SHRIMP U‐Pb zircon dating from Sulu‐Dabie dolomitic marble, eastern China: constraints on prograde, ultrahigh‐pressure and retrograde metamorphic ages
SHRIMP U‐Pb zircon dating from Sulu‐Dabie dolomitic marble, eastern China: constraints on prograde, ultrahigh‐pressure and retrograde metamorphic ages
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DOI：
10.1111/j.1525-1314.2006.00655.x
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发表时间：
2006-09
影响因子：
3.4
通讯作者：
F. Liu;Axel Gerdes;J. Liou;H. Xue;F. Liang
中科院分区：
文献类型：
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作者：
F. Liu;Axel Gerdes;J. Liou;H. Xue;F. Liang
激光拉曼光谱和阴极发光 (CL) 图像显示，苏鲁-大别白云质大理岩中的锆石具有继承性（碎屑）、顺行、超高压 (UHP) 和逆行变质生长的独特特征。继承的锆石域在 CL 图像中呈暗发光，并包含 Qtz + Cal + Ap 矿物包裹体。前变质域在 CL 图像中呈白色发光，并保留了 Qtz + Dol + Grt + Omp + Phe + Ap 的石英榴辉岩相组合，形成于 542–693 °C 和 1.8–2.1 GPa。相比之下，超高压变质域在 CL 图像中呈灰色发光，保留了 Coe + Grt + Omp + Arg + Mgs + Ap 的超高压组合，并记录了 739–866 °C 和 >5.5 GPa 的超高压条件。最外层逆退边缘具有暗发光的 CL 图像，并含有方解石等低磷矿物，与区域角闪岩相退缩有关。激光烧蚀 ICP-MS 微量元素数据显示，主要是岩浆起源的继承核心与响应顺行、超高压和逆行变质作用而生长的锆石域之间存在显着差异。对这些分区锆石进行 SHRIMP U-Pb 定年，确定了四个离散的 206Pb/238U 年龄组：1823-503 Ma 记录在源自各种元古代原岩的继承（碎屑）锆石中，前行域记录了 254-239 Ma 的石英榴辉岩相变质作用，超高压生长域发生在 238-230 Ma，晚最外缘的角闪岩相退行叠印仅限于 218-206 Ma。因此，扬子克拉通元古代大陆物质在早三叠世俯冲到55-60公里深度，并在石英榴辉岩相条件下重结晶。随后这些变质岩在中三叠世进一步俯冲至165~175 km深度，经历超高压变质作用，最后在晚三叠世这些超高压变质岩被挖出至地壳中层（约30 km），并受到区域角闪岩相变质作用的叠印。从SHRIMP数据和变质P-T条件推导出来的俯冲和折返速率分别为9-10 km Myr−1和6.4 km Myr−1，这些快速的俯冲-折返速率可以解释所获得的P-T-t路径。如此快速的折返表明，返回地壳深处的苏鲁-大别超高压岩石是由浮力驱动的，而浮力是地幔深度板片断裂的结果。
Laser Raman spectroscopy and cathodoluminescence (CL) images show that zircon from Sulu‐Dabie dolomitic marbles is characterized by distinctive domains of inherited (detrital), prograde, ultrahigh‐pressure (UHP) and retrograde metamorphic growths. The inherited zircon domains are dark‐luminescent in CL images and contain mineral inclusions of Qtz + Cal + Ap. The prograde metamorphic domains are white‐luminescent in CL images and preserve a quartz eclogite facies assemblage of Qtz + Dol + Grt + Omp + Phe + Ap, formed at 542–693 °C and 1.8–2.1 GPa. In contrast, the UHP metamorphic domains are grey‐luminescent in CL images, retain the UHP assemblage of Coe + Grt + Omp + Arg + Mgs + Ap, and record UHP conditions of 739–866 °C and >5.5 GPa. The outermost retrograde rims have dark‐luminescent CL images, and contain low‐P minerals such as calcite, related to the regional amphibolite facies retrogression. Laser ablation ICP‐MS trace‐element data show striking difference between the inherited cores of mostly magmatic origin and zircon domains grown in response to prograde, UHP and retrograde metamorphism. SHRIMP U‐Pb dating on these zoned zircon identified four discrete 206Pb/238U age groups: 1823–503 Ma is recorded in the inherited (detrital) zircon derived from various Proterozoic protoliths, the prograde domains record the quartz eclogite facies metamorphism at 254–239 Ma, the UHP growth domains occurred at 238–230 Ma, and the late amphibolite facies retrogressive overprint in the outermost rims was restricted to 218–206 Ma. Thus, Proterozoic continental materials of the Yangtze craton were subducted to 55–60 km depth during the Early Triassic and recrystallized at quartz eclogite facies conditions. Then these metamorphic rocks were further subducted to depths of 165–175 km in the Middle Triassic and experienced UHP metamorphism, and finally these UHP metamorphic rocks were exhumed to mid‐crustal levels (about 30 km) in the Late Triassic and overprinted by regional amphibolite facies metamorphism. The subduction and exhumation rates deduced from the SHRIMP data and metamorphic P–T conditions are 9–10 km Myr−1 and 6.4 km Myr−1, respectively, and these rapid subduction–exhumation rates may explain the obtained P–T–t path. Such a fast exhumation suggests that Sulu‐Dabie UHP rocks that returned towards crustal depths were driven by buoyant forces, caused as a consequence of slab breakoff at mantle depth.