文档介绍：
北秦岭混合岩锆石 U-Pb 年龄及其地质意义#
刘倩,吴元保,王浩
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(中国地质大学地球科学学院,武汉 430074)
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摘要:混合岩在北秦岭造山带内广泛发育,然而其形成时代却没有得到有效约束。本文对北秦岭混合岩中
锆石开展了详细的形态学,阴极发光图像,微量元素以及 U-Pb 年代学研究。CL 图像揭示北秦岭混合岩中
锆石多是新生的均质锆石,少数锆石含继承核并发育变质增生边。形态学以及微量元素特征显示多数锆石
为形成于部分熔融过程中的变质锆石。继承锆石核具有震荡环带特征并给出了约 900Ma 的 U-Pb 年龄,应代
表了混合岩原岩的形成时代。新元代早期的花岗质原岩可能形成于活动大陆边缘环境,该期事件可能是
Rodinia 超大陆聚合的响应。变质深熔锆石获得 455 ± 5 到 420 ± 4 Ma 的变质年龄,主峰在 ca. 435Ma。
结合前人结果,我们认为北秦岭混合岩化作用主要发生在 455-400Ma 之间。这期混合岩化作用比北秦岭约
490Ma 的超高压变质作用晚约 50Ma,暗示这两期变质作用是由独立的构造事件引起。北秦岭混合岩化作用
与麻粒岩相变质作用以及花岗质岩浆作用同期,共同指示了商丹洋的向北俯冲。超高压变质岩主要发现于
秦岭群的北部,少量产出于秦岭群中部,指示超高压岩石沿秦岭群北缘折返之后在后期混合岩化作用过程
中与基底岩石发生了拆离。
关键词:锆石 U-Pb 年龄,混合岩,秦岭群,商丹洋
中图分类号:P597+.3
Zircon U-Pb ages of migmatites from the North Qinling
terrane and their geological implications
LIU Qian, WU Yuanbao, WANG Hao
(Faculty of Earth Sciences, China University of Geosciences, Wuhan 430074)
Abstract: Migmatites are predominant in the North Qinling orogen. However, the formation ages of
the migmatites are poorly constrained. This paper presents bined study of CL image, U-Pb age,
and trace element of zircon in migmatites from the North Qinling 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 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 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 to 400 Ma. The migmatization was about 50 Ma later than the c. 490 Ma ultra-high pressure
metamorp