McPhee Cove conglomerate, Tautuku, south-east Otago

Miller, Hannah Elizabeth, 1977-

The nature and provenance of the Jurassic McPhee Cove Conglomerate, Murihiku Terrane, New Zealand, has been investigated using clast petrography, geochemistry and age. The McPhee Cove Conglomerate forms three strike belts within a 12 sqkm area in the Catlins, southeast Otago. The outcrop studied here is based mainly on 50m high cliffs immediately south-west of Tautuku Peninsula at l.athyrus Bay. The McPhee Cove Conglomerate is a 50m thick, polymict, mostly metre bedded, unfossiliferous, poorly sorted orthoconglomerate, with minor sandstone interbeds. Clasts are rounded to well rounded and range in size from granule to boulder. The conglomerate is Ururoan to LowerTemaikan in age and is underlain by the Otekura Formation and overlain by the Purakauiti Formation. It is intetpreted as debris flow deposits, deposited on a shallow submarine-fan. The McPhee Cove Conglomerate consists of plutonic (31% ), volcanic (33% ), volcaniclastic (sedimentary) (34%) and lesser metamorphic (2%) clasts. The plutonic clasts have island arc geochemical characteristics, are calc-alkaline in composition, have 1-type plutonic characteristics and range from granite, granodiorite to monzodiorite, but are dominated by high Si02 (70-76 wt%) granites. The volcanic, volcaniclastic and metamorphic clasts also have island arc geochemical characteristics. Volcanic clasts range from rhyolite to basalt and the volcaniclastic dasts are weakly metamorphosed, dominantly crystal-lithic volcanic sandstones to breccias. The metamorphic clasts are quartzofeldspathic gneisses. Geochemical analyses have defined two clast populations. The majority of the plutonic (81% ), volcanic, volcaniclastic and metamorphic clasts have similar geochemical compositions and representative samples from this group have yielded Permian U-Pb dates of 260±5 Ma and 268±5 Ma This suggests that this group as whole may have been derived from a dissected Permian arc. A secondary population, although similar petrographically to the group discussed above, has geochemical characteristics that deviate from the main population. This smaller group is composed of alkali feldspar granites, samples with adakitic geochemical affinities, and monzonites to monzodiorites. Samples from this group have yielded Jurassic dates of 186±6 Ma and 181±6 Ma. Assuming that the dated samples adequately represent the two populations then at least two suites of arc rocks have supplied detritus to the McPhee Cove Conglomerate. Although the clasts show a broad resemblance to igneous rocks of the Median Tectonic Zone, on the combined basis of petrography, geochemistry and geochronology no source terranes for either the Permian or Jurassic groups are presently recognised in New Zealand.

xii, 198 p. : ill. (some col.), maps (one folded) ; 26 cm.

1999Miller

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