闽西南基性岩脉中捕获锆石SIMSU-Pb年龄及Hf、O同位素特征

Zircon SIMS U-Pb Age,Hf and O Isotopes of Mafic Dikes,Southwest Fujian Province

测定了闽西南地区5件基性岩脉的全岩主微量元素,锆石U-Pb年龄和Hf,O同位素。5件岩脉主要为辉绿岩,其全岩SiO_2含量为45%~53%,稀土元素显示轻稀土富集的右倾配分模式。基性岩脉中大部分锆石具明显振荡环带和扇状环带,为典型岩浆结晶锆石特征。锆石U-Pb年龄(96~2 400 Ma)分布较为分散。除少量锆石年龄(96~142 Ma)可能指示岩脉形成年龄外,其余均为捕获锆石。捕获锆石年龄主要分布在4个范围:早元古代(2 467~1 796 Ma),中晚元古代—震旦纪(1 343~647 Ma),志留纪—晚三叠世(427~225 Ma),晚侏罗世(159~140 Ma)。Hf-O同位素显示早元古代锆石来源于接近球粒陨石均一储库的地幔。中晚元古代以后年龄的锆石其Hf-O同位素均具有亏损地幔岩浆与地壳组分混合的特征;志留纪—晚侏罗世锆石主要来源于S型壳源花岗岩的重熔;早白垩世晚期的锆石ε_(Hf)(t)值与δ^(18)O值清晰地显示出亏损地幔与地壳岩浆混合的趋势。闽西南基性岩脉中锆石的二阶段Hf亏损地幔模式年龄(TDM2)峰值主要分布在1.6~1.9 Ga,说明早元古代晚期幔源岩浆作用形成的基性岩地壳可能是形成后期花岗岩的主要源区。

The study in this paper determined whole rock major and trace elements,zircon U-Pb age and Hf,O isotopes of 5 mafic dikes in the southwestern Fujian province. The 5 dikes are mainly diabase and the whole rock SiO_2 content are between 45% ~ 53%. Most zircons of the mafic dikes display obvious oscillatory zoning and fanshaped zoning,and have the typical magmatic zircon crystallization characteristics. Zircon U-Pb age is dispersed with 96 ~ 2 400 Ma range. In addition to the minimum age（ 96 ~ 142 Ma） which might be the age of the formation of dikes,the remaining are captured zircon. The captured zircon age was mainly distributed in 4 groups： Early Proterozoic（ 2 467 ~ 1 796 Ma）; Middle and late Proterozoic（ 1 343 ~ 647 Ma）; Silurian to late Triassic Epoch（ 427 ~ 225 Ma）; and Late Jurassic（ 159 ~ 140 Ma）. Hf-O isotope shows that the early Proterozoic zircon was derived from the mantle of the homogeneous chondrite reservoir,and the others show magmatic mixing characteristics between depleted mantle and crust. Zircon＇s ε_（Hf）（ t） and δ^（18）O of the early Late Cretaceous clearly show the mixing trend of depleted mantle and crustal magma. The peak of zircon Hf two-stage depleted mantle model age TDM2 was mainly distributed in the 1. 6 ~ 1. 9 Ga. The Early Proterozoic mafic crust might be the main source for latter granite.