苏鲁超高压变质带含褐帘石正、副片麻岩中变质锆石的地球化学性质及其SHRIMP U-Pb定年

Geochemical characteristics and SHRIMP U-Pb dating of metamorphic zircons from allanitebearing para-and ortho-gneiss in Sulu ultrahigh-pressure metamorphic (UHP) belt.

含褐帘石正、副片麻岩是苏鲁地体重要的超高压变质岩石。矿物包体激光拉曼鉴定、阴极发光图像分析、锆石微区LA-ICP-MS成分测试及SHRIMP U-Pb定年结果表明,含褐帘石正、副片麻岩中的锆石具有明显的三层结构,即弱发光效应的继承性(碎屑或岩浆结晶)锆石的核、强发光效应的含柯石英变质增生幔部和弱发光效应的退变边。继承性岩浆结晶锆石核部记录了新元古代(>810Ma)的原岩形成年龄,含柯石英锆石微区记录了241～223Ma的超高压变质年龄(正、副片麻岩的加权平均年龄分别为230±7Ma和229±7Ma),锆石的退变边记录了217～200Ma的角闪岩相退变质年龄(正、副片麻岩的加权平均年龄分别为211±3Ma和210±2Ma),由此限定苏鲁地体构造抬升速率为5.3km/Myr,表明苏鲁超高压岩石经历了一个相对快速抬升的变质演化过程。与苏鲁-大别其它超高压岩石变质增生锆石对比,含褐帘石正、副片麻岩中的超高压变质增生锆石微区具有十分独特的地球化学性质,主要表现为重稀土元素(HREE)明显富集,U含量明显偏低(10～29×10^(-6)),而Th/U比值则明显偏高(0.45～0.92)。HREE的明显富集与寄主岩石普遍存在超高压矿物褐帘石(富集轻稀土元素)有关,而低U含量和高Th/U比值的特点则表明超高压峰期变质阶段处在高氧逸度(f_o_2)的体系环境。

Laser Raman spectroscopy and cathodoluminescence (CL) images show that most zircons separated from allanite-bearing paragneiss (S27) and orthogneiss (S28) in drill hole CCSD-PP2 at Maobei, southwestern Sulu UHP terrane, retain low-P mineral-bearing inherited (detrital or magmatic) cores, coesite-hearing UHP mantles and retrogressive rims. LA-ICP-MS and SHRIMP U-Pb analyses of these zoned zircons identify three discrete and meaningful age groups: Neoproterozoic protolith ages (> 810Ma) are recorded in the inherited cores, the UHP metamorphic event in the coesite-bearing zircon domains occurred at 241 similar to 223Ma (the weighted mean age is 229 +/- 7Ma and 230 +/- 7Ma for S27 and S28, respectively), and the late amphibolite-facies retrogression recorded in the outmost rims was at 217 similar to 200Ma (the weighted mean age is 210 +/- 2Ma and 211 +/- 3Ma for S27 and S28, respectively). Thus, the exhumation rate deduced from the SHRIMP and metamorphic P-T conditions for Sulu UHP terrane is 5.3km/Myr. Compared with the UHP zircon domains from common eclogite and its country-rocks in the Sulu-Dabie UHP terrane, the coesite-bearing zircon domains from S27 and S28 have unique trace element and rare earth element (REE) compositions. They are characterized by higher heavy rare earth element (HREE) content and lower U content (10 similar to 29 X 10(-6)) with higher Th/U ratios (0.45 similar to 0.92), which are genetically related to the compositions of UHP metamorphic allanite (being rich in light rare earth element) from the host-rocks. Moreover, lower U content and higher Th/U ratio of the coesite-bearing zircon domains are evidently controlled by higher oxygen fugacity during the UHP peak metamorphism.