乌拉特中旗克布岩体的地球化学特征及SHRIMP定年：早二叠世华北克拉通底侵作用的证据被引量：60

Geochemistry and SHRIMP dating of the Kebu massif from Wulatezhongqi,Inner Mongolia:evidence for the Early Permian underplating beneath the North China Craton

下载PDF

导出

摘要乌拉特中旗克布岩体位于内蒙古中西部,属于华北克拉通北缘中段。克布岩体具有埃达克岩的特征:SiO_2= 57.66～66.94%(>56%),Al_2O_3=16.81～18.47%(>15%),MgO=1.33～3.28%(<3%);Y=7.25～13.46×10^(-6) (≤18×10^(-6)),Yb=0.72～1.42×10^(-6)(≤1.9×10^(-6)),Sr=540.4～655×10^(-6)(>400×10^(-6));高场强元素(HFSEs,如Nb、Ta、Ti、Zr)含量低,LRRE富集(La/Yb=12.11～28.56),Eu为弱负异常至正异常(δEu~*=0.95～1.09)。高Sr/Y(48.66～74. 57)、La/Yb以及中等Mg~#(38.72～46.16)、K_2O(1.11～2.17)近似于壳源埃达克岩。克布岩体具有低ε_(Nd)(t)(=-15.1),高(^(87)Sr/^(86)Sr)_i(=0.7073),也表明了它们的源岩来自大陆下地壳,明显不同于板片熔融来源的埃达克岩同位素特征,而与中国东部的埃达克质岩近似。与华北克拉通晚侏罗世下地壳拆沉作用形成的埃达克岩相比,克布岩体具有低的过渡金属元素含量。克布岩体可能为玄武质液体底侵,导致变厚的下地壳基底部分熔融而形成。克布岩体中锆石的SHRIMP U-Pb锆石年龄为291±4Ma,表明早二叠世初期华北克拉通下的底侵作用已经存在。 The Kebu massif is located in Wulatezhongqi, west centre of Inner Mongolia and belongs to the midsection of the northern margin of the North China Craton. The Kebu massif is characterized of adakite. They have SiO2 = 57.66 -66.94% （〉 56% ）, Al2O3 = 16.81-18.47% （〉15%）, MgO = 1.33-3.28% （〈3%）, Y = 7.25-13.46x10^-6（≤18×10^-6）, Yb =0.72-1.42× 10-6（≤1.9 × 10^-6） and low in high-fields strength elements （HFSEs, e.g. Nb, Ta, Ti, Zr） relative to continental arc andesitedacite-rhyolite （ADR）, Sr =540.4 -655.0×10^-6（〉400 ×10^-6） , rich in LRRE （La/Yb = 12.11 -28.56）, slight negative to positive Eu anomaly （δEu ＊ = 0.95 - 1.09）. High Sr/Y （48.66 - 74.57 ）, La/Yb and moderate Mg^# （ 38.72 - 46.16） , K2O （ 1.11 -2.17） is similar to the adakite derived from the thickened lower crust. Low εNd（t） values （ - 15. 1） and high （^87Sr/^86Sr）i（0.7073） are also inconsistent with an origin by slab melting, but resemble the adakitic rock from the East China. However, the Kebu massif is low in Cr, Ni and Mg^# relative to the adakite formed by lower crust foundering from the North China craton during the Late Jurassic. This leads us to conclude that the Kebu massif was formed by partial melting of marie material at the base of ancient continental crust due to basaltic liquid underplating. The SHRIMP U-Pb zircon age of the Kebu massif is 291±4Ma, which means that underplating in the North China craton has been existed since the Early Permian.

作者罗红玲吴泰然李毅

机构地区教育部造山带与地壳演化重点实验室(北京大学)