Geochemistry and zircon U–Pb geochronology of Paleoproterozoic arc related granitoid in the Northwestern Yangtze Block and its geological implications
Geochemistry and zircon U–Pb geochronology of Paleoproterozoic arc related granitoid in the Northwestern Yangtze Block and its geological implications
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DOI：
10.1016/j.precamres.2011.12.015
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发表时间：
2012-04
影响因子：
3.8
通讯作者：
Y. Wu;Shan Gao;Hongfei Zhang;Jianping Zheng;Xiaochi Liu;Hao Wang;H. Gong;Lian Zhou;
中科院分区：
文献类型：
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作者：
Y. Wu;Shan Gao;Hongfei Zhang;Jianping Zheng;Xiaochi Liu;Hao Wang;H. Gong;Lian Zhou;
由于太古代至古元古代岩石露头稀少，扬子地块的早期历史尚未得到很好的约束。本文报道了扬子地块西北部后河杂岩中灰色片麻岩的锆石U-Pb年龄、主量元素和微量元素数据以及Sr-ND-Hf同位素组成的综合研究。片麻岩的锆石U-Pb定年结果为2081±9 Ma。这一时代被解释为后河杂岩的形成时代，因此后河杂岩代表了扬子地块西部发现的最古老的岩石。后河杂岩中灰色片麻岩的SiO_2含量为58.63%~68.59%，Na2O含量为3.88%~5.28%，Fe2O_3含量为2.86~6.69%，Al_2O_3含量为16.01~18.88%，MgO含量为0.97~2.65%。这些岩石的锶(149-390ppm)、铬(9.07-45.1ppm)和镍(4.97-21.3 ppm)含量较低，而Y(12.9-32.7ppm)和Yb(0.95-2.25ppm)含量较高。它们的特征是相对富含LILS和LREE，但富含HFSE。这些特征与钙碱性花岗岩的特征相似，表明它们的形成可能与俯冲相关的过程或先前存在的弧岩的重熔有关。汇编的太古代至古元古代锆石颗粒年龄谱显示，扬子地块西部和东部具有相似的太古代和早古元古代年龄范围，表明两部分都存在古老的大陆核。另一方面，扬子地块西部出露了大量约2050-2400 Ma的锆石颗粒，表明扬子地块西部可能存在古元古代具有活动型大陆边缘的微大陆。由于扬子地块太古代至古元古代岩石露头较差，目前还不可能确定这种活动边缘的确切性质。在扬子地块东部发现了约2.0Ga的孔兹岩和变质沉积岩，表明扬子地块东部可能存在被动型大陆边缘。约2.0Ga时，西部微大陆与东部的碰撞可能导致了最终的弧-陆碰撞和扬子地块的拼合。
The early history of the Yangtze Block has not been well constrained yet, due to the scarce outcrops of Archean to Paleoproterozoic rocks. In this study, we report an integrated study of zircon U–Pb age, major and trace element data and Sr–Nd–Hf isotope compositions for gray gneisses from the Houhe complex in the northwestern part of the Yangtze Block. Zircon U–Pb dating yields a weighted mean207Pb/206Pb age of 2081±9Ma for a gneiss. This age is interpreted as the formation age of the Houhe complex and thus the Houhe complex represents the oldest rocks found in the western part of the Yangtze Block. The gray gneisses from the Houhe complex range in SiO2contents from 58.63% to 68.59% and Na2O from 3.88% to 5.28%, and have relatively high Fe2O3contents of 2.86–6.69%, Al2O3of 16.01–18.88%, and MgO of 0.97–2.65%. These rocks show low Sr (149–390ppm), Cr (9.07–45.1ppm) and Ni (4.97–21.3ppm) contents, but high Y (12.9–32.7ppm) and Yb (0.95–2.25ppm). They are characterized by a relative enrichment in LILEs and LREEs, but a depletion in HFSEs. These features are similar to those of calc-alkaline granitoids, suggesting that their formation might be related to a subduction-related process or remelting of preexisted arc rocks. Compiled age spectra of Archean to Paleoproterozoic zircon grains reveal that the western and eastern parts of the Yangtze Block have similar Archean and early Paleoproterozoic age ranges, indicative of the occurrence of an old continental nucleus in both parts. On the other hand, there are large amounts of ca. 2050–2400Ma zircon grains revealed in the western part of the Yangtze Block, implying that there might be a microcontinent with an active-type continental margin during the Paleoproterozoic times in the western part of the Yangtze Block. Because of the poor outcrops of Archean to Paleoproterozoic rocks in the Yangtze Block, it is no possible to establish the exact nature of such an active margin yet. The occurrence of ca. 2.0Ga khondalitic and metasedimentary rocks has been documented in the eastern part of the Yangtze Block, implying that there might be a passive-type continental margin in the eastern part of the Yangtze Block. At ca. 2.0Ga, the collision between the western microcontinent and the eastern part may have resulted in the final arc-continental collision and the amalgamation of the Yangtze Block.