**Dun Mountain–Maitai Terrane**
**Definition**
The Dun Mountain–Maitai terrane is a geologically significant terrane located in the South Island of New Zealand, characterized by a complex assemblage of ultramafic and mafic rocks, sedimentary sequences, and volcanic formations. It represents a fragment of oceanic crust and island arc material accreted onto the Gondwana margin during the Paleozoic to early Mesozoic eras.
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## Dun Mountain–Maitai Terrane
### Introduction
The Dun Mountain–Maitai terrane is a prominent geological unit within the New Zealand geologic framework, particularly in the Nelson and Marlborough regions of the South Island. It is named after two key localities: Dun Mountain near Nelson, known for its distinctive ultramafic rock outcrops, and the Maitai Valley, where associated sedimentary and volcanic rocks are well exposed. This terrane is part of a larger mosaic of accreted terranes that form the basement geology of New Zealand, providing critical insights into the tectonic evolution of the southwestern Pacific margin.
### Geological Setting
The Dun Mountain–Maitai terrane is situated within the Median Batholith zone, a major geological province that separates the Western and Eastern provinces of New Zealand’s South Island. It is bounded by other terranes such as the Caples and Brook Street terranes to the west and the Rakaia terrane to the east. The terrane is interpreted as a fragment of oceanic lithosphere and island arc complexes that were accreted to the Gondwana continental margin during the Late Paleozoic to Early Mesozoic, approximately 250 to 200 million years ago.
### Lithology and Rock Types
The Dun Mountain–Maitai terrane is notable for its diverse lithological assemblage, which includes:
– **Ultramafic Rocks:** The terrane is famous for its extensive outcrops of ultramafic rocks, primarily serpentinites and peridotites, derived from the Earth’s mantle. These rocks are exposed prominently at Dun Mountain and are often associated with distinctive greenish and reddish weathering colors due to oxidation of iron-rich minerals.
– **Mafic and Ultramafic Intrusions:** Alongside ultramafic rocks, the terrane contains gabbros and other mafic intrusions, representing oceanic crustal material.
– **Volcanic and Sedimentary Sequences:** The Maitai component of the terrane includes volcanic rocks such as basalts and andesites, as well as sedimentary rocks like sandstones, siltstones, and mudstones. These sequences are interpreted as remnants of island arc volcanism and associated sedimentation.
– **Metamorphic Overprint:** The terrane has undergone low- to medium-grade metamorphism, with some rocks exhibiting greenschist facies mineral assemblages, reflecting tectonic burial and deformation during accretion.
### Tectonic Evolution
The Dun Mountain–Maitai terrane records a complex tectonic history linked to the assembly of the supercontinent Gondwana and the evolution of the southwestern Pacific margin. Key aspects of its tectonic evolution include:
– **Oceanic Crust Formation:** The ultramafic and mafic rocks originated as part of oceanic lithosphere, likely formed at a mid-ocean ridge or supra-subduction zone setting during the Late Paleozoic.
– **Island Arc Development:** Volcanic and sedimentary sequences in the Maitai area represent island arc volcanism, indicating subduction-related magmatic activity.
– **Terrane Accretion:** During the Late Triassic to Early Jurassic, the Dun Mountain–Maitai terrane was accreted onto the Gondwana margin through subduction and collision processes, contributing to the growth of the continental crust.
– **Deformation and Metamorphism:** Subsequent tectonic events, including the Rangitata Orogeny, caused deformation, metamorphism, and uplift of the terrane, shaping its present-day geology.
### Economic and Scientific Significance
The Dun Mountain–Maitai terrane holds both economic and scientific importance:
– **Mineralization:** The ultramafic rocks are associated with deposits of chromite and other economically valuable minerals. Historically, chromite mining occurred in the Dun Mountain area.
– **Geochemical Studies:** The terrane provides a natural laboratory for studying mantle-derived rocks, oceanic crust processes, and island arc magmatism.
– **Tectonic Reconstructions:** Understanding the terrane’s origin and accretion contributes to broader models of plate tectonics, terrane accretion, and continental growth in the southwest Pacific region.
### Key Localities and Outcrops
– **Dun Mountain:** The type locality for the ultramafic rocks, featuring extensive serpentinite outcrops with characteristic red and green coloration due to weathering.
– **Maitai Valley:** Exposes volcanic and sedimentary sequences that complement the ultramafic assemblages, providing a more complete picture of the terrane’s geological history.
– **Nelson Region:** The terrane is well exposed throughout the Nelson area, with numerous road cuts, quarries, and natural outcrops accessible for study.
### Research History
The Dun Mountain–Maitai terrane has been the subject of geological research since the early 20th century, with initial mapping and petrological studies focusing on the unusual ultramafic rocks. Subsequent work has integrated geochronology, geochemistry, and structural geology to refine understanding of the terrane’s origin and tectonic evolution. Modern research continues to explore its role within the complex terrane collage of New Zealand and its implications for regional tectonics.
### Summary
The Dun Mountain–Maitai terrane is a fundamental geological unit in New Zealand’s South Island, representing an accreted fragment of oceanic crust and island arc material. Its diverse lithology, complex tectonic history, and economic mineralization make it a key focus for geological research and understanding the tectonic evolution of the southwestern Pacific margin.
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**Meta Description:**
The Dun Mountain–Maitai terrane is a geologically significant accreted terrane in New Zealand’s South Island, composed of ultramafic, mafic, volcanic, and sedimentary rocks formed during the Paleozoic to Mesozoic eras. It provides important insights into oceanic crust formation, island arc volcanism, and continental growth along the Gondwana margin.