吉林晚古生代榆木川基性岩的地球化学特征及其岩石成因

Geochemical characteristics and petrogenesis of late Paleozoic mafic rocks from Yumuchuan,Jilin Province

位于兴蒙造山带张广才岭块体的榆木川辉绿玢岩，K—AT年龄测定表明其形成年龄为262～264Ma，主微量元素研究表明，辉绿玢岩SiO2含量为47.02％～52.60％，贫K2O（0.13％～0.47％），Na2O〉K2O，属于低钾拉斑系列，稀土分布模式属于轻稀土略亏损的平坦型，微量元素原始地幔标准化图解中大离子亲石元素（Rh、Ba、Sr）较为富集，但不存在高场强元素（Nb、Ta）的亏损，同位素组成显示Sr同位素初始比值在0.704958～0.705631之间，εNd（t）值（72～81）较高且变化较小，表明其来源于亏损地幔的部分熔融且成岩过程中未受到地壳物质的混染。基性岩形成于造山后的伸展拉张背景，并结合前人的研究成果，认为晚古生代（约270Ma）兴蒙造山带已经进入晚期，此时加厚的岩石圈由于重力不稳定发生拆沉减薄作用，导致软流圈的大量上涌和岩石圈拉张，从而引起先存亏损岩石圈地幔的部分熔融，最后侵入地壳深部形成研究区辉绿玢岩。

Yumuchuan sillites located in the Zhangguangcailing Block of the Xingmeng orogenic Belt, K-Ar dating yields the ages ranging from 262 to 264 Ma, geochemical data show that the SiO2 contents of the sillites vary from 47.02% to 52.60%, these rocks belong to low potassium tholeiite series, and are characterized by low K20 （0.13% 0.47%） and Na20 〉 K20. Chondrite-normalized patterns for the sillites exhibit flat but moderate depletion in LREE （（La/Yb）N 0.65 0.87）, and the primitive mantle-normalized spidergrams show relatively enrichment in Rb and Ba, but without depletion in the HSFE, such as Nb and Ta. Strontium and Nd isotopic compositions of the representative sillites are characterized by low initial 87Sr/86Sr ratios （0.704958-0.705631）, high and constant εNd（t） values （7.2 8.1）, suggesting that they were derived from a depleted mantle source without significant crustal contamination. It is generally accepted that mafic intrusions are formed in a post-orogenic extensional tectonic setting, based on the available studies. We thus propose a petrogenetic model for the studied sillites, i.e., in the late period of the Xingmeng orogenic belt, lithospheric delamination occurred due to the gravitational instability of the thickened lithosphere, and then resulted in the upwelling of hot asthenosphere, lithospheric extension, as well partial melting of pre-existing depleted lithospheric mantle, meanwhile, extensive fractional crystallization of minerals, such as olivine and clinopyroxene during primary magma ascent, finally, intruded and formed Yumuchuan sillites.