3.45Ga granitic gneisses from the Yangtze Craton, South China: Implications for Early Archean crustal growth
3.45Ga granitic gneisses from the Yangtze Craton, South China: Implications for Early Archean crustal growth
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DOI：
10.1016/j.precamres.2013.12.018
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发表时间：
2014-03
影响因子：
3.8
通讯作者：
Jing‐Liang Guo;Shan Gao;Y. Wu;Ming Li;Kang Chen;Zhaochu Hu;Zhengwei Liang;Yongsheng Liu;
中科院分区：
文献类型：
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作者：
Jing‐Liang Guo;Shan Gao;Y. Wu;Ming Li;Kang Chen;Zhaochu Hu;Zhengwei Liang;Yongsheng Liu;
采用西姆斯、LA-ICP-MS和LA-MC-ICP-MS等方法测定了扬子区孔岭地体两个花岗质片麻岩的锆石U-Pb-Lu-Hf-O同位素组成，并对两个样品进行了全岩主量元素和微量元素分析。西姆斯和LA-ICP-MS数据揭示了两种片麻岩相似的5个锆石年龄组，分别为3.4、3.3、2.9、2.7和2.0 Ga。根据CL图像，将3.4 Ga锆石分为岩浆A组、变质B组和增生C组。这三个组具有不可区分的年龄和Th/U比值。A组和B组的176 Hf/177 Hf（t）值完全相同，而C组的锆石太薄，无法用LA-ICP-MS分析。两个样品的年龄一致性为98-102%，加权平均西姆斯年龄为3434.3 ± 9.6 Ma（2σ，MSWD = 13，n= 8），B组为3446.0 ± 8.8 Ma（2σ，MSWD = 10.7，n = 15），C组为3479 ± 26 Ma（2σ，MSWD = 0.49，n = 2）。A组和B组的上截距年龄为3457 ± 14 Ma（2σ，MSWD = 0.85，n = 23）。LA-ICP-MS测得A组和B组的加权平均年龄分别为3442 ± 19 Ma（2σ，MSWD = 0.17，n = 7）和3435 ± 11 Ma（2σ，MSWD = 0.44，n = 16）。上截距年龄为3443 ± 13 Ma（2σ，MSWD = 0.63，n = 23）。这些西姆斯和LA-ICP-MS年龄是一致的。我们认为，A、B组的西姆斯和LA-ICP-MS年龄是花岗岩岩浆作用及其后续变质作用的最佳估计。受年代误差的限制，变质作用应发生在花岗质岩浆作用之后的几千万年内。因此，这两种花岗质片麻岩代表了目前已知的中国南方最古老的岩石。它们比前人报道的3300 Ma奥长质片麻岩早150 Ma。3.4Ga的锆石显示出近变质的Hf（t）（−0.7 ± 1.0，2σ，MSWD = 1.14，n = 8），低于同时代的亏损地幔值。这表明花岗岩浆中含有先存大陆地壳的物质。它们的δ ~（18）O值（6.1-6.4‰）高于地幔，表明它们与地表水有过相互作用。3.4Ga锆石的地壳形成年龄（TDM 2）为3.9 ~ 3.6Ga，加权平均值为3703 ± 27 Ma（2σ，MSWD = 1.05，n = 7）。研究结果支持了前人关于扬子陆壳可能包含3.8Ga大陆地壳的观点，在较年轻的年龄组中，3.3Ga锆石的176 Hf/177 Hf（t）和δ 18 O值与3.4Ga锆石相似，表明它们是由3.4Ga锆石蚀变而来的。两个样品中的2.9Ga和2.7Ga锆石均为稀有的岩浆成因锆石。它们的176 Hf/177 Hf（t）比值与3.4Ga锆石不同，表明它们的源区不同。这两个年龄组与2.9Ga的奥长英云闪长质花岗闪长质岩浆活动和2.7Ga的A型花岗质岩浆活动相一致。2.0Ga变质锆石无论是整合还是不整合，其176 Hf/177 Hf（t）比值与2.7Ga变质锆石的176 Hf/177 Hf（t）比值重叠，表明它们是同一源区。而2.0Ga锆石的δ 18 O变化很大，与年龄一致性呈正相关。低δ 18 O（低至3.1‰）则需要与热液流体的相互作用。这些结果表明，至少有一些2.0 Ga的锆石可能已从2.7 Ga的锆石热液流体的改变。
Zircon U–Pb–Lu–Hf–O isotopic compositions of two granitic gneisses from the Kongling Terrain in the Yangtze Craton, South China were determined by SIMS, LA-ICP-MS and LA-MC-ICP-MS. Whole rocks of the two samples were analyzed for major and trace element compositions. The SIMS and LA-ICP-MS data reveal similar five zircon age groups of 3.4, 3.3, 2.9, 2.7, and 2.0 Ga for both gneisses. Three groups (magmatic Group A, metamorphic Group B, and overgrowth Group C) of the 3.4 Ga zircons were identified based on their CL images. These three groups have indistinguishable ages and Th/U ratios. Groups A and B show identical176Hf/177Hf (t), although Group C was too thin to be analyzed by LA-ICP-MS. Taken together, zircons from the two samples with 98–102% age concordance give weighted average SIMS ages of 3434.3 ± 9.6 Ma (2σ, MSWD = 13,n= 8) for Group A, 3446.0 ± 8.8 Ma (2σ, MSWD = 10.7,n= 15) for Group B, and 3479 ± 26 Ma (2σ, MSWD = 0.49,n= 2) for Group C. Groups A and B together yield an upper intercept age of 3457 ± 14 Ma (2σ, MSWD = 0.85,n= 23). The LA-ICP-MS data yield weighted average ages of 3442 ± 19 Ma (2σ, MSWD = 0.17,n= 7) for Group A and 3435 ± 11 Ma (2σ, MSWD = 0.44,n= 16) for Group B. They yield an upper intercept age of 3443 ± 13 Ma (2σ, MSWD = 0.63,n= 23). These SIMS and LA-ICP-MS ages are consistent. We propose that the above SIMS and LA-ICP-MS ages of Groups A and B are the best estimates of the granitic magmatism and the subsequent metamorphism. The metamorphism must have occurred after the granitic magmatism within a few tens of million years, as constrained by their age errors. Accordingly, these two granitic gneisses represent the oldest rocks currently known in South China. They predate the previously reported 3300-Ma-old trondhjemitic gneiss from the Kongling Terrain by 150 Ma.The 3.4 Ga zircons show near chondritic ɛHf(t) (−0.7 ± 1.0, 2σ, MSWD = 1.14,n= 8), which is below the coeval value of the depleted mantle. This suggests that the granitic magma contained materials of pre-existing continental crust. Their higher-than-mantle δ18O values (6.1–6.4‰) imply that such materials must have been interacted with surface water. Crust formation ages (TDM2) of the 3.4 Ga zircons vary from 3.9 to 3.6 Ga with a weighted average of 3703 ± 27 Ma (2σ, MSWD = 1.05,n= 7). Our results support previous studies that the Yangtze Craton may have contained the continental crust as old as 3.8 Ga.Among the younger age groups, the 3.3 Ga zircons exhibit176Hf/177Hf (t) and δ18O values similar to the 3.4 Ga zircons, suggesting that they were altered from the 3.4 Ga zircons. The 2.9 and 2.7 Ga zircons in both samples are rare and magmatic. Their176Hf/177Hf (t) ratios are distinct from the 3.4 Ga zircons, indicating different sources. These two age groups are consistent with the 2.9 Ga trondhjemitic–tonalitic–granodioritic and the 2.7 Ga A-type granitic magmatism in the Kongling Terrain. The 2.0 Ga metamorphic zircons, regardless of being concordant or discordant, have176Hf/177Hf (t) ratios overlapping those of the 2.7 Ga zircons, suggesting a common source. In contrast, δ18O of the 2.0 Ga zircons is strongly variable and positively correlated with age concordance. The low δ18O (down to 3.1‰) requires interaction with hydrothermal fluid. These results suggest that at least some of the 2.0 Ga zircons were likely to have been altered from the 2.7 Ga zircons by hydrothermal fluid.