南秦岭佛坪地区麻粒岩相岩石锆石U-Pb年龄

U-Pb Zircon Dating of Granulite Facies Rocks from the Foping Area in the Southern Qinling Mountains

对南秦岭佛坪地区麻粒岩相岩石中锆石的特征进行了详细的研究和U-Pb同位素年龄测定。中酸性麻粒岩中3个变质成因锆石的^(256)Pb/^(238)U表面年龄统计权重平均值(谐和年龄)为221.4±3.6Ma,是这些变质锆石较为准确的生成年龄;该样品4个锆石颗粒组成的不一致线上、下交点年龄分别为822±272Ma和218±14Ma;石榴夕线黑云片麻岩中4个锆石颗粒构成的不一致线的上下交点年龄分别为2283±162Ma和271±15Ma。两个样品的数据点均比较靠近下交点而远离上交点,其上交点年龄不具明确地质意义。综合各方面因素,认为麻粒岩相峰变质作用时代为印支期—海西晚期(220～270Ma左右),这是迄今为止秦岭地区所报道过的最年轻的麻粒岩地质体。

This paper presents a detailed study on the characteristics and U-Pb analysis of zircon from the granulite facies rocks in the Foping area in the southern Qinling Mountains. The weighted average 206Pb/238U apparent age of 3 fractions of metamorphic zircons from intermediate-acid granulite is 221. 4±3. 6 Ma (concordia age), which indicates their crystallization age; the upper and lower intercept ages of discordia constructed using 1 fractions of zircons are 822-272 Ma and 218±14 Ma respectively. A discondia line constructed through 4 fractons of zircons from garnet- sillimanite-biotite plagiogneiss defines an upper intercept age of 2283±162 Ma and a lower intercept age of 271±15 Ma. All the data points of the two samples are close to the lower intercept , so their upper intercept ages have no explicit geological significance. The granulite facies is Indosinian-late Hercynian(ca. 220-270 Ma), and this is the youngest granulite in the Qinling area.