汝城地区晚中生代火山岩地球化学特征及其对源区属性的指示

GEOCHEMISTRY OF LATE MESOZOIC VOLCANIC ROCKS IN RUCHENG AREA AND ITS IMPLICATION FOR THE SOURCE NATURE

湘东南汝城地区发育一套由基性玄武岩和中酸性安山质-英安质岩石组成的火山岩建造,属于低钾拉斑系列,该火山岩系中两个玄武岩的K-Ar年龄分别为124.5±2.5Ma和127.6±1.9Ma,属晚侏罗—早白垩世产物。在主、微量元素上两者成分存在明显差异。其中安山质-英安质岩石具有高MgO特征,属高MgO岩石,LILE富集、Nb-Ta、Sr-P亏损强烈,(La/Yb)N=6.7～7.9,Eu*/Eu=0.74～0.85,具岛弧型微量元素配分型式,87Sr/86Sr(t)=0.71079～0.71118,εNd(t)=-7.64～-8.16,与adakites高Mg岩石有着明显的差别,可能是富集岩石圈地幔熔融后直接分异的产物;玄武岩LILE富集,Nb-Ta富集,(La/Yb)cn=4.0～4.3,Eu*/Eu=1.00～1.16,具OIB型微量元素配分型式,87Sr/86Sr(t)=0.70812～0.70832,εNd(t)=0.48～1.03,其源区具二元混合趋势,其源区可能是富集型岩石圈地幔端员与亏损的软流圈地幔端员的混合产物。汝城地区晚中生代玄武岩和高Mg安山质-英安质岩石源区属性的限定及其相互的空间依存关系表明该区晚中生代时有着较薄的岩石圈厚度,处于岩石圈伸展减薄的大地构造背景。

The volcanic rocks in Rucheng, southeast of Hunan Province, composed of basalts and andesite dacites, belong to low K calc alkaline series. The two basalt samples from this volcanic sequence yield the K Ar ages of 124.5±2.5Ma and 127.6±1.9 Ma, respectively. These rocks from basalt and andesites dacites have distinct elemental and isotopic compositions. Among them, andesites dacites have high MgO (MgO=3.93%-5.07%) content, and show enrichment in LILE and depletion in Nb Ta Ti P with (La/Yb)N=6.7 7.9, Eu*/Eu=0.74 0.85. The basalts display enrichment in LILE and enrichment in Nb Ta relative to La with (La/Yb)cn=4.0 4.3, Eu*/Eu=1.00 1.16. The high MgO andesite dacites characterized by arc island type trace element patterns while OIB patterns for basalts. Corrected age initial 87Sr/86Sr ratios are that high MgO andesites dacites range from 0.71079 to 0.71118 and εNd(t) from -7.64 to -8.16, whereas basalts show 87Sr/86Sr(t)=0.70812 0.70832 and εNd(t)=0.48 1.03. It is concluded that high MgO andesites dacites are derived from the fractionation of the enriched lithospheric mantle melting and that the basalts are originated from the mixture of enriched lithospheric mantle and asthenospheric mantle in response to intra continental lithospheric extension and thinning.