The Loch Burn Formation in eastern Fiordland is the metamorphosed and
eroded effusive product of a long-lived Jurassic to Early Cretaceous
volcanic arc. Relict sedimentary features within meta-volcaniclastic
components indicate sedimentation in a mostly terrestrial or
shallow-water environment that was fed by debris flows from proximal
granitic and volcanic high points. In the Murchison Mountains,
deposition is constrained by a c. 342 Ma granite, which is unconformably
overlain by the Loch Burn Formation, and a c. 158 Ma quartz diorite that
intrudes the Loch Burn Formation. This latter age is 8-9 m.y. older than
a volcanic clast from Loch Burn Formation and a sandstone horizon
previously dated from the Stuart Mountains, and therefore supports
previous suggestions that the Loch Burn Formation is a long-lived and
time-transgressive unit. The Carboniferous basement provides a potential
source for detrital zircon in metasediment, inherited zircon in the
intrusive quartz diorite, and some granitoid clasts within the
formation. Geochemical similarities between the quartz diorite, volcanic
clasts within the Loch Burn Formation, and nearby Jurassic to Early
Cretaceous Darran Suite plutons imply that the Loch Burn Formation is
the volcanic equivalent of the Darran Suite. The distinctive
lithological, geochemical, age, and internal relationships of the Loch
Burn Formation are also seen in the volcano-sedimentary Largs Group of
northern Fiordland and Paterson Group of Stewart Island, suggesting that
these three units are lithological and chronological equivalents to one
another.
The Loch Burn Formation provides a comprehensive record of the tectonic
evolution of eastern Fiordland, with several episodes of uplift and
burial. These are: uplift and erosion of a Carboniferous plutonic
basement by c. 195 Ma; deposition of the older part of the Loch Burn
Formation sequence in the Jurassic before burial and intrusion by Darran
Suite plutons at 158 Ma; deposition continuing until at least 148 Ma;
metamorphism of the entire Loch Burn Formation at greenschist and
amphibolite facies conditions; uplift, erosion, and then deposition of
the overlying Eocene sediments; reburial to zeolite facies depths
beneath the Tertiary Te Anau and Waiau Basins; final uplift in the
Pliocene.