A new data compilation shows that in intermediate to felsic rocks, zircon Eu/Eu* [chondrite normalized Eu/ ( ) Sm Gd × ] correlates with whole rock La/Yb, which has been be used to infer crustal thickness. The resultant positive correlation between zircon Eu/Eu* and crustal thickness can be explained by two processes favored during high-pressure differentiation: (1) supression of plagioclase and (2) endogenic oxidation of Eu2+ due to garnet fractionation. Here we calibrate a crustal thickness proxy based on Eu anomalies in zircons. The Eu/Eu*in-zircon proxy makes it possible to reconstruct crustal thickness evolution in magmatic arcs and orogens using detrital zircons. To evaluate this new proxy, we analyzed detrital zircons separated from modern river sands in the Gangdese belt, southern Tibet. Our results reveal two episodes of crustal thickening (to 60–70 km) since the Cretaceous. The first thickening event occurred at 90–70 Ma, and the second at 50–30 Ma following Eurasia-India collision. These findings are temporally consistent with contractional deformation of sedimentary strata in southern Tibet. INTRODUCTION Crustal thickness profoundly influences a number of geologic processes including magmatic differentiation (Ducea, 2002; Ducea et al., 2015; Farner and Lee, 2017; Tang et al., 2018), ore formation (Kay, 2001; Lee and Tang, 2020), and erosion and weathering (Larsen et al., 2014). However, resolving the evolution of crustal thickness has been challenging. Because of erosion, weathering, and tectonic processes, the continental crust has been subject to constant destruction and overprinting after its formation, so the preserved continental crust may not reflect how thick it was when the crust was first generated. Recent studies found that certain element ratios (e.g., Sr/Y and La/Yb) in intermediate to felsic rocks correlate with crustal thickness (Chapman et al., 2015; Profeta et al., 2015). As the crust thickens, the increased pressure of magmatic differentiation enhances amphibole and/or garnet fractionation relative to plagioclase. Amphibole and garnet fractionations deplete middle and heavy rare earth elements (REEs), respectively, relative to light REEs and Sr. The La/Yb and Sr/Y approaches extract differentiation pressure and thus crustal thickness from magmatic records, and their effectiveness has been demonstrated by a number of studies of Phanerozoic orogens (Chapman et al., 2015; Farner and Lee, 2017; Haschke et al., 2002). Despite their successful applications, these whole rock chemistry methods require extensive sampling over large areas in the field, and sample availability can be strongly biased by accessibility; for deep time, their applications are further limited by the increasingly significant issue of preservation of rock records. Here we extend the arena of rock chemistry–based crustal thickness proxies by linking zircon Eu anomaly to whole rock La/Yb ratio. Zircon is ubiquitous in the continental crust and can survive most metamorphic, erosional, and weathering processes due to its refractory nature. Detrital zircons naturally sample large areas exposed to erosion and may fill gaps in Earth’s history where rock records are missing (Balica et al., 2019; McKenzie et al., 2016, 2018; Zhu et al., 2020). We first calibrate the zircon crustal thickness proxy against the whole rock La/Yb proxy. Then, we evaluate the zircon proxy with a case study of the Gangdese belt, southern Tibet, where independent geologic constraints on crustal thickening and/or shortening exist. RATIONALE Europium anomaly [Eu/Eu*; chondrite normalized Eu/ ( ) Sm Gd × ] in zircon is dictated by Eu/Eu* in the melt and Sm-Eu-Gd partitioning between zircon and melt. Unlike other REEs which are trivalent (except Ce), Eu exists as both Eu2+ and Eu3+ in most magmatic systems. Eu2+ is geochemically similar to Sr2+ and thus strongly partitions into plagioclase (Ren, 2004). Plagioclase fractionation causes Eu depletion relative to neighboring Sm and Gd in the residual melt. As a consequence, zircons crystallizing from the residual melt would have negative Eu anomalies, or Eu/Eu* <1 (Holder et al., 2020). Eu/Eu* in zircon is also sensitive to Eu2+/Eu3+ in the melt because Eu2+ is significantly more incompatible than Eu3+ in zircon lattice, as inferred from Sr2+ partitioning in zircon (Thomas et al., 2002). As the crust thickens, the intracrustal differentiation through anatexis or fractional crystallization takes place at higher pressures, which suppresses plagioclase (e.g., Green, 1982) and *E-mails: mingtang@pku.edu.cn, jiweiqiang@ mail.iggcas.ac.cn Downloaded from https://pubs.geoscienceworld.org/gsa/geology/article-pdf/doi/10.1130/G47745.1/5144012/g47745.pdf by Ming Tang on 05 September 2020 2 www.gsapubs.org | Volume XX | Number XX | GEOLOGY | Geological Society of America thus prevents Eu depletion in the melt. In addition, intense crustal thickening stabilizes garnet, which preferentially sequesters Fe2+ over Fe3+ from the melt. The elevated Fe3+/∑Fe (ferric Fe to total Fe ratio) in the melt would then oxidize multivalent trace elements such as Eu (Tang et al., 2018, 2019). Europium oxidation converts the less-compatible Eu2+ to morecompatible Eu3+, which enhances Eu partitioning in zircon. The collective effect is that zircon Eu/Eu* increases with increasing crustal thickness (Fig. 1). CALIBRATING THE EU/EU*-INZIRCON CRUSTAL THICKNESS PROXY We compiled 120 igneous samples with paired zircon–whole rock composition data. These samples are from 29 localities around the world. The whole rocks generally have intermediate to felsic compositions with SiO2 contents of 55–75 wt%. We did not include more mafic samples because mafic rocks are not important sources of zircon; we also ignored high-silica samples (SiO2 >75 wt%) because Eu systematics may be complicated by extreme differentiation (Fig. S3 in the Supplemental Material1). We divided our compiled samples into postArchean and Archean groups. Post-Archean samples were further divided into I-type (igneous protoliths), S-type (sedimentary protoliths), A-type (shallow emplacement) granitoids based on the classification in the source literature. The Archean samples are represented by TTGs (tonalite-trondhjemite-granodiorite), which belong to I-type by definition. I-type, including Archean TTGs, and A-type samples all fall on a positive correlation between zircon Eu/Eu* and whole rock [La/Yb]N (the subscript N denotes chondrite normalized) (Fig. 2A). This prominent relationship suggests that, on the first order, Eu/Eu* in zircon is controlled by the differentiation pressure which correlates with crustal thickness (McKenzie et al., 2018), despite the various complexities associated with global sample compilation. In particular, redox conditions seem to play a less-important role in determining Eu/Eu* in zircons from felsic magmas, or redox conditions of felsic magmas also correlate with differentiation pressure in most Iand A-type granitoids (Tang et al., 2018, 2019). S-type samples show no correlation between zircon Eu/Eu* and whole 1Supplemental Material. Methods, supplemental figures, and data. Please visit https://doi .org/10.1130/ GEOL.S.12869660 to access the supplemental material, and contact editing@geosociety.org with any questions.

中文翻译：

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从碎屑锆石中的铕异常重建地壳厚度演化

新的数据汇编表明，在中长英质岩石中，锆石 Eu/Eu* [球粒陨石归一化 Eu/ ( ) Sm Gd × ] 与整块岩石 La/Yb 相关，后者已被用于推断地壳厚度。由此产生的锆石 Eu/Eu* 与地壳厚度之间的正相关可以通过高压分异过程中的两个有利过程来解释：（1）斜长石的抑制和（2）由于石榴石分馏引起的 Eu2+ 的内生氧化。在这里，我们根据锆石中的 Eu 异常校准地壳厚度代理。Eu/Eu*in-zircon 代理使得利用碎屑锆石重建岩浆弧和造山带中地壳厚度演化成为可能。为了评估这个新的替代物，我们分析了从藏南冈底斯带现代河砂中分离出来的碎屑锆石。我们的结果揭示了自白垩纪以来的两次地壳增厚（至 60-70 公里）。第一次增厚事件发生在 90-70 Ma，第二次发生在欧亚-印度碰撞后的 50-30 Ma。这些发现在时间上与藏南沉积地层的收缩变形一致。导言 地壳厚度对包括岩浆分异（Ducea，2002；Ducea 等，2015；Farner 和 Lee，2017；Tang 等，2018）、矿石形成（Kay，2001；Lee 和 Tang， 2020），以及侵蚀和风化（Larsen 等，2014）。然而，解决地壳厚度的演变一直具有挑战性。由于侵蚀、风化和构造过程，大陆地壳形成后一直受到不断的破坏和叠加，因此，保存下来的大陆地壳可能无法反映地壳最初生成时的厚度。最近的研究发现，中长英质岩石中的某些元素比率（例如 Sr/Y 和 La/Yb）与地壳厚度相关（Chapman 等，2015；Profeta 等，2015）。随着地壳增厚，岩浆分异压力的增加增强了角闪石和/或石榴石相对于斜长石的分馏。角闪石和石榴石分馏相对于轻稀土和 Sr 分别消耗中重稀土元素 (REE)。La/Yb 和 Sr/Y 方法从岩浆记录中提取分异压力，从而提取地壳厚度，其有效性已被证明通过对显生宙造山带的大量研究（Chapman 等人，2015 年；Farner 和 Lee，2017 年；Haschke 等人，2002 年）。尽管它们的应用很成功，但这些全岩石化学方法需要在现场大面积进行广泛采样，并且样品可用性可能会受到可及性的严重影响；在很长一段时间内，它们的应用受到越来越重要的岩石记录保存问题的限制。在这里，我们通过将锆石 Eu 异常与整个岩石 La/Yb 比值联系起来，扩展了基于岩石化学的地壳厚度代理的领域。锆石在大陆地壳中无处不在，由于其耐火性质，可以在大多数变质、侵蚀和风化过程中存活下来。碎屑锆石自然对暴露于侵蚀的大面积区域进行采样，并可能填补地球历史上岩石记录缺失的空白（Balica 等人，2019 年；McKenzie 等人，2016 年，2018 年；Zhu 等人，2020 年）。我们首先针对整个岩石 La/Yb 代理校准锆石地壳厚度代理。然后，我们通过藏南冈底斯带的案例研究评估锆石替代品，那里存在地壳增厚和/或缩短的独立地质约束。基本原理 铕异常 [Eu/Eu*; 锆石中球粒陨石归一化 Eu/ ( ) Sm Gd × ] 由熔体中的 Eu/Eu* 和锆石和熔体之间的 Sm-Eu-Gd 分配决定。与其他三价稀土元素（Ce 除外）不同，Eu 在大多数岩浆系统中同时以 Eu2+ 和 Eu3+ 的形式存在。Eu2+ 在地球化学上与 Sr2+ 相似，因此强烈地划分为斜长石 (Ren, 2004)。斜长石分馏导致残余熔体中相对于相邻 Sm 和 Gd 的 Eu 耗尽。因此，从残余熔体中结晶的锆石将具有负的 Eu 异常，或 Eu/Eu* < 1（Holder 等人，2020 年）。锆石中的 Eu/Eu* 也对熔体中的 Eu2+/Eu3+ 敏感，因为 Eu2+ 比锆石晶格中的 Eu3+ 明显更不相容，正如从锆石中的 Sr2+ 分配推断的那样（Thomas 等，2002）。随着地壳增厚，在更高的压力下发生通过精熔或分步结晶的地壳分异，这抑制了斜长石（例如，Green，1982）和 *E-mails: mingtang@pku.edu.cn, jiweiqiang@mail.iggcas.ac .cn 唐明于 2020 年 9 月 5 日从 https://pubs.geoscienceworld.org/gsa/geology/article-pdf/doi/10.1130/G47745.1/5144012/g47745.pdf 下载 2 www.gsapubs.org | 卷XX | 编号 XX | 地质 | 美国地质学会因此防止了熔体中的欧盟消耗。此外，强烈的地壳增厚稳定了石榴石，它优先从熔体中螯合 Fe2+ 而不是 Fe3+。熔体中升高的 Fe3+/∑Fe（三价铁与总铁之比）会氧化多价微量元素，如 Eu（Tang 等人，2018 年，2019 年）。铕氧化将相容性较差的 Eu2+ 转化为相容性较高的 Eu3+，从而增强了锆石中 Eu 的分配。集体效应是锆石Eu/Eu*随着地壳厚度的增加而增加（图1）。校准 EU/EU*-INZIRCON 地壳厚度代理 我们编制了 120 个火成岩样品，其中包含成对的锆石-全岩石成分数据。这些样本来自全球 29 个地区。整个岩石通常具有中等至长英质成分，SiO2 含量为 55-75 wt%。我们没有包括更多的镁铁质样品，因为镁铁质岩石不是锆石的重要来源；我们也忽略了高硅样品（SiO2 > 75 wt%），因为 Eu 系统可能因极端分化而变得复杂（补充材料中的图 S3）。我们将我们编译的样本分为后太古代和太古代组。根据来源文献中的分类，太古代后样品进一步分为 I ​​型（火成岩原岩）、S 型（沉积原岩）、A 型（浅层侵位）花岗岩。太古代样品由TTGs（tonalite-trondhjemite-granodiorite）代表，根据定义属于I型。I 型，包括太古代 TTG 和 A 型样品都属于锆石 Eu/Eu* 与整个岩石 [La/Yb]N 之间的正相关（下标 N 表示球粒陨石归一化）（图 2A）。这种显着的关系表明，在第一阶，锆石中的 Eu/Eu* 受与地壳厚度相关的分异压力控制（McKenzie 等，2018），尽管与全球样本编制相关的各种复杂性。特别是，氧化还原条件似乎在确定来自长英质岩浆的锆石中 Eu/Eu* 的作用不太重要，或者长英质岩浆的氧化还原条件也与大多数 I 和 A 型花岗岩的分化压力相关（Tang et al., 2018 , 2019)。S 型样品显示锆石 Eu/Eu* 和整个 1 补充材料之间没有相关性。方法、补充图和数据。请访问 https://doi .org/10.1130/GEOL.S.12869660 获取补充材料，如有任何问题，请联系editing@geosociety.org。特别是，氧化还原条件似乎在确定来自长英质岩浆的锆石中 Eu/Eu* 的作用不太重要，或者长英质岩浆的氧化还原条件也与大多数 I 和 A 型花岗岩的分化压力相关（Tang et al., 2018 , 2019)。S 型样品显示锆石 Eu/Eu* 和整个 1 补充材料之间没有相关性。方法、补充图和数据。请访问 https://doi .org/10.1130/GEOL.S.12869660 获取补充材料，如有任何问题，请联系editing@geosociety.org。特别是，氧化还原条件似乎在确定来自长英质岩浆的锆石中 Eu/Eu* 的作用不太重要，或者长英质岩浆的氧化还原条件也与大多数 I 和 A 型花岗岩的分化压力相关（Tang et al., 2018 , 2019)。S 型样品显示锆石 Eu/Eu* 和整个 1 补充材料之间没有相关性。方法、补充图和数据。请访问 https://doi .org/10.1130/GEOL.S.12869660 获取补充材料，如有任何问题，请联系editing@geosociety.org。S 型样品显示锆石 Eu/Eu* 和整个 1 补充材料之间没有相关性。方法、补充图和数据。请访问 https://doi .org/10.1130/GEOL.S.12869660 获取补充材料，如有任何问题，请联系editing@geosociety.org。S 型样品显示锆石 Eu/Eu* 和整个 1 补充材料之间没有相关性。方法、补充图和数据。请访问 https://doi .org/10.1130/GEOL.S.12869660 获取补充材料，如有任何问题，请联系editing@geosociety.org。