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 extensional setting of the Early Cretaceous magmatism in eastern China: example from the Yinmawanshan pluton in southern Liaodong Peninsula.

饮马湾山岩体位于辽东半岛南部,侵位于辽南变质核杂岩之中。岩体主要由三种岩石组成,由外向里分别是片麻状花岗岩、斑状花岗岩和中细粒二长花岗岩类。其中片麻状花岗岩类主要岩石类型为石英二长闪长岩和花岗闪长岩,具有明显的片麻理构造,部分岩石具有明显的变形组构,其片麻理方向与变质核杂岩中拆离断层方向一致。似斑状花岗岩仅显示微弱片麻理,而细粒二长花岗岩则为明显的块状构造,表明该岩体为同伸展变形就位产物。锆石U-Pb同位素测定显示,该岩体不同岩相岩石具有大体一致的年龄,其形成于120～130Ma的早白垩世,而不是以前认为的三叠纪。该岩体岩石具有较大的地球化学成分变化范围,其中片麻状和似斑状岩石具有高Sr含量(>600ppm)、低Y、Yb含量、轻重稀土分馏强烈的特点;而细粒二长花岗岩则相对低Sr高Rb、低^(87)Rb/^(86)Sr比值。Sr-Nd同位素综合分析表明,饮马湾山花岗岩主要来源于下地壳,但较大的范围Mg~#、Sr、Nd同位素组成则表明可能有其它物质(如岩石圈地幔、新生地壳)参与成岩作用。上述研究充分地显示,我国东部早白垩世岩浆活动发育在伸展构造背景,并可能与岩石圈减薄存在密切的联系。

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 similar to 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 tow Sr concentrations, high Rb contents, and low Sr-87/(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.