小秦岭华山复式岩基大夫峪岩体锆石U-Pb年龄、Hf同位素和地球化学特征

Zircon U_Pb ages, Hf isotopic composition and geochemistry of Dafuyu granitoid poluton from Huashan complex batholith in Xiaoqinling

大夫峪岩体位于华北陆块南缘小秦岭地区,是小秦岭地区华山复式岩基的重要组成部分,其LA-ICPMS锆石U-Pb定年结果显示:粗粒黑云母二长花岗岩和中细粒黑云母二长花岗岩的成岩年龄分别为(142.6±1.4)Ma和(140.1±1.2)Ma。岩体的w(Si O2)=69.12%~73.58%,w(Na2O)=3.95%~4.40%,大于3.2%,K2O/Na2O=0.92~1.22,A/CNK=1.03~1.04,小于1.1,属于弱过铝质高钾钙碱性I型花岗岩。岩石的ΣREE含量较高,LREE富集,LREE、HREE分馏明显,有较弱的负Eu异常。此外,岩石富集K、Rb、Ba、Sr等大离子亲石元素,而相对亏损Nb、Ta、Hf、Ti、P等高场强元素。粗粒黑云母二长花岗岩的εHf(t)主要为-24^-18,TDM2主要为2.3~2.7 Ga;中细粒黑云母二长花岗岩的εHf(t)主要为-26^-16,其TDM2主要为2.3~2.9 Ga。岩石地球化学和Hf同位素组成特征表明,岩体主要由古老下地壳部分熔融形成,源区物质可能为新太古宙太华群,并在成岩过程中有幔源或新生地壳组分的参与。根据区域地质和动力地质背景的演化历史,笔者认为,大夫峪岩体的形成时代,与东秦岭地区的其他岩体的形成时代基本一致,均形成于晚侏罗世—早白垩世,与华北克拉通东部以及华南、长江中下游等地区晚中生代岩浆作用的时间基本一致,说明整个中国东部晚中生代岩浆活动可能受控于统一的大地构造背景,即古太平洋板块俯冲欧亚板块,从而使整个中国东部发生了构造体制的大转变。大夫峪岩体就是在这种构造体制转换过程中形成的。

Tectonically located on the south margin of the North China Craton, the Dafuyu granitoid pluton is an im- portant component part of the Huashan complex batholith in the Xiaoqinling region, Henan Province. LA-ICP- MS zircon U-Pb dating yielded weighted average age of （142.6 ± 1.4） Ma for coarse-grained biotite monzonitic granite and （140.1 ±1.2） Ma for fine-medium-grained biotite monzonitic granite. The pluton is weakly peralu- minous and high-K calc-alkaline, characterized by the data w （SiO2） = 69.12 % -- 73.58 %, w （Na20） = 3.95 % --4.40% 〉3.2%, K20/Na20= 0.92-- 1.22, and A/CNK = 1.03 -- 1.04, 〈 1.1. Y, REE is high, and there exists enrichment of LREE. The fractionation between LREE and HREE is obvious. It is enriched in such LILE as K, Rb, Ba and Sr and depleted in HFSE like Nb, Ta, Hf, Ti and P, with weak negative anomalies of Eu. The coarse-grained biotite monzonitic granite shows a large variation in zircon Hf compositions with ~Hf（ t ） being -24-- -18 and two-stage model ages being 2.3-2.7 Ga; the fine-medium-grained biotite monzonitic granite also shows a large variation, with εHf（ t ） being -26-- - 16 and two-stage model ages being 2.3- 2.9 Ga. Geochemistry and zircon Hf isotopic compositions of the pluton suggest that it was probably derived from the ancient continental crust of the Archaean Taihua Group; nevertheless, in the rock-forming process, juvenile components might have contributed to its formation, as evidenced by the mafic magmatic enclaves in the pluton. According to the regional geological features and geodynamic considerations, the authors hold that the emplace- ment of the Dafuyu pluton in Huashan complex batholith took place in late Jurassic and early Cretaceous, with the same formation age as the other big granite plutons in east Qinling region. It also has the same formation time as the late Mesozoic magmatism in eastern North China Craton, South China and middle and lower Yangtze River region. It is thus considered that late Mesozoic magmatic activity of the whole eastern China was probably controlled by the unified tectonic background, i.e., the subduction of the Pacific plate toward the Eurasian con- tinent. This can explain the considerable shift of structure in the whole eastern China. The Dafuyu pluton in Huashan complex batholith might have been formed in this process.