Metamorphism and metamorphic K-Ar ages the mesozoic accretionary complex in Northland, New Zealand

A southwest dipping Mesozoic accretionary complex, which consists of tectonically imbricated turbiditic mudstone and sandstone, hemipelagic-siliceous mudstone, and bedded cherts and basaltic rocks of pelagic origin, is exposed in northern North Island, New Zealand. Interpillow limestone is sometimes contained in the basaltic rocks. The grade of subduction-related metamorphism increases from northeast to southwest, indicating an inverted metamorphic gradient dip. Three metamorphic facies are recognized largely on the basis of mineral parageneses in sedimentary and basaltic rocks: zeolite, prehnite-pumpellyite and pumpellyite-actinolite. From the apparent interplanar spacing d(002) data for carbonaceous material, which range from 3.642 to 3.564 Angstrom, the highest grade of metamorphism is considered to have attained only the lowermost grade of the pumpellyite-actinolite facies for which the highest temperature may be approximately 300degreesC. Metamorphic white mica K-Ar ages are reported for magnetic separates and <2 mum hydraulic elutriation separates from 27 pelitic and semipelitic samples. The age data obtained from elutriation separates are approximately 8 m.y. younger, on average, than those from magnetic separates. The age difference is attributed to the possible admixture of nonequilibrated detrital white mica in the magnetic separates, and the age of the elutriation separates is considered to be the age of metamorphism. If the concept, based on fossil evidence, of the subdivision of the Northland accretionary complex into north and south units is accepted, then the peak age of metamorphism in the north unit is likely to be 180-130 Ma; that is, earliest Middle Jurassic to early Early Cretaceous, whereas that in the south unit is 150-130 Ma; that is, late Late Jurassic to early Early Cretaceous. The age cluster for the north unit correlates with that of the Chrystalls Beach-Taieri Mouth section (uncertain terrane), while the age cluster for the south unit is older than that of the Younger Torlesse Subterrane in the Wellington area, and may be comparable with that of the Nelson and Marlborough areas (Caples and Waipapa terranes).