Loch Burn Formation, Fiordland, New Zealand: SHRIMP U‐Pb ages, geochemistry and provenance

Abstract

New SHRIMP U‐Pb ages and geochemical data have been obtained for the volcano‐sedimentary Loch Burn Formation (LBF). A rhyolitic clast from the tops of the Stuart Mountains gave a SHRIMP age of 150.3 ± 1.9 Ma, and a very fine sandstone from the same area was dominated by 147.9 ±2.1 Ma zircons. These ages imply a <148 Ma depositional age for the LBF in this area, in contrast to a previous 195‐1 +3Ma age determination for an intercalated felsic flow in the North Fiord area, c. 1 km distant. Two tonalitic clasts from Cumbrae Island gave ages of 354.6 ± 2.6 and 326.8 ±3.2 Ma, respectively. The c. 47 m.y. difference between depositional ages in the Stuart Mountains and North Fiord indicates that the LBF as currently mapped includes more than one unit. however, volcanic clasts and lavas from the two areas are indistinguishable in terms of major and trace element geochemistry, and there is currently insufficient information to discriminate between the two units. informal units are therefore proposed: the < 148 Ma LBF‐2 unit and the c. 195 Ma LBF‐1 unit. volcanic clasts and lavas from both units are probably derived from the Darran Suite arc. The c. 327 Ma tonalite clast has moderate Zr/TiO2 and chemistry similar to most of the other LBF tonalite samples, whereas the c. 355 Ma tonalitic clast has unusual, very high Zr chemistry. Neither appear to be related to any New Zealand plutonics currently well characterised in the literature, based on differences in major and trace element chemistry. however, recent work indicates that possible correlatives with appropriate age and geochemistry exist for both groups of tonalites. No Western Province material is recorded in any of the LBF samples, but this is not considered to exclude formation adjacent to or within the gondwana margin.