华北克拉通减薄的岩浆岩响应:来自河北武安洪山含霓石斑状正长岩的证据

Magmatic response to lithospheric thinning of the North China Craton:evidence from porphyritic aegirite-bearing syenite in Wu’an,Hebei,China

洪山碱性杂岩体出露于河北省邯郸市永年县,地处太行山南段,出露面积约53 km^2,岩性以正长岩类为主体,侵位于晚古生代—中生代地层中。洪山碱性杂岩体中正长岩属于钾质的碱性系列岩石,形成于拉张环境,其形成年龄为(120.1±4.6) Ma,是华北克拉通岩浆作用峰期的产物。岩体稀土元素配分图中无Eu异常,表现为轻稀土元素富集的配分型式。富集Rb、K、Sr等大离子亲石元素,亏损Nb、Ta、Ti等高场强元素,具有大陆地壳特征。野外观察到岩石具有从深成到浅成-超浅成的特征,电子探针发现长石出现反环带,正长岩中不含继承锆石等特征,结合若要使地壳内岩石部分熔融产生无铕异常的正长质岩浆,那么压力应大于20 kbar,说明洪山碱性杂岩体形成于加厚地壳局部熔融,后经历了少量幔源岩浆混合及地幔流体的注入等过程,导致正长岩出现一定的地幔Hf、Nd同位素印记,而未改变其主微量元素特征,随后在拉张环境下快速上升至地表浅处。

The Hongshan alkaline complex is located in the southern section of the Taihang Mountains,Hebei Province with an outcrop area of about 53 km^2.Its lithology is dominated by syenite and intruded in the Late Paleozoic-Mesozoic strata.Our study showed that the alkaline intrusive complex belongs to alkaline series,formed in an extensional environment at the age of 120.1±4.6 Ma as a product of the peak period of the North China Craton magmatism.The REE patterns showed no Eu anomaly.The complex is enriched in light rare earth and large ion lithophile elements,such as Rb,K,Sr,and depleted in high field strength elements,such as Nb,Ta and Ti,exhibiting continental crustal characteristics.We observed in the field that the rocks are characterized by deep and shallow-ultra shallow formation.We also observed through electron microprobe analysis that the feldspar has a reverse belt that contained no inherited zircon.These observations,combined with the notion that partial melting of Earth’s crust is needed to produce syenitic magma without Eu abnormalities,suggested a formation pressure of greater than 20 kbar.That is,the Hongshan syenite formed by partial melting of the thickened crust with some mantle derived magma mixing and mantle fluid injection to produce its mantle Hf and Nd isotopic signatures without altering the major and trace elemental characteristics,and then rose rapidly to the Earth surface under extensional condition.