RELATIVE AGE AND FORMATION OF A PHREATOMAGMATIC DIATREME IN THE MANDAMUS IGNEOUS COMPLEX, NEW ZEALAND

New Zealand’s cretaceous igneous complexes provide unique insights into the early rifting processes of Gondwanaland. These complexes are only sparsely studied however, due to their small sizes and brief periods of activity. Adjacent to the Mandamus Igneous Complex (MIC) in northern Canterbury is the Glens of Tekoa Breccia (GTB), a genetically unclassified and undated volcanic deposit. Field observations along a transect of the breccia, supplemented by thin section and geochemical data extracted from collected samples, were used to reconstruct a relative chronology of igneous processes within the region.

Jointing surfaces dipping at 33° N run parallel throughout the cyndrillically outcropping breccia structure. Trachytic dikes cross cutting the outer margins of the breccia were observed and correlated with petrographically similar dikes crosscutting other units within the complex. The highly weathered breccia is polymict, with degassing structures identified within its matrix. Thin sections of the breccia show pyroxene and olivine phenocrysts enclosed in palagonitized tachylite (juvenile) clasts. Petrographic characteristics of xenoliths within the GTB were used to identify clast provenance. Sandstone clasts were correlated with the Pahau terrane, and volcanic clasts were traced to MIC lava flows. Major oxide weight percentages of xenolith clasts were found to match MIC compositions, further correlating the GTB and MIC.

We interpret the GTB deposit to be an exhumed phreatomagmatic diatreme structure. Within the context of the complex, these results contradict a previous narrative presented by Tappenden (2004). We postulate that the GTB is the source vent of a misidentified lahar deposit, rather than the vent of basaltic and trachytic lava flows in the complex as detailed by Tappenden. Additionally, using crosscutting relationships identified in the field, we suggest that the complex formed over a longer period of time, with multiple intrusive and extrusive phases.