PB-SR-ND-O ISOTOPIC CONSTRAINTS ON THE ORIGIN OF RHYOLITES FROM THE TAUPO VOLCANIC ZONE OF NEW-ZEALAND - EVIDENCE FOR ASSIMILATION FOLLOWED BY FRACTIONATION FROM BASALT

A comprehensive Sr-Nd-Pb-O isotopic study is reported for rhyolites from the Maroa Volcanic Centre in the Taupo Volcanic Zone (TVZ) of New Zealand. The Sr-Nd isotopic compositions of the rhyolites (Sr-87/Sr-86=0.705236 to 0.705660 and epsilon(Nd)=2.0 to 0.2) are intermediate between those of primitive basalts (Sr-87/Sr-86 = 0.70387 and epsilon(Nd) = 5.3) and the Torlesse basement (Sr-87/Sr-86 = 0.709 and epsilon(Nd) = -4.5). The relatively low 'mantle-like' oxygen isotopic compositions of deltaO-18 = 7 +/- 0.5 are consistent with the Nd-Sr isotopic constraints in that they can be accounted for by approximately 15% to 25% crustal contamination of a basaltic parent by relatively deltaO-18-rich Torlesse metasediment. High precision Pb isotopic analyses of plagioclase separates from the Maroa rhyolites show that they have essentially the same compositions as the Torlesse metasedimentary terrane which is itself distinctive from the Western or Waipapa metasediments. Due to the high concentration of Pb in the Torlesse metasediments ( > 20 ppm) compared to the basalts (< 2 ppm), the Pb isotopic composition of the volcanics may be controlled by relatively small amounts (> 10%) of crustal contamination. All these results are shown to be consistent with derivation of the rhyolites by approximately 15% to 25% contamination of relatively primitive basaltic magmas with Torlesse metasedimentary crust, followed by extensive, essentially closed system fractionation of the 'basalt' to a magma of rhyolite composition. It is argued that the processes of assimilation and fractionation are separated in both space and time. The voluminous high silica rhyolites, which make up >97% of the exposed volcanism in the continental margin back-arc basin environment of the TVZ, therefore appear to be a product of predominantly new additions to the crust with assimilation-recycling of pre-existing crust being of secondary importance.