北京西山官地杂岩的形成时代及构造意义

A geochronological constraint to the Guandi complex, Western Hills of Beijing, and its implications for the tectonic evolution.

出露于北京西山房山岩体南北两侧的官地杂岩,主要由正片麻岩、斜长角闪岩组成,局部具混合岩化特征。对官地杂岩的形成时代及出露原因一直存在很大的争议。一种观点认为官地杂岩形成时代为太古宙,出露于中生代早期的区域伸展体制下,另一种观点则认为官地杂岩是中新元古界或古生界泥质变质岩,在房山岩体侵位过程中发生接触变质作用的产物。研究表明,官地杂岩是一套正片麻岩,其中的锆石核部为岩浆成因,而外部普遍发育较窄的浅色边。SHRIMP II锆石铀铅年龄测定获得锆石的一致曲线与不一致曲线上交点年龄值为(2 521 ±20) Ma,代表了新太古代的岩浆结晶年龄,从而证明官地杂岩原岩形成于新太古代。构造分析表明,官地杂岩与上覆中元古代蓟县系至早古生代地层间为剥离断层接触关系,并为房山岩体侵位和改造,证明是一个形成于房山岩体侵位前的变质核杂岩构造。但房山岩体的侵位并未对锆石的岩浆年龄和变质年龄产生明显影响。

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.