鲁西沂水地区早前寒武纪混合岩锆石U-Pb年龄与Hf同位素组成及其构造意义

Early Pre-Cambrian migmatites in Yishui area of Luxi: Zircon U-Pb ages and Hf isotopic compositions and their geological significance

沂水地区位于鲁西花岗绿岩区东部沂水-汤头断裂带内。本文对区内九层岭与大山两地的混合岩样品进行了锆石U-Pb年龄与Hf同位素分析研究。结果显示九层岭混合岩有(2693±11)Ma和(2558±9)Ma两个年龄记录,分别代表早期熔融年龄和混合岩化年龄;锆石Hf同位素组成较为均一,ε_(Hf)(t)=+2.42^+5.46并且沿^(176)Lu/^(177)Hf=0.015平均地壳演化线分布,相应二阶段锆石Hf模式年龄都在2.80~2.93 Ga之间。而两个来自大山的样品则给出了(2475±14)Ma和(2474±14)Ma一致的结果,代表了混合岩化年龄;Hf同位素组成变化较大,ε_(Hf)(2475 Ma)=–1.48^+8.0,ε_(Hf)(2474 Ma)=–2.76^+6.43,二阶段锆石Hf模式年龄整体在2.48~3.15 Ga之间。结合区域演化背景,我们认为以上特征指示沂水地区很可能残存2.8 Ga的古老基底于地壳深部,经历并且记录了2.8~2.9 Ga新生地壳形成,及新太古代两期重要的2.7 Ga和2.5 Ga的岩浆作用事件。同时该地区存在多个时期形成的地壳物质(2.48~2.99 Ga)复合的源区,在板片俯冲角度变大(~2.54 Ga)后构造热作用影响加强使得源区在2.48 Ga重熔并有少量古老地壳物质(3.15 Ga)的贡献。

The Yishui area is located in the east of the Luxi granite-greenstone belt, and is bounded by the Yishui-Tangtou Fault zone to the east. In this article we present zircon U-Pb ages and Hf isotopic results of migmatites from the Jiucengling （08YS-50） and Dashan （08YS-14 and 08YS-21） in the Yishui area. The Jiucengling sample gives two ages of （2693±11） Ma and （2558±9） Ma, representing the time of previous partial melting and recent metamorphic anatexis, respectively. Their zircon Hf isotopic compositions vary slightly with eHf（t） values in the range from ＋2.42 to ＋5.46, and define a line as the evolution tendency of the average curst （with 176Lu/177Hf = 0.015）, while the corresponding two-stage Hf model age gives similar ages of 2.80-2.93 Ga. However, two samples from the Dashan migmatites yield identical results of （2475±14） Ma and （2474±14） Ma within errors, which means that their protolith was remelted at （2475±14） Ma. Their zircon Hf compositions show a wide range of enf（2475 Ma） from -2.76 to ＋8.0, with the two-stage Hf model ages varying from 2.48 to 3.15 Ga. Based on previous data, the aforementioned characteristics probably indicate the survival of 2.8 Ga basement at depth and witnessed the formation of juvenile crust aged at 2.8-2.9 Ga, and two other important magmatic events in the Neoarchean at 2.7 Ga and 2.5 Ga, respectively. These signatures indicate that a heterogeneous source region including juvenile materials with different ages （2.48-2.99 Ga） and an ancient crust aged 3.15 Ga were present beneath the study area. Therefore, it is likely that the tectono-thermal event at 2.48 Ga was related to slab subduction and these older basement rocks were the results of the transition of slab subduction angle in this area from a flat subduction to a steeper one which probably occur at -2.54 Ga.