江南造山带东段桃岭-段莘带花岗岩锆石定年及年代学格架

ZIRCON AGE DATING AND GEOCHRONOLOGICAL FRAMWORK OF THE GRANITES IN THE TAOLING-DUANXIN AREA, EASTERN JIANGNAN OROGENIC BELT

桃岭-段莘带是位于江南造山带东段的一条东西向展布的燕山期花岗岩带,由7个岩体构成。通过锆石LA-ICPMS定年方法得到该带6个岩石样品的年龄分别为131.2Ma±1.9Ma,131.3Ma±1.9Ma,132.0Ma±1.8Ma,129.7Ma±1.6Ma,149.1Ma±1.8Ma和152.4Ma±2.6Ma。定年结果表明,该带花岗岩存在早、晚两期岩浆活动。早期花岗岩形成时代为晚侏罗世(152.4Ma^149.1Ma),岩性以黑云母二长花岗岩和二云母二长花岗岩为主;晚期花岗岩形成时代为早白垩世(132.0Ma^129.7Ma),岩性以二云母花岗岩和二云母二长花岗岩为主。江南造山带东段燕山期岩浆活动可划分为早、晚两个期次(155Ma^137Ma和137Ma^121Ma),早期岩浆作用和W,Mo多金属成矿密切相关,而晚期岩浆作用成矿作用不显著。桃岭-段莘带燕山早、晚期花岗岩年代学上分别与江南造山带东段燕山早、晚期岩浆岩相对应,这些花岗岩可能是在伸展构造背景下形成。

7 east-west trending rock bodies of granite occur along the Taoling-Duanxin magmatic belt in the eastern Jiangnan orogenic belt. LA-ICP-MS zircon U-Pb dating for 6 samples of granite yield weighted mean ages of 131.2 Ma±1.9 Ma,131.3 Ma±1.9 Ma,132.0 Ma±1.8 Ma,129.7 Ma±1.6 Ma,149.1 Ma±1.8 Ma and 152.4 Ma±2.6 Ma respectively.It shows that the emplacement of these granitic rocks can be divided into two major active stages. The early stage granitic activity occurred in the late Jurassic period(152.4 Ma^149.1 Ma) and formed two-mica granite and two-mica monzogranite. The late stage emplacement of granites acted in the early Cretaceous period(132.0 Ma^129.7 Ma) and produced two-mica monzogranite and biotite monzogranite.Regional geological study reveals that the Yanshanian magmatism in the eastern Jiangnan orogenic belt also experienced two active stages,155 Ma^137 Ma and 137 Ma^120 Ma respectively.The early stage magmatic event is closely related to W and Mo polymetallic mineralization. While there is no significant mineralization occurred in the late stage magmatic activity. In terms of geochronology,the Yanshanian early and late stage granites in the Taoling-Duanxin belt are correspond to those in the eastern Jiangnan orogenic belt. It is considered that these granites were formed in an extensional environment.