A geochonological constraint on the the origin of the Guandi complex, Western Hilss of Beijing with implications for the tectonic evolution

Abstract

The Guandi complex outcrops on both the north and south sides of the Fangshan pluton, and consists of orthogneiss and amphibolite. There have been different interpretations for the origin of the Guandi Complex owing to the lack of age constraints. The controversy also resulted in different interpretations for the tectonic framework and evolution of the Western Hills of Beijing. This study shows that the zircon grains of Guandi orthogneiss have igneous cores and probably metamorphic rims. SHRIMP II results yield an upper intercept age of (2 521 ± 20) Ma (96% confidence, MSWD is 1.4). This upper intercept age is considered as the best estimate of the crystallization age, which represents an intrusion that occurred in the Late Archean. Therefore, the Guandi complex is interpreted as a part of the Archaean basement. A basement detachment fault separates the Archean Guandi Complex from its overlying strata (Mesoproterozoic Jixian system to the Lower Paleozoic strata). The Fangshan pluton intruded the detachment fault but did not affect the Zircon U-Pb age of the Guandi Complex. This evidence supports the conclusion that the Guandi Complex is a Metamorphic Core Complex formed before the Fangshan intrusion.

出露于北京西山房山巖體南北兩側的官地雜巖,主要由正片麻巖、斜長角閃巖組成,局部具混合巖化特征。對官地雜巖的形成時代及出露原因一直存在很大的爭議。一種觀點認為官地雜巖形成時代為太古宙,出露于中生代早期的區域伸展體制下,另一種觀點則認為官地雜巖是中新元古界或古生界泥質變質巖,在房山巖體侵位過程中發生接觸變質作用的產物。研究表明,官地雜巖是一套正片麻巖,其中的鋯石核部為巖漿成因,而外部普遍發育較窄的淺色邊。SHRIMP II鋯石鈾鉛年齡測定獲得鋯石的一致曲線與不一致曲線上交點年齡值為(2 521 ±20) Ma,代表了新太古代的巖漿結晶年齡,從而證明官地雜巖原巖形成于新太古代。構造分析表明,官地雜巖與上覆中元古代薊縣系至早古生代地層間為剝離斷層接觸關系,并為房山巖體侵位和改造,證明是一個形成于房山巖體侵位前的變質核雜巖構造。但房山巖體的侵位并未對鋯石的巖漿年齡和變質年齡產生明顯影響。