| Zoned Zircon from Eclogite Lenses in Marbles from the Dabie-Sulu UHP Terrane, China： A Clear Record of Ultra-deep Subduction and Fast Exhumation |
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| 作者姓名： | Liu Fulai |
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| 作者单位： | Ministry of Land and Mineral Resources of China |
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| 基金项目： | We are grateful for Prof. Jian Ping and Dr. Shi Yeruo for their assistance with SHRIMP U-Pb dating. This study was supported by the National 973 Project of Chinese Ministry of Science and Technology （Grant No. 2003CB716502）, the Natural Science Foundation of China （Grant No. 40399143） and the German Science Foundation （DFG; grant No. GE 1152/2-2 and WE2850/3- 1）. |
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| 摘 要： | Eclogite lenses in marbles from the Dabie-Sulu ultrahigh-pressure (UHP) terrane are deeply subducted meta-sedimentary rocks. Zircons in these rocks have been used to constrain the ages of prograde and UHP metamorphism during subduction, and later retrograde metamorphism during exhumation. Inherited (detrital) and metamorphic zircons were distinguished on the basis of transmitted light microscopy, cathodoluminescence (CL) imaging, trace element contents and mineral inclusions. The distribution of mineral inclusions combined with CL imaging of the metamorphic zircon make it possible to relate zircon zones (domains) to different metamorphic stages. Domain 1 consists of rounded, oblong and spindly cores with dark-luminescent images, and contains quartz eclogite facies mineral inclusion assemblages, indicating formation under high-pressure (HP) metamorphic conditions of T = 571-668℃and P = 1.7-2.02 GPa. Domain 2 always surrounds domain 1 or occurs as rounded and spindly cores with white-luminescent images. It contains coesite edogite facies mineral inclusion assemblages, indicating formation under UHP metamorphic conditions of T = 782-849℃and P > 5.5 GPa. Domain 3, with gray-luminescent images, always surrounds domain 2 and occurs as the outermost zircon rim. It is characterized by low-pressure mineral inclusion assemblages, which are related to regional amphibolite facies retrograde metamorphism of T = 600-710℃and P = 0.7-1.2 GPa. The three metamorphic zircon domains have distinct ages; sample H1 from the Dabie terrane yielded SHRIMP ages of 245±4 Ma for domain 1, 235±3 Ma for domain 2 and 215±6 Ma for domain 3, whereas sample H2 from the Sulu terrane yielded similar ages of 244±4 Ma, 233±4 Ma and 214±5 Ma for Domains 1, 2 and 3, respectively. The mean ages of these zones suggest that subduction to UHP depths took place over 10-11 Ma and exhumation of the rocks occurred over a period of 19-20 Ma. Thus, subduction from～55 km to > 160 km deep mantle depth took place at rates of approximately 9.5-10.5 km/Ma and exhumation from depths >160 km to the base of the crust at～30 km occurred at approximately 6.5 km/Ma. We propose a model for these rocks involving deep subduction of continental margin lithosphere followed by ultrafast exhumation driven by buoyancy forces after break-off of the UHP slab deep within the mantle.
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| 关 键 词： | 大别-苏鲁超高压变质带 地体 大理岩 榴辉岩透镜体 环带锆石 超深俯冲 剥露 测年 |
| 收稿时间： | 2006-12-25 |
| 修稿时间： | 2007-01-24 |
Zoned Zircon from Eclogite Lenses in Marbles from the Dabie-Sulu UHP Terrane,China: A Clear Record of Ultra-deep Subduction and Fast Exhumation |
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| Authors: | Liu Fulai |
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| Institution: | Ministry of Land and Mineral Resources of China |
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| Abstract: | Abstract: Eclogite lenses in marbles from the Dabie‐Sulu ultrahigh‐pressure (UHP) terrane are deeply subducted meta‐sedimentary rocks. Zircons in these rocks have been used to constrain the ages of prograde and UHP metamorphism during subduction, and later retrograde metamorphism during exhumation. Inherited (detrital) and metamorphic zircons were distinguished on the basis of transmitted light microscopy, cathodoluminescence (CL) imaging, trace element contents and mineral inclusions. The distribution of mineral inclusions combined with CL imaging of the metamorphic zircon make it possible to relate zircon zones (domains) to different metamorphic stages. Domain 1 consists of rounded, oblong and spindly cores with dark‐luminescent images, and contains quartz eclogite facies mineral inclusion assemblages, indicating formation under high‐pressure (HP) metamorphic conditions of T = 571–668°C and P = 1.7‐2.02 GPa. Domain 2 always surrounds domain 1 or occurs as rounded and spindly cores with white‐luminescent images. It contains coesite eclogite facies mineral inclusion assemblages, indicating formation under UHP metamorphic conditions of T = 782–849°C and P > 5.5 GPa. Domain 3, with gray‐luminescent images, always surrounds domain 2 and occurs as the outermost zircon rim. It is characterized by low‐pressure mineral inclusion assemblages, which are related to regional amphibolite facies retrograde metamorphism of T = 600–710°C and P = 0.7‐1.2 GPa. The three metamorphic zircon domains have distinct ages; sample H1 from the Dabie terrane yielded SHRIMP ages of 245 ± 4 Ma for domain 1, 235 ± 3 Ma for domain 2 and 215 ± 6 Ma for domain 3, whereas sample H2 from the Sulu terrane yielded similar ages of 244 ± 4 Ma, 233 ± 4 Ma and 214 ± 5 Ma for Domains 1, 2 and 3, respectively. The mean ages of these zones suggest that subduction to UHP depths took place over 10–11 Ma and exhumation of the rocks occurred over a period of 19–20 Ma. Thus, subduction from ～ 55 km to > 160 km deep mantle depth took place at rates of approximately 9.5–10.5 km/Ma and exhumation from depths >160 km to the base of the crust at ?30 km occurred at approximately 6.5 km/Ma. We propose a model for these rocks involving deep subduction of continental margin Iithosphere followed by ultrafast exhumation driven by buoyancy forces after break‐off of the UHP slab deep within the mantle. |
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| Keywords: | zoned zircon SHRIMP U-Pb dating ultra-deep subduction fast exhumation eclogite lenses in marble Dabie-Sulu UHP belt |
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