Geology of the Serpentine Saddle area, Western Otago.
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The Dun Mountain ophiolite at Serpentine Saddle consists of a coherent but tectonically thinned sequence comprising from east to west of serpentinite melange with tectonic inclusions of chromitite and serpentinised dunite and wehrlite (400 m); altered and sometimes pervasively sheared hornblende gabbros and microgabbros, foliated amphibolite and at least two generations of mafic and felsic dikes (500 m); a discontinuous tectonised zone of pillowed and possibly massive metabasalts (0- SO m); a breccia sequence dominantly derived from the non~ultramafic of these lithologies (250 m).
The breccias, which rest in depositional contact on spilitic lavas and transgress onto sheared metagabbros, are extremely poorly sorted, contain clasts up to boulder size and possible exotic material. They are interbedded with sandstone and pillow lava horizons and intruded by granophyre sills. The top of the breccia sequence is finer grained, contains no evidence for contemporaneous volcanism or intrusion, and is conformably overlain by arc-derived Atomodesma limestone with interbedded volcanogenic sandstone.
Early deformation and low P/T metamorphism in the ophiolite suite are comparable to events at recent spreading ridges. Later moderately high P/T metamorphism has affected these rocks together with overlying Bryneira strata.
The ophiolite is situated entirely on the sub-vertical- steeply overturned east limb of the Key Summit Regional Syncline. The Dvorak Fault is a newly recognised major dip-slip fault which offsets lower Bryneira rocks. The Livingstone Fault forms a simple steeply-dipping contact between the easternmost ultramafics of the Dun Mountain ophiolite and rocks of the Caples terrane further east. A major anticline is recognised in the Caples rocks and is correlated with the Poseidon Anticline {Bishop et. al., 1976).
The breccias, which rest in depositional contact on spilitic lavas and transgress onto sheared metagabbros, are extremely poorly sorted, contain clasts up to boulder size and possible exotic material. They are interbedded with sandstone and pillow lava horizons and intruded by granophyre sills. The top of the breccia sequence is finer grained, contains no evidence for contemporaneous volcanism or intrusion, and is conformably overlain by arc-derived Atomodesma limestone with interbedded volcanogenic sandstone.
Early deformation and low P/T metamorphism in the ophiolite suite are comparable to events at recent spreading ridges. Later moderately high P/T metamorphism has affected these rocks together with overlying Bryneira strata.
The ophiolite is situated entirely on the sub-vertical- steeply overturned east limb of the Key Summit Regional Syncline. The Dvorak Fault is a newly recognised major dip-slip fault which offsets lower Bryneira rocks. The Livingstone Fault forms a simple steeply-dipping contact between the easternmost ultramafics of the Dun Mountain ophiolite and rocks of the Caples terrane further east. A major anticline is recognised in the Caples rocks and is correlated with the Poseidon Anticline {Bishop et. al., 1976).
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124 leaves : illus., (part col.) 3 maps (in pocket) ; 30 cm.
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1978Hyslop
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Hyslop, Kenneth James., “Geology of the Serpentine Saddle area, Western Otago.,” Otago Geology Theses, accessed December 7, 2024, https://theses.otagogeology.org.nz/items/show/111.