大别山北淮阳构造带柳林辉长岩:新元古代晚期裂解事件的记录

Liulin Gabbro in the Beihuaiyang Tectonic Belt of the Dabie Orogen: A Witness of the Late Neoproterozoic Rifting Event

大别山北坡北淮阳构造带的柳林-定远-王母观一线,断续分布着带状产出的辉长岩体,构成一条NWW向的基性岩带,以往被认为是形成于海西期的造山后伸展环境.对该带西缘鸡公山北坡柳林辉长岩进行了锆石SHRIMPU-Pb定年和辉石、斜长石的矿物化学研究.辉长岩中的岩浆锆石SHRIMPU-Pb年龄为(611±13)Ma,代表岩体结晶时代,这一年龄远远老于其围岩上奥陶统定远组的时代.岩体主要矿物辉石属普通辉石,Na2O、FeO和MnO较高,长石属中长石,低CaO,这些特征明显区别于岛弧或活动大陆边缘环境中的同类矿物,而与板内伸展环境中形成的辉长岩的矿物特征相似.研究表明,大别山北部新元古代晚期(610Ma左右)发育了一次陆内裂解事件.

A number of gabbrie plutons discontinuously crop out along the Liulin-Dingyuan-Wangrnuguan region in the Beihuaiyang tectonic belt of north Dabie. They were previously considered to have been developed in a post-orogenic extensional setting. In this paper we present new data of zircon SHRIMP dating and EPMA chemical analysis results of pyroxenes and plagioelases from the Liulin gabbro at the northern foot of Jigongshan Mountain. SHRIMP U-Pb dating on magrnatic zircons yielded an age of （611±13）Ma, which is interpreted as the crystallization time of the Liulin gabbro. The date is significantly older than the immediately adjacent Upper Ordovieian Dingyuan Formation, revising the contact relationship between the gabbro body and its wall rock. Pyroxene is exclusively augite with high Na2O, FeO and MnO contents, and plagioelase is low CaO andesine, which share characteristics with their counterparts from gabbros formed in an intra-plate setting and are largely different from those from gabbros associated with island are or active continental margin. The results indicate that there may have been a continental rifting event in the north of the Dabie orogen during the late Neoproterozoic period.