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近期在北京密云沙厂北东的大龙门村附近发现一条花岗斑岩岩脉,该岩脉侵入于新太古代密云群角闪斜长片麻岩当中,其顶部则与片麻岩一起被常州沟组含砾砂岩沉积不整合覆盖。对该岩脉采用LA-MC-ICPMS进行锆石U-Pb同位素年龄测定,获得了(1 673±10)Ma的侵位年龄。这表明华北北部的常州沟组底界(也即长城系的底界)年龄小于1 670 Ma,很可能接近1 650 Ma,比传统的底界年龄1 800 Ma至少年轻150Ma左右。这是迄今为止华北北部长城系底界最为直接而精确的年龄约束。结合上覆大红峪组、高于庄组等测年资料,可进一步将新厘定的长城系(含常州沟组、串岭沟组、团山子组、大红峪组)的形成时代限定在1 650~1 600 Ma,归属于固结纪(Statherian Period,1 800~1 600 Ma)最晚期。这一新进展表明,华北北部中—新元古界的底界与西伯利亚里菲系(Riphean)和印度温地亚(Vindhyan)超群等沉积序列的底界几乎完全一致,可能暗示华北与这些古陆在哥伦比亚超大陆旋回中有着紧密的亲缘关系。
Recently, a granite porphyry dike was discovered near the Great Longmen Village in the north east of Miyun Sand Factory in Beijing, which invaded the Neo-Archean Miyun Triassic amphibifer gneiss, Changzhou ditch group gravel sandstone unconformity sedimentary cover. LA-MC-ICPMS was used to measure zircon U-Pb isotopic ages of this dyke, and the emplacement age of (1673 ± 10) Ma was obtained. This indicates that the bottom of the Changzhou Gouzi Formation (ie, the bottom of the Great Wall System) in northern North China is less than 1670 Ma, probably close to 1650 Ma, which is at least 150 Ma younger than the conventional bottom boundary of 1800 Ma. This is by far the most direct and accurate age constraint of the Great Wall System in northern North China. Combined with the dating data of Dahongyu and Gaoyuzhuang groups, the formation age of the newly determined Great Wall (including Changzhou Gou, Tan Linggou, Tanshanzi and Dahongyu formations) can be further limited to 1 650 ~ 1 600 Ma, which belongs to the last phase of Statherian Period (1800 ~ 1600 Ma). This new progress shows that the bottom boundary of the Meso-Neoproterozoic in northern North China is almost completely consistent with the sedimentary sequences of the Riphean and Vindhyan superdeads in Siberia and may suggest that the North China and these ancient Lu has a close genetic relationship in the Columbia supercontinental cycle.