Abstract:

The volcanic rocks of Early Cretaceous Yingcheng Formation contain abundant perlitic rocks and they are important petroliferous layers in Songliao basin. Fresh perlitic rocks are found in the Jiutai area of the southeastern uplift of Songliao basin. In the perlitic rocks, phenocrysts are well preserved and they are mainly composed of plagioclase, biotite, amphibole, and mafic clinopyroxene and orthopyroxene. We conducted geochemical analyses on the perlitic rocks as well as vitreous rocks after removing phenoscrysts, and probe analyses on the phenocrysts. The results indicate that the perlitic rocks are felsic high-K calc-alkali series rocks and their trace-and-REE characteristics are similar to upper crust. The clinopyroxene phenocryst is rhombic-hypersthene and/or salite, and hypersthene for orthopyroxene phenocryst. Both clinopyroxene and orthopyroxene phenocrysts are xenocrysts of metamorphic origin. Whereas in the neighbor basaltic layers of perlitic rocks, the plagioclase phenocryst is labradorite, and Fe-salite for clinopyroxene phenocryst, and both are of magmatic origin. All these characteristics suggest that the perlitic rocks might have originated from upper crust, and their pyroxene xenocrysts may be the results of assimilation and contamination of wall rocks during the migration and ascent of magma.

Key words: Yingcheng Formation, perlitic rocks, phenocryst, southeastern uplift of Songliao basin, sedimentology