Geology and geochemistry of mafic rocks from ophiolites of East Nelson, New Zealand

In the East Nelson area, ophiolitic rocks of the Dun Mountain Ophiolite Belt (East Nelson ophiolites) have been described by past workers as representing either a single disrupted ophiolite suite or a number of distinct ophiolite suites. -- Through geochemical evaluation of mafic rocks of the East Nelson ophiolites, it has been determined that potentially three separate ophiolite suites exist; the Dun Mountain Ophiolite, the Patuki melange and the Croisilles melange. -- The Dun Mountain Ophiolite represents a semi-complete ophiolite suite. Geochemical evidence indicates that mafic rocks of this ophiolite are of island-arc tholeiite composition and have been produced by partial melting of depleted mantle above a subduction zone. The disrupted and relatively incomplete nature of the ophiolite is considered to be the result of tectonic activity associated with obduction and later orogenesis. -- Unconformably overlying the Dun Mountain Ophiolite are local accumulations of conglomeratic material, known as the Upukerora Formation. Clasts within this formation closely resemble lithologies observed within the Dun Mountain Ophiolite and pyroxenes analyzed from clasts are found to be compositionally similar to those observed within the ophiolite. -- Rocks of the Patuki and Croisilles melanges lie in fault contact with, and underlie the Dun Mountain Ophiolite. Blocks within the melanges consist of sedimentary, mafic to ultramafic volcanic and plutonic rocks suspended in matrices dominated by sheared serpentinite. Although these rocks are highly disrupted, they are considered to represent vestiges of true ophiolitic assemblages, as representative lithologies of an ophiolite are observed within the blocks. These rocks likely represent fragments of oceanic crust subducted beneath the Dun Mountain Ophiolite. -- Basaltic rocks of the Fatuki and Croisilles melanges are divided into two petrographically and geochemically defined suites; a mid-ocean ridge suite and an alkaline within-plate suite. Geochemical evidence suggests mid-ocean ridge basalts of the melanges are indistinguishable and therefore it is suggested that the melanges may represent dislocated portions of the same oceanic basement. -- A fore-arc environment of formation is favoured for the origin of the Dun Mountain Ophiolite. By this model the Patuki and Croisilles melanges are considered fragments of normal ocean crusts sheared off the subducted slab during subduction.