从岩石Sm-Nd同位素模式年龄论北秦岭地壳增生和地壳深部性质

Crustal Growth and Lower Crust Nature of North Qinling:Study of Sm-Nd Isotopic Model Ages

北秦岭各岩类１１６个样品Ｓｍ－Ｎｄ同位素模式年龄（ｔＤＭ）变化于０．９～２．４Ｇａ之间，反映北秦岭地壳主要形成于元古代，０．９Ｇａ之后没有明显大规模新生地壳的形成。在Ｎｄ模式年龄分布图上出现２．０５Ｇａ、１．４０Ｇａ和１．０５Ｇａ三个明显的峰值，它们相应地代表北秦岭地壳的三个增生期。与华北克拉通南缘地壳存在２．６５Ｇａ、２．１０Ｇａ和１．４０Ｇａ三个增生期相对比，表明从华北克拉通南缘到北秦岭，地壳侧向增生是逐渐发展的，这是一个统一典型陆块的地壳增长过程，而北秦岭原来应属于华北的一部分。另外，从北秦岭花岗岩揭示的北秦岭地壳深部性质看，北秦岭地壳深部在１．０Ｇａ～１．２Ｇａ左右的板底垫托作用是相当明显的，而在晚古生代后可能又受到区域折离层作用的影响。

The Nd isotopic model ages (tDM) are used to estimate crust--formation ages and thereby to investigate crustal growth. 116 samples of rocks from the North Qinling yield tDM model ages from 0.9Ga to 2. 4Ga indicating that the crust--formation period of the North Qinling is mainly at the Proterozoic with no volume of new crustal formation after 0. 9 Ga, and there are three distinct peaks, namely, 2. 05Ga, 1.40Ga and 1. 05Ga in histogram of ton model ages, which correspond to three crustal growth episodes of the North Qinling. Samples of rocks from southern margin of the North China Craton also yield three distinct peaks (or three crustal growth episodes) in histogram of ton model ages, but they are 2. 65Ga, 2. 10Ga and 1.40Ga. These reflect the lateral crustal growth process from the southern margin of the North china craton to the North Qinling. This is a typical crustal growth process of a united continental block. So, the North Qinling continental block would belong to a part of the North China continental block. According to the nature of the deep crust revealed by by granitoid in the deep crust of the North Qinling there existed underplating about 0.9Ga~ 1. 2Ga.