天津蓟县孙各庄碱性杂岩体年代学和岩石地球化学特征

Geochronology and Geochemistry of Sungezhuang Alkaline Complex in Jixian County,Tianjin

通过对天津蓟县孙各庄碱性杂岩体开展系统的主量、稀土和微量元素岩石地球化学研究和高精度的锆石SHRIMP U-Pb测年研究,厘定岩体的形成时代,全面认识该区碱性岩浆活动的性质、物质来源和构造背景,并探讨其重要的地质意义。研究表明:孙各庄碱性杂岩体中正长岩的锆石SHRIMP U-Pb年龄为(225.6±1.1)Ma,形成于晚三叠世,为印支期碱性岩浆活动的产物。岩体主要岩石类型为碱性正长岩和正长辉石岩:前者高Al,富碱,贫Ca、Mg和Fe,为铝过饱和岩石;后者富碱,高Ca、Mg、Fe;均属于碱性-强碱性岩石,且相对富K。稀土元素总量高,轻、重稀土元素分馏明显,具极微弱的负Eu异常。微量元素富集大离子亲石元素Sr、Ba和轻稀土元素La、Ce、Pr、Nd、Sm,相对亏损高场强元素Nb、Ta、Hf和Th,正长辉石岩富集过渡族元素Cr、Ni和Co。由此表明:孙各庄碱性杂岩体形成于由挤压向伸展转变的引张型后碰撞构造环境,其岩浆物质来源与富集地幔相关,进而推测华北克拉通北缘早中生代岩石圈地幔具富集性。综合对比研究结果显示,华北克拉通北缘早中生代碱性岩浆活动的时间为238～220Ma。华北克拉通早中生代大规模的碱性岩浆活动可能与华北克拉通的破坏特别是岩石圈减薄具有相同的深部地球动力学背景。可以认为:华北克拉通北缘印支期碱性岩体的侵位时代238～220Ma可能代表华北克拉通岩石圈伸展减薄的起始时间,而且,岩石圈伸展减薄是从华北克拉通北缘西部开始的。

Zircon SHRIMP U-Pb age（225.6±1.1） Ma of alkaline syenite from the Sungezhuang alkaline complex in Jixian County of Tianjin is reported for the first time,which suggests that the complex is a product of Late Triassic alkaline magmatism.A systematic study on the composition of major element,trace element,REE of the alkaline syenite and ortho-pyroxenite from the complex reveals their lithogeochemical feature,source material,tectonic setting and geological significance.According to the bulk compositions,the syenite has high Al,low Ca,Fe and Mg content with aluminum over-saturation,and the ortho-pyroxenite has high Ca,Fe and Mg content.Both are characterized by enrichment in alkaline with K2O/Na2O1.It is suggested that the Sungezhuang alkaline complex is resulted from the magmatism in a post-orogenic setting.These rocks are all of high REE content,LREE enrichment,HREE depletion and no obviously Eu anomaly.In the PM-normalized diagram,trace element diagram exhibits LILE（Sr,Ba） and LREE（La,Ce,Pr,Nd,Sm） enrichment,relative depletion in HFSE（Nb,Ta,Hf and Th） with negative U,Nb,Ce and Ti anomalies.Furthermore,transitional element（Cr,Ni and Co） contents in the ortho-pyroxenite are relatively low,which implies that the Sungezhuang alkaline complex has a deep magma source related to enriched mantle.The comprehensive research on Indosinian alkaline intrusive belt in northern margin of the North China craton shows that the emplacement ages of these alkaline complexes range from 238 Ma to 220 Ma.It can speculate that the geodynamic setting of large-scale Early Mesozoic alkaline magmatism in northern margin of the North China craton and destruction of the North China craton,specially lithospheric thinning in North China,are basically the same.The Early Mesozoic alkaline rocks（238-200 Ma） is possible to represent the beginning of lithospheric thinning in North China.Moreover,the lithospheric extension and thinning in northern margin of the North China craton may start from the western section.