U–Pb geochronology of Permian plutonic rocks, Longwood Range, New Zealand: implications for Median Batholith–Brook Street Terrane relations

Abstract

We present U–Pb sensitive high-resolution ion microprobe (SHRIMP) zircon ages from gabbros and associated intrusive rocks of the Longwood Range, Southland. U–Pb zircon ages of 257.6 ± 2.5 Ma, 256.5 ± 1.8 Ma and 251.6 ± 2.0 Ma (2σm) have been obtained from the layered, platiniferous Hekeia Gabbro. These are 5–10 Ma older than cooling ages previously obtained by Ar–Ar dating of hornblendes. A revised U–Pb zircon age of 252.8 ± 2.7 Ma is reported for a sample of western Pourakino Trondhjemite [previously dated at 292 ± 8 Ma by multicrystal 207Pb/206Pb thermal ionization mass spectrometry (TIMS) methods] and a dike of the trondhjemite that cuts the Hekeia Gabbro is dated at 254.8 ± 2.6 Ma. Our new zircon ages show that the Hekeia Gabbro and Pourakino Trondhjemite were intruded within the same, relatively narrow interval from 261 to 252 Ma. This age range is substantially younger than Early Permian (c. 276–288 Ma) ages inferred from fossil evidence of major Takitimu Group volcanism in the adjacent Brook Street Terrane. We make the novel suggestion that the Hekeia and Pourakino bodies, along with plutons at Bluff and Oraka Point, constitute a new Longwood Suite of the Median Batholith that is spatially and temporally distinct from allochthonous Brook Street Terrane volcanic magmatism. A U–Pb zircon age of 232.3 ± 1.5 Ma from a western Longwood Range granodiorite is the oldest Darran Suite pluton so far recognized and represents the start of a major, isotopically more evolved, pulse of the Median Batholith.