PETROLOGY OF THE GABBRO AND SHEETED BASALTIC INTRUSIVES AT NORTH CAPE, NEW-ZEALAND

The North Cape massif consists of a semiconformable sequence of serpen tinite, gabbro, sheeted sill and dike units, and pillow lavas. Althoug h structurally disrupted, they can be interpreted in terms of an ideal ised ophiolite sequence and represent the most complete sequence in th e Northland Ophiolite. Their age is considered to be Late Cretaceous - Paleocene on the basis of microfossils in associated sediments. Early Miocene K-Ar ages from igneous rocks are thought to reflect the time of emplacement as a thrust sheet of oceanic crust and upper mantle. Th e gabbros are divided into a lower unit characterised by well-develope d cumulate layering and an upper unit which is massive; the sheeted si lls and dikes are quartz-diorite and microgabbro interleaved with mino r pillow lava. Two phases of alteration are observed, a pervasive low- grade greenschist metamorphism attributed to sea-water interaction aft er formation as oceanic crust, and an overprinting zeolitic alteration which is possibly post-emplacement. Their tholeiitic nature as well a s overlapping geochemical compositions suggest that the gabbros and sh eeted dikes and sills represent different components of a single magma tic system related by simple fractionation processes. Several lines of evidence suggest that the magmas that formed the North Cape gabbro an d sheeted intrusives have subduction-related chemical characteristics. In the gabbro, calcic plagioclase (An(86-92)) and depleted Zr and Y a bundances, and in the sheeted intrusives depleted high field strength element abundances relative to typical MORE, is indicative of a subduc tion signature. The presence of subduction-related characteristics wit hin the Northland Ophiolite suggests that it may have originated at a back-are basin rather than a major ocean ridge spreading centre.