江西广丰碱性橄榄玄武岩的成因:SHRIMP锆石U-Pb年龄和元素、Sr-Nd-Pb同位素制约

Petrogenesis of Alkali Olivine Basalt from Guangfeng Basins in Jiangxi Province: SHRIMP zircon U-Pb Dating and Element、Sr-Nd-Pb Isotope Constraints

广丰盆地红色沉积岩系下部夹有一层玄武岩,玄武岩的锆石具有明显的环带结构,显示岩浆成因的特征;SHRIMP锆石U-Th-Pb分析结果显示：Th/U=0.28～1.28,13个分析点的206Pb/238U年龄变化为88.5～102.5 Ma,加权平均年龄为（99.0±0.7）Ma,MSWD=1.3,地质时代属晚白垩世早期。在Si O2-（K2O＋Na2O）和Nb/Y-Zr/Ti O2图解上均落入碱性玄武岩范围,显示碱性系列的特征,在TFe O/Mg O-Si O2图解上显示拉斑系列的特征,属典型的碱性橄榄玄武岩。较低的Nb/Th（=2.21～4.98,平均3.43）、Ti/Yb（=3 125～5 753,平均3975）,较高的Nb/U（=6.63～25.82,平均16.24）和δ18O值（=8‰～9.6‰,平均值为8.9‰）,指示碱性橄榄玄武岩有地壳物质的成分。（87Sr/86Sr）i-Si O2,εNd（t）-Si O2,（87Sr/86Sr）i-10 000/Sr和εNd（t）-100/Nd图解显示,地壳物质的加入主要是源区混合的结果。在（143Nd/144Nd）i-（206Pb/204Pb）i,（87Sr/86Sr）i-（206Pb/204Pb）i和（143Nd/144Nd）i-（87Sr/86Sr）i图解上,碱性橄榄玄武岩均显示是由亏损地幔（DMM）和岩石圈富集地幔（EMⅡ）端元混合而成,且亏损地幔成分所占比例高于浙东玄武岩,与浙西玄武岩相近。

There is a layer of basalt at the bottom of Guangfeng basin of red sedimentary rocks, the zircon of basalt has obvious band structure which display a characteristics of magma genesis. The SHRIMP zircon U-Th- Pb analytical results shows：Th/U =0.28 - 1.28,13 analysis points of ^206pb/^238U age ranges of 88.5 - 102.5 Ma, with a weighted average of （99.0 ＋ 0.7 ） Ma, MSWD - 1.3, belonging to The early Late Cretaceous of Product of volcanic activity. Plotted within the field of alkaline series in SiO2-（ K2O ＋ Na2O）diagram and in Nb/Y-Zr/TiO2 diagram, dispalys the Characteristics of alkaline series ; the values fall the field of tholeiitic basaltic rock in the TFeO/MgO-SiO2 diagram. Therefore, it belongs to the typical alkaline olivine basalt at the bottom of Guangfeng Basin of Red sedimentary rocks. Alkali olivine basalt with a low Nb/Th and Ti/Yb value, Nb/Th = 2.21 -4.98 （3.43 on average）,Ti/Yb =3125 -5753（3 975 on average）;a high Nb/U and δ^8O value, Nb/U =6.63 - 25.82（ 16.24 on average）, δ^18O = 8‰ - 9.6‰（ 8.9‰ on average）, Indicates alkaline olivine basalt have the contribution of crustal material. In （^87Sr/^86Sr） i-SiO2, εNd（t） -SiO2, （^87Sr/^86Sr） i-10000/Sr and εNd（t） - 100/ Nd diagrams dispaly that joined crustal matter is the result of the magma source area mixxing . In the （^143 Nd/ ^144Nd） i- （ 206 ^Pb/^204 Pb ） i, （ ^87 Sr/^86 Sr ） i- （ ^206 Pb/^204 Pb ） i and （ ^143 Nd/^144 Nd ） i- （^ 87 Sr/^86 Sr ） i diagrams, the values fall exclusively between the DMM and EM I1 positions, and there is a higher proportion of DMM than the eastern Zhejiang basalt.