吕梁-五台地区晚太古宙—古元古代花岗质岩石锆石U-Pb年代学和Hf同位素性质及其地质意义

Zircon U-Pb ages,Hf isotopes and significance of the late Archean-Paleoproterozoic granitoids from the Wutai-Luliang terrain,North China.

报道了五台地区一个晚太古代片麻状富钾花岗岩的锆石U-Pb年龄为2509±7．4Ma。该年龄与前人获得的年代学数据一起,进一步限定了晚太古期间五台-吕梁地区与俯冲有关的弧岩浆活动的年代框架。为了探讨晚太古宙—古元古代不同类型的花岗岩的源区性质和演化,特别是当时的陆壳增生机制和古老陆壳性质,本文在锆石U-Pb年代学的基础上对相应的锆石进行了细致的Hf同位素原位测量。结果表明,晚太古代片麻状花岗岩可能代表发育在古老克拉通(东部陆块?)上的弧岩浆作用产物。碰撞后(1．8 Ga)花岗岩的形成与加厚的造山带发生垮塌性伸展有关,后者导致幔源物质底侵,并引起下地壳物质的部分熔融而形成花岗质岩石。这些壳源花岗质岩浆再以不同比例与幔源岩浆混合而形成吕梁-五台地区成分复杂的碰撞后岩浆岩(花岗质为主)。锆石Hf同位素还表明,吕梁-五台地区可能存在老于2．7Ga的古老克拉通物质(2．9Ga?),虽然目前还没有发现这么老的碎屑锆石。

We obtained a zircon U-Pb age of 2509 7.4Ma for a gneissic K-rich granitic pluton from eastern Wutai terrain, North China. The newly obtained age data, along with previously published data, define a better age framework for the late Archean subduction-related magmatism in the Wuta-Luliang terrain. We report in this paper zircon U-Pb ages and zircon in situ Hf isotopic analytical data in order to trace the source of the late Archean-Paleoproterozoic granitoids and, particularly, to study the nature and mechanism of continental growth at that time. Our data suggest that the late Archean gneissic K-rich granitoids likely represent products of arc magmatism that was developed upon the North China craton. The post-collisional granitoids （ ca. 1.8 Ga） were genetically related to a process of extensional collapse of the thickened orogen, which triggered partial melting of lithospheric mantle and subsequently underplating of resultant basaltic magmas in the lower crust. Underplating of the basaltic magmas, in turn, induced partial melting of lower crustal material, generating granitic melts. The granitic melts subsequently mixed in varied proportions with the mantle-derived basaltic magma to produce the compositionally complicated post-collisional magmas （mainly granitic in composition） in the Wutai-Luliang terrain. Zircon Hf isotopic data reveal that crustal component of older than 2.7Ga （2.9Ga？） may exist in the WutaiLuliang terrain, although zircon debris of this age have not yet been found.