吉林南部三棵榆树组火山岩的成因:锆石U-Pb年龄和Hf同位素证据

Petrogenesis of Volcanic Rocks from Sankeyushu Formation in Southern Jilin:Evidences from Zircon U-Pb Ages and Hf Isotopes

锆石LA-ICP-MS U-Pb测年结果显示,吉林南部地区三棵榆树组粗面岩形成于118 Ma的早白垩世晚期。岩石地球化学研究表明,三棵榆树组粗面岩的SiO2含量为65.11%～65.61%,富碱(K2O=6.29%～7.28%,Na2O=3.71%～4.21%),属于钾玄岩系列,弱过铝质(A/CNK=1.05～1.12),含镁指数Mg#=9～16;稀土元素总量较高(∑REE=317.72×10-6～371.30×10-6),轻重稀土分馏明显(LaN/YbN=19.50～22.80),Eu负异常较弱(δEu=0.75～0.81);微量元素富集Rb、Th、Zr、K,明显亏损Nb、Ta和Sr等,显示典型的大陆地壳特征。锆石的176Hf/177Hf比值介于0.282 206～0.282 309之间,εHf(t)值介于-13.32～-16.97之间,Hf模式年龄(TDM2)介于2.0～2.6 Ga之间,平均为2.2 Ga。上述特征表明,吉林南部地区早白垩世粗面岩主要来源于古元古代地壳物质的部分熔融,形成于伸展构造环境。

Zircon U-Pb dating by LA-ICP-MS shows that Sankeyushu Formation trachyte in southern Jilin area formed in 118 Ma of late Early Cretaceous.Study of petrogeochemistry suggests that SiO2 contents from Sanke-yushu Formation trachyte are 65.11%-65.61%,rich in alkali（K2O=6.29%-7.28%,Na2O=3.71%-4.21%）.It belongs to shoshonite series with weak peraluminous（A/CNK=1.05-1.12） and low Mg index（Mg#=9-16）.The total amount of rare earth elements is relatively high（∑REE=317.72×10-6-371.30×10-6）.Light and weight lanthanon differentiation evidence（LaN/YbN=19.50-22.80）is noticeable.Eu negative anomaly is weak（δEu=0.75-0.81）.The trace elements Rb,Th,Zr and K enrichment,Nb,Ta and Sr obvious depletion,show the emblematical character of continent lithosphere.176Hf/177Hf from zircons range from 0.282,206 to 0.282,309,εHf（t） from-13.32 to-16.97,Hf model ages are 2.0-2.6 Ga,average is 2.2 Ga.The above characteristics indicate that the Early Cretaceous trachyte in the study area mainly came from Paleoproterozoic lithosphere partial melting and was formed in an extensional tectonic environment.