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

Taupo is one of the most productive and frequently active rhyolite complexes in the world. Its early history from 320 to ca. 65 ka is recorded by numerous lava domes with associated locally dispersed pyroclastic deposits, together with two poorly welded ignimbrites of greater extent, but is not well constrained by radiometric age data. From ca. 65 to ca. 28 ka, dome-building activity began to be accompanied by increasingly powerful explosive events, culminating in the major phreatomagmatic Oruanui eruption at 22.6 C-14 ka (26.5 ka calibrated age), during which the structure of the modem volcano was established. Twenty eight eruptions are recognised in the post-Oruanui sequence, with ages from 20.5 ka to 1740 calibrated years B.P., of which 27 were explosive (many 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 majority of the rhyolites can be grouped into magma types on the basis of mineral assemblages, major-and trace-element compositions, and Sr and Nd isotopes. Domes erupted between 320 and ca. 65 ka are in total isotopically and geochemically diverse, but often occur in small spatially associated groups that share the same isotopic signature, suggesting a common source or magma chamber. Domes and widespread pyroclastics erupted from ca. 65 ka to ca. 28 ka record the growth of an isotopically homogeneous (Sr-87/Sr-86=0.70550-0.70559) magma chamber beneath the area now occupied by the northern part of Lake Taupo. This chamber produced the ca. 400 km(3) (magma) Oruanui eruption at 26.5 ka. Development of this large chamber was accompanied by eruption of distinct magma types in other geographic areas of the volcano. All magma erupted in the post-Oruanui sequence is compositionally distinct from any observed Oruanui magma (e.g., Sr-87/Sr-86 = 0.70597-0.70622). Compositional data allow eruptives from this sequence to be subdivided into four magma types, one dacitic, forming the first three post-Oruanui eruptions between 20.5 and 17 ka, and three subtly distinct rhyolite compositions. Detailed stratigraphic and chronological controls reveal that these three 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 variations are stepwise not gradual. Ranges of chemical and isotopic compositions within the products of individual post-26.5 ka magma eruptions are trivial compared to variations between the three groups. Taupo volcanic centre is both highly productive (ca. 6.5 km(3) ky(-1)) and frequently active, and in the latter characteristic represents an extreme style of behaviour among rhyolite volcanoes. This unusually high frequency of activity is paralleled by rapid, stepwise changes in the compositions of the rhyolites erupted, that often appear to reflect tapping of distinct batches of magma rather than evolution of a single gradually-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 and 26.5 ka.