华北地台南缘铅同位素演化

THE EVOLUTION OF LEAD ISOTOPE IN THE SOUTHERN MARGIN OF THE HUABEI PLATFORM

通过对华北地台南缘金、钼、铅锌矿床的91个方铅矿,5个黄铁矿,8个钾长石和7个全岩样品铅同位素组成的研究,认为本区地壳中铅同位素经历了四个演化阶段:2600Ma前为地幔系统,2600～1000Ma左右为“幔质”地壳系统,1000～700Ma前为热液对流系统,700～200Ma左右出现岩浆—地壳混合系统。这些重要时限反映了本区的热历史。

The samples are collected from the main Au-W ore deposits including the quartz-vein type and the altered-rock type in the Xiao Qinling Gold Metallogenic belt, the Mo-multiraetallic ore deposits including the porphyry type and the carbonate rock type in the Jindui Cheng-the Luan Chuan Mo-m etallogenic belt and the Pb-Zn-Au-Ag ore deposits in the Tieluzi-.the western Mangiing granite metallogenic belt near the Heigou Fault zone in the area. The ore-bearing stratigraphic horizons belong separately to the Tai Hua Group ( granulite-the gniss facies, volcanic rocks and the volcanic debris ) , the Xioner Group and the kuanping Group ( the green schist iacies volcanic-terrigenous clastic rocks) .Their forming time was the Archean era to the middle Proterozoic era. The two main stages of the gold mineralization took place in the courses of the mixed rock formation during 0.8-1 billian years and in the early Yenshan magmatic activities, but it is suggested that the source beds occured in the Taihua Group. The mineralization oi the Mo ore, deposits related to the porphyry intrusion activity during 0. 1-0. 2b. years and the source of rocks was thought of as the Taihua Group and its basement. The lead-zinc minerolization in the Kuanping Group happened during the Pha-nerozoic Qinling rifting period which lasted about. 0.7-0.8 billion years. The ores were formed by the hydrothermal convection in the fault zone, but the lead came mainly from the older basement. The data illustrated in this paper include 91 galenites, 5 pyrites, 8 k folds-pars and 7 bulk rock samples etc. Their distribution is well distributed in this area and very typical. The analytical precision of the main data obtained by using the mass spectrometry can reach per the thousand grade, thus being able to meet the needs of this discussion. The lead isotope compositions of the gold ore show stronger homogeneity and the model ages of Doe(l974)are between 0.8 b.years and 1 b.years, but the abundance of Pb204 is 1.37 to 1.41. The values are 9 to 9.5. The litho-logical character of the source bed is similar to the green stone belt. So, after a high level of homogenization, the first stage ( in the mantle ) and the second stage ( in the mantle crust ) of evolution might be considered as a single stage evolutionary system. Therefore, 0.8 to 1 billion years might be regarded as a main metallogenic age. Although the molybdennurn ore deposits show a certain type of homogeneity, there is a remarkable linear thend. This indicated the limits between 0.2 b. years and 2.3 to 2.6 b. years. The abundance of Pb204 is mostly restricted within 1.40 to 1.44, so that in lead isotope compositions there are more old lead and it is so clear that the compositions from the crust depth occupy high propotion. In the hochschildite ore deposits the linear trend of the lead isotope is most obvious, but the compositions of old lead are least. The two-time limits wera defined as; 0.7 to 0.8 b.years and 2.2 to 2.6 b.years. Based on other geochronologic data, it is suggested thas the lead isotope in this area should go through four evolutional stages, the first stage evolution in the mantle system before 2.6 b.years; the second in the mantle crust system during about 2.6 b. years to 1 b. years; the third in the hydrothermal convection system before 0.7 b. years and the fourth in the magma crust mixing system about 0.1 to 0.2 b. years ago. These main time limits show the history of heat in this area.