Pb-Nd Isotopes Indicate the Origin of Island Arc Terranes in the Early Paleozoic Pacific.

Abstract

The Takaka Terrane in New Zealand is one of the best exposed arc fragments of the early Paleozoic Australian-Antarctic convergent margin and constitutes one of the most outboard terranes of this margin in paleogeographic reconstructions. Pb-Nd isotope compositions of clinopyroxenes from the Cambrian Devil River Volcanics of the Takaka Terrane enable identification of the location of the terrane in the Paleo-Pacific Ocean. The Devil River Volcanics, a suite of primitive arc and back-arc rocks, are interbedded with the partly continent-derived Haupiri Group sediments. Extremely radiogenic Pb and unradiogenic Nd compositions in the arc rocks cannot be explained by assimilation of the Haupiri Group sediments or a continental basement of such a composition. Pb isotope compositions of the Takaka Terrane sediments are much less radiogenic and overlap with crustal compositions of the Lachlan Fold Belt in Australia, suggesting that both units are derived from one source, the Australian-Antarctic Pacific margin. Pb-Nd isotope compositions in the Devil River Volcanics reflect contamination of their mantle sources by subducted sediments derived from Archean provinces in either Antarctica or Laurentia. Both provinces show characteristically high 207Pb/204Pb500 and were located at the Pacific rim in the Cambrian. Mixing between mantle and Proterozoic continental material from present western South America or eastern Laurentia cannot explain the high 207Pb/204Pb500 in the New Zealand rocks. As in New Zealand, extreme spreads in Pb-Nd isotope compositions in other Cambrian volcano-sedimentary sequences in southeast Australia and Tasmania can be explained by the same model, suggesting that all these fragments originated along the Australian-Antarctic Gondwana margin. Pb isotope compositions of arc rocks, therefore, provide a new tool for terrane analysis in the early Paleozoic Pacific ocean.