太行山南段中生代杂岩体的岩石成因:元素和Nd-Sr-Pb同位素地球化学证据

Petrogenesis of the Mesozoic Intrusive Complexes from the Southern Taihang Orogen, North China Craton: Elemental and Sr-Nd-Pb Isotopic Constraints

通过太行山南段三个中生代杂岩体(西戍、武安和洪山)的元素和同位素地球化学特征的研究,讨论其成因和地球动力学环境。结果表明,西戍和武安杂岩体主要由从二长辉长岩到二长岩的一系列岩石组成,其地球化学性质相似(高Mg#,具微弱至正Eu异常的REE模式等)。西戍杂岩体的ENd(135 Ma)=-12.3- -16.9,ISr= 0.7056-0.7071,与武安杂岩体稍有不同。西戍杂岩体的(206Pb/204Pb)i=16.92-17.3,(207Pb/204Pb)i=15.32- 15.42,(208Pb/204Pb)i=37.16-37.63,较武安杂岩体的略高。西戊-武安杂岩体都起源于EM1型富集地慢,但被下地壳物质不同程度混染。洪山杂岩体(正长岩-花岗岩)也来自EM1型富集地幔的部分熔融,但属于不同的岩浆事件,并仅受轻微的下地壳混染。太行山岩浆作用的发生可能与古太平洋板块的水平俯冲消减而形成的弧后伸展环境有关。

Geochemical and isotopic study was carried out for three Mesozoic intrusive suites (the Xishu. Wu'an and Hongshan suites) from the North China Craton (NCC) to understand their genesis and geodynamic implications. The Xishu and Wu'an suites are gabbroic to monzonitic rocks shaning many common geochemical features like high Mg# and minor to positive Eu anom alies in REE patterns. Initial Nd-Sr isotopic compositions of Xishu suite are εNd (135 Ma) - 12. 3 to - 16. 9 and from 0. 7056 to 0. 7071 for ISr respectively; whereas those of the Wu'an suite are slightly different. Pb isotopic ratios of the Xishu suite are 16. 92-17. 3 for (206 Pb/201 Pb), 15. 32- 15. 42 for (207 Ph/204Pb), and 37. 16-37. 63 for (208Pb/204Pb), respectively. They are slightly higher than those of the Wu'an suite. It is considered that the Xishu-Wu'an complexes were originated from partial melting of an EM1 type mantle source, followed by significant contamination of lower crustal components. The εNd values ( -8 to -11)of the Hongshan suite (mainly syenite and granite) are distinctly higher than those of the Xishu-Wu'an suites, whereas the Pb isotopic ratios of the Hongshan suite are only slightly higher than those of the Xishu-Wu'an suites. It is believed that the Xishu-Wu'an complex was formed via fractionation of a parental magma originated from the EM1-type mantle source, with various extent contamination of lower crustal components, whereas the Hongshan suite was formed via fractionation of a parental magma originated from the EM1 - type mantle source in another magmatic movement, with very weak contamination of lower crustal component. It is believed that the Taihang magmatic movements could be related with the back-arc extensional regime caused by the subduction of the paleo-Pacific slab beneath the NCC,