关键词: 地质年代学, 岩石地球化学, 锆石SHRIMP U-Pb测年, 蓟县孙各庄碱性杂岩体, 华北克拉通北缘, 早中生代

Abstract:

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/Na₂O>1. 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.

Key words: geochronology, geochemistry, ziron SHRIMP U-Pb dating, Sungezhuang alkaline complex in Jixian County of Tianjin, northern margin of the North China craton, Early Mesozoic