浙西开化地区新元古代(~800 Ma)洋陆俯冲:来自活动陆缘弧火山岩序列组合的制约

Neoproterozoic(~800 Ma) Subduction of Ocean-Continent Transition:Constraint from Arc Magmatic Sequence in Kaihua, Western Zhejiang

浙西开化地区处于江南造山带东段,沿下庄-树范断裂北西侧发育一套浅变质的玄武岩-安山岩-英安岩-流纹岩组合.地球化学分析结果显示,玄武岩、安山岩和英安岩、流纹岩表现为连续演化的岩浆序列,岩石多富集Ba、K、Rb,亏损Sr等大离子亲石元素,富集Pb,亏损P、Ti、Ta、Nb等高场强元素.玄武岩Nb含量介于11.8×10-6~15.2×10-6,Nb/Ta=15.36~18.10,Nb/U=8.90~19.32,具有富Nb特点;安山岩MgO含量为5.31%~8.56%,Mg#值为56.89~68.83,FeOT/MgO介于0.82~1.36,显示高Mg特征;英安岩和流纹岩Ga/Al比值高,且FeO^T/MgO多介于5.66~18.50,锆石饱和温度为837~920℃,表现出A型酸性火山岩特征.锆石U-Pb定年结果表明,玄武岩、安山岩和流纹岩的成岩年龄分别为800.5±9.2 Ma、799.3±7.1 Ma和798.3±6.2 Ma,均系新元古代(~800 Ma)构造岩浆活动的产物.富Nb玄武岩和高Mg安山岩组合为活动陆缘弧的典型代表,而英安岩和流纹岩则可能形成于俯冲机制下的拉张环境,进一步表明新元古代(~800 Ma)左右,古华南洋北西向扬子陆块的俯冲仍在继续.

As a record of the process for ocean-continent transition related oceanic subduction,the magmatic rocks consisting of the basalts,andesites,dacites and rhyolites,were identified near the northwastern Xiazhuang-Shufan fault in Kaihua County,western Zhejiang,which is the eastern segment of Jiangnan orogeny.Geochemical analyses indicate that the basalts and andesites are mostly enriched in Ba,K,Rb,Th,U,Pb,but depleted in Sr,P,Nb,Ta,Ti.The basalts show high Nb contents of 11.8×10-6 to 15.2×10-6,Nb/Ta=15.36-18.10,and Nb/U=8.90-19.32.The andesites have higher Mg values with MgO contents ranging from 5.31%to 8.56%,Mg#ranging from 56.89 to 68.83,and FeOT/MgO=0.82-1.36.The dacites and rhyolites have higher Ga/Al ratios,FeOT/MgO ranging from 5.66 to 18.50 mostly,and high magma temperatures(837-920℃),reflecting the characteristics of A-type rhyolites.The U-Pb dating of zircon yields age of 800.5±9.2 Ma for the Nb-enriched basalt,799.3±7.1 Ma for the high-Mg andesite,and 798.3±6.2 Ma for the A-type rhyolite,confirming that the magmatic activity of continental marginal arc occurred when the paleo-South-China Plate subducted northwestwardly in Neoproterozoic,and suggesting that the subducting movement continued to 800 Ma or later.