Petrogenesis of Early Cretaceous Laoshan A-type granites and the implications for the tectonic evolution of Jiaodong Peninsula

Laoshan granite, a typical A-type granite in eastern China, is located in the middle of Sulu ultrahigh pressure metamorphic orogenic belt. The magma source and petrogenesis are still controversial, and the systematic research on their diagenetic conditions is also lacking. On the basis of detailed field work and petrography, the major element compositions of biotite and amphibole were in-situ examined to estimate the temperature, pressure and oxygen fugacity conditions of magma crystallization. Furthermore, the major and trace element compositions, and Sr-Nd isotopic compositions for Laoshan granite were determined to explore the characteristics of magma source and petrogenesis. New results show that Laoshan granite has a high magma formation temperature (>850 degrees C) and oxygen fugacity conditions. Magma crystallized under low pressure in the shallow crust with magma solidified temperature around 700 degrees C and then went through rapid cooling. Laoshan granite is rich in SiO2 and alkali, but low in Ca and Mg. The contents of high field strength elements (i.e., Th, Zr, Nb, Y) and some large ion lithophilic elements (Rb and K) are high, while the contents of Ba and Sr are low. The content of rare earth elements is enriched, with the Eu negative anomaly, showing a right leaning "seagull type" pattern. In terms of isotopic composition, all samples show enriched Sr and Nd isotopic characteristics, with (Sr-87/Sr-86)i=0.706447 similar to 0.707876, epsilon Nd(117Ma)=-14.8 similar to-17.4, and tDM2=2112 similar to 2325Ma. Combined with the former research results in Jiaodong area, the Laoshan A-type granite is considered to be derived from remelting of K-rich alkaline rock, and its magma source might be related to the Triassic alkaline magmatic activity in Jiaodong area. The formation of Laoshan A-type granite indicates that the Yangtze plate, at the bottom of the collision orogenic belt, collapsed completely. Simultaneously, the lithosphere thinning reached its peak.