Emplacement depths of Paleozoic-Mesozoic plutons from western New Zealand estimated by hornblende-Al geobarometry

Abstract

Emplacement depths of 33 rocks from 27 Paleozoic and Mesozoic plutons from western New Zealand have been estimated utilising the hornblende-A1 barometer of Anderson and Smith. Emplacement depths for plutons from the Median Batholith/Median Tectonic Zone (Median and Separation Point Suites) plutons range from 8 to 27 km. The greatest emplacement depths are found along the western edge of the batholith, in areas dominated by plutons of the Separation Point Suite; together with geochronological data they suggest significant uplift in the interval 118 to c. 110 Ma. Emplacement depths of c. 8-10 km are typical along the eastern side of the batholith in mainly Triassic plutons. No trend along the length of the batholith is apparent. Plutons associated within the lower plates of metamorphic core complexes were emplaced in the range 19-23 km, and were uplifted over a similar, but possibly slightly later, time interval. No significant consistent difference in amphibole type or chemistry was observed between the suites. Epidote believed to be of magmatic origin occurs in 11 plutons which record emplacement pressures of 3.1 kbar or greater, in conflict with the notion that such epidote indicates pluton emplacement pressures ≥6-8 kbar.