扬子克拉通北缘新元古代岛弧花岗岩类成分极性及成因的地球化学探讨

Geochemical Study of Compositional Polarity and Causes of Late Proterozoic Island-Arc Granitoids from Northern Margin of Yangtze Craton

对扬子克拉通北缘新古代岛弧花岗岩类成分极性的地球化学研究表明，在ＳｉＯ２含量相近的条件下，由北向南岩石中Ｋ２Ｏ、Ｋ２Ｏ＋Ｎａ２Ｏ，Ｋ２Ｏ／Ｎａ２Ｏ逐渐增加，而ＣａＯ逐渐降低；从北向南，微量元素Ｒｂ，Ｂａ，Ｔｈ，Ｚｒ，Ｎｂ，Ｔａ，ΣＲＥＥ及Ｒｂ／Ｔｈ，Ｒｂ／Ｎｂ，ΣＬＲＥＥ／ΣＨＲＥＥ（Ｌａ／Ｙｂ）Ｎ等逐渐增加或具增加趋势，而Ｓｒ，Ｃｏ，Ｎｉ和Ｓｒ／Ｂａ具逐渐降低或具降低趋势；

The island-are granitoids of the northern margin of the Yangtze Craton related to the plate subduction during the Late Proterozoic exhibt a clear compositional polarity with systemat-ic lateral increase towards the interior of the Yangtze Craton in K2O, K2O+Na2O contents and K2O/Na2O ratio at similar SiO2 levels. Similar compositional polarity exists in the trace ele-ments and isotopes. From the north belt to the south belt, Rb, Ba, Th, Zr, Nb, Ta, REE, Rb/Th, Rb/Nb, ∑LREE/∑HRE, (La/Yb)N, δ18O and εSr(t) show a gradually increase or increase trend, while Sr, Co, Ni, Sr/Ba and εNd(t) tend to gradually decrease in granitic rocks. These spacial framework and compositional variation resemble that found in the Circum-Pacific island-arcs or active Continental margins. The mechanism of the compositional polarity can be revealed by the ratio of Continental crustal material to the mantle material in the source of the island -arc granitic magma. This is one of the important factors affacting the compositional polarity of magma. The ratio of the Continental crust material in the magma of subducting zone gradually increases towards the Continental interior.