Petrochemical and Sr Isotopic Studies of Lavas and Xenoliths from Tongariro Volcanic Centre - Implications for Crustal Contamination of Calc-Alkaline Magmas

Abstract

Petrogenesis of Tongariro Volcanic Centre lavas, particularly those from Mount Ruapehu and nearby vents, is investigated through a detailed petrochemical and Sr isotope study. The importance and nature of crustal contamination as a process is assessed from metasedimentary xenoliths and their relationship to local sedimentary basement lithologies. These results are tested by least squares modelling of processes such as crystal fractionation, crystal accumulation, combined assimilation and fractional crystallisation (AFC) and magma mixing.

Sedimentary basement lithologies near the TVC provide a guide to the composition of potential crustal contaminants and a genetic link to some xenoliths. The rocks are of three main types (i) Torlesse terrane greywackes and argillites, (ii) Waipapa terrane greywackes· and (iii) Late Tertiary marine sandstones, siltstones and conglomerates.

Torlesse terrane flysch sediments, which form the Kaimanawa ranges on the eastern margin, have dominantly granitic bulk compositions and comprise a chemical continuum between Si-, Na-, Sr-rich greywackes and Al-, Fe- and K-rich argillites. A Rb-Sr whole-rock isochron age for the latter of 141 (3) Ma is interpreted as the time of low-grade metamorphism; similar rocks from 0taki Forks, north of Wellington yield an age of 182 (13) Ma which is some 40 Ma younger than the depositional age as inferred from fossil evidence. These data suggest that metamorphism and uplift of Torlesse terrane sequences are local events unrelated to major phases of the Rangitata Orogeny.

Waipapa terrane greywackes occur to the NW and west of the TVC. These have intermediate calc-alkaline chemistries and low Sr isotopic ratios (.70500 to -70850) which give a whole-rock isochron age of 205 (17) Ma. By comparison with a similar rock suite from Coffs Harbour Block, northern N.S.W., Australia, this age is interpreted to be the timing of low-grade (prehnite-pumpellyite facies) metamorphism.

Late Tertiary marine sediments form a thin veneer over older basement on the margins of TVC. These rocks are chemically similar to Torlesse terrane metasediments and have Sr isotopic compositions ranging between .707 and .710.

Xenoliths of metasedimentary, igneous and metaigneous origin occur in most TVC lavas and are abundant in some. They are classified according to petrography and presumed origin, and are of six main types: (1) Upper crustal xenoliths (TYPE UCX) include porcellanite, metagreywacke and calcsilicate and can be related to known sedimentary basement on the basis of mineralogy, bulk-rock chemistry and Sr isotopic composition (2) Vitrified xenoliths (TYPE VX) occur only in Ngauruhoe 1954 and Pukeonake lavas and are of two chemically distinct types. Of these, TYPE VX.a xenoliths are chemically similar to Torlesse terrane metasediments and usually contain more than 50% glass, representing advanced partial melting. Retention of their original bulk-rock chemistry implies derivation from shallow depths and rapid transport to the surface. TYPE VXb xenoliths are less vitrified and are chemically different from any known basement lithology (3) Quartz-rich xenoliths (TYPE QX) are conspicuous and abundant in most lavas and are mainly quartzo-feldspathic gneisses (TYPE QXa) or quartzites (TYPE QXb). The latter probably represent TYPE QXa xenoliths modified by extraction of partial granitic melt and subsequent recrystallisation. Both are interpreted to be restite assemblages derived initially from greywacke-gneiss (probably from the Torlesse terrane). Several rare TYPE QX xenoliths (TYPES QXc to QXf) with unusual mineral assemblages and obscure origins also occur (4) Quartz-poor xenoliths (TYPE QPX) have biotite-, spinel-rich or feldspar-rich assemblages and are interpreted to be restites after partial melting micaceous layers of greywacke-gneiss (5) Igneous of the feldspathic and xenoliths (TYPE IX), include variably altered blocks of surface volcanics, a natroalunitebearing nodule and a variety of cognate cumulate nodules and show little evidence of pyrometamorphism or of an extended history (6) Meta-igneous xenoliths (TYPE MIX) have broadly calc-alkaline chemistries but are texturally, mineralogically and isotopically different from host lavas. Some (TYPE MIXa) are coarse-grained with high Cr and Ni contents and may be basic cumulates. Others (TYPES MIXb and MIXc) are finer-grained and are chemically similar to low-K orogenic andesites. All may have originated from the base of the continental crust and represent the original oceanic crust on which the Torlesse terrane was deposited.

Cation exchange equilibria pertaining to certain key assemblages indicates that equilibration of most xenoliths occurred at temperatures of 800 °C to 1000 °C (reliable pressure estimates are unattainable). The occurrence of granitic partial melt in some xenoliths and the dominance of quartz-rich and quartz-poor xenolith types indicate that crustal contamination (by assimilation of partial melt) is a widespread phenomenum.

Ruapehu lavas and those of nearby vents are dominantly calc-alkaline, medium-K andesites. They are porphyritic with phenocrysts of plagioclase, augite, olivine (mainly in basalts and basic andesites), orthopyroxene (mainly in acid andesites and dacites) and titanomagnetite (or chromian spinel in basic lavas). Hydrous minerals are rare. The lavas can be catagorised into six petrographically and chemically distinct groups: TYPE are plagioclase-pyroxene-rich and are the dominant type, being represented by all Ruapehu Group Formations, Red Crater basalt and Ngauruhoe 1954 andesite. Compositions range from basalt to dacite and show decreasing Fe, Mg, Ca, Cr, Ni, constant Ti, Al, Na, Sr, increasing LILE and increasing 87 sr/86 sr with increasing silica. TYPE 2 are characterised by high modal plagioclase and TYPES 3 & 4 by high mafic mineral contents, but each is otherwise similar to TYPE 1. TYPE 5 lavas are olivine andesites from Pukekaikiore, Ohakune and Hauhungatahi. They characteristically contain no plagi.oclase phenocrysts and have high Mg, Ca and Sr contents and low 87sr/86sr ratios. TYPE 6 lavas show disequilibrium features (such as strongly reversed-zoned phenocrysts) which are normally considered evidence for magma mixing. All have high Cr and Ni contents, and low 87sr/86sr ratios.

Potential parental magmas for TVC lavas include high-alumina basalts restricted to the extensional zone of rhyolitic volcanism in Taupo Volcanic Zone, low-alumina basalts occurring at the southern end of the extensional zone and directly associated with andesites and one example of magnesian quartz tholeiite (Waimarino basalt). The latter, from east of Lake Taupo, has primary chemical characteristics but is highly porphyritic and contains quartzose xenoliths. Compositional data indicate that neither low-alumina nor high-alumina basalt can be generated directly from tholeiite by any reasonable process and therefore each represents a distinct magma type.

Petrochemical and isotopic data of TVC lavas and xenoliths provide an excellent framework for petrogenetic modelling. Least squares analysis shows that evolved TYPE 1 lavas can be generated from low-alumina basalt (e.g. Ruapehu basalt) or from less-evolved TYPE lavas by AFM, involving FOAM fractionation plus assimilation of granitic partial melt of greywackegneiss. Additional selective contamination may be required to explain the high Sr isotopic ratios of some lavas (e.g. Ngauruhoe 1954). TYPE 2 lavas can be generated from TYPE either by FOAM fractionation (where plagioclase removal is suppressed) or, better, by plagioclase addition. TYPE 3 can be generated from TYPE 1 by olivine+ clinopyroxene addition. However, their higher LILE and lower 87sr/86sr ratios may rather suggest an alternative genesis from an unknown parent. TYPE 5 lavas can be generated from a Waimarino basalt-type parent by FOAM fractionation without addition of a crustal contaminant. The somewhat higher 87sr;86sr ratio of Waimarino basalt indicates that although tholeiitic basalt has occurred in small amounts throughout the history of the TVC, its isotopic composition (and

LILE content) has varied with time. TYPE 4 are chemically and isotopically intermediate to TYPES 3 and 5 and, although internally consistent these lavas cannot be easily generated from any known basalt type (Red Crater basalt gives the best-fit model). For TYPE 6 lavas, the high Cr and Ni contents and low 87sr/86sr imply that a Waimarino basalt-type parent must be one endmember; best-fit models are achieved when Mangawhero Formation dacite is the other.

Each petrogenetic model is consistent with petrographic, chemical and Sr isotopic constraints. They show that it is feasible to generate most Ruapehu lavas from low-alumina basalt by processes of crystal fractionation with or without crustal assimilation. However, some spatially and volumetrically restricted lava types are better derived from a more tholeiitic parent by crystal fractionation or hybridisation with dacite.