Modelling magmatic trends in time and space: Eruptive and magmatic history of Tongariro Volcanic Complex, New Zealand

Abstract

Detailed mapping of volcanic stratigraphy in conjunction with 41 new K-Ar ages have been used to constrain a petrological model of Tongariro Volcanic Complex within a time-space framework. Over 400 samples form an extensive geochemical data base comprising whole-rock major and trace element data, together with whole-rock REE and isotopic analyses and mineral compositions for selected samples. These data have been used to investigate the complex interplay between magma batches on a variety of time scales and the time-space relationships of the subvolcanic plumbing system. This new integrated approach to the study of Tongariro has not been applied to many other composite volcanoes but is crucial in linking petrological models to realistic geological relationships. Tongariro is a large (c.60 km3), active, basaltic andesite to dacite composite cone complex situated near the southern end of the Taupo Volcanic Zone, New Zealand. A virtually continuous eruptive history of the Tongariro complex has been divided into 17 small (<2 km3) to large (12 km3) nested and overlapping volcano-stratigraphic or cone-forming units which represent the products (aa and block lava flows, welded scoria-spatter deposits, volcaniclastic tuff breccias) of many recurrent styles of eruptions from at least 30 discrete vents, active for varying periods of time (a few to tens of thousands of years) throughout Tongariro's known lifetime from c.275 ka to the present. Episodes of rapid cone growth have occurred at 210-200 ka (Tama 2 cone eruptive rate = 1 km3/ka) and 130-80 ka (NE Oturere, SW Oturere and Tongariro Trig cones under construction), with development of the complex most recently dominated by the rapid growth (0.9 km3/ka) of Ngauruhoe cone since 2.5 ka. There is no orderly time-space relationship between cone-building events; the locus of activity shifted non-systematically over the lifetime of the complex within a 13 km-long and 5 km-wide SW-NE aligned vent corridor. Tongariro eruptive products vary in composition almost continuously from 53.0 to 64.2 wt% SiO2 and 1.1 to 9.2 wt% MgO, forming a calc-alkaline, medium-K suite. Two-pyroxene andesites are volumetrically dominant, but hornblende is a significant phase confined to the older southern cones of Tama 1 and 2 and Pukekaikiore, and olivine is particularly prominent in the young (post-25 ka) eruptives, suggesting an overall time-space relationship with petrography and mineralogy. Tongariro rocks exhibit features typical of subduction-related magmas such as light-REE enriched patterns ([Ce/Yb]N=1.8-3.9), relatively low high field strength abundances (e.g. Nb=2.7-6.7), and strongly spiked patterns on incompatible element spider diagrams. A wide range in 87Sr/86Sr (0.704442 - 0.706193) is accompanied by less variability in 143Nd/144Nd (0.512629-0.512862), Pb isotopic ratios (206Pb/204Pb= 18.781-18.854, 207Pb/204Pb= 15 .594-15.645, 208Pb/204Pb=3 8.5 88- 38.802), and δ 18O (+6.16-6.59). Variation diagrams for the Tongariro cone-forming units reveal major diversity in the absolute elemental abundances for given SiO2 contents, in the length, steepness and shape of chemical trends, and in the distribution of chemical groups within compositional space. However, certain volcano-stratigraphic units (e.g. the amphibole-bearing andesites of the older southern cones) do share similar patterns of chemical ordering which suggest derivation from a common 'parental' magma reservoir and a similar early history. Nonetheless, considerable chemical variability in closely-related samples is common, and provides strong evidence for a higher level influence on magma compositions. Nonsystematic relationships with age (compositional breaks, reversals and loops) along geoche mical trends on variation diagrams and chemical stratigraphy plots cannot be modelled by fractional crystallization. These coincide with petrographic and mineralogical evidence for disequilibrium including plagioclase sieve textures, strong reverse zoning, coexisting high-temperature and low-temperature phases exhibiting reaction rims or corrosion, olivine phenocrysts with much higher Fo contents than expected from wholerock Mg#, and bimodal or very widely ranging crystallization temperature estimates for some samples. These features suggest the involvement of multiple small batches of magma, rising from the various 'parental' reservoirs by differing ascent paths, within a complex subvolcanic plumbing system which has allowed varying degrees of interaction and magma mixing. As indicated by the presence of quartzite xenoliths of the Torlesse metasedimentary basement, these magmas have also interacted with the crust, which represents an important means of introducing often wide compositional diversity of magmas on time scales as short as decades or centuries. A considerable variation in incompatible trace element ratios (e.g. Ba/Zr, Nb/Ta, La/Yb, Zr/Hf) and radiogenic isotopic compositions (e.g. Ngauruhoe 87Sr/86Sr ratios increase from 0.705470 in 1954 to 0.706165 in 1975) for closely-related samples, and the marked scatter of data evident on assimilation-fractional crystallization (AFC) process diagrams indicate that magmas have not evolved along a unique AFC trend but can more aptly be characterised as forming a 'family' of AFC trends or perhaps a series of 'one-off AFC 'events' produced by contamination of numerous small and frequent Tongariro magma batches with varying amounts of compositionally heterogeneous Torlesse crust. This investigation of chemically and isotopically diverse lavas related on time scales of 100 years (historic Ngauruhoe eruptives), 1000 years (Ngauruhoe, Red Crater and Te Mari Crater vents) and 10 000 years (0-10 ka young eruptives, 120-130 ka NE Oturere, 200-210 ka Tama 2) has demonstrated that the observed compositional variability is mainly developed at comparatively high crustal levels within a complex subvolcanic plumbing system by fractional crystallization and the mixing of numerous, small (<<0.1 km3 ), short-lived (c. 1 ka) magma batches coupled with variable amounts of crustal assimilation of a compositionally diverse Torlesse metasedimentary contaminant. These processes have been operating to similar effect throughout the lifetime of the Tongariro complex, although subtle differences in geochemical patterns suggest that the regularity or rate of magma batch production may have varied slightly. The young Ngauruhoe cone has been highlighted as a cameo study for which the well-constrained stratigraphy, enabled by the preservation of numerous young lava flows erupted over the last thousand years, provides a unique opportunity for detailed petrological modelling paying careful attention to age relationships. Five geochemically and isotopically distinct groups of magma reflect variable crustal contamination of many discrete magma batches within several independent shallow plumbing systems beneath Ngauruhoe.