Nd, Sr and Pb isotope geochemistry of Yanshanian granitoids in eastern Inner Mongolia and their origins

Abstract：For lack of systematic isotopic study, the study of the origin of Yanshanian granitoids in eastern Inner Mongolia has been focused on their petrology and geochemistry, which leads to controversial views on the origin. There are mainly three views,all of which emphasize that the Yanshanian magmatic rocks originated from mixing of mantle and crust. The authors restudied the origin of the Yanshanian granitoids in the area by using the Nd, Sr and Pb isotope methods. The study shows that all the granitoids have positive εNd（t）values, ranging from +0.75 to +8.12 with an average of +3.07, which indicates that they are genetically associated with the depleted mantle. The whole-rock initial Sr ratios range from 0.7028 to 0.7096, with an average of 0.7063, being intermediately between that (0.702-0.706) of MORB (0.706-0.718) of continental crust, but close to that of MORB. The initial lead ratios of the granitoids are 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb, with their respective averages of 18.3742, 15.5500 and 38.1810, all of which are higher. The μ value calculated from the K-feldspar lead isotope ratios ranges from 9.51 to 8.91, which is lower than the isochronic evolution line (μ = 9.74) of the continent crust. Based on the above data, With Combination of the isotopic study of magmatic rocks in the Hinggan -Mongolia-northern Xinjiang region (a Late Paleozoic geosyncline) adjacent to the study area, the authors think that the Yanshanian granitic magma may have originated from partial melting of the depleted mantle and partial melting of the late Variscan paleo-Mongolian ocean crust derived from the depleted mantle in the area, i.e. the ultimate source of the Yanshanian granitic magma in the area is the depleted mantle. Therefore, a“three-step” evolutional model is brought forward, that is the evolution model of depleted mantle – the ancient Mongolian ocean crust – the marginal continental rejuvenation.