山西繁峙新生代玄武岩地幔源区及成因探讨:元素及Sr-Nd-Pb-Hf同位素地球化学证据

Mantle sources and petrogenesis of the Cenozoic basalts in Fanshi,Shanxi Province: Geochemical and Sr-Nd-Pb-Hf isotopic evidence

山西省繁峙玄武岩位于华北克拉通重力梯度带附近，是华北克拉通中部新生代玄武岩重要组成部分。前人全岩K-Ar测年结果为26．3-24．3Ma。对繁峙地区苏孟庄和应县两地玄武岩的地球化学特征研究表明，其微量元素和同位素均具有类OIB特征，即富集不相容元素，轻、重稀土元素分馏明显（（La／Yb）N=8．42～21．60），不存在Sr、Eu负异常，Sr同位素比值（^87Sr／^86Sr=0．703848-0．704870）较低，Nd（^143Nd／^144Nd=0．512617—0．513057）和Hf（^176Hf/^177Hf=0．282873～0．283001）同位素比值较高，Pb同位素比值分别^206Pb／^2014Pb=17．2—17．9，^207Pb／^204Pb=15．3～15．4和^208Pb／^204Pb=37．5—37．9。结合岩相学特征和主量元素特征，我们推断繁峙新生代玄武岩是软流圈低程度部分熔融结果，并存在岩石圈物质的加入，岩浆上升时在岩石圈地幔条件下的岩浆房内经历了以橄榄石、单斜辉石为主的分离结晶作用，岩浆因快速上升而地壳混染程度甚低。苏孟庄碱性玄武岩具有较深的熔融深度和较低的熔融程度，而应县亚碱性玄武岩熔融深度较浅，熔融程度较高。结合重力梯度带附近其他地区的新生代玄武岩的研究，我们推测重力梯度带附近新生代的火山活动可能起源于西部软流圈地幔向东流动越过重力梯度带时的减压部分熔融，该地区广泛分布的断裂带为岩浆上涌提供了通道。本文为中国东部新生代玄武质火山活动的岩石成因学研究提供了新的视角。

The Fanshi basalts located in the vicinity of the Da Hinggan Ling-Taihangshan gravity gradient zone, represent a major component of Cenozoic basalts of the central North China Craton. Previous studies gave the Fanshi basalts whole rock K-Ar ages of 26.3 - 24. 3Ma. The Fanshi basalts from Sumengzhuang and Yingxian two locations all show OIB-like trace element and isotopic signatures, i. e. , they are enriched in incompatible elements with highly fractionated LREEs and HREEs （（La/Yb） N = 8.42 - 21.60） without negative Sr and Eu anomalies. They show relatively low Sr （^87 Sr/^86 Sr = 0. 703848 -0. 704870） and high Nd （ ^143 Nd/^144 Nd = 0. 512617 - 0. 513057）, Hf （ ^176 Hf/^177 Hf = 0. 282873 - 0. 283001 ） isotope ratios, and Pb isotope ratios were ^206Pb/^204 Pb = 17.2 - 17. 9, ^207Pb/^204Pb = 15.3 - 15.4 and ^208Pb/^204 Pb = 37.5 - 37.9. All these geochemical features, combined with petrographic observations and major element data, allow us to suggest that the Fanshi basalts were derived from small degree partial melting of asthenospherie mantle with contributions of metasomatized lithospheric mantle. Olivine and clinopyroxene dominated fractional crystallization is an important process in the petrogenesis of these basalts, which may have taken place in magma chamber of lithospheric mantle condition. The rapid ascent explains the lack of crustal contamination. Sumengzhuang samples are characterized by relatively deeper and lower extent of melting, whereas the Yingxian samples have the signature of slightly shallower and higher extent of melting. Together with studies of Cenozoic basalts in other areas near the gravity gradient zone, we suggest that the Cenozoic basalts near the Gravity Gradient zone may be originated from decompression melting of eastward asthenosphere flow when crossing the gravity gradient zone. This study offers some new perspectives on the petrogenesis of the Cenozoic basaltic volcanism in eastern China in general.