AN OUTLINE GEOCHEMISTRY OF RHYOLITE ERUPTIVES FROM TAUPO VOLCANIC CENTER, NEW-ZEALAND

Taupo is one of the most productive and frequently active rhyolite com plexes in the world. Its early history from 320 to ca. 65 ka is record ed by numerous lava domes with associated locally dispersed pyroclasti c deposits, together with two poorly welded ignimbrites of greater ext ent, but is not well constrained by radiometric age data. From ca. 65 to ca. 28 ka, dome-building activity began to be accompanied by increa singly powerful explosive events, culminating in the major phreatomagm atic Oruanui eruption at 22.6 C-14 ka (26.5 ka calibrated age), during which the structure of the modem volcano was established. Twenty eigh t eruptions are recognised in the post-Oruanui sequence, with ages fro m 20.5 ka to 1740 calibrated years B.P., of which 27 were explosive (m any with inferred or suspected accompanying effusive activity) and one purely effusive. Eruption products at Taupo are dominantly rhyolitic, and the ranges of Sr-87/Sr-86 (0.70514 to 0.70725) and epsilon Nd (-3 .1 to +2.0) preclude a common origin for all the rhyolites. The majori ty of the rhyolites can be grouped into magma types on the basis of mi neral assemblages, major-and trace-element compositions, and Sr and Nd isotopes. Domes erupted between 320 and ca. 65 ka are in total isotop ically and geochemically diverse, but often occur in small spatially a ssociated groups that share the same isotopic signature, suggesting a common source or magma chamber. Domes and widespread pyroclastics erup ted from ca. 65 ka to ca. 28 ka record the growth of an isotopically h omogeneous (Sr-87/Sr-86=0.70550-0.70559) magma chamber beneath the are a now occupied by the northern part of Lake Taupo. This chamber produc ed the ca. 400 km(3) (magma) Oruanui eruption at 26.5 ka. Development of this large chamber was accompanied by eruption of distinct magma ty pes in other geographic areas of the volcano. All magma erupted in the post-Oruanui sequence is compositionally distinct from any observed O ruanui magma (e.g., Sr-87/Sr-86 = 0.70597-0.70622). Compositional data allow eruptives from this sequence to be subdivided into four magma t ypes, one dacitic, forming the first three post-Oruanui eruptions betw een 20.5 and 17 ka, and three subtly distinct rhyolite compositions. D etailed stratigraphic and chronological controls reveal that these thr ee rhyolite types were erupted in three discrete periods from 11.8 to 9.95, 7.05 to 2.75 and 2.15 to 1.74 ka, and that compositional variati ons are stepwise not gradual. Ranges of chemical and isotopic composit ions within the products of individual post-26.5 ka magma eruptions ar e trivial compared to variations between the three groups. Taupo volca nic centre is both highly productive (ca. 6.5 km(3) ky(-1)) and freque ntly active, and in the latter characteristic represents an extreme st yle of behaviour among rhyolite volcanoes. This unusually high frequen cy of activity is paralleled by rapid, stepwise changes in the composi tions of the rhyolites erupted, that often appear to reflect tapping o f distinct batches of magma rather than evolution of a single graduall y-evolving magma chamber. New rhyolite magma bodies of volumes ca. 35 km(3) and ca. 400 km(3) were apparently generated in only about 1.8 ky and 40 ky, respectively, prior to caldera-forming eruptions at 1.77 a nd 26.5 ka.