Two-Stage Exhumation of Ultrahigh-Pressure Metamorphic Rocks from the Western Dabie Orogen, Central China
Two-Stage Exhumation of Ultrahigh-Pressure Metamorphic Rocks from the Western Dabie Orogen, Central China
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DOI：
10.1086/657259
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发表时间：
2011-01
期刊：
影响因子：
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通讯作者：
Y. Wu;Shan Gao;Xiaochi Liu;Jing Wang;M. Peng;H. Gong;Honglin Yuan
中科院分区：
文献类型：
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作者：
Y. Wu;Shan Gao;Xiaochi Liu;Jing Wang;M. Peng;H. Gong;Honglin Yuan
通过对大别造山带西段新县超高压单元榴辉岩的锆石微量元素、U-Pb年龄和Lu-Hf同位素组成的测定，研究了榴辉岩的折返过程。阴极发光成像显示，大多数锆石是变质的，而有些样品中继承了岩浆核。岩浆岩核具有较高的Th/U和Lu/Hf比值，微量元素含量高，Eu负异常明显，与岩浆成因一致。它们的上交点年龄为Ma，正的δ Hf（t）值为8.0，表明大别造山带西部中元古代早期地壳在新元古代中期受到改造。变质锆石中含有石榴石、绿辉石、金红石和石英等矿物包裹体。它们的特征是Nb、Ta、Y和重稀土元素含量低，重稀土配分模式近平坦，Eu负异常不明显。这表明变质锆石形成于高压石英榴辉岩相变质条件下。两个样品中的变质锆石的锆石包钛温度分别为0.001 ℃和0.001 ℃，表明它们形成于不同的高压榴辉岩相变质阶段。这两个样品中变质锆石的U-Pb加权平均年龄分别为100 Ma和100 Ma，表明大别造山带西部超高压岩石的折返作用起始于1090 km和1700 °C，并在1050 km和1600 °C发生高压石英榴辉岩相退变质作用。由此得出的最大折返速率和平均冷却速率分别为0.33 cm/yr和8°C/Ma。根据已发表的本区超高压岩石Rb-Sr和Ar-Ar年龄值为212 Ma，后期折返的最大折返速率和平均冷却速率分别为0.67cm/yr和65°C/Ma。这两个阶段的冷却过程可能是大陆碰撞带高压-超高压变质岩石普遍存在的。
Zircon trace element, U-Pb age, and Lu-Hf isotope composition were determined for two eclogite samples from the Xinxian ultrahigh-pressure (UHP) unit in the western Dabie orogen to constrain its exhumation processes. Cathodoluminescence imaging reveals that most zircons are metamorphic, while some have inherited magmatic cores in one sample. The magmatic cores have high Th/U and Lu/Hf ratios, high trace element contents, and a clear negative Eu anomaly, consistent with their igneous genesis. They yield an upper intercept age of Ma and a positive ϵHf(t) value of 8.0, arguing for reworking of early Mesoproterozoic crust during the middle Neoproterozoic in the western Dabie orogen. The metamorphic zircons contain mineral inclusions of garnet, omphacite, rutile, and quartz. They are characterized by low Nb, Ta, Y, and heavy rare earth elements (HREE) contents, a nearly flat HREE pattern, and an insignificant negative Eu anomaly. These indicate that the metamorphic zircons formed under high-pressure (HP) quartz eclogite-facies metamorphic conditions. The metamorphic zircons in the two samples have Ti-in-zircon temperatures of and C, respectively, suggesting that they formed in different HP eclogite-facies metamorphic stages. The metamorphic zircons in the two samples yield weighted mean U-Pb ages of and Ma, which may date the initial exhumation to ∼90 km and ∼700°C and a subsequent HP quartz eclogite-facies retrogression at ∼50 km and ∼600°C for ultrahigh-pressure (UHP) rocks in the western Dabie orogen. This yields a maximum exhumation and average cooling rates of ∼0.33 cm/yr and 8°C/Ma, respectively. Based on the published Rb-Sr and Ar-Ar ages of ∼212 Ma for the UHP rocks in the area, the subsequent stage of exhumation of UHP rocks has a maximum exhumation and average cooling rates of 0.67 cm/yr and 65°C/Ma, respectively. This two-stage cooling process may be common for HP-UHP metamorphic terranes in continental collision zones.