Zircon U–Pb ages and Hf isotope compositions of migmatites from the North Qinling terrane and their geological implications
Zircon U–Pb ages and Hf isotope compositions of migmatites from the North Qinling terrane and their geological implications
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DOI：
10.1111/jmg.12065
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发表时间：
2014-02
影响因子：
3.4
通讯作者：
Q. Liu;Y.‐B. Wu;H. Wang;Shang Gao;Zhen Qin;X. Liu;S. Yang;H. Gong
中科院分区：
文献类型：
--
作者：
Q. Liu;Y.‐B. Wu;H. Wang;Shang Gao;Zhen Qin;X. Liu;S. Yang;H. Gong
混合岩在北秦岭（NQ）造山带中占主导地位，但其形成年龄很少受到限制。本文对来自 NQ 装置的混合岩中锆石的阴极发光成像、U-Pb 年龄、微量元素和 Hf 同位素进行了综合研究。在混合岩中，大多数锆石颗粒以新的均质晶体形式出现，而有些则以继承核心周围的过度生长边缘形式出现。形态和微量元素特征表明，锆石晶体是变质的，是在部分熔融过程中形成的。继承的核心具有振荡分区并产生 c 的 U-Pb 年龄。 900 Ma，代表它们的原岩年龄。新元古代早期的原岩可能形成于活跃的大陆边缘，是对罗迪尼亚超大陆聚集的反应。混合岩锆石产生的 Hf 模型年龄为 1911 ± 20 至 990 ± 22 Ma，表明原岩源自古元古代到新元古代地壳物质的再造。深熔锆石的形成年龄范围为 455 ± 5 至 420 ± 4 Ma，峰值为 c。 435马。结合前人的研究结果，我们认为NQ地体的混合作用发生在c。 455–400 Ma。迁移是c。比c晚50 Ma。 490 Ma超高磷（UHP）变质作用，表明它们发生在两次独立的构造事件中。相比之下，混合岩化作用与NQ单元中的麻粒岩相变质作用和花岗质岩浆作用同时发生，共同认为它们的形成是由于商丹洋向北俯冲所致。超高压岩石主要分布在北缘，偶尔也分布在NQ单元的内部，这表明它们是沿北缘被挖出的，并在随后的混合岩作用过程中与基底分离。
Migmatites are predominant in the North Qinling (NQ) orogen, but their formation ages are poorly constrained. This paper presents a combined study of cathodoluminescence imaging, U–Pb age, trace element and Hf isotopes of zircon in migmatites from the NQ unit. In the migmatites, most zircon grains occur as new, homogeneous crystals, while some are present as overgrowth rims around inherited cores. Morphological and trace element features suggest that the zircon crystals are metamorphic and formed during partial melting. The inherited cores have oscillatory zoning and yield U–Pb ages of c. 900 Ma, representing their protolith ages. The early Neoproterozoic protoliths probably formed in an active continental margin, being a response to the assembly of the supercontinent Rodinia. The migmatite zircon yields Hf model ages of 1911 ± 20 to 990 ± 22 Ma, indicating that the protoliths were derived from reworking of Palaeoproterozoic to Neoproterozoic crustal materials. The anatexis zircon yields formation ages ranging from 455 ± 5 to 420 ± 4 Ma, with a peak at c. 435 Ma. Combined with previous results, we suggest that the migmatization of the NQ terrane occurred at c. 455–400 Ma. The migmatization was c. 50 Ma later than the c. 490 Ma ultra‐high‐P (UHP) metamorphism, indicating that they occurred in two independent tectonic events. By contrast, the migmatization was coeval with the granulite facies metamorphism and the granitic magmatism in the NQ unit, which collectively argue for their formation due to the northward subduction of the Shangdan Ocean. UHP rocks were distributed mainly along the northern margin and occasionally in the inner part of the NQ unit, indicating that they were exhumed along the northern edge and detached from the basement by the subsequent migmatization process.