The Petrology and Geochemistry of a Suite of Intrusive Igneous Rocks from the Southern Kermadec Arc-Havre Trough, SW Pacific
The study of intrusive igneous rocks can provide insights into deep crustal processes. In active intraoceanic arc environments, the opportunity to study of these intrusive igneous rocks usually comes from xenoliths entrained within eruptive products, as accessibility to in situ intrusive rocks is limited. This thesis documents a suite of the first intrusive samples dredged from the Havre Trough, which provide insights into deep magmatic processes in this intraoceanic back-arc system. The suite of ten intrusive igneous rocks were dredged from Basin E, a back-arc basin (BAB) in the Kermadec Arc-Havre Trough (KAHT) and consist of in situ gabbroic meso- to orthocumulates. Four representative samples were selected from the suite of ten on the basis of grainsize, and from them a petrogenetic model was built to determine the associations of the samples within the magmatic system of the region. The four samples all exhibit comparable mineral assemblages of plagioclase, clinopyroxene, and magnetite, with olivine and orthopyroxene absent. Texturally the samples appear to have formed in (a) magma chamber(s) where the minerals cooled slowly and formed relatively large, euhedral crystals that trapped interstitial melt between them. The interstitial melts crystallised forming more evolved intercumulus material (plagioclase + quartz ± amphibole ± apatite). Three of the four samples have coarser grainsizes (1-2 mm), and exhibit similar magnetite temperature estimates, indicating that they formed from similar melts. The other sample has a finer grainsize (<1 mm), and exhibits lower temperature estimates, indicating that this sample formed from a lower temperature, faster cooling melt. Plagioclase compositions follow a similar trend to plagioclase phenocrysts from modern back-arc volcanoes which indicates that these samples have an association with the modern magmatic system rather than the now extinct Miocene (Colville) Arc. Clinopyroxene trace element data are also consistent with these samples being associated with the modern subduction system. The magma chamber(s) that the samples formed in, comes from a mid-lower crustal depth, 3-6 km based on pressure estimates from amphibole crystal chemistry. The exposure of rocks from this depth would have been facilitated by normal faulting associated with rifting and opening of the Havre Trough. Petrologic and geochemical analyses of these cumulates suggest that the deep, back-arc basins consist of entirely new magmatic material formed from BAB volcanism, with no evidence for pre-existing crust.