Re-initiation of plutonism at the Gondwana margin after a magmatic hiatus: The bimodal Permian-Triassic Longwood Suite, New Zealand

The Cambrian to Cretaceous Tuhua Intrusives, New Zealand, preserve an igneous record of Phanerozoic subduction and crustal growth at the margin of Gondwana. Within the Tuhua Intrusives, the coeval gabbroic and trondhjemitic intrusions of the c. 261-243 Ma Longwood Suite stand out as being isotopically more primitive and chemically distinct from all other New Zealand plutonic suites. We present new U-Pb crystallization ages, trace element analyses and Sr-Nd isotope compositions of the Longwood Suite. U-Pb SHRIMP zircon ages of 258.5 +/- 2.5 Ma, 256.0 +/- 1.8 Ma, 247.8 +/- 2.7 Ma and 243.2 +/- 2.4 Ma obtained from plutons on Ruapuke Island, and a dike at Bluff, affirm the restricted time range and expand the known areal extent of the Longwood Suite. Longwood Suite granitoids are I-type and sodic (K/Na < 0.4), with distinctive low Rb and Nb/Ta, flat rare earth element patterns (La/YbN < 10), unradiogenic 87Sr/86Sr(t) (0.7029 to 0.7032) and radiogenic e143Nd(t) (+6.3 to +8.2), compared to the nearby, calc-alkaline, Late Triassic Darran Suite I-type plutons of the Tuhua Intrusives. Stable Nd isotope ratios of Longwood Suite samples are highly variable (d146/144Nd = 233 ppm) compared to global plutonic rocks (d146/144Nd = 44 ppm) and reflect the removal of phosphate minerals. Collectively, these geochemical characteristics are consistent with generation of the granitoids by shallow (garnet-absent) melting of an amphibolitic residue, from which we infer relatively thin lithosphere. The Longwood Suite has a maximum areal addition rate of 43 km2/Ma, substantially less than the subsequent plutonic suites when the magmatic arc was fully established. We suggest a petrotectonic model whereby Gondwana continental margin crust was tectonically underplated by Permian intra-oceanic island arc crust and mantle lithosphere, which subsequently melted to generate the isotopically primitive gabbro and trondhjemite plutons of the Longwood Suite. (c) 2021 International Association for Gondwana Research. Published by Elsevier B.V. All rights reserved.

Automated summary

The Tuhua Intrusives in New Zealand provide evidence of Phanerozoic subduction and crustal growth at the Gondwana margin. Among them, the Longwood Suite of gabbroic and trondhjemitic intrusions stands out for its isotopic and chemical characteristics. New U-Pb ages, trace element analyses, and Sr-Nd isotope compositions of the Longwood Suite are presented. The results confirm the restricted time range and expand the known distribution of the Longwood Suite. The geochemical characteristics indicate that the granitoids of the Longwood Suite were generated by shallow melting of an amphibolitic residue, suggesting relatively thin lithosphere. The petrotectonic model suggests that the Gondwana continental margin crust was underplated by Permian intra-oceanic island arc crust and mantle lithosphere, which subsequently melted to form the isotopically primitive gabbroic and trondhjemitic plutons of the Longwood Suite.