湖南攸县新市玄武岩锆石LA-ICP-MS U-Pb定年及其地球化学特征

The Zircon LA-ICP-MS U-Pb Ages and Geochemistry Features of Basalt at Xinshi,Youxian,Hunan Province

新市玄武岩呈狭长带状分布,出露宽50-200m,延伸25 km以上,厚35.8m,由两个不完整喷发旋回的玄武质熔岩组成,顶底由气孔—杏仁状玄武岩组成,中部为致密状玄武岩;夹于下白垩统神皇山组中;经LA-ICP-MS锆石U-Pb定年,成岩年龄为132.7±1.2 Ma(MSWD=0.058),为早白垩世产物。通过岩石学、岩石地球化学等综合研究,岩石类型主要为亚碱性的玄武安山岩和玄武粗安岩;SiO2含量<53.88%,Na2O+K2O较低,属高钾钙碱性-钙碱性拉斑玄武岩系列;ΣREE较高,均大于281.79×10-6,δEu值较大,均大于1.05;Isr平均0.7069,TDMⅡ平均0.74 G a,εNd(T)平均+2.2,表明其物质来源于原始地幔,形成于陆内拉张环境。

Xinshi basalt was long and narrow banded outcrop width 50 - 200 m, extending more than 25 km long and a thickness of 35.8 m, composed of basaltic lava from two incomplete eruption cycles, top and bottom are stomata - almond-shaped basalts, central is the dense basalt, and occurred in the Lower Cretaceous Huangshenmiao Formation. LA-ICP-MS U-Pb dating of zircons from Xinshi basalt are yielded age of 132.7 ± 1.2 Ma （MSWD=O.058） which means it erupted in Early Cretaceous. Comprehensive study of petrology, geochemistry, Xinshi basalt are composing of subalkaline basaltic andesite and basaltic trachyandesite; SiO2 content of less than 53.88%, lower Na2O ＋ K2O indicate it is belongs to high-K calc-alkaline to calc-alkaline tholeiite series; ∑ REE, are greater than 281.79, 8 Eu value is usually larger than 1.05; the average Isr is 0.7069, the average TDM I is 0.74 Ga, the average εNd （T） is ＋2.2. All the geochemical evidence indicate that the basaltic magma is derived from the primitive mantle in the intracontinental extensional environment.