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±0.6 Ma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％...

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.