中国东部早白垩世岩浆作用的伸展构造性质: 以辽东半岛南部饮马湾山岩体为例

The Yinmawanshan pluton is located in the southern part of the Liaodong Peninsula and emplaced in the Liaonan metamorphic core complex. Petrographically, three types of rocks can be distinguished from the outside to the center, i. e. , the gneissic, porphyritic and fine/medium-grained granites. The gneissic rocks, mainly composed of quartz monzodiorite and granodiorite, have evident gneissic structures and experienced ductile deformation in some locations. The direction of foliations and lineations are in accord with those of the mylonites at the lower plate of Liaonan metamorphic core complex. While the porphyritic granodiorite has weakly gneissic structure, and the fine/medium-grained monzogranite has evident massive structure. Above-mentioned characteristics show that this pluton is syn-deformed and emplaced in an extensional setting. Zircon U-Pb isotopic dating for different petrographic rocks gives similar emplacement ages ( 130 ∼ 120 Ma) , i. e. , the Early Cretaceous, not the Triassic as previously thought. The gneissic and the porphyritic rocks have high Sr contents ( > 600ppm) , low Y and Yb contents, and intense fractionation of LREE (light rare earth elements) and HREE (heavy rare earth elements), while the fine/medium-grained monzogranites have relatively low Sr concentrations, high Rb contents, and low ⁸⁷ Sr/⁸⁶ Sr ratio. The Sr-Nd-isotopic data indicate that the Yinmawanshan pluton was mainly derived from ancient lower crust. However, the relatively high Mg^# values and large scale of Sr and Nd isotopic compositional variations suggest that materials derived from other sources (for example, lithosphere mantle, juvenile crust) had contributed to the genesis. Therefore, it is suggested that the giant Early Cretaceous magmatism in the eastern China developed in an extensional setting, which was possibly induced by lithospheric thinning in the eastern China.