Geochemical characteristics of the Dushan complex and their geological significance

The Dushan complex consists of granitic rocks with different lithologic types, which are fine-grained quartz monzonite, medium-fine-grained biotite monzogranite and coarse-medium-grained biotite syenogranite. Previous chronological data indicate that the emplacements of these rocks were almost synchronous. Through field work and indoor research, the authors have concluded that this complex was a product of comagmatic evolution. Geochemically, the Dushan complex shows high- silica (69.90% on average), high-aluminum (14.82% on average), low-magnesia (1.09% on average), high-strontium (917.18 μg/g on average), lowyttrium (5.44 μg/g on average), low- ytterbium (0.50 μg/g on average), (La/Yb)N higher than 1 (50.95 on average) and negatively abnormal Nb/Ta. Features of plots on the Sr/Y-Y and Sr/Yb-Yb discrimination diagrams show that they are adakitic rocks. Strong Nb-Ta negative anomaly in samples implies that the Dushan complex may inherit the characteristics of protoliths which are possibly TTGs in this region. Although some metabasaltic and metasedimentary components were involved in protoliths, the TTGs might be the most important. Through geochemical analysis, the strong Nb/Ta anomaly, high (La/Yb)N and high Sr as well as inconspicuous Eu anomaly indicate that the residual mineral assemblages were amphibole + garnet ± plagioclase, and the residual phase was amphibole eclogite facies without rutile. The estimated thickness of the crust was 50~70 km, which in turn reflected the height of the paleomountains of 3000~5000 m. In the light of data of other batholiths in the region, the authors have concluded that the orogenesis between Siberia plate and Sino- Korean paleoplate was occurring during mid- late Triassic. At that time, towering mountain chains appeared in the northeast margin of North China Craton.