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 +/- 11.9 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)(+3) Ma age determination
for an intercalated felsic flow in the North Fiord area, c. I 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.