclosing Jurassic grey wacke of the Tararua Formation (Torlesse Terrane). The chert contains abundant, moderately well preserved radiolaria of Middle Jurassic (Baj ocian) age, and a Late Jurassic age is indicated for associated limest one. Immobile elements indicate the spilitic rocks are strongly alkali ne in character, and very different from many other basaltic lavas in the Torlesse Terrane. They can probably be classified as oceanic alkal i basalts, but their original tectonic setting is not evident from the limited geochemical data. Vesicularity of the lavas suggests they may have been erupted at water depths of less than 500 m. A depositional model is proposed in which lava erupted on the outer shelf or upper sl ope provided a temporary barrier against terrigenous sedimentation, en abling radiolarian chert to be deposited on the seaward (protected) si de, and relatively pure bioclastic limestones to form on top of the vo lcanic high. This model does not require the chert-limestone-spilite a ssociation to be tectonically emplaced into the terrigenous elastic se quence as has been inferred for similar associations elsewhere in New Zealand.