The geochemistry of tholeiitic and alkalic plutonic suites within the Northland ophiolite, northern New Zealand; magmatism in a back arc basin

Abstract

The Northland ophiolite of New Zealand consists mainly of tholeiitic basalts with subordinate alkalic volcanic rocks. The alkalic rocks occur together with, but are generally younger than the tholeiitis. Both the tholeiitic and alkalic suites include minor plutonic intrusives which range from gabbro to plagiogranite. Variations within the tholeiitic plutonic suite can be modelled in terms of low pressure crystal fractionation. The evolution of the alkalic suite is more problematic, because each massif appears to be derived from a slightly different parental magma. New trace element chemistry presented in this study shows that the tholeiitic rocks formed from a mantle source that was compositionally modified by supra-subduction zone processes but that the alkalic suite has a different chemical signature. Nevertheless both suites of rocks are interpreted as having formed in a back arc spreading system. In this environment, the addition of fertile mantle to the supra-subduction wedge, facilitated by subduction roll-back, provided a source for the alkalic rocks with mixed geochemical characteristics.