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.

中文翻译：

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Pb-Nd同位素表明早古生代太平洋岛弧地体的起源

新西兰的 Takaka 地体是早古生代澳大利亚-南极会聚边缘出露最好的弧形碎片之一，是古地理重建中该边缘最外侧的地体之一。来自 Takaka 地体寒武纪魔鬼河火山的单斜辉石的 Pb-Nd 同位素组成能够确定该地体在古太平洋中的位置。魔鬼河火山是一套原始弧和弧后岩石，与部分源自大陆的 Haupiri 群沉积物互层。弧形岩石中极放射成因的铅和非放射成因的 Nd 成分不能用 Haupiri 群沉积物的同化作用或这种成分的大陆基底来解释。Takaka Terrane 沉积物的 Pb 同位素组成的放射成因要少得多，并且与澳大利亚拉克兰褶皱带的地壳组成重叠，这表明这两个单元都来自一个来源，即澳大利亚-南极太平洋边缘。魔鬼河火山中的 Pb-Nd 同位素组成反映了来自南极洲或劳伦西亚太古宙省的俯冲沉积物对其地幔来源的污染。这两个省都显示出 207Pb/204Pb500 的特征性高值，并且都位于寒武纪的太平洋边缘。来自现在南美洲西部或劳伦西亚东部的地幔和元古代大陆物质之间的混合不能解释新西兰岩石中的高 207Pb/204Pb500。和新西兰一样，澳大利亚东南部和塔斯马尼亚的其他寒武纪火山沉积序列中 Pb-Nd 同位素组成的极端扩散可以用相同的模型来解释，这表明所有这些碎片都起源于澳大利亚-南极冈瓦纳大陆边缘。因此，弧形岩石的铅同位素组成为早古生代太平洋地体分析提供了新的工具。