栾川群沉积-岩浆记录的年代学、成因及其构造背景研究

In the global models, the Earth experienced a critical tectonic evolution during late Mesoproterozoic to early Neoproterozoic, meanwhile the North China Craton (NCC) was characterized by “Unnamed System (or Building System)” and multi-rifting magmatism. However, in Jixian and other areas, a prolonged age gap of 0.3 Ga exists between the “Unnamed System” and the upper “Qingbaikou System”, with limited outcrop of magmatic records for the Rodinia assembly. Preliminary chronological data shows that, the Luanchuan Group, which distributes in the southern margin of the NCC, records abundant information of the “Unnamed system” and the Grenvillian tectono-thermal event. Especially, volcanic interlayers have been found near the lower Luanchuan Group, what are used to definite the stratigraphic columns. This proposal will be started with these deposition features and magmatic records. High resolution dating technology (for zircon, baddeleyite, monazite and xenotime) and stratigraphic correlation study will be proceeded to restrict the deposition time, to construct the spatio-temporal framework of magmatism and deposition. Fe-Nd-Hf isotopic analysis with systematic petrology and elemental geochemistry methods will be utilized to investigate their material source, petrogenesis and tectonic background. The ultimate goal is to make it clear that whether the “Unnamed System” exists in the southern NCC and to provide more clues for how it evolved during the late Precambrian.

中元古代晚期-新元古代早期是全球超大陆构造演化过程的关键阶段，华北克拉通以发育“待建纪”（1.4-1.0 Ga）沉积和多期裂谷岩浆作用为特征。然而，蓟县剖面及其它相当层位的“待建系”间断长达3亿年，揭示的响应于新元古代超大陆聚合过程的岩浆记录还比较有限。新的资料表明，华北克拉通南缘栾川群记录有“待建纪”沉积作用和“格林威尔”期岩浆活动的丰富信息，尤其出露了可以高精度标定地层时代的火山岩夹层。本项目拟以栾川群沉积特征和岩浆记录为切入点，通过锆石/斜锆石、沉积自生矿物的原位微区定年和层序对比研究，精确厘定栾川群的沉积时限，建立岩浆-沉积作用的时空格架；通过Fe-Nd-Hf同位素组成研究，结合传统的岩石学、元素地球化学分析方法，剖析岩浆作用和沉积序列的源区性质、岩石成因及其大地构造背景，为查证华北南部“待建纪”的沉积-火山记录、解析晚前寒武纪构造体制和演化过程提供依据。

蓟县剖面及燕辽地区下马岭组（ca. 1.4~1.35Ga）曾被认为是华北克拉通中元古界“待建系”的唯一沉积记录，十余年内未发现其他可对比层位，限制了对华北克拉通延展纪（1.4~1.2Ga）沉积体系特征的进一步认识。本次研究在华北克拉通南缘栾川地区白术沟组上段碳硅质板岩-千枚岩中识别出凝灰岩夹层，获得其岩浆锆石的LA- MC- ICPMS U-Pb年龄分别为(1330±10)Ma（MSWD=0.43，N=38）和(1332±10)Ma（MSWD=0.90，N=24）。二者在误差范围内一致，精确限定白术沟组主体应沉积于1.33Ga前后，表明该组也属于华北中元古界“待建系”。分别位于华北克拉通北部和南缘的“待建系”下马岭组及白术沟组，凝灰岩夹层的年龄（~1.38Ga 及~1.33Ga）相差~50Myrs，但其整体岩性组成特点及上、下地层叠覆关系等高度相似，表明它们的沉积背景和演化过程具有可对比性。白术沟组沉积时代的准确限定，为理清华北克拉通南缘中元古界和新元古界相关沉积序列提供了关键资料，是构建华北克拉通中-新元古界年代学格架，进而深入剖析华北克拉通及全球延展纪阶段地质演化的重要依据。而中元古代~1.4Ga期间，大气氧水平相对较高，但短时气候极端变化使得营养物质供应不稳定，延滞了生命演化。