大别山超高压变质带花岗岩两期变质作用和造山带抬升的年代学证据

Geochronological Evidence of Two Stage Metamorphism of Meta-Granitoids in Dabie UHPM Belt and Exhumation of the Orogenic Zone

大别山超高压变质带中变质花岗岩常与榴辉岩密切伴生。单颗粒石U Pb定年确定变质花岗岩的原岩为新元古代 (75 5～ 714Ma)花岗岩 ,并经历古生代 (384～ 2 84Ma)和早中生代 (2 2 4.5～ 197Ma)两期变质作用。角闪石 (2 0 3.9Ma)、白云母 (196 .4Ma)和黑云母 (180 .8Ma)Ar Ar年龄表明 ,在早中生代超高压变质作用之后 ,变质花岗岩与榴辉岩一样立即快速隆升冷却 ,直到 180Ma前后这一次快速折返运动才告一段落。双河和碧溪岭变质花岗岩中的榍石、锆石和磷灰石裂变经迹 (FT)年龄表明 ,两个变质花岗岩自 180Ma至 5 4Ma(双河岩体 )或至 5 7Ma(碧溪岭岩体 )为大别山超高压变质带稳定的缓慢抬升期。从 5 4Ma(或从 5 7Ma)～ 5 0Ma(或至5 3.6Ma)进入第二次也是最后一次快速隆升冷却时期。由此 ,大别山的造山运动结束 ,转入新的缓慢隆升阶段至今。

As the important member in Dabie ultrahigh pressure metamorphic belt, meta-granitoids are often associated with coesite-bearing eclogites. TIMS zircon U-Pb dating of the rocks show that their protoliths were later Proterozoic (755-714 Ma) granitoids; and then they were strongly metamorphosed in two periods of Paleozoic (384-284 Ma) and Early Mesozoic (224.5-97 Ma), respectively. 40Ar/ 39Ar ages of amphibole, muscovite and biotite and fission track (FT) ages of sphene, zircon and apatite indicate the meta-granitoids started and developed during the first rapid exhumation process after early Mesozoic deep-subducting metamorphism from 224.5 Ma to 180.8 Ma. The corresponding cooling rates of the rocks were 10.2℃/Ma (Shuanghe block) and 26.6℃/Ma (Bixiling block), respectively. The second rapid uplift movement, that started from 53.4 Ma (Shuanghe block) and 57 Ma (Bixiling block) to 50 Ma and 53.6 Ma, was more rapid than the first, the cooling rates were 28.2℃/Ma and 32.3℃/Ma, respectively. The modern orographic shape of Dabie orogenic belt appeared in an embryonic form as early as the age of 50 Ma.