松辽盆地早白垩世双峰式火山岩的成因和地质意义

Bimodal volcanism from rift basins is important for understanding processes of basin extension and crust-mantle interaction. During late Mesozoic, hundreds of intracontinental rift basins developed in Northeast Asia. The Songliao basin, located in the center of NE China, is a unique case among them as it evolved into the largest rift basin. Bimodal volcanism of YingCheng Formation was developed at the late stage of basin rift and aroused great interest as part of the widespread early Cretaceous magma activities in eastern China. Although lots of studies have been focused on this bimodal volcanism but the petrogenesis and tectonic affiliations of them are still controversial. Because of the volcanic sequence often has the phenomenon of diachronous and inversion of the lithostratigraphic correlation, and the mafic endmember of the bimodal volcanism has not been well constrained. These make it difficult to identify the properties of the volcanic rocks with regular analysis. In this study, we plan to focus on typical profiles of the bimodal volcanism of YingCheng Formation in Songliao basin, and combine detailed field research with high-precision SIMS zircon/baddeleyite U-Pb dating to clarify the temporal-spatial relationship of the volcanic rocks. Based on petrology and geochemistry studies, Sr-Nd-Pb and Mg isotopic researches are designed to trace characteristics of the magma source and recycling processes of crustal material. The combination of multiple isotopic systems will provide new insights into the crust-mantle interaction and magma evolution during lithospheric extension. It is anticipated to address questions regarding the early Cretaceous basin development and tectonic evolution of NE China.

裂陷盆地发育的双峰式火山岩是岩浆源区、局部熔融条件和岩浆演化机理的综合反映，是探讨壳-幔相互作用和盆地伸展过程的重要研究对象。松辽盆地早白垩世营城组双峰式火山岩，是盆地伸展断陷晚阶段形成的产物，同时是中国东部地区早白垩世构造-岩浆活动的重要组成，一直备受关注。但由于火山岩地层易混淆，基性火山岩的研究不充分，制约了对该期火山活动的认识，亟需新的思路和方法提供约束。基于此，本项目拟对松辽盆地营城组双峰式火山岩进行系统的岩石学和地球化学研究。在前期工作的基础上，通过锆石/斜锆石SIMS原位U-Pb定年以及全岩Sr-Nd-Pb-Mg同位素分析，结合系统的岩相学观察和全岩主微量元素分析，厘清双峰式火山岩的形成时代，明确基性-酸性火山岩的成因关系，示踪火山岩的源区性质和地壳物质的循环过程，进而深入探讨盆地伸展过程中的源区组成变化和壳-幔相互作用特点，并据此对该期强烈的岩浆活动以及区域构造演化进行限定。

裂陷盆地的形成往往伴随着与伸展过程相关的火山作用的发育，而盆地和山岭作为岩石圈动力学作用下两种主要地貌形态和地壳结构，是岩石圈变形的两个侧面，它们之间的耦合特征受控于壳-幔相互作用等深部过程。裂陷盆地发育的双峰式火山岩是岩浆源区、局部熔融条件和岩浆演化机理的综合反映，是探讨壳-幔相互作用和盆地伸展过程的重要研究对象。本项目以松辽盆地及周缘晚中生代火山岩为研究对象，利用锆石/斜锆石SIMS U-Pb年代学和Sr-Nd-Pb-Mg多同位素联合分析，并结合盆地沉积作用特征，解析浅表沉积-构造运动和岩浆活动、深部动力学机制的耦合关系，制约盆地从断陷向坳陷转变的构造机制。主要得到如下主要认识和成果：1）盆地断陷晚期基性火山岩斜锆石SIMS U-Pb 年龄与酸性火山岩锆石SIMS U-Pb 定年结果基本一致，指示岩石圈强烈伸展过程中地幔和地壳熔融同时发生，产生了巨量的岩浆活动；2）松辽盆地断陷期火山岩具有相对均一的Sr-Nd-Pb同位素组成，但可进一步划分为碱性和亚碱性系列，埃达克质岩石和I型流纹岩反映了岩石圈由厚变薄的伸展过程，而具有OIB特征的碱性玄武质岩石和A型流纹岩指示岩石圈伸展接近尾声，盆地由此进入坳陷阶段；3）松辽盆地和东部大三江盆地群在断陷向坳陷转变过程中沉积物质来源都发生了明显改变，指示白垩纪中期东亚大陆边缘受到挤压作用影响，可能与鄂霍茨克板块随着Izanagi板块向东亚大陆俯冲并与东亚大陆发生碰撞相关，进而对中国东北地区盆-岭构造格局的形成产生深刻影响。上述成果对认识和理解东北亚地区的构造与地壳演化及认识松辽盆地的成因和深部动力学机制具有重要意义。