【摘要】 矿洞沟 -梁屯正长岩体侵位年龄为 2 175.4Ma ,是年代最古老的正长岩体之一。正长岩浆演化过程中发生了分离结晶作用。岩浆起源于亏损地幔与下地壳组分参予的混合源区 ,有上地壳物质的混染。该岩体是研究岩石圈发展早期碱性岩浆作用的极好标本。更多还原

【Abstract】 Kuangdonggou-Liangtun syenite mass in the southern margin of Yingkou-Kuandian anteclise within Sino-Korean paraplatform was emplaced in Gaixian Formation of Paleoproterozoic upper Liaohe Group. The isotope age of this intrusion is 2175.4 Ma, suggesting that it is one of the oldest syenite mass formed during the development of rifts after the formation of Paleoproterozoic cratonized crust. Fractional crystallization once took place in the evolution, as shown by the variation of chemical and mineral compositions of syenite magma; in addition, contamination of the upper crust material must have happened in the formation of quartz syenite. The syenite magma was probably generated from the depleted mantle mixed with materials of the lower crust, and was contaminated by materials of the upper crust. The evidence is as follows: ε Nd t value is -0.54, ε Sr t values are +26.78～+42.19; projections of Pb isotopic composition of feldspar lies between the lead isotopic growth curve of the mantle and that of the upper crust; there exists a large proportion of EMII in the magmatic source; a few sample points are located near the EMI branch; the syenite is enriched in U, Th and, especially, Pb but is deficient in Nb, Sr, P and Ti. The source region might have experienced weak metasomatism before the generation of magma, and this might accounts for the formation of weakly alkaline syenite magma.更多还原