河南箕山登封群单颗粒锆石^(207)Pb/^(206)Pb同位素年代研究

SINGLE-GRAIN ZIRCON ^(207)Pb/^(206)Pb DATING OF THE DENGFENG GROUP, JISHAN, HENAN PROVINCE

本文对河南箕山登封群老袋窑组交流纹斑岩进行了单颗粒锆石^(207)Pb/^(206)Pb同位素年龄测定。研究表明锆石属典型的岩浆结晶产物,其年龄值为2501.94±0.70Ma,它与登封群下部火山岩的Sm-Nd年龄(2514±42Ma)非常接近,这说明箕山地区登封群火山作用时限很短(10Ma±),是一套完整的花岗-绿岩建造,和嵩山地区登封群属同一期构造-岩浆活动的产物,并不能划分出早元古代岩石建造。

The Dengfeng Group in Jishan is a Archean granite-greenstone formation located on the south margin of the North China platform. The authors determined single-grain zircon the 207Pb/206Pb isotope age of the metarhyolite porphyry of the Laodaiyao Formation in the upper part of the Dengfeng Group. The research indicates that the assemblage of accessory minerals in metarhyolite is apatite + zircon + magnetite. The zircon crystal is long prismatic-dipyramidal in shape the prismatic planes {100} and {110} are developed well, the pyramid planes {311} and {111} are not developed, and the ratio of length to width is from 4. 5 : 1 to 2. 5 : 1, so the rock belongs to the typical product of magmatic crystallization. The average age of 207Pb/206Pb of selected 11 grains is 2501. 94±0. 70 Ma, very close to the Sm-Nd isochron age (2514±42 Ma) of the volcanic rocks in the lower part of the Dengfeng Group. This demonstrates that the time span of the volcanic activity of the Dengfeng Group in Jishan was probably very short (10 Ma±) and that the group is a granite-greenstone formation formed by sedimentation in a very short time. This group and the Dengfeng Group in the Mount Songshan area both belong to the product of the tectono-magmatism of the same episode; so the early Proterozoic formations can not be defined.