Geology of the Pigroot Creek : insight into petrology and thermochronology of the deep crust beneath Otago, New Zealand

Beinlich, Andreas.

The Cenozoic "mafic phonolite" from the Pigroot Creek, eastern Otago, New Zealand, has often attracted its attention for petrological research due to its fractionated appearance and the concurrent occurrence of abundant spinal lherzolite nodules. In this locality, an undeformed cumulate gabbro was found during this study, composed of intermediate plagioclase, pargasitic hornblende, Fe-Ti oxides, titanite and accessory apatite and zircon. Major and trace elements indicate crystallisation from primitive alkaline magma and subsequently equilibration under amphibolite facias cl conditions (0.5-0.9 GPa) as revealed by amphibole-plagioclase geobarometry (Holland & Blundy, 1994; Ernst & Liu, 1998). The combination of LA-ICP-MS U-Pb dating and Ti-Zr geothermometry resulted in an almost complete t-T path from 100-20 Ma. That is in detail: 1) zircon: 100.7±2.4- 88.3 ± 1.5 M a 208Pb-corrected 206PbP38U zircon ages (n = 35) at 710 ± 20 oc {Ti-inzircon geothermometry of Watson et al., 2006); 2) titanite: 84.9 ± 1 Ma and 825°C to 33.6 ± 1.4 Ma and 770°C e07Pb-corrected 206PbP38U ages; n = 49; Zr-in-titanite geothermometry of Hayden et al., 2006 at 0.7 GPa); 3) apatite: 20±3 Ma (n = 27) projected from measured 207Pb/206Pb in plagioclase. Titanite age and temperature data are inconsistent with diffusive loss of Pb and Zr (Cherniak, pers. comm.), and indicate, as also revealed by REE data and reaction textures, slow cooling and contemporaneous crystal growth at the expense of ilmenite and plagioclase. Apatite ages clearly overlap the K-Ar age of the phonotephritic host rock (Coombs, pers. comm.) and indicate therefore thermal perturbation during transport to the surface. This record indicates 1) crystallisation of alkaline mafic melt during the early break-up of Zealandia from Gondwana from 100-88 M a, 2) heating during progressive lithospheric thinning from 88-84 Ma, followed by 3) a period (84-34 Ma) of slow cooling (:s; 2°/Ma) subsequent to separation of the Zealandia microcontinent and opening of the Tasman Sea, and finally 4) transport to the surface in the phonotephritic melt. The long period of slow cooling indicates tectonothermal stability from 80-35 Ma which is perfectly supported by evidences from southern Zealandia mid-crust (Ar thermochronology) and surface (Waipounamu erosion surface).

vi, 114 leaves : ill., map ; 30 cm. + 1 map.

2006Beinlich

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