陕西小秦岭地区太华群的锆石U-Pb年龄和Hf同位素组成

U-Pb ages and Hf isotope compositions of zircons of Taihua Group in Xiaoqinling area,Shaanxi Province

本文对陕西华山岩体南侧一个太华群黑云斜长片麻岩进行了地球化学、锆石U-Pb年龄和Hf同位素分析。地球化学分析显示其原岩为中酸性钙碱性花岗质岩石,稀土含量较低(∑REE=83.13×10-6),但富集Pb和LILE元素(如Rb、Ba)。锆石的176Hf/177Hf比值变化于0.281258～0.281404,具明显负的εHf(t)值(-6.86～-11.23)。锆石的Hf同位素模式年龄(2.96～3.24Ga)表明原岩是由中太古代地壳演化而来。对比显示小秦岭太华群黑云斜长片麻岩与鲁山地区的太华群具有相似的中太古代地壳源区,但它们的形成时代可能不同。锆石内部结构、Th/U比值以及定年结果显示小秦岭地区的太华群在～1.91Ga经历了一期重要的变质热事件,该事件与Columbia超大陆拼合时的全球性碰撞造山事件相关联。小秦岭太华群为华北克拉通块体南缘的地质单元,但各地区太华群在原岩组成、形成时代和变质时代上具有不同的特征,太华群应是一个杂岩体,至少可以解体为新太古代和古元古代两部分。

This paper presents new geochemical,LA-ICPMS U-Pb zircon dating and LA-MC-ICPMS zircon Hf isotopic results for a biotite plagioclase gneiss from the Taihua Group at the South of Huashan pluton,Shaanxi Province.Geochemical results indicate that the protolith is intermediate-acidic calc-alkaline granitic rock,with low REE content（∑REE=83.13×10-6） and high Pb and LILE element contents（e.g.Rb,Ba）.The 176Hf/177Hf ratio of zircons from this sample varies from 0.281258 to 0.281404 with obviously negative εHf（t） values（-6.86^-11.23）.The Hf model ages vary from 2.96 to 3.24Ga,indicating that the original magma of the protolith derived from Mesoarchean crust.Comparisons show that the biotite plagioclase gneiss of Xiaoqinling area has similar source of Mesoarchean crust with the Taihua Group in Lushan area,but the timing of formation may be different.Structure of zircons,the Th/U ratio and age indicate that the Taihua Group in the Xiaoqinling area experienced an important thermal event at ~1.91Ga,which has relationship with the global collision event during convergence of Columbia Supercontinent.The Taihua Group of Xiaoqinling area is part of the southern margin of North China craton,but the Taihua Group in different area has distinction in composition,timing of formation and metamorphic age.Therefore,the Taihua Group is the complex and may be at least separated into two units with Neoarchean and Paleoproterozoic ages,respectively.