河南舞阳南部张士英岩体的地球化学与成因及其构造意义

LA-ICPMS Zircon U-Pb Geochronology and Geochemical Characteristics of the Zhangshiying Syenite From South of Wuyang,Henan Province

张士英岩体岩石类型以石英二长岩、石英正长岩为主。岩体属于准铝质的高钾-钾玄岩系列(Al2O3为14.8%～16.7%,A/CNK=0.77～1.02,K2O/Na2O在1.00～1.43之间),稀土元素总量在261.7～381.0μg/g之间,轻稀土元素相对富集,稀土元素球粒陨石标准化模式呈右倾平滑曲线,(La/Yb)N比值为28～50,具有轻微的Eu负异常,Eu/Eu?比值在0.73～0.87之间。微量元素组成上表现为Sr、Ba、Nb、Ta的亏损,而Th、U相对富集。(87Sr/86Sr)i初始平均值为0.709,εNd(t)值为-13.9～-19.9,Nd的模式年龄为1.48～2.10Ga。岩体锆石LA-ICP-MSU-Pb年龄为(124.2±0.5)Ma,为早白垩世岩浆作用的产物。岩体侵位的区域地质背景为华北克拉通白垩纪大规模岩石圈转型和减薄时期,岩石的微量元素及同位素特征表明,岩体主要来源于下地壳物质的部分熔融,但有年轻地幔物质的加入。反映在区域强烈伸展拉张和岩石圈减薄背景下,软流圈地幔岩浆底侵所造成的下地壳部分熔融及壳幔混合过程。

The Zhangshiying syenite is located to the south of Wuyang,Henan Province,and tectonically on the southern margin of the North China craton.The syenite intrusion consists mainly of quartz monzonite and quartz syenite.The rocks are of metaluminous and high-K-shoshonitic（Al2O3=14.8%~16.7%,A/CNK=0.98~1.10,K2O/Na2O=1.00~1.43）,which are rather high in rare earth elements（ranging from 261.7μg/g to 381.0μg/g）,and characterized by relative enrichment of LREE with（La/Yb）N ratio of 28~50 and weak negative Eu anomalies（Eu/Eu=0.73 ~0.87）,and show smooth right dipping chondrite normalized REE patterns.The rocks are significantly depleted in Sr,Ba,Nb,Ta and relatively enriched in Th and U.The isotopic compositions of the rocks strongly suggest a crustal origin regarding the high（^87Sr/^86Sr）i ratios（0.709）,low εNd（t） value（-13.9^-19.9） with Nd model ages range from 1.48Ga to 2.10Ga.LA-ICP-MS U-Pb zircon dating gives a crystallization age of（124.2±0.50）Ma.The syenite was formed in early Cretaceous under large-scale lithospheric thinning and transformation tectonic regime of the North China craton.Trace elements and isotopic characteristics of the rocks suggest that the magma was generated through partial melting of the lower crust with involvement of juvenile mantle material.The syenite might have been formed through partial melting of the lower crust coupled with involvement of juvenile mantle material and high degree fractional crystallization,which was triggered by underplating of asthenospheric mantle magma under the extensive extension and lithospheric thinning tectonic settings.