宁芜盆地火山-侵入岩的时代、地球化学特征及其地质意义

Geochrology,Geochemistry of Volcanic-Intrusive Rocks in the Ningwu Basin and Its Geological Implications

本文系统选取了宁芜盆地四个火山旋回的典型火山岩以及与铁铜矿成矿有关的岩体进行了精确的锆石定年和Hf同位素分析以及全岩主微量元素测试,研究结果显示龙王山组、大王山组、姑山组火山岩形成时代为134~127Ma,娘娘山组火山岩形成时代稍晚,为118.8±1.6Ma,一个中基性闪长玢岩样品形成时代为130.6±1.3Ma。三个中酸性斑岩样品的形成时代为124~128Ma。龙王山、大王山、姑山旋回火山岩及同期潜火山岩-侵入岩的Hf同位素特征相似,εHf(t)值范围大多在-2^-5之间,暗示其源区性质相似,娘娘山旋回εHf(t)值稍高,反映源区可能有更多幔源物质。结合岩石地球化学特征,认为岩浆的源区为一个富集地幔,而造成中生代火山岩源区地幔交代的流体/熔体可能与更早期的俯冲事件有关,而陆源物质是深海沉积物被俯冲的大洋板片带入地幔深部的。侵入岩与同期火山岩具有相似的形成时代、岩石地球化学和锆石Hf同位素组成,表明它们是同源异相体。宁芜盆地广泛的岩浆活动为中国东部岩石圈强烈的减薄作用下的一个均一富集地幔熔融的产物。

This study presents the accurate zircon dating results,Hf isotopic compositions and major-trace element analysis for typical volcanic rocks and intrusive rocks related to Fe and Cu deposits during four volcanism periods in the Ningwu basin.LA-ICP-MS zircon dating results indicates that the volcanic rocks in the Longwangshan,Dawangshan and Gushan formations formed at 134～127 Ma,while the volcanic rock in the Niangniangshan Formation has a younger crystallization age at 118.8 ± 1.6 Ma.A mafic porphyry has an age of 130.6±1.3 Ma,and three moderate felsic porphyry formed at 124～128 Ma.Zircons from the volcanic rocks and intrusive rocks in the Longwangshan,Dawangshan and Gushan Formations have similar Hf isotopic characteristics,with εHf（t） values of-2 ～-5,suggesting similar magmatic sources,while the volcanic rocks from the Niangniangshan Formation have zircons with higher εHf（t）,suggesting more mantle derived materials involved in the source.Combing the geochemistry characteristics,it is suggested that these magmatic rocks were derived from an enriched mantle.The fluid/melt in mantle metasomatism could be related to an older subduction event,while the continent sourced materials were carried in deep mantel by the oceanic plate as oceanic sediments.Similar formation time,geochemistry and Hf isotopic compositions between intrusive rocks and coeval volcanic rocks indicate that they were derived from similar source with different phases.The magmatism in the Ningwu basin was produced from the remelting of enriched mantle at the tectonic setting of lithosphere thinning in eastern China.