Eocene adakitic porphyries in the central‐northern Qiangtang Block, central Tibet: Partial melting of thickened lower crust and implications for initial surface uplifting of the plateau
Eocene adakitic porphyries in the central‐northern Qiangtang Block, central Tibet: Partial melting of thickened lower crust and implications for initial surface uplifting of the plateau
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DOI：
10.1002/2016jb013259
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发表时间：
2017-02
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通讯作者：
Quan Ou;Qiang Wang;D. Wyman;Haixiang Zhang;Jin-Hui Yang;Ji‐Peng Zeng;Lu‐Lu Hao;Yi-Wei Chen
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作者：
Quan Ou;Qiang Wang;D. Wyman;Haixiang Zhang;Jin-Hui Yang;Ji‐Peng Zeng;Lu‐Lu Hao;Yi-Wei Chen
青藏高原地壳增厚和地表初始隆升的时间和机制一直存在很大争议。本文报道了青藏高原中部羌塘地块中北方拉日错地区始新世埃达克质斑岩的锆石U-Pb年龄、矿物、元素和Sr-Nd同位素组成数据。激光烧蚀-电感耦合等离子体-质谱锆石U-Pb年龄测定表明，斑岩形成于42-41 Ma。所有样品均显示埃达克岩地球化学特征，如高SiO2和Al 2 O3，低Y和Yb含量，高Sr/Y和La/Yb比值，Sr为正，Eu异常可忽略，Nb-Ta亏损。它们还具有低镁值（Mg #）（16-45）和高K2 O（4.0-4.6 wt %）值，以及高（87 Sr/86 Sr）i（0.7076-0.7080）和低εNd（t）（−5.9至−4.4）值。这些埃达克质岩石很可能是由石榴子石、角闪石和金红石稳定区内增厚的下地壳部分熔融而成，但斜长石相对较少。此外，该地块下地壳增厚的过程可能发生在中始新世之前，很可能是由松潘-甘孜地块在新生代早期向南俯冲造成的。此外，出露埃达克质斑岩的出现明确地表明，折返过程发生在其形成之后。因此，这些~42 Ma埃达克质斑岩表明42 Ma以来发生了显著的折返作用。结合热年代学和古高程资料，我们认为，羌塘地块的初始地表隆升是在中始新世（~42 Ma）由地壳增厚触发的。
The time and mechanism of crustal thickening and initial surface uplift of the Tibetan Plateau remain highly controversial. Here we report on zircon U‐Pb age, and mineral, element, and Sr‐Nd isotope composition data for Eocene adakitic porphyries in the Laorite Co area of central‐northern Qiangtang Block, central Tibet. Laser ablation‐inductively coupled plasma‐mass spectrometry zircon U‐Pb age dating shows that the porphyries were generated at 42–41 Ma. All samples display adakitic geochemical characteristics, such as high SiO2 and Al2O3, and low Y and Yb contents, and high Sr/Y and La/Yb ratios with positive Sr and negligible Eu anomalies and Nb‐Ta depletion. They also have low magnesium number (Mg #) (16–45) and high K2O (4.0–4.6 wt %) values, as well as high (87Sr/86Sr)i (0.7076–0.7080) and low εNd(t) (−5.9 to −4.4) values. These adakitic rocks were most probably generated by partial melting of thickened lower crust in the stability field of garnet, amphibole, and rutile but with relatively minor plagioclase. Moreover, such a crustal thickening process beneath this block likely occurred no later than the middle Eocene and was most probably caused by the southward subduction of the Songpan‐Ganzi Block in the early Cenozoic. In addition, the occurrences of exposed adakitic porphyries unambiguously indicate that exhumation process occurred after their formation. Thus, these ~42 Ma adakitic porphyries indicate the occurrence of significant exhumation since 42 Ma. Integrated with the thermochronological and paleoelevation data, we suggest that the initial surface uplift of the Qiangtang Block, triggered by crustal thickening, was underway by the middle Eocene (~42 Ma).