江南造山带东段新元古代九岭复式岩体锆石U-Pb年代学及构造意义

Zircon U-Pb ages of the Neoproterozoic Jiuling complex granitoid in the eastern segment of the Jiangnan orogen and its tectonic significance

江南造山带东段的九岭岩体为华南分布面积最大的新元古代花岗质侵入体。据其岩石组合、结构构造及野外侵入关系,可将其解体为由早到晚3个侵入序次的复式岩体,依次为黑云母花岗闪长岩、英云闪长岩及黑云母二长花岗岩。其中,黑云母花岗闪长岩分布面积最广,黑云母二长花岗岩次之,英云闪长岩分布面积最小,围岩为新元古代双桥山群浅变质岩系。LA-ICP-MS锆石U-Pb定年结果表明,黑云母花岗闪长岩、英云闪长岩、黑云母二长花岗岩分别形成于821.6~824.0 Ma、819.5~823.6 Ma、820.4~824.5 Ma,指示它们基本同时侵位,但三者均具有自SE向NW时代变新的趋势。九岭岩体与围岩(双桥山群)的侵入接触面具有南陡北缓、角岩化带南窄北宽且围岩捕掳体及捕获锆石也呈南少北多的特征,表明九岭岩体SE侧岩体剥蚀深度强于NW侧,可能暗示了新元古代华夏板块向扬子板块碰撞拼贴过程中,研究区SE侧岩浆起源深度较深,剥蚀程度较高,且形成时代较早,并逐渐向NW侧迁移(岩浆起源深度变浅、时代变新)。

Located in eastern Jiangnan orogen, the Jiuling pluton is the largest Neoproterozoic granitoid intrusion in South China. According to mineral assemblage, structure and intrusive contact relationships, Jiuling pluton can be divided into complex massif with 3 intrusion orders, followed by biotite-granodiorite, tonalite and biotite-monzogranite from early to late respectively. Among them, granodiorite are distributed most widely, followed by biotite-monzogranite, while tonalite is only distributed in a minimum area. They intruded into the surrounding epimetamorphic rocks of Neoproterozoic period. The LA-ICP-MS zircon U-Pb dating results show that the biotite-granodiorite, tonalite and biotite-monzogranite were formed in 821.6-821.6 Ma, 819.5-823.6 Ma and 820.4-824.5 Ma, respectively,which indicates that they belong to the product of contemporary magmatic activity. On the whole, however, the ganitoids tend to be younger from the southeast side to the northwest side. Besides, the dip angle of contact interface between Jiuling pluton and surrounding rocks (Shuangqiaoshan Group) is steep in the south and smooth in the north, hornfelsic belt is narrow in the south and wide in the north, and the xenoliths from the surrounding rock (Shuangqiaoshan Group) and the captured zircons also tend to decrease in size from south to north. These characteristics show that the rocks on the southeast side was eroded deeper than those on the northwest side, probably implying that, during the collision between Cathaysia plate and Yangtze plate in Neoproterozoic, the original depth of the granitic magma on the southeast side was deeper,the denudation was more intense, and the magma was formed earlier than the magma on the northwest side and gradually migrated northwestward, i.e., the origin depth of magma became shallow and the formation time became younger.