黔南基性岩墙岩石地球化学、SHRIMP锆石U-Pb年代学及地质意义

Petrogeochemistry,Zircon SHRIMP U-Pb Geochronology of Mafic Dykes in South Guizhou and Their Geological Implications

基性岩墙,与层状、环状基性杂岩体和高Ti、低Ti玄武岩共同组成了峨眉山大火成岩省岩石组合.为进一步确定大火成岩省及相关生物灭绝事件的时间联系,及更深化研究大火成岩省的成因,对分布于贵州省南部的基性岩墙进行了主、微量元素、Sr-Nd同位素测定和锆石SHRIMP U-Pb年代学研究.黔南基性岩墙∑REE=135.66×10-6~280.59×10-6,LREE/HREE为6.42~7.54,(La/Yb)N为7.94~9.85,轻重稀土分异明显,δEu为1.0~1.3,具有Ba、Sr、K等LILE富集,Nb、Ta、Zr、Hf等HFSE亏损特征,显示与峨眉山高钛玄武岩相似的地球化学特征.Th/Ta(1.80~1.94)、Nb/U(30.8~39.88)、Th/La(0.08~0.10)、Nb/Th(7.89~8.40)比值与原始地幔相似,较低的初始(87Sr/86Sr)i比值(0.705 278~0.706 052)、εNd(t)(-0.5^+1.6)、以及Th/Ta比值(<2.13)显示岩浆无明显的地壳混染,岩浆可能形成于受地幔柱作用的富集石榴石地幔源区10%~12%的部分熔融.SHRIMP锆石206Pb/238 U加权平均年龄为261.2±2.6Ma,反映峨眉山大火成岩的喷发时间可能集中在260Ma左右,并可能与瓜德鲁普末期的生物灭绝有关.

The rocks of Emeishan large igneous province（ELIP）are mainly composed of mafic dykes,layered and ring mafic complex,high Ti and low Ti basalts.The major elements,trace elements,Sr-Nd isotopes and zircon SHRIMP U-Pb ages of mafic dykes from South Guizhou Province were studied in order to further determine the temporal link between ELIP and related mass extinction events and research the genesis of ELIP in this paper.Results show ∑REE=135.66×10^-6-280.59×10^-6,LREE/HREE=6.42-7.54,（La/Yb）N=7.94-9.85,relatively enriched LREE to HREE,δEu=1.0-1.3,enriched LILE,such as Ba,K,Sr,depleted HFSE,such as Nb,Ta,Zr,Hf,displaying similar geochemical characteristics of Emeishan high Ti basalts.The similar ratios of Th/Ta（1.80-1.94）,Nb/U（30.8-39.88）,Th/La（0.08-0.10）,Nb/Th（7.89-8.40）to primitive mantle,low（^87Sr/^86Sr）i（0.705 278-0.706 052））,εNd（t）（-0.5-＋1.6）and Th/Ta（2.13）imply no obvious crustal contamination,and the mafic rocks may have been caused by 10%-12% partial melting of the garnet peridotite in their source region that interacted with the upwelling Emeishan plume at the periphery of the plume.The 261.2±2.6 Ma age of zircon SHRIMP U-Pb may imply that the ELIP formed by a mantle plume at-260 Ma and was one of the likely causes of the end-Guadalupian mass extinction.