甘肃西秦岭新生代钾霞橄黄长岩和碳酸岩的微量、稀土和Sr,Nd,Pb同位素地球化学:地幔柱-岩石圈交换的证据

Trace elements, REE and Sr, Nd, Pb isotopic geochemistry of Cenozoic kamafugite and carbonatite from west Qinling,Gansu province: Implication of plume-lithosphere interaction

西秦岭新生代钾霞橄黄长岩和碳酸岩具有强烈富集LILE和LREE的特征,经球粒陨石标准化的REE分配模式与OIB十分相似。钾霞橄黄长岩和碳酸岩的(^(87)Sr/^(86)Sr)i分别在0.70381～0.70940和0.70529～0.71332之间,^(144)Nd/^(143)Nd分别介于0.512404～0.512924和0.512210～0.512928之间。经计算获得多数样品的ε_(Nd)落在-3.4～5.58范围内,与OIB的ε_(Nd)值一致。这两类岩石的^(208)Pb/^(204)Pb、^(207)Pb/^(204)Pb和^(206)Pb/^(204)Pb分别为37.613～39.330和38.060～38.995,15.842～16.441和15.545～15.677,以及18.418～22.4和18.149～19.062。采用主量元素MgO-Ni和ε(Nd)-(^(87)Sr/^(86)Sr)相关图,以及高场强元素比值Zr/Nb-La/Nb和Ba/La-Ba/Nb相关图以及^(208)Pb/^(204)Pb-^(206)Pb/^(204)Pb,^(207)Pb/^(204)Pb-^(206)Pb/^(204)Pb相关图,一致证明本区火山岩具有与洋岛玄武岩(OIB)相似的地球化学特征,且源区具有EM1和EM11富集端员的混合。但是本区火山岩高的 Nb/Ta比和强烈富集Nb等高场强元素,以及较高的^(144)Nd/^(143)Nd值,表明该火山岩地球化学具有某种特殊性。结合对西秦岭深部地球物理资料及地质构造背景和演化历史的分析,提出西秦岭新生代钾霞橄黄长岩和碳酸岩的成因与地幔柱的活动有关,源区包含了EM1?

The kamafugites and carbonatites in West Qinling, Gansu province have the characteristics rich in LILE and LREE. The distribution pattern of the REE by chondrite-normalized is very like to that one of OIB. (Sr-87/Sr-86) i of the kamafugite and carbonatite is 0.70381 similar to 0.70940 and 0.70529 similar to 0.71332 respectively, Nd-144/Nd-143 of the volcanic rocks is 0.512404 similar to 0.512924 and 0.512210 similar to 0.512928 respectively. epsilon(Nd) calculated is in the range of -3.4 similar to 5.58, and very like to that epsilon(Nd) of OIB. Pb-208/Pb-204 Pb-207/Pb-204 and Pb-206/Pb-204 of the two type of volcanic rocks are 37.613 similar to 39.330 and 38.060 similar to 38.995, 15.842 similar to 16.441 and 15.545 similar to 15.677, and 18.418 similar to 22.4 and 18.149 similar to 19.062 respectively. It proved that West Qinling Cenozoic volcanic rocks have the geochemical characteristics similar to OIB, and the end-menbers of EM1 and EM11 in the sources in the diagrams of MgO vs. Ni, epsilon(Nd) vs. (Sr-87/Sr-86), Zr/Nb vs. La/Nb, Ba/La vs. Ba/Nb, Pb-208/Pb-206 vs. Pb-206/Pb-204 and Pb-207/Pb-204 VS. Pb-206/Pb-204. However, there are some specific characteristics, such as very high Nb/Ta and Nb, and higher Nd-144/Nd-143 than that one of OIB in in the volcanic rocks. In summary, according to the data of deep-geophysics, geological background and evolution history of West Qinling, the genesis of the end-menbers of EM1 and EM11 may be related to the fluid action from mantle plume/asthenosphere, dehydration of the ocean platform and detachment of the continental lithosphere. For this reason, the genesis of the Wesi Qinling Cenozoic volcanic rocks may be related to interaction of mantle plume-and lithosphere.