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

Mary-Elizabeth Connors Samuel Hampton Dept. of Geology and Environmental Geosciences at Lafayette College and Dept of Geological Sciences at University of Canterbury Lafayette College 730 High St, Easton, PA 18042, USA University of Canterbury 20 Kirkwood Ave, Upper Riccarton, Christchurch 8041

Located in the Canterbury foothills of New Zealand’s Southern Alps is a cylindrically outcropping volcanic breccia deposit hundreds of meters in diameter, called the Glens of Tekoa Breccia. The massive, polymict breccia is surrounded by sandstones and argillites of the Pahau Terrane, which is offset from the Mandamus Igneous Complex (MIC) by the Island Hills Fault. The complex formed in the Cretaceous as rifting of the NZ-Gondwana margin began, with evidence of both plutonic and volcanic phases of activity. No prior research has proven the ambiguous association between the complex and breccia, or the genetics of the deposit itself. Field observations made along a transect of the breccia revealed macro and microstructures indicative of emplacement mechanisms, and trachytic dikes found crosscutting the outer margins of the breccia and the complex have constrained its relative timing. Samples were collected for thin section and major oxide geochemical analyses in an effort to link the breccia with the MIC. These analyses showed that the breccia clasts share mineralogical and chemical characteristics, implying the same magmatic source, but a different eruptive style. The geometry and texture of the breccia deposit support an interpretation that this feature is an exhumed phreatomagmatic diatreme of a maar volcano, forming from the post-eruptive collapse of overlying strata mixing with juvenile eruptives. The identification of this feature and its relationship to the complex challenge the previous interpretation of the breccia pipe as a central vent associated with lava flows in the complex. We argue that this feature represents a separate, hydromagmatic phase during the later development of the MIC. Furthermore, fieldwork conducted in this study questions the notion that the complex was initially volcanic, transitioning to purely plutonic. Instead, we argue in favor of a more complex sequence of pulses at multiple vents over a much longer time period.


Additional Abstract Information

Presenter: Mary Connors

Institution: Lafayette College

Type: Poster

Subject: Geography & Earth Sciences

Status: Approved


Time and Location

Session: Poster 7
Date/Time: Fri 1:20pm-2:20pm
Location: University Center Ballroom - Tripod 49 Side A