七宝山早白垩世火山岩地球化学特征及其构造意义

七宝山晚中生代火山岩位于郯庐断裂带山东段管帅盆地东缘，出露于青山群八亩地组中。本文通过对火山岩样品TYS48和TYS52中锆石LA-ICP MS U-Pb测年，获得其年龄分别为120.1±0.6Ma和118.5±1.1 Ma，显示该区火山岩为早白垩世岩浆活动的产物。研究区火山岩SiO2含量为56.02％～66.45％，富钠、富碱、偏碱性（σ平均为4.01），为一套高钾钙碱性系列的火山岩。与区内埃达克质岩相比，除TYS49样品外，其余样品均具有较低的MgO含量为（0.52％～2.02％）和Mg#（16～42）。地球化学特征显示，火山岩富集Rb、Ba、Sr等大离子亲石元素（LILE）和轻稀土元素（LREE），相对亏损Nb、Ta、Ti等高场强元素（HFSE）和重稀土元素，具有弱的Eu负异常（δEu平均为0.85）； 具有富集的Sr-Nd-Pb同位素组成：87Sr/86Sr(t)=0.7084～0.7098，εNd(t)=-20.4～-16.2，206Pb/204Pb(t)=16.44～17.19，207Pb/204Pb(t)=15.39～15.47， 208Pb/204Pb(t)=36.82～37.65。研究表明，在郯庐断裂带伸展活动和岩石圈减薄背景之下，软流圈物质上涌使得地温升高而造成下地壳部分熔融，熔融的下地壳与岩石圈地幔相互作用形成了七宝山火山岩。断裂带的存在与活动，对华北克拉通可能具有破坏作用，使得岩石圈的稳定性和整体性遭到破坏，从而成为华北岩石圈减薄中的有利位置。

Geochemistry of Early Cretaceous volcanic rocks from the Qibaoshan area and their tectonic implications

Late Mesozoic volcanic rocks from the Qibaoshan area，called the Bamudi Formations，occur along the eastern margin of the Guanshuai Basin in the Shandong segment of the Tan Lu fault zone.Two volcanic rock samples yielded LA ICP MS 206Pb/238U zircon weighted mean ages were 120.1±0.6 Ma and 118.5±1.1 Ma respectively，belonging to the Early Cretaceous.These rocks are rich in alkali and Na2O concentrations with variable SiO2 contents(56.02％～66.45％).They are to calc alkaline to alkaline series with an average σ of 4.01 and most of them plot in the trachyte field with minor trachyandesite.Compared with adakites in studied area，these volcanic rocks have lower MgO contents(0.52％～2.02％) and Mg#(16～42)except TYS49 samples.The Qibaoshan volcanic rocks are enriched in LILEs(e.g. Rb，Ba，Sr)and LREEs and depleted in HFSEs(Nb，Ta，Ti)and HREEs，with weak negative Eu anomalies(average=085).They have enriched Sr Nd Pb compositions(ISr=0.708 4～0.7098，εNd(t)=-20.4～-16.2，206Pb/204Pb(t)=16.44～17.19，207Pb/204Pb(t)=15.39～15.47，208Pb/204Pb(t)=36.82～37.65).The geochemical characteristics of these rocks indicate that the primitive magma was dominantly derived from the lower crust of the North China Craton(NCC)with an involvement of materials from the lithospheric mantle and asthenosphere.The Qibaoshan volcanic rocks fromed in a crust extensional setting.The asthenosphere upwelling heated lithosphere and induced partial melting of lower crust，produced lithospheric mantle derived and crustal derived magmas.They mixed and solidified to form the Qibaoshan volcanic rocks.This paper further suggest that the Tan Lu fault zone is an intensive thinning zone，its occurrence and activities provides an inducing and promoting role during the lithosphere thinning and plays an important role in the destruction of the North China Craton.