GEMOC ARC National Key Centre

Richard C. Price¹, Ian E. M. Smith², Anthony Reay³, and Richard J. Arculus⁴

1. School of Science and Technology, The University of Waikato, Hamilton, New Zealand.

2. Department of Geology, University of Auckland, Auckland, New Zealand.

3. Department of Geology, University of Otago, Dunedin, New Zealand.

4. GEMOC ANU

Petrological research concerned with andesitic volcanism is seldom linked with work on I-type granite complexes and yet at least some intrusive complexes must represent intracrustal magmatic systems that at one time underlay volcanic arcs. We report here some of the outcomes of a study of active andesitic volcanoes in the North Island of New Zealand and seek to integrate this information with data we have obtained for a Permian-Triassic aged intrusive complex in the South Island of New Zealand.

The largest, active, andesitic volcano in New Zealand is Ruapehu. At 2797 m it is also the highest mountain in the North Island. It has been active over at least 250 ka years, with the most recent eruption occurring during the period September to June 1996. The magmatic history of the volcano is preserved in laharic deposits and tephra units making up an extensive ring plain (Donaghue et al., 1995) and in lava flow sequences of a complex central cone (Hackett, 1985). Ruapehu eruptives show a change with time to progressively more potassic compositions. The isotopic compositions also show temporal changes, and this is interpreted to reflect an increasing involvement of crustal material. With time Ruapehu magmas have become more evolved and more variable in terms of overall geochemistry and isotopic compositions. Collectively, data for samples from the volcano show a rough correlation between 87Sr/86Sr isotopic ratios and SiO2 abundance and this has been interpreted (eg. Graham and Hackett, 1987) to reflect assimilation of crustal material, accompanied by crystal fractionation - assimilation crystal fractionation or AFC.

Detailed mapping of lava flows on Ruapehu has defined complex stratigraphic sequences and geochemical data have been collected to examine the fine scale petrological variation occurring within these. A 400m thick section of flows is exposed along the upper Whangaehu gorge on east Ruapehu and, within this section several lava flow sequences each containing three or four conformable flows have been distinguished. Geochemical variation within these sequences shows an overall cyclic pattern from relatively low SiO2 contents to higher values with time and, although more variable, strontium isotopic compositions tend to show a similar pattern. The data are consistent with a model whereby magmas evolve within and erupt through a system of dykes probably located a few kilometres below the volcano. Eruption of discrete flow packages is possibly associated with recharge of dykes with fresh magma. Variation within the packages probably reflects crystal fractionation and mixing between fresh, recharging magma and magma remaining in the dyke system from previous recharge/fractionation events. Recharging magma batches vary considerably in composition, are all geochemically evolved, and show evidence for interaction with crust. They probably evolve in a deep magma reservoir located within the lower or middle crust.

Intrusive rocks, ranging in composition from ultramafic cumulates through gabbros and quartz diorites to granites, are exposed along the Southland coast at the southern end of the Longwood Ranges, in the south of New Zealand's South Island (Price and Sinton, 1978). All rocks show the trace element characteristics typical of subduction-related magmas and they are believed to represent the remnants of a Permian- Triassic volcanic arc (Kimborough et al., 1994). Dykes are abundant within the complex and many of these are composite with compositions ranging from dolerite through andesite to dacite. Within some dykes, pillow like mafic material is contained within an envelope of more felsic rock and the margins of the pillows commonly show what appear to be chilled margins. Intermediate compositions within these dykes have compositions consistent with a derivation by mixing between mafic and felsic components. Dykes of this type could represent the feeder conduits for andesitic volcanoes that once overlay the intrusive complex. The felsic component in the dykes could represent either a residual evolved magma left from an earlier recharge event or melt derived by anatexis of the host quartz diorite

Dioritic rocks contain mafic enclaves in abundance and these are most common in complex zones marking the boundaries with gabbroic rocks. Enclaves of this type are commonly argued to represent mafic magma blobs that have mingled or mixed with the host (eg. Didier, 1973; Vernon et al., 1988). Enclaves from the Longwoods quartz diorites have very distinctive and uniform compositions. They are enriched in rare earth elements (REE) relative to their hosts and the chondrite normalised REE patterns are characterised by depletion of the light relative to intermediate REE. They all show distinctive Eu depletions and they are all relatively depleted in Ni compared to other rocks with similar SiO2 contents. They appear to be derived by crystal fractionation from magmas represented by the gabbroic rocks of the complex and they were probably incorporated when quartz diorite magmas were intruded into and disrupted evolving basaltic magma chambers. If the quartz diorites are mixed magmas, then the enclaves cannot represent the mafic component of this mixing trajectory.
 
 
 
 

REFERENCES

Didier, J, 1973. Granites and their enclaves: The bearing of enclaves on the origin of granites. Developments in Petrology 3, 393 pages.

Donoghue, S L, Neall, V E, and Palmer, A S, 1995. Stratigraphy and chronology of late Quaternary andesitic tephra deposits, Tongariro Volcanic Centre, New Zealand. Journal of the Royal Society of New Zealand 25, 115-206.

Graham, I J, and Hackett, W R, 1987. Petrology of calc-alkaline lavas from Ruapehu Volcano and related vents, Taupo Volcanic Zone, New Zealand. Journal of Petrology 28, 531-567.

Hackett, W R, 1985. Geology and petrology of Ruapehu Volcano and related vents. Unpublished PhD Thesis, Victoria University of Wellington, New Zealand.

Kimborough, D L, Tulloch, A J, Coombs, D S, Landis, C A, Johnston, M R, and Mattinson, J M, 1994. Uranium-lead zircon ages from the Median tectonic Zone, New Zealand. New Zealand Journal of Geology and Geophysics 37, 393-419.

Price, R C, and Sinton, J M, 1978. Geochemical variations in a suite of granitoids and gabbros from Southland, New Zealand. Contributions to Petrology and Geochemistry 67, 267-278.

Vernon, R H, Etheridge, M E, and Wall, V J, 1988. Shape and microstructure of microgranitoid enclaves: indicators of magma mingling and flow. Lithos 22, 1-11.