Petrology and geochronology of the volcaniclastic and volcanogenic Mesozoic Loch Burn Formation in eastern Fiordland, New Zealand

Abstract

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.