A tectonic synthesis of the Dun Mountain Ophiolite Belt

Abstract

The Dun Mountain Ophiolite Belt (DMOB) is an Early-Permian ophiolite sequence exposed in the South Island of New Zealand. The ophiolite is overlain by the thick deep-marine sedimentary Maitai Group. The Alpine Fault divides the DMOB into two sections, one in Nelson and the other in Southland. The DMOB is divided into three different units based on lithology and geochemistry: the Livingstone Ophiolite, which is a typical ophiolite sequence dipping sub-vertically and facing west; the Otama Mélange, a deformed ocean-floor assemblage with no ultramafics or serpentinites and a greater amount of felsic rocks than the other two units; and the Patuki Mélange, a highly deformed ophiolite structurally beneath the Livingstone Ophiolite.

The Lvingstone Ophiolite has three phases of igneous activity. The first phase is represented by cumulates, massive gabbro, and extensive pillow lavas. It has a MORB-like geochemistry with a subtle above-subduction signature. The age of this phase is 277.6 ± 3.3 Ma using U/Pb in zircon. The second phase locally intrudes the first with dykes which are feeders for extensive non-pillowed lava flows of variable thickness. The age of the second phase (275.2 ± 5.4 Ma) cannot be distinguished from the first. The second phase has a stronger above-subduction geochemical signature than the first phase. The third phase comprises felsic and intermediate dykes that cut the first two phases and intrude into the sediments overlying the DMOB. This phase has not been directly dated but has the same geochemistry as the felsic rocks in the Otama Mélange. The igneous rocks of the Otama Mélange are 50% felsic and have an age of 269.3 ± 4.5 Ma. The mafic and felsic rocks from the Otama Mélange have a strong above-subduction geochemistry, but are not typical of arcs.

The Patuki Mélange contains both MORB-like and OIB igneous rocks in a serpentinite matrix. The MORB-like Patuki Mélange is similar to the first stage of igneous activity in the Livingstone Ophiolite. Sediment blocks within the Patuki Mélange have been correlated with the Maitai Group, based on their petrology and detrital zircon age pattern. These sediments have a youngest detrital zircon age of Late Permian through to the Early Triassic. The Maitai Group sediment are distal in character within the Patuki Mélange and more proximal above the Livingstone Ophiolite.

I infer that the Livingstone Ophiolite represents a fore-arc, and the Otama Mélange a localization of the Livingstone ophiolites stage three igneous activity in that fore-arc (possibly due to ridge subduction). The Patuki Mélange is either an off-scraping of a subducted slab or part of the trench wall of the above-subduction crust. The DMOB may have been part of the same ocean-crust as the Brook Street Terrane during its formation, but there is no specific evidence for this. Detrital zircons from the Caples Terrane are almost exclusively Triassic in age. The Maitai Group may have some time overlap with the oldest Murihiku Terrane. The DMOB is identical in geology and age to the Yakuno Ophiolite in Japan which may have once been part of the same subduction-zone before the opening of the Neo-Tethys. Detrital zircons from the Aspiring Terrane have a Jurassic age 154.1 ± 2.0 Ma, which constrains the age of the metamorphism of the Haast Schist.

The DMOB has been highly deformed with evidence for extensional structure reactivated in compression on the sea-floor during igneous activity; however, most of the observed internal deformation in the DMOB is Cenozoic in age. The serpentinites are completely overprinted by the oblique compression through New Zealand since the Miocene.