Geochemical Characteristics and Tectonic Significance of A-type Granite in the South Margin of Zhaobishan, Beishan Area

The biotite monzogranite, located in the south margin of Zhaobishan, was intruded in Hongliuhe ophiolite, which are characterized by high-K and calc-alkaline composition with SiO₂ 68.86%~73.27%, high alkali (K₂O+Na₂O=7.03~7.98), coupled with midium aluminum (Al₂O₃=13.75%~14.1%) and weakly peraluminous (A/NKC=1.03~1.07). These rocks have high ratios of Ga×104/Al(5.22 ~ 6.48)and Y/Nb (2.71 ~ 3.43) in trace elements, which also show evidently loss in Nb, Ta, Sr, P, Ti as well as the enrichments of K, La, Nd, Zr, Hf and Sm. They are also rich in LREE with (La/Yb)_N=7.19 ~ 13.15, and negative Eu anomalies(δEu=0.82 ~ 0.84). The results indicate that this biotite monzogranite belongs to A(A2) granite, which was formed by partial melting of young oceanic crust and island arc after had being used to be heated by basaltic magma in the extensional environment of early Devonian. Represented by the west part of Hongliuhe-Niujuanzi-Xichangjing ophiolitic belts, the paleo-ocean was closed between the late Ordovician and early Devonian(446.4~404.8Ma). After that, there was a dynamical evolution which was characterized by the end of the squeeze and the beginning of stretch, with the conversion time may be around 415~404.8 Ma.